JPH0241568B2 - - 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 excellent comprehensive press formability including deep drawability, stretchability, and roughening resistance. (Prior Art) Al-killed steel sheets have traditionally been used as cold-rolled steel sheets with excellent deep drawability and stretchability. The manufacturing technology is known as Al
Killed steel is hot-rolled and coiled at a low temperature.
Preventing AlN precipitation and during the heating process of box annealing.
The feature is that deep drawability is improved by synchronizing the precipitation of AlN due to the combination of Al and N with recovery and recrystallization. The crystal structure of the steel sheet thus obtained has a form that is elongated in the rolling direction, that is, a so-called expanded grain, and has an axial ratio of 1.8 or more. (Problem to be solved by the invention) However, in recent years in the automobile industry, there has been a shift from the conventional forming method mainly based on deep drawing to a method mainly based on overhanging, and there has also been a shift in processing methods aimed at reducing process steps. Integral molding of high-quality parts is also actively progressing. With the development of such press forming methods, there has been a demand for improved stretchability while maintaining the conventional deep drawability for applied steel sheets. In response to these demands, cold-rolled steel sheets for deep drawing manufactured according to conventional methods sometimes lack ductility, and attempts have been made to improve ductility by reducing the amount of C, etc. However, due to the coarsening of crystal grains, the skin becomes rough, which is called "orange peel", and appearance defects frequently occur.
Therefore, a cold-rolled steel sheet with excellent overall press formability that is compatible with recent new forming methods is desired. (Means for Solving the Problems) The present invention has been made to improve the problems of the prior art described above, and involves specifying the steel components and setting the heating temperature of the steel billet during hot rolling to a low temperature of 1000 to 1150°C. , and at least 400 in batch annealing after cold rolling.
The synergistic effect of heating the heating rate from 30°C to 550°C at 30°C/hr or less transforms the crystal structure from the conventional drawn grain to an equiaxed grain direction (axis ratio 1.6 or less), and improves deep drawability. We have newly discovered that it is possible to produce a cold-rolled steel sheet with excellent overall press formability that does not cause orange peel after press working and has excellent stretchability. The gist of the present invention is that C: more than 0.010
0.035%, Mn: 0.08~0.40%, P: 0.005~0.030
%, Sol.Al: over 0.025 ~ 0.080%, N: 0.0020 ~
A steel piece containing 0.0060% and Cr; 0.070% or less, with the remainder consisting of iron and unavoidable impurities, is heated as a hot piece or after being made into a cold piece, heated to 100 to 1150℃, and heated at 820℃ or higher. After finish rolling, winding at 700℃ or less, and cold rolling, at least 400℃
A press with an average axial ratio of crystal grains of 1.6 or less, characterized by heating at a heating temperature of 30℃/hr or less in a temperature range of 550℃ or less, and annealing at a temperature of 800℃ or higher than the recrystallization temperature. A method for producing cold-rolled steel sheets with excellent formability. In the present invention, the values representing deep drawability are targeted to be 1.6 or more and the elongation to be 46% or more. The present invention will be explained in detail below. When C exceeds 0.035%, deep drawability and stretchability deteriorate. On the other hand, if it is less than 0.010%, press formability will improve, but the in-plane anisotropy of the steel sheet will increase, and there is a concern that orange peel will occur.
The preferred range is 0.015-0.025%. Mn is 0.08 to prevent embrittlement during hot rolling.
% or more, but if it exceeds 0.40%, press formability will deteriorate. 0.15-0.25% is preferred. The smaller the amount of P, the better for extensibility, but a certain amount of P is preferable for lowering the axial ratio of crystal grains and for deep drawability, so in order to achieve a good balance between the two, the lower limit is set to 0.005% and the upper limit is set to 0.030%. Sol.Al needs to exceed 0.025% to ensure deep drawability, but too much will have the opposite effect, so the upper limit is set at 0.080%. The preferred range is 0.040-0.060%
It is. Like Al, N is 0.0020% to ensure deep drawability.
However, if it exceeds 0.0060%, the steel plate will harden and its stretchability will decrease. More than 0.0030 to 0.0050% is preferred. In the present invention, within the range of Sol.Al and N mentioned above,
A good value can be obtained by regulating the value of Sol.Al (%) x N (%) to 1.5 x 10 ^-4 or more. Cr is an effective component that lowers the axial ratio of crystal grains and prevents rough skin, but if its content is too large, deep drawability deteriorates, so the upper limit is set at 0.070%. The chemical composition of the present invention is as described above, with the remainder consisting of iron and inevitable impurities. In the present invention, after being melted in a converter or the like,
It is made into steel billet by ingot blooming or continuous casting. The steel billet may remain in a red-hot state, or after it has been cooled down to near room temperature and becomes a cold billet.
It is heated to 1000-1150℃, finish rolled at 820℃ or higher, and wound up at 700℃ or lower. If the heating temperature of the steel billet is less than 1000°C, the finishing temperature will drop too much and the deep drawability will deteriorate. On the other hand, if it is too high, the reduction in the axial ratio that is the goal of the present invention cannot be achieved;
The upper limit is set at 1150°C since the overhang property deteriorates. The finishing temperature must be 820°C or higher to ensure deep drawability. If the winding temperature exceeds 700°C, deep drawability will deteriorate. After descaling, the hot rolled coil is cold rolled and recrystallized annealed. Cold rolling can be carried out within the normal cold rolling rate range, but in order to improve deep drawability,
70-90%, preferably under high pressure. In the next annealing, gradual heating is performed at a temperature range of at least 400°C or more and 550°C or less at a heating rate of 30°C/hr or less. In order to have good deep drawability as well as stretchability, it is very important to have a heating rate within the above minimum temperature range, preferably 400℃ or higher, up to the soaking temperature.
At heating rates exceeding ℃/hr, deep drawability and stretchability deteriorate. Slow heating at 15°C/hr is more desirable. The annealing temperature must be higher than the recrystallization temperature, but if it is too high, the crystal grains will grow too much and cause orange peel, so the upper limit is set at 800°C. The axial ratio of the crystal grains should be 1.6 or less since orange peel is likely to occur if it exceeds 1.6. The present invention can be applied not only to cold-rolled steel sheets but also to a method for producing original sheets for surface treatments such as galvanizing and tin plating. (Example) Steel having the components shown in Table 1 is melted and made into steel slabs, then hot-rolled to a thickness of 3.6 mm under the same hot-rolling conditions shown in Table 1, and then cold-rolled to a thickness of 0.8 mm after pickling. did. Further, as shown in Table 2, batch annealing was performed at 700°C for 6 hours by heating in a specific temperature range at a specified heating rate.
The mechanical properties were then investigated after 0.8% temper rolling. We also investigated the surface roughness of the steel plate after 30% tensile prestrain. The results of those investigations are shown in Table 2. In No. 5 produced by the conventional method, the axial ratio of the crystal grains was too large, resulting in rough skin. On the other hand, the method according to the present invention has excellent mechanical properties, has a small axial ratio, and does not cause rough skin, and fully satisfies the object of the present invention.

【table】

[Table] ○ marks are based on the method of the present invention.
The number in parentheses of the heating rate indicates the temperature range of the specified heating rate.
(Effects of the Invention) As is clear from the above examples, the method of the present invention has excellent deep drawability and stretchability that can be adapted to the molding styles that have been changing in the recent automobile industry, and has a low temperature resistance that does not cause rough skin. It is of great industrial significance because it not only allows us to provide rolled steel sheets, but also from the perspective of manufacturing technology, it allows us to enjoy energy-saving effects when heating steel slabs.

Claims (1)

[Claims] 1 In weight%, C: more than 0.010% to 0.035% Mn: 0.08 to 0.40% P: 0.005 to 0.030% Sol.Al: more than 0.025% to 0.080% N: 0.0020 to 0.0060% Cr: 0.070 % or less, with the remainder consisting of iron and unavoidable impurities.
Heated to 1150℃, hot finish rolled at 820℃ or higher,
After being coiled at 700°C or less and then cold rolled, the temperature range of at least 400°C or more and 550°C or less is 30°C/30°C.
Heating at a heating rate below hr and above the recrystallization temperature 800
A method for producing a cold-rolled steel sheet having excellent press formability and having an average crystal grain axial ratio of 1.6 or less, which is characterized by annealing at a temperature of ℃ or lower.