JPH0418015B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for manufacturing a cold rolled steel sheet having extremely excellent deep drawability. [Conventional technology] Conventionally, in the manufacturing process of cold-rolled steel sheets, a material that has been hot-rolled at a finishing temperature of _A3 transformation point or higher is pickled, cold-rolled at a cold rolling rate of about 75%, and then annealed. It is common to do so. In such a conventional method, in order to improve the rank-ford value (hereinafter referred to as r value) of the cold rolled steel sheet, the cold rolling rate is set to 75.
It has been found that increasing the rolling rate from about 10% to about 90% is sufficient, but in this case, it has been difficult to adopt such a high cold rolling rate due to the capacity of the cold rolling mill. Furthermore, even if it were possible to roll at the above-mentioned high cold rolling rate by adopting a reverse rolling mill, in reality, the finished hot-rolled sheet thickness would be limited to a maximum thickness of about 6 mm that can pass through the pickling line. Therefore, when manufacturing a cold-rolled steel sheet of, for example, 0.8 mmt, a cold-rolling rate of 87% cannot be obtained, and in the end, it is difficult to achieve the above-mentioned high cold-rolling rate, and sufficient cold-rolling texture cannot be expected to develop. . As a result, a texture effective for deep drawability did not develop after recrystallization annealing, and the r value was around 2.0. [Problems to be solved by the invention] As described above, in the conventional manufacturing method of cold-rolled steel sheets, an ideal cold-rolling rate cannot be obtained and a sufficient cold-rolling texture cannot be obtained. There was a problem that there was a limit to the drawability, that is, the r value. The object of the present invention was made in view of the above-mentioned conventional situation, and it is an object of the present invention to provide a method for manufacturing a cold-rolled steel sheet having extremely good deep drawability. In order to solve the above-mentioned conventional problems, the inventor of the present invention conducted intensive research, created a part of the texture of cold rolling in the hot rolling stage, perfected it in the subsequent cold rolling, and The inventors have discovered that the shape, precision, and surface quality required for a steel plate can be achieved during the cold rolling process, and have thus completed the present invention. [Means for solving the problem] Therefore, the specified invention of the present application contains C: 0.01% by weight or less, N: 0.01% by weight or less, Ti: 0.2% by weight or less, Nb: 0.2% by weight or less, and (C/12+
For steel to which one or both of Ti and Nb is added in an amount such that N/14)<(Ti/48+Nb/93), the slab heating temperature is 1000℃ or higher, and the heating temperature is 900℃ to 1200℃.
In addition to rough rolling, Ti and Nb carbon and nitride are precipitated to reduce the solid solution (C, N) to 20 ppm or less,
In the non-recrystallized temperature range of 800°C to 600°C, the roll diameter D ₁ is D ₁ > 100t ₁ with respect to the finished hot rolled plate thickness _{t 1 so} that the finished plate thickness t 1 is less than 4.5 mm. Finish rolling with a reduction ratio of R ₁ using rolling rolls, winding at 600℃ or less, and after pickling, the cold rolled finish plate thickness t ₂ is 1.5 mm.
To ensure that _the rolling reduction ratio _{R 2 >} 50% (however, the sum _of R ₁ and _{R 2} The rolling reduction ratio R is
This manufacturing method involves cold rolling (95%>R>75%) and then annealing. Further, a related invention of the present application is that the steel having the chemical composition in the above specific invention contains B: 0.0005 to 0.005% by weight, and this is applied to the above manufacturing method. In addition, the total rolling reduction ratio R is calculated|required here by the following formula. R=1-(1- _R1 )(1- _R2 ) Further, the rough rolling may be conventional reheat rolling or direct rolling in which rolling is performed immediately after casting. Here, the reason for limiting the components and manufacturing conditions of the present invention will be explained. First, the reason for limiting the components in the specific invention will be explained. If C and N are added in excess of 0.01% each, precipitates such as TiC, TiN, and NbC will increase, resulting in poor workability of the product, and the amount of Ti and Nb to fix them will increase. Therefore, C≦0.01% and N≦0.01% were set. By adding Ti and Nb, the charcoal,
By forming nitrides, the solid solution (C, N) in the steel can be reduced, and the ferrite recrystallization temperature can be significantly increased, making it possible to delay recovery during processing. From a metallurgical point of view, this makes it possible to extend the state similar to cold rolling to a high temperature range of 800°C or lower, making it possible to create part of the cold texture during hot rolling. . And this
The amount of Ti and Nb added is the amount necessary to form carbon and nitrides and fix the solid solution (C, N) in the steel, that is, (C/12 + N/14) < (Ti/48 + Nb/93). In order to satisfy the requirements and also to take economic efficiency into account, Ti: 0.2% by weight or less and Nb: 0.2% by weight or less. In addition, there are no restrictions on other ingredients, but
Ingredients normally included in cold rolled steel sheets for processing, such as Mn < 0.3%, Si < 0.2%, P < 0.02%, S <
0.02%, Al<0.1%, etc. Next, the reason for limiting the components in the related invention will be explained. Note that the reason for C, N, Ti, and Nb is the same as that explained in the above-mentioned specific invention, so the explanation thereof will be omitted. By adding B, the warp cracking resistance can be improved. That is, by adding Ti and Nb, it is possible to reduce solid solution C, but this reduction in solid solution C weakens the bonding strength of grain boundaries and deteriorates the warp cracking resistance during secondary processing. let Therefore, by adding B, it is possible to segregate this B at the grain boundaries and strengthen the bonding force at the grain boundaries. Here, regarding the amount of B added, if it is less than 0.0005%, the above effects cannot be obtained, and if it is added more than 0.005%, it is economically disadvantageous, and addition of excessive B has a negative effect on the deep drawability of the product. From 0.0005% to 0.005%. Next, manufacturing conditions will be explained. The point of performing carbon and nitride precipitation treatment for Ti and Nb: Even if rolling is performed in the unrecrystallized ferrite region in a state where a large amount of solid solution (C, N) is included, the The (111) texture does not develop, and the (200) texture develops, which has a negative effect on deep drawability.
In order to obtain high deep drawability through cold rolling, it is necessary to precipitate carbon and nitrides by coiling at a high temperature after hot rolling. Before non-recrystallization rolling in the temperature range of 600℃, especially below 800℃, it is necessary to precipitate carbon and nitrides of Ti and Nb to reduce the solid solution (C, N) in the steel. It is. Here, before hot rolling finishing, Ti (C, N), Nb (C,
The following three methods can be considered to sufficiently precipitate N). For reheat rolling, the slab heating temperature is 900℃
A method that uses a low temperature of ~1100℃ to prevent precipitates from forming a solid solution during the slab heating stage. By lowering the rough rolling temperature to 900-1000℃, Ti,
A method for rolling-induced precipitation of carbon and nitride of Nb. A method in which carbon and nitrides are precipitated after a period of time between the end of rough rolling and the finishing of hot rolling (in this case, the waiting time is 400 yen in the precipitate formation temperature range).
(preferably more than 1 minute). The point that the warm rolling temperature was 880 to 600℃, and the total reduction ratio with cold rolling was 75% or more and 95% or less: When rolling at a temperature exceeding 800℃, ferrite recrystallizes and the processed structure changes. No rolling texture remains.
The amount of reduction in this temperature range has no relation to the properties of the final product. In addition, the rolling reduction in the unrecrystallized ferrite region below 800°C is important for forming the rolling texture during the next cold rolling process, and therefore it is important to ensure that the rolling reduction rate in the unrecrystallized ferrite region is 75 to 95% in total. Control the amount of reduction. On the other hand, rolling at temperatures below 600°C is not practical because the deformation resistance of the material is too high. Therefore, the temperature was set at 800℃ to 600℃. Further, in order to obtain a texture effective for deep drawability by recrystallization annealing, the cold rolling texture must be sufficiently developed. For this purpose, the sum of the rolling rate in the ferrite non-recrystallized area and the cold rolling rate must be 75 to 95%. In other words, the rolling reduction ratio R ₁ in warm rolling is the total rolling reduction ratio R [=1-(1-R ₁ )(1-R ₂ )] including the rolling reduction ratio R ₂ in the subsequent cold rolling.
The ratio should be 75% or more and 95% or less. In addition, the winding temperature is set under conditions that do not recrystallize during the winding stage, that is,
It is necessary to keep the temperature below 600℃. This is because if it exceeds this, the cumulative effect of cold rolling cannot be expected. In this case, in cold rolling, a reduction ratio of 50% or more is required in order to perfect the texture created by the hot rolling and to adjust the surface shape. Lubrication during hot rolling: Regarding lubrication during hot rolling, the better the lubrication, the better the properties equivalent to those of cold rolling, but even if the lubrication conditions are poor, as long as the total reduction with cold rolling is within the range of the present invention. Since it is possible to obtain a higher r value than a cold-rolled steel sheet with a rolling reduction of less than , there is no need to specify it in particular. Points for specifying the roll diameter: If the roll diameter is too small compared to the finished sheet thickness, plastic strain will not reach the center of the sheet even if the cold rolling rate is increased, and a sufficient cold rolling texture will not be obtained. Therefore, the roll diameter D is set so that D>100t with respect to the finished plate thickness t. Note that this also applies to cold rolling rolls. [Function] In the method for manufacturing a cold rolled steel sheet according to the present invention,
In addition to precipitating carbon and nitrides of Ti and Nb to reduce the solid solution (C, N) in the steel, warm rolling and cold rolling are performed at a prescribed reduction rate using rolling rolls with a prescribed roll diameter. From a metallurgical point of view, the condition is almost the same as cold rolling only, but in the high temperature range 800℃.
The cold rolling texture, which affects deep drawability, can be developed by both rolling processes, resulting in high deep drawability. In addition, in the related invention of the present application, since B is added, in addition to the above-mentioned effects, deterioration of warp cracking resistance due to reduction of solid solution (C, N) in steel can be prevented. . [Example] Hereinafter, an example of the present invention will be described with reference to the drawings. First, embodiments of the specific invention of the present application will be described. Table 1 shows the chemical components of the steel of the present invention (C, D in the table) and the comparative steel (A, B in the table). In this example, the four steel types shown in Table 1 were melted in a converter, a slab was manufactured using the normal process, and then hot rolled under the conditions shown in Table 2, wound into a coil, and After washing, cold rolling was performed under the conditions shown in Table 2, and continuous annealing (CAL) at 850°C for 1.5 minutes or batch annealing (BOX) at 750°C for 3 hours was performed to determine the r value of the material. It was measured. The results are shown in Table 2. As is clear from Table 2, the steel of the present invention (columns 3, 4, 6, 7, and 10) has a high r value of 2.0 or more in all cases, and good deep drawability is obtained. I understand that. In addition, the inventor of the present invention has determined that rolling reduction and deep drawability (r value)
We conducted an experiment on the relationship between The figure is a characteristic diagram showing the above experimental results. This experiment consisted of 0.0025%C-0.01%Si-0.15%Mn-
Hot-rolled sheets of 0.001%S-0.08%Ti-0.003%N were cold-rolled with varying reduction ratios and recrystallized at 850°C for 2 minutes, after which the r value was measured (○ in the figure). (indicated by a symbol). In addition, half of the total rolling reduction of the above hot-rolled sheet was reduced to 800
The material is hot-rolled at temperatures below ℃, and the remaining half is cold-rolled.
Similarly, after recrystallization annealing at 850°C for 2 minutes, the r value was measured (indicated by ● in the figure). As is clear from the figure, the total rolling reduction rate is 75 to 95.
% range, it can be seen that r values of 2.0 or more were obtained in all cases (○, ●). Next, embodiments of related inventions of the present application will be described. Table 3 shows the chemical components of the steel of the related invention of the present application (E in the table) and the comparative steel (C in the table). This example

【table】

【table】

[Table] In the table, * indicates outside the manufacturing conditions of the present invention.
Then, we melted the two steel types shown in Table 3, produced a slab using the normal process, and then reheated it to 100℃ to produce a slab in the γ range.
Roughly rolled to a thickness of 30mm, finish rolled when the temperature reached 850℃, and the rolling reduction was 60% below 800℃.
Finished to a thickness of 2.9mm, finishing temperature is both steel (E, C)
Both temperatures were set at around 730℃. Next, this was rolled up at 500°C, pickled, and cold rolled. Since the finished plate thickness of this cold rolling is 0.8 mm, the total rolling reduction in the temperature range of 800°C or less is 89%. The roll diameter for hot rolling is φ800, and the roll diameter for cold rolling is φ580. Then, the steel plate obtained by the above manufacturing method is
After continuous annealing at 850°C for 1.5 minutes, the r value was measured and the warp cracking resistance test during secondary processing was conducted. This vertical cracking resistance test adopted the cup vertical cracking test, and after blanking the steel plate with φ145, the drawing ratio α:
A test cup was prepared using 2.0, and the cup was placed over a conical punch and fractured by applying a load from the bottom of the cup at a temperature ranging from liquid nitrogen temperature to room temperature. The transition temperature was measured from the brittle fracture rate at that time.

【table】

〔Effect of the invention〕

As described above, according to the method for producing an ultra-deep drawable cold rolled steel sheet according to the present invention, carbon and nitrides of Ti and Nb are precipitated to reduce the solid solution (C, N) in the steel, and A part or most of the cold rolling texture is created during the hot rolling stage, and this texture is completed and the surface texture is adjusted during the cold rolling stage, resulting in excellent surface texture and deep drawability. This has the effect of producing extremely superior cold-rolled steel sheets. Further, according to the related invention of the present application, since B is further added, there is an effect of improving warp cracking resistance in addition to the above-mentioned effect.

[Brief explanation of drawings]

The drawing is a characteristic diagram showing the relationship between rolling reduction and r value.

Claims (1)

[Claims] 1 Contains C: 0.01% by weight or less, N: 0.01% by weight or less, Ti: 0.2% by weight or less, Nb: 0.2% by weight or less, and (C/12+N/14)<(Ti /48+Nb/
93) For steel consisting of residual Fe and unavoidable impurities with the addition of one or both of Ti and Nb in amounts such as While rolling, carbides and nitrides of Ti or Nb are precipitated to reduce the total amount of solute C and solute N to 20 ppm or less,
The finished hot rolled plate thickness t ₁ is less than 4.5 mm in the non _- recrystallized temperature range of 800°C to 600°C.
After finish rolling with a reduction ratio R ₁ in the non-recrystallized area using a roll with a roll diameter D ₁ of D ₁ > 100t ₁ , it is wound up at 600°C or less and pickled.
Further, so that the cold rolled finished plate thickness t ₂ is less than 1.5 mm, the roll diameter D ₂ is set to D ₂ > with respect to the finished plate thickness t _{2 .}
Rolling reduction ratio R ₂ > 50% using a rolling roll of 100t ₂
(However, the total reduction ratio R of R ₁ and R ₂ is 95%>R>75
%), and then annealing is performed. 2 C: 0.01% by weight or less, N: 0.01% by weight or less,
Contains B: 0.0005 to 0.005% by weight, Ti: 0.2% by weight or less, Nb: 0.2% by weight or less, and (C/12+
N/14) < (Ti/48+Nb/93).
Remaining Fe with one or both of Ti and Nb added
For steel containing unavoidable impurities, the slab heating temperature is set to 1000°C or higher, and rough rolling is performed in a temperature range of 900°C to 1200°C, and carbides and nitrides of Ti or Nb are precipitated to form solid solution C. and the total amount of solid solution N is 20 ppm or less, and the finished hot rolled plate thickness t ₁ is less than 4.5 mm in the non _- recrystallized temperature range of 800 ° C to 600 ° C. Roll diameter D ₁
After finish rolling with a rolling reduction ratio of R ₁ in the non-recrystallized area using rolling rolls with D ₁ > 100t ₁ , winding is performed at 600℃ or less, pickling is performed, and the final cold rolled plate thickness is
In order to make t ₂ less than 1.5 mm, use a rolling roll whose roll diameter D ₂ is D ₂ > 100t ₂ for the finished plate thickness t ₂ and set the rolling reduction ratio R ₂ > 50% (however, R ₁ and R After _cold rolling with a total reduction ratio R of 95%>R>75%),
A method for producing a cold-rolled steel sheet for ultra-deep drawing, characterized in that annealing is performed.