JPH0567684B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a high-strength cold-rolled steel sheet that has high strength and is excellent in press forming. The cold-rolled steel sheet according to the present invention is used, for example, in automobiles, home appliances, steel structures, etc. after being subjected to appropriate surface treatment and press working, and in particular, it is important to simultaneously impart formability and strength required for these products. is possible. As a result, these products can be made thinner or lighter. (Conventional technology) High-strength steel is made by adding Si, Mn, Cr, and P to increase strength based on ultra-low carbon Ti-added steel, which is made by thoroughly decarburizing at the steel manufacturing stage to make it extremely low carbon and then adding Ti. There are already many proposals for tensile cold-rolled steel sheets. For example, Japanese Patent Publication No. 57-57945 discloses a cold-rolled steel sheet in which a large amount of P is added to the ultra-low carbon Ti-added steel. In this case, Mn is
Since it may contain less than 0.90%, the r value that can be obtained is limited to 1.6 to 1.9. Also,
There is no mention of N and S contents or secondary processing embrittlement. In addition, Japanese Patent Publication No. 58-29129 discloses an example in which a large amount of Mn is added alone to the above-mentioned ultra-low carbon Ti-added steel, but in this case too, the r
As a result, it is necessary to use water quenching for cooling after continuous annealing, making it impractical. Other methods have been disclosed, such as those in which Si is added to the ultra-low carbon Ti-added steel and those in which Cr is added, but these have not been put into practical use because of the problem of oxidation on the surface of the steel sheet. This is the reality. On the other hand, it is well known that adding alloying elements to such ultra-low carbon Ti-added steel tends to cause secondary work embrittlement, and therefore B is generally added in combination to reduce secondary work embrittlement. Measures are being taken to prevent this. However, adding a large amount of B may cause cracks in the slab.
It is difficult to reliably add , and it may lead to operational instability, so it has not been used as a definitive means for preventing secondary processing embrittlement. (Problems to be Solved by the Invention) From the above, the present inventors, as well as the industry, believe that the tensile strength is 38 Kgf/mm ² or more, the yield stress is the tensile strength - 12 Kgf/mm ² or less, r value
A high-strength cold-rolled steel sheet with a tensile strength of 2.2 or higher and less susceptible to secondary work embrittlement, and a method for manufacturing it using conventional continuous annealing and low-cost alloy addition, have been studied and sought after for many years. Therefore, the object of the present invention is to have a tensile strength of 38 Kgf/mm ² or more, a yield stress of (tensile strength - 12 Kgf/mm ² ) or less, an r value of 2.2 or more, and secondary work brittleness is unlikely to occur. An object of the present invention is to provide a method for producing high-strength cold-rolled steel sheets. (Means for Solving the Problems) In order to achieve the above object, the present inventors have continued intensive research focusing on the above-mentioned ultra-low carbon Ti-added steel. Here, the knowledge newly discovered by the present inventors is based on the ultra-low carbon Ti-added steel.
The coexistence of Mn and P not only increases the tensile strength after cold rolling and annealing, but also significantly increases the r value, and furthermore, a small amount of solid solution C remains. The residual solid solution C effectively prevents secondary work brittleness. This is due to the interaction between Ti, Mn, P, S, and C. For example, in steel where Mn and P do not coexist, TiC and MnS are each formed as stable precipitates. , Ti≧4(C+
If Ti of 12/14N) is added, no solid solution C will remain, but if a large amount of Mn and P coexist as in the present invention, part of the TiC will be decomposed, and TiC, It is thought that precipitates such as MnS, TiP, TiS, and MnP are formed, and C in a solid solution state exists. When recrystallization annealing is performed under such conditions, a recrystallization texture favorable to the r value develops due to this small amount of solid solute C, and the r value is significantly improved. It also remains in the steel, strengthens grain boundaries and prevents secondary work brittleness, and also makes it possible to exhibit a small amount of bake hardenability. Furthermore, in the case of hot-rolled steel sheets as described above, the present inventors have determined that the coiling temperature after hot rolling is
It has been found that the r value is further improved by significantly lowering the temperature from 550 to 700°C. Here, the gist of the present invention is as follows: C: 0.001 to 0.012%, N: 0.001 to 0.008%, sol.Al: 0.08% or less, S≦0.010%, Ti: 0.01 to 0.15%, and , Ti≧4(C+12/
14N), and Mn: more than 1.0%, less than 3.0% and P: 0.05
~0.15% is added in combination, and if desired, B:0.0001~
A steel with a composition of 0.0004% Fe and unavoidable impurities is hot rolled, the coiling temperature after hot rolling is room temperature to 450°C, and then cold rolling and recrystallization annealing are carried out. This is a method for producing high-strength cold-rolled steel sheets with good formability. (Function) Here, the reason why the steel composition and manufacturing conditions are limited as described above in the present invention will be further explained. C: C is naturally contained in steel. The amount of C necessary for the above-mentioned grain boundary strengthening is about 0.0005%, but since lowering the amount of C would lead to increased costs, the lower limit was set at 0.001%. When C increases, it contributes to strengthening, but the required Ti increases, which increases the cost. Therefore, in the present invention, the upper limit of C is
It was set at 0.012%. N: The smaller the number of N, the better. However, since reducing it is costly, the lower limit was set at 0.001%.
On the other hand, if it is too large, a large amount of Ti needs to be added, so the upper limit was set at 0.008%. sol.Al: Added to deacidification adjustment. Although it is not necessary to add Ti, the yield of Ti addition decreases in that case. If there is too much sol.Al, the cost will increase, so the upper limit was set at 0.08%. S: In the present invention, it is particularly desirable that S: be reduced.
When the amount of S exceeds 0.010%, MnS is formed, which deteriorates workability and makes it difficult to form the above-mentioned MnP. Ti: Ti% is determined by Ti≧4 (C+12/14N).
This is the conventional formula, where C and N are TiC,
This indicates that an amount of Ti should be added that will be fixed as TiN. Less than 0.01% is not realistic based on the above formula, and adding more than 0.15% not only increases costs but also makes it difficult for TiC to decompose as described above, so it was limited to 0.01 to 0.15%. Mn: This is necessary to form MnS and MnP. If it is less than 1.0%, its formation is insufficient and a high r value and grain boundary strengthening cannot be obtained. On the other hand, if it exceeds 3.0%, too much MnP is formed and the r value decreases. Therefore, it was limited to more than 1.0% and less than 3.0%. Preferably it is 1.2 to 2.0%. P: P is also necessary to form MnP and TiP. In particular, it has the effect of capturing Ti and dissolving C more than TiC. If it is less than 0.05%, such effects are insufficient and a high r value and grain boundary strengthening cannot be achieved. On the other hand, 0.15
If it exceeds %, P segregation in the steel will increase, making slab cracks more likely to occur. Therefore, 0.05
Limited to ~0.15%. B: B segregates at grain boundaries and has the effect of strengthening the grain boundaries. The present invention is characterized in that C segregates at grain boundaries, strengthens the grain boundaries, and prevents secondary work embrittlement, but even if a small amount of B is added in combination as necessary, the effects of the present invention may be reduced. However, in order to ensure the strengthening of grain boundaries, it may be added as necessary. In this case, less than 0.0001% is meaningless, and 0.0004%
Adding B in excess not only increases addition cost and causes slab cracking, but also in high Mn materials like the present invention, adding too much B actually deteriorates deep drawability, so it is set at 0.0001 to 0.0004%. One of the features of the present invention is that a smaller amount of B is required compared to the conventional case. Next, the reason for limiting the conditions in the manufacturing method will be described. Hot rolling, cold rolling, annealing: The coiling temperature after hot rolling is usually 550 to 700℃, and even if fluctuations due to coil position are included, the coiling temperature is 500 to 750℃.
It is ℃. However, we found an even lower 450°C
It has been found that the r value is further improved when the winding temperature is set to ~room temperature. This is achieved by low-temperature winding.
It is presumed that this is because precipitates such as TiC have a desirable size to increase the r value. At temperatures above 450°C, the effect is small and is no different from that under normal winding conditions, but below 450°C, the r value becomes noticeably improved. On the other hand, since winding cannot be performed below room temperature, the lower limit of low temperature winding in the present invention is set at room temperature. After hot rolling, cold rolling and annealing are performed, and even in this case, normal manufacturing methods for cold rolled steel sheets and surface-treated steel sheets are applied. Note that continuous annealing is preferable for annealing. In that case, the annealing temperature is preferably 700 to 920°C. The same applies to continuous annealing in a continuous hot-dip galvanizing line.
In the case of batch annealing, it is preferable to perform it at 700 to 750°C. Thereafter, an appropriate amount of temper rolling is performed to produce the product. Next, the present invention will be explained in detail by way of examples. Example 1 Steel having the composition shown in Table 1 was melted and made into a slab. After heating at 1100°C for 1 hour, hot rolling was immediately started, and hot rolling with a thickness of 3.2 mm was carried out at a finishing temperature of 880°C. Finished with steel plate. After pickling, these were cold rolled to a thickness of 0.8 mm, and then soaked at 820°C at a temperature increase rate of 40°C/sec.
Continuous annealing was performed for 60 seconds at a cooling rate of 40°C/sec.
After that, it is temper rolled with an elongation rate of 0.3% and then
A JIS No. 5 tensile test piece was taken and a tensile test was conducted. Here, the aging index is 100 after adding 8% pre-strain.
℃ for 1 hour, and then re-stretched, and the yield stress was determined from the increase in yield stress at this time. It is known that this aging index shows a high value when the amount of solid solute carbon in the steel sheet is large. In addition, blanks with a diameter of 50 mm are punched out from temper-rolled steel plates, and then deep drawn into cup shapes using a punch with a diameter of 33 mm.The blanks are then subjected to drop weight tests at various temperatures to determine the temperature at which brittle fracture occurs. Examined. This is the method of secondary processing brittleness test. Table 1 also summarizes these results. It can be seen that the steel plate according to the present invention has a tensile strength of 35 Kgf/mm ² or more and a yield stress of less than the tensile strength -12 Kgf/mm ² , and has good elongation compared to its strength and has a very high r value of 2.2 or more. . On the other hand, comparative steel 15 has poor elongation due to a large amount of S, comparative steel 16 has a low r value due to a lack of Mn, comparative steel 17 has a low r value due to a lack of P, and comparative steel 18 has a low r value due to a lack of P. The elongation of Comparative Steel 19 was low because of too much P, and the elongation of Comparative Steel 19 was low because the amount of Ti added was insufficient. Regarding secondary work embrittlement, all of the examples of the present invention have a temperature of −40° C. or lower, which poses no practical problem, whereas the comparative steels have problems at temperatures of 0° C. or higher. Comparative Steels 1 and 11 are data obtained when the coiling temperature after hot rolling is high. It can be seen that the r value is lower than that of the rolled material at 450°C or lower. Furthermore, as shown in Steel No. 20, when B is added beyond the range of the present invention, the r value decreases significantly, making the material unsuitable for deep drawing applications.

[Table] (Effects of the invention) As described above, according to the present invention, high-strength steel with excellent formability was obtained using a low-cost manufacturing method, resulting in cost reduction and simplification of the manufacturing line. Given the current situation where there is a strong need for In particular, when the steel plate according to the present invention is used for automobile frames and other major structural members, it greatly contributes to reducing the weight of the vehicle, and its industrial effects are significant.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between Mn%, P%, r value, and tensile strength; and FIG. 2 is a graph showing each data point of yield stress, r value, and tensile strength of the steel plate according to the present invention. It is a graph shown on the relationship diagram of r value, yield stress, and tensile strength of the high tensile strength cold-rolled steel plate manufactured conventionally.

Claims (1)

[Claims] 1% by weight, C: 0.001 to 0.012%, N: 0.001 to 0.008%, sol.
Al: 0.08% or less, S≦0.010%, Ti: 0.01-0.15%
Contains Ti≧4 (C+12/14N), Mn: more than 1.0%, less than 3.0%, and P: 0.05
A steel with a composite addition of ~0.15% and the balance consisting of Fe and unavoidable impurities is hot rolled, the coiling temperature after hot rolling is room temperature ~450℃, and then cold rolling and recrystallization annealing are performed. A method for producing high-strength cold-rolled steel sheets with good formability. 2% by weight, C: 0.001-0.012%, N: 0.001-0.008%, sol.
Al: 0.08% or less, S≦0.010%, Ti: 0.01-0.15%
Contains Ti≧4 (C+12/14N), Mn: more than 1.0%, less than 3.0%, and P: 0.05
~0.15% and B: 0.0001 ~ 0.0004%, the balance is Fe and unavoidable impurities, and the coiling temperature after hot rolling is room temperature ~ 450°C, and then cold rolling. A method for producing a high-strength cold-rolled steel sheet with good formability, which comprises rolling and recrystallization annealing.