JPH06287642A - Production of low-strength cold rolled steel sheet for deep drawing excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a low-strength steel sheet for deep drawing improved in corrosion resistance and workability by subjecting a steel, prepared by using a extra low carbon Ti-Nb steel having P- and Cu-added specific composition as a base and further controlling the contents of O and H2, to annealing under specific conditions. CONSTITUTION:This steel has a chemical composition consisting of, by weight, <=0.0025% C, <=0.05% Si, 0.04-0.15% Mn, 0.50% Ni, 0.0003-0.0020% B, 0.20-0.50% Cu, 0.005-0.070% Al, 0.03-0.06% P, <=0.004% S, <=0.0030% N, <=0.0040% O, <=0.0002% H, further 0.01-0.05% Ti and/or 0.001-0.010% Nb, and the balance Fe and satisfying Ti>=4C+3.4N+1.5S. This steel is hot-rolled, cold-rolled, and annealed at 860-950 deg.C. By this method, the low-strength cold rolled steel sheet for deep drawing, having <35kgf/mm<2> tensile strength and excellent in workability and corrosion resistance, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of cold-rolled steel sheets suitable for automobile steel sheets, etc., which have excellent deep drawability and low corrosion strength (TS <35 kgf / mm ² ). It is a suggestion about the method.

[0002]

2. Description of the Related Art In recent years, steel sheets for automobiles have a tensile strength of 35 kgf /
A high-strength cold-rolled steel sheet having mm ² or more and excellent deep drawability has been demanded. By adopting such a high-tensile cold-rolled steel sheet, it has become possible to reduce the gauge of automobile steel sheets. On the other hand, however, corrosion resistance, especially pitting corrosion resistance, tends to decrease as the thickness of automobile steel sheets decreases, and prevention of this has become an important issue. For this reason, in recent years, it has become inevitable to develop a high-strength cold-rolled steel sheet having excellent corrosion resistance as well as deep drawability. On the other hand, there is an attempt to replace a member that has conventionally used a surface-treated steel sheet with a corrosion-resistant cold-rolled steel sheet for the purpose of cost reduction. Furthermore, in order to further improve the corrosion resistance, surface treatment of the corrosion-resistant cold-rolled steel sheet is also under study. In order to meet such demands, it is necessary to develop a cold-rolled steel sheet that has deep drawability equivalent to that of conventional ones and at the same time has higher corrosion resistance. By the way, heretofore, there have been some proposals for a method for producing a high-strength cold-rolled steel sheet having excellent deep drawability. For example, Japanese Shokoku 63
JP-A-95799 discloses a method for producing a high-strength cold-rolled steel sheet having a good surface property by adding P and Cu in combination.

[0003]

However, in the high-strength cold-rolled steel sheet for deep drawing according to the prior art, there is no consideration for improving the corrosion resistance, and as a result of adding P and Cu, The tensile strength was 35 kgf / mm ² or more, and it was hardened more than necessary, which was an adverse effect. Moreover, with respect to the steel sheet, there are still problems such as inclusion defects and blistering defects depending on whether annealing is appropriate or not, and improvement thereof is strongly desired.

As described above, the prior art lacks consideration for corrosion resistance, and the tensile strength is unnecessarily increased, which may affect deep drawability.
There are two problems, and an object of the present invention is to establish a technique capable of overcoming such problems. This object is achieved by devising the compositional composition of steel and the manufacturing conditions described in detail below.

[0005]

[Means for Solving the Problems] In the research for realizing the above-mentioned object, the inventors have found that a cold-rolled steel sheet for low strength deep drawing excellent in corrosion resistance has the following concept. The present invention has been conceived based on the finding that it can be obtained by determining the chemical composition of steel and the manufacturing conditions. In order to soften the material that is hardened by the addition of P and Cu, which imparts corrosion resistance, to minimize the addition of other solid solution strengthening elements, and to improve the formability, ultra-low carbon Ti −Nb type IF (I
nterstitial Free) alloy as a base, continuous annealing in the range of 860 ° C directly below Ac ₃ to 950 ° C in the γ region in order to grow crystal grains and obtain soft steel sheets, In order to prevent the generation of blistering defects generated as a result, total oxygen is set to 40 ppm or less to reduce inclusions and H ₂ is controlled to 2 ppm or less. Then, under such an idea, the present invention has a material property that the corrosion weight loss, the maximum pitting corrosion depth is reduced by 20 to 30% compared with the conventional cold-rolled steel sheet, and the workability is a deep-drawn steel sheet. El ≧ 46%. TS: 31 ~ 34kgf / mm ² , YS: 19 ~ 21kgf / mm
It was developed aiming at the material which shows ² , γ ≧ 1.9.

The present invention developed on the basis of such an idea has the following essential constitution. That is, in the present invention, C: 0.0025 wt% or less, Si: 0.05 wt% or less, Mn: 0.04 to 0.15 wt%, Ni: 0.50 wt% or less, B:
0.0003 to 0.0020wt%, Cu: 0.20 to 0.50wt%, Al: 0.005
~ 0.070 wt%, P: 0.03-0.06 wt% or less, S: 0.004 wt%
% Or less, N: 0.0030 wt% or less, O: 0.0040 wt% or less, H: 0.0002 wt% or less, and Ti ≧ 4C +
Satisfying 3.4N + 1.5S, Ti: 0.01-0.05
wt%, Nb: Any of 0.001 to 0.010 wt% 1
860-95 after being hot-rolled and cold-rolled a steel containing one or two kinds and the balance being Fe and unavoidable impurities
Tensile strength can be improved by annealing in the temperature range of 0 ° C.
A method for producing a cold-rolled steel sheet for low strength deep drawing having excellent corrosion resistance, which is characterized in that the steel sheet has a weight of less than 35 kgf / mm ² .

[0007]

The following is a description of experimental studies that have led to the present invention. C: 0.0020wt%, Si: 0.01wt%, Mn: 0.08wt
%, P: 0.05 wt%, S: 0.003 wt%, Al: 0.04 wt%,
N: 0.002 wt%, Ti: 0.03 wt%, Nb: 0.003 wt%, B:
0.001 wt%, Cu: (0.01 and 0.30) wt%, Ni: 0.2 wt%
%, O: (0.002 to 0.010) wt% 12 seat bar composition
After heating to 50 ° C.-soaking, hot rolling was performed at a finishing temperature of 880 ° C. Then, after pickling and cold rolling, 900 ℃ ×
Recrystallization annealing was performed for 20 seconds. Figure 1 shows the effect of steel components, especially Cu and O, on corrosion resistance. The corrosion resistance is evaluated by immersing it in 0.5% NaCl corrosive solution for 8 hours,
Carry out a corrosion cycle of drying for 16 hours (number of cycles
30), and performed by measuring the maximum pitting depth after the corrosion test. As can be seen from the results shown in this figure, the corrosion resistance strongly depends on the steel components, especially Cu and O content.
It was revealed that the corrosion resistance of the (0.30 wt% Cu) steel was remarkably improved when O ≦ 0.004 wt%.

As a result of various examinations based on the above experimental results, the present inventors determined the following steel composition and treatment conditions. (1) Steel component composition; The reasons for limiting the composition of each component are explained below.

C: 0.0025 wt% or less C is preferably as small as possible in order to improve the deep drawability, but if its content is up to 0.0025 wt%, it does not exert a bad influence so much. % Or less.

Si: 0.05 wt% or less Since Si has the effect of strengthening steel, Si is contained in a necessary amount according to the desired strength. However, in the present invention, considering the addition of P and Cu in combination, The amount of Si added was suppressed as much as possible, and the upper limit was limited to 0.05 wt% or less.

Mn: 0.04 to 0.15 wt% Since Mn has the effect of strengthening steel, 0.04 wt% is contained according to the desired strength, but in the present invention, P and Cu are added in combination, The upper limit was set to 0.15 wt% or less in consideration of Mn / S ≧ 10 in order to prevent hot cracking.

Ni: 0.50 wt% or less Ni is added to improve the Cu-added steel sheet since hot rolling causes brittle cracks and impairs the surface properties of the steel sheet. If the addition amount exceeds 0.50 wt%, the deep drawability is adversely affected, so the content was limited to 0.50 wt% or less.

B: 0.0003 to 0.0020 wt% B is added in order to prevent the secondary work brittleness resistance of the steel due to the decrease in the grain boundary bonding strength of the ultra-low carbon steel, and the addition amount is 0.0003. If it is less than wt%, the effect of addition is poor. On the other hand, if it exceeds 0.0020 wt%, the deep drawability is adversely affected, so the range was limited to 0.0003 to 0.0020 wt%.

Cu: 0.2-0.5 wt% Cu is an element that plays an important role in the present invention.
It is added to improve the corrosion resistance. The amount added is 0.2
If it is less than wt%, it is less effective in improving the corrosion resistance, while 0.5%
If added in excess of wt%, the deep drawability will be adversely affected, so the range was limited to 0.2-0.5 wt%. Incidentally, it is desirable that Cu be coexistent with P in order to improve the corrosion resistance.

Al: 0.005 to 0.07 wt% Al is used as a deoxidizing element during steelmaking, and is also included for improving the yield of carbon / nitride forming elements. If the content is less than 0.005 wt%, the effect of addition is small, while that of 0.07 wt%
Even if added in excess of wt%, the deoxidizing effect and the above-mentioned yield improving effect saturate, so the range was limited to 0.005 to 0.07 wt%.

P: 0.03 to 0.06 wt% or less P is an element which plays an important role in the present invention,
Since it has the effect of strengthening steel and the effect of improving corrosion resistance, a predetermined amount is added. The addition amount is 0.03wt
If it is less than%, it has little effect on improving the corrosion resistance, while it is 0.
If it exceeds 06wt%, it will adversely affect the deep drawability.
It was limited to the range of ~ 0.06wt%.

S: 0.004 wt% or less As the content of S decreases, the deep drawability and corrosion resistance improve. If the content is up to 0.004 wt%, there is no adverse effect, so a content of 0.004 wt% or less is allowed.

N: 0.0030 wt% or less N is the smaller, the deeper drawability is improved. However, if the N content is 0.003 wt% or less, it does not exert a bad influence so much. Therefore, the N content is 0.0030 wt% or less. Is acceptable.

O: 0.0040 wt% or less O is an element that plays the most important role in the present invention, and in order to improve the corrosion resistance, this O must be 0.0040 wt% or less. Further, this effect of reducing O is effective for the Cu-added steel as shown in FIG. 1, and the Cu-free steel has almost no O-reducing effect on the corrosion resistance. In addition, the effect of this O is that Ar that gathers around inclusions during high temperature annealing in order to suppress softening during continuous annealing,
Since H ₂ bubbles expand at high temperature and cause blistering defects,
Therefore, it is necessary to keep it low, and preferably
Keep it to 0.0040wt% or less.

Ti: 0.01 to 0.05 wt% Ti is a carbon / nitride forming element, and is a solid solution (C,
N) is precipitated and fixed as (TiC, TiN), and is added to form a {111} orientation that is advantageous for deep drawability. If the addition amount is less than 0.01 wt%, the effect of addition is poor, while on the other hand,
In addition to adding more than 05wt%, the effect is saturated,
Rather, it may lead to deterioration of surface quality, so 0.01
Limited to ~ 0.05wt%. In addition, when Ti satisfies the following expression: Ti ≧ 4C + 3.4N + 1.5S in relation to C, N and S, the deep drawability is particularly good.

Nb: 0.001 to 0.010 wt% Nb is a carbide-forming element. In order to precipitate and fix the solid solution C in the steel as NbC, preferentially forming the {111} orientation advantageous for deep drawability. Added. This amount is 0.001 wt%
If less than 0.010 wt%, the effect is not manifested, while if added over 0.010 wt%, ductility deteriorates, so 0.001 to 0.010 wt%
Limited to%.

H ₂ : In order to prevent generation of blisters during high temperature annealing, it is necessary to limit it to 2 ppm or less.

(2) Hot Rolling Conditions In the hot rolling of the present invention, from the viewpoint of energy saving, the continuously cast slab is immediately reheated or subjected to heat treatment without being cooled to a temperature below the Ar ₃ transformation point after continuous casting. It is desirable to do it later. The finishing temperature of this hot rolling is Ar ₃
It is preferable for deep drawing to be performed at a temperature above the transformation point,
From the viewpoint of preventing hot brittle cracking, low temperature hot rolling is rather preferable.
Then, it is necessary to heat-roll under conditions that prevent coarsening of crystal grains during hot rolling by partially precipitating TiC, AlN, and TiN precipitates. The slab heating temperature that can meet this requirement is 1250 ° C or lower, and the finish rolling temperature is preferably 870 ° C or higher, which is higher than the Ar ₃ transformation point. The coil winding temperature is preferably 700 ° C or lower in order to improve the deep drawability due to the coarsening of Ti precipitates due to high temperature winding.

(3) Cold rolling condition: In order to obtain a high r value, the high pressure reduction ratio is preferable, and is 50% or more.

(4) Annealing conditions Annealing conditions are important in the present invention, and it is necessary to anneal in the temperature range of 860 ° C to 950 ° C in order to obtain excellent deep drawability. Excellent deep drawability cannot be obtained at annealing temperatures below 860 ° C. On the other hand, if annealing is performed in a temperature range exceeding 950 ° C, the texture becomes random due to α → γ transformation, resulting in poor deep drawability. Therefore, the annealing temperature is limited to 860 ° C to 950 ° C.

In the present invention, a continuous annealing line or a continuous hot dip galvanizing line is suitable as the annealing process. The reason for this is that since the annealing time in the above line is short, Cu does not concentrate on the surface of the steel sheet and the surface treatability becomes good. As the above continuous hot-dip galvanizing method, non-alloyed hot-dip galvanizing and alloyed hot-dip galvanizing are well suited. Further, the steel sheet of the present invention may be subjected to a special treatment after annealing or galvanizing to improve chemical conversion treatment property, weldability, press formability, corrosion resistance and the like.

[0027]

EXAMPLES Steel slabs having chemical compositions shown in Table 1 (A to G)
Was subjected to heating-soaking-rough rolling-finish rolling under the conditions shown in Table 2, and after pickling the obtained hot-rolled sheet, it was cold-rolled at a reduction rate shown in Table 1 to give a 0.8 mm sheet. After the thickness was increased, recrystallization annealing was performed on the continuous annealing line at a temperature in the range of 840 ° C to 930 ° C. The material properties of the test materials (A to G) thus obtained were tested. The tensile properties were measured using JIS No. 5 tensile test pieces. The r value was measured by the 3-point method after 15% tensile prestrain, and was measured in the L direction (rolling direction), D direction (45 ° direction to rolling direction) and C direction (90 ° direction to rolling direction). ) Was obtained as r = (r _l + 2r _D + r _C ) / 4. We also tested corrosion resistance,
This was evaluated by the same method as described above.

The results of these tests are also shown in Table 2.
As is clear from the results shown in Table 2, the cold-rolled steel sheets (A, D, E, F) obtained in conformity with the method of the present invention were found to have excellent deep drawability and corrosion resistance. .

[0029]

[Table 1]

[0030]

[Table 2]

FIG. 2 shows the corrosion resistance of the steel No. A in Table 1 evaluated by the maximum pitting depth. The steel sheet (A) of the present invention has about 30% more pitting corrosion than the ordinary steel sheet. The results show that the depth is improved.

Next, FIG. 3 shows the annealing temperatures 860 to 95 for the contents of oxygen and hydrogen within and outside the range of the present invention.
The effect on blistering rate in the 0 ° C. range was investigated, and the result was that only in the case of the steel (d) conforming to the present invention, blistering was not observed.

[0033]

As described above, according to the present invention, Cu
By subtle alloy design of combined addition of P and P and accompanying suppression of solid solution strengthening element and controlling annealing conditions, it has better corrosion resistance than before and tensile strength of less than 35 kgf / mm ² . It is possible to manufacture a certain low-strength cold-rolled steel sheet for deep drawing. In addition, the anti-corrosion steel plate plated with zinc is easily corroded on the non-plated plate end face side, but according to the present invention, such a defect can be minimized. Furthermore, according to the present invention, it becomes possible to manufacture an inexpensive anticorrosion steel sheet having a thin coating, and in a portion where the corrosive environment is not severe, it is possible to reduce the corrosion weight loss by 10 to 30% compared to the conventional cold-rolled steel sheet, It is possible to provide an inexpensive corrosion-resistant steel plate. Further, in the present invention, since a mild steel sheet capable of deep drawing can be obtained, it can be expected that the field of application will be expanded as a steel sheet for automobiles, for example.

[Brief description of drawings]

FIG. 1 is an explanatory view showing the influence of Cu and O contents on corrosion resistance.

FIG. 2 is a combined view of the results of the maximum pitting depth comparison test of the steel sheet of the present invention and a normal steel sheet.

FIG. 3 is an explanatory view showing the influence of oxygen and hydrogen contents in an annealing temperature range of 860 to 950 ° C. as a blistering rate.

Claims (1)

[Claims] 1. C: 0.0025 wt% or less, Si: 0.05 wt% or less, Mn: 0.04 to 0.15 wt%, Ni: 0.50 wt% or less, B: 0.0003 to 0.0020 wt%, Cu: 0.20 to 0.50 wt%, Al: 0.005-0.070 wt%, P: 0.03-0.06 wt% or less, S: 0.004 wt% or less, N: 0.0030 wt% or less, O: 0.0040 wt% or less, H: 0.0002 wt% or less, and Ti ≧ 4C + 3.4N + 1.5S is satisfied, and Ti: 0.01 to 0.05 wt% and Nb: 0.001 to 0.010 wt% are contained, and the balance is Fe and inevitable impurities. Steel is hot-rolled, cold-rolled, and then annealed in the temperature range of 860 to 950 ° C to obtain a steel sheet with a tensile strength of less than 35 kgf / mm ^2. A method for producing an excellent cold rolled steel sheet for low strength deep drawing.