JPS6386819A - Production of cold rolled steel sheet for deep drawing - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet having excellent mechanical properties together with deep drawability by subjecting a specifically composed low-C steel to lubricative hot rolling at a specified temp. region, strain rate and draft and subjecting the rolled steel to coiling, pickling and cold working, then to recrystallization annealing. CONSTITUTION:The steel consisting of the compsn. contg. <=0.05wt% C, 0.005-0.10% Si, 0.01-0.3% Mn, 0.001-0.05% P, <=0.020% S, <=0.10% Al, and <=0.005% N, and contg. >=1 kinds among <=0.08% Ti, <=0.05% Nb, and <=0.003% B. The steel is subjected to at least one pass of the lubricative hot rolling under the conditions of the Ar3 transformation point - 600 deg.C range, 500(s<-1>) strain rate and >=35% draft. After the rolled steel is subjected to coiling and pickling, the steel is subjected to the cold working at 50-95% draft. The steel is successively subjected to the recrystallization annealing. The cold rolled steel sheet suitable for an automobile panel material, etc., having excellent deep drawability is obtd. by the above-mentioned method.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for producing cold-rolled steel sheets for deep drawing, and in particular to production of cold-rolled steel sheets exhibiting excellent deep-drawing properties for use as panel materials for automobiles. This is a proposal regarding technology.

(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 steel sheets require high ductility and high Lankford value (T 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 been increasing.

For example, due to the complex shape of automobile oil pans, they are actually completed by welding.
There is a strong demand from automobile manufacturers for integrated molding. on the other hand,
Car designs are becoming more complex to meet diversifying needs, and as a result, more and more parts are difficult to form using conventional steel sheets.

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 for improving deep drawing housing. By the way, the deep drawability of a steel sheet is closely related to its texture, and the more grains there are in the (222) orientation, the more grains in the (20
0) It is known that the smaller the number of oriented grains, the higher the value obtained. Conventional methods for obtaining this high T value include, for example,
Special Publication No. Sho 44-17268, Special Publication No. Sho 44-1726
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. 9 and Japanese Patent Publication No. 44-17270.

According to this two-step cold rolling method, the final product has many grains with (222+ orientation) and few grains with (200) orientation. Compared to the steel sheet, the (222) oriented grains increase, while the (200) oriented grains decrease, so when cold rolling and annealing are performed again, the (2
This is because while the 22) oriented grains further increase, the (200) oriented grains further decrease. Therefore,
Steel plates with high T values can be manufactured.

On the other hand, in JP-A-56-62926, C:
0.008wtχ (The following is simply expressed as "%")
, Si: 0.57%, Mn: 0.35%, Al
: 0.43% and Nb: 0.061%, after normal hot rolling and cold rolling, box annealing at 950'C for 1 hour results in an extremely high T value of r = 4.73. We are proposing a technology to obtain this.

(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 There was a problem in that the process had to be performed one more time, which required enormous amounts of energy, personnel, and costs.

In addition, the latter of the above conventional techniques has a metamorphosis set m
Because it utilizes the weave formation mechanism, the crystal annealing temperature can be reduced to A.
3 transformation point or higher, and therefore, compared with recrystallization annealing below the A3 transformation point, there is an increase in energy cost and equipment and technical difficulties due to high temperature annealing. Furthermore, it is necessary to add a large amount of Si or AI, which causes the problem 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) In order to solve the problems of the above-mentioned conventional technology, the present invention provides C: 0.05 wt% or less and Si: 0.005.
~0.10wt%Mn: 0.01~0.3wtχ
, P: 0.001-0.05wtχ, S: 0.
0.020 wt% or less, Al: 0.10 wt% or less, N: 0.005 wt% or less, and Ti
: 0.08 wt% or less, Nb: 0.05 wt% or less, and B: 0.003 wt% or less, as necessary, and the remainder is substantially Fe.
A rope consisting of at least one bath at Ar3 metamorphosis point ~ 60
At a temperature range of 0°C, strain rate: 500 (s-1)
As mentioned above, hot rolling with lubrication is performed at a rolling reduction ratio of 35% or more to obtain a hot-rolled sheet of a predetermined thickness, this hot-rolled sheet is wound up at 600°C or higher, and after passing through a pickling process, a rolling reduction ratio of 50~ A method for producing a cold-rolled steel sheet for deep drawing, which is characterized by subjecting it to 95% cold rolling 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 1 (wt’, 1) Table 1 shows the sample materials (ultra-low carbon steel) used in the research.

A copper piece of this test steel was heated to 700℃ - After soaking, 1 pass 4
Lubricated and non-lubricated hot rolling was performed at rolling reductions of 0% and 30%, and winding treatment was performed at 650°C for 1 hour. After cold rolling the hot-rolled sheet with a rolling reduction of 75%,
Continuous annealing was performed at -40 sec. The relationship between the strain rate, rolling reduction, presence or absence of lubrication, and the lower value at this time is shown in FIG.

As you can see from this figure, the strain rate is 500 (s-1)
An improvement in the lower value was observed in the above cases, and the rolling reduction ratio was 40% (better) than 30%, and lubricated rolling was 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 reason why the materials and manufacturing conditions are limited as in the above-mentioned solution method will be explained in order. do.

(Regarding the 11fi1 composition, the steel composition is important in the present invention, c: o, os% or less, Si: 0.005 to 0
．． 10%, Mn: 0.01-0.3%, P: 0.
001-0.05%, S: 0.020% or less, A
Contains L: 0.10% or less and N: 0.005% or less, and if necessary, Ti: 0.08% or less, Nb: 0.05% or less, B: 0.003% or less, depending on the type of work or It is necessary to contain two or more kinds, with the remainder consisting of Fe and unavoidable impurities.

(a) C50.05% C is preferable because the less it is, the better the deep drawability of the cold-rolled sheet is, but when it exceeds 0.05%, it has a negative effect on the deep drawability.

(b) 0.005%≦Si≦0.10%Si is an effective element for forming a set "mm" shadow, which is preferable for the strength or deep drawability of steel sheets.If its content is less than o, oos%, the desired effect is not achieved. On the other hand, if the content exceeds 0.10%, the surface properties of the steel sheet will deteriorate and the ductility will also decrease.
, oos%≦Si≦0.10%.

fc) 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%, the ductility decreases;
It was set as 1%≦Mn≦0.3%.

(dlo, 001%≦P≦0.05% P has the effect of strengthening the rope. Its content is 0.00%
If the content is less than 1%, the desired effect cannot be obtained, while if the content exceeds 0.05%, the ductility deteriorates, so 0.001%
It was set as ≦P≦0.05%.

(aSS≦0.020% The smaller the amount of S, the more advantageous it is for deep drawability.

If the content is less than 0.020%, it will not adversely affect deep drawability, so it was set as S≦0.020%.

(fl A I≦0.10% A1 is added to perform deoxidation, but if it is contained in an amount exceeding 0.10%, it will exhibit a coarse grain structure, cause roughness during press processing, and further deteriorate the surface texture. also deteriorates.

(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 the deep drawability, so it was set as N≦0.005%.

(hlTi, 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, their content is 0.08% each.

If it exceeds 0.05% or 0.003%, the ductility deteriorates, so TiS2.08%, NbS2.05%, B≦0
．． It was set as 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 piece is suitably 800 to 1250°C, particularly preferably 950 to 1150°C. It is also applicable to those obtained by 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 5011 or less from molten steel (sheet bar caster method, strip caster method) also has great economic advantages from the viewpoint of energy saving and process saving, and is particularly useful as a means of manufacturing the rolled material of this invention. It is.

(3) Hot rolling is the most important step in this invention, and during finish rolling, rolling is performed at least one pass in the temperature range of Ar3 transformation point to 600°C and at a strain rate of 500 (s to I) or more. Rate: After lubricated rolling of 35(X) or more, 600
It is necessary to wind it up at a temperature above ℃.

In a high temperature range where the rolling temperature is higher than the Ar4 transformation point, even if high strain rate and large reduction rolling is performed, the crystal orientation will be randomized due to r-α transformation and a (222) orientation, which is advantageous for drawability, will be formed. is not possible. On the other hand, when rolled at 600° C. or lower, it is impossible to wind up at 600° C. or higher, so the steel sheet exhibits a processed Mi weave, and self-annealing cannot occur during winding.

The number of rolling passes and the distribution of the rolling reduction ratio may be arbitrary as long as the condition is satisfied that the product can be finished with lubrication in at least one pass, and of course the arrangement structure of the rolling mill, roll diameter, tension, etc. are not particularly limited.

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. 6
If the winding temperature is less than 00°C, recrystallization will not proceed during self-annealing during winding, which is disadvantageous for deep drawability (2
00) orientation grains remain.

(4) About cold rolling This step 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 A cold rolled steel strip that has undergone a cold rolling process requires 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.

The heating temperature ranges from the recrystallization temperature (approximately 600°C) to 950°C. In addition, 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. It is effective to improve the material quality, especially the ductility.

After annealing, tPl award rolling of 10% or less may be added to the belt 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 zinc plating (including alloy-based plating), 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 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 copper plate is wound up at 600° C. or higher, recrystallization is completed by self-annealing. Therefore, high strain rate, large reduction rolling, winding and self-annealing alone have the same effect as performing 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 having a thickness of 3.2 mm using a finish rolling mill consisting of 7 stands. At this time, lubricated rolling was performed using the rolling mills of all stands, and high strain rate and large reduction rolling was performed using the rolling mill of the last stand, followed by winding at varying temperatures.

After continuing pickling, the plate thickness is 0.8 t* (cold rolling reduction ratio near 5%) or 1.7111 plate thickness (cold rolling reduction ratio = 47% -
A cold rolled sheet of Comparative Example) was obtained, and then recrystallization annealing (annealing temperature near 60 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. Tensile properties are JIS
It was determined using a No. S test piece, and was determined as the average value in three directions: L (rolling direction), C (rolling direction 90"), and D (rolling direction 45"). Also, the T value was determined with 15% tensile prestrain. After that, L and C were measured using the 3-point method.

It was determined as an 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 T value and ductility than the comparative example.

(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 performing high strain rate and large reduction rolling, deep drawability is significantly superior to that of the conventional method. Cold-rolled steel sheets with excellent mechanical properties as well as other properties can be manufactured at low cost.

[Brief explanation of the drawing]

Figure 1 shows the “strain rate”, “reduction rate”, and
It is a graph showing the influence of "lubrication".

Claims (1)

[Claims] 1, C: 0.05 wt% or less, Si: 0.005 to 0.
10wt%Mn: 0.01-0.3wt%, P: 0.0
01-0.05wt%S: 0.020wt% or less, Al
: 0.10 wt% or less, N: 0.005 wt% or less, Ti: 0.08 wt% or less, Nb: 0.
0.05wt% or less and B: 0.003wt% or less as necessary, and the remainder substantially consists of Fe, at least one pass at a temperature between the Ar_3 transformation point and 600°C. Strain rate: 500 (s
^-^ 1) Lubricated hot rolling is carried out at a rolling reduction rate of 35% or more to obtain a hot-rolled sheet of a predetermined thickness, and this hot-rolled sheet is heated at 600℃.
After winding up with the above, and then going through the pickling process, rolling reduction ratio: 50
A method for producing a cold-rolled steel sheet for deep drawing, characterized by subjecting it to ~95% cold rolling and then recrystallization annealing.