CN104903484A - Hot-rolled steel sheet with excellent cold workability and surface hardness after processing - Google Patents

Abstract

The hot-rolled steel sheet of the present invention has a sheet thickness of 3 to 20mm, and a composition of C: 0.3% or less (not containing 0%), Si: 0.5% or less (not containing 0%), Mn: 0.2-1%, P: 0.05% or less (0% or less), S: 0.05% or less (not containing 0%), Al: 0.01-0.1%, N: 0.008-0.025%, and the balance of iron and inevitable impurities, wherein the solid solution ratio of N: 0.007% or more, and the content of C and N satisfies the relationship of 10C + N.ltoreq.3.0, the structure being pearlite in terms of area ratio to the entire structure: less than 20%, balance: ferrite having an average grain size within a range of 3 to 35 μm, which exhibits good cold workability during working and a predetermined surface hardness after working.

Description

Hot-rolled steel sheet with excellent cold workability and surface hardness after processing

technical field

The present invention relates to a hot-rolled steel sheet that exhibits good cold workability during working and a predetermined surface hardness after working.

Background technique

In recent years, from the viewpoint of environmental protection, in order to improve the fuel efficiency of automobiles, there has been an increasing demand for the reduction of the weight of various parts of automobiles, such as transmission parts such as gears and casings, that is, higher strength. soaring. In order to meet such demands for weight reduction and high strength, generally used steel materials are steel materials obtained by hot forging bar steel (hot forging materials). In addition, in order to reduce CO ₂ emissions in the parts manufacturing process, the demand for cold forging of parts such as gears processed by hot forging has been increasing.

Then, cold working (cold forging) has the advantages of high productivity, dimensional accuracy, and steel product yield compared with hot working and warm working. However, there is a problem when manufacturing components by such cold working. In order to ensure the strength of the cold-worked component to be more than a desired predetermined value, it is necessary to use a steel material with high strength, that is, high deformation resistance. However, the higher the deformation resistance of the steel used, the shorter the life of the die for cold working, and the difficulty that cracks are likely to occur during cold working.

Therefore, it has been conventionally practiced to produce a high-strength component that can secure a predetermined strength (hardness) by cold forging a steel material into a predetermined shape and then performing heat treatment such as quenching and tempering. However, heat treatment after cold forging inevitably changes the dimensions of the part, so machining such as secondary cutting is required to correct it, and a solution that can omit heat treatment and subsequent processing is desired.

In order to solve the above-mentioned problems, for example, it is disclosed that solid-solution C suppresses the progress of aging at room temperature in low-carbon steel, ensures a predetermined amount of age hardening due to strain aging, and obtains wire rods and steel bars for cold forging excellent in strain aging characteristics. (Refer to Patent Document 1).

However, in this technique, strain aging is controlled only by the amount of solid solution C, and it is difficult to obtain a steel material that satisfies sufficient cold workability and required hardness and strength after working.

Therefore, the present applicant has conducted various studies focusing on the difference in the effects of solute C and solute N contained in steel materials on deformation resistance and static strain aging. , can obtain a steel material for machine structural use that exhibits good cold workability during processing and exhibits a predetermined surface hardness (strength) after cold working (cold forging), and a patent application has been made (see Patent Document 2).

This steel material achieves both cold workability and higher hardness (higher strength) after working, but it is a hot-forged material like the wire rod and bar steel described in Patent Document 1 above, and has a problem of high manufacturing cost. Therefore, in order to further reduce the manufacturing cost, research has been conducted on manufacturing automobile parts by cold working from hot-rolled steel sheets instead of conventional hot-forged materials.

For example, a hot-rolled steel sheet for nitriding treatment capable of obtaining a high surface hardness and a sufficient depth of hardening after nitriding treatment has been proposed (see Patent Document 3).

However, this technique has a problem in that nitriding treatment is required after cold working, and sufficient cost reduction cannot be achieved.

In addition, there is proposed a hot-rolled steel sheet whose composition contains C: 0.10% or less, Si: less than 0.01%, Mn: 1.5% or less, and Al: 0.20% or less, and has a ratio of (Ti+Nb)/2: 0.05 Contain in the range of ~0.50%, the total of S, N and O is 0.0100% or less, and contain S: 0.005% or less, N: 0.005% or less, O: 0.004% or less, and make the microstructure 95% or more. The hot-rolled steel sheet has a ferrite single-phase structure, and the precision-pressed surface of this hot-rolled steel sheet has excellent dimensional accuracy, and the surface hardness of the pressed surface after processing is extremely high, and is also excellent in red rust resistance (see Patent Document 4).

However, in this hot-rolled steel sheet, N is restricted to an extremely low content as a harmful element, and it is completely different in terms of technical concept from the hot-rolled steel sheet of the present invention that actively utilizes N.

prior art literature

patent documents

Patent Document 1: Japanese Patent Application Laid-Open No. 10-306345

Patent Document 2: Japanese Patent Laid-Open No. 2009-228125

Patent Document 3: Japanese Patent Laid-Open No. 2007-162138

Patent Document 4: Japanese Patent Laid-Open No. 2004-137607

Contents of the invention

The problem to be solved by the invention

The present invention was made with the above circumstances in mind, and an object of the present invention is to provide a hot-rolled steel sheet that exhibits good cold workability during working and exhibits a predetermined surface hardness after working.

means to solve the problem

The first invention is a hot-rolled steel sheet excellent in cold workability and surface hardness after working, characterized in that,

The plate thickness is 3~20mm,

Ingredient composition is in mass % (hereinafter, it is the same as referring to chemical composition.) For,

C: less than 0.3% (excluding 0%),

Si: 0.5% or less (excluding 0%),

Mn: 0.2～1%,

P: less than 0.05% (excluding 0%),

S: less than 0.05% (excluding 0%),

Al: 0.01 to 0.1%,

N: 0.008～0.025%,

The balance consists of iron and unavoidable impurities,

Solid solution N: 0.007% or more,

And the content of C and N satisfies the relationship of 10C+N≤3.0,

The tissue is calculated as the area ratio relative to the total tissue,

Pearlite: less than 20%, balance: ferrite,

The average grain size of the ferrite is in the range of 3-35 μm.

The second invention is the hot-rolled steel sheet according to the first invention, the composition of which further contains

Cr: 2% or less (excluding 0%) and/or Mo: 2% or less (excluding 0%).

The third invention is the hot-rolled steel sheet according to the first or second invention, the composition of which further contains from

Ti: 0.2% or less (excluding 0%),

Nb: 0.2% or less (excluding 0%),

V: At least one selected from the group consisting of 0.2% or less (excluding 0%).

A fourth invention is the hot-rolled steel sheet according to any one of the first to third inventions, wherein the composition further contains

B: 0.005% or less (excluding 0%).

A fifth invention is the hot-rolled steel sheet according to any one of the first to fourth inventions, wherein the composition further contains:

Cu: 5% or less (excluding 0%),

Ni: 5% or less (excluding 0%),

Co: at least one selected from the group consisting of 5% or less (excluding 0%).

A sixth invention is the hot-rolled steel sheet according to any one of the first to fifth inventions, wherein the composition further contains:

Ca: 0.05% or less (excluding 0%),

REM: less than 0.05% (excluding 0%),

Mg: 0.02% or less (excluding 0%),

Li: 0.02% or less (excluding 0%),

Pb: 0.5% or less (excluding 0%),

Bi: at least one selected from the group consisting of 0.5% or less (excluding 0%).

Invention effect

According to the present invention, in the structure of the ferrite main body having a predetermined average grain size, by ensuring the amount of solid solution N and making the content of C and the content of N satisfy a predetermined relationship, the deformation resistance during cold working can be reduced. Small, the life of the metal mold is extended, and the steel plate is difficult to crack, and the parts obtained after processing can ensure the predetermined surface hardness of the hot-rolled steel plate.

Detailed ways

Hereinafter, the hot-rolled steel sheet of the present invention (hereinafter also referred to as "steel sheet of the present invention", or simply referred to as "steel sheet") will be described in more detail. The steel plate of the present invention is in common with the hot-forged material described in Patent Document 2 in terms of ensuring the solid solution amount of N and satisfying a predetermined relationship between the C content and the N content. It is different in that it becomes a ferrite-pearlite complex structure and makes the ferrite grains finer.

[Thickness of the steel plate of the present invention: 3 to 20 mm]

First, the steel plate of the present invention is intended to be a steel plate with a plate thickness of 3 to 20 mm. When the board thickness is less than 3 mm, rigidity as a structural body cannot be ensured. On the other hand, if the plate thickness is higher than 20 mm, it will be difficult to achieve the structure defined by the present invention, and the desired effect cannot be obtained. The preferred plate thickness is 4 to 19 mm.

Next, the component composition constituting the steel sheet of the present invention will be described. Hereinafter, the units of chemical components are all mass %.

[Ingredient composition of the steel sheet of the present invention]

<C: Less than 0.3% (excluding 0%)>

C is an element that greatly affects the formation of the structure of the steel plate. Although the structure is a ferrite-pearlite composite structure, it is an element that needs to be limited in order to obtain a ferrite-based structure with as little pearlite as possible. . When C is contained excessively, the pearlite fraction in the steel plate structure increases, and the deformation resistance may become too large due to the work hardening of pearlite. Therefore, the C content in the steel sheet is 0.3% by mass or less, preferably limited to 0.25% or less, more preferably 0.2% or less, particularly preferably 0.15% or less. However, if the content of C is too small, deoxidation of steel during smelting becomes difficult, so it is preferably 0.0005% or more, more preferably 0.0008% or more, and particularly preferably 0.001% or more.

<Si: 0.5% or less (excluding 0%)>

Si is an element that needs to be reduced as much as possible because it increases the deformation resistance of the steel sheet by solid solution in the steel. Therefore, in order to suppress an increase in deformation resistance, the Si content in the steel sheet is limited to 0.5% or less, preferably 0.45% or less, more preferably 0.4% or less, particularly preferably 0.3% or less. However, if the Si content is extremely small, deoxidation during dissolution becomes difficult, so it is preferably 0.005% or more, more preferably 0.008% or more, and particularly preferably 0.01% or more.

<Mn: 0.2 to 1%>

Mn is an element with deoxidation and desulfurization effects in the steelmaking process. In addition, if the content of N in the steel is increased, cracks are likely to occur due to dynamic strain aging caused by exothermic heat during processing, but on the other hand, Mn improves the workability at this time and has the effect of suppressing cracks. In order to exert these functions effectively, the Mn content in the steel material is 0.2% or more, preferably 0.22% or more, and more preferably 0.25% or more. However, if the Mn content becomes excessive, the deformation resistance becomes too large and segregation causes structural inhomogeneity, so it is 1% or less, preferably 0.98% or less, more preferably 0.95% by mass or less.

<P: 0.05% or less (excluding 0%)>

P is an impurity element that is unavoidably contained in steel. If it is contained in ferrite, it will segregate at the ferrite grain boundary to deteriorate the cold workability, and also solid-solution strengthen the ferrite. The element that causes the deformation resistance to increase. Therefore, it is desirable to reduce the content of P as much as possible from the viewpoint of cold workability, but an extreme reduction will lead to an increase in steelmaking costs, so considering engineering capabilities, it is made 0.05% or less, preferably 0.03% or less.

<S: 0.05% or less (excluding 0%)>

S, like P, is an unavoidable impurity, and is an element that precipitates as FeS in the form of a film at the crystal grain boundary and degrades workability. In addition, it also has the effect of causing thermal brittleness. Therefore, from the viewpoint of improving deformability, the S content in the present invention is 0.05% or less, preferably 0.03% or less. However, it is industrially difficult to make the S content zero. In addition, since S has the effect of improving machinability, it is recommended to contain preferably 0.002% or more, and more preferably 0.006% or more from the viewpoint of improving machinability.

<Al: 0.01 to 0.1%>

Al is an element effective for deoxidation in the steelmaking process. In order to obtain this deoxidizing effect, the Al content in the steel material is 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, if the content of Al is excessive, the toughness will decrease and cracks will easily occur, so it is 0.1% or less, preferably 0.09% or less, more preferably 0.08% by mass or less.

<N: 0.008～0.025%>

N is an important element for obtaining predetermined strength through static strain aging after processing. Therefore, the N content in the steel material is 0.008% or more, preferably 0.0085% or more, and more preferably 0.009% or more. However, if the content of N is excessive, in addition to static strain aging, the influence of dynamic strain aging during processing will become significant, and the deformation resistance will increase unsuitably, so it is 0.025% or less, preferably 0.023% by mass or less, and more preferably 0.023% by mass or less. 0.02% or less.

<Solid solution N: 0.007% or more>

Then, by securing a predetermined amount of solid solution N in the steel sheet (hereinafter, referred to as "solid solution N amount"), it is possible to accelerate static strain aging without increasing deformation resistance so much. In order to ensure the required strength after cold working, the amount of solid solution N needs to be 0.007% or more. However, if the amount of solid solution N is excessive, the cold workability will deteriorate, so it is preferably 0.03% or less. In addition, since the content of N in the steel material is 0.025% or less, the amount of solid solution N does not substantially reach 0.025% or more.

Here, the amount of solid solution N in the present invention is an amount obtained by subtracting the total amount of N compounds from the total amount of N in the steel according to JIS G 1228. A practical measurement method of the amount of solid solution N is exemplified below.

(a) Inert gas melting method-thermal conductivity method (determination of total N content)

Put the sample cut from the test material into the crucible, melt it in the inert gas flow to extract N, transport the extract to the thermal conductivity unit, measure the change of thermal conductivity, and obtain the total N amount.

(b) Ammonia distillation and separation of indophenol blue absorptiometry (determination of total N compound amount)

Dissolve the sample cut from the test material in 10% AA-based electrolyte, perform constant current electrolysis, and measure the total N compound content in the steel. The 10% AA electrolyte used is a non-aqueous solvent electrolyte composed of 10% acetone, 10% tetramethylammonium chloride, and methanol as the balance. It is a solution that will not form a passive film on the steel surface.

About 0.5 g of a sample of the test material was dissolved in the 10% AA-based electrolyte, and the resulting insoluble residue (N compound) was filtered through a filter made of polycarbonate with a pore size of 0.1 μm. The obtained insoluble residue was heated and decomposed in sulfuric acid, potassium sulfate and pure copper chips, and the decomposed product was made to match the filtrate. After the solution was made alkaline with sodium hydroxide, steam distillation was performed, and the distilled ammonia was absorbed in dilute sulfuric acid. In addition, phenol, sodium hypochlorite, and sodium pentacyanonitrosylferrate were added to form a blue complex, and the absorbance was measured using an absorptiometer to obtain the total amount of N compounds.

Then, the amount of solid solution N can be obtained by subtracting the total amount of N compounds obtained by the method of (b) above from the total amount of N obtained by the method of (a) above.

<The content of C and N satisfies the relationship of 10C+N≤3.0>

In the steel of the present invention, the solid solution C greatly increases the deformation resistance and does not contribute much to the static strain aging. On the other hand, the solid solution N can promote the static strain aging without increasing the deformation resistance so much. After the increase in hardness. Therefore, in the steel of the present invention, in order to increase the hardness after processing without increasing the deformation resistance during processing, the content of C and N must satisfy the relationship of 10C+N≤3.0, preferably 0.009≤10C +N≤2.8, more preferably 0.01≤10C+N≤2.5, particularly preferably 0.01≤10C+N≤2.0. From the viewpoint of refining the crystal grains in the hot-rolled steel sheet and ensuring the formability of the steel sheet, a certain amount of C content and solid solution C amount is required, but when 10C+N>3.0, the amount of C and/or N is excessive , The deformation resistance is too large. Here, in the above inequality, the reason why the coefficient of the C content is set to be 10 times the coefficient of the N content is to consider that the strength of the hot-rolled steel sheet of the present invention can be increased even if the solid solution C is the same as the solid solution N. And the degree of increase in deformation resistance will still be about an order of magnitude (10 times) larger.

The steel of the present invention basically contains the above-mentioned components, and the balance is iron and unavoidable impurities. In addition, the following allowable components can also be added within the range that does not impair the effect of the present invention.

<Cr: 2% or less (excluding 0%) and/or Mo: 2% or less (excluding 0%)>

Cr is an element that increases the strength of crystal grain boundaries and improves the deformability of steel. In order to effectively exert such an effect, it is preferable to contain 0.2% of Cr. However, if Cr is contained in excess, deformation resistance may increase and cold workability may decrease, so the content is recommended to be 2% or less, more preferably 1.5% or less, and particularly recommended to be 1% or less.

In addition, Mo is an element that has the function of increasing the hardness and deformability of the processed steel material. In order to effectively exert such a function, Mo is preferably contained in an amount of 0.04% or more, more preferably in an amount of 0.08% or more. However, if Mo is contained excessively, the cold workability may deteriorate, so the content is recommended to be 2% or less, more preferably 1.5% or less, particularly recommended 1% or less.

<At least one selected from the group consisting of Ti: 0.2% or less (excluding 0%), Nb: 0.2% or less (excluding 0%), and V: 0.2% or less (excluding 0%)>

These elements have a strong affinity with N, coexist with N to form N compounds, refine the crystal grains of steel, improve the toughness of processed products obtained after cold working, and also have the effect of improving crack resistance. . However, even if each element is contained in excess of the upper limit, the property improvement effect cannot be obtained. The content of each element is recommended to be 0.2% or less, more recommended to be 0.001-0.15%, and especially recommended to be 0.002-0.1%.

<B: 0.005% or less (excluding 0%)>

Like Ti, Nb, and V above, B has a strong affinity with N, and forms N compounds when it coexists with N, which makes the crystal grains of steel finer and improves the toughness of processed products obtained after cold working. The element of crack resistance. Therefore, when the steel plate of the present invention contains B, the required amount of solid solution N can be ensured and the strength after cold working can be improved, so its content is recommended to be 0.005% or less, more recommended to be 0.0001 to 0.0035%, and especially recommended to be 0.0002 to 0.0002%. 0.002%.

<At least one selected from the group consisting of Cu: 5% or less (excluding 0%), Ni: 5% or less (excluding 0%), and Co: 5% or less (excluding 0%)>

These elements all have the effect of strain-aging and hardening the steel material, and are elements effective in improving the strength after processing. In order to effectively exert such an action, each of these elements is preferably contained in an amount of 0.1% or more, more preferably in an amount of 0.3% or more. However, if the content of these elements is excessive, the effect of strain aging and hardening of the steel material and the effect of increasing the strength after processing may be saturated, and cracking may be promoted, so it is recommended to be 5% or less, and 4% is more recommended. % or less, especially recommended to be less than 3%.

<From Ca: 0.05% or less (excluding 0%), REM: 0.05% or less (excluding 0%), Mg: 0.02% or less (excluding 0%), Li: 0.02% or less (excluding 0%), At least one selected from the group consisting of Pb: 0.5% or less (excluding 0%), Bi: 0.5% or less (excluding 0%)>

Ca spheroidizes sulfide compound-based inclusions such as MnS, improves the deformability of steel, and is an element that contributes to the improvement of machinability. In order to effectively exert such an effect, Ca is preferably contained in an amount of 0.0005% or more, more preferably in an amount of 0.001% or more. However, even if it is contained in excess, the effect is saturated, and the effect corresponding to the content cannot be expected, so 0.05% or less is recommended, 0.03% or less is more recommended, and 0.01% or less is especially recommended.

Like Ca, REM is an element that spheroidizes sulfide compound-based inclusions such as MnS, improves the deformability of steel, and is an element that contributes to the improvement of machinability. In order to effectively exert such an effect, REM is preferably contained in an amount of 0.0005% or more, more preferably in an amount of 0.001% or more. However, even if it is contained in excess, the effect is saturated, and the effect corresponding to the content cannot be expected, so 0.05% or less is recommended, 0.03% or less is more recommended, and 0.01 mass% or less is especially recommended.

In addition, in the present invention, REM means to contain lanthanide elements (15 elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements, at least one element selected from the group consisting of La, Ce, and Y is preferably contained, and La and/or Ce are more preferably contained.

Like Ca, Mg is an element that spheroidizes sulfide compound-based inclusions such as MnS, improves the deformability of steel, and is an element that contributes to the improvement of machinability. In order to effectively exhibit such an effect, Mg is preferably contained in an amount of 0.0002% or more, preferably in an amount of 0.0005% or more. However, even if it is contained in excess, the effect is saturated, and the effect corresponding to the content cannot be expected, so 0.02% or less is recommended, 0.015% or less is more recommended, and 0.01% or less is especially recommended.

Like Ca, Li is an element that spheroidizes sulfide compound-based inclusions such as MnS to improve the deformability of steel, and lowers the melting point of Al-based oxides to make them harmless, and is an element that contributes to the improvement of machinability. In order to effectively exhibit such an effect, Li is preferably contained in an amount of 0.0002% or more, more preferably in an amount of 0.0005% or more. However, even if it is contained in excess, the effect is saturated, and the effect corresponding to the content cannot be expected, so 0.02% or less is recommended, 0.015% or less is more recommended, and 0.01% or less is especially recommended.

Pb is an effective element for improving machinability. In order to effectively exert such an action, Pb is preferably contained in an amount of 0.005% or more, more preferably in an amount of 0.01% or more. However, if it is contained in excess, production problems such as the occurrence of rolling marks will occur, so it is recommended to be 0.5% or less, more preferably 0.4% or less, and especially recommended to be 0.3% by mass or less.

Bi, like Pb, is an effective element for improving machinability. In order to effectively exert such an effect, Bi is preferably contained in an amount of 0.005% or more, more preferably in an amount of 0.01% or more. However, even if it is contained in excess, the effect of improving machinability is saturated, so 0.5% by mass or less is recommended, more preferably 0.4% or less, particularly 0.3% or less.

Next, the structure that characterizes the steel sheet of the present invention will be described.

[Structure of steel plate of the present invention]

As described above, the steel sheet of the present invention is characterized in that it is based on steel with a ferrite-pearlite complex structure, and in particular, the size of ferrite grains is controlled within a specific range.

<Pearlite: less than 20%, balance: ferrite>

The structure of the steel plate of the present invention is composed of a multiphase structure of ferrite and pearlite. Excessive pearlite degrades the formability of the steel sheet, so the area ratio of pearlite is less than 20%, more preferably 19% or less, further preferably 18% or less, particularly preferably 15% or less. The balance is ferrite.

<Average grain diameter of the ferrite: in the range of 3 to 35 μm>

In order to improve the workability of the steel sheet and satisfy the surface texture after working, the average grain size of ferrite constituting the ferrite structure needs to be in the range of 3 to 35 μm. If the ferrite grains are too fine, the deformation resistance becomes too high, so the average grain size is 3 μm or more, preferably 4 μm or more, more preferably 5 μm or more. On the other hand, if the ferrite is too coarse, the surface texture after working will deteriorate, and the toughness, fatigue properties, etc. will deteriorate, so the average grain size is 35 μm or less, preferably 30 μm or less, more preferably 25 μm or less.

[Measuring method of area ratio of each phase]

Regarding the area ratio of each of the above phases, each test steel plate is corroded with a nital etching solution, and five fields of view are taken with a scanning electron microscope (SEM; magnification: 1000 times), and the ferrite and pearlite can be obtained by the counting method. of each ratio.

〔Measurement method of average crystal grain diameter〕

The average grain size of the above-mentioned ferrite can be measured as follows. That is, the grain diameters of ferrite existing in three places, namely, the outermost layer portion, the plate thickness 1/4 portion, and the plate thickness center portion, were measured. Regarding the particle diameter of one ferrite particle, the side portion in the rolling direction of each measurement position was etched with nital etching solution, and five fields of view were photographed at the position with a scanning electron microscope (SEM; magnification 1000 times), The center-of-gravity diameter obtained by image analysis of ferrite grains was used as the average grain diameter.

Next, a preferable manufacturing method for obtaining the above-mentioned steel sheet of the present invention will be described below.

[Preferable manufacturing method of the steel sheet of the present invention]

The production of the steel sheet of the present invention may be carried out by any method as long as it can form the raw material steel having the above composition into a desired thickness. For example, it can be carried out by preparing molten steel having the above composition in a converter under the conditions shown below, making it into a slab by ingot casting or continuous casting, and then rolling it into a hot-rolled steel sheet with a desired thickness.

[Preparation of molten steel]

The N content in the molten steel can be adjusted by adding a raw material containing an N compound to the molten steel during smelting in a converter, and/or by controlling the atmosphere of the converter to an N ₂ atmosphere.

[heating]

Heating before hot rolling is performed at 1100 to 1300°C. In this heating, high-temperature heating conditions are required in order to dissolve as much N as possible without forming N compounds. A preferable lower limit of the heating temperature is 1100°C, and a more preferable lower limit is 1150°C. On the other hand, temperatures higher than 1300°C are operationally difficult.

[hot rolled]

Hot rolling is performed so that the finish rolling temperature may be 880° C. or higher. If the finish rolling temperature is too low, the ferrite transformation will occur at a high temperature, the precipitated carbides in the ferrite will be coarsened, and the fatigue strength will be deteriorated. Therefore, a certain level or higher finish rolling temperature is required. In order to coarsen the austenite grains and increase the ferrite grain size to some extent, the finish rolling temperature is more preferably 900° C. or higher. In addition, since it is difficult to secure the temperature, the upper limit of the finish rolling temperature is 1000°C.

[Hot rolling pass schedule]

The hot-rolled steel sheet of the present invention has a thickness of 3 to 20 mm. However, in order to refine the ferrite crystal grains and control the average grain diameter within a predetermined grain size range, it is necessary not only to control the above-mentioned rolling temperature, but also to It is necessary to make the final rolling reduction of the continuous rolling of the final rolling 15% or more. Usually, 5 to 7 passes of continuous rolling are carried out in the finish rolling, but the pass schedule is set from the viewpoint of the bite control of the strip, and the final rolling reduction is limited to about 12 to 13%. The above-mentioned final rolling reduction is preferably 16% or more, more preferably 17% or more. The higher the above-mentioned final rolling reduction is 20% or 30%, the more fine grains can be obtained. However, from the viewpoint of rolling control, the upper limit is made about 30%.

[Quick cooling after hot rolling]

After the finish rolling is completed, quenching is performed at a cooling rate of 20°C/s or higher (first quenching rate) within 5s, and quenching is stopped at a temperature of 580°C or higher and lower than 670°C (quenching stop temperature). This is to obtain a ferrite-pearlite multiphase structure with a predetermined phase fraction. When the cooling rate (quick cooling rate) is lower than 20°C/s, the pearlite phase transformation is promoted, or when the quenching stop temperature is lower than 580°C, the pearlite phase transformation or bainite phase transformation is accelerated to obtain a predetermined phase fraction All ferritic-pearlitic steels become difficult, and the bending workability deteriorates. On the other hand, when the quenching stop temperature is 670° C. or higher, the precipitated carbides in the ferrite are coarsened, and the fatigue strength is deteriorated. The quenching stop temperature is preferably 600 to 650°C, more preferably 610 to 640°C.

[Slow cooling after rapid cooling stops]

After the above-mentioned rapid cooling is stopped, it is slowly cooled by standing cooling or air cooling at a cooling rate (slow cooling rate) of 10° C./s or less for 5 to 20 seconds. As a result, the formation of ferrite proceeds sufficiently, and the precipitated carbides in ferrite are moderately fined. When the cooling rate is higher than 10°C/s, or the slow cooling time is lower than 5s, the amount of ferrite formed is insufficient. On the other hand, if the slow cooling time is longer than 20 s, the precipitated carbides are not coarsened, and the fatigue strength is deteriorated.

[Rapid cooling and coiling after slow cooling]

After the above slow cooling, it is rapidly cooled again at a cooling rate of 20°C/s or higher (second rapid cooling rate), and coiled at a temperature higher than 300°C and lower than 450°C. This is to ensure bending workability by forming a ferrite-based structure. When the cooling rate (second rapid cooling rate) is lower than 20°C/s or the coiling temperature is higher than 450°C, a large amount of pearlite is formed. On the other hand, martensite and retained austenite are formed when the cooling rate is lower than 300°C. deteriorating.

Hereinafter, the present invention is described in more detail through the examples, but the following examples do not limit the nature of the present invention, and can also be appropriately changed and implemented within the scope of being able to meet the gist of the foregoing and the following, and these are all included in the scope of the present invention. within the technical range.

【Example】

Steel having the composition shown in Table 1 below was smelted by a vacuum melting method, cast into a steel ingot with a thickness of 120 mm, and hot-rolled under the conditions shown in Table 2 below to produce a hot-rolled steel sheet. In any of the tests, the cooling rate until the rapid cooling stop after finishing rolling was 20°C/s or more, and the cooling condition after the rapid cooling stop was slow cooling at a cooling rate of 10°C/s or less for 5 to 20 s.

For the hot-rolled steel sheet obtained in this way, the amount of solid solution N, the area ratio of each phase of the structure in the steel sheet, and the average grain size of ferrite were obtained by the respective measurement methods described in the above [Detailed Embodiments].

In addition, regarding the workability of the above-mentioned hot-rolled steel sheet, since it is a steel sheet with a thickness of about 10 mm, the 90° bending workability was evaluated by a 90° V-block test. Assuming that the thickness of the steel plate is t and the minimum bending radius inside the punch is R, use a 90-degree punch with a curvature ratio of R/t=1, press the test piece into the 90-degree die, and take it out For the test piece, observe the outside of the bend visually. As a result of visual observation, when a crack is visible, it is marked as ×, when no crack occurs, but a visually detectable crack is visible, it is marked as △, when there is no crack even though minute unevenness (wrinkle) can be seen, it is marked as ○, and when there is even a wrinkle When it happened and was not seen, it is ◎. In addition, the difference between a "crack" and a "crack" is that a gap with a maximum width of 1 mm or more is defined as a "crack", and a gap with a maximum width of less than 1 mm is defined as a "crack".

Moreover, the hardness of the surface part of the bent part after the said bending test was measured, and the surface hardness after processing was evaluated. Using a Vickers hardness testing machine, the Vickers hardness of each processed test piece was measured under the conditions of load: 1000g, measurement position: D/4 position central part of the cross section of the test piece (D: component diameter), and number of measurements: 5 times (Hv).

The results of these measurements are shown in Table 3 below.

【Table 1】

(one: no addition, underline: outside the scope of the present invention)

【Table 2】

(Mark: underline = outside the scope of the present invention, * = outside the recommended range)

【table 3】

(Mark: underline = outside the scope of the present invention, * = outside the recommended range,

Evaluation of 90°bending workability; ◎Very good, ○Good, △Surface cracks, ×Cracks

-: No measurement due to crack,

Invention example: [90° bendability = ◎ or ○] and [surface hardness after processing ≥ 250Hv],

Comparative example: the case where the conditions of the above-mentioned invention steel are not satisfied)

As shown in Table 3, Steel Nos. 1 to 3, 7 to 14, and 25 to 28 were manufactured using steel grades satisfying the compositional requirements of the present invention under recommended hot rolling conditions. Invented steel with sufficient requirements stipulated by the organization of the invention, 90° bending workability and surface hardness after processing all meet the qualified standards, and can obtain not only good cold workability during processing, but also a predetermined surface hardness after processing ( Strength) hot-rolled steel plate.

On the other hand, Steel Nos. 4 to 6, 15 to 24, and 29 are comparative steels that do not satisfy at least one of the composition and structure requirements specified in the present invention, and do not satisfy 90° bending workability and processed surface At least one passing standard among the hardness.

For example, Steel No. 4 satisfies the composition requirements, but the heating temperature before hot rolling is too low outside the recommended range, the amount of solid solution N is insufficient, and the surface hardness after processing is poor.

In addition, Steel No. 5 satisfies the compositional requirements, but the plate thickness after hot rolling is too large outside the specified range, the ferrite grains are coarsened, and both 90° bending workability and surface hardness after working are poor.

In addition, although Steel No. 6 satisfies the composition requirements, the final rolling reduction during hot rolling is too small outside the recommended range, the ferrite grains are coarsened, and both 90° bending workability and surface hardness after working are poor.

In addition, Steel No. 15 (steel type j) had low N content and poor surface hardness after working, although the hot rolling conditions were within the recommended range.

On the other hand, Steel No. 16 (steel type k) has too high an N content and poor at least 90° bending workability, although the hot rolling conditions are within the recommended range.

In addition, steel No. 17 (steel type 1) has too high a C content and does not satisfy the requirement of 10C+N≤3.0, although the hot rolling conditions are within the recommended range, excessive pearlite is formed, and at least 90° bending workability is poor.

In addition, Steel No. 18 (steel type m) had an excessively high Si content and poor at least 90° bending workability, although the hot rolling conditions were within the recommended range.

In addition, Steel No. 19 (steel type n) had low Mn content and poor surface hardness after working, although the hot rolling conditions were within the recommended range.

On the other hand, Steel No. 20 (steel type o) had too high a Mn content and poor at least 90° bending workability, although the hot rolling conditions were within the recommended range.

In addition, Steel No. 21 (steel grade p) had too high a P content and was inferior in at least 90° bending workability, although the hot rolling conditions were within the recommended range.

In addition, steel No. 22 (steel type q) had too high an S content and poor at least 90° bending workability, although the hot rolling conditions were within the recommended range.

In addition, Steel No. 23 (steel grade r) had too low Al content and poor at least 90° bending workability, although the hot rolling conditions were within the recommended range.

On the other hand, Steel No. 24 (steel type s) had an excessively high Al content and poor at least 90° bending workability, although the hot rolling conditions were within the recommended range.

On the other hand, steel No. 29 (steel type x) does not satisfy the requirement of 10C+N≦3.0, although the hot rolling conditions are within the recommended range, and at least 90° bending workability is poor.

From the above, the applicability of the present invention can be confirmed.

Although this invention was demonstrated in detail with reference to the specific embodiment, it is clear for those skilled in the art that various changes and correction can be added without deviating from the mind and range of this invention.

This application is based on Japanese Patent Application (Patent Application No. 2013-002640) filed on January 10, 2013 and Japanese Patent Application (Patent Application No. 2013-056658) filed on March 19, 2013, the contents of which are incorporated herein by reference.

Industrial availability

The hot-rolled steel sheet of the present invention is suitable for various parts for automobiles, such as transmission parts such as gears and casings.

Claims (2)

1. A hot-rolled steel sheet excellent in cold workability and surface hardness after processing, characterized in that, The plate thickness is 3~20mm, Ingredient composition is calculated in mass %, C: Less than 0.3% and excluding 0%, Si: 0.5% or less and 0% or less, Mn: 0.2～1%, P: less than 0.05% and excluding 0%, S: less than 0.05% and excluding 0%, Al: 0.01 to 0.1%, N: 0.008～0.025%, The balance consists of iron and unavoidable impurities, Solid solution N: 0.007% or more, And the content of C and N satisfies the relationship of 10C+N≤3.0, The tissue is calculated as the area ratio relative to the total tissue, Pearlite: less than 20%, balance: ferrite, The average grain size of the ferrite is in the range of 3 to 35 μm. 2. The hot-rolled steel sheet according to claim 1, wherein the composition further contains at least one of the following elements in mass %, Cr: less than 2% and not including 0%, Mo: 2% or less and 0% or less, Ti: 0.2% or less and excluding 0%, Nb: 0.2% or less and 0% or less, V: 0.2% or less and excluding 0%, B: 0.005% or less and excluding 0%, Cu: less than 5% and not including 0%, Ni: 5% or less and excluding 0%, Co: 5% or less and 0% or less, Ca: 0.05% or less and 0% or less, REM: less than 0.05% and excluding 0%, Mg: 0.02% or less and 0% or less, Li: 0.02% or less and 0% or less, Pb: 0.5% or less and 0% or less, Bi: 0.5% or less and 0% is not included.