CN1598032A - High-carbon hot-rolled steel plate,cold-rolled steel plate and making method thereof - Google Patents

Abstract

High-carbon hot-rolled steel sheet, containing C: 0.20-0.48% by mass percentage, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% or less , N: 0.005% or less, B: 0.001-0.005%, Cr: 0.05-0.3%, the rest is composed of iron and unavoidable impurities, with a ferrite structure with an average particle size of 6 μm or less and an average particle size of 0.1 μm Above carbides less than 1.20μm. The ferrite structure includes ferrite grains substantially free of carbides, and the volume ratio of ferrite grains substantially free of carbides is 10% or less. A manufacturing method comprising the steps of cooling the hot-rolled steel sheet at a cooling rate greater than 120°C/s and at a cooling end temperature of 620°C or lower, and coiling the cooled hot-rolled steel sheet at a coiling temperature of 600°C or lower The step is a step of annealing the coiled hot-rolled steel sheet at an annealing temperature of 640° C. or higher and an Ac ₁ transformation point or lower. The C content of the high-carbon cold-rolled steel sheet is 0.20-0.58%.

Description

High-carbon hot-rolled steel sheet, cold-rolled steel sheet, and their manufacturing methods

technical field

The present invention relates to high-carbon hot-rolled steel sheets, cold-rolled steel sheets and their manufacturing methods. In particular, it relates to high-carbon hot-rolled steel sheets and cold-rolled steel sheets used for automobile structural parts and the like, and their manufacturing methods.

Background technique

High carbon steel sheets are used for tools or automotive parts such as gears and transmissions. High-carbon steel plates are subjected to heat treatment such as quenching and tempering after punching and forming. One of the demands of users who process these components is to improve burring performance in forming after punching. The hole-expanding performance was evaluated by stretch-flange formability as the press-formability. Therefore, a material excellent in stretch flanging properties is desired. In addition, in the case of forming a complex shape, good tensile properties as an index of ductility are also required.

Thus, several techniques have been studied for improving the tensile flanging properties of high carbon steel sheets. For example, Japanese Unexamined Patent Publication No. 11-269552 discloses a method for producing a medium/high carbon steel sheet with excellent stretch flanging performance in a cold rolling process. This method uses steel containing C: 0.1 to 0.8% by mass, the metal structure is essentially ferrite + pearlite structure (ferrite+pearlite structure), and the area ratio of pro-eutectoid ferrite is determined by the C content (mass) as required. %) above the specified value determined, and the pearlite lamellar spacing (pearlite lamellar spacing) is above 0.1 μm. The hot-rolled steel sheet is cold-rolled at a rolling reduction of 15% or more, and then the manufactured cold-rolled steel sheet is subjected to three-stage or two-stage annealing. In the three-stage or two-stage annealing, the above-mentioned cold-rolled steel sheet is kept for a long time in the three-stage or two-stage temperature range.

In addition, Japanese Patent Application Laid-Open No. 11-269553 discloses that steel containing C: 0.1 to 0.8% by mass, proeutectoid ferrite area ratio (%) is more than a specified value determined by C content, proeutectoid ferrite The hot-rolled steel plate with pro-eutectoid ferrite+pearlite structure is annealed by continuously performing the first stage of heating and holding and the second stage of heating and holding.

Further, JP-A-2003-13145 discloses a high-carbon hot-rolled steel sheet excellent in stretch flanging performance. Steel containing 0.2 to 0.7% by mass of C is hot-rolled above the finish rolling temperature (Ar ₃ transformation point -20°C), cooled at a cooling rate greater than 120°C/s and at a cooling termination temperature of 650°C or less, and then Coiling is performed at a coiling temperature of 600°C or lower, and after pickling, annealing is performed at an annealing temperature of 640°C or higher and an Ac ₁ transformation point or lower. The average grain size of carbides is controlled to be more than 0.1 μm and less than 1.2 μm, and the volume ratio of ferrite grains not containing carbides is less than 10%.

In addition, JP-A-2003-13144 discloses a high-carbon cold-rolled steel sheet excellent in stretch flanging performance. Steel containing 0.2 to 0.7% by mass of C is hot-rolled above the finish rolling temperature (Ar ₃ transformation point -20°C), cooled at a cooling rate greater than 120°C/s and at a cooling termination temperature of 650°C or less, and then Coil at a coiling temperature below 600°C, and after pickling, cold-roll with a cold rolling reduction of 30% or more, and the cold-rolled steel sheet produced is annealed at an annealing temperature above 600°C and below the Ac ₁ transformation point annealing. The average grain size of carbides is controlled to be 0.1 μm or more and less than 2.0 μm, and the volume ratio of ferrite grains without carbides is 15% or less.

In the steel plates described in JP-A-11-269552 and JP-A-11-269553, the ferrite structure is composed of proeutectoid ferrite, and since it does not substantially contain carbides, it is soft and has excellent ductility. The flanging performance may not be good. The reason for this is considered as follows. During punching, due to the large deformation of the pro-eutectoid ferrite near the punching end face, there is a great difference in the amount of deformation between the pro-eutectoid ferrite and the ferrite containing spherical carbides. As a result, stress concentration occurs near the grain boundaries of these crystal grains with large differences in strain, and cavities occur at the interface between the spheroidized structure and ferrite. As it develops into cracks, the tensile flanging performance is deteriorated as a result.

As a solution, it is considered to soften the entire body by strengthening spheroidizing annealing. However, in the case of enhanced spheroidizing annealing, the spheroidized carbides become coarser and become the starting point of cavities during processing. At the same time, carbides are difficult to dissolve in the heat treatment stage after processing, resulting in a decrease in quenching strength.

In addition, recently, from the viewpoint of improving productivity, the requirements for processing levels have become more stringent. Therefore, even if the hole expansion process is performed on high carbon steel plate, due to the increase in the degree of processing, etc., cracks are likely to occur on the punched end surface. Therefore, high carbon steel sheets are also required to have high tensile flanging properties.

In view of these circumstances, the present inventors developed Japanese Patent Application Laid-Open No. 2003-13145 for the purpose of providing a high-carbon steel sheet that can be produced without long-term multi-stage annealing, that is less prone to cracking at the punched end surface, and that has excellent stretch flanging performance. No. 2003-13144 Publication and the technology described in the Japanese Patent Application Laid-Open No. 2003-13144. These technologies can be used to manufacture high-carbon hot-rolled steel sheets or high-carbon cold-rolled steel sheets with excellent stretch flanging properties.

Recently, for applications such as drive system components, from the viewpoint of good durability and light weight, the strength can be improved even with parts such as integrally formed parts without heat treatment. Strength above 440MPa. In addition to such a request, hot-rolled steel sheets are required in order to reduce the manufacturing cost of components.

There are more than 10 stamping processes in the one-piece forming. Since it is not only flanging processing, but also a complex combination of forming modes such as auxetic and bending, it requires both stretch flanging performance and ductility.

However, if the technology described in JP-A-2003-13145 and JP-A-2003-13144 is to achieve TS≥440 MPa (converted into HRB hardness above 73 degrees), sufficient stretch flanging performance may not be obtained. That is to say, the expansion rate (λ) is used to evaluate the performance of stretch flanging. It is hoped that λ≥70%, preferably λ≥75%, but the above-mentioned technology cannot stably ensure the requirements of TS and stretch flanging performance at the same time. Furthermore, the ductility problem is not addressed with the above techniques.

Contents of the invention

The object of the present invention is to provide a high-quality high-strength flange with excellent tensile flanging performance that satisfies the hole expansion ratio λ≥70% and elongation of 35% or more while the punched end face is difficult to produce cracks and has a tensile strength of 440 MPa or more. Carbon hot-rolled steel sheet, cold-rolled steel sheet, and their manufacturing methods.

In order to achieve the above object, the present invention provides, by mass percentage, essentially composed of C: 0.20-0.48%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al High-carbon hot-rolled steel sheet consisting of: 0.1% or less, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to 0.3%, and the balance is iron and unavoidable impurities. The high-carbon hot-rolled steel sheet has a ferrite structure with an average particle size of 6 μm or less and carbides with an average particle size of 0.1 μm or more and less than 1.20 μm. The ferrite structure includes ferrite grains substantially free of carbides, and the volume ratio of ferrite grains substantially free of carbides is 10% or less.

It is desirable that the carbide has an average particle diameter of 0.5 μm or more and less than 1.20 μm. It is desirable that the ferrite grains have a volume ratio of 5% or less. Further, it is desirable that the ferrite grains have a volume ratio of 5% or less, and that the carbides have an average grain size of 0.5 μm or more and less than 1.20 μm.

In addition, the present invention provides a material consisting of C: 0.20-0.58%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% or less in terms of mass percentage. High-carbon cold-rolled steel sheet consisting of less than, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to 0.3%, and the rest is iron and unavoidable impurities. The high-carbon cold-rolled steel sheet has a ferrite structure with an average particle size of 6 μm or less and carbides with an average particle size of 0.1 μm or more and less than 1.20 μm. The ferrite structure includes ferrite grains substantially free of carbides, and the volume ratio of ferrite grains substantially free of carbides is 15% or less.

It is desirable that the carbide has an average particle diameter of 0.5 μm or more and less than 1.20 μm. It is desirable that the ferrite grains have a volume ratio of 10% or less. Further, it is desirable that the ferrite grains have a volume ratio of 10% or less, and that the carbides have an average grain size of 0.5 μm or more and less than 1.20 μm.

The present invention provides a method for manufacturing a high-carbon hot-rolled steel sheet comprising a hot rolling process, a cooling process, a coiling process, and annealing.

The hot rolling process is substantially composed of C: 0.20-0.48%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% in terms of mass percentage Below, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to 0.3%, and the rest is composed of iron and unavoidable impurities, and the finish rolling temperature is above (Ar ₃ transformation point -10°C) (finishing temperature) is formed by hot rolling. In the cooling step, the hot-rolled steel sheet is cooled at a cooling rate of more than 120°C/sec and at a cooling end temperature of 620°C or lower. In the coiling step, the cooled hot-rolled steel sheet is coiled at a coiling temperature of 600° C. or lower. The annealing step is constituted by annealing the coiled hot-rolled steel sheet at an annealing temperature of not less than 640° C. and not more than the Ac ₁ transformation point.

It is desirable that this cooling process is composed of a process of cooling the hot-rolled steel plate at a cooling rate greater than 120° C./second and at a cooling termination temperature below 600° C. The process configuration of coiling is performed at a coiling temperature below ℃.

This annealing step is preferably constituted by annealing the coiled hot-rolled steel sheet at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower.

Furthermore, it is desirable that the cooling process is composed of a process of cooling the hot-rolled steel sheet at a cooling rate greater than 120° C./second and at a cooling termination temperature of 600° C. or lower. The step of coiling at a coiling temperature of 500°C or lower is constituted, and the annealing step is constituted of annealing the coiled hot-rolled steel sheet at an annealing temperature of 680°C or higher and an Ac ₁ transformation point or lower.

Furthermore, the present invention provides a method for producing a high-carbon cold-rolled steel sheet comprising a hot-rolling step, a cooling step, a coiling step, a cold-rolling step, and an annealing step. The hot rolling process is substantially composed of C: 0.20-0.58%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% in terms of mass percentage Below, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to 0.3%, and the rest is composed of iron and unavoidable impurities, and the finish rolling temperature is above (Ar ₃ transformation point -10°C) Process configuration for hot rolling. The cooling step is composed of a step of cooling the hot-rolled steel sheet at a cooling rate of more than 120°C/sec and at a cooling end temperature of 620°C or lower. The coiling step is composed of a step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600° C. or lower. The cold-rolling step is composed of a step of cold-rolling the coiled hot-rolled steel sheet at a rolling reduction of 30% or more after pickling. The annealing step is constituted by annealing the coiled hot-rolled steel sheet at an annealing temperature of 640° C. or higher and an Ac ₁ transformation point or lower.

It is desirable that this cooling process is composed of a process of cooling the hot-rolled steel plate at a cooling rate greater than 120° C./second and at a cooling termination temperature below 600° C. The process configuration of coiling is performed at a coiling temperature below ℃.

This annealing step is preferably constituted by annealing the hot-rolled steel sheet at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower.

The method for producing a high-carbon cold-rolled steel sheet further preferably includes a step of annealing at an annealing temperature of 640° C. or higher and an Ac ₁ transformation point or lower after the coiling step and before the cold-rolling step. The aforementioned annealing step is desirably constituted by annealing the coiled hot-rolled steel sheet at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower. In the manufacturing method of the high-carbon cold-rolled steel sheet, it is desirable that the cooling step is composed of a step of cooling the hot-rolled steel sheet at a cooling rate greater than 120°C/s and at a cooling termination temperature of 600°C or lower. The coiling step is constituted by coiling the cooled hot-rolled steel sheet at a coiling temperature of 500° C. or lower.

Description of drawings

FIG. 1 is a graph showing the relationship between the Mn content and the hardness after quenching in Embodiment 1. FIG.

FIG. 2 is a graph showing the relationship between the Mn content and the hardness after quenching in Embodiment 2. FIG.

Detailed ways

Implementation mode 1:

Embodiment 1 provides that the composition contains C: 0.20-0.48%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% or less in terms of mass percentage , N: 0.005% or less, B: 0.001-0.005%, Cr: 0.05-0.3%, and the rest are iron and unavoidable impurities. The average particle size of ferrite is below 6 μm, and the average particle size of carbide is 0.1 A high-carbon hot-rolled steel sheet having a micrometer of not less than 1.20 μm and a structure in which the volume ratio of ferrite grains substantially free of the aforementioned carbides is 10% or less. It is desirable that the average particle size of carbides be more than 0.5 μm and less than 1.20 μm. It is desirable that the volume ratio of ferrite crystal grains be 5% or less.

Furthermore, Embodiment 1 provides that the steel having the above-mentioned composition is hot-rolled at a finishing temperature above (Ar ₃ transformation point - 10°C), and then cooled at a cooling rate of more than 120°C/sec and below 620°C. The production method of high-carbon hot-rolled steel sheet is cooled at the termination temperature, then coiled at a coiling temperature below 600°C, and then annealed at an annealing temperature above 640°C and below the Ac ₁ transformation point. It is desirable to perform the above-mentioned annealing at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower. It is desirable to perform the cooling at a cooling end temperature of 600°C or lower, and to perform the coiling at a coiling temperature of 500°C or lower.

The high-carbon hot-rolled steel sheet of Embodiment 1 and its manufacturing method are obtained through intensive research on the effects of composition and microstructure on the tensile flanging performance and ductility of the high-carbon steel sheet. During this process, it was found that the factors affecting the tensile flanging performance and ductility of the steel plate are not only the composition and the shape and amount of carbides, but also the dispersed form of carbides.

In addition, it was found that the tensile strength of the high-carbon hot-rolled steel sheet can be improved by separately controlling the average grain size of carbides in the shape of carbides and the volume ratio of ferrite grains in the dispersed state of carbides that do not substantially contain carbides. side performance issues. Furthermore, it was found that by controlling the composition and ferrite particle size, the tensile flanging performance and strength can be stably balanced at a high level, and the elongation can be stably increased by specifying and controlling the carbide particle size. On the basis of this understanding, the manufacturing method of controlling the above-mentioned structure was studied, and the manufacturing method of high-carbon hot-rolled steel plate with excellent tensile flanging performance and ductility was determined.

The constituent elements of Embodiment 1 will be described below.

C content: 0.20 to 0.48 (mass%, the same below)

C is an important element that forms carbides and affects the hardness after quenching. However, if the C content is less than 0.20%, pro-eutectoid ferrite is obviously formed in the structure after hot rolling, increasing ferrite grains substantially free of carbides, and the distribution of carbides becomes uneven. In addition, ferrite grains also become coarse. Furthermore, even in this case, sufficient strength as a mechanical structural member cannot be obtained after quenching. On the other hand, if the C content exceeds 0.48%, the tensile flanging property and ductility are low even after annealing. Therefore, the C content is specified to be not less than 0.20% and not more than 0.48%.

Si: 0.1% or less

Si is an element that improves hardenability and improves material strength by solid solution strengthening (solid solution strengthening), so it is desirable to contain 0.005% or more. However, if the content exceeds 0.1%, pro-eutectoid ferrite is likely to be formed, ferrite grains substantially free of carbides increase, and the tensile flanging performance deteriorates. Therefore, the Si content is limited to 0.1% or less.

Mn: 0.20～0.60%

Like Si, Mn is an element that improves hardenability and improves material strength by solid solution strengthening. In addition, it is an important element for fixing S in the form of MnS and preventing hot cracking of the slab. It is well known that the Mn content has a great influence on the hardenability. Therefore, the effect of the Mn content on the hardenability of the B and Cr-added steel of the present invention was studied.

Composed of C: 0.34%, Si: 0.04%, Mn: 0.10～0.90%, P: 0.01%, S: 0.005%, sol.Al: 0.03%, N: 0.0040%, B: 0.0025%, Cr: 0.25% After the constituent steel was dissolved, hot rolling was performed at a heating temperature of 1250°C, a finishing temperature of hot rolling of 880°C, and a coiling temperature of 560°C. Then annealed at 710° C. for 40 hours to make a steel plate with a thickness of 5.0 mm. After cutting the obtained steel plate into a size of 50×100 mm, the temperature was raised to 820° C. for 10 seconds with a heating furnace, and then quenched into oil at about 20° C. Hardenability was evaluated by measuring 10-point hardness on the quenched test piece with Rockwell hardness C (HRc). An average hardness (HRc) of 50 or more was evaluated as good. The obtained results are shown in Fig. 1 .

FIG. 1 is a graph showing the relationship between the Mn content and the hardness after quenching. According to Figure 1, it can be seen that the Mn content above 0.20% can ensure the hardness (HRc) above 50, and the Mn content above 0.35% can reach the hardness (HRc) of 55, which can obtain higher quenching hardness more stably.

In addition, from the perspective of improving material strength, fixing S in the form of MnS, and preventing slab thermal cracking, these effects are small when the Mn content is less than 0.20%, and it is conducive to the formation of pro-eutectoid ferrite, making ferrite Bulky.

On the other hand, if it is more than 0.60%, the tensile strength can be obtained, but manganese bands of segregated bands are remarkably easy to form, and the tensile flanging performance and elongation deteriorate.

For the above reasons, the Mn content is specified to be 0.20% to 0.60%, preferably 0.35% to 0.60%.

P: less than 0.02%

P is an element that should be reduced as much as possible because it segregates at grain boundaries and lowers toughness. However, the P content is allowed below 0.02%, so the P content is limited to below 0.02%.

S: less than 0.01%

S and Mn form MnS, which deteriorates the stretch flanging performance, so it is an element that should be reduced as much as possible. However, the S content is allowable below 0.01%, so the S content is limited to below 0.01%.

sol.Al: less than 0.1%

Al is used as a deoxidizer to improve the cleanliness of steel, so it is added in the steelmaking stage. Generally, sol.Al contains more than 0.005% in steel. On the other hand, even if Al is added to such an extent that the sol.Al content exceeds 0.1%, the effect of improving cleanliness is saturated, and the cost increases. Therefore, the sol.Al content in steel is stipulated below 0.1%.

N: 0.005% or less

N forms BN, which reduces the solid solution B content effective for hardenability and reduces hardenability, so it is an element that should be reduced as much as possible. However, since the N content of 0.005% or less is acceptable, the N content is limited to 0.005% or less.

B: 0.001～0.005%

B suppresses the formation of proeutectoid ferrite during the cooling process after hot rolling, and is an important element for improving the hardenability while improving the tensile flanging performance. However, when the B content is less than 0.001%, sufficient effects cannot be obtained. On the other hand, if it exceeds 0.005%, the effect will be saturated, the load of hot rolling will increase, and workability will fall. Therefore, the B content is specified to be not less than 0.001% and not more than 0.005%.

Cr: 0.05～0.3%

Like B, Cr suppresses the formation of pro-eutectoid ferrite in the cooling process after hot rolling, and is an important element for improving the hardenability while improving the tensile flanging performance. However, when the Cr content is less than 0.05%, sufficient effects cannot be obtained. On the other hand, even if it exceeds 0.3%, the hardenability improves, but the effect of suppressing the formation of pro-eutectoid ferrite is saturated, and the cost increases. Therefore, the Cr content is specified to be not less than 0.05% and not more than 0.3%.

Next, the structure of the steel sheet in Embodiment 1 will be described.

Average grain size of ferrite: below 6μm

The average particle size of ferrite is an important factor affecting the tensile flanging performance and material strength, and is an important condition in Embodiment 1. By making the ferrite crystal grains finer, the strength can be improved without deteriorating the tensile flanging performance. That is, by keeping the average particle size of ferrite below 6 μm, the tensile strength of the material can be ensured above 440 MPa, and excellent tensile flanging performance can be obtained at the same time. On the other hand, if fine crystal grains smaller than 1.0 μm are formed, the strength will increase significantly and the load may increase during press working, so the lower limit is preferably 1.0 μm or more. The ferrite grain size can be controlled by manufacturing conditions, especially finish rolling temperature and cooling end temperature.

Average particle size of carbides: 0.1 μm or more and less than 1.20 μm

The average particle size of carbides generally has a great influence on the machining performance and the generation of cavities in the reaming process. Carbide miniaturization can suppress the generation of cavities, but if the average particle size of carbides is less than 0.1 μm, the ductility decreases with the increase of hardness, so the tensile flanging performance also decreases. On the other hand, as the average particle size of carbides increases, the processing performance generally improves, but if it is more than 1.20 μm, the tensile flanging performance decreases due to the generation of cavities in the hole expansion process. Therefore, the average particle size of carbides should be controlled above 0.1 μm and less than 1.20 μm. Furthermore, by controlling the average particle size of carbides to be more than 0.5 μm and less than 1.20 μm, the increase in strength can be suppressed, while the elongation can be increased, and excellent tensile properties can be obtained. Therefore, it is preferably not less than 0.5 μm and less than 1.20 μm. In addition, the average particle size of carbides can be controlled by manufacturing conditions, especially cooling termination temperature, coiling temperature and annealing temperature. Here, regarding the particle size of carbides, the average value of the long and short diameters of carbides is taken as the particle size of each carbide, and the average value of the particle size of each carbide is defined as the average particle size of carbides.

Dispersion state of carbides: the volume ratio of ferrite grains substantially free of carbides is 10% or less

By making the dispersed state of the carbides uniform, the stress concentration on the punched end surface during hole expansion can be alleviated as described above, and the generation of cavities can be suppressed. By setting the volume ratio of ferrite grains substantially free of carbides to 10% or less, the dispersion state of carbides can be made uniform, and the stretch flanging performance can be remarkably improved. Therefore, the volume ratio of ferrite grains substantially free of carbides is defined to be 10% or less. Furthermore, by setting the volume ratio of ferrite grains substantially free of carbides to 5% or less, the dispersed state of carbides can be made more uniform, and very excellent stretch flanging performance can be obtained. Therefore, it is desirable to regulate it at 5% or less. On the other hand, this composition system is a hypoeutectoid steel, and considering that it is difficult to completely suppress proeutectoid ferrite, it is desirable to set the lower limit of the volume ratio of ferrite grains substantially free of carbides to 1%. Furthermore, the dispersed state of carbides, that is, the volume ratio of ferrite grains substantially free of carbides, can be determined by manufacturing conditions, especially the final rolling temperature, cooling rate after rolling, cooling termination temperature and coiling temperature Take control.

Among them, ferrite grains substantially free of carbides refer to ferrite grains in which carbides cannot be detected by observing the metal structure with a general optical microscope, and mean ferrite grains in which no carbides can be detected even with a scanning electron microscope at low magnification. to the ferrite grains of carbides. That is, ferrite grains substantially free of carbides in the present invention are defined as grinding the thick section of a steel plate sample, corroding it with nital etching solution, and observing it at 1000 times with a scanning electron microscope. Ferrite grains of carbides were also not detectable. Such ferrite grains are parts formed as pro-eutectoid ferrite after hot rolling, and carbides are not observed in the grains even in the state after annealing, that is, it can be said that they do not contain carbides substantially. ferrite grains.

Reasons for limiting the manufacturing conditions of Embodiment 1 will be described below.

Finishing temperature of hot rolling: (Ar ₃ transformation point -10°C) or above

When the finishing temperature of the steel during hot rolling is less than (Ar ₃ transformation point -10°C), due to the phase transformation of part of the ferrite, the grains of ferrite that do not substantially contain carbides increase, and the stretching is reversed. Edge performance deteriorates. In addition, since ferrite crystal grains are remarkably coarsened, and the average ferrite grain size exceeds 6 μm, tensile flanging performance deteriorates and strength decreases. Therefore, the finishing temperature of hot rolling is specified to be (Ar ₃ transformation point - 10°C) or higher. In this way, uniformity and miniaturization of the structure can be realized, and the performance and strength of the stretch flanging can be improved. On the other hand, the upper limit of the finish rolling temperature is not particularly limited, but at a high temperature exceeding 1000°C, scale-like defects are likely to occur, so it is preferably 1000°C or lower. In addition, the Ar ₃ transformation point (° C.) can be calculated by the following formula.

Ar ₃ =930.21-394.75C+54.99Si-14.40Mn+5.77Cr (1)

However, the element symbols in the formula represent the content (mass %) of each element, respectively.

Cooling conditions after hot rolling: Cooling rate > 120°C/s

In the present invention, rapid cooling (cooling) is performed after rolling in order to reduce the volume ratio of pro-eutectoid ferrite grains after phase transformation. If the cooling method after hot rolling is slow cooling, the degree of undercooling of austenite is small, and a large amount of proeutectoid ferrite is formed. When the cooling rate is less than 120° C./sec, pro-eutectoid ferrite is obviously formed, and the ferrite grains substantially free of carbides exceed 10%, and the tensile flanging performance deteriorates. Therefore, the cooling rate for post-rolling cooling is specified to be greater than 120°C/sec. On the other hand, the upper limit of the cooling rate is desirably 700° C./sec from the viewpoint of facility capacity.

Here, the cooling rate refers to the average cooling rate from the start of cooling after finish rolling to the stop of cooling. In addition, cooling generally begins within 3 seconds after finish rolling, but from the viewpoint of further refining the ferrite grains and pearlite after phase transformation and further improving workability, it is desirable that the cooling time be greater than 0.1 seconds and less than 1.0 seconds after finish rolling. Cooldown starts in seconds.

Cooling termination temperature: below 620°C

When the cooling end temperature in cooling after hot rolling is high, ferrite is formed in cooling to coiling, and at the same time, the aggregated structure of pearlite and the interlamellar spacing increase. Therefore, after annealing, the ferrite grains become coarse, and at the same time, fine carbides cannot be obtained, the strength decreases, and the tensile flanging performance deteriorates. When the cooling end temperature is higher than 620° C., the ferrite grains substantially free of carbides exceed 10%, and the tensile flanging performance deteriorates. Therefore, the cooling termination temperature of post-rolling cooling is specified to be 620°C or lower. Furthermore, in the case of making ferrite grains substantially free of carbides 5% or less, it is desirable to set the cooling end temperature at 600° C. or less. On the other hand, the lower limit of the cooling end temperature is not particularly limited, but the lower the temperature, the worse the shape of the steel sheet, so it is desirably set at 200°C.

Coiling temperature: below 600°C

After the cooling is terminated, the steel plate is coiled. The higher the coiling temperature, the larger the interlamellar spacing of pearlite. Therefore, carbides after annealing are coarsened, and when the coiling temperature exceeds 600° C., the stretch flanging performance deteriorates. Therefore, the coiling temperature is specified to be 600°C or lower. Furthermore, by setting the coiling temperature below 500°C, the dispersed state of carbides can be made more uniform, and very excellent stretch flanging performance can be obtained, so it is desirable to set it below 500°C. On the other hand, the lower limit of the coiling temperature is not particularly specified, but the lower the temperature, the worse the shape of the steel sheet is, so it is desirably set at 200°C or higher.

In order to make the dispersion of carbides more uniform and obtain excellent stretch flanging performance, it is desirable to cool at a cooling termination temperature below 600°C and at the same time perform coiling at a coiling temperature below 500°C.

Annealing temperature: above 640°C and below Ac ₁ transformation point

The hot-rolled steel sheet obtained above was annealed in order to spheroidize carbides. When the annealing temperature is less than 640°C, the spheroidization of carbides is insufficient or the average particle size of carbides is less than 0.1 μm, and the tensile flanging performance deteriorates. On the other hand, when the annealing temperature exceeds the Ac ₁ transformation point, a part of the steel is austenitized, and pearlite is formed again during cooling, so the tensile flanging performance deteriorates. In addition, elongation also deteriorated. According to the above reasons, the annealing temperature is specified to be above 640°C and below the Ac ₁ transformation point. Furthermore, by setting the annealing temperature above 680°C, the average particle size of the carbides can be made above 0.5 μm, a high elongation rate can be obtained, and more excellent stretch flanging performance can be obtained. Therefore, it is desirable to regulate the temperature at 680°C or higher and the Ac ₁ transformation point or lower. In addition, the Ac ₁ transformation point (° C.) can be calculated by the following formula.

Ac ₁ =754.83-32.25C+23.32Si-17.76Mn+17.13Cr (2)

The symbols of the elements in the formula represent the content (% by mass) of each element, respectively.

In the composition preparation of the high carbon steel in Embodiment 1, either a converter or an electric furnace can be used. The high-carbon steel prepared with such a composition is made into a billet by ingot casting-slab rolling or continuous casting. The steel slab is hot-rolled. At this time, the heating temperature of the slab is desirably set at 1300° C. or lower in order to avoid deterioration of the surface condition due to scale.

In hot rolling, rough rolling may be omitted and finish rolling may be performed, or continuous casting slab may be directly rolled, or direct rolling may be performed in which rolling is carried out while holding heat to prevent temperature drop. It is hoped that in order to ensure the final rolling temperature, the strip heater can also be used to heat the rolled piece during hot rolling. In order to promote spheroidization or reduce hardness, the steel coil after coiling can also be kept warm by slow cooling cover and other means.

After being coiled into a hot-rolled steel sheet, it is desirable to perform pickling and then annealing in the usual manner. For annealing, either box annealing or continuous annealing may be used. After that, level it as needed. Since flattening has no effect on hardenability, there are no restrictions on the conditions for flattening.

According to the above method, a high-carbon hot-rolled steel sheet excellent in stretch flanging performance or further in ductility can be obtained. What has been described above is an embodiment of the manufacturing method of the present invention, and is not limited thereto.

The reason why the high-carbon hot-rolled steel sheet thus produced has excellent tensile flanging properties is considered as follows. The internal structure of the punched end face has a great influence on the performance of stretch flanging. In particular, when there were many ferrite grains substantially free of carbides (proeutectoid ferrite after hot rolling), it was confirmed that cracks occurred at the interface with the spheroidized structure portion.

Observing the behavior of the microstructure, it is found that a cavity is obviously generated at the carbide interface due to stress concentration during blanking. The larger the carbide size and the more ferrite grains substantially free of carbides, the greater the stress concentration. These cavities are connected into cracks during hole reaming.

In this way, stress concentration can be reduced and the formation of cavities can be reduced by controlling not only the manufacturing conditions but also the average particle size of carbides and the proportion of ferrite grains substantially free of carbides.

Example 1

The continuous casting slab of steel with the chemical composition shown in Table 1 is heated at 1250°C, the finishing temperature of hot rolling is 880°C, the time from finishing rolling to the beginning of cooling is 0.7 seconds, and the cooling rate after hot rolling is Hot rolling was performed under conditions of 150°C/sec, cooling end temperature of 610°C, and coiling temperature of 560°C. Thereafter, it was pickled, and box annealed at 710° C. for 40 hours to manufacture a steel plate with a thickness of 5.0 mm. Among them, the chemical components (compositions) of Steel Nos. A to E and N are within the range of Embodiment 1, and Steel Nos. F to M are comparative examples whose compositions deviate from the range of Embodiment 1.

Samples were taken from these steel sheets, and the average particle size of ferrite, the average particle size of carbides, and the dispersion state of carbides were measured, evaluation of tensile flanging performance, and tensile tests were performed. Each test, measurement method and conditions are as follows.

(i) Average particle size of ferrite, average particle size of carbide and their dispersion state

After grinding the thick section of the sample plate and corroding it with nitric acid alcohol etching solution, the microstructure was photographed with a scanning electron microscope, and the characteristic value of the mark was determined.

First, the average grain size of ferrite was measured by the cut-off method in the test method for ferrite grain size specified in JIS Standard G0552 on the microstructure photograph taken with the above-mentioned scanning electron microscope at 1000 magnification.

Regarding the average particle size of carbides, also use the structure photos taken at 3000 times, draw 20 lines of 100mm in the thickness direction within the actual area of ^0.01mm2 , and compare the long diameter and The short diameter is measured, and the average value of the two is taken as the particle size of the carbide, and the average value of all the measured carbide particle sizes is obtained as the average particle size of the carbide.

In addition, regarding the dispersed state of carbides, the area ratio of ferrite grains in which no carbides were observed was measured for the microstructure photograph taken at a magnification of 1000, and this was regarded as ferrite grains substantially free of carbides. The volume ratio serves as an indicator of the dispersed state of carbides.

(ii) Evaluation of tensile flanging performance

A punching tool with a punch diameter d ₀ =10 mm and a die hole diameter of 12 mm (gap 20%) was used for punching, and then a hole expansion test was performed. The hole expansion test is carried out with a flat-bottomed cylindrical punch (50mmφ, 5R (shoulder radius: 5mm)) by means of pressing, and the hole diameter db (mm) is measured when cracks through the plate thickness occur on the edge of the hole. The expansion rate λ (%) defined by the formula.

λ=100×(db-d ₀ )/d ₀ (3)

(iii) Tensile test

Take the JIS No. 5 test piece along the 90° direction (C direction) with the rolling direction, and perform a tensile test at a tensile speed of 10mm/min to measure the tensile strength and elongation.

Table 2 shows the average particle size of ferrite, the average particle size of carbides, the dispersed state of carbides, the tensile flanging performance, and the tensile strength obtained from the above test results. Here, the stretch flanging performance was evaluated by the hole expansion ratio λ of the above-mentioned formula (3). In the present invention, the tensile strength TS is 440 MPa or more, and the hole expansion ratio λ is 70% or more (the plate thickness is 5.0 mm) as targets. In addition, when good ductility is required, the elongation rate should be 35% or more.

In Table 2, the chemical components (compositions) of steel Nos. A to E and N are within the range of Embodiment 1, and the average particle size of ferrite is 6 μm or less, and the average particle size of carbide is 0.1 μm or more and less than 1.20 μm. . An example of the invention in which the volume ratio of ferrite grains substantially free of carbides is 10% or less. They achieved the goals of Embodiment 1 in which the tensile strength (TS) was 440 MPa or more and the hole expansion ratio λ was 70% or more. In addition, since the average grain size of carbides is 0.5 μm or more, the elongation reaches 35% or more.

On the contrary, steel No.F to M in Table 2 are comparative examples whose chemical components (compositions) deviate from the range of the first embodiment. Steel No.F has a low C content, the average particle size of ferrite, the average particle size of carbides, and the volume ratio of ferrite grains substantially free of carbides exceed the range of Embodiment 1, and the tensile strength is less than 440 MPa, The hole expansion ratio was also lower than the target. Steel No. G has a high C content and a structure within the range of Embodiment 1, but the hole expansion rate is lower than the target. In addition, the elongation rate is also low. Steel No.H has high Si and P, and Steel No.L and M have low B and Cr, respectively, and a large amount of proeutectoid ferrite is produced in both, and the volume ratio of ferrite grains substantially free of carbide exceeds that of the embodiment The upper limit of 1 is 10%, and the hole expansion rate is lower than the target.

Steel No.1 of the comparative example has a large amount of proeutectoid ferrite due to its low Mn, and the volume ratio of ferrite grains substantially free of carbides is higher than the range of Embodiment 1, and the average ferrite grain size More than 6 μm, the strength and hole expansion rate are lower than the target. Since steel No. J has high Mn, a banded structure is formed, so the hole expansion rate is lower than the target. In addition, the elongation rate is also low. Steel No. K has high S, increased MnS, and greatly decreased the hole expansion rate.

Example 2

Among the steels shown in Table 1 above, after heating the continuously cast slabs of Steel No.A and C of Invention Example to 1250°C, hot rolling was carried out under the conditions shown in Table 3, pickled and annealed to produce A steel plate with a thickness of 5.0mm. Among them, steel plate Nos. 1 to 8 are inventive examples in which the manufacturing conditions are within the range of the first embodiment, and steel plate Nos. 9 to 16 are comparative examples in which the manufacturing conditions deviate from the range of the first embodiment.

Samples were taken from these steel sheets, and in the same manner as in Example 1, the average particle size of ferrite, the average particle size of carbides, and the dispersion state of carbides were measured, the tensile flanging performance was measured, and a tensile test was performed. Each test, measuring method and condition are the same as in Example 1. The results are shown in Table 4.

In Table 4, steel sheets Nos. 1 to 8 whose production conditions fall within the range of Embodiment 1 have an average particle size of ferrite of 6 μm or less, an average particle size of carbides of 0.1 μm to less than 1.20 μm, and substantially no carbide The volume ratio of ferrite grains is 10% or less, which is the steel sheet of the inventive example.

The steel sheets of these inventive examples achieved the goals of the present invention that the tensile strength (TS) was 440 MPa or more and the hole expansion ratio λ was 70% or more. Among them, the annealing temperature of steel plates No. 1, 3, 5, and 7 is above 680 ° C, which is the ideal range of the manufacturing conditions of the present invention, because the average particle size of carbides is above 0.5 μm, and high elongation (more than 35%) can be obtained . Among them, steel plates No. 3 and 7 have cooling termination temperatures below 600°C, coiling temperatures below 500°C, and annealing temperatures above 680°C, which are the ideal ranges for the production conditions of the present invention, and iron that does not substantially contain carbides. The volume ratio of the element crystal grains is less than 5%, and the average particle size of carbides is more than 0.5 μm, so that a high elongation rate (more than 35%) can be obtained while obtaining a high hole expansion rate (more than 85%).

On the contrary, steel plate Nos. 9 to 16 in Table 4 are comparative examples in which the production conditions deviated from the range of Embodiment 1. In steel sheets No. 9 and 13, the finish rolling temperature is lower than the range of Embodiment 1, the average grain size of ferrite, and the volume ratio of ferrite grains substantially free of carbides exceed the upper limit of the range of Embodiment 1, and the tensile strength Strength and hole expansion ratio are lower than target. The cooling rate after rolling of steel sheets No. 10 and 14 was lower than the range of Embodiment 1, the volume ratio of ferrite grains substantially free of carbides also exceeded the upper limit of the range of Embodiment 1, and the hole expansion rate was lower than the target.

Steel sheets Nos. 11 and 15 of the comparative examples have a cooling end temperature and a coiling temperature higher than those of Embodiment 1, and the average ferrite grain size, average carbide grain size, and ferrite grains substantially free of carbide The volume ratio exceeds the upper limit of the range of Embodiment 1, and the tensile strength and hole expansion rate are lower than the target. In steel plate No. 12, the annealing temperature was higher than the range of the present invention, the average particle size of carbides, and the volume ratio of ferrite grains substantially free of carbides also exceeded the upper limit of the range of Embodiment 1, and the hole expansion rate was lower than the target. In addition, the elongation rate is also low. In steel plate No. 16, the annealing temperature was lower than the range of Embodiment 1, the spheroidization of carbides was insufficient, and the particle size could not be accurately measured. However, the average particle size of carbides was significantly more than 1.2 μm, and the hole expansion ratio was greatly reduced. In addition, the elongation rate is also low.

By adopting the high-carbon hot-rolled steel sheet of Embodiment 1, the degree of processing can be increased in the processing of transmission components such as pinion gears. As a result, manufacturing steps can be omitted, and components can be manufactured at low cost.

Table 1 Mass % Steel No. C Si mn P S Sol.Al N B Cr Ar ₃ (°C) Ac ₁ (°C) Remark A 0.28 0.02 0.52 0.010 0.002 0.03 0.0042 0.0021 0.23 815 741 Example of the invention B 0.22 0.05 0.56 0.018 0.006 0.03 0.0039 0.0026 0.27 840 744 Example of the invention C 0.46 0.03 0.45 0.012 0.001 0.02 0.0045 0.0029 0.19 745 736 Example of the invention D. 0.34 0.02 0.32 0.009 0.003 0.04 0.0041 0.0034 0.16 793 741 Example of the invention E. 0.40 0.07 0.24 0.011 0.002 0.02 0.0033 0.0016 0.25 774 744 Example of the invention f 0.18 0.03 0.50 0.013 0.008 0.03 0.0042 0.0023 0.27 855 745 comparative example G 0.55. 0.06 0.52 0.015 0.005 0.04 0.0037 0.0021 0.24 710 733 comparative example h 0.31 0.17 0.48 0.032 0.006 0.02 0.0043 0.0024 0.16 811 743 comparative example I 0.28 0.05 0.14 0.011 0.003 0.04 0.0037 0.0023 0.25 822 749 comparative example J 0.34 0.04 0.83 0.009 0.005 0.03 0.0042 0.0025 0.21 787 734 comparative example K 0.27 0.05 0.54 0.013 0.018 0.04 0.0033 0.0021 0.26 820 742 comparative example L 0.36 0.02 0.50 0.010 0.005 0.03 0.0039 0.0004 0.20 783 738 comparative example m 0.42 0.04 0.43 0.015 0.006 0.05 0.0047 0.0028 0.01 760 735 comparative example N 0.23 0.04 0.55 0.015 0.005 0.03 0.0037 0.0023 0.28 835 743 Example of the invention

Underlined part : outside the scope of the invention

Table 2 Steel No. Average grain size of ferrite (μm) Carbide Average Particle Size (μm) Volume ratio of ferrite substantially free of carbides (%) Tensile strength (MPa) Elongation (%) Hole expansion rate λ(%) Remark A 4.2 0.76 6 452 41 83 Example of the invention B 5.7 1.17 10 445 43 74 Example of the invention C 2.1 0.68 6 510 36 71 Example of the invention D. 3.0 0.82 7 473 39 80 Example of the invention E. 2.7 0.79 8 498 37 78 Example of the invention f 12.0 1.25 17 437 40 43 comparative example G 1.8 0.85 3 558 29 39 comparative example h 5.3 1.04 14 443 38 46 comparative example I 7.8 1.13 15 429 39 43 comparative example J 2.9 0.78 4 504 29 40 comparative example K 4.1 0.81 5 458 37 twenty one comparative example L 5.6 0.73 19 468 36 34 comparative example m 3.2 0.88 15 483 35 42 comparative example N 5.5 1.12 10 448 43 76 Example of the invention

Underlined part : outside the scope of the invention

table 3 Steel plate No. Steel No. Ar ₃ (°C) Ac ₁ (°C) Finishing temperature (℃) Time until cooldown starts (seconds) Cooling rate (°C/s) Cooling end temperature (℃) Coiling temperature (℃) Annealing conditions Remark 1 A 815 741 850 0.7 130 620 570 680℃×40hr Example of the invention 2 A 815 741 850 0.7 130 610 560 640℃×40hr Example of the invention 3 A 815 741 830 0.5 180 550 480 710℃×40hr Example of the invention 4 A 815 741 850 0.5 180 550 480 660℃×40hr Example of the invention 5 C 745 736 780 0.3 150 610 550 680℃×40hr Example of the invention 6 C 745 736 780 0.3 150 610 550 650℃×40hr Example of the invention 7 C 745 736 790 0.3 200 530 480 710℃×40hr Example of the invention 8 C 745 736 790 0.3 200 540 470 670℃×40hr Example of the invention 9 A 815 741 785 0.3 130 610 570 680℃×40hr comparative example 10 A 815 741 840 0.5 60 600 560 680℃×40hr comparative example 11 A 815 741 850 0.5 130 650 630 680℃×40hr comparative example 12 A 815 741 840 0.5 130 610 570 750℃×40hr comparative example 13 C 745 736 725 0.3 130 590 550 680℃×40hr comparative example 14 C 745 736 780 0.3 50 590 550 680℃×40hr comparative example 15 C 745 736 780 0.3 130 700 650 680℃×40hr comparative example 16 C 745 736 790 0.3 130 590 550 600℃×40hr comparative example

Underlined part : outside the scope of the invention

Table 4 Steel plate No. Steel No. Average grain size of ferrite (μm) Carbide Average Particle Size (μm) Volume ratio of ferrite substantially free of carbides (%) Tensile strength (MPa) Elongation (%) Hole expansion rate λ(%) Remark 1 A 3.7 0.73 7 458 40 79 Example of the invention 2 A 2.1 0.36 8 479 33 75 Example of the invention 3 A 4.4 0.86 3 447 42 90 Example of the invention 4 A 3.4 0.43 2 447 34 88 Example of the invention 5 C 2.5 0.70 6 502 36 73 Example of the invention 6 C 1.7 0.38 6 526 32 70 Example of the invention 7 C 3.1 0.82 2 513 37 86 Example of the invention 8 C 2.3 0.46 3 529 33 85 Example of the invention 9 A 7.3 1.18 26 426 35 35 comparative example 10 A 5.7 0.97 14 444 36 42 comparative example 11 A 8.4 2.11 twenty one 420 37 31 comparative example 12 A 4.6 3.62 18 526 31 twenty three comparative example 13 C 6.8 1.03 16 438 36 30 comparative example 14 C 4.3 0.92 12 476 35 36 comparative example 15 C 7.5 2.87 13 437 36 25 comparative example 16 C 3.8 NG ^* 7 578 25 17 comparative example

Underlined part : outside the scope of the invention NG ^* : cannot be accurately measured due to insufficient spheroidization (average carbide particle size > 1.2 μm)

Implementation mode 2:

Embodiment 2 provides that the composition contains C: 0.20-0.58%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% or less in terms of mass percentage , N: 0.005% or less, B: 0.001-0.005%, Cr: 0.05-0.3%, the rest is iron and unavoidable impurities, with an average particle size of ferrite below 6 μm and an average particle size of carbides above 0.1 μm 1.20 μm high-carbon cold-rolled steel sheet with a volume ratio of 15% or less of ferrite grains substantially free of the aforementioned carbides. It is desirable that the average particle size of carbides be more than 0.5 μm and less than 1.20 μm. It is desirable that the volume ratio of ferrite grains be 10% or less.

In addition, Embodiment 2 provides steel with the above composition, which is hot-rolled at a finish rolling temperature (Ar ₃ transformation point - 10°C) or higher, and then cooled at a cooling rate of more than 120°C/sec and 620°C or lower. Cooling at the cooling end temperature, followed by coiling at a coiling temperature below 600°C, cold rolling at a reduction rate of 30% or more, and annealing at an annealing temperature above 640°C and below the Ac ₁ transformation point Manufacturing method of rolled steel plate. It is desirable to perform the above-mentioned annealing at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower. It is desirable to perform the cooling at a cooling end temperature of 600°C or lower, and to perform the coiling at a coiling temperature of 500°C or lower. In the above-mentioned production method, the annealing may be performed at an annealing temperature of 640° C. or higher and an Ac ₁ transformation point or lower before cold rolling after coiling.

The high-carbon cold-rolled steel sheet of the embodiment and its manufacturing method are obtained through intensive research on the effects of composition and microstructure on the tensile flanging performance and ductility of the high-carbon steel sheet. During this process, it was found that the factors affecting the tensile flanging performance and ductility of the steel plate are not only the composition and the shape of the carbide, but also the dispersed form of the carbide.

In addition, it is clear that the tensile strength of high-carbon cold-rolled steel sheet can be improved by separately controlling the average grain size of carbides in the form of carbides and the volume ratio of ferrite grains in the dispersed state of carbides that do not substantially contain carbides. side performance issues. Furthermore, it was found that by controlling the composition and ferrite particle size, the tensile flanging performance and strength can be stably balanced at a high level, and further regulation and control of the carbide particle size can stably increase the elongation. On the basis of this understanding, the manufacturing method of controlling the above-mentioned structure was studied, and the manufacturing method of high-carbon hot-rolled steel plate with excellent tensile flanging performance and ductility was determined.

The main elements constituting Embodiment 2 will be described below.

C content: 0.20 to 0.58 (mass%, the same below)

C is an important element that forms carbides and affects the hardness after quenching. However, when the C content is less than 0.20%, pro-eutectoid ferrite is obviously formed in the structure after hot rolling, the ferrite grains substantially free of carbides increase, and the distribution of carbides becomes uneven. In addition, ferrite grains also become coarse. Furthermore, even in this case, sufficient strength as a mechanical structural member cannot be obtained after quenching. On the other hand, if the C content exceeds 0.58%, the tensile flanging property and ductility are low even after annealing. Therefore, the C content is specified to be not less than 0.20% and not more than 0.58%.

Si: 0.1% or less

Si is an element that improves hardenability and improves material strength by solid solution strengthening, so it is desirable to contain 0.005% or more. However, if the content exceeds 0.1%, pro-eutectoid ferrite is likely to be formed, ferrite grains substantially free of carbides increase, and the tensile flanging performance deteriorates. Therefore, the Si content is limited to 0.1% or less.

Mn: 0.20～0.60%

Like Si, Mn is an element that improves hardenability and improves material strength by solid solution strengthening. In addition, forming MnS with S to fix S is an important element for preventing hot cracking of slabs. It is well known that the Mn content has a great influence on the hardenability. Therefore, the effect of the Mn content on the hardenability of the B and Cr-added steel of the present invention was studied.

Composed of C: 0.36%, Si: 0.03%, Mn: 0.10～0.90%, P: 0.01%, S: 0.003%, sol.Al: 0.03%, N: 0.0040%, B: 0.0025%, Cr: 0.25% After the constituent steel was dissolved, hot rolling was performed at a heating temperature of 1250°C, a hot rolling finishing temperature of 880°C, and a coiling temperature of 560°C. Then cold rolling was carried out at a reduction rate of 50%, and annealing was carried out under the condition of 710° C. for 40 hours to make a steel plate with a thickness of 2.5 mm. The obtained steel plate is cut into a size of 50×100 mm, then heated to 820° C. for 10 seconds with a heating furnace, and then quenched into oil at about 20° C. Hardenability was evaluated by measuring 10-point hardness on the quenched test piece with Rockwell hardness C (HRc). An average hardness (HRc) of 50 or more was evaluated as good. The obtained results are shown in Fig. 2 .

Fig. 2 is a graph showing the relationship between the Mn content and the hardness after quenching. According to Figure 2, it can be seen that when the Mn content is above 0.20%, the hardness (HRc) can be guaranteed to be above 50, and when the Mn content is above 0.35%, the hardness (HRc) can reach 55, and a higher quenching hardness can be stably obtained. .

In addition, from the perspective of improving material strength, forming MnS to fix S, and preventing slab thermal cracking, these effects are small when the Mn content is less than 0.20%, and it is conducive to the formation of pro-eutectoid ferrite, making ferrite coarse change.

On the other hand, if it exceeds 0.60%, the tensile strength is obtained, but the manganese band of segregated bands is remarkably easy to form, and the tensile flanging performance and elongation deteriorate.

For the above reasons, the Mn content is specified to be 0.20% to 0.60%, preferably 0.35% to 0.60%.

P: less than 0.02%

P segregates at the grain boundaries to lower the toughness, and is an element to be reduced as much as possible. However, since a P content of 0.02% or less is acceptable, the P content is limited to 0.02% or less.

S: less than 0.01%

S and Mn form MnS, which deteriorates the stretch flanging performance, so it is an element that should be reduced as much as possible. However, since the S content is allowable at 0.01% or less, the S content is limited to 0.01% or less.

sol.Al: less than 0.1%

Al is used as a deoxidizer to improve the cleanliness of steel, so it is added in the steelmaking stage. Generally, sol.Al contains more than 0.005% in steel. On the other hand, even if Al is added to such an extent that the sol.Al content exceeds 0.1%, the effect of improving cleanliness is saturated, and the cost increases. Therefore, the sol.Al content in steel is stipulated below 0.1%.

N: 0.005% or less

N forms BN, which reduces the solid solution B content effective for hardenability and reduces hardenability, so it is an element that should be reduced as much as possible. However, since the N content of 0.005% or less is acceptable, the N content is limited to 0.005% or less.

B: 0.001～0.005%

B suppresses the formation of proeutectoid ferrite during the cooling process after hot rolling, and is an important element for improving the hardenability while improving the tensile flanging performance. However, when the B content is less than 0.001%, sufficient effects cannot be obtained. On the other hand, if it exceeds 0.005%, the effect will be saturated, and the load of hot rolling will increase, and the workability will fall. Therefore, the B content is specified to be not less than 0.001% and not more than 0.005%.

Cr.0.05～0.3%

Like B, Cr suppresses the formation of pro-eutectoid ferrite in the cooling process after hot rolling, and is an important element for improving the hardenability while improving the tensile flanging performance. However, when the Cr content is less than 0.05%, sufficient effects cannot be obtained. On the other hand, even if it exceeds 0.3%, the hardenability improves, but the effect of suppressing the formation of proeutectoid ferrite is saturated, and the cost increases. Therefore, the Cr content is specified to be not less than 0.05% and not more than 0.3%.

Next, the structure of the steel plate will be described.

Average grain size of ferrite: below 6μm

The average particle size of ferrite is an important factor affecting the tensile flanging performance and material strength, and is an important condition in Embodiment 2. By making the ferrite crystal grains finer, the strength can be improved without deteriorating the tensile flanging performance. That is, by keeping the average particle size of ferrite below 6 μm, the tensile strength of the material can be ensured above 440 MPa, and excellent tensile flanging performance can be obtained at the same time. On the other hand, if the fine crystal grains smaller than 1.0 μm are formed, the strength will be remarkably improved and the load may increase during press working, so it is desirable to be 1.0 μm or more. The ferrite grain size can be controlled by manufacturing conditions, especially finish rolling temperature and cooling end temperature.

Average particle size of carbides: 0.1 μm or more and less than 1.20 μm

The average particle size of carbides generally has a great influence on the machining performance and the generation of cavities in the reaming process. Carbide miniaturization can suppress the generation of cavities, but if the average particle size of carbides is less than 0.1 μm, the ductility decreases with the increase of hardness, so the tensile flanging performance also decreases. On the other hand, as the average particle size of carbides increases, the processing performance generally improves, but if it is more than 1.20 μm, the tensile flanging performance decreases due to the generation of cavities in the hole expansion process. Therefore, the average particle size of carbides should be controlled above 0.1 μm and less than 1.20 μm. Further, by controlling the average particle size of the carbides to be more than 0.5 μm and less than 1.20 μm, the increase in strength can be suppressed, while the elongation can be increased, and excellent tensile properties can be obtained. Therefore, it is desirable to specify the thickness to be not less than 0.5 μm and less than 1.20 μm. In addition, the average particle size of carbides can be controlled by manufacturing conditions, especially cooling termination temperature, coiling temperature and annealing temperature. Here, regarding the particle size of carbides, the average value of the long and short diameters of carbides is taken as the particle size of each carbide, and the average value of the particle size of each carbide is defined as the average particle size of carbides.

Dispersion state of carbides: the volume ratio of ferrite grains substantially free of carbides is 15% or less

By making the dispersed state of the carbides uniform, the stress concentration on the punched end surface during hole expansion can be alleviated as described above, and the generation of cavities can be suppressed. By setting the volume ratio of ferrite grains substantially free of carbides to 15% or less, the dispersion state of carbides can be made uniform, and the stretch flanging performance can be remarkably improved. Therefore, the volume ratio of ferrite grains substantially free of carbides is defined to be 15% or less. Furthermore, the volume ratio of ferrite grains substantially free of carbides is kept below 10%, so that the dispersed state of carbides is more uniform, and very excellent stretch flanging performance can be obtained. Therefore, it is desirable to set it at 10% or less. On the other hand, this composition system is a hypoeutectoid steel, and considering that it is difficult to completely suppress proeutectoid ferrite, it is desirable to set the lower limit of the volume ratio of ferrite grains substantially free of carbides to 1%. In addition, the dispersed state of carbides, that is, the volume ratio of ferrite grains substantially free of carbides, can be determined by manufacturing conditions, especially the final rolling temperature, cooling rate after rolling, cooling termination temperature and coiling temperature and annealing The temperature is controlled.

Among them, ferrite grains substantially free of carbides refer to ferrite grains in which carbides cannot be detected by observing the metal structure with a general optical microscope, and mean ferrite grains in which no carbides can be detected even with a scanning electron microscope at a low magnification. to the ferrite grains of carbides. That is, the so-called ferrite grains substantially free of carbides in the present invention stipulate that the plate thickness section of the steel plate sample is ground, corroded with nitric acid alcohol corrosion solution, and then observed under 1000 times with a scanning electron microscope. Ferrite grains of carbides were also not detectable. Such ferrite grains are parts formed as pro-eutectoid ferrite after hot rolling, and carbides are not observed in the grains even in the state after annealing, that is, it can be said that they do not contain carbides substantially. ferrite grains.

The reasons for limiting the manufacturing conditions will be described below.

Finishing temperature of hot rolling: (Ar ₃ transformation point -10°C) or above

When the finishing temperature of the steel during hot rolling is less than (Ar ₃ transformation point -10°C), due to the phase transformation of part of the ferrite, the grains of ferrite that do not substantially contain carbides increase, and the stretching is reversed. Edge performance deteriorates. In addition, since ferrite crystal grains are remarkably coarsened, and the average ferrite grain size exceeds 6 μm, tensile flanging performance deteriorates and strength decreases. Therefore, it is stipulated that the finishing temperature of hot rolling should be above (Ar ₃ transformation point - 10°C). In this way, the uniform micronization of the tissue can be realized, and the performance and strength of the stretch flanging can be improved. On the other hand, the upper limit of the finish rolling temperature is not particularly limited, but at a high temperature exceeding 1000°C, scale-like defects are likely to occur, so it is preferably 1000°C or lower. In addition, the Ar ₃ transformation point (°C) can be calculated by the following formula.

Ar ₃ =930.21-394.75C+54.99Si-14.40Mn+5.77Cr (1)

The symbols of the elements in the formula represent the content (% by mass) of each element, respectively.

Cooling conditions after hot rolling: Cooling rate > 120°C/s

In Embodiment 2, rapid cooling (cooling) is performed after rolling in order to reduce the volume ratio of pro-eutectoid ferrite grains after phase transformation. If the cooling method after hot rolling is slow cooling, the degree of undercooling of austenite is small, and a large amount of proeutectoid ferrite is formed. When the cooling rate is below 120°C/sec, pro-eutectoid ferrite is obviously formed, and the ferrite grains substantially free of carbides exceed 15%, and the tensile flanging performance deteriorates. Therefore, the cooling rate for post-rolling cooling is specified to be greater than 120°C/sec. On the other hand, the upper limit of the cooling rate is desirably 700° C./sec from the viewpoint of facility capacity.

Here, the cooling rate is the average cooling rate from the start of cooling after finish rolling to the stop of cooling. In addition, cooling generally begins within 3 seconds after finish rolling, but from the viewpoint of further refining the ferrite grains and pearlite after phase transformation and further improving workability, it is desirable that the cooling time be greater than 0.1 seconds and less than 1.0 seconds after finish rolling. Cooldown starts in seconds.

Cooling termination temperature: below 620°C

When the cooling end temperature in cooling after hot rolling is high, ferrite is formed in cooling to coiling, and at the same time, the aggregated structure of pearlite and the interlamellar spacing increase. Therefore, after cold rolling and annealing, the ferrite grains are coarsened, fine carbides cannot be obtained, the strength is lowered, and the tensile flanging performance is deteriorated. When the cooling end temperature is higher than 620° C., the ferrite grains substantially free of carbides exceed 15%, and the tensile flanging performance deteriorates. Therefore, the cooling termination temperature of post-rolling cooling is specified to be below 620°C. Furthermore, in the case of making ferrite grains substantially free of carbides 10% or less, the cooling end temperature is desirably set to be 600° C. or less. On the other hand, the lower limit of the cooling end temperature is not particularly limited, but the lower the temperature, the worse the shape of the steel sheet, so it is desirably set at 200°C.

Coiling temperature: below 600°C

After the cooling is terminated, the steel plate is coiled. The higher the coiling temperature, the larger the interlamellar spacing of pearlite. Therefore, carbides after cold rolling and annealing are coarsened, and when the coiling temperature exceeds 600° C., the stretch flanging performance deteriorates. Therefore, it is stipulated that the coiling temperature should be below 600°C. Furthermore, by keeping the coiling temperature below 500°C, the dispersed state of carbides can be made more uniform, and very excellent stretch flanging performance can be obtained, so it is desirable to set it below 500°C. On the other hand, the lower limit of the coiling temperature is not particularly specified, but the lower the temperature, the worse the shape of the steel sheet is, so it is desirably set at 200°C or higher.

In order to make the dispersion of carbides more uniform and to obtain excellent stretch flanging performance, it is desirable to cool at a cooling termination temperature below 600°C and at the same time perform coiling at a coiling temperature of 500°C.

Furthermore, the hot-rolled steel sheet after coiling is desirably pickled to remove scale before cold rolling. In particular, when annealing a hot-rolled steel sheet, in order to eliminate the influence of scale on the surface of the steel sheet, it is desirable to perform pickling before the above-mentioned annealing. Pickling can be carried out according to the general method.

Annealing temperature of hot-rolled steel sheet: above 640°C and below the Ac ₁ transformation point during annealing

Cold rolling is performed after hot rolling, and it is desirable to perform annealing (primary annealing) in order to spheroidize carbides beforehand. The primary annealing at this time may be box annealing or continuous annealing. When the annealing temperature of primary annealing is less than 640 degreeC, the effect of annealing cannot be acquired. On the other hand, when the annealing temperature exceeds the Ac ₁ transformation point, a part of the steel is austenitized and pearlite is formed again during cooling, so the effect of annealing cannot be obtained. Therefore, the annealing temperature in the case of performing one annealing is specified to be 640° C. or higher and the Ac ₁ transformation point or lower. In addition, in order to obtain excellent stretch flanging properties, it is desirable to specify an annealing temperature above 680°C.

Cold rolling reduction rate: more than 30%

Cold rolling refines and evenly disperses the carbides, which improves the stretch flanging performance. However, when the reduction ratio of cold rolling is less than 30%, not only this effect cannot be obtained, but also the remaining non-recrystallized portion after annealing deteriorates the stretch flanging performance. In addition, the elongation rate is also low. Therefore, the cold rolling reduction rate is stipulated above 30%. The upper limit of the rolling reduction is not particularly limited, but it is desirably set at 80% or less in view of the rolling load.

Annealing temperature of cold-rolled steel sheet: above 640°C and below Ac ₁ transformation point

In order to promote recrystallization and carbide spheroidization after cold rolling, annealing is carried out. When the annealing temperature is less than 640°C, the spheroidization of carbides is insufficient or the average particle size of carbides is less than 0.1 μm, and the tensile flanging performance deteriorates. On the other hand, when the annealing temperature exceeds the Ac ₁ transformation point, a part of the steel is austenitized, and pearlite is formed again during cooling, so the tensile flanging performance deteriorates. In addition, the elongation also deteriorated. According to the above reasons, the annealing temperature is specified to be above 640°C and below the Ac ₁ transformation point. Furthermore, by setting the annealing temperature above 680° C. and making the average particle size of carbides above 0.5 μm, a high elongation rate can be obtained, and a more excellent stretch flanging performance can also be obtained. Therefore, it is desirable to regulate the temperature at 680°C or higher and the Ac ₁ transformation point or lower. In addition, the Ac ₁ transformation point (° C.) can be calculated by the following formula.

Ac ₁ =754.83-32.25C+23.32Si-17.76Mn+17.13Cr (2)

The symbols of the elements in the formula represent the content (% by mass) of each element, respectively.

Composition preparation of the high carbon steel in Embodiment 2 can be performed using either a converter or an electric furnace. The high-carbon steel prepared with such a composition is made into a billet by ingot casting-slab rolling or continuous casting. The steel slab is hot-rolled. At this time, the heating temperature of the slab is desirably set at 1300° C. or lower in order to avoid the surface state caused by scale.

In addition, during hot rolling, rough rolling may be omitted and finish rolling may be performed, or continuous casting slab may be directly rolled, or direct rolling may be performed in which rolling is carried out while holding heat to prevent temperature drop. It is hoped that in order to ensure the final rolling temperature, the strip heater can also be used to heat the rolled piece during hot rolling. In order to promote spheroidization or reduce hardness, the steel coil after coiling can also be kept warm by slow cooling cover and other means.

After being coiled into a hot-rolled steel plate, pickling is carried out according to the general method according to the situation. It is then annealed after cold rolling. For annealing, either box annealing or continuous annealing may be used. After annealing after cold rolling, tempering is performed as necessary. Since flattening has no effect on hardenability, there are no restrictions on the conditions for flattening.

According to the above method, a high-carbon cold-rolled steel sheet with excellent stretch flanging performance or excellent ductility can be obtained. What has been described above is an embodiment of the manufacturing method of the present invention, and is not limited thereto.

The reason why the high-carbon cold-rolled steel sheet thus produced has excellent stretch flanging performance is considered as follows. The internal structure of the punched end face has a great influence on the performance of stretch flanging. In particular, when there are many ferrite grains substantially free of carbides (corresponding to the pro-eutectoid ferrite portion after hot rolling), it has been confirmed that cracks occur at the interface with the spheroidized structure portion.

Observing the behavior of the microstructure, it is found that a cavity is obviously generated at the carbide interface due to stress concentration during blanking. The larger the carbide size and the more ferrite grains substantially free of carbides, the greater the stress concentration. These cavities are connected into cracks during hole reaming.

In this way, stress concentration can be reduced and the formation of cavities can be reduced by controlling not only the manufacturing conditions but also the average diameter of carbide particles and the proportion of ferrite grains substantially free of carbides.

Example 1

The continuous casting slab of steel with the chemical composition shown in Table 5 is heated at 1250°C, the finishing temperature of hot rolling is 880°C, the time from finishing rolling to the start of cooling is 0.7 seconds, and the cooling rate after hot rolling is Hot rolling was performed under conditions of 150°C/sec, cooling end temperature of 610°C, and coiling temperature of 560°C. Thereafter, it was pickled, cold-rolled at a reduction ratio of 50%, and box annealed at 710° C. for 40 hours to manufacture a steel plate with a thickness of 2.5 mm. Among them, the chemical components (compositions) of Steel Nos. A to E are inventive examples within the range of the second embodiment, and steel Nos. F to M are comparative examples whose components deviate from the range of the second embodiment.

Samples were taken from these steel sheets, and measurements of the average particle size of ferrite, average particle size of carbides, and dispersion state of carbides, evaluation of tensile flanging performance, and tensile tests were performed. Each test, measurement method and conditions are as follows.

(i) Average particle size of ferrite, average particle size and dispersion state of carbides

Measured by the same method as Embodiment 1.

(ii) Evaluation of tensile flanging performance

A punching tool with a punch diameter d ₀ =10 mm and a die hole diameter of 11 mm (gap 20%) was used to punch out the sample, and then a hole expansion test was performed. The hole expansion test is carried out with a flat-bottomed cylindrical punch (50mmφ, 5R (shoulder radius: 5mm)) by means of pressing, and the hole diameter db (mm) is measured when cracks through the plate thickness occur on the edge of the hole. The expansion rate λ (%) defined by the formula.

λ=100×(db-d ₀ )/d ₀ (3)

(iii) Tensile test

Take the JIS No. 5 test piece along the 90° direction (C direction) with the rolling direction, and perform a tensile test at a tensile speed of 10mm/min to measure the tensile strength and elongation.

Table 6 shows the average particle size of ferrite, the average particle size of carbides, the dispersed state of carbides, the tensile flanging performance, and the tensile strength obtained from the above test results. Here, the stretch flanging performance was evaluated by the hole expansion ratio λ of the above-mentioned formula (3). In the present invention, the tensile strength TS is 440 MPa or more, and the hole expansion ratio λ is 80% or more (the plate thickness is 2.5 mm) as targets. In addition, when good ductility is required, the elongation rate should be 35% or more.

In Table 6, the chemical components (compositions) of Steel Nos. A to E are within the range of Embodiment 2, and the average grain size of ferrite is 6 μm or less, and the average grain size of carbides is 0.1 μm or more and less than 1.20 μm. An example of the invention in which the volume ratio of ferrite grains without carbides is 15% or less. They have reached the goal of the present invention that the tensile strength (TS) is above 440 MPa and the hole expansion ratio λ is above 80%. In addition, since the average particle size of carbides is above 0.5 μm, the elongation rate reaches above 35%.

On the contrary, steel No.F to M in Table 6 are comparative examples whose chemical components (compositions) deviate from the range of the second embodiment. Steel No. F has a low C content, the average ferrite grain size, average carbide grain size, and ferrite grain volume ratio substantially free of carbides exceed the range of Embodiment 2, and the tensile strength is less than 440 MPa. The porosity was also lower than targeted. Steel No. G has a high C content and a structure within the range of Embodiment 2, but the hole expansion rate is also lower than the target. In addition, the elongation rate is also low. Steel No.H has high Si and P, and steel No.L and M have low B and Cr respectively, and a large amount of proeutectoid ferrite is produced in both, and the ferrite grain volume ratio substantially free of carbide exceeds the present invention The upper limit of the range is 15%, and the hole expansion rate is lower than the target.

Steel No.1 of the comparative example has a large amount of proeutectoid ferrite due to its low Mn, and the volume ratio of ferrite grains substantially free of carbides is higher than the range of Embodiment 2. In addition, the average ferrite grain size exceeds 6μm, the strength and porosity are lower than the target. Since steel No. J has high Mn, a banded structure is formed, so the hole expansion rate is lower than the target. In addition, the elongation rate is also low. Steel No. K has high S, increased MnS, and greatly decreased the hole expansion rate.

Example 2

Among the steels shown in Table 5 above, after heating the continuously cast slabs of Steel No.A and C of Invention Example to 1250°C, hot rolling was carried out under the conditions shown in Table 7, followed by pickling, cold rolling and Annealed to produce a steel plate with a plate thickness of 2.5 mm. An annealing is performed on a part of the steel plate after pickling. Among them, steel plate Nos. 1 to 12 are invention examples in which the manufacturing conditions are within the range of the second embodiment, and steel plate Nos. 13 to 19 are comparative examples in which the manufacturing conditions are out of the range of the second embodiment.

Samples were taken from these steel sheets, and in the same manner as in Example 1, the average particle size of ferrite, the average particle size of carbides, and the dispersion state of carbides were measured, the tensile flanging performance was evaluated, and a tensile test was performed. The results are shown in Table 8.

In Table 8, steel sheets No. 1 to 12 whose production conditions fall within the range of Embodiment 2 have an average particle size of ferrite of 6 μm or less, an average particle size of carbides of 0.1 μm to less than 1.20 μm, and substantially no carbide The ferrite grain volume ratio is 15% or less, which is the steel sheet of the inventive example. The steel sheets of these inventive examples achieved the goals of Embodiment 2 in which the tensile strength (TS) was 440 MPa or more and the hole expansion ratio λ was 80% or more.

Among them, steel plates No. 3, 4, 5, 6, 11, and 12 have cooling termination temperatures below 600°C and coiling temperatures below 500°C, and steel sheets No. 5, 6, 9, 10, 11, and 12 are An example in which annealing is performed once is within the ideal range of the manufacturing conditions in the second embodiment. They all obtained a high hole expansion rate (above 85%). In addition, steel sheets Nos. 1, 3, 5, 7, 9, and 11 were annealed at 680° C. or higher after cold rolling, and high elongations were obtained in all of them.

On the contrary, steel plate Nos. 13 to 19 in Table 8 are comparative examples in which the manufacturing conditions (Table 7) deviate from the range of the second embodiment. In steel plate No. 13, the finish rolling temperature is lower than the range of the present invention, the average grain size of ferrite, and the volume ratio of ferrite grains substantially free of carbides exceed the upper limit of the range of Embodiment 2, and the tensile strength and hole expansion rate is lower than the target. The cooling rate after rolling of steel plate No. 14 was lower than the range of Embodiment 2, the volume ratio of ferrite grains substantially free of carbides also exceeded the upper limit of the range of Embodiment 2, and the hole expansion rate was lower than the target.

In steel sheet No. 15 of the comparative example, the cooling end temperature is higher than the range of Embodiment 2, and the average particle size of ferrite, the average particle size of carbides, and the volume ratio of ferrite grains substantially free of carbides exceed that of Embodiment 2. At the upper end of the range, the tensile strength and porosity were lower than the target. Comparative example steel plate No. 16, the coiling temperature is higher than the range of Embodiment 2, the average particle size of carbides exceeds the upper limit of the range of Embodiment 1, and the hole expansion rate is lower than the target.

In steel plate No. 17, the cold rolling reduction ratio was lower than the range of Embodiment 2, the unrecrystallized structure remained, the ferrite grains were not refined, the tensile strength was also high, and the elongation and hole expansion ratio were lower than the target. The annealing temperature after cold rolling of steel plate No. 18 was higher than the range of Embodiment 2, the average particle size of carbides, and the volume ratio of ferrite grains substantially free of carbides exceeded the upper limit of the range of Embodiment 2, and the hole expansion ratio was Aim low. In addition, the elongation rate is also low. In steel plate No. 19, the annealing temperature after cold rolling was lower than the range of Embodiment 2, the spheroidization of carbides was insufficient, and the particle size could not be accurately measured, but the average particle size of carbides clearly exceeded 1.20 μm, and the hole expansion rate was lower than the target. Low. In addition, the elongation rate is also low.

Table 5 Mass% Steel No. C Si mn P S Sol.Al N B Cr Ar ₃ (°C) Ac ₁ (°C) Remark A 0.27 0.03 0.48 0.008 0.003 0.04 0.0038 0.0023 0.21 820 742 Example of the invention B 0.23 0.03 0.58 0.013 0.005 0.02 0.0041 0.0027 0.25 834 742 Example of the invention C 0.56 0.02 0.43 0.016 0.002 0.03 0.0043 0.0018 0.20 705 733 Example of the invention D. 0.36 0.04 0.35 0.010 0.004 0.03 0.0039 0.0032 0.12 786 740 Example of the invention E. 045 0.06 0.22 0.014 0.001 0.02 0.0031 0.0012 0.28 754 743 Example of the invention f 0.17 0.02 0.48 0.014 0.006 0.04 0.0040 0.0025 0.23 859 745 comparative example G 0.65 0.04 0.55 0.012 0.004 0.02 0.0044 0.0019 0.26 669 729 comparative example h 0.29 0.16 0.50 0.035 0.005 0.03 0.0036 0.0021 0.15 818 743 comparative example I 0.28 0.04 0.15 0.013 0.005 0.03 0.0040 0.0025 0.24 821 748 comparative example J 0.36 0.03 0.80 0.010 0.003 0.02 0.0039 0.0022 0.19 779 733 comparative example K 0.28 0.06 0.57 0.008 0.017 0.02 0.0043 0.0034 0.22 816 741 comparative example L 0.34 0.03 0.47 0.014 0.006 0.05 0.0046 0.0004 0.27 792 741 comparative example m 0.51 0.02 0.41 0.017 0.003 0.04 0.0035 0.0037 0.02 724 732 comparative example

Underlined part : outside the scope of the invention

Table 6 Steel No. Average grain size of ferrite (μm) Carbide Average Particle Size (μm) Volume ratio of ferrite substantially free of carbides (%) Tensile strength (MPa) Elongation (%) Hole expansion rate λ(%) Remark A 4.8 0.78 12 448 40 81 Example of the invention B 5.5 1.12 14 441 42 84 Example of the invention C 1.9 0.57 11 514 35 81 Example of the invention D. 3.2 0.88 11 462 37 82 Example of the invention E. 2.5 0.73 9 507 36 86 Example of the invention f 14.2 1.38 26 428 41 39 comparative example G 1.3 0.72 8 564 28 33 comparative example h 5.4 1.08 20 440 38 48 comparative example I 8.6 1.14 twenty three 421 42 41 comparative example J 3.3 0.81 9 494 29 43 comparative example K 4.5 0.83 11 451 39 30 comparative example L 5.8 0.76 25 458 37 38 comparative example m 2.3 0.81 19 501 35 28 comparative example

Underlined part : outside the scope of the invention

Table 7 Steel plate No. Steel No. Finishing temperature (℃) Time until cooldown starts (seconds) Cooling rate (°C/s) Cooling end temperature (℃) Coiling temperature (℃) primary annealing condition Cold rolling reduction (%) Annealing conditions after cold rolling Remark 1 A 850 0.7 150 610 560 - 50 710℃×40hr Example of the invention 2 A 850 0.7 150 610 570 - 50 650℃×40hr Example of the invention 3 A 850 0.7 150 550 480 - 50 700℃×40hr Example of the invention 4 A 850 0.7 150 540 470 - 50 670℃×40hr Example of the invention 5 A 830 0.5 170 530 470 680℃×20hr 60 680℃×20hr Example of the invention 6 A 830 0.5 170 550 480 680℃×20hr 60 640℃×20hr Example of the invention 7 C 725 0.6 140 610 550 - 30 680℃×40hr Example of the invention 8 C 725 0.6 140 610 560 - 30 660℃×40hr Example of the invention 9 C 720 0.6 150 610 540 660℃×40hr 50 710℃×20hr Example of the invention 10 C 720 0.6 150 610 540 660℃×40hr 50 650℃×20hr Example of the invention 11 C 715 0.5 180 550 460 680℃×40hr 50 710℃×20hr Example of the invention 12 C 715 0.5 180 560 470 680℃×40hr 50 640℃×20hr Example of the invention 13 A 800 0.2 150 600 580 - 50 710℃×40hr comparative example 14 A 850 0.7 50 590 540 680℃×40hr 50 680℃×40hr comparative example 15 A 840 0.5 130 640 600 - 40 710℃×40hr comparative example 16 A 850 0.6 150 620 610 660℃×20hr 60 680℃×40hr comparative example 17 C 730 0.5 150 600 580 - 20 710℃×40hr comparative example 18 C 740 0.6 180 610 570 680℃×20hr 50 740℃×40hr comparative example 19 C 730 0.5 130 600 560 - 50 600℃×40hr comparative example

Underlined part : outside the scope of the invention

Table 8 Steel plate No. Steel No. Average grain size of ferrite (μm) Carbide Average Particle Size (μm) Volume ratio of ferrite substantially free of carbides (%) Tensile strength (MPa) Elongation (%) Hole expansion rate λ(%) Remark 1 A 4.5 0.75 11 451 40 83 Example of the invention 2 A 3.2 0.39 12 474 34 81 Example of the invention 3 A 3.9 0.68 8 457 41 87 Example of the invention 4 A 2.7 0.32 7 481 33 85 Example of the invention 5 A 5.3 0.94 6 442 43 94 Example of the invention 6 A 4.0 0.44 5 467 34 91 Example of the invention 7 C 2.1 0.62 12 508 36 81 Example of the invention 8 C 1.3 0.33 13 525 30 80 Example of the invention 9 C 3.6 0.93 11 485 37 85 Example of the invention 10 C 2.7 0.46 12 498 31 85 Example of the invention 11 C 3.3 0.87 3 497 36 87 Example of the invention 12 C 2.1 0.39 2 507 31 85 Example of the invention 13 A 8.2 1.14 twenty four 431 40 37 comparative example 14 A 5.4 1.02 19 446 41 43 comparative example 15 A 8.1 1.97 twenty two 423 39 29 comparative example 16 A 5.2 2.18 13 482 38 40 comparative example 17 C 8.3 0.72 11 616 33 15 comparative example 18 C 4.8 3.05 18 574 26 18 comparative example 19 C 4.2 NG ^* 5 564 twenty four twenty one comparative example

Underlined part : outside the scope of the invention NG ^* : cannot be accurately measured due to insufficient spheroidization (average carbide particle size > 1.2 μm)

Claims (18)

1. High-carbon hot-rolled steel plate, in terms of mass percentage, essentially consists of C: 0.20-0.48%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al : 0.1% or less, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to 0.3%, the rest is composed of iron and unavoidable impurities, Ferrite structure with an average particle size of 6 μm or less and carbide with an average particle size of 0.1 μm or more and less than 1.20 μm, The ferrite structure includes ferrite grains substantially free of carbides, The volume ratio of ferrite grains substantially free of the carbide is 10% or less. 2. The high-carbon hot-rolled steel sheet according to claim 1, wherein the carbide has an average particle size of not less than 0.5 μm and less than 1.20 μm. 3. The high-carbon hot-rolled steel sheet according to claim 1, wherein the ferrite grains have a volume ratio of 5% or less. 4. The high-carbon hot-rolled steel sheet according to claim 1, wherein the ferrite grains have a volume ratio of 5% or less, and the carbides have an average particle size of 0.5 μm or more and less than 1.20 μm. 5. High-carbon cold-rolled steel sheet, in terms of mass percentage, essentially consists of C: 0.20-0.58%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al : 0.1% or less, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to 0.3%, the rest is composed of iron and unavoidable impurities, Ferrite structure with an average particle size of 6 μm or less and carbide with an average particle size of 0.1 μm or more and less than 1.20 μm, The ferrite structure includes ferrite grains substantially free of carbides, The volume ratio of ferrite grains substantially free of the carbide is 15% or less. 6. The high-carbon cold-rolled steel sheet according to claim 5, wherein the carbide has an average particle size of not less than 0.5 μm and less than 1.20 μm. 7. The high-carbon cold-rolled steel sheet according to claim 5, wherein the ferrite grains have a volume ratio of 10% or less. 8. The high-carbon cold-rolled steel sheet according to claim 5, wherein the ferrite grains have a volume ratio of 10% or less, and the carbides have an average particle size of 0.5 μm or more and less than 1.20 μm. 9. The manufacturing method of high-carbon hot-rolled steel plate is composed of C: 0.20-0.48%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% or less, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to _0.3 %, and the rest is composed of iron and unavoidable impurities. The process of hot rolling at rolling temperature, A process of cooling the hot-rolled steel sheet at a cooling rate exceeding 120°C/sec and at a cooling termination temperature of 620°C or less, the process of coiling the cooled hot-rolled steel sheet at a coiling temperature of 600°C or less; and The step of annealing the coiled hot-rolled steel sheet at an annealing temperature of not less than 640°C and not more than the Ac ₁ transformation point is constituted. 10. The method for manufacturing a high-carbon hot-rolled steel sheet as claimed in claim 9, wherein the cooling step is performed by cooling the hot-rolled steel sheet at a cooling rate exceeding 120° C./second and at a cooling termination temperature below 600° C. process composition, The coiling step is composed of a step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 500° C. or lower. 11. The method for manufacturing a high-carbon hot-rolled steel sheet according to claim 9, wherein the annealing step is constituted by annealing the coiled hot-rolled steel sheet at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower. . 12. The method for manufacturing high-carbon hot-rolled steel sheets according to claim 9, wherein the cooling step is performed by cooling the hot-rolled steel sheets at a cooling rate exceeding 120° C./second and at a cooling termination temperature of 600° C. or lower. process composition, The coiling step is composed of a step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 500° C. or lower, The annealing step is constituted by annealing the coiled hot-rolled steel sheet at an annealing temperature of 680° C. or more and not more than the Ac ₁ transformation point. 13. The manufacturing method of high-carbon cold-rolled steel sheet is composed of C: 0.20-0.58%, Si: 0.1% or less, Mn: 0.20-0.60%, P: 0.02% or less, S: 0.01% or less, sol.Al: 0.1% or less, N: 0.005% or less, B: 0.001 to 0.005%, Cr: 0.05 to _0.3 %, and the rest is composed of iron and unavoidable impurities. The process of hot rolling at rolling temperature, A process of cooling the hot-rolled steel sheet at a cooling rate exceeding 120°C/sec and at a cooling termination temperature of 620°C or less, The process of coiling the cooled hot-rolled steel sheet at a coiling temperature below 600°C, A step of cold-rolling the coiled hot-rolled steel sheet at a rolling reduction of 30% or more after pickling, and The step of annealing the coiled hot-rolled steel sheet at an annealing temperature of not less than 640°C and not more than the Ac ₁ transformation point is constituted. 14. The method for manufacturing high-carbon cold-rolled steel sheets as claimed in claim 13, wherein the cooling step is performed by cooling the hot-rolled steel sheets at a cooling rate exceeding 120° C./second and at a cooling termination temperature below 600° C. process composition, The coiling step is composed of a step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 500° C. or lower. 15. The method for manufacturing high-carbon cold-rolled steel sheet according to claim 13 or 14, wherein the annealing step is composed of annealing the hot-rolled steel sheet at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower. 16. The manufacturing method of high-carbon cold-rolled steel sheet according to claim 13, further, after the coiling step and before the cold rolling step, there is a step of annealing at an annealing temperature of 640° C. or more and Ac ₁ transformation point or less . 17. The method for manufacturing high-carbon cold-rolled steel sheet according to claim 16, wherein the annealing step is composed of annealing the cold-rolled steel sheet at an annealing temperature of 680° C. or higher and an Ac ₁ transformation point or lower. 18. The method for manufacturing high-carbon cold-rolled steel sheets as claimed in claim 16 or 17, wherein the cooling process consists of cooling the hot-rolled steel sheets at a cooling rate exceeding 120° C./second and at a cooling termination temperature below 600° C. The step of cooling is constituted, and the coiling step is constituted by a step of coiling the cooled hot-rolled steel sheet at a coiling temperature of 500° C. or lower.