JPH0774412B2 - High-strength thin steel sheet excellent in workability and resistance to placement cracking and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention has a tensile strength of 100 to 150 kgf / mm ² , and is excellent in workability and resistance to misplacement cracking, and its production. It is about the method.

(Prior Art) In recent years, in the automobile industry, high-strength thin steel sheets have been used because of cost increase of a vehicle body for improving fuel efficiency or safety of passengers at the time of collision. Especially, from the viewpoint of safety at the time of collision, steel sheets having a tensile strength of 100 kgf / mm ² or more, which is extremely high and unprecedented, have begun to be used. Steel sheets for automobiles do not simply need to have high strength, but workability and weldability are required depending on the application. Especially, for steel sheets with a tensile strength of 100 kgf / mm ² or more, bendability is among the workability. , Drawability and resistance to cracking are required.

Generally, the workability of a high-strength thin steel sheet is arranged in a balance between strength and elongation, and a steel sheet having high strength and good elongation is said to be excellent as a high-strength thin steel sheet. However, the high strength thin steel sheet for automobiles is often processed by bending, and it can be said that a steel sheet having excellent bendability is excellent as a high strength thin steel sheet. Further, drawing processing is often performed, and its importance is increasing. Furthermore, resistance to crack placement after drawing has been regarded as a problem.

On the other hand, regarding the high-strength thin steel sheet having a tensile strength of 100 kgf / mm ² or more, conventionally, a method by water cooling as described in JP-A-58-22327, or an alloy when the cooling rate is slow such as box annealing is used. Those manufactured by a method of increasing the amount of addition to increase the strength are used. With the water cooling method,
Since the cooling rate is too fast, the amount of strain accumulated in the steel sheet during the cooling process is large, and excellent drawability cannot be obtained.In the case of box annealing, where the cooling rate is extremely slow, 100 kgf / In order to obtain a tensile strength of mm ² or more, it is necessary to add a large amount of alloying elements, which impairs weldability and becomes economically expensive. There was no high-strength thin steel sheet having a tensile strength of 100 kgf / mm ² or more that satisfies the workability and the resistance to cracking.

(Problems to be Solved by the Invention) The present invention improves the above-mentioned drawbacks, has a tensile strength of 100 to 150 kgf / mm ² or more, and has a high strength excellent in drawability and anti-deposition cracking resistance. A thin steel sheet and a method for manufacturing the same are provided.

(Means for Solving Problems) The gist of the present invention is as follows.

(1) C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5 to 2.6
%, The balance Fe and unavoidable impurities, and the residual austenite content is 5% or less 100 to 1
A high-strength thin steel sheet with a tensile strength of 50 kgf / mm ² and excellent workability and resistance to placement cracking.

(2) C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5 to 2.6
%, In addition to Ti: 0.01 to 0.25% and Nb: 0.01 to 0.30%, one or two, and the balance Fe and inevitable impurities, and the residual austenite content is 5% or less. A high-strength thin steel sheet with a tensile strength of 100 to 150 kgf / mm ² and excellent workability and resistance to placement cracking.

(3) C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5 to 2.6
%, Ti: 0.01 to 0.25%, Nb: 0.01 to 0.30%, one or two kinds and B: 0.0003 to 0.01%, and the balance Fe and unavoidable impurities and the residual austenite content is 5%. A high-strength thin steel sheet having a tensile strength of 100 to 150 kgf / mm ² and being excellent in workability and anti-deposition cracking, which is characterized below.

(4) C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5 to 2.6
%, The steel and the balance Fe and unavoidable impurities, hot rolling at A ₃ transformation point or more finishing temperature, subjected to a rolling cold rolling ratio 30% to 70% of the cold, in the annealing step, the 750 to 900 ° C. After keeping in the temperature range for 1 second to 5 minutes, the average cooling rate is 100 to 500 ℃ / s
Cool to 100-500 ° C with ec, then 1-20 at 200-500 ° C
Hold for 5 minutes and then cool to reduce the amount of retained austenite to 5%.
A method for producing a high-strength thin steel sheet having a tensile strength of 100 to 150 kgf / mm ² and being excellent in workability and resistance to placement cracking, which is characterized as follows.

(5) C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5 to 2.6
% In addition to Ti: 0.01 to 0.25% and Nb: 0.01 to 0.30% of one or and B: containing 0.0003 to 0.01% consisting ,, the balance Fe and unavoidable impurities Steel, A ₃ transformation point Hot rolling at the above finishing temperature, cold rolling with a cold rolling rate of 30 to 70%,
In the annealing process, after maintaining the temperature range of 750-900 ℃ for 1 second-5 minutes, the average cooling rate is 100-500 ℃ / sec.
Workability having a tensile strength of 100 to 150 kgf / mm ² , characterized in that the retained austenite amount is 5% or less by cooling to 200 ° C, then holding at 200 to 500 ° C for 1 to 20 minutes, and then cooling. And a method for producing a high-strength thin steel sheet having excellent resistance to misplacement cracking.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have excellent drawing workability and resistance to placement cracking.
As a result of diligent studies on high-strength thin steel sheets having a tensile strength of 0 to 150 kgf / mm ² , a steel strip was formed by ordinary hot rolling, and after descaling, ordinary cold rolling was performed in the high temperature range during the annealing process. It has been found that the high strength thin steel sheet excellent in drawing work and anti-deposition cracking can be obtained by reducing the concentration of C and reducing the amount remaining as austenite to 5% or less by performing the annealing of. .

Fig. 1 shows C: 0.13%, Si: 0.5%, Mn: 2.1%, Ti: 0.04%, B: 0.0
Steel containing 010% is smelted and hot-rolled by a conventional method at a finishing temperature of 850 ° C and a cooling rate from finishing to winding of 50 ° C / sec.
As a hot-rolled steel sheet with a thickness of 2 mm, it was wound at a winding temperature of 630 ° C and cold-rolled at a cold rolling rate of 40% according to the usual method.
After forming a 1.2 mm steel plate, it is continuously annealed and held at a temperature of 750 ° C. and 900 ° C. for 40 seconds, then cooled to 300 ° C. at a cooling rate of 200 ° C./sec, and held at 350 ° C. for 3 minutes and then cooled. It is the figure which investigated the relationship between the retained austenite amount of the steel sheet manufactured by changing various conditions on the basis of time, and the drawability and the cracking resistance. Here, the amount of retained austenite is (200) of γ at a position of 1/4 in the plate thickness direction of the steel plate by X-ray diffraction,
It is obtained by measuring the three surfaces (220) and (311) and comparing them with a standard sample. In addition, the drawability is 60
A sample as punched into a mm disk was drawn with a drawing ratio of 2.25, and it was determined whether or not it could be processed without cracking. Further, as for the placement cracking property, the sample after the drawing process for which the above-mentioned drawing processability was required was immersed in ethyl alcohol, and the length (mm) of the crack per sample that was broken by 7 days was determined. The drawability and resistance to placement cracking of high strength thin steel sheets are improved when the amount of retained austenite is 5% or less. It is considered that this is because the retained austenite undergoes work-induced transformation in martensite due to processing, resulting in volume expansion, increase in strain, deterioration due to hydrogen intrusion after processing, etc. Conceivable.

Conventionally, increasing retained austenite is said to improve workability. The residual austenite amount is about 2 to 3% in a steel plate having a tensile strength of about 60 kgf / mm ² , and the carbon equivalent increases as the tensile strength increases. Therefore, the residual amount is large, and the retained austenite has a tensile strength of about 100 kgf / mm ^2.
It is about 10%. It has been said that if the amount of retained austenite is large, the elongation is improved and the workability is improved. However, due to the difference in the processing method, the inventors of the present invention have excellent drawing workability and resistance to delamination cracking as shown in FIG. I found that I can not say that it is present. That is, contrary to the conventional knowledge, when the amount of retained austenite is large, the drawability and the resistance to delamination cracking tend to deteriorate. The challenge is how to reduce the amount of austenite. In order to reduce the amount of retained austenite, it is important to suppress the enrichment of C to γ in the α + γ two-phase region during heat treatment. As a result, it is possible to suppress the concentration of C in γ by leaving C in the α phase and suppressing the concentration of γ in γ, and annealing in the high temperature region of the γ single phase region and α + γ two-phase region as the heat treatment temperature. is important. Furthermore, the cooling rate is high because the time spent in the α + γ two-phase region is shortened, and it is effective to adjust the strength by tempering after cooling and promote the reduction of retained austenite by bainite transformation. is there.

As described above, by reducing the retained austenite, the drawability and the cracking resistance of the high-strength thin steel sheet are improved, and the rapid cooling from high-temperature annealing increases the tensile strength and strengthens C, Si, Mn and other strengthening elements. Can be reduced, which is not only effective in improving weldability but also economically effective.

In the present invention, the reasons for limiting the components as described above are as follows.

C is an element necessary to obtain strength by utilizing precipitation strengthening and transformation strengthening. If the content of C is less than 0.03%, precipitation strengthening and transformation strengthening cannot be sufficiently utilized and desired tensile strength cannot be obtained. The lower limit is 0.03%. Also, 0.20%
If it is contained in excess of 10%, C segregates and the concentration of C in γ in the α + γ two-phase region occurs during heat treatment, the retained austenite increases, and the drawability and anti-deposition cracking resistance deteriorate. Therefore, the upper limit is 0.20%.

Si has a small concentration of C in the α phase, inevitably causes concentration in the γ phase, and increases the amount of retained austenite, so it is desirable to reduce it. Uniformity of the structure is obtained, and the lower limit is 0.15%, and if it is less than that, the effect is small and the lower limit is 0.15%. The upper limit is 1.5
%, The generation of scale is significant in the hot rolling process and the surface quality of the steel sheet deteriorates, so its upper limit is 1.5%.
%.

Mn is an important element for obtaining strength by utilizing transformation strengthening, and if its content is less than 0.5%, the amount of martensite produced is small and the desired tensile strength cannot be obtained, so its lower limit is set to 0.5%. To do. If it exceeds 2.6%, not only the weldability is significantly impaired, but also the uniformity of the structure, which is a feature of the present invention, is impaired and the bending characteristics are deteriorated, so the upper limit is made 2.6%.

Ti and Nb are elements necessary for obtaining strength by utilizing precipitation strengthening. Further, it is an element that is indispensable for effectively using B, and is an element that further improves weldability. B is an element that easily binds to N and forms a solid solution N in steel.
If present, B precipitates as BN and B cannot be effectively used. In order to utilize B effectively, Ti and Nb have the ability to be fixed as a nitride, so they are important together with obtaining strength. If the content of Ti is less than 0.01%, the fixing of N and the improvement of weldability are insufficient, so the lower limit is made 0.01%, and if it is contained in excess of 0.25%, the effect is saturated. The upper limit for this is 0.25%. The content of Nb is 0, similar to Ti.
If it is less than 01%, the fixing of N and the improvement of weldability are insufficient, so the lower limit is made 0.01%, and even if it exceeds 0.30%, the effect is saturated, so the upper limit is made 0.30%. .

B is a quenching strengthening element by quenching in the annealing process, and if its content is less than 0.0003%, its effect is lost, so its lower limit is set to 0.0003%, and if it exceeds 0.01%, it is used in the hot rolling process. Defects are likely to occur and the surface properties of the steel sheet are significantly impaired, so the upper limit is made 0.01%.

Next, the conditions of the manufacturing process of the present invention will be described.

To a finish temperature of hot rolling step A ₃ transformation point or above retains distortion rolling to less can not be homogeneous tissue, becomes lamellar structure, exist thickened portion and C, This is because it also affects the concentration of C after cold rolling and annealing and the amount of retained austenite increases.

The cooling rate from finishing to winding in the hot rolling process should be as fast as possible to make the structure uniform, preferably 50 ° C / sec.
After the above, it is wound, but the winding temperature is not limited. However, by lowering the coiling temperature, the structure is made uniform, and the subsequent continuous annealing makes it difficult for the concentration of C to occur, so that the retained austenite can be reduced. desirable.

When heat treatment is performed before cold rolling, the heat treatment before cold rolling is set to 650 ° C. or higher in order to soften the steel sheet before cold rolling and facilitate cold rolling.

The cold rolling rate is set to 30 to 70% in order to make the final sheet thickness thinner, and the lower limit is set to 30% when the final sheet thickness is made thin because of the ability of the hot rolling mill. This is because it exceeds the limit that can reduce the thickness. Further, the upper limit is set to 70% because even if the steel sheet is softened by high-temperature winding or heat treatment, it exceeds the limit of reducing the thickness of the steel sheet due to the capability of the cold rolling mill.

The reason for setting the annealing temperature range to 800 ° C to 900 ° C is that at a low temperature of less than 800 ° C, C is concentrated in the γ phase in the α + γ two-phase region and residual austenite is likely to remain. This is because it is possible to suppress the concentration of C and reduce the amount of retained austenite, thereby improving drawability and resistance to misplacement cracking. Further, since it becomes difficult to pass the strip in the continuous annealing step at a temperature exceeding 900 ° C, the upper limit is set to 900 ° C.

If the cooling rate is less than 100 ° C / sec, not only the effect of B cannot be effectively utilized, but also martensitic transformation due to rapid cooling cannot be performed, and the alloy addition amount must be increased to obtain the desired tensile strength. In particular, when C is increased, the retained austenite is increased and the drawability and the resistance to placement cracking are deteriorated, so the lower limit is made 100 ° C./sec. Also, 500 ℃ / sec
If it exceeds, the amount of strain accumulated in the steel sheet during cooling increases, and it is difficult to obtain good drawability and resistance to misplacement cracking, so the upper limit is made 500 ° C / sec. The temperature after cooling is set to 100-500 ° C, and then retained at 200-500 ° C after martensitic transformation and then tempered to form bainite, which reduces the hardness difference from ferrite and reduces drawability and anti-cracking resistance. If the temperature after cooling is less than 100 ° C, the hardness of martenside increases and the drawability deteriorates, and heating to 200-500 ° C is required, which is economically disadvantageous. The lower limit is 100 ° C. Also 500 ° C
If it exceeds, it is disadvantageous to the martensitic transformation and it is difficult to obtain the desired strength, so the upper limit is made 500 ° C.

Further, the reason for holding at 200 to 500 ° C. is to improve the drawability and the resistance to delamination cracking as bainite by tempering after martensite transformation, but the lower limit of 200 ° C. is tempering and bainite. For the sake of recommendation
The reason why the temperature is 500 ° C. is that if the temperature exceeds 500 ° C., it will be too tempered and it will be difficult to obtain the desired strength.

Further, the present invention, by reducing the retained austenite, excellent in drawability and resistance to placement cracking, other, bendability, secondary workability, stretch flangeability, fatigue characteristics,
Further, since the amount of C for obtaining the same tensile strength may be small, it is possible to obtain a high strength thin steel sheet having excellent weldability.

As described above, the steel sheet produced according to the present invention is 100 to 150 kgf /
It is a high-strength thin steel sheet that has a tensile strength of mm ² and is excellent in workability and resistance to placement cracking, and that can be economically manufactured.

(Examples) Next, the present invention will be described in detail with reference to Examples.

Example Steel produced by the ingot making method or continuous casting method was continuously hot-rolled in Table 1 and hot-rolled, pickled, under the production conditions shown in Table 2.
The steel sheets obtained after annealing, including those that were cold-rolled, annealed, and heat-treated before cold-rolling, were investigated for tensile strength, drawability and resistance to crack placement.

As can be seen from Table 2, in the comparative examples other than the present invention, the desired tensile strength, drawability and resistance to delamination cracking were not obtained, and in contrast, according to the present invention, the desired tensile strength, drawability and resistance to drawing were found. It can be seen that misplacement cracking is obtained.

(Effects of the Invention) As described above, according to the present invention, 100 to 150 kg
It is possible to economically provide a high-strength thin steel sheet having a tensile strength of f / mm ² and excellent in drawability and resistance to placement cracking.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between the amount of retained austenite of a high-strength thin steel sheet, drawability, and cracking resistance.

Claims (5)

[Claims] 1. C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5
˜2.6%, balance Fe and unavoidable impurities and residual austenite content of 5% or less
A high-strength thin steel sheet with a tensile strength of 100 to 150 kgf / mm ² and excellent workability and resistance to placement cracking. 2. C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5
~ 2.6%, Ti: 0.01-0.25% and Nb: 0.01-0.30%
1 or 2 of the above, consisting of the balance Fe and unavoidable impurities, and having a residual austenite amount of 5% or less, having a tensile strength of 100 to 150 kgf / mm ² ; High-strength thin steel plate with excellent crackability. 3. C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5
~ 2.6%, Ti: 0.01-0.25% and Nb: 0.01-0.30%
100 to 150 kgf /, characterized in that it contains 1 or 2 of B and 0.0003 to 0.01% of B, consists of the balance Fe and unavoidable impurities, and has a residual austenite amount of 5% or less.
A high-strength thin steel sheet with a tensile strength of mm ² and excellent in workability and resistance to placement cracking. 4. C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5
~ 2.6%, the balance Fe and unavoidable impurities steel, A
Hot rolling at a finishing temperature of ₃ transformation points or higher, cold rolling rate 30-70%
Cold-rolling, and in the annealing process, after maintaining the temperature range of 750 ~ 900 ℃ for 1 second ~ 5 minutes, the average cooling rate 100 ~ 5
100 to 150 kgf / mm ² characterized in that the amount of retained austenite is 5% or less by cooling at 100 ° C / sec to 100 to 500 ° C, then holding at 200 to 500 ° C for 1 to 20 minutes, and then cooling. Method for producing a high-strength thin steel sheet having excellent tensile workability and excellent workability and anti-deposition cracking resistance. 5. C: 0.03 to 0.20%, Si: 0.15 to 1.5%, Mn: 0.5
In addition to to 2.6% by Ti: 0.01 to 0.25% and Nb: 0.01 to 0.30% of one or and B: containing 0.0003 to 0.01% consisting ,, the balance Fe and unavoidable impurities Steel, A ₃ Hot rolling at a finishing temperature above the transformation point, cold rolling at a cold rolling rate of 30 to 70%, and holding in the temperature range of 750 to 900 ° C for 1 second to 5 minutes in the annealing step, then the average cooling rate 10 at 100-500 ° C / sec
The tensile strength of 100 to 150 kgf / mm ² is characterized in that the amount of retained austenite is set to 5% or less by cooling to 0 to 500 ° C, then holding at 200 to 500 ° C for 1 to 20 minutes, and then cooling. A method for producing a high-strength thin steel sheet having excellent workability and resistance to placement cracking.