JPS6338525A - Manufacture of cold-rolled steel sheet excellent in press formability - Google Patents

Abstract

PURPOSE:To manufacture a cold-rolled steel sheet excellent in deep drawability and bulge formability, by subjecting a steel slab in which composition ratio is specified to hot rolling at a temp. lower than heretofore and also by specifying a heating velocity in annealing after cold rolling so as to reduce the axial ratio of a crystalline grain. CONSTITUTION:The steel slab consisting of, by weight ratio, >0.010-0.035% C, 0.08-0.40% Mn, 0.005-0.030% P, 0.025-0.080% SolAl, 0.0020-0.0060% N, and the balance Fe with inevitable impurities and containing, if necessary, <=0.070% Cr is prepared. This steel slab is heated, is its state of hot slab or in the state of cold slab, to 1,000-1,150 deg.C and subjected to hot finish rolling at 820 deg.C, which then is wound up at <=700 deg.C and cold-rolled. Subsequently, this cold-rolled sheet is heated slowly through the temp. range from at least 400 deg.C to 550 deg.C at <=30 deg.C/hr heating rate and then is subjected to recrystallization annealing at a temp. between the recrystallization temp. and 800 deg.C. In this way, the cold-rolled steel sheet having excellent resistance to surface roughness can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a cold-rolled steel sheet having excellent comprehensive press formability including deep drawability, stretchability, and roughening resistance.

(Conventional technology) A cold-rolled steel sheet with excellent deep drawability and stretchability has traditionally been used.
l-killed steel plate has come into use. The manufacturing technology is disclosed in, for example, JP-A-59-13030 and JP-A-60-221.
As disclosed in Japanese Patent No. 526, the precipitation of AβN is prevented by coiling l-killed steel at a low temperature during hot rolling, and A7! due to the combination of A2 and N during the heating process of box annealing. The feature is that deep drawability is improved by synchronizing H precipitation 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 may lack ductility, and C
Efforts have been made to improve ductility by reducing the amount, but due to the coarsening of the crystal grains, a rough texture known as ``orange beer'' occurs 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 reducing the heating temperature of the steel billet during hot rolling to a low temperature of 1000 to 1150°C. Furthermore, due to the synergistic effect of setting the heating rate from at least 400 degrees to 550 degrees in the batch annealing after cold rolling to 30 degrees C/hr or less, the crystal structure changes from the conventional expanded grain to the equiaxed direction ( It has been newly discovered that it is possible to produce a cold-rolled steel sheet with excellent overall press formability, which does not produce orange beer after press working, and which has excellent deep drawability and stretchability.

The gist of the present invention is that C: more than 0.010 to 0
．． 035%, Mn: 0.08-0.40%, P:
0.005-0.030%, Sol, Aj! : 0
．． More than 0.025 to 0.080%, N: 0.0020 to 0.025 to 0.080%, N: 0.0020 to 0.
A steel slab containing 0Q60% and, if necessary, Cr; 0.070% or less, with the balance consisting of iron and unavoidable impurities, is heated as a hot slab or after being made into a cold slab.
Heating to 1150°C and hot finish rolling at 820°C or higher,
After winding and cold rolling at 700°C or less, heating at a temperature range of at least 400°C to 550°C at a heating rate of 30°C/hr or less, and annealing at a temperature of at least recrystallization temperature and 800°C or less. The present invention is characterized by a method for producing a cold-rolled steel sheet having an average crystal grain axial ratio of 1.6 or less and excellent press formability. In the present invention, the f value representing deep drawability is targeted to be 1.6 or more, and the elongation is targeted 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 beer may occur.

The preferred range is 0.015-0.025%.

Mn requires 0.08% or more to prevent embrittlement during hot rolling, but if it exceeds 0.40%, press formability deteriorates. 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 o, oos%, and the upper limit is set to 0.030. %.

5oji! , AA is 0.025% to ensure deep drawability.
Although it is necessary to have an amount exceeding 0.0%, too much amount will have the opposite effect, so the upper limit is set to o, oao%. The preferred range is 0.040~
It is 0.060%.

Like A1, N is 0.0020% to ensure deep drawability.
However, if it exceeds 0.0060%, the steel plate will harden and the stretchability will decrease. More than 0.0030 to 0.0050
% is preferred.

In the present invention, the above Soj! , AjLN within the range of 5a11. The value of Al (%) x N (%) is 1.5 x 1
A good t value can be obtained by regulating it to 0-' or more.

The basic components of the present invention are as described above, and within the above range of components, addition of Cr is effective in further lowering the axial ratio of crystal grains and preventing rough skin. However, Cr
If the amount is too large, the deep drawability deteriorates, so the upper limit of its addition is set at 0.070%.

In the present invention, after being melted in a converter or the like, it is made into a steel billet by ingot blooming or continuous casting.

One piece is heated as a red-hot piece, or after it has been cooled to near room temperature and becomes a cold piece, it has a temperature of 1000 to 115
Heated to 0℃, finish rolled at 820℃ or higher, 700℃
Can be wound at temperatures below ℃.

If the heating temperature of the steel slab is less than 1000°C, the finishing temperature will drop too much and the deep drawability will deteriorate. On the other hand, if the temperature is too high, the reduction in the axial ratio targeted by the present invention will not be achieved and the overhang properties will deteriorate, so the upper limit is set at 1150°C.

The finishing temperature needs to be 820°C or higher to ensure deep drawability. When the winding temperature exceeds 700°C, deep drawability deteriorates.

The hot rolled coil is cold rolled after descaling,
Recrystallized and annealed. Cold rolling may be carried out at a normal cold rolling rate, but in order to improve deep drawability, a high rolling rate of 70 to 90% is preferable. In subsequent annealing, at least 40
Gradual heating is performed in a temperature range of 0°C to 550°C at a heating rate of 30°C/hr or less. In order to improve deep drawability while also having stretchability, it is very important to maintain a heating rate within the above minimum temperature range, preferably 400°C or higher to the soaking temperature. Deep drawability and stretchability deteriorate. Slow heating at 15°C/hr is even 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 orange peel will occur, so the upper limit is set at 800°C. The axial ratio of the crystal grains is set to 1.6 or less since orange peel tends 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 a steel billet, then the same hot rolling conditions as shown in Table 1 are used to make the plate thickness 3.61, and then after pickling, the plate thickness is 0.8 The fins were cold-rolled.Furthermore, as shown in Table 2, the specified temperature range was heated at the specified heating rate, and 6
Batch annealing was performed for hours. Then, after 0.8% temper rolling, the mechanical properties were investigated. In addition, the roughness of the steel plate after 30% tensile prestrain was also investigated.

The survey results are shown in Table 2. In case 5 obtained 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 (?hl-1゜1-2.2, 3.
4) has excellent mechanical properties, has a small axial ratio, and does not cause rough skin, and fully satisfies the object of the present invention. ml-3 has a low t value due to its fast 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 (2)

[Claims] (1) In weight%, C: more than 0.010 to 0.035%, M
n: 0.08-0.40%, P: 0.005-0.03
0%, Sol. Al: 0.025-0.080%, N:
A steel piece containing 0.0020 to 0.0060% and the remainder consisting of iron and unavoidable impurities is heated to 1000 to 1150°C as a hot piece or after being made into a cold piece, and heated to 82
After hot finish rolling at 0°C or higher, coiling at 700°C or lower, and then cold rolling, at least 400°C or higher and 550°C
℃ or less at a heating rate of 30℃/hr or less, and annealing at a temperature higher than the recrystallization temperature and lower than 800℃, and the axial ratio of crystal grains is 1.6 or less on average. A method for producing cold-rolled steel sheets with excellent formability. (2) In weight%, C: more than 0.010 to 0.035%, M
n: 0.08-0.40%, P: 0.005-0.03
0%, Sol. Al: 0.025-0.080%, N:
A steel billet containing 0.0020 to 0.0060%, Cr: 0.070% or less, and the balance consisting of iron and inevitable impurities is heated as a hot billet or after being made into a cold billet.
Heating to 1150°C and hot finish rolling at 820°C or higher,
After winding at 700℃ or less and then cold rolling, the temperature range of at least 400℃ or more and 550℃ or less is 30℃/30℃.
Heating at a heating rate of hr or less to 800℃ above the recrystallization temperature
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 comprises annealing at a temperature of: