JP2003034825A - Manufacturing method of high strength cold rolled steel sheet - Google Patents

Abstract

(57) [Problem] To provide a method for producing a high-strength cold-rolled steel sheet having excellent workability and weldability and having a tensile strength exceeding 340 MPa. The A low carbon steel, finishing temperature Ar ₃ hot rolled in to Ar ₃ + 80 ° C., then cooled over 80 ° C. above the temperature range at 100 ° C. / s or more cooling rate within the final rolling after 1s, A high-strength cold-rolled steel sheet characterized by being rolled at 650 ° C or less, pickled and cold-rolled, and then annealed at a heating rate of more than 100 ° C / s from 600 ° C to the end of recrystallization. Manufacturing method.
For low carbon steel, mass%, C: 0.2% or less, Si: 2.0% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, so
l. A method for producing a high-strength cold-rolled steel sheet, comprising: Al: 0.1% or less, N: 0.01% or less, and the balance substantially consisting of iron.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a high-strength cold-rolled steel sheet having excellent workability and weldability.

[0002]

2. Description of the Related Art In recent years, high-strength cold-rolled steel sheets having high tensile strength have been attracting attention as steel sheets used for automobile parts and home electric appliances. As a steel strengthening mechanism,
・ As shown on p.41 of 75th Nishiyama Memorial Technology Course (1981, Japan Iron and Steel Institute), solid solution strengthening, precipitation strengthening,
Five mechanisms are known: transformation strengthening, grain refinement, and work hardening. Among these, since strengthening by work hardening significantly deteriorates the workability of steel, the strengthening mechanisms of are usually used for the production of high strength thin steel sheets.

Among the strengthening mechanisms (1) to (3), especially fine graining is the most effective means as a method for increasing the strength without significantly impairing the workability and weldability. When further strengthening is required with this grain refining mechanism as a base strengthening mechanism, solid solution strengthening, precipitation strengthening, or transformation strengthening can be combined.

In the case of a hot-rolled steel sheet, the fine graining of the structure is realized by carrying out hot strong working and quenching. However, in the case of a cold-rolled steel sheet that is thin and has excellent surface roughness and plate thickness accuracy, it is necessary to perform recrystallization annealing after cold rolling, and it has been generally difficult to achieve grain refinement. Therefore, some techniques have been proposed for the purpose of making the structure finer in the high-strength cold-rolled steel sheet.

For example, Japanese Patent No. 3046128 proposes a method of manufacturing a hard surface-treated original plate having excellent workability. Here, steel containing 0.5 to 1.5% of Mn is a hot-rolled steel strip under normal hot rolling conditions, and after cold rolling, when performing continuous annealing, a temperature range of 500 ° C. or higher is 300 to 2000 ° C. / Heat at s, stay at 730 ~ 830 ℃ for 2 seconds or less, and cool at a cooling rate of 100 ~ 500 ℃ / s. In this way, by ultra-rapid heating,
By suppressing the diffusion of C, the two-phase separation does not proceed, and by suppressing the grain growth of ferrite, ultrafine grains can be obtained.

Japanese Patent No. 2688384 proposes a method for producing a high-strength cold-rolled steel sheet having excellent stretch flangeability.
This is a method of hot rolling and cold rolling steel with Nb added at 0.005 to 0.045 wt%, followed by heating and annealing at a heating rate of 5 ° C / s or more. Is intended to be realized. In addition, a higher heating rate is advantageous for miniaturization, so 5 ° C /
s or more, preferably 10 ℃ / s or more, in the example maximum 20 ℃ / s
Examples are shown up to. At this time, the ferrite grain size has been reduced to an average of 11 μm.

Japanese Patent Publication No. 2-1210 proposes a method for producing a high-strength cold-rolled steel sheet having good ductility. In this technology, at the time of continuous annealing after hot rolling, pickling, and cold rolling, the temperature range from at least 600 ° C to the Ac ₁ transformation point is heated at a heating rate of 5 ° C / s or more, and the soaking is performed for 10s to 10min. After holding, cooling is performed while controlling. This is because by shortening the heating time from the recrystallization start temperature of ferrite grains from 600 ℃ to the Ac ₁ transformation point, the ferrite structure becomes fine recrystallized grains or unrecrystallized state, and the Ac ₁ transformation occurs in that state. It is intended to make the austenite grains that undergo reverse transformation finer by heating to a temperature above the point. Although the upper limit of the heating rate is not particularly defined, the example shows a maximum of 30 ° C./s.

Japanese Unexamined Patent Publication (Kokai) No. 10-237549 proposes a method of manufacturing a steel sheet for a can, which has excellent non-earing properties and has uniform properties in the longitudinal and width directions of the coil. In this technology, after hot rolling at a finish rolling temperature of 800 to 1000 ° C, primary cold rolling is performed, then a heating rate of 10 ° C / sec or more, a soaking temperature of Ac ₁ transformation point to 800 ° C, and a soaking temperature. 60 s heat time
Continuous annealing at ec or less is performed, and secondary cold rolling is performed at a reduction rate of 20% or less.

Japanese Patent Publication No. 1-25381 discloses a Ti content of 0.002 to 0.
A method of quenching steel containing 020% after hot rolling, winding at a temperature of 270 ° C or lower, and performing continuous annealing at a heating rate of 1 to 100 ° C / s and a soaking temperature of 650 to 850 ° C after cold rolling. Is proposed.
This method consists of solid solution C in steel by low temperature winding in hot rolling.
It is possible to achieve a high r-value by allowing the cementite to remain and finely depositing cementite in the annealing process after cold rolling. An example is shown in which the time from hot rolling to winding is 10 to 20 seconds.

[0010]

However, the above-mentioned prior art has the following problems. That is,
As in the technique described in Japanese Patent No. 3046128, by suppressing the diffusion of C by performing ultra-rapid heating, although the generation and growth of austenite grains are suppressed, the nucleation site of ferrite grains increases, and grain growth. Is insufficient for the suppression of Therefore, it is considered that the ferrite grains are not sufficiently refined, and the workability is insufficient.

In the technique described in Japanese Patent No. 2688384, it is supposed that the carbonitrides can be formed by the addition of Nb, but the grain size can be reduced, but at the same time, the recrystallization temperature is increased.
Therefore, it is necessary to set the annealing temperature higher, and it is rather difficult to achieve sufficient grain refinement. Further, although it is said that the particles can be made finer by increasing the heating rate, the actual maximum is about 20 ° C./s, which is only an effect of suppressing the grain growth. Therefore, as a result, sufficient grain refinement has not been achieved, and the balance between strength and ductility is poor.

The technology described in Japanese Examined Patent Publication No. 2110-2 is also said to be able to make fine particles by increasing the heating rate, but practically only a maximum of about 50 ° C./s is assumed, and It has only the effect of suppressing growth. Therefore, it is considered that sufficient grain refinement cannot be achieved. According to this publication, higher strength and higher ductility can be achieved, but this has a large effect of the low temperature transformation phase generated by cooling control after annealing, leaving a problem in HAZ softening resistance during welding. ing.

In the technique described in Japanese Patent Laid-Open No. 10-237549, it is said that fine particles can be formed by increasing the heating rate, but in reality, only a maximum of about 30 ° C./s is assumed, and It has only the effect of suppressing growth. Therefore,
Even in this case, it is considered that sufficient grain refinement cannot be achieved, and as a result, it can be said that workability is insufficient.

In the technique disclosed in Japanese Patent Publication No. 1-25381, it is unlikely that the rapid cooling is carried out immediately in a short time such as within 1 second after the end of hot rolling. In addition, the heating rate during annealing is up to 100 ° C / s, and it may not be intended for fine graining, so graining is considered to be insufficient, and workability is expected to be poor. .

The present invention solves the above problems and realizes grain refinement in a cold-rolled steel sheet, which is usually difficult to grain refine, has excellent workability and weldability, and has a tensile strength of more than 340 MPa. An object is to provide a method for manufacturing a high strength cold rolled steel sheet.

[0016]

The above problems can be solved by the following inventions. The invention uses low-carbon steel at a finishing temperature
Ar ₃ to Ar ₃ + at 80 ° C. and hot rolling, 100 ° C. within the final rolling after 1s
After cooling at a cooling rate of / s or more over a temperature range of 80 ° C or more, winding at 650 ° C or less, pickling and cold rolling, the temperature range from 600 ° C to the end of recrystallization is 100 ° C / s. A method for producing a high-strength cold-rolled steel sheet, characterized by annealing at a higher heating rate.

The present invention has been made as a result of intensive studies to solve the above-mentioned problems. In the process of research, from the viewpoint of workability and weldability using ductility and hole expandability as indicators, we aimed to increase the strength by making the structure finer. That is, in the hot rolling stage, immediate rapid cooling after rolling causes ferrite transformation from unrecrystallized austenite, and further rapid heating in the recrystallization annealing process after cold rolling leads to a dramatically finer structure. It has been found that a high-strength cold-rolled steel sheet that is granulated and has good workability and weldability can be obtained.

The cold-rolled steel sheet targeted by the present invention includes a steel sheet having a surface treatment such as a hot dip galvanized material or an electrogalvanized material. Hereinafter, the manufacturing conditions of the present invention will be described.

Final rolling temperature of hot rolling: Ar _{3 to} Ar ₃ +8
When the final rolling temperature (finishing temperature) of 0 ° C hot rolling is below the Ar ₃ transformation point, coarse ferrite grains are generated, so the microstructure of the cold rolled steel sheet is refined in the subsequent manufacturing process. Things will be difficult. On the other hand, if the temperature of the final rolling exceeds Ar ₃ + 80 ° C., recrystallization of the processed austenite proceeds, so that it becomes impossible to generate fine ferrite grains by transformation from unrecrystallized austenite. Therefore, the final rolling temperature (finishing temperature) of hot rolling is Ar _{3 to} Ar ₃ +80.
Within the range of ° C.

Cooling start time after rolling: Within 1 s after final rolling The cooling condition after rolling is one of the technologies forming the basis of the present invention. If the time to start cooling after rolling exceeds 1 s after the final rolling, the strain accumulated during rolling in the austenite region may be recovered or recrystallized, which is not preferable for grain refinement due to ferrite transformation. Therefore, the cooling start time after rolling should be within 1 s after the final rolling.

Cooling condition after rolling: Cooling rate over a temperature range of 80 ° C. or more After cooling starts at 100 ° C./s or more, it is necessary to secure a cooling rate and a temperature difference before and after cooling in order to refine the structure. is there. When the cooling rate is less than 100 ° C / s, the strain accumulated in austenite is recovered or recrystallized, and the ferrite grains after transformation are not refined. When the temperature difference (falling temperature) before and after cooling is less than 80 ° C, the strain is recovered or recrystallized, and the ferrite grains after transformation are not refined. Therefore, the cooling condition after rolling is set to a cooling rate of 100 ° C./s or more over a temperature range of 80 ° C. or more.

Winding temperature: 650 ° C. or less In winding after hot rolling, if the winding is carried out at a high temperature exceeding 650 ° C., grain growth of ferrite grains occurs,
In the subsequent manufacturing process, the structure of the cold rolled steel sheet cannot be refined. Therefore, after hot rolling, the coil is wound at 650 ° C or lower. The lower limit of the coiling temperature is not particularly specified, and the coiling may be carried out at room temperature from the viewpoint of material properties, but it may be determined from the viewpoints of cooling capacity, operability and the like.

Heating Rate in Annealing: Greater than 100 ° C./s from 600 ° C. to the end of recrystallization The heating process in recrystallization annealing after cold rolling is a part that forms the basis of the present invention together with the cooling conditions after hot rolling. is there.
When this heating rate is slow, the recovery of strain progresses during heating, and when the annealing target temperature is reached, the driving force for generating recrystallization nuclei decreases, and it becomes impossible to obtain fine grains. This problem becomes remarkable in the temperature range of 600 ° C or higher.

By making the heating rate higher than 100 ° C./s, the recovery of strain during heating can be suppressed, and recrystallized nuclei from ferrite grain boundaries can be randomly generated at a target annealing temperature. A fine structure is obtained. From the viewpoint of suppressing the recovery of strain, this heating rate needs to be secured up to the recrystallization end temperature.

Therefore, the heating rate in annealing is 600 ° C.
To 100 ° C / s from the end to recrystallization. From the viewpoint of productivity, it is preferable to heat from room temperature to the annealing temperature at a heating rate higher than 100 ° C./s, but in the range from room temperature to 600 ° C., the strain recovery amount itself even at low speed heating. Since it is small, 100 ° C / s or less is acceptable. The heating method is not particularly limited, but the heating may be performed by induction heating, direct energization, or the like.

Further, in the above invention, the low carbon steel is
Mass%, C: 0.2% or less, Si: 2.0% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03% or less, sol.Al: 0.1% or less, N: 0.01% or less Also, the manufacturing method of the high-strength cold-rolled steel sheet may be characterized in that the balance is substantially made of iron.

In the present invention, since the chemical composition is further specified, the high strength cold rolled steel sheet can be manufactured more efficiently. The chemical components will be described below.

C: 0.2% or less C is an inexpensive and effective element for increasing the strength of steel. However, addition of a large amount of C exceeding 0.2% causes an increase in the amount of pearlite, which not only deteriorates the ductility and stretch flangeability but also adversely affects the weldability. Therefore, the C content is 0.2% or less.

Si: 2.0% or less Si solid-solution strengthens ferrite without deteriorating workability and improves the balance between strength and workability, so Si is added according to the required strength level. However, addition of a large amount of Si exceeding 2.0% deteriorates toughness and weldability.
Therefore, the upper limit of Si content is 2.0%.

Mn: 2.5% or less Mn is a solid solution strengthening element and is effective in increasing strength. It is preferable to add Mn according to the required strength level. However, addition of a large amount of Mn exceeding 2.5% causes deterioration of weldability. Therefore, the upper limit of Mn content is 2.5%.

P: 0.1% or less P is a solid solution strengthening element and is effective for increasing strength. Further, in the case of Si-added steel, it suppresses the generation of red scale, so P is added as necessary. Is preferred. However, addition of a large amount of P exceeding 0.1% promotes segregation at grain boundaries, and reduces ductility and toughness. Therefore, the upper limit of P content is 0.1%.

S: 0.03% or less S significantly reduces hot ductility, induces hot cracking, and significantly deteriorates surface properties. Furthermore, S
Not only contributes to the strength, but also forms coarse MnS as an impurity element and, in the case of Ti-added steel, reduces the ductility and stretch flangeability by forming a large amount of coarse Ti-based sulfide. Let These problems become remarkable when the S content exceeds 0.03%, and it is desirable to reduce them as much as possible. Therefore, the upper limit of S content is 0.03%. Furthermore, from the viewpoint of improving ductility and stretch flangeability, the S content is 0.01%.
The following is preferable.

Sol.Al: 0.1% or less sol.Al has a function of reducing inclusions in steel as a deoxidizing element, but when added in excess of 0.1%, alumina-based inclusions The number of products increases and the ductility decreases. Therefore, the upper limit of the sol.Al content is 0.1%.

N: 0.01% or less When N is contained in a large amount exceeding 0.01%, slab cracks may occur during hot rolling and surface defects may occur. Therefore, the upper limit of N content is 0.01%.

In these means, "the balance is substantially iron" means that the content of inevitable impurities and other trace elements is included in the present invention unless the action and effect of the present invention are impaired. It means that it is included in the range of.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION In carrying out the invention, a low carbon steel having a chemical composition corresponding to a desired strength level or a steel having the above chemical composition is melted. The melting method is a normal converter method,
The electric furnace method or the like can be appropriately applied. Other elements are not particularly specified, but the following components can be adjusted or added in the production of the high strength cold rolled steel sheet of the present invention.

Si: preferably 0.5%, more preferably 0.2% Generally, addition of a large amount of Si promotes the formation of firelite on the surface of the slab during heating by hot rolling, and promotes the generation of a surface pattern called so-called red scale. When it is used as a hot-dip galvanized steel sheet, it also induces a non-plating defect due to Si, so in the case of a steel sheet or hot-dip galvanized steel sheet that requires surface texture, the upper limit is about 0.5%, and Desirably, the upper limit is about 0.2%.

Ti, Nb, V, Cu, Ni, Cr, Mo, B: added as required, and further Ti, Nb, V, depending on the required strength level.
Additional elements such as Cu, Ni, Cr, Mo and B may be added. The upper limit of the addition amount is 1% or less for Ti, V, Cu, Ni, Cr, Mo, and Nb
It is desirable that the content be 1.5% or less and B be 0.01% or less.

After the molten steel is cast into a slab, it is heated as it is or after cooling and hot rolling. The hot-rolled steel sheet after finish rolling is wound at the above-mentioned winding temperature and subjected to ordinary cold rolling.

For annealing, rapid heating is performed under the above heating conditions. Although the annealing temperature (recrystallization temperature) is not particularly specified, it is preferable to raise the temperature of each chemical component to the temperature required for recrystallization annealing, and it is preferable to make the temperature as low as possible within that range. The annealing time is not specified, but the time to stay in the temperature range above the recrystallization temperature is not particularly required.

Cooling after annealing may be either cooling or quenching. Especially when manufactured as a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, the heat history in the process may be used.

As described above, according to the present invention, it is possible to manufacture a cold rolled steel sheet having a low yield ratio, excellent workability and weldability, and high tensile strength. The reason for this is not limited to the scope of the claims of the present invention, but is considered as follows.

That is, in the recrystallization annealing process after cold rolling, when gradual heating is performed, recovery of strain progresses during heating, and most of the strain accumulated in cold rolling disappears. However, recrystallization requires holding at a high temperature for a long time. Therefore, generation of large recrystallized grains is unavoidable, and the size of the recrystallized grains becomes nonuniform due to grain growth.

On the other hand, when the rapid heating is performed, the recrystallization temperature range can be reached without the recovery of the strain during the heating. Then, since the strain immediately shifts to the high temperature range without recovery, the driving force for the recrystallization nucleation becomes enormous, and not only from the ferrite grain boundary with a large tilt angle, but also the strain at the time of cold rolling is relatively high. It is considered that nucleation proceeds instantaneously even from a small-angled ferrite grain boundary where it is hard to enter.

Here, unlike the usual case where a large tilt ferrite grain boundary corresponds to the nucleation from the austenite grain boundary, a small tilt ferrite grain boundary is inside the unrecrystallized austenite grain during hot rolling. It corresponds to the nuclear generation from. In the present invention, the structure is controlled by hot rolling and rapid cooling after rolling as described above, and the structure of the hot-rolled steel sheet is a ferrite structure having a low-angle grain boundary.

The recrystallized nuclei formed in the ferrite grain boundaries with a small tilt angle tend to grow randomly without any preferential orientation because the orientation difference between adjacent grains is small. However, since recrystallized nuclei are simultaneously produced in an enormous number, recrystallized grains try to grow equally, and as a result, they can hardly grow each other and an ultrafine structure is formed. It is estimated to be.

On the other hand, unlike the present invention, when a hot-rolled steel sheet having a ferritic structure having a normal high-angle grain boundary is cold-rolled and recrystallized by rapid heating, the adjacent grains are Since the difference in orientation is large, the grain growth proceeds with the preferred orientation. As a result, even if the recrystallized nuclei are generated all at once, some of the recrystallized grains preferentially grow, so the overall grain size is nonuniform, and it is considered that a fine grain structure cannot be formed. To be

[0048]

EXAMPLES Examples of the present invention will be described. The present invention is not limited to these examples.

First, steels having the components shown in Table 1 were melted in a laboratory vacuum melting furnace and once cooled to room temperature.

[0050]

[Table 1]

Then, the steel ingot was reheated at 1250 ° C. and subjected to lab hot rolling. After rolling, it was once cooled under various water-cooling conditions, then continuously air-cooled to a temperature equivalent to the coiling temperature, held in a furnace at that temperature for 1 hour, and then cooled in the furnace to obtain a heat treatment equivalent to the coiling. After the hot rolling, cold rolling was performed at a cold rolling rate of 60% to obtain a cold rolled steel sheet having a sheet thickness of 1.2 mm. Then, recrystallization annealing was performed under various heating conditions. Table 2 shows the experimental conditions.

[0052]

[Table 2]

Here, the steel plates No. 1, 2, 5, 8, 10, 12, 14 to 16 are invention steel plates. The others are comparative steel plates, and the steel plate No.
3 and 4 are the final temperature of finish rolling, No. 6 is the cooling start time after finish rolling, No. 7 is the cooling rate after finish rolling, Steel plate No. 9 is the cooling down temperature, and steel plate No. 11 is winding. For temperature and No. 13, the heating rate in annealing is outside the scope of the present invention. For steel sheets Nos. 1 to 15, after annealing, quenching was performed at 1000 ° C / s, and 350
The steel sheet No. 16 was annealed at 60 ° C. for 60 s, and after annealing, it was cooled to room temperature at 10 ° C./s.

Tensile properties, hole expansion properties (stretch flange properties), and weldability were investigated using the annealed samples. Here, the weldability was evaluated by examining the hardness distribution of the welded portion by TIG bead-on welding and by the hardness ratio of the HAZ softened portion to the base material portion (HAZ softened portion hardness / base material hardness). Table 3 collectively shows the characteristic values obtained by the test investigation.

[0055]

[Table 3]

The relationship between the manufacturing conditions and the characteristic values will be described below with reference to the drawings based on the test survey results. Here, as the hot rolling conditions, the final finishing temperature is 830 ° C and the cooling start time is 0.8
s, cooling rate is 150 ° C / s, falling temperature is 100 ° C, coiling temperature is 600 ° C, heating rate is 150 ° C / s, annealing temperature is 760 ° C, annealing time is 1s, cooling after annealing Speed is 1000 ℃ /
The influence of each manufacturing condition on the tensile strength when s is used as a base (standard condition) is shown.

FIG. 1 shows the influence of the final finishing temperature on the tensile strength. Here, only the final finishing temperature is changed,
The other manufacturing conditions were the above-mentioned base conditions (standard conditions). The Ar ₃ transformation point of the steel used is 800 ° C., and the finishing temperature range of the present invention for hot rolling is 800 to 880 ° C. As shown in the figure, the tensile strength is the lower limit of the final finish temperature of the present invention range 800
When the temperature is below ℃ or above the upper limit of 880 ℃, it is 540 to 550MPa, whereas in the range of the present invention 800 to 880 ℃, it is 600 to 610MPa.
It became higher around a and 60MPa.

FIG. 2 shows the influence of the cooling start time in the quenching after the final rolling on the tensile strength. As shown in the figure, the tensile strength was 550 MPa when the cooling start time exceeded the upper limit of 1 s in the range of the present invention, whereas it was 610 to 620 MPa and 60 MPa or higher at 1 s or less of the range of the present invention.

FIG. 3 shows the influence of the cooling rate in the quenching after hot rolling on the tensile strength. As shown in the figure, the tensile strength is 540 MPa when the cooling rate is below the lower limit of 100 ° C./s of the present invention range, whereas it is 610 to 620 MPa and 70 MPa or more at 100 ° C./s or more of the present invention range. It was

FIG. 4 shows the influence of the temperature drop during quenching after hot rolling on the tensile strength. As shown in the figure, the tensile strength is 555 MPa when the falling temperature is below the lower limit of 80 ° C. of the present invention range, while it is 61 at 80 ° C. or higher of the present invention range.
It became higher around 0 to 620MPa and 60MPa.

FIG. 5 shows the influence of the winding treatment temperature on the tensile strength. As shown in the figure, the tensile strength is 545 MPa when the winding temperature exceeds the upper limit of 650 ° C. of the present invention range, whereas it is 610 to 620 MPa at 650 ° C. or less of the present invention range.
And it became high around 70MPa.

FIG. 6 shows the influence of the heating rate during annealing after cold rolling on the tensile strength. As shown in the figure, the tensile strength is 550 MPa when the heating rate is lower than the lower limit of 100 ° C./s of the present invention range, whereas it is 610 to 620 MPa and 60 MPa or more when the heating rate exceeds 100 ° C./s of the present invention range. It became high.

Further, as shown in Table 3, the steel sheet of the present invention has superior ductility, hole expandability, and HAZ softening resistance to the comparative steel under any of the manufacturing conditions. Further, among the invention steel sheets, as shown in Steel sheet No. 15, when the annealing temperature after cold rolling is 720 ° C., when the annealing time is 10 s, and as shown in Steel sheet No. 16, the cooling rate after annealing is shown. Even at 10 ° C / s, an excellent balance between tensile strength and ductility, hole expandability, and HAZ softening resistance are obtained.

[0064]

As described above, according to the present invention, by limiting the hot rolling finishing condition, the cooling condition, the winding condition and the annealing condition after the cold rolling, in the hot rolling stage, unrecrystallized austenite By generating a fine ferrite structure and performing rapid heating in the subsequent recrystallization annealing process after cold rolling, the structure is dramatically fine-grained and a large increase in strength can be obtained. As a result, a method for producing a high-strength cold-rolled steel sheet excellent in workability and weldability is provided, and an industrially effective effect is brought about.

[Brief description of drawings]

FIG. 1 is a diagram showing the influence of final finishing temperature on tensile strength.

FIG. 2 is a diagram showing the influence of cooling start time in quenching after final rolling on tensile strength.

FIG. 3 is a diagram showing the influence of the cooling rate of quenching after hot rolling on the tensile strength.

FIG. 4 is a diagram showing the influence of the temperature drop during quenching after hot rolling on the tensile strength.

FIG. 5 is a diagram showing the influence of the winding treatment temperature on the tensile strength.

FIG. 6 is a diagram showing an influence of a heating rate during annealing after cold rolling on tensile strength.

Continued front page    F-term (reference) 4K037 EA01 EA02 EA05 EA06 EA11                       EA13 EA15 EA16 EA17 EA18                       EA19 EA20 EA23 EA25 EA27                       EA28 EA31 EA32 EB06 EB07                       EB08 EB09 FC03 FC04 FD04                       FE01 FE02 FH00 FJ01

Claims (2)

[Claims] 1. A low carbon steel is hot-rolled at a finishing temperature of Ar _{3 to} Ar ₃ + 80 ° C. and cooled within a temperature range of 80 ° C. or more at a cooling rate of 100 ° C./s or more within 1 s after final rolling. , High strength cold rolling characterized by winding at 650 ° C or less, pickling, cold rolling, and annealing in a temperature range from 600 ° C to the end of recrystallization at a heating rate higher than 100 ° C / s. Steel plate manufacturing method. 2. The low carbon steel is mass%, C: 0.2% or less,
Si: 2.0% or less, Mn: 2.5% or less, P: 0.1% or less, S: 0.03
% Or less, sol.Al: 0.1% or less, N: 0.01% or less,
The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein the balance is substantially iron.