JPH04116137A - High toughness high carbon cold rolled steel sheet and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a high carbon cold rolled steel sheet excellent in toughness, in a steel having a specified compsn. in which each content of Nb, Ti, P and Cu is prescribed, by incorporating fine spheroidized cementite having specified average grain size. CONSTITUTION:A high carbon cold rolled steel sheet having a compsn. contg., by weight, 0.30 to 0.90% C, 0.05 to 0.70% Si, 0.05 to 1.00% Mn, <=0.020% P, 0.005 to 0.10% Ti, 0.005 to 0.100% Nb, <=0.08% solAl and 0.0020 to 0.010% N, furthermore suitably contg., at need, one or more kinds among 0.005 to 0.020% Ca 0.05 to 0.50% Cu, <=0.30% Cr and 0.0003 to 0.0030% B and the balance Fe with inevitable impurities and contg. fine spherical cementite having <=0.8mu average grain size is prepd. In this way, the high carbon steel sheet having excellent hydrogen cracking resistance and toughness in an are with high hardness of about >=45 HRc can be obtd.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a high-toughness, high-carbon cold-rolled steel sheet and a method for manufacturing the same. , has excellent wear resistance, and has an excellent effect of preventing cracking caused by hydrogen intrusion during use, and has good manufacturability and processability, making it suitable for chain parts, gear parts,
The present invention relates to a high-toughness, high-carbon cold-rolled steel sheet suitable for use in clutch parts, leaf spring hose bands, seatbelt buckles, and washers, and a method for manufacturing the same.

(Prior art) In general, chain parts, gear parts, hose clips, clutch parts, seat belt punctures, washer parts, etc.
S30CM to SS7 specified in SG 3311
It is usually manufactured by using 0C high carbon steel plate, SK7M, SK5M high carbon cold rolled steel plate as a material, forming it, and then hardening it through heat treatment such as quenching, tempering, and austempering.

Here, the raw material steel plates for each of the above products are soft and easy to process before forming, and the desired strength can only be obtained through heat treatment after forming, and have sufficient impact resistance when used as a product. It is required to exhibit wear resistance. For this reason, materials with a high carbon content as described above are transported, and generally, when thin steel sheets are shipped from steel manufacturers, they are subjected to spheroidizing annealing to make them soft. After shipping, the raw material thin steel sheet is formed into a desired shape by the user, and then subjected to heat treatment such as quenching and tempering to impart necessary properties. In this case, the impact resistance and abrasion resistance of the product are particularly affected by the tempering temperature, so the tempering temperature of "as-quenched" or "up to 650°C" is carefully selected depending on the form and situation of use. (Usually 180-450 degrees). In addition, for austempering treatment, after soaking to an austenite region of 800°C or higher, the material is cooled in a molten salt bath or a molten lead bath at 250 to 380°C.

(Problem to be Solved by the Invention) However, despite careful selection of heat treatment conditions, the hardened and tempered high carbon steel sheets specified in JIS have insufficient impact resistance and hydrogen cracking resistance. for example,
High-strength steel of TS: 200 or higher is used for hose clips that fix connections such as fuel pipes or gas pipes in automobile engines because high spring properties are required. To ensure this strength, conventional CM is 0.70-
0.85% high carbon steel (S70C, SK5M, SK7
M, etc.) has been used after being austempered. However, when such steel is used, there is a problem that cracks occur in areas that receive stress concentration during use, and the fracture surfaces of these cracks appear to be intergranular fractures. It is presumed that the cause was hydrogen that entered the fracture.

In order to prevent such hydrogen cracking, it is necessary to refine the austenite grain size and adjust the chemical components so that the austenite grain boundaries are strengthened. To refine the austenite grain size, A+ precipitates during slab heating or quenching, or soaking in the austenite region
A common method is to suppress it using fine particles such as N.

However, in order to obtain finer austenite crystal grains than conventional ones, in addition to the precipitation of these fine particles, it is necessary to precipitate many other fine particles.

In addition, with regard to post-forming heat treatment, there is a demand for shortening the soaking time in the austenite region for the purpose of improving manufacturing efficiency. For this purpose, it is necessary to refine the cementite grains that have become spheroidized after cold rolling and annealing, and in order to refine the cementite grains, the pearlite structure in the hot rolled sheet must be made uniform and fine. Must be. Several proposals have also been made regarding hot rolling methods to solve this problem.

The present inventors invented a B-added steel containing Ti or Ti-Nb (hereinafter referred to as "prior application steel") in a previous application (Japanese Patent Application No. 1-177335), but this steel type was developed for the purpose of improving toughness. Since Cr steel or Cr-no steel is used as the base, the cost increases, and the solid solution of cementite into the steel during heat treatment is significantly delayed due to the solid solution of Cr and Mo into the cementite.

Therefore, it is desirable to reduce the amount of Cr added as much as possible to an extent that does not impede hardenability, and to eliminate the addition of Mo.

Furthermore, considering the rolling conditions of hot-rolled sheets, the addition of B is essential, which poses a problem of increased costs.

Further, knowledge regarding hot rolling conditions is disclosed in Japanese Patent Application No. 2-14723.
No. 3 was obtained, but the target steel at this time was general high carbon steel, and no measures regarding hydrogen cracking resistance etc. were taken into consideration, and
-570), it is necessary to improve the hydrogen embrittlement resistance for use in the high strength range.

The general object of the present invention is to provide a high-carbon cold-rolled steel sheet that is less expensive and has excellent wear resistance and hydrogen cracking resistance after quenching, tempering, or austempering, as well as excellent toughness. An object of the present invention is to provide a manufacturing method.

The specific purpose of the present invention is to solve the above-mentioned problems, to make the austenite structure extremely fine after heat treatment, to improve impact resistance, wear resistance, and to prevent cracking caused by hydrogen penetration during use. It is an object of the present invention to provide a high-toughness, high-carbon cold-rolled steel sheet having excellent manufacturability and workability, and a method for manufacturing the same.

(Means for solving the problem) In order to achieve the above object, the present inventors have repeatedly conducted various tests,
We are conducting research to provide cold-rolled steel sheets that have sufficient hardness and tensile strength to be used as materials for these high-strength steel sheets, have good workability, and do not cause problems such as cracking during the rolling process or forming process into final products. As a result, I obtained the following findings.

(a) Previously, it was thought that hydrogen embrittlement and fatigue embrittlement, which tend to occur in steel types with high material strength, could not be completely prevented, but even in such steel types, the ingredients must be strictly adjusted. A specific amount of Nb (0,005~o,io
o%), the austenite grains are effectively refined and cracking due to hydrogen embrittlement is significantly suppressed.

(b) In addition, 0.005 to 0.100% T
When i is added to U, a T1Nb-based composite nitride or carbonitride is formed during slab heating or soaking during quenching, and austenite grain growth is effectively suppressed.

(C) Furthermore, when the P content in steel is reduced below a certain value, the amount of P segregated at austenite grain boundaries is reduced, suppressing grain boundary embrittlement that causes brittle fracture, and further improving the toughness of the material. To be.

(d) Furthermore, the addition of an appropriate amount of Cu forms a sulfide film on the surface of moving gears, chains, etc., and has the property of suppressing the intrusion of hydrogen from the surface, which prevents cracks due to hydrogen embrittlement. It must be effective.

(eJ After hot rolling, when cooling is divided into an accelerated cooling zone (finishing mill side) and an air cooling zone (winding machine side) on the hot run table, the cooling rate in the accelerated cooling zone is set to 5~
Accelerated cooling stop temperature (hereinafter referred to as "
"Intermediate temperature" (displayed as "Tll1") is 550
By cooling to above °C, a uniform and fine pearlite structure is formed over the entire length of the coil, and by adjusting the annealing time in box annealing after cold rolling,
A steel plate having uniform and fine spheroidized cementite is obtained. As a result, when soaking in the onystenite region after forming, cementite is easily dissolved into austenite, shortening the heat treatment time and efficiently preventing cementite from remaining in the steel after heat treatment.

The present invention has been completed based on the findings shown in (a) to (e), and its gist is as follows:
C: 0.30-0.90%, Si: 0.05-0 in weight percentage
,70%, Mn: 0.05-1.00%, P:'
0.020% or less, Ti: 0.005 to 0.10%,
Nb: 0.005-0.100%, sol', Al
: O, 08% or less, N: 0.0020-0.0
10%, and if necessary, add one or more of the following ■Ca: 0.005 to 0.020%, ■Cu: 0.05
~0.50%, ■Cr: 0.30% or less: ■B:
It is a high-toughness, high-carbon cold-rolled steel sheet having a steel composition consisting of O,0O03 to 0.0030% balance Fe and unavoidable impurities, and having fine spheroidized cementite with an average grain size of 0.8p or less in the steel.

In addition, from another aspect, the present invention cools the steel having the above steel composition by dividing it into an accelerated cooling zone on the finish rolling mill side and an air cooling zone on the winding machine side on the hot run table after hot rolling. When doing so, the cooling rate in the accelerated cooling zone is set to 5~
Accelerated cooling stop temperature (intermediate temperature,
Two) is cooled to 550°C or higher, then air cooled to complete the pearlite transformation, and then coiled to form a hot rolled steel sheet.The hot rolled steel sheet is pickled and descaled, and if necessary,
After box annealing in the temperature range of 00°C to AC++30°C, cold rolling at a reduction rate of 30 to 80% and Ac, -7
By carrying out box annealing in the temperature range of 0° (-Ac++ 30°C) once or twice or more, an average grain size of 0.
This is a method for producing a high-toughness, high-carbon cold-rolled steel sheet, which is characterized by precipitating fine spheroidized cementite of 8 μm or less.

Thus, according to the present invention, quenching and tempering after forming,
Or, during heat treatment such as austempering, the soaking time is effectively shortened, and the austenite grains after heat treatment are effectively refined, resulting in improved wear resistance, hydrogen cracking resistance C, and of course toughness. An excellent high carbon cold rolled steel sheet can be obtained.

(The reason why the composition and manufacturing conditions of the cold-rolled steel sheet according to the present invention are numerically limited as described above will be explained.

(Chemical composition) (a) C In order to obtain the desired hardness, strength, hardenability and wear resistance of the steel plate, it is necessary to add 0.30% or more of C. Further, when the C content [1] exceeds 0.90%, not only the workability before quenching deteriorates but also the brittleness after quenching increases, so the amount of C added was set at 0.30 to 0.90%.

(b) Si It is not particularly necessary to add Si cumulatively, but if it is added in excess of 0.70%, the steel plate tends to become hard and brittle, so the Si content is set at 0.70% or less. Ta.

Further, in order to ensure hardenability, it is necessary to add 0.05% or more, preferably 0.15% or more.

(C) 11 n One application of the high carbon steel sheet targeted by the present invention is gears, chains, etc., and unlike -S wear-resistant steel sheets, it is necessary to reduce Mn in order to improve toughness.

In particular, in the steel sheet of the present invention, if it is contained in an amount exceeding 1.0%, it is likely to be hardened by heat treatment, becoming too hard and causing a decrease in toughness. On the other hand, if the Mn content is less than 0.05%, the amount of solid solution S will increase, causing embrittlement during hot working and impairing the manufacturability of the steel sheet. 05
-1.00%, preferably limited to 0.80% or less.

(d) P The lower the P content, the better from the viewpoint of toughness, and in the present invention, P
The content is set at 0.020% or less, preferably 0.020% or less.
It is preferable to limit it to 0.015% or less.

(e) Nb Nb has the effect of refining austenite grains and improving the toughness of steel, and this effect is also very effective in preventing fracture due to hydrogen embrittlement. Therefore, Nb is added for the purpose of preventing the occurrence of these cracks, but if the content is less than 0.005%, the desired effect of the above action cannot be secured; on the other hand, if the content is less than 0.100%, However, since these effects reach a saturated state, the Nb content was determined to be 0.005 to 0.100%. Further, in order to form a T1Nb-based composite precipitate, it is preferable to add about twice the amount of Ti added.

(f) Ti Ti has the effect of improving the hardenability of steel and increasing the hardness and tensile strength of steel by forming and finely dispersing TiN or TiC. Furthermore, TiNb (CN) is formed as a composite precipitate with Nb, and it also has the effect of promoting refinement of austenite crystal grains. However, if the Ti content is less than 0.005%, the desired effect due to the above action cannot be obtained;
If Ti is contained in excess of more than 0.00%, it not only increases costs but also leads to hardening of the steel and loses its benefits. Therefore, the Ti content is set at 0.005 to 0.100%. Further, in order to form a T1Nb-based composite precipitate, it is desirable that the amount of Nb added is not exceeded.

(g) sol, Al Al is a component added as necessary as a deoxidizing agent for steel, but sol. If the Al content exceeds 0.08%, it will not only increase the cost but also cause hardening of the steel plate, so there is no advantage. In this way, sol.

Because AI content is allowed up to 0.08%,
Its content was determined to be 0.08% or less.

(h) N The inclusion of N is effective in improving the hardness and tensile strength of steel.
Forming AlN, TiN, etc. prevents austenite from becoming coarse and helps improve toughness. In order to ensure this effect, 0.0020% or more of N is required. On the other hand, if the content exceeds 0.010%, the hardness increases and the workability before quenching is inhibited, so the content is reduced to 0.010%.
0.010% or less.

(i) Ca Even in ordinary steels, deterioration of toughness due to S is a problem. This is due to the formation of coarse MnS, but Ca
By adding P during the steelmaking stage, this formation can be controlled or the toughness can be significantly improved by forming fine spherical precipitates. However, if the amount added exceeds 0.02%, this effect will be saturated and the cost will be increased, so the upper limit of the amount added is set at 0.02%.

(j) Cu Cu does not have a very large effect on hardenability, but
A sulfide film is formed on the surface, and the effect of controlling hydrogen intrusion is remarkable. This effect suppresses the occurrence of crack starting points in the surface layer. This effect has been confirmed at 0.05% or more, but this effect is saturated at 0.50% or more. By the way, in ordinary steel, Cu is unavoidably contained in an amount of less than 0.05%, although this differs depending on whether the steel is a converter steel or an electric furnace steel.

However, since 0.05% or more is required to prevent cracking due to hydrogen embrittlement, the component range was set at 0.05 to 0.50%.

(k) Cr Cr is allowed to be added up to 0.30% in order to improve hardenability and temper softening resistance in the 0.3 to 0.6% C range. Adding more than 0.30% increases material cost and makes it difficult to achieve the intended purpose of the present invention.

(1) B: B is 0.0003 to improve hardenability and toughness
Loadings of ~0.0030% are acceptable.

Note that S as an impurity causes deterioration of toughness due to the formation of MnS, and also reduces the required Ca to prevent this.
S
The upper limit of 0.015% is desirable.

(Average particle size in steel) In the present invention, if the average particle size of spheroidized cementite precipitated in steel after cold rolling exceeds 0.8/Jl11, it becomes coarse particles and the toughness after heat treatment is improved. Therefore, the average grain size of spheroidized cementite in the steel after cold rolling is limited to 0.8p or less.

(Manufacturing Process) In the present invention, a high-toughness, high-carbon cold-rolled steel sheet is manufactured through the steps of hot rolling, controlled cooling, cold rolling, and annealing.

In that case, the hot rolling conditions are not particularly limited and may be carried out under conventional conditions, but the rolling end temperature must be ^r3 in order to sufficiently complete pearlite transformation by controlled cooling, which will be described later.
point [generally 9to-2o3. /i〒15.2(χN
i) +44.7(χSi) +31.5(!Mo)”
It is preferable to use a grade of 1 or higher than Cl.

(m) Cooling rate The cooling rate in the accelerated cooling zone is aimed at completing the pearlite transformation before winding, and the lower limit of the cooling rate is specified as 57 (:/sec) due to constraints on the sheet threading speed and the length of the cooling zone. Further, if the cooling rate is increased excessively, the temporary hardness increases and the cold rolling property is adversely affected, so the upper limit of the cooling rate is set to 50° C./sec.

(n) Limitation of intermediate temperature Even if the pearlite is passed through the accelerated cooling zone at the above-mentioned cooling rate, the morphology of pearlite will differ depending on the C and other alloy components of the steel.

Therefore, it is necessary to set an intermediate temperature corresponding to the amount of C, Si, and Mn.

The reason for setting the intermediate temperature (Tm) to be 650°C or lower and 550°C or higher is because the results obtained from a simulation using a rolling mill indicate that if the coiling is performed at a temperature higher than 650°C, the transformation will not be completed before winding. As the characteristic value fluctuation increases,
The average particle size of spheroidized cementite after cold rolling and annealing is 1 μm.
As a result, it takes a very long time to soak the austenite region during heat treatment of the product. In addition, if the sheet is wound at a temperature lower than 550° C., low-temperature transformation phases such as bainite appear during accelerated cooling, resulting in a hard hot-rolled sheet and deteriorating cold workability.

The specific configuration of the above-mentioned accelerated cooling zone and air cooling zone may be such that, for example, a cooling zone by water cooling is provided on the exit side of the hot finishing rolling mill to form the accelerated cooling zone, and the air cooling zone may be configured by a winding machine. .

(0) Preliminary annealing conditions A steel plate hot-rolled according to the limited intermediate temperature conditions shown in (n) increases its hardness to some extent in order to obtain uniformity of its structure. As this increases the rolling load during cold rolling, it is preferable to carry out preliminary annealing before cold rolling as necessary to reduce the load.The annealing conditions at this time include converting extremely fine cementite into coarse particles. The lower limit temperature is set at 500°C to ensure that the temperature is reduced. Further, in order to suppress excessive coarsening and coarse lamellar formation of cementite, it is preferable to carry out box annealing in which the upper limit temperature is in the temperature range of Acl+30°C for 1 to 24 hours.

(p) The lower limit of the cold rolling reduction rate is set at 3 in order to improve the workability of the product by promoting recrystallization and spheroidization in the subsequent finish annealing.
It was set to 0%. Further, the upper limit was set at 80% to prevent the occurrence of edge cracks.

(q) Final annealing conditions In order to obtain uniformity of structure in the product, the annealing conditions are such that the lower limit temperature is Ac, -70"C in order to coarsen the fine spheroidized cementite to some extent.

In addition, excessive coarsening of spheroidized cementite and coarse lamellae
It is preferable to perform box annealing in which the upper limit temperature is soaked for 1 hour or more in a temperature range of Acl+30° C. in order to suppress this.

Further, if soaking is carried out for over 24 hours, the cementite will become coarse and the soaking time in the austenite region will increase during heat treatment of the product, so preferably the soaking time in the annealing step is within 24 hours.

Note that the above cold rolling and finish annealing are performed at least once.
Do it twice. Under normal conditions, cold rolling and subsequent finish annealing only need to be performed once, but if the carbon content is large, it is usually performed once or twice until the hardness condition: H*a<90 is achieved. .

Through the above process, a high carbon cold rolled steel sheet in which spheroidized cementite with an average grain size of 0.8 μm or less is precipitated can be obtained.

(Example) Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto.

Example 1 In this example, in order to examine the correlation between hardness and hydrogen cracking properties in the high hardness range, the hardness properties and hydrogen cracking durability time of a high carbon cold rolled steel sheet obtained under the manufacturing conditions according to the present invention were measured. evaluated. For comparison, a sample other than the i+inu configuration of the present invention was used.

Note that each steel sample in the above table contained approximately 0.005% S.

After soaking the steel in A- above for 1 hour at 1200°C,
A 4 mm hot rolled sheet was prepared at a finishing temperature of 0''C. This was acceleratedly cooled to 580°C at a rate of 15°C/sec, and then air cooled and coiled at 520°C after completion of transformation. This hot rolled sheet was After pickling and descaling, the plate was soaked at 650°C for 6 hours to soften it, then cold rolled at a reduction rate of 70% to a thickness of 1.2 m+*, and then soaked at 690°C for 16 hours to form a spherical shape. Chemical annealing was performed.

The high carbon cold rolled steel sheet obtained in this way has a width of 51 and a length of 1
Punched out as a plate material of 00IIll and heated at 860°C for 15
Immediately after soaking, an austempering process was performed by holding the sample in a salt bath at 270 to 360°C.

Next, a depth of 0.3n++n was placed in the longitudinal center of this test piece.
, a slit with a radius of 0.1mm and a tip of 0.05+u+R is machined, and the bending angle is 20° as shown in Figure 1.
A hydrogen cracking resistance test was conducted by holding the sample in a dilute sulfuric acid aqueous solution of C, 0,02N. The correlation between the hardness of the test piece at this time and the time until hydrogen cracking occurs in the slit portion is shown graphically in FIG.

Steel outside the steel composition range of the present invention 1. It can be seen that J and K are inferior in both material hardness and cracking durability.

Example 2 In this example, the influence of hot rolling conditions on hydrogen cracking resistance in a high hardness region was considered.

Using steel types B and D of Example 1, the controlled cooling and spheroidizing annealing (finish annealing) conditions after hot rolling are shown in Table 2.
level (a = f), and the cementite average grain size after cold rolling and final annealing based on this and 10 m1n at 850 ° C.
After soaking, the temperature is 280°C for steel type B and 340'' for steel type
Austemper treatment was carried out by soaking at C for 40 ml.

The hardness at this time is for both steel types B and D! (49.
5-50.5 (Hv ~500-520).

Thereafter, hydrogen cracking resistance was evaluated using the same test method as in Example 1.

The results are shown in graphs in FIGS. 3 and 4 for steel types B and D, respectively.

As can be seen from Figures 3 and 4, when the cooling rate in accelerated cooling is less than 5°C/sec, or when the intermediate temperature is over 650'C, or when the cold If the cementite grains become coarse during the annealing treatment after rolling, the cementite remains in the steel even after the austempering treatment, and the hydrogen cracking resistance at the same hardness level is significantly deteriorated.

Table 2 Note: *2 Example 3 outside the scope of the present invention In this example, each test steel having the steel composition shown in Table 3 was hot rolled (slab soaking condition I2) under the manufacturing conditions shown in Table 4.
00"cxlh), controlled cooling, preliminary annealing, plate thickness 3.0m
Rolling ratio 60 from a hot-rolled sheet of m to a cold-rolled sheet with a thickness of 1.2 mm
The grain size and hardness of spheroidized cementite were evaluated for cold rolled steel sheets obtained by cold rolling % and spheroidizing annealing as final annealing.

The results are also summarized in Table 4.

Each of the test materials thus obtained was subjected to austempering treatment under the conditions shown in Table 5, and the test materials obtained were evaluated for hardness, presence of residual cementite, toughness, and hydrogen cracking resistance. I did it.

The results are summarized in Table 5.

Currently, the characteristics required in industry are: ■Hardness 49
．． 0 (H*C) or more, ■ No residual cementite, ■ Impact resistance value of 1 kgf-m or more, ■ Hydrogen cracking durability of 50 hours or more, and all of the materials according to the present invention are It's satisfying.

(Effects of the Invention) Since the present invention is configured as described above, a steel plate with excellent hydrogen cracking resistance and toughness in a high hardness range of H+tc: 45 or more can be obtained compared to conventional high carbon steel plates. Furthermore, the time required for quenching, tempering, or soaking in the austenitic region such as in austenite is shortened, resulting in high productivity as a product, and is extremely useful industrially.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram of a test piece of the steel plate of the present invention; Figure 2 is a graph showing the relationship between material strength and cracking durability time of the steel plate of the present invention; and Figures 3 and 4 are diagrams of the steel plate of the present invention. TI due to cracking durability and hydrogen cracking! It is a bar graph showing the time until disconnection.

Claims (7)

[Claims] (1) Weight percentage: C: 0.30-0.90%, Si: 0.05-0.70
%, Mn: 0.05-1.00%, P: 0.020% or less, Ti: 0.005-0.10%, Nb: 0.005
~0.100%, sol. Al: 0.08% or less, N:
A high-toughness, high-carbon cold-rolled steel sheet having a steel composition of 0.0020 to 0.010%, the balance being Fe and unavoidable impurities, and having fine spheroidized cementite with an average grain size of 0.8 μm or less in the steel. (2) In terms of weight ratio, Ca: 0.005 to 0.
The high toughness, high carbon cold rolled steel sheet according to claim 1, containing 0.020%. (3) In terms of weight ratio, further Cu: 0.05 to 0.5
The high-toughness, high-carbon cold-rolled steel sheet according to claim 1 or 2, containing 0%. (4) The high-toughness, high-carbon cold-rolled steel sheet according to any one of claims 1 to 3, further containing Cr: 0.30% or less in weight proportion. (5) In terms of weight ratio, B:0.0003 to 0.
The high toughness, high carbon cold rolled steel sheet according to any one of claims 1 to 4, containing 0.030%. (6) The steel having the steel composition according to any one of claims 1 to 5 is divided into an accelerated cooling zone on the finishing rolling mill side and an air cooling zone on the winding machine side on the hot run table after hot rolling. When cooling, the cooling rate in the accelerated cooling zone is set to 5 to 50 °C/sec, and the accelerated cooling stop temperature (intermediate temperature, Tm) is 650 °C or lower and 550 °C or higher, and then air cooling is performed. After the pearlite transformation is completed, the hot-rolled steel sheet is coiled and then pickled and descaled, followed by cold rolling at a rolling reduction of 30 to 80% and Ac_1-70°C.
Cooling box annealing in the temperature range of ~Ac_1+30℃ or 2
By performing this process more than once, the average grain size is 0.8 μm in the steel.
A method for producing a high-toughness, high-carbon cold-rolled steel sheet, characterized by precipitating the following fine spheroidized cementite. (7) The high-toughness, high-carbon cold-rolled steel sheet according to claim 6, wherein after pickling and descaling the hot-rolled steel sheet, box annealing is performed in a temperature range of 500°C to Ac_1+30°C prior to cold rolling. manufacturing method.