JPH0542486B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention is a proposal for a manufacturing technology for a steel plate suitable for use as an automobile steel plate, particularly a bake-hardenable cold-rolled steel plate that has excellent deep drawability and can be subjected to low-temperature baking treatment. (Conventional technology) In recent years, steel sheets for automobiles have been required to have high strength with the aim of improving natural cost through weight reduction, but from the viewpoint of press formability, properties such as low descending strength, high elongation, and high r value are required. The reality is that they are also desired. Against this background, steel sheets have been developed that are soft and have good press formability during press forming, and have the property of increasing yield strength and tensile strength during subsequent coating baking, that is, have bake hardenability. It's on. Incidentally, bake hardenability is generally defined as the increase in yield strength due to baking equivalent treatment at 170°C for 20 minutes after imparting a 2% prestrain. This simulates the paint baking treatment conditions in the automobile production process.
In order to obtain this bake hardenability, it is necessary to allow solid solution C and N to remain in the steel sheet to an extent that does not impair the non-aging property at room temperature. Conventionally, methods for manufacturing cold-rolled steel sheets with bake hardenability have been made using Ti-added steel as disclosed in JP-A No. 53-114717, and Ti-added steel as disclosed in JP-A-57-70258. Nb-added steel or as disclosed in Japanese Patent Application Laid-open No. 59-31827.
Steels with improved compositions, such as Ti and Nb composite steels, have been proposed. In all of the steel plates obtained by these conventional methods, even when subjected to baking equivalent treatment at 170°C for 20 minutes, the bake hardenability obtained was at most 3 to 5 kgf/mm ² . The reason for this is that in order to ensure non-aging properties at room temperature, solid solution C and N must remain.
This is because there is naturally an upper limit to the amount, and if a high amount of bake hardening and excellent non-aging properties at room temperature are to be achieved at the same time, the amount of bake hardening will be 5 Kgf/mm ² at most. (Problem to be solved by the invention) The automobile industry is now in a situation where it is inevitable to move towards lowering the baking temperature due to energy conservation by lowering the baking temperature of paints and the increasing use of plastics. . Therefore, an object of the present invention is to provide a technology for manufacturing a bake-hardenable cold-rolled steel sheet that is non-aging at room temperature, has excellent deep drawability, and can be subjected to low-temperature baking treatment. This objective can be advantageously achieved by a method having the following points as its gist. (Means for Solving the Problems) The gist of the present invention is based on ultra-low carbon steel containing C; 0.001 to 0.008 wt%, and an aging index of 4 Kgf/mm ² by selecting pure box annealing or continuous annealing conditions. Exceed 8
Kgf/mm ² and elongation rate by subsequent temper rolling:
By adding 2-5%, aging index: 4Kg
Even at high aging indexes such as over f/mm ² and up to 8 kgf/mm ² , where aging at room temperature was a problem, we have developed a bake-hardenable steel sheet that is not aged at room temperature, has excellent deep drawability, and can be baked at low temperatures. It depends on manufacturing technology. To describe in more detail the method of the present invention which is suitable as a means for solving the above problems, C: 0.001~
0.008wt%, Si≦0.5wt%, Mn≦0.5wt%, P≦
0.15wt%, Al: 0.005~0.10wt%, S≦0.010wt
%, N≦0.005wt%, and 0.05wt% or less of one or two of Ti and Nb,
A steel whose balance is essentially Fe is hot-rolled, cold-rolled and heat-treated, and the aging index is reduced to 4 kg by selecting the heating temperature and cooling conditions in the heat treatment.
deep drawing, which consists of adjusting the f/ ^mm2 to a range exceeding 8Kgf/ ^mm2 , and subjecting the cold-rolled sheet that has undergone this heat treatment to skin pass rolling with an elongation rate in the range of 2 to 5% to suppress room temperature aging. This is a method for producing bake-hardenable steel sheets with excellent properties, and preferably the work roll diameter used for temper rolling is
The size is 400 mm or more, and the surface roughness of the obtained steel plate is controlled to be 0.5 to 5 μm. The aging index here is a value evaluated by the amount of increase in the lower yield point when accelerated aging treatment is performed at 100°C for 30 minutes after applying a prestrain of 7.5% in a tensile test. (Function) The details of the experimental study that led to the present invention will be explained below. If solid solution (C, N) remains in the steel sheet, stretch strain will occur during press forming, so it is common to perform temper rolling to eliminate the yield elongation that causes this. If there is a large amount of solid solution (C, N), room temperature aging will occur and yield elongation will appear again, so efforts are being made to suppress the remaining solid solution (C, N) as much as possible in steel sheets for pressing. On the other hand, bake-hardenable steel sheets utilize the solid solution C and N to increase yield strength by accelerating aging at high temperatures.
For this reason, residual solid solution C can be maintained to the extent that aging at room temperature does not occur.
Controlling this is important in the manufacturing process. For this reason, the aging index is used as a guideline for the amount of solid solution C and N in a steel sheet. In order not to cause room temperature aging, the aging index is at most 3 to 4 Kgf/mm ² . The amount of bake hardening obtained at this time is at most 3 to 5 kgf/mm ² . Naturally, temper rolling is performed after annealing, but the elongation rate is also
As shown in Japanese Patent No. 70258, less than 2% is common. This is a restriction because the yield strength increases as the elongation rate in skin pass rolling increases, which is disadvantageous to press formability. Now, the inventors of the present invention are aware of the need for low-temperature bake-hardenable steel sheets in the future, and aim to further improve the bake-hardening amount. First, the aging index and skin pass rolling conditions are The impact of this study was examined. FIG. 1 shows the results of this study, and is a graph showing the influence of aging index (AI) and baking equivalent temperature on the amount of bake hardening (BH amount). From this figure, it can be seen that steel plates with AI>4Kgf/mm ² can produce a high amount of BH even when baked at low temperatures. Moreover, the amount of BH obtained is at a level higher than the conventional level of 3 to 4 Kgf/mm ² . Therefore, by setting AI>4Kgf/Γ, it is possible to obtain a steel plate with high BH properties that can be subjected to low temperature baking treatment. Figure 2 shows the results of aging materials with different aging index and temper rolling conditions at 30°C for 2 months to determine the yield elongation occurrence limit. indicates that yield elongation occurs. This shows that even with a steel sheet with a high aging index (AI), by setting the elongation rate to 2% or more, yield elongation can be suppressed and non-aging properties can be obtained. Incidentally, even at the same elongation rate, the larger the average roughness of the steel sheet was, the more advantageous it was in terms of aging resistance. The reason for this is not clear, but it is thought that by increasing the roughness, the local strain introduced into the steel sheet surface has the same effect as increasing the temper rolling elongation rate. As will be described later, as the elongation rate increases, the yield strength increases, which is disadvantageous to press formability, so there is a limit to increasing the elongation rate. Therefore, the rougher the average roughness of the steel plate, the better. On the other hand, if the roughness is too rough, unevenness of the roughness will be noticeable even after coating, which is unfavorable in terms of appearance. Therefore, the average roughness is preferably in the range of 0.5 to 5 μm. FIG. 3 shows the effect of roll diameter on aging properties during skin pass rolling with the same roughness. It can be seen that a larger roll diameter is advantageous for aging resistance. As the roll diameter increases, the length of contact with the steel plate increases.
It is thought that the shear strain exerted on the steel plate concentrates near the surface of the steel plate, further promoting the effect of roughness. As shown in the figure, the difference is particularly noticeable for roll diameters of 400 mm or more.
The roll diameter is preferably 400 mm or more. FIG. 4 shows the relationship between elongation, yield strength, and tensile strength. Here, the increase in strength was based on the elongation rate of 1%. It can be seen from the figure that as the elongation rate increases, both yield strength and tensile strength increase. Tensile strength reaches almost saturation at elongation rate of 5%, so 5%
Even if the amount is increased, the yield strength only increases significantly, and it is preferable to limit the elongation rate to 2 to 5%. Aging index (AI) that does not impair aging resistance at this time
The upper limit is up to 8Kgf/ ^mm2 , and AI is up to 4Kgf/mm2.
The range exceeds mm ² and exceeds 8Kgf/mm ² . As explained above, by subjecting a steel plate with an aging index of more than 4 kgf/mm ² to 8 kgf/mm ² to skin pass rolling in accordance with the aging index, it is possible to achieve a deep bake hardenability that is non-aging at room temperature and can be baked at low temperatures. It was found that a cold-rolled steel sheet for drawing could be obtained. Next, the reason for limiting the composition of the steel used as a starting material when carrying out the production method of the present invention will be described. Usually, the elements contained in steel sheets, especially C, Si, Mn, P
etc., the target range is determined by the required strength, and the more the above-mentioned total elements increase, the higher the strength and the lower the elongation. Therefore, the basic idea of the present invention is to obtain the low drop strength and high elongation necessary for press formability by reducing the content of these elements more than usual by the amount that increases the temper rolling elongation rate. shall be. C: The lower the content of this element, the better the material quality, especially the elongation and r value necessary for press formability. In the present invention, it is desirable to lower the
If it is less than 0.001 wt%, the bake hardenability that is the objective of this invention cannot be obtained. On the other hand, if it exceeds 0.008 wt%, the elongation and r value will deteriorate, which is not suitable for the present invention, so the C content is set to 0.001 to 0.008 wt%. Si, Mn: Both Si and Mn effectively contribute to increasing the strength of steel sheets without deteriorating deep drawability, but both are limited to 0.5wt% or less to prevent deterioration of elongation. do. P: For the same reason as Si and Mn, the content of P should be 0.15wt% or less. Note that unlike Mn and S, the addition of P improves the r value, so it is rather preferable as a reinforcing element. Al: Al requires addition of 0.005wt% or more for purposes such as deoxidation, but addition of more than 0.10wt% has a negative effect on surface properties, so 0.005 to 0.10wt%
shall be. S, N: Since S and N form precipitates, the smaller the amount, the more elongation occurs, which is advantageous for the r value. From this, it is assumed that S≦0.01wt% and N≦0.005wt%. Ti, Nb: Ti and Nb are added to the extent that an appropriate amount of solid solution C remains. Ti is [48/14N+48/32S<Ti48/14N+48/32S+48/
12C] range is desirable. This means that impurities such as N and S
Necessary for precipitation as TiN, TiS,
On the other hand, it is sufficient to precipitate all the C as TiC.
When Ti is added, bake hardenability cannot be obtained because solid solution C cannot remain. Therefore, Ti is set to 0.05wt% or less. Regarding Nb, it precipitates as NbC, but by setting the annealing temperature to 800° C. or higher, solid solution C can be dissociated from NbC and dissolved, so there is no particular restriction. but,
Adding more than 0.05wt% of Nb inhibits grain growth during annealing and is disadvantageous for elongation, so 0.05wt%
Do the following. While fixing some of S, N, and C with Ti, precipitating solid solution C as NbC sufficient to obtain the desired aging index is a
Particularly preferred for improving value. This is thought to be because the solid solution Nb itself, which is dissociated by high-temperature annealing, stretches and has the effect of improving the anisotropy of the r value. Furthermore, Cr, Cu, V, Zr or less than 0.5wt%
Addition of 0.05wt% or less of Sb and Ca does not deteriorate BH properties or deep drawability, so these can be added according to the purpose of each addition. The manufacturing process for cold-rolled steel sheets with the above-mentioned composition is not particularly regulated, but for example, steel melted in a converter or electric furnace may be turned into a slab or sheet bar by an ingot-blooming method or a continuous casting method. , a method of forming a cold-rolled sheet by conventional hot rolling and cold rolling is adopted. Recrystallization annealing may be either box annealing or continuous annealing. In this annealing, annealing conditions are selected to obtain a desired aging index. (Example) Steel having the composition shown in Table 1 was melted in a converter, and after being subjected to vacuum degassing treatment, it was made into a slab by continuous casting. These slabs were subjected to conventional hot rolling and cold rolling to obtain cold rolled plates with a thickness of 0.8 mm, and then continuous annealing was performed. In continuous annealing, the annealing temperature and cooling rate were varied to obtain the desired aging index. Thereafter, temper rolling was performed in accordance with each aging index and target material quality. Table 2 shows the results of examining the mechanical properties of each product thus obtained. Table 1 shows the aging index AI and the bake hardening amount BH, but these are for a temper rolling elongation of 2%. (AI and
There is almost no effect of temper rolling elongation on BH. ) Here, all mechanical properties were measured using JIS No. 5 test pieces. In each of the above embodiments of the present invention,
After 2 months at 30°C, YEl was 0% in all cases, confirming non-aging property at room temperature, and yielding steel sheets for low temperature baking with excellent deep drawability.

【table】

【table】

【table】

[Table] (Effects of the Invention) As explained above, according to the present invention, it is possible to easily produce a cold-rolled steel sheet for deep drawing that is non-aging at room temperature and can be baked at low temperatures.

[Brief explanation of the drawing]

Fig. 1 is a graph showing the aging index and the baking temperature dependence of BH amount, Fig. 2 is a graph showing the relationship between the aging index and skin pass rolling elongation rate and the influence of steel sheet roughness to obtain room temperature aging properties. FIG. 3 is a graph showing the relationship between yield elongation and roll diameter, and FIG. 4 is a graph showing the relationship between temper rolling elongation and strength increase.

Claims (1)

[Claims] 1 C: 0.001 to 0.008wt%, Si≦0.5wt%, Mn≦
0.5wt%, P≦0.15wt%, Al: 0.005-0.10wt%,
Contains S≦0.010wt%, N≦0.005wt%, and
One or two of Ti and Nb below 0.05wt%
A steel containing seeds with the remainder essentially having a composition of Fe is hot-rolled, cold-rolled and heat-treated, and the aging index ^exceeds 4 Kgf/mm2 by selecting the heating temperature and cooling conditions in the heat treatment. Baking with excellent deep drawability consists of adjusting the elongation to a range of up to 8 Kgf/ ^mm2 , and applying temper rolling to an elongation rate of 2 to 5% on the cold-rolled sheet after this heat treatment to suppress room temperature aging. Method for manufacturing hardenable steel plate.