JPH11323489A - High strength cold rolled steel sheet having superior workability and excellent in shape fixability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet excellent in shape fixability in bending and hat bending. SOLUTION: The surface layer in the range between the surface and a position at a depth of 5 to 35% of sheet thickness from the surface has an average composition in which C is contained in an amount higher by 0.05 to 0.5% by weight than the average C content in the central part of sheet thickness and the other chemical components are similar to those in the central part of sheet thickness and also has a metallic structure consisting of, by volume ratio, 10-35% retained austenite, <=2% martensite, and the balance ferrite or bainite. The inner layer in the range between a position at a depth of 5% of sheet thickness from the surface and the central part of sheet thickness has an average composition consisting of, by weight, 0.06-0.4% C, 0.5-4.0% of either or both of Si and Al, 0.5-2.0% Mn, and the balance Fe with inevitable impurities and also has a metallic structure consisting of, by volume, 3-20% retained austenite, <=2% martensite, and the balance ferrite or bainite.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-workability, high-strength cold-rolled steel sheet having excellent shape freezing properties and a method for producing the same.

[0002]

2. Description of the Related Art The status of automobiles as a convenient and comfortable means of transportation is increasing year by year, and it is necessary to reduce fuel consumption and fossil fuel consumption in order to prevent environmental destruction and global warming. It is more important than ever. For this reason, it is required to reduce the weight of the vehicle body while improving the engine performance, and it is required to further increase the strength of the steel sheet, which is a main constituent material of the vehicle body, without impairing the formability. In recent years, laws and regulations on collision safety in anticipation of car accidents have been rapidly expanding and strengthening.
Expectations for high-strength steel sheets are increasing. here,
Index values of formability include n-value and r-value, including elongation by a tensile test, but simplification of the pressing process by integral molding has become an issue in recent years. Among them, it is becoming important.

[0003] Therefore, utilization of transformation-induced plasticity of retained austenite has been proposed, and without using expensive alloying elements.
Only C of about 0.06 to 0.4%, Si of about 0.5 to 2.0%, and Mn of about 0.2 to 2.5% are used as basic alloying elements in a two-phase coexisting temperature range. 300-600 ° C after annealing
Japanese Patent Application Laid-Open No. 1-230715 discloses a steel sheet containing retained austenite in a metal structure by a heat treatment characterized by performing bainite transformation at a temperature of about the same. This is hereinafter referred to as residual γ hyten. As another component, a steel plate using Al instead of Si is disclosed in Japanese Patent Laid-Open No. 6-145788.
No. 6,086,045. Such a steel sheet is not only a cold-rolled steel sheet manufactured by continuous annealing, but also disclosed in JP-A-1-79345.
By controlling the cooling at the run-out table and the winding temperature as in the publication, hot rolled steel sheets can also be obtained.
In addition, the inventor has found that the excellent work hardening property leads to excellent absorption energy at the time of vehicle collision.
No. 8296. Thus, the residual γ hyten is
Extensive practical use is expected.

[0004] Such a high-strength steel sheet is considered to be used for members and the like among automobile components, and required forming characteristics include shape freezing during bending and hat bending. Material factors that affect this shape freezing property include yield strength and tensile strength,
It is shown in the "Press Forming Difficulty Handbook" (published by Nikkan Kogyo Shimbun) that generally the shape freezing property tends to decrease as the strength increases. Therefore, when a high-strength steel sheet is used, in order to press-mold a part that sufficiently satisfies the dimensional accuracy, the number of mold adjustments increases, which is disadvantageous in cost.

[0005] The knowledge to improve the shape freezing property of the high-strength steel sheet is disclosed in Japanese Patent Application Laid-Open Nos. 7-148527 and 7-185563 as a forming method. However, these findings are not desirable because the molding method is limited and the degree of freedom in design is reduced. As a steel sheet, there is a technique using a laminated steel sheet, which is disclosed in JP-A-62-259839 and JP-A-7-268484. However, these are not desirable because they increase the production cost of the steel sheet. Further, Japanese Patent Application Laid-Open No. 7-275938 discloses a finding that the shape freezing property is improved by using a multi-layer steel sheet having different inner layer strengths. However, this finding does not mention anything about a specific method for producing a multilayer steel sheet.
No mention is made of the shape freezing property, but a steel sheet in which the surface of the residual γ-hyten was strengthened by carburization was disclosed in
No. 92 is disclosed. However, it does not mention the amount of carburization required for reinforcement.

[0006] That is, an object of a high-workability high-strength steel sheet used for automobile parts is to improve the shape freezing property during bending or hat bending regardless of the forming method.

[0007]

SUMMARY OF THE INVENTION The present invention relates to a high-workability, high-strength cold-rolled steel sheet capable of overcoming the problem of shape freezing during bending and hat bending, regardless of the forming method, and a method for producing the same. , To provide.

[0008]

Means for Solving the Problems The present inventor studied a good workability high strength cold rolled steel sheet having excellent shape freezing properties. As a result, it was found that the C content of the surface inner layer of the steel sheet was appropriately controlled and heat treatment conditions were improved. By controlling the metal structure of the surface layer, the surface layer is strengthened more than the inner layer, and the stress state of the plate thickness section at the time of bending is changed, thereby improving the shape freezing property. Also, by taking appropriate heat treatment conditions,
It has been found that the steel sheet can be manufactured industrially stably.

That is, in bending, the material expands from the center of the thickness of the sheet to the outside, so that shrinkage stress occurs after release, and from the center of the thickness of the sheet, the material shrinks conversely, and the elongation stress occurs. A moment is generated so as to reduce the stress difference in the thickness direction, and an angle change occurs. This is springback. As a method for reducing the springback, tensile bending is well known. This is a method of applying tension to the steel sheet during bending, causing the material inside from the center of the sheet thickness to also elongate and deform, reducing the stress difference in the thickness direction after release and reducing springback. is there. At this time, when the applied tension is increased, the neutral plane, which is a plane where the distortion is 0, moves from the center of the sheet thickness to the inside of the bend, and eventually all of the steel sheet is elongated and deformed. By controlling the stress state of the plate thickness section in this way, the neutral surface is moved from the plate thickness center, and springback can be reduced.

Regarding the method of reducing the springback by moving the neutral surface of the strain from the center of the plate thickness regardless of the forming method, a difference in strength between the surface layer having large deformation and the inner layer in which the neutral surface moves is provided. It is mentioned.

This is explained as follows.

[0012] In the case of bending work for forming a component such as a member, it may be considered that the bent portion is substantially yield deformed, and the stress is substantially proportional to the yield strength of the steel sheet. When the strength of the inner surface layer is different, the stress at the time of bending is different between the outer layer and the inner layer. Here, the thickness variation is considered. That is, the sheet thickness increases due to compression deformation inside the bend, and decreases due to elongation deformation outside the bend. for that reason,
The compression part increases and the elongation part decreases. At this time, the neutral plane moves in consideration of the balance of stress, but moves inward when the surface layer is higher in strength than the inner layer, and moves outward when the surface layer is lower in strength than the inner layer. As described above, the neutral surface moves from the center of the plate thickness, and the springback is reduced.

This method was applied to residual gamma hyten. The strength of the residual γ-hyten is greatly affected by the C content in the steel. Therefore, the C content was changed in the inner layer to cause a difference in strength. The amount of C in the surface layer is as described in JP-A-8-120.
As disclosed in JP-A-341-341, it is possible to increase the amount by continuous casting and the like by adding a wire. In addition, since the content can be increased by carburization, the C content in the surface layer is set to be larger than that in the inner layer.

[0014] As a method for producing such residual γ-hyten, a steel plate obtained by hot rolling a slab having an increased surface C content by continuous casting or the like by adding a wire or a clad steel plate having a surface C content larger than that of the inner layer is used. Is subjected to recrystallization annealing in a two-phase coexisting temperature range after cold rolling, followed by rapid cooling and holding for an appropriate time in a bainite transformation region, and then cooling to room temperature, or there is no difference in the C content of the surface layer. After cold rolling, the hot-rolled sheet is recrystallized and annealed in a two-phase coexisting temperature range, and thereafter, at the same temperature as the recrystallization annealing, is kept in a carburizing atmosphere to be carburized to a predetermined C content. A method of rapidly cooling and maintaining the bainite transformation temperature range for an appropriate time and then cooling to room temperature can be applied.

The gist of the present invention is as follows: (1) The average composition of the surface layer from 5% to 35% of the plate thickness from the surface is as follows:
In terms of weight ratio, C: contains an amount of 0.05% or more and 0.5% or less higher than the average C amount in the center of the plate thickness, and other chemical components are the same as those in the center of the plate thickness. 10% to 35% by volume of retained austenite, 2% martensite
Hereinafter, the balance is ferrite or bainite, and the average composition of the inner layer from the inside of the surface thickness of 5% to the center of the thickness is C: 0.06% or more, 0.4% or less, and
3. 0.5% or more of at least one of i and Al;
0% or less, Mn: 0.5% or more and 2.0% or less,
Good workability and high strength excellent in shape freezing, consisting of the balance of Fe and unavoidable impurities, containing 3-20% by volume of retained austenite, 2% or less of martensite, and the balance of ferrite or bainite in the metal structure. The average composition of the cold-rolled steel sheet and the surface layer of 5% or more and 35% or less of the sheet thickness from the surface is 0.05% or more and 0.5% or less as the weight ratio of the average C content at the center of the sheet thickness. Contains only high C content,
The other chemical components are as follows: The cold-rolled steel sheet, which is the same as the central part of the sheet thickness, is recrystallized at a temperature of (Ac ₁ transformation point + 10 ° C.) or more and (Ar ₃ transformation point −5 ° C.) or less for 20 seconds or more. Anneal, 300 ° C to 600 ° C at a cooling rate of 3 ° C / s or more
A method for producing a high-strength cold-rolled steel sheet having good shape-freezing properties, characterized in that it is cooled to a temperature of 60 to 600 seconds and then cooled to room temperature; 3) The cold-rolled steel sheet is (Ac ₁ transformation point +1)
0 ° C.) and (Ar ₃ transformation point −5 ° C.) or less.
After recrystallization annealing for more than 2 seconds, at the same temperature, 5% of the sheet thickness from the surface by a carburizing atmosphere in a continuous annealing furnace.
The average composition of the surface layer of not less than 35% or less contains a C amount that is 0.05% or more and 0.5% or less higher than the average C amount at the center of the sheet thickness by weight, and the other chemical components are the sheet thickness. After carburizing so as to be the same as the central part, it is cooled at a cooling rate of 3 ° C./s or more from 300 ° C. to 600 ° C., kept at this temperature for 60 seconds to 600 seconds, and then cooled to room temperature. A method for producing a high-workability, high-strength cold-rolled steel sheet having excellent shape freezing properties.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the components and metal structure of the present invention are as follows.

C is an austenite stabilizing element, and 2
In the phase coexisting temperature range and the bainite transformation temperature range, it moves from ferrite and concentrates in austenite. as a result,
3 to 35% of chemically stabilized austenite remains after cooling to room temperature, and improves formability by transformation induced plasticity. If C is less than 0.06%, it is difficult to secure 3% or more of retained austenite, and if C is 0.4% or more, it is easy to secure retained austenite. Since bainite in which carbides of a target size are densely present is mainly used, the toughness is deteriorated and it is not practical.

Si and Al are essential elements for retaining austenite, and promote the formation of ferrite and suppress the formation of carbides, so that they have the effect of securing the retained austenite, and at the same time have the function of deoxidizing and strengthening elements. Also acts as. Therefore, the lower limit of the addition of at least one of Si and Al needs to be 0.5% or more. However, even if Si and Al are excessively added, the above-mentioned effect is saturated and the steel is rather embrittled. Therefore, the upper limit of the addition of at least one of Si and Al is set to 4.0%.

Mn has an effect of stabilizing austenite and securing retained austenite and is a strengthening element. From this viewpoint, the lower limit of Mn addition must be 0.5% or more. However, even if Mn is excessively added, the above effect is saturated, and adverse effects such as suppression of ferrite transformation are caused. Therefore, the upper limit of the amount of Mn added is set to 2.0% or less.

It is needless to say that it is desirable that no element other than those specified above be added in principle.
For a and REM, the sulfide-based inclusions are spheroidized to improve the hole expandability, so that the upper limit is 0.01%. N
One or two or more of b, Ti, Cr, Cu, Ni, V, and B may be added to secure the strength and reduce the grain size, but if the total amount exceeds 0.2%. Since it becomes difficult to obtain the metallographic structure of the present invention and the cost increases, the upper limit is acceptable up to 0.2%.

Although P is effective in securing retained austenite, it reduces toughness, so that it can be tolerated to 0.02% or less.

S is acceptable up to 0.01% or less due to a reduction in formability such as hole expandability due to sulfide-based inclusions.

Mo is an element that stabilizes retained austenite as well as Mn, and can be tolerated up to 0.3%. If it is more than that, carbides become apparent in the metallographic structure, causing deterioration of press formability.

The volume fraction of retained austenite in the inner layer is 3
The reason for setting it to 20% is that if the amount of retained austenite is less than the limit, the effect of deformation-induced plasticity cannot be sufficiently obtained. The upper limit is the limit of the amount of retained austenite obtained from the upper limit of the C content.

The C content of the surface layer is 0.05% of the average C content.
The reason why the content is set to 0.5% or less is that if the strength is less than the limit, a difference in strength required to cause the movement of the neutral plane during bending is not obtained. If the strength is more than the limit, the strength is too high.
This is because cracks occur during bending.

The reason for making the surface layer 5% or more and 35% or less of the plate thickness from the surface is that if it is less than the limit, a change in stress necessary for the movement of the neutral surface during bending cannot be obtained.
This is because the effect of multi-layering is reduced when the amount is more than the limit.

The volume fraction of retained austenite in the surface layer is 1
The reason for setting the amount to 0 to 35% is that if the amount of retained austenite is less than the limit, the strength with the inner layer becomes small, and a change in stress necessary for movement of the neutral surface during bending cannot be obtained. The upper limit is the limit of the amount of retained austenite obtained from the upper limit of the C content.

The reason why the volume fraction of martensite in the inner layer is set to 2% or less is that the workability is deteriorated when the amount of martensite is more than that.

The reasons for limiting the manufacturing process of the present invention are as follows.

First, the cold-rolled steel sheet is recrystallized and annealed in a temperature range where two phases of austenite and ferrite coexist. At this time,
Austenite stabilizing elements such as C and Mn are concentrated in austenite, facilitating stabilization of retained austenite by subsequent heat treatment. The reason why the recrystallization annealing temperature is not less than (Ac ₁ transformation point + 10 ° C.) and not more than (Ar ₃ transformation point −5 ° C.) is that if it is less than the limit, a sufficient amount of austenite will not be formed, and the dissolution of carbides will not occur. This is because the concentration of C in the austenite is not sufficient and the ferrite is not sufficient, and if it is more than the limit, only a very small amount of ferrite is present, and further, it is a single phase of austenite without any ferrite. This is because, as a whole, the distribution becomes thin, and the amount that can stabilize retained austenite is not concentrated.
The reason for setting the recrystallization annealing time to 20 seconds or more is that if the time is less than the limit, undissolved carbides may be present.

If the C content of the surface layer does not satisfy the condition of claim 1, after the recrystallization annealing is completed, carburizing is performed at a temperature equivalent to that of the recrystallization annealing. The carburizing method is not particularly limited,
Carburization in a carburizing atmosphere in a continuous annealing furnace is desirable in terms of productivity.

When the C content of the surface layer satisfies the condition of claim 1, the bainite transformation temperature range from 300 ° C. to 600 ° C. at a cooling rate of 3 ° C./s or more from the two-phase coexistence temperature range.
Cool to the temperature range. The cooling rate was limited to 3 ° C./s or more in order to cool the austenite formed in the two-phase coexisting temperature range to the bainite transformation temperature range without causing pearlite transformation. When cooling is completed at 600 ° C. or more, decomposition to pearlite occurs rapidly, and austenite cannot remain. Further, when cooling is completed at a temperature lower than 300 ° C., the majority of austenite is transformed into martensite, so that press formability is deteriorated.

Then, at 300 to 600 ° C., 60 to
Hold for 600 seconds and then cool to room temperature. The purpose of this is to further promote the enrichment of C in untransformed austenite at the time of bainite transformation to stabilize retained austenite. The reason why the holding time was limited to 60 seconds or more and 600 seconds or less was that the concentration of C in the untransformed austenite required to stabilize the retained austenite was insufficient below the limit. This is because bainite transformation proceeds and untransformed austenite disappears.

By satisfying the above conditions, a high-workability, high-strength steel sheet having excellent shape freezing properties can be realized.

[0035]

EXAMPLE A continuous cast slab having the composition shown in Table 1 was heated at about 1200 ° C., finish rolled at 910 ° C., cooled, and then rolled up at about 550 ° C. to obtain a hot rolled steel sheet having a thickness of 4 mm. Cold rolled. Here, steel N is a slab to which only the surface layer of C is added in a continuous casting mold, and steel O
Is a slab for a clad steel sheet manufactured by hot pressing. After the cold rolling, continuous annealing was performed. The samples required during continuous annealing were carburized. Then, after performing 0.8% skin pass rolling, the tensile properties were measured with a JIS No. 5 tensile test piece, and the product of the tensile strength and the total elongation of 20,000 or more was defined as good workability. The shape freezing property was evaluated by hat molding. Hat molding was performed using a mold as shown in FIG. The molding conditions are as follows.

Tool conditions (1) Punch side length: 100 mm, shoulder R: 5 mm (2) Die shoulder R: 5 mm (3) Clearance: 0.7 mm (4) Wrinkle holding force: 150 kN (5) Lubrication: rust prevention Oil (6) Molding height: 75 mm · Evaluation Width spread amount: ΔW = W-W ₀ W: punch side length 100 mm W ₀ : width at 65 mm from the bottom of the hat

[0037]

[Table 1] The measurement site is shown in FIG. From this test, the relationship between ΔW and the tensile strength of the steel sheet not having a multilayer structure was determined. If ΔW is equal to or less than 0.85 times ΔW of the non-multi-layered steel sheet at the same tensile strength, it is determined that the shape freezing property has been improved. FIG. 3 shows the relationship between the tensile strength and ΔW.

Tables 2 and 3 show annealing conditions, surface layer C content after carburization, surface layer thickness, austenite volume ratio (γ ratio) and martensite volume ratio (M ratio) of the inner layer, tensile strength, total elongation, shape freezing. The evaluation of sex is shown.

In Experiment No. 1, the workability was not good because the C content of the inner layer was less than the limit. In Experiment No. 2, workability was not good because the retained austenite was not stabilized because the Si content was less than the limit. Also, the difference in strength between the inner and outer layers was considered to be small, and the shape freezing property was not good.

Experiment Nos. 3 to 10 are experiments in which steel C was used and the surface layer thickness and C content were changed. Experiment number 4
Nos. 10 and 10 had poor shape freezing properties because the surface layer thickness was out of the limit. Experiment Nos. 5 to 9 all satisfied the conditions of the present invention, and thus a good workability high-strength cold-rolled steel sheet having excellent shape freezing properties was realized.

Experiment Nos. 11 to 17 were performed using steel C,
The effects of heat treatment conditions were studied. In Experiment No. 11, since the annealing temperature was higher than the limit, retained austenite did not remain, and workability was not good. Also, the difference in strength between the inner and outer layers was considered to be small, and the shape freezing property was not good. Experiment number 1
In Sample No. 2, since the annealing time was shorter than the limit and undissolved carbide remained, the concentration of C into austenite was not sufficient, so that no residual austenite remained and workability was poor. Also, the difference in strength between the inner and outer layers was considered to be small, and the shape freezing property was not good. In Experiment No. 13, the cooling rate after annealing was low, and pearlite transformation occurred during cooling, so that no retained austenite remained and workability was poor. Also, the difference in strength between the inner and outer layers was considered to be small, and the shape freezing property was not good. In Experiment Nos. 14 and 15, the influence of the retention temperature was examined. In Experiment No. 14, since the holding temperature was lower than the limit, the majority of the austenite was transformed into martensite, and the workability was not good. In Experiment No. 15, since the retention temperature was higher than the limit, austenite underwent pearlite transformation and no residual austenite remained, so that workability was poor. Experiment No. 16, 1
7 examined the effect of retention time. In Experiment No. 16, since the retention time was shorter than the limit, the concentration of elements sufficient for stabilization of retained austenite was insufficient, and martensite remained without much retained austenite, resulting in poor workability. Was. In Experiment No. 17, since the retention time was longer than the limit, much retained austenite did not remain, and the workability was poor.

In Experiment Nos. 18 to 20, the components were examined under the same C content. Experiment Nos. 18 and 19 all satisfied the conditions of the present invention, and thus a high-workability, high-strength cold-rolled steel sheet having excellent shape freezing properties was realized. In Experiment No. 20, since the Mn content was higher than the limit, the toughness was deteriorated.

Experiment Nos. 21 to 25 are experiments using steel G and changing the surface layer thickness and C content. Experiment number 2
In Nos. 1 and 25, since the surface layer thickness was out of the limit, the shape freezing property was not good. Since Experiment Nos. 22 to 24 satisfied all the conditions of the present invention, a good-workability high-strength cold-rolled steel sheet having excellent shape freezing properties was realized.

In Experiment Nos. 26 to 28, the C content was 0.2%.
In this case, the effect of steel composition was examined. Experiment No. 26 satisfies all the conditions of the present invention, so that a good workability high-strength cold-rolled steel sheet having excellent shape freezing properties was realized. Experiment number 2
8 is because the sum of the contents of Si and Al is more than the limit,
Workability was not good. In Experiment No. 27, since the Mn content was larger than the limit, the toughness was deteriorated.

Experiment Nos. 29 to 33 are experiments using steel K and steel L and changing the surface layer thickness and the C content.
In Experiment No. 29, since the surface layer thickness was below the limit, the shape freezing property was not good. Experiment Nos. 30 to 32 satisfy all the conditions of the present invention, and thus a good workability high-strength cold-rolled steel sheet having excellent shape freezing properties was realized. In Experiment No. 33, the shape freezing property was not good because the surface layer thickness was more than the limit. In addition, since the C content of the surface layer was large, the workability was also deteriorated.

In Experiment Nos. 34 and 35, the toughness was deteriorated because the content of the inner layer C was large.

Experiment Nos. 36 and 37 are experiments made from multilayer slabs, and all the conditions of the present invention were satisfied. Therefore, good workability and high strength cold-rolled steel sheets having excellent shape freezing properties were realized.

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

According to the present invention, it is possible to provide a high-workability, high-strength cold-rolled steel sheet having excellent shape freezing properties and used for automobile parts and the like, which is an industrially valuable invention.

[Brief description of the drawings]

FIG. 1 is a diagram showing a mold for evaluating the effect of the present invention.

FIG. 2 is a diagram showing a measurement site of a test piece for evaluating the effect of the present invention.

FIG. 3 is a diagram showing a relationship between tensile strength and ΔW.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Punch 2 Die 3 Test piece W width

Claims (3)

[Claims] An average composition of a surface layer having a thickness of 5% to 35% of the sheet thickness from the surface is 0.05% to 0.5% higher than the average C content at the center of the sheet thickness by weight. The other chemical components are the same as those in the central part of the thickness of the sheet, the retained austenite in the metal structure is 10 to 35% by volume, the martensite is 2% or less, and the balance is ferrite or bainite. The average composition of the inner layer from the inside of the plate thickness 5% to the center of the plate thickness is C: 0.06% or more by weight ratio,
0.4% or less, at least one of Si and Al is 0.5% or more, 4.0% or less, Mn: 0.5% or more,
2.0% or less, with the balance being Fe and unavoidable impurities.
Good workability, high-strength cold-rolled steel sheet excellent in shape freezing property, having a martensite content of 2% or less and a balance of ferrite or bainite. 2. The average composition of the surface layer of 5% or more and 35% or less of the sheet thickness from the surface is 0.05% or more and 0.5% or less higher than the average C amount at the center of the sheet thickness by weight. Containing quantity,
The other chemical components are as follows: The cold-rolled steel sheet, which is the same as the central part of the sheet thickness, is recrystallized at a temperature of (Ac ₁ transformation point + 10 ° C.) or more and (Ar ₃ transformation point −5 ° C.) or less for 20 seconds or more. Anneal, 300 ° C to 600 ° C at a cooling rate of 3 ° C / s or more
A method for producing a high-workability, high-strength cold-rolled steel sheet having excellent shape freezing properties, comprising cooling to a temperature of 60 to 600 seconds, and then cooling to room temperature. 3. The cold-rolled steel sheet is referred to as (Ac ₁ transformation point +
10 ° C. or more and (Ar ₃ transformation point −5 ° C.) or less
The recrystallization annealing is performed for 0 second or more, and then, at the same temperature, in a continuous annealing furnace, in a carburizing atmosphere, a thickness of 5 mm from the surface.
% Or more of 35% or less of the average composition of the surface layer contains 0.05% or more and 0.5% or less of the C content by weight in comparison with the average C content at the center of the sheet thickness. After carburizing so as to be the same as the thick center part, it is cooled from 300 ° C. to 600 ° C. at a cooling rate of 3 ° C./s or more, kept at this temperature for 60 seconds to 600 seconds, and then to room temperature. A method for producing a high-workability, high-strength cold-rolled steel sheet having excellent shape freezing properties, characterized by cooling.