JPH02166259A - Cold rolled steel sheet excellent in workability and surface characteristic after working - Google Patents

Abstract

PURPOSE:To provide a cold rolled steel sheet excellent in workability, such as press formability, and free from deterioration in surface characteristics even after subjected to working of this kind by controlling compositional limit values. CONSTITUTION:A cold rolled steel sheet having a composition consisting of, by weight, <=0.01% C, <=0.1% Si, <=1.0% Mn, <=0.5% Al, <=0.01% N, 0.0001-0.01% Be, 0.0005-0.1% Ti, and the balance Fe with inevitable impurities is prepared. If necessary, one or more kinds among 0.0005-0.03% Nb, 0.0005-0.03% V, 0.0005-0.01% Mo, 0.0005-0.50% Cr, 0.0010-0.10% Cu, 0.0010-0.10% Sb, and 0.0001-0.01% B are further added to the above composition.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is particularly suitable for processing such as bending, press forming, drawing, deep drawing, or ultra-deep drawing, and is suitable for processing such as bending, press forming, drawing, deep drawing, ultra-deep drawing, etc. This relates to rolled steel plates.

(Prior Art) Cold-rolled steel sheets have been widely used for processing such as press forming and deep drawing due to their excellent workability. However, in recent years, various properties other than workability have come to be required due to its usefulness. For example, when cold-rolled steel plates are used as exterior panel materials, not only is the surface required to be beautiful, but at the same time, the surface is also required to have surface irregularities resulting from processing, which often results in a deterioration of the steel plate's appearance. . As a document related to this point, for example, JP-A-62-151208 proposes a manufacturing technique for cold-rolled steel sheets that employs a surface roughness pattern with excellent appearance and workability.

(Problems to be Solved by the Invention) However, in the technology disclosed in the above publication, when the shape of a press-formed product is complex, the processing degree and processing mode of each part are often extremely different, and this affects the surface of each part. The disadvantage was that the properties were significantly different.

In order to prevent surface unevenness due to processing, it is not enough to improve the surface properties alone; it is necessary to improve the workability of the steel sheet itself and to suppress surface deterioration due to processing as much as possible. There were none so far.

It is an object of the present invention to propose a new cold-rolled steel sheet that has excellent workability in press forming and the like and does not cause deterioration in surface quality even after such processing.

(Means for Solving the Problems) According to the present invention, C: O, Oht% (hereinafter simply expressed as %) or less, Si: 0.1% or less, Mn: 1.0% or less, A
L: 0.5% or less, N: 0.01% or less, Be:
0.0001-0.01% and Ti: 0.00
This invention is a cold-rolled steel sheet with excellent workability and surface quality after processing, which contains 05 to 0.1% of C and the balance is Fe and unavoidable impurities.
: 0.1% or less, Mn: 1.0% or less, AI:
0.5% or less, N: 0.01% or less, Be: 0
．． 0001-0.01% and Ti: 0.0005
~0.1%, further Nb: 0.0005~
0.03%, V: 0.0005-0.03%, Mo
: O,0O05~0.01%, Cr: 0.00
05-0.50%, Cu: 0.0010-0.10%,
Sb: 0.0010-0.10% and B: 0.0
The cold rolled steel sheet contains at least one of the following: 0.001 to 0.01%, with the remainder being Fe and unavoidable impurities, and has excellent workability and surface quality after working.

(Function) The ideal crystal structure of a cold-rolled steel sheet for processing should be uniform, whether the grain size is fine (excellent in strength, toughness, etc.) or relatively coarse (excellent in ductility, etc.). This was an important point.

In other words, under the conventional technology, a non-uniform crystal structure, especially a mixed grain structure, has poor uniformity on the surface of a steel sheet, causes surface defects such as surface roughness, and is easily damaged locally during processing.
Furthermore, the texture (crystal orientation) was also thought to be inferior.

However, according to the findings of the inventors, the above-mentioned defects are mainly caused by the generation of extremely coarse recrystallized grains during co-grain formation, and the co-grain structure itself is not the fundamental defect. In addition, it was found that in some cases, the component elements that cause mixed grains simply deteriorate the material directly. Considering these points, the mixed grain structure itself is rather advantageous in maintaining good surface properties of the steel sheet after processing, and may be able to maintain uniformity of the surface of the steel sheet against uneven processing. Based on the idea,
First, we investigated methods for obtaining an ideal mixed grain structure.

Here, the mixed grain structure is obtained mainly by growing specific grains during recrystallization, or conversely by suppressing the growth of grains other than specific grains. The former can be achieved mainly by removing factors that inhibit grain boundary movement, such as reducing solid solution C1N, and removing or coarsening fine precipitates.
As a result, extremely coarse recrystallized grains are inevitably produced, which significantly deteriorates workability. On the other hand, the latter is difficult to achieve using precipitate distribution, and therefore it is more promising to use solid solution elements that have a strong tendency to segregate at grain boundaries. However, when the solid solution elements are uniformly distributed, the crystal structure tends to be uniformly atomized, and when they are unevenly distributed, the distribution itself tends to cause deterioration of the material quality of the seeds. In particular, most of the elements that are effective in adjusting the particle size are harmful to processability, such as BlP, and the use of such elements is disadvantageous.

In this respect, addition of an appropriate amount of Be is extremely effective in overcoming this situation, that is, in obtaining a fine and appropriate mixed grain crystal structure without adversely affecting the workability of the steel sheet.

FIG. 1 shows the results of an investigation on the elongation and roughening properties of Be-added steel sheets, along with the properties of ordinary cold-rolled steel sheets without Be added. Note that the Be-added steel sheet used here was C:
0.0015-0.0035%, Si: 0.01%,
MnO, 1%, Al:o, 03 ~ 0.07%, N
: 0.002 to 0.005%, Ti: 0.02
~0.05%, Be: O, OO1~0.005%, and the steel sheet without Be added contains the same components as above except for Be, and the steel sheet winding temperature after hot rolling is 550 ° C. ~720°C1 continuous annealing temperature 750°C
-890'C to obtain structures with different grain sizes. From FIG. 1, it can be seen that the Be-added steel sheet has excellent elongation characteristics even with the same grain size, and although it is undesirable for roughness to occur during processing, the elongation until roughness occurs is also large. That is, it can be seen that the Be-added steel sheet not only has excellent surface properties after working, but also exhibits superior properties in terms of workability compared to the comparative copper sheet.

Here, Be is considered to be an element that tends to segregate at grain boundaries;
Since its segregation effect is relatively weak, it becomes a fine mixed grain structure with a wide grain size distribution, and this is considered to be a major factor in obtaining a material with excellent surface texture and workability. In addition, Be segregates at grain boundaries to suppress the embrittlement of processed materials, and it also concentrates on the surface of steel sheets to suppress surface defects such as roughness during processing, and is also an important factor during processing. It is thought to be effective in improving the characteristics of sliding properties and galling resistance, as well as strengthening the heat affected zone during welding of steel plates.

It has been said that the larger the grain size, the better the elongation of steel sheets, but this is actually mainly controlled by relatively large crystal grains, and is therefore strongly influenced by the average grain size as well as the degree of mixed grains. Ru. On the other hand, surface defects such as roughness due to processing are considered to be caused by reducing the average grain size rather than by making the grain size uniform or non-uniform unless coarse grains are present as described above. Of course, for the same average grain size, the larger the mixed grain size, the larger the maximum grain size, which is disadvantageous, but in a fine mixed grain structure, the fine crystal grains prevent the concentration of surface micro defects due to the rotation of larger grains. This is thought to be offset by the protective effect.

Next, the reason for limiting the component composition will be described.

Be: Be is the most important element in this invention. Be is an element that inherently has a tendency to segregate at grain boundaries, and if the amount added exceeds 0.01%, it will cause a significant deterioration in workability, while if it is less than 0.0001%, no effect can be expected. Therefore, the amount of Be added was set in the range of 0.0001 to 0.01%.

In order to obtain good elongation and r-value, CTC must be an extremely low carbon material that is lower than conventional low carbon materials.

In particular, if it exceeds 0.01%, good deep drawability cannot be obtained even if the amount of Ti added is increased, as will be described later.

Therefore, the amount of C added was set to 0.01% or less. In order to obtain excellent processability, the content is preferably 0.005% or less.

Si: If the amount of Si added exceeds 0.1%, it deteriorates the elongation and drawability of the steel sheet, so it was set to 0.1% or less.

Mn: Like Si, excessive addition of Mn deteriorates the elongation and drawability of the steel sheet, so the content was set to 1.0% or less.

Al: Some amount of Al remains in the steel for deoxidation, etc., but addition of more than 0.5% has a negative effect on the surface quality, so it must be kept below 0.5%. Preferably 0.
1% or less is preferable.

Since NUN deteriorates deep drawability, it should be kept at 0.01% or less. Preferably it is 0.005% or less.

Ti: Ti has a great effect on improving material quality by forming TiS, TiN, TiC, etc., and is used in this invention to help obtain good workability. For this purpose, it is necessary to add a small amount of Ti (both 0.0005%).

On the other hand, the effect is saturated at about 0.1%, and adding more than that leads to deterioration of the surface properties of the steel sheet and is also disadvantageous in terms of cost. Therefore, the amount of Ti added is 0.0005 to 0.1
% range. The optimum amount of Ti added is SXN
Although it depends on the amount of C, it is approximately 0.01 to 0.05%, and in particular, Ti is added in atomic ratio to prevent Be from becoming nitride.
It is desirable that /N≧1.

In addition, in this invention, S, P as general components in steel
Although there is no need to specifically limit the amount of S and P, it is desirable to reduce the amount within a range commensurate with cost for a cold-rolled steel sheet for processing, and it is preferable that both of the above S and P components be 0.02% or less.

In addition to the above, in this invention, one or more of carbonitride-forming elements Nb, V, Mo, and Cr are added to Ti in order to enhance the effect of Ti and obtain a higher r value. It is also possible to add them in combination, and even in that case, the features of the present invention are not lost. Here, 0 for Nb and V
．． 03%, 0.01% for Mo and 0.5 for Cr.
Even if the addition exceeds 0.0005%, the effect will be saturated and it will be disadvantageous in terms of cost.On the other hand, in order to expect the effect of each element, it is necessary to add 0.0005% or more. Therefore,
0.0005-0.03% for Nb, 0.0005-0.03% for V, 0.0 for Mo
005~0.01%, 110005~0 for Cr
．． 5%.

It is also effective to add one or both of Sb and Cu to promote carbide precipitation, but neither of these is effective.
The effect is expressed at 0.001% or more, and the effect is saturated at 0.1%. Therefore, sb and Cu are each 0
．． 001 to 0.1%.

Furthermore, it is also effective in this invention to add B to improve brittleness and assist in grain refinement. here B
The lower limit of the effective addition amount is o, oooi%, and on the other hand, if it exceeds 0.01%, excessive grain refinement will occur. Therefore, B is set at 0.0001 to 0.01%.

In addition, the above-mentioned Nb, V, MoXCr, Sb,
There is no problem in adding Cu and B in any combination.

Next, we will discuss desirable conditions when manufacturing a steel plate using steel having the above-mentioned composition as a raw material.

First, steel manufacturing, hot rolling, and cold rolling may be carried out according to conventional methods, and the present invention does not particularly require limitations on these conditions. The annealing temperature for continuous annealing after cold rolling may be at least the recrystallization temperature as usual, but more preferably at least the primary recrystallization temperature +30°C. This is because it is easier to obtain an ideal mixed grain structure when the primary recrystallization progresses sufficiently and some of the grains become slightly coarse secondary recrystallized grains. In addition, in the case of steel sheets containing Be, the secondary recrystallized grains are less likely to become coarse compared to conventional steels, so there is no particular upper limit for the annealing temperature, but common sense suggests that annealing should be carried out below the 3rd transformation point. desirable.

Incidentally, the skin pass rolling after annealing may also be carried out within the range of common sense (approximately (plate thickness (mm) %) for the purpose of straightening the plate shape, etc.

(Example) After heating a steel slab with the composition shown in Table 1 to 1250°C, finishing temperature 900°C, winding temperature 600°C
A steel plate having a thickness of 1.0 mm (7) was obtained by hot rolling under these conditions and cold rolling at a rolling reduction of 65 to 75%, followed by annealing.

Note that test steel No. 2 was annealed at 780°C (primary recrystallization temperature +10°C), and the others were annealed at 830°C for 1 minute each. Further, for test steel No. 12, in order to prevent a large amount of solid solution C from remaining, over-aging treatment was performed at 300° C. for 5 minutes.

After cutting out a sample of the size shown in Figure 2(a) from each of the obtained steel plates, it was press-formed to the size shown in Figure 2(b).
After deep drawing, the surface texture of each sample was visually inspected and analyzed for surface roughness.
A graded evaluation was performed. The results are shown in Table-2. Note that the n and r values of each sample are shown in Table 1.

Visual inspection of surface texture and surface roughness analysis were evaluated in four stages: ◎:
Same as before processing, ○: Fading occurs, wavelength 50
If there is an amplitude increase exceeding 2 μm among the components of ~200 μ■ (excluding those corresponding to Δ and ×), △
: When unevenness is observed on the back surface, and some of the components with a wavelength of 200 to 1000 μm have an amplitude increase of more than 10 μm (excluding those corresponding to ×), ×: Skin roughness or wrinkles occur, and the wavelength is 1000 to 5000 μm Among the components, there is a case where the amplitude increase exceeds 10011 m.

The above surface roughness analysis is based on a profile measured along a line of 20 randomly taken opening lengths of 10 to 5,000.
The amplitude (8M) of each wavelength component of pm was determined, and the amplitude increase was determined from the difference between the amplitude after processing and the amplitude before processing (μm).

As is clear from Tables 1 and 2, many of the comparative materials have poor workability and the surface quality deteriorates significantly after processing, but the steel sheet according to the present invention has good workability and surface quality after processing. It was confirmed that.

(Effects of the Invention) According to the present invention, a cold-rolled steel sheet that has excellent workability such as press forming, has a uniform surface texture after processing, and is free from deterioration.
It can be manufactured easily in terms of process.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between grain size and total elongation, and FIGS. 2(a) and 2(b) are illustrations of processing procedures for steel sheets in Examples. Figure 1 Crystal a skin (Cr. N,)

Claims (1)

[Claims] 1. C: 0.01wt% or less, Si: 0.1wt% or less, Mn: 1.0wt% or less, Al: 0.5wt% or less, N: 0.01wt% or less, Be: 0.0001-0.01wt% and Ti:0.
0005 to 0.1 wt% with the remainder being Fe and unavoidable impurities, the cold rolled steel sheet has excellent workability and surface quality after processing. 2. C: 0.01 wt% or less, Si: 0.1 wt% or less, Mn: 1.0 wt% or less, Al: 0.5 wt% or less, N: 0.01 wt% or less, Be: 0.0001 to 0. 01wt% and Ti:0.
0005 to 0.1 wt%, and further contains Nb: 0.0005 to 0.03 wt%, V: 0.0005 to 0.03 wt%, Mo: 0.0005 to 0.01 wt%, Cr: 0.0005 to 0.50wt%, Cu: 0.0010-0.10wt%, Sb: 0.0010-0.10wt% and B: 0.0
001 to 0.01 wt%, with the remainder being Fe and unavoidable impurities, and having excellent workability and surface quality after processing.