JP2001064748A - High strength cold rolled steel sheet excellent in workability and weldability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel sheet for an automotive structural member having high elongation characteristics, good weldability, high strength in a high speed strain range, high energy absorptivity or the like. SOLUTION: As to the method for producing a high strength cold rolled steel sheet excellent in workability and weldability, a steel sheet obtd. by subjecting a slab contg., by mass, 0.04 to 0.14% C, 0.2 to 2.0% Si, 0.5 to 2.0% Mn,<=0.008% P, 0.0003 to 0.0050% B and 0.010 to 2.00% Al, and the balance Fe with inevitable impurities to hot rolling, thereafter coiling it at <=550 deg.C and subsequently executing cold rolling at a draft of <=70% is heated at the temp. of the Ac1 point to Ac3 point for >=1 sec, is thereafter cooled to 350 to 550 deg.C at a cooling rate of <=100 deg.C/sec and is held to the temp. for 2 to 30 min.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of high-strength cold-rolled steel sheets mainly used for structural members of automobiles, and in particular, has extremely excellent workability, particularly excellent elongation characteristics, and excellent It belongs to the technical field of high strength cold rolled steel sheet having weldability.

[0002]

2. Description of the Related Art Recently, steel sheets for structural members such as members that play a role of absorbing energy at the time of collision as frame members of automobiles have been developed from the viewpoint of safety improvement and to improve fuel efficiency as a measure against environmental problems. From the viewpoint of weight reduction, those having high strength are rapidly being used. On the other hand, an increase in strength leads to deterioration of workability and makes it difficult to press the part itself. Therefore, a steel sheet having both strength and workability is strongly desired.

[0003] In response to such demands, structurally strengthened steels utilizing hard phases such as martensite and bainite are widely used. These structurally strengthened steels also have excellent strength characteristics under high-speed deformation on the assumption of collisions, which have recently attracted attention, and have become the mainstream instead of conventional precipitation-strengthened steels. On the other hand, recently, by utilizing the work-induced transformation of retained austenite, a high-strength steel sheet that has excellent elongation and excellent strength properties under high-speed deformation, a so-called TRIP steel sheet, has been developed. It is disclosed in Japanese Unexamined Patent Publication No. Hei 1-230715 and Japanese Unexamined Patent Publication No. Hei 2-101117.

[0004]

The excellent elongation characteristics of these TRIP steels are almost correlated with the amount of unstable retained austenite left to room temperature. Increasing the amount greatly affects the processing characteristics of the steel sheet. It is valid. Therefore, in order to increase the amount of retained austenite, JP-A-1-230715, JP-A-2-101117, and the like, as a component, as a component, C, Si, it is possible to add a large amount of Mn, It is an effective means directly. However, considering the use as a steel sheet for a structural member of an automobile, a remarkable increase in C 2, Si, and Mn is not preferable because it lowers the weldability of the steel sheet. Also,
Steel sheets with a high content of these elements have high deformation resistance of the steel itself, making it difficult to roll.For example, in order to facilitate cold rolling, the winding temperature of a hot-rolled steel sheet is set relatively high. In addition, it is necessary to make it softer. Further TRIP
When a high Si content steel such as steel is wound at a high temperature of about 600 ° C. or more, for example, in a manufacturing method disclosed in Japanese Patent Application Laid-Open No. 6-145808, grain boundaries are oxidized, and Problems such as remarkably deteriorating the surface properties newly arise.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has a relatively low element content, high elongation characteristics, good weldability, and high strength in a high-speed strain range. The purpose of the present invention is to provide a high-strength cold-rolled steel sheet that has all the necessary properties as a steel sheet for use. Is to adjust the components and the structure before continuous annealing to an optimum state, and to appropriately control the subsequent continuous annealing conditions.

[0006]

The gist of the present invention is as follows.
C: 0.04-0.14%, Si: 0.2-2.0%, Mn: 0.5-2.0%, P:
0.008% or less, B: 0.0003-0.0050%, Al: 0.010-2.00%
High strength with excellent workability and weldability characterized by containing Fe and unavoidable impurities, with a residual austenite having an area ratio of 4 to 15%, and a composite structure of ferrite and bainite It is a cold rolled steel sheet.

[0007] In mass%, C: 0.04 to 0.14%, Si: 0.2 to 2.0
%, Mn: 0.5 to 2.0%, P: 0.008% or less, B: 0.0003 to 0.00
A steel slab containing 50%, Al: 0.010 to 2.00%, the remainder consisting of Fe and unavoidable impurities is rolled at a temperature of 550 ° C or less after hot rolling, and then cold rolled at a rolling reduction of 70% or less. After heating the rolled steel sheet to the temperature of Ac ₁ point to Ac ₃ point for 1 sec or more, 10
This is a method for producing a high-strength cold-rolled steel sheet excellent in workability and weldability as described above, wherein the steel sheet is cooled to a temperature of 350 to 500 ° C. at a cooling rate of 0 ° C./sec or less and kept at this temperature for 2 to 30 minutes.

[0008] A high-strength cold-rolled steel sheet which is controlled to 0.005% or less by mass S and contains 0.010% or less Ca and has excellent workability and weldability, and a method for producing the same.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reasons for limiting the range of components of steel to which the present invention is applied will be described.

[0010] In order to enrich C in austenite during continuous annealing after cold rolling and to generate retained austenite in an area ratio of 4% or more, a content of C is required to be 0.04% or more. However, an excessive content greatly impairs weldability and hinders practicality. Therefore, the upper limit of the content is set to 0.14%.
A more preferred C content is in the range of 0.08 to 0.12%.

[0011] Si is required to have a content of 0.2% or more in order to suppress the formation of carbides, promote the concentration of C in austenite, and generate stable retained austenite. However, when the content exceeds 2.0%, the susceptibility of the steel slab to cracking and oxidation of the surface grain boundaries is increased, and the cost is increased. Therefore, the Si content is in the range of 0.2 to 2.0%.

Mn is an element useful for stably obtaining a high strength and stabilizing the austenite structure. To obtain the effect, a content of 0.5% or more is required. However, when the content exceeds 2.0%, a band-like structure harmful to ductility tends to be formed, which causes a reduction in elongation.
Therefore, the Mn content is in the range of 0.5 to 2.0%.

[0013] Since P is an element that generates a crack defect in the nugget that causes a break in the nugget of the spot welded joint, the content of P is positively reduced in the present invention. To achieve its full effect,
Limited to 0.008%. More preferably, the P content is 0.
005% or less.

[0014] Al is used for deoxidation, and a content of 0.010% or more is required to obtain the effect. Also, Al
Is effective in suppressing the formation of carbides like Si and generating stable retained austenite. But 2.00%
If the content exceeds, the austenite is stabilized more than necessary, so-called transformation-inducing composition (TRIP effect) does not occur, and the elongation is reduced. Therefore, the Al content is 0.010
2.00%.

[0015] B is to improve the weldability, C,
Even when the amounts of Si and Mn are relatively reduced, at least 0.0003% is contained as a solid solution element in order to stably generate retained austenite. However, even if the content exceeds 0.0050%, the effect is saturated and the cost is simply increased. Therefore, the B content is in the range of 0.0003 to 0.0050%.

Further, in the present invention, S is regulated to 0.005% or less and Ca is contained to 0.010% or less as required for improving stretch flangeability. This aims at reducing the sulfide inclusions and controlling the form of the sulfide inclusions.

Next, the reasons for limiting the manufacturing process will be described. The high-strength cold-rolled steel sheet according to the present invention is obtained by hot rolling a steel slab containing the above-mentioned components according to a conventional method, and after rolling, 5
Wind at a temperature of 50 ° C or less. If the winding temperature exceeds 550 ° C, grain boundary oxidation is likely to occur on the surface of the hot-rolled steel sheet, and a beautiful surface cannot be obtained. In addition, by winding at a low temperature (550 ° C or less), the structure becomes a mixed structure mainly composed of ferrite and bainite (may contain some martensite), and C, which is a solid solution element at the stage of hot-rolled steel sheet, is hardened. By concentrating in the phase bainite, retained austenite can be efficiently generated during continuous annealing in a subsequent step. In order to more efficiently generate retained austenite during continuous annealing, the temperature at the time of winding is preferably 520 ° C. or less.
The hot-rolling end temperature is not particularly defined, but if it is lower than the Ar ₃ point, the rolling load becomes excessively large, so that a temperature lower than this is not preferable.

Further, after pickling, cold rolling is performed at a rolling reduction of 70% or less, and then continuous annealing is performed. Cold rolling exceeding the rolling reduction of 70% imposes an excessive load on the rolling equipment, so the rolling reduction is limited to 70%.

The soaking temperature of the continuous annealing is in a ferrite-austenite two-phase region where the temperature is higher than ₁ point of Ac and lower than ₃ points of Ac. If the soaking temperature is below the Ac ₁ point, austenitization does not occur, so that formation of retained austenite is no longer possible. Also, A
c If it exceeds ₃ points, the concentration of C in austenite during the cooling process after soaking is not sufficient, and it is difficult to stably generate retained austenite, leading to a decrease in elongation. The soaking time may be such that the bainite undergoes a reverse transformation, and there is no particular problem if it is 1 sec or more. However, long-time soaking is not preferable because not only the effect is saturated, but also the length of the annealing furnace is increased and equipment cost is increased.

The cooling following the soaking is effective for efficiently forming bainite and enriching C in austenite.
It may be cooled to a temperature range of 500 ° C at a cooling rate of 100 ° C / sec or less. Cooling at a cooling rate higher than this does not make any difference in the effect, but rather increases the equipment cost. Although there is no particular lower limit on the cooling rate, it is desirably 1 ° C./sec or more from the viewpoint of preventing pearlite generation.

In order to efficiently obtain a retained austenite structure, it is very important to efficiently transform austenite into bainite by maintaining the isothermal state for at least 2 minutes in a temperature range of 350 to 500 ° C. If the isothermal holding temperature exceeds 500 ° C., bainite or pearlite transformation occurs in subsequent cooling, and it is no longer possible to expect formation of sufficient retained austenite. On the other hand, if the isothermal holding temperature is less than 350 ° C,
Pearlite transformation occurs, or martensitic transformation occurs depending on the component system, and similarly, formation of retained austenite cannot be expected. Therefore, the isothermal holding temperature is 350
The temperature range is ~ 500 ° C.

If the holding time is less than 2 minutes, the concentration of C in austenite is not sufficient, and improvement in elongation cannot be expected. Only in the case of the above component ranges and manufacturing conditions, a structure having a sufficient amount of retained austenite phase to satisfy desired properties by continuous annealing can be obtained by hot rolling, with ferrite bainite as a main structure, The amount of retained austenite is specified to be at least 4% in area ratio. Below this, it is impossible to secure high elongation characteristics.
The upper limit of the amount of retained austenite is set to 15% in area ratio, but the elongation reaches a saturated state with this amount. In order to obtain more retained austenite, the maximum content is consequently approached even within the composition range of the present invention, and excellent weldability can no longer be maintained. The high-strength cold-rolled steel sheet according to the present invention does not impair workability and weldability even when various types of electroplating are applied to the surface. The amount of retained austenite was determined by quantitative analysis using an X-ray diffraction method. Further, the upper limit of the holding time is not more than 30 min because not only the effect is saturated but also the bainite transformation progresses so much that a predetermined structure cannot be obtained.

Now, the present invention will be described in further detail with reference to Examples.

[0024]

Example 1 A slab having the chemical composition shown in Table 1 was heated to 1150 ° C, hot-rolled to a thickness of 2.4 mm at a finishing temperature of 850 to 880 ° C, and then at an average cooling rate of 40 ° C / sec. It was cooled and wound up at 450 ° C. each. After pickling, they are cold-rolled to a thickness of 1.4 mm, annealed at 830 ° C for 60 seconds, and then annealed at 15 ° C.
Cooled to 440 ° C at an average cooling rate of / sec. Subsequently, the sample was kept at this temperature (440 ° C.) for 5 minutes, cooled to 230 ° C. at an average cooling rate of 1 ° C./sec, and then continuously annealed at an average cooling rate of 10 ° C./sec to room temperature.

After subjecting the cold-rolled steel sheet manufactured as described above to temper rolling at an elongation of 0.2%, a test piece was taken, and the amount of retained austenite, mechanical properties, spot weldability, and high-speed strain range. The strength characteristics were investigated. Table 2 shows the results.
Shown in The amount of retained austenite was measured by the X-ray diffraction method using a rolled surface of a cold-rolled steel plate as a test piece as a measurement surface.

The spot weldability is determined by the combination of the same materials.
Conduct 1000 continuous RBIs, and then from 1001 RBI
Using 10 samples of 1010 hitting materials, the frequency of occurrence of cracks or blowhole defects in the nugget was evaluated. The evaluation は is a defect generation rate of 10% or less, the evaluation △ is 10 to 80%, and the evaluation X is 80% or more. The spot welding conditions are as follows.

Electrode: Dome radius type tip diameter 6mm Pressing force: 4.9kN (500kgf) Energizing time: 17 cycles Current value: 10.0kA

The strength characteristics in the high-speed strain range are as follows:
It was evaluated by the ratio between the tensile strength at ^{0 s -1} and the static tensile strength.
The high-speed tensile test was performed by using an electrohydraulic servo-type high-speed tensile tester, and the load was detected by a strain gauge attached to the front and back surfaces of a grip portion of the test piece. The specimen shape is parallel part length: 7.5mm,
Parallel part width: 3.5 mm.

As shown in Table 2, Examples 1 and 2 of the present invention
3, 4, and 5 are high strength and good strength-ductility balance (TS × El ≧ 2300) when the amount of retained austenite is in the range of 5 to 10%.
0 MPa ·%), and both the spot weldability and the strength characteristics in the high-speed strain range are good. In contrast, Comparative Example No.
In 6, 7, 8, 9, and 10, since the chemical component range is out of the limited range of the present invention, any one of the strength-ductility balance, spot weldability, and strength characteristics in the high-speed strain range is not improved. It is significantly inferior to the example.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

Example 2 A slab having the chemical composition of No. 5 shown in Table 1 was heated to 1200 ° C., hot rolled to a thickness of 2.4 mm at a finishing temperature of 900 ° C., and then average cooled at 60 ° C./sec. Cool at a speed,
The film was wound at the winding temperature shown in Table 3. After pickling, these were cold-rolled to a thickness of 1.4 mm, annealed at 800 ° C. for 60 seconds, and then cooled to an isothermal holding temperature shown in Table 3 at an average cooling rate shown in Table 3 and shown in Table 3. Isothermal holding was performed under isothermal holding conditions. Then, it was cooled to room temperature at an average cooling rate of 10 ° C./sec. Table 3 shows the individual winding temperatures, cooling rates after annealing, and isothermal holding conditions.

After the cold-rolled steel sheet thus produced was subjected to temper rolling at an elongation of 0.2%, test pieces were sampled and examined for the amount of retained austenite and mechanical properties. The results are also shown in Table 3.

As shown in Table 3, Example No. 11 of the present invention had a retained austenite content of 8.2% and exhibited high strength and good strength.
It shows the ductility balance (TS × El ≧ 23000 MPa ·%).
On the other hand, in Comparative Examples Nos. 12, 13, 14, 15, and 16, the manufacturing conditions were out of the limited range of the present invention, so that the strength-
The ductility balance is remarkably inferior to the examples of the present invention. Furthermore, it should be noted that in Comparative Example No. 12, since the winding temperature in hot rolling was as high as 700 ° C., grain boundary oxidation occurred and the surface properties of the final cold-rolled steel sheet were poor.

[0035]

[Table 3]

As described above, according to the present invention, a high-strength cold-rolled steel sheet having extremely excellent elongation characteristics and excellent weldability and suitable as a structural member for automobiles can be easily obtained.

[0037]

As is apparent from the above description,
According to the present invention, in order to stably and efficiently generate retained austenite with a small element content in consideration of weldability, it has high elongation characteristics, good weldability, high strength in a high-speed strain region, and the like. A high-strength cold-rolled steel sheet excellent in workability and weldability suitable as a steel sheet for automobile structural members can be obtained.

Continued on the front page F term (reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA15 EA23 EA25 EA27 EA28 EB05 EB09 EB11 FA02 FC04 FC07 FD04 FE01 FG01 FH01 FJ05 FK03 FL01 FL02 FL05 FM02 JA06

Claims (3)

[Claims] C .: 0.04 to 0.14% by mass, Si: 0.2 to 2.% by mass%.
0%, Mn: 0.5-2.0%, P: 0.008% or less, B: 0.0003-0.0
Processing characterized by containing 050%, Al: 0.010 ~ 2.00%, balance being Fe and unavoidable impurities, and consisting of a composite structure of ferrite and bainite with retained austenite at an area ratio of 4 ~ 15% High strength cold rolled steel sheet with excellent weldability and weldability. 2. C .: 0.04 to 0.14% by mass, Si: 0.2 to 2.% by mass.
0%, Mn: 0.5-2.0%, P: 0.008% or less, B: 0.0003-0.0
A steel slab containing 050%, Al: 0.010-2.00%, the remainder consisting of Fe and unavoidable impurities is rolled at a temperature of 550 ° C or less after hot rolling, and then cold rolled at a rolling reduction of 70% or less. After heating the rolled steel sheet to the temperature of Ac ₁ point to Ac ₃ point for 1 sec or more,
The method for producing a high-strength cold-rolled steel sheet according to claim 1, wherein the steel sheet is cooled to a temperature of 350 to 500 ° C at a cooling rate of 100 ° C / sec or less and maintained at this temperature for 2 minutes to 30 minutes. . 3. The high-strength cold-rolled steel sheet having excellent workability and weldability according to claim 1, wherein the S content is specified as 0.005% or less in mass% and the Ca content is 0.010% or less. 2. A method for producing a high-strength cold-rolled steel sheet having excellent workability and weldability according to item 1.