JPH034607B2 - - 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 for deep drawing, and in particular is a proposal for a technology for manufacturing a cold-rolled steel sheet that exhibits excellent deep-drawing characteristics and is used as an automobile panel material. (Prior Art) Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order to improve deep drawability, the mechanical properties of the steel sheet need to be high ductility and high Rankford value (γ value).
Furthermore, the surface properties of steel sheets are also an important factor when used as outer panels for automobiles. By the way, in the past, when assembling an automobile body, a large number of press parts were individually spot welded, but recently there has been a demand to reduce the number of parts and welds by making some of these parts larger and integrating them. It's getting higher. For example, due to the complicated shape of automobile oil pans, they are actually completed by welding, but there is a strong demand from automobile manufacturers for integral molding. Meanwhile, car designs are becoming more complex in order to meet diversifying needs, and as a result, an increasing number of parts are difficult to form using conventional steel plates.
In order to meet these demands, cold-rolled steel sheets with better deep drawability than conventional ones have become necessary. Conventionally, various methods have been proposed to improve deep drawability. By the way, it is known that the deep drawability of a steel sheet is closely related to its texture, and the greater the number of {222} oriented grains and the fewer {200} oriented grains, the higher the value obtained. Conventional methods for obtaining this high price include, for example, Japanese Patent Publication No. 17268, No. 17269, Japanese Patent Publication No. 17270,
A so-called two-stage cold rolling method has been proposed in which cold rolling is performed in two steps on a low carbon rimmed steel sheet, as disclosed in Japanese Patent Publication No. According to this two-step cold rolling method, the final product is {222}
There are many oriented grains, and there are few {200} oriented grains. This is because the primary cold rolling and annealing treatment increases the {222} oriented grains in the steel sheet compared to the hot rolled sheet before cold rolling.
On the other hand, since the {200} oriented grains decrease, when cold rolling and annealing are performed next, the {222} oriented grains further increase, whereas the {200} oriented grains further decrease. Therefore, steel sheets with high prices can be manufactured. On the other hand, in Japanese Patent Application Laid-open No. 56-62926,
C: 0.008wt% (hereinafter simply expressed as "%"), Si: 0.57%, Mn: 0.35%, Al: 0.43% and Nb: 0.061%, after normal hot rolling and cold rolling,
By performing box annealing at 950℃ for 1 hour, = 4.73
We are proposing a technology to obtain extremely high prices. (Problems to be Solved by the Invention) Among the above-mentioned conventional techniques, the former two-stage cold rolling method has achieved the improvement of deep drawability, but compared to the conventional process, the cold rolling-annealing process Step 1
There was a problem in that the process had to be performed many times, and the energy, manpower, and cost required for this process were enormous. Furthermore, since the latter of the above conventional techniques utilizes the formation mechanism of a transformation texture, the crystal annealing temperature must be raised to the _A3 transformation point or higher.
Therefore, compared to recrystallization annealing below the _A3 transformation point,
Increased energy costs and equipment and technical difficulties due to high temperature annealing are also involved. Furthermore, Si
Alternatively, a large amount of Al must be added, which poses a problem in that the surface properties of the steel sheet deteriorate. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to overcome the above-mentioned problems of performing two cold rolling operations and the above-mentioned problems of dealing with only the component composition by adopting a new method mainly based on the interaction between hot-rolling conditions and component composition. At the same time, the present invention proposes an advantageous method for producing cold-rolled steel sheets having exceptionally excellent deep drawability. (Means for solving the problems) Regarding the problems of the above-mentioned conventional technology,
In the present invention, C: 0.05wt% or less, Si: 0.005~
0.10wt%, Mn: 0.01~0.3wt%, P: 0.001~
0.05wt%, S: 0.020wt% or less, Al: 0.10wt% or less, N: 0.005wt% or less, and Ti:
0.08wt% or less, Nb: 0.05wt% or less and B:
Steel containing one or more of 0.003wt% or less as necessary, and the remainder substantially consisting of Fe,
At least one pass in the temperature range of Ar ₃ transformation point to 600℃, strain rate: 500 (s ^-1 ) or more, rolling reduction: 35
Lubricated hot rolling is performed at a temperature of 50% to 95%, followed by hot rolling at a temperature of 600°C or higher, followed by a pickling process, followed by cold rolling at a rolling reduction of 50 to 95%. The present invention provides a method for producing a cold-rolled steel sheet for deep drawing, which is characterized by subjecting the steel sheet to recrystallization annealing and then recrystallization annealing. (Function) Below, the results of research on the influence of hot rolling on deep drawability, which led to the idea of the method of the present invention, will be explained.

[Table] Table 1 shows the test materials (ultra-low carbon steel) used in the research. After heating the specimen steel piece to 700℃ and soaking it,
Lubricated and non-lubricated hot rolling was performed at rolling reductions of 40% and 30% in one pass, and winding treatment was performed at 650°C for 1 hour. The hot-rolled sheet was cold-rolled at a rolling reduction of 75%, and then continuously annealed at 830°C for 40 seconds. Figure 1 shows the relationship between the strain rate, rolling reduction rate, presence or absence of lubrication, and values at this time. As you can see from this figure, the strain rate is 500 (s ^-1 )
An improvement in the value was observed above, and the reduction rate was 30%.
40% is better, and lubricated rolling is better than non-lubricated rolling. Therefore, regarding the method of the present invention, which was completed based on the above-mentioned new knowledge about the hot rolling method, the reasons why the materials and manufacturing conditions are limited as in the above-mentioned solution are explained in order below. explain. (1) Regarding the steel composition, the steel composition is important in the present invention: C: 0.05% or less, Si: 0.005 to 0.10%, Mn: 0.01 to 0.3%, P: 0.001 to 0.05%, S: 0.020% or less, Contains Al: 0.10% or less and N: 0.005% or less, and if necessary, contains one or more of Ti: 0.08% or less, Nb: 0.05% or less, B: 0.003% or less, and the remainder is
It must consist of Fe and unavoidable impurities. (a) C≦0.05% C is preferable because the less it is, the better the deep drawability of the cold-rolled sheet is, but if it exceeds 0.05%, it will have a negative effect on the deep drawability. (b) 0.005%≦Si≦0.10% Si is an effective element for forming a texture that is favorable for the strength or deep drawability of steel sheets. If the content is less than 0.005%, the desired effect cannot be obtained, while if the content exceeds 0.10%, the surface properties of the steel sheet will deteriorate and the ductility will also decrease, so it was set as 0.005%≦Si≦0.10%. (c) 0.01%≦Mn≦0.3% Mn has the effect of improving the brittleness of the steel sheet. If the content is less than 0.01%, the desired effect cannot be obtained, while if it exceeds 0.3%, ductility decreases, so it was set as 0.01%≦Mn≦0.3%. (d) 0.001%≦P≦0.05% P has the effect of strengthening steel. If the content is less than 0.001%, the desired effect cannot be obtained,
On the other hand, if the content exceeds 0.05%, ductility deteriorates, so it was set as 0.001%≦P≦0.05%. (e) S≦0.020% The smaller the S content, the more advantageous it is to deep drawability. If its content is less than 0.020%, it will not adversely affect deep drawability, so S≦
It was set at 0.020%. (f) Al≦0.10% Al is added to perform deoxidation, but
If it is contained in an amount exceeding 0.10%, a coarse grain structure will be exhibited, causing roughness during press working and further deteriorating the surface quality. (g) N≦0.005% The smaller the amount of N, the better the deep drawability.
If it is 0.005% or less, it will not adversely affect deep drawability, so it was set as N≦0.005%. (h) Ti, Nb, and B Ti, Nb, and B are all carbonitride-forming elements, and are effective in reducing solid solution C and N in steel and forming a texture that is advantageous for deep drawability. . However, the content is 0.08 each
%, 0.05%, 0.003%, the ductility deteriorates, so Ti≦0.08%, Nb≦0.05%, B≦
It was set at 0.003%. (2) Regarding the rolled material As the rolled material, a steel billet obtained by a general method such as an ingot-blowing method or a continuous casting method can be used. In this case, the heating temperature of the steel billet is 800
-1250°C is suitable, and a range of 950-1150°C is particularly suitable. It is also applicable to those obtained by the continuous casting-direct rolling method (so-called CC-DR method) in which rolling is started without reheating the steel billet from continuous casting. On the other hand, the method of directly casting rolled material of 50 mm or less from molten steel (sheet bar caster method, strip caster method) also has great economic merits from the viewpoint of energy saving and process saving, and the manufacturing method of the rolled material of this invention This is particularly advantageous. (3) About hot rolling This is the most important process in this invention, and during finish rolling, at least one pass is performed in the temperature range of Ar ₃ transformation point to 600°C at a strain rate of 500°C.
It is necessary to carry out lubricated rolling at a rolling reduction rate of 35 (%) or more at a temperature of (s ^-1 ) or more, and then wind it at a temperature of 600°C or more. In the high temperature range where the rolling temperature is higher than the Ar ₃ transformation point, even if high strain rate and large reduction rolling is performed, γ→
Due to the α transformation, the crystal orientation becomes random, and it is impossible to form the {222} orientation, which is advantageous for drawability. On the other hand, when rolling at 600℃ or less, 600℃
Since it is impossible to wind up the steel sheet at a temperature higher than that, the steel sheet exhibits a processed structure, and self-annealing cannot occur during winding. The number of rolling passes and the distribution of rolling reduction ratio are at least 1
As long as it satisfies the conditions for finishing with lubrication in each pass, it can be arbitrarily selected, and of course, the arrangement structure of the rolling mill, roll diameter, etc.
Tension and the like are not particularly limited either. The type of lubricating oil and the method of dispersing it are also not limited in this invention. For example, suspension oils based on mineral oils and the like can be applied in the usual manner. It is essential that the winding temperature be 600°C or higher. This is because if the winding temperature is less than 600°C, recrystallization will not proceed during self-annealing during winding, and therefore {200} oriented grains, which are disadvantageous for deep drawability, will remain. (4) About cold rolling This process is important in this invention in order to obtain a high γ value and to reduce in-plane anisotropy, and it is essential that the cold rolling reduction rate be 50 to 95%. It is. If the cold rolling reduction is less than 50% or more than 95%, excellent deep drawability cannot be obtained. (5) Regarding annealing Cold rolled steel strips that have gone through the cold rolling process require recrystallization annealing. The annealing method may be either a box annealing method or a continuous annealing method, but the latter is advantageous from the viewpoint of homogeneity and productivity. Heating temperature ranges from recrystallization temperature (approximately 600℃) to 950℃
The range shall be . In the case of continuous annealing, there are no particular limitations on the thermal cycle, that is, the cooling rate after soaking, and the presence or absence of overaging treatment, but
Slow cooling below ℃/see or overaging treatment at around 350℃ is effective for improving material quality, especially ductility. The steel strip after annealing may be subjected to temper rolling of 10% or less in order to correct the shape and adjust the surface roughness. In addition, the cold-rolled steel sheet obtained by this invention can also be applied to the original plate of a surface-treated steel sheet for processing. Surface treatments include galvanizing (including alloys), tin plating, and enameling. The effects of high strain rate, large reduction rolling in the present invention are considered as follows. First, high strain rate, large reduction rolling has the effect of substantially lowering the rolling temperature. That is, even in a high temperature range from the Ar ₃ transformation point to 600°C, when high strain rate large reduction rolling is performed, a deformed texture equivalent to that obtained by cold rolling is formed. When this steel plate is rolled up at 600°C or higher, recrystallization is completed by self-annealing. Therefore, high strain rate, large reduction rolling and self-annealing alone have the same effect as primary cold rolling and annealing. Therefore, the deep drawability after cold rolling and annealing is much better than that of conventional hot rolling. (Example) Steel slabs having the composition shown in Table 2 were manufactured by the method shown in Table 3. This was continuously processed into a hot-rolled plate with a thickness of 3.2 mm using a finishing rolling mill consisting of 7 stands. At this time, lubricated rolling was performed using the rolling mills of all stands, and high strain rate, large reduction rolling was performed using the rolling mill of the last stand, followed by winding at various temperatures. After subsequent pickling, the plate thickness is 0.8mm (cold rolling reduction:
75%) or 1.7 mm thick (cold rolling reduction ratio: 47% → comparative example), and then recrystallization annealing (annealing temperature: 760 to 830°C) was performed. Table 3 summarizes the hot rolling conditions and material properties after continuous annealing of the inventive examples and comparative examples. The tensile properties were determined using JIS No. 5 test pieces, L (rolling direction), C (90° in the rolling direction), D (45° in the rolling direction) 3
It was calculated as the average value in the direction. In addition, the values were measured by a three-point method after applying 15% tensile prestrain, and L, C,
It was calculated as the average value in the D3 direction. As is clear from Table 3, the steel plate obtained according to the method of the present invention has a higher γ value and ductility than the comparative example.

【table】

【table】

[Table] (Effects of the invention) As explained above, according to the present invention, by adopting the steel composition and hot rolling conditions, especially the conditions of high strain rate and large reduction rolling, deep drawing is much superior to that of the conventional method. It is possible to produce cold-rolled steel sheets at low cost that exhibit excellent mechanical properties as well as mechanical properties.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of "strain rate", "reduction ratio", and "lubrication" on the values.

Claims (1)

[Claims] 1 C: 0.05wt% or less, Si: 0.005-0.10wt%,
Mn: 0.01-0.3wt%, P: 0.001-0.05wt%, S:
0.020wt% or less, Al: 0.10wt% or less and N:
0.005wt% or less, and contains one or more of Ti: 0.08wt% or less, Nb: 0.05wt% or less, and B: 0.003wt% or less, as necessary, and the remainder is substantially Fe. At least one pass of the steel is lubricated and hot rolled in the temperature range from the Ar ₃ transformation point to 600°C at a strain rate of 500 (s ^-1 ) or more and a rolling reduction of 35% or more to form a specified plate. A thick hot-rolled plate is formed, this hot-rolled plate is rolled up at a temperature of 600℃ or higher, and then subjected to a pickling process and then cold-rolled at a reduction rate of 50 to 95%.
Furthermore, it is characterized by subsequent recrystallization annealing,
A method for producing cold-rolled steel sheets for deep drawing.