JPH04280943A - High strength cold rolled steel sheet excellent in deep drawability, chemical conversion treating property, secondary working brittleness resistance, and spot weldability and its production - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention provides a high-strength cold-rolled steel sheet with a tensile strength of over 40 Kgf/mm2, which is suitable for use in relatively severe processing applications such as automobiles, and a method for producing the same. This is what I am trying to propose. The purpose of increasing the tensile strength of automotive steel plates is to improve safety and reliability by increasing the strength of each component, and to reduce the weight of the vehicle by reducing the required plate thickness, thereby reducing fuel consumption. Therefore, in these days when there is a growing interest in preserving the global environment, this issue is attracting attention as an issue that should be greatly promoted.

0002

[Prior Art] Conventionally, methods for producing cold-rolled steel sheets with excellent formability, such as those disclosed in Japanese Unexamined Patent Publication No. 56-139654, have been based on ultra-low C steel with reduced C content. A method has been proposed and disclosed in which carbonitride-forming components such as Ti and Nb are added to improve aging properties, and further, components such as P that do not impair workability are added to increase strength. The limit of its strength was approximately 40 kgf/mm2 in terms of tensile strength. In addition, in order to further increase the strength of these materials, for example,
As in Publication No. 193221, a method for manufacturing steel with the addition of a large amount of Si has also been proposed and disclosed, but there are problems inherent to the addition of a large amount of Si, mainly surface treatment such as deterioration of chemical conversion properties and deterioration of plating properties. Due to the unavoidable problem of compatibility, the steel sheet could not withstand the intended use as a steel sheet for automobiles.

0003

[Problems to be Solved by the Invention] This invention aims to advantageously solve the above-mentioned problems.Among the high-strength steel plates used for automobiles, especially those with a tensile strength of 40 kg
f/mm2, and of course has high strength that satisfies all required properties such as processability, chemical conversion treatment properties, secondary work brittleness resistance, and spot weldability. The purpose of this paper is to propose cold-rolled steel sheets and their manufacturing methods.

0004

[Means for Solving the Problems] This invention advances the study to increase the tensile strength to 40 Kgf/mm2 or more based on ultra-low carbon steel, optimizes the amount of residual solid solution C, and improves the tensile strength of Ni and B.
By adding P, chemical conversion treatment properties, secondary work brittleness resistance, spot weldability, etc. are improved, and by adding a relatively large amount of P, the deterioration of surface properties due to Si, which is also added as a reinforcing component, is improved. This is based on the knowledge that it is possible to minimize the

■ As the first invention, C: 0.0005 wt% or more and 0.0050 wt% or less, Si: 0.30 wt% or more and 1.20 wt% or less, Mn: 0.05 wt% or more and 0.30 wt% or less,
Ni: 0.02 wt% or more 0.20 wt% or less, T
i: 0.0 as effective *Ti calculated by the following formula (1).
010 wt% or more 0.30 wt% or less, B: 0.0
003 wt% or more 0.0020 wt% or less, P: 0
．． 040 wt% or more 0.150 wt% or less, Al:
0.060 wt% or more 0.150 wt% or less, S:
Contains 0.010 wt% or less and N: 0.0040 wt% or less, and the remainder is composed of iron and unavoidable impurities.It has good deep drawability, chemical conversion treatment property, secondary work brittleness resistance, and spot weldability. Strength cold rolled steel plate.
Note Effective * Ti (wt%) = Ti (w
t%) -48/32 S(wt%) -48/14 N(
wt%) --------(1)

0006]■
As a second invention, C: 0.0005 wt% or more and 0.0050 wt% or less, Si: 0.30 wt% or more and 1.20 wt% or less, Mn: 0.05 wt% or more and 0.30 wt% or less,
Ni: 0.02 wt% or more 0.20 wt% or less, T
i: 0.0 as effective *Ti calculated by the following formula (1).
010 wt% or more and 0.30 wt% or less, Nb: 0.
002 wt% or more 0.005 wt% or less B: 0.0
003 wt% or more 0.0020 wt% or less, P: 0
．． 040 wt% or more 0.150 wt% or less, Al:
0.060 wt% or more 0.150 wt% or less, S:
Contains 0.010 wt% or less and N: 0.0040 wt% or less, and the remainder is composed of iron and unavoidable impurities.It has good deep drawability, chemical conversion treatment property, secondary work brittleness resistance, and spot weldability. Strength cold rolled steel plate.
Note Effective * Ti (wt%) = Ti (w
t%) -48/32 S(wt%) -48/14 N(
wt%) --------(1), and further,

[0007] ■ As a third invention, a steel slab prepared to have the above-mentioned composition (①), and as a fourth invention, a steel slab prepared to have the above-mentioned composition (①) were used as raw materials, and 1150
After starting hot rolling at a temperature range of 1280°C or higher and finishing finish rolling at a temperature of 750°C or higher and 950°C or lower, air cooling for 3 seconds or more and 10 seconds or less is followed by 30°C.
Rapidly cooled at a cooling rate of 350 °C or more at a cooling rate of 350 °C or more 600
It is rolled up at a predetermined temperature of ℃ or less, then cold rolled at a reduction rate of 65% or more, and after this cold rolling, heated to 650℃ at a heating rate of 20℃/second or more, and then heated to 650℃ at a heating rate of 20℃/second or more. Continuous annealing is performed by heating to a predetermined temperature of 750°C to 900°C at a heating rate of 2 seconds or more and soaking for 40 seconds or less, followed by rapid cooling to a temperature of 450°C at a cooling rate of 20°C/second or more. This is a method for producing a high-strength cold-rolled steel sheet with good deep drawability, chemical conversion treatment properties, secondary work brittleness resistance, and spot weldability. Here, in the first invention and the third invention, the Nb content includes cases where it is unavoidably contained, and its value is less than 0.002 wt%.

[0008]

[Operation] In order to improve workability, the present invention uses an ultra-low carbon steel with a C content of 0.0005 wt% to 0.0050 wt% as a base, and increases the strength by adding P and Si. After chemical conversion treatment, trace amounts of Ni and B are added to ensure spot weldability, and Ti
The aim is to keep the addition amount to a small level to ensure an appropriate amount of solid solution C, and to improve chemical conversion treatment properties and secondary processing brittleness resistance. Also,
In addition to the above, the amount of Al 2 added is increased to stabilize the fixation of solid solution N and to strengthen it by making the particles finer.

[0009] The reason for limiting the composition range of steel and manufacturing conditions will be described below. ■ Steel composition C: 0.0005wt% to 0.0050wt% A small amount of C is desirable from the viewpoint of improving elongation and r-value, but if the content is less than 0.0005wt%, secondary work embrittlement resistance and the strength of the welded zone (heat-affected zone).In addition, it is industrially disadvantageous in terms of cost to reduce the temperature to a lower level. On the other hand, if the content exceeds 0.0050 wt%, no improvement in material quality can be expected even if equivalent amounts of Ti and Nb are added, and there is a high risk of problems occurring in hot rolling and other intermediate manufacturing processes. Therefore, its content is set to 0.0005 wt% or more and 0.0050 wt% or less.

[0010]Si:0.30wt%~1.20wt%S
i is added as a reinforcing component. Basically, it can be adjusted according to the target tensile strength level, but if the content is less than 0.3 wt%, the deterioration in spot weldability caused by the addition of P cannot be compensated for. On the other hand, 1.
If it exceeds 20 wt%, the hot-rolled base plate will harden significantly, resulting in poor cold rollability, and chemical conversion treatment properties will also be markedly degraded, and various internal defects will also tend to increase, which is not preferable. Therefore, its content is 0.30
The content should be from wt% to 1.20wt%.

[0011] Mn: 0.05wt% to 0.30wt
%Mn has a solid solution strengthening function and is a component that is advantageous for hot brittleness, but the solid solution strengthening ability due to the addition of Mn is not so large, and in low S and Ti containing steels like this invention steel, hot Brittleness is unlikely. In addition, although the cause is not necessarily clear, especially when the amount of C added is high, the Mn content increases.
If it exceeds 0.30 wt%, the elongation and r value will deteriorate significantly. On the other hand, since there is no need to reduce it more than necessary, the lower limit is set at 0.05 wt%, which can be reduced without difficulty. Therefore, its content is 0.05 wt% or more 0
．． The content shall be 30 wt% or less.

[0012] Ni: 0.02wt% to 0.20wt%N
Although i has not been particularly added in the past because it is an expensive component, it has a particularly important meaning in this invention. That is, addition of Ni improves chemical conversion treatment properties. Although the detailed mechanism is unknown, in high Si-added steel as in the present invention, the content needs to be 0.20 wt% or more in order to improve chemical conversion treatability. Note that the addition of Ni is also effective in improving the resistance to secondary work brittleness. On the other hand, if the content exceeds 0.20 wt%, the effect tends to be saturated, which also leads to an increase in cost. Therefore, the content is set to 0.02 wt% or more and 0.20 wt% or less.

[0013] Ti: effective* calculated by the above formula (1)
0.010 wt% to 0.030 wt% Ti is an important component in this invention. It fixes some of the solid solution C and N in the steel and improves the material quality, especially the r value. Also,
In this invention, the interaction with Al, which is added in a larger amount than in conventional steels, improves chemical conversion treatment properties and secondary workability. The content as effective *Ti amount is 0.
If the amount is less than 0.010 wt%, improvements in chemical conversion treatment properties and secondary workability cannot be stably achieved. On the other hand, when the effective *Ti amount is 0.
If it exceeds 0.030 wt%, the deterioration of chemical conversion treatment properties will be most noticeable, and the strength of spot welded joints will also deteriorate, which is not preferable. Therefore, the effective *Ti amount is
0.010wt% or more and 0.030wt% or less. Although it is not particularly novel that the effective amount of Ti is determined by the relationship between S and N, Ti is greatly influenced by Al, P, and Si that are added at the same time. In addition to properties, chemical conversion treatment properties, spot weldability,
The effective amount of Ti was determined by taking into consideration the resistance to secondary processing brittleness and the ease of each manufacturing process condition.

[0014] Nb: In the second invention, 0.002 w
t% to 0.005 wt% Nb is disadvantageous in BH properties (bake hardenability), but is effective in improving spot weldability. Therefore, in the first invention, which emphasizes BH properties, no additive is used. However, it is unavoidably included. There is no problem with less than 0.002 wt%. On the other hand, the second method focuses on improving spot weldability.
In the invention, in order to obtain the effect, 0.002
Although it is necessary to contain at least 0.005wt%
If the content exceeds %, it causes deterioration in secondary processing brittleness and workability (especially r value). Therefore, the content in this case is 0.002 wt% or more and 0.005 wt% or less.

[0015] B: 0.0003wt% to 0.0020w
t%B is one of the important components in this invention. By containing B at 0.0003 wt% or more, good chemical conversion treatability, secondary work embrittlement resistance, and spot weldability can be obtained despite the steel containing high P and high Si content. However, if the content exceeds 0.0020 wt%, the in-plane anisotropy of the tensile properties after cold rolling and annealing increases, which is not desirable as a steel sheet for press working. Therefore, its content is 0.0003 wt% or more 0.0020 wt
% or less.

[0016] P: 0.040 wt% to 0.150 w
The addition of t%P has an extremely important meaning in this invention. In ordinary ultra-low C steel to which P is not added, the surface cleanliness is excellent, and on the contrary, chemical conversion treatment properties are deteriorated. By containing 0.040 wt% or more of P, the surface is activated and chemical conversion treatment properties are improved, and this effect is also effective when a component that causes deterioration of chemical conversion treatment properties, such as Si, is contained. It is. On the other hand, 0.150
If the content exceeds wt%, the deterioration of resistance to secondary work brittleness becomes significant, which is not preferable. Therefore, its content is set to 0.040 wt% or more and 0.150 wt% or less.

[0017] Al: 0.060 wt% to 0.150
wt%Al is also one of the important components for this invention. Although the detailed mechanism is unknown, by increasing the Si content to 0.060 wt% or more compared to conventional steel,
It acts effectively to suppress the deterioration of chemical conversion treatment properties caused by the addition of P and the 2-work brittleness caused by the addition of P. However, if the content exceeds 0.150 wt%, it is not preferable because it not only increases the cost of melting but also deteriorates the rollability during hot rolling. Therefore, the Al content is 0.060 w
t% or more and 0.150wt% or less.

S: 0.010 wt% or less S is a component to be removed in this invention. Reducing its content reduces precipitates in the steel, improving workability, and contributes to improving the yield of Ti, which fixes C. This processability improvement effect can be obtained by setting the content to 0.010 wt% or less. However, the content is desirably 0.005 wt% or less, which is advantageous for weldability.

N: 0.0040wt% or less In this invention, N is a component that should be removed like S, and by reducing it, the quality of the material can be expected to improve. It also contributes to improving yield. This material quality improvement effect can be achieved by keeping the amount below 0.004wt%.
A nearly satisfactory effect can be obtained.

[0020] Manufacturing conditions Slab temperature at the start of hot rolling: 1150°C to 1280°C In the hot rolling of the slab after continuous casting, it is desirable from the viewpoint of the material that the hot rolling start temperature is lower as long as a predetermined finishing temperature can be secured. , the equipment load during hot rolling increases. Therefore, assuming the current equipment, the lower limit of the hot rolling start temperature is 1150°C. On the other hand, if the hot rolling start temperature exceeds 1280°C, the surface quality of the steel sheet will eventually deteriorate significantly. therefore,
The upper limit is set to 1280°C.

[0021] Finish rolling temperature: 750°C to 950°C
The finish rolling temperature needs to be at least 750°C in order to improve the workability represented by the r value after cold rolling and annealing. If the hot rolled steel sheet is rolled at a temperature lower than this, the residual rolling structure will be noticeable in the hot rolled steel sheet, and a texture that is unfavorable to workability will eventually be formed, which is not preferable. On the other hand, if the finish rolling temperature exceeds 950° C., the damage to the hot rolling rolls will be large and will be a major hindrance to actual production. Therefore, the finish rolling temperature of hot rolling is set to 750°C or more and 950°C or less.

Cooling after finish rolling: After air cooling for 3 seconds to 10 seconds, quenching to a predetermined coil winding temperature at a cooling rate of 30°C/sec or more If quenching is performed immediately after finish rolling, re-cooling after hot rolling will occur. Since the crystals do not progress sufficiently, this ultimately results in increased non-uniformity and deterioration of the material. Even in a steel type in which recrystallization progresses relatively slowly, such as this invention steel, it is desirable to allow sufficient recrystallization to proceed.In order to obtain such a hot-rolled base plate, the air cooling time after finish rolling is 3 seconds. or more is required. On the other hand, if the air cooling time exceeds 10 seconds, Ti
Phosphides and the like are precipitated, and the strengthening effect and the effect of improving chemical conversion treatment properties due to the addition of P are not fully exhibited. Therefore, after finishing hot rolling, the hot rolled product is air cooled for 3 seconds or more and 10 seconds or less, and then rapidly cooled to a predetermined coiling temperature at a cooling rate of 30° C./second or more.

[0023] Winding temperature: 350°C to 600°C If the winding temperature is less than 350°C, the plate shape will be disturbed and the next process of pickling and cold rolling will be hindered. On the other hand, if the temperature exceeds 600° C., not only will phosphides of Ti be produced, leading to deterioration of the material, but also the deterioration of pickling properties will become significant as the scale thickness increases. Furthermore, various problems associated with surface concentration of Si and the like become apparent. Therefore, the winding temperature of hot rolling is set at 350°C or higher and 600°C or lower.

Cold rolling reduction: 65% or more In order to obtain sufficient deep drawability, the cold rolling reduction needs to be 65% or more. Note that it is preferably 75% or more. Heating rate during continuous annealing: 20℃ from room temperature to 650℃
/ seconds or more, from 650 °C to the specified annealing temperature at 5 °C / seconds.
The lower limit of the heating rate of seconds or more is defined from the viewpoint of workability. In the temperature range from room temperature to 650°C where recrystallization partially starts, rapid heating at 20°C/sec or more is required. If the heating rate is lower than this, the processability (particularly the r value) deteriorates significantly, although the cause is not necessarily clear. In the range from 650° C. to a predetermined annealing temperature, the strength decreases and the target high strength cannot be obtained unless heated at 5° C./sec or more. Therefore, the heating rate during continuous annealing is 20°C/second or more from room temperature to 650°C, and 5°C/second or more from 650°C to a predetermined annealing temperature.

Annealing temperature: 750°C to 950°C The annealing temperature requires a minimum of 750°C to complete recrystallization. On the other hand, if annealing is performed at an unnecessarily high temperature, T. S. The upper temperature limit is set at 950°C because not only will the target high strength not be obtained due to the decrease in the chemical conversion treatment, but also the surface concentration of the additive components will become significant, resulting in deterioration of various surface treatment properties after chemical conversion treatment. . Therefore, the annealing temperature is set to 750°C or more and 950°C or less.

Cooling rate: 450°C from predetermined annealing temperature
Cooling rate of 20℃/sec
Processability deteriorates. Therefore, the cooling rate from the predetermined annealing temperature to 450°C is set to 20°C/second or more.

[0027]

[Example] Example 1 An actual machine was manufactured using 23 steel types in total, including 7 compatible steel types of the present invention shown in Table 1 and 16 comparative steel types shown in Table 2, under manufacturing conditions compatible with the present invention shown in Table 3. Tables 4 and 5 show the results of the investigation of tensile properties, average r value, LDR, chemical conversion treatability, secondary work brittleness, spot weldability, etc. for each steel plate, respectively.

[0028]

[Table 1]

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

[0032]

[Table 5]

The measurement conditions for each of the above survey items are as follows. Tensile properties: Investigated using a JIS No. 5 test piece using a conventional method. Average r value: After applying a 15% tensile prestrain, it was measured using a three-point method, in the L direction (rolling direction), D direction (45° direction to the rolling direction), and C direction (90° direction to the rolling direction). ) was determined as average r value=(rL + 2rD + rC)/4.

[0034] LDR (limit drawing ratio): 33mm
The maximum drawing ratio without cracking or wrinkles was investigated under the conditions of a φ punch and anti-rust oil coating.

Chemical conversion treatment property: Using a commercially available treatment liquid,
After degreasing → washing with water → surface conditioning → immersion in treatment liquid → washing with water → drying, the film crystal size, film adhesion amount, and P ratio were investigated and comprehensively evaluated.

[0036] Secondary processing resistance: After flange-cutting a conical cup drawn at a drawing ratio of 2.0, a 5 kg weight was dropped from a height of 80 cm at various temperatures to apply an impact load to prevent brittle cracking. Evaluation was made based on the upper limit temperature at which . In practice, it is considered that there is no problem if this upper limit temperature is -45°C or lower.

Spot weldability: Welding was performed under the following conditions and evaluated by cross tensile strength. This strength is practically 400kg
It is thought that strength greater than f is required. Electrode tip: Cu-Cr alloy, CF tip diameter: 4.8 mmφ Current application time: 8 cycles Optimum value of pressurizing force: 150 kgf to 800 kgf Welding current: Optimum value of 4 KA to 8 KA Evidently from Table 4 (compatible example) and Table 5 (comparative example) As shown, the suitable examples exhibit excellent properties in terms of tensile properties, deep drawing formability, chemical conversion treatment properties, secondary work brittleness resistance, and spot weldability.

Example 2 Steel No. 1 shown in Table 1 above was used. Using the steel material No. 2, prototype production was performed on an actual machine under various manufacturing conditions shown in Table 6 (2 conditions for compatible examples, 13 conditions for comparative examples), and the tensile properties, average r value, and LDR of these prototype steel plates were determined. , chemical conversion treatment property,
Table 7 shows the results of investigating the secondary work brittleness under the same measurement conditions as in Example 1.

[0039]

[Table 6]

[0040]

[Table 7]

As is clear from Table 7, all the examples adapted to the present invention exhibit good characteristics. However, in comparative examples that deviate from the manufacturing conditions of this invention, some of the properties may be good, but some of the other properties may be deteriorated, and when trying to apply it to actual press-formed products. It becomes a big obstacle.

Example 3 Six steel types of the present invention shown in Table 8 were used to obtain the results shown in Table 3 above.
Table 9 shows the results of investigating the tensile properties, average r value, and BH properties of steel sheets manufactured under the manufacturing conditions of this invention shown in Table 9.
Shown below.

[0043]

[Table 8]

[0044]

[Table 9]

Here, the tensile properties and average r value were measured under the same conditions as in Example 1, and the BH properties were calculated using the following method. BH property: Stress at 2% tensile strain (σ2) and 2%
After applying a tensile prestrain of 170
The yield stress (σ
Y) was measured and calculated using the following formula. BH property = (σY) - (σ2)

[0046] Table 9 shows all conforming examples, but by adjusting the Nb content to 0.002wt% or less, 3.
It can be seen that a steel plate having BH properties exceeding kgf/mm2 can be manufactured. In addition, it has been confirmed that other properties of these products have not deteriorated at all. In this way, the ability to control BH properties by adjusting the extremely small amount of Nb can only be realized by applying the steel having the composition and manufacturing conditions of the present invention.

[0047]

[Effect of the invention] This invention is based on ultra-low C steel,
The purpose of this invention is to increase the tensile strength to 40 kgf/mm2 or more, and the first invention is to improve deep drawability by containing appropriate amounts of Si, Ni, B, P, Al, etc. as main components. , a high-strength cold-rolled steel sheet with good chemical conversion treatment properties, secondary workability, spot weldability, and BH properties, and the second invention is based on containing an appropriate amount of Nb in addition to the component composition of the first invention. The third invention is a high-strength cold-rolled steel sheet that has good deep drawability, chemical conversion treatment property, and secondary workability, and is also excellent in spot weldability. The fourth invention is to produce a high-strength cold-rolled steel sheet having each of the above characteristics by hot rolling, cold rolling, and annealing each steel material having the composition of the second invention under optimized conditions. The high-strength steel sheet obtained by the manufacturing method of the present invention is suitable for use in applications where relatively severe processing is performed, such as in automobiles.

Claims

[Claims] [Claim 1] C: 0.0005wt% or more 0.0
050 wt% or less, Si: 0.30 wt% or more 1.
20wt% or less, Mn: 0.05wt% or more 0.3
0 wt% or less, Ni: 0.02 wt% or more 0.20
wt% or less, Ti: 0.010 wt% or more as effective *Ti calculated by the following formula (1) 0.30 wt%
Below, B: 0.0003wt% or more 0.0020w
t% or less, P: 0.040 wt% or more 0.150w
t% or less, Al: 0.060 wt% or more 0.150
wt% or less, S: 0.010 wt% or less, and N: 0.0040 wt% or less, with the remainder consisting of iron and unavoidable impurities. Deep drawability, chemical conversion treatment resistance, secondary work brittleness, and spot resistance. High strength cold rolled steel plate with good weldability.
Note Effective * Ti (wt%) = Ti (w
t%) -48/32 S(wt%) -48/14 N(
wt%) --------(1) [Claim 2]
C: 0.0005 wt% or more and 0.0050 wt% or less, Si: 0.30 wt% or more and 1.20 wt% or less, Mn: 0.05 wt% or more and 0.30 wt% or less,
Ni: 0.02 wt% or more 0.20 wt% or less, T
i: 0.0 as effective *Ti calculated by the following formula (1).
010 wt% or more and 0.30 wt% or less, Nb: 0.
002 wt% or more 0.005 wt% or less B: 0.0
003 wt% or more 0.0020 wt% or less, P: 0
．． 040 wt% or more 0.150 wt% or less, Al:
0.060 wt% or more 0.150 wt% or less, S:
Contains 0.010 wt% or less and N: 0.0040 wt% or less, and the remainder is composed of iron and unavoidable impurities.It has good deep drawability, chemical conversion treatment property, secondary work brittleness resistance, and spot weldability. Strength cold rolled steel plate.
Note Effective * Ti (wt%) = Ti (w
t%) -48/32 S(wt%) -48/14 N(
wt%) --------(1) [Claim 3]
C: 0.0005wt% or more 0.0050wt
% or less, Si: 0.30wt% or more 1.20wt% or less, M
n: 0.05 wt% or more 0.30 wt% or less, Ni
: 0.02wt% or more and 0.20wt% or less, Ti:
Effective *Ti calculated using the following formula (1) is 0.01
0 wt% or more 0.30 wt% or less, B: 0.000
3 wt% or more 0.0020 wt% or less, P: 0.0
40 wt% or more and 0.150 wt% or less, Al: 0.
060 wt% or more and 0.150 wt% or less, S: 0.
Using a steel slab prepared to a composition containing 0.010 wt% or less and N: 0.0040 wt% or less, with the remainder being iron and unavoidable impurities, hot rolling is started in a temperature range of 1150 ° C. or higher and 1280 ° C. or lower. , 750℃ or higher 9
After finish rolling at a temperature range of 50°C or less, air cooling for 3 seconds or more and 10 seconds or less, followed by rapid cooling at a cooling rate of 30°C/second or more, and coiling at a predetermined temperature of 350°C or more and 600°C or less, Thereafter, it is cold rolled at a reduction rate of 65% or more, and after this cold rolling, it is heated to 650°C at a heating rate of 20°C/sec or more.
75 °C, followed by a heating rate of 5 °C/sec or more.
Deep drawing characterized by continuous annealing in which the material is heated to a predetermined temperature of 0°C to 900°C and soaked for 40 seconds or less, and then rapidly cooled to a temperature of 450°C at a cooling rate of 20°C/second or more. A method for producing a high-strength cold-rolled steel sheet with good properties such as hardness, chemical conversion treatment properties, secondary work brittleness resistance, and spot weldability.
Note Effective * Ti (wt%) =
Ti (wt%) -48/32 S (wt%) -48/1
4 N (wt%) ----------(1) [Claim 4] C: 0.0005wt% or more 0.0050
wt% or less, Si: 0.30wt% or more 1.20wt
% or less, Mn: 0.05wt% or more 0.30wt
% or less, Ni: 0.02 wt% or more 0.20 wt%
Hereinafter, Ti: 0.010 wt% or more and 0.30 wt% or less as effective *Ti calculated by the following formula (1), N
b: 0.002 wt% or more 0.005 wt% or lessB
: 0.0003 wt% or more and 0.0020 wt% or less, P: 0.040 wt% or more and 0.150 wt% or less, Al: 0.060 wt% or more and 0.150 wt% or less, S: 0.010 wt% or less, and Using a steel slab prepared to a composition containing N: 0.0040 wt% or less, with the remainder being iron and unavoidable impurities, hot rolling is started at a temperature range of 1150 °C or higher and 1280 °C or lower, and the temperature is 750 °C or higher.9
After finish rolling at a temperature range of 50°C or less, air cooling for 3 seconds or more and 10 seconds or less, followed by rapid cooling at a cooling rate of 30°C/second or more, and coiling at a predetermined temperature of 350°C or more and 600°C or less, Thereafter, it is cold rolled at a reduction rate of 65% or more, and after this cold rolling, it is heated to 650°C at a heating rate of 20°C/sec or more.
75 °C, followed by a heating rate of 5 °C/sec or more.
Deep drawing characterized by continuous annealing in which the material is heated to a predetermined temperature of 0°C to 900°C and soaked for 40 seconds or less, and then rapidly cooled to a temperature of 450°C at a cooling rate of 20°C/second or more. A method for producing a high-strength cold-rolled steel sheet with good properties such as hardness, chemical conversion treatment properties, secondary work brittleness resistance, and spot weldability.
Note Effective * Ti (wt%) =
Ti (wt%) -48/32 S (wt%) -48/1
4 N (wt%) --------(1)