JPS59143047A - High-strength cold-rolled steel plate having favorable surface property - Google Patents

Abstract

PURPOSE:To obtain a high-strength cold-rolled steel plate for a car, etc. having favorable surface properties, high press formability and a reduced weight by adding a specified amount of Cu to a low C steel contg. a restricted amount of P and by specifying the relation between the Ti and C equivs. CONSTITUTION:A steel is composed of, by weight, 0.001-0.020% C, <=1.0% Si, 0.01-1.00% Mn, 0.04-0.20% P, 0.05-0.18% Cu, <=0.10% sol. Al, <=0.008% N, one or more among <=0.20% each of Ti, Nb and Zr, and the balance Fe with inevitable impurities. To the steel may be further added one or more among 0.03-0.25% Mo, 0.03-0.20% REM and 0.0003-0.0050% B. The relation between the Ti equiv. of the steel represented by equation I and the C equiv. represented by equation II is regulated so as to satisfy an equation (C equiv.)-1/4(Ti equiv.) <=0.0010%.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cold rolled steel sheet having good surface properties, press formability and high strength.

In recent years, various high-tensile strength cold-rolled steel sheets have been developed and put into practical use for the purpose of reducing the weight of passenger cars.

However, as a steel plate that can be used in the body of a passenger car to reduce the weight of the vehicle while exhibiting the same performance as conventional mild steel plates, it has the following characteristics: (1) The manufacturing cost is as low as that of mild steel plates.

(2) Press workability is as good as that of mild steel sheet.

(3) The surface quality is as good as that of a mild steel plate.

(4) The strength of the steel plate used as a panel is high.

Although it is necessary to have the above four properties, the steel sheets currently in practical use do not necessarily have all of these properties.

That is, for example, as a steel plate with good press workability, C
So-called IF, in which carbon is reduced as much as possible, carbonitride is added, and a sufficient amount of alloying elements are added to precipitate and fix.
Free steel has been developed.

For example, when T1 is used as a carbonitride-forming element, this IF steel contains T1 in an amount that satisfies the following.

In addition, hereinafter, ``chi'' refers to weight percentage.

However, this IF steel has problems such as a decrease in strength due to its low C and a soft steel plate, which conflicts with the recent demand for high-strength steel plates.

By the way, by including an appropriate amount of P in steel, 5
It is generally known that the strength of steel sheets can be increased at low cost, but if such a method is applied to cold-rolled steel sheets, non-uniform pickling occurs during pickling before cold rolling, resulting in poor cold-rolled steel sheets. The inventors' studies have revealed that it is impossible to obtain desirable results because the surface quality of steel sheet products is significantly impaired and the deep drawability of cold-rolled steel sheet products is deteriorated.

Further research by the present inventors has revealed that the addition of Cu is effective in improving the surface quality and deep drawability of P-containing cold-rolled steel sheets.

That is, by containing a specific amount of Cu, during pickling, Cuja precipitates in the area before local overpickling occurs, preventing further overpickling, resulting in uniform pickling. By carrying out thorough pickling, the surface properties of the steel sheet after cold rolling are significantly improved, and Cu also facilitates the growth of crystal grains during annealing, thereby increasing the r value.

This invention was made based on the above findings, and includes a cold rolled steel sheet having c: o, 601 to 0.020%, and Si: 1.0% or less.

Mn: 0.01-1.00%, P, 004-020%
.

Cu, 005~○18%, sol, Al: 0.
Less than 10%.

N: 0.008% or less.

including, Ti: 0.20% or less.

Nb 0.20% or less.

Zr, 020% or less.

Contains one or more of the following, and (C equivalent) --; (Ti equivalent)≦0.0010 (
%)...■ Ti equivalent calculated by the above ■ formula and the above 0
The relationship with the amount of C4 calculated by the formula satisfies the above formula 0, and if necessary, Mo: 0.03 to 0.25%.

REM: 0.03-0.20%.

B: 0.0003-o, oo-5o%.

By configuring the composition to contain one or more of the above, press moldability is improved and excellent strength is also ensured.

Next, the reason why the composition of the cold-rolled steel sheet of the present invention is limited to 11F as described above will be explained.

■ C The C component has the effect of forming fine carbides and improving the strength of the steel sheet, but if its content is less than 0.001%, the desired effect will not be obtained and the steel will become too soft. It is difficult to melt with 9, while 0. '02'O
If the content exceeds %, not only will a large amount of carbonitride-forming elements be required, but the amount of carbonitride precipitation will be too large and press workability will deteriorate.
Its content was set to 0.001 to 0.020%.

■ In addition to deoxidizing the steel, the 5I C1 component has the effect of ensuring the strength of the steel plate, but if it exceeds 10%, the toughness of the steel plate deteriorates and scale adhesion during annealing occurs on the surface of the steel plate. Since it imparts color, its content was set to 1.0% or less.

■ Mn The Mn component has the effect of improving the toughness of steel sheets, but if the content is less than 1.001%, hot brittleness will occur, while if the content exceeds 1.00%, it will cause problems in melting. Since this becomes difficult and causes an increase in cost, the content is set to 0.01 to 100 inches.

■PP The P component is an unavoidable alloying element for increasing the strength of steel at low cost, but if it is contained in an amount exceeding 0.20%, it significantly impairs the ductility of the steel sheet. On the other hand, if it is less than 0.04, the curing ability is too small, so the content should be reduced from 0.04 to 0.
．． It was set at 20%.

■ cu Cu is an alloying element that is the essence of the present invention. Cu has the effect of preventing the deterioration of surface properties caused by the local overacid phenomenon that occurs on the steel plate surface during pickling due to the addition of a large amount of P, and facilitates the growth of crystal grains during annealing, increasing the r value. It has an effect.

If the content is less than 0.05%, the desired effect cannot be obtained, while if it exceeds 0.18%, the melting point of Cu is low during heating in hot rolling, so that Cu is absorbed into the austenite crystal grains. Since it crystallizes in the field and makes the steel brittle and causes hexagonal cracking during hot rolling, its content is reduced to 0.05 to 0.1.
It was set at 8%.

■ SOMU AA soL AA is included to sufficiently deoxidize and improve the yield of carbide-forming elements, but soL AL contains 0.
Even if the content exceeds 10%, no further deoxidizing effect will be obtained and the cost will increase, so the upper limit is set to 0.10%.
%.

＠N The smaller the amount of N, the less the added content of carbonitride-forming elements, so it is preferable that the content of L is O, OO.
If it exceeds 80%, even if carbonitride-forming elements are included, r
Since a decrease in value is unavoidable, the content is reduced to o,
o o so o% or less.

(2) Ti, Nb, and Zr As mentioned above, these components have the uniform effect of forming fine carbonitrides to improve press formability and strength, and each of these components has a Ti content of 0.20%.

Even if the content exceeds Nb: 0.20% and Zr: 0.620%, no further improvement effect is seen due to the above action,
Since the cost is high, each upper limit value is set as Ti:
0.20%, Nb: 0.20%, and Zr.

It was set to 0.20%.

In addition, the above formulas 0 to 0 calculate the solid solution (C+N) by 000 kO(
%1 or less and the remaining C-)N is precipitated as carbonitride.

In addition, the upper limit of (C equivalent) - T - (Tl equivalent) is 0, O
The reason for setting O10'' (%) is that if this upper limit is exceeded, the amount of solid solution (C+N) increases and the press formability of the steel sheet deteriorates.

■MO+RE2vi and B These components have a uniform effect of improving the secondary processing brittleness of the steel sheet, so they are included as necessary, but 1vio: less than 0.03%, REMo, less than 0.3% and B : If the content is less than 0.0O03-%, the desired effect cannot be obtained; on the other hand, lJc, 025%,
If the content exceeds REM: 0.20% and B: 0.0050%, the weldability of the steel plate will deteriorate, so the respective contents are set as M.

(J, Q 3-0.2.5%, REM: 0.0
3-0.20% and B: 0.0003-0.00
It was set at 50%.

After continuous casting of steel having the chemical composition as shown in ``X'' or a reheated slab, a hot-rolled steel sheet was produced, and a product was manufactured using the normal cold-rolled steel sheet manufacturing process and conditions.

As a result, a high-strength cold-rolled steel sheet with good surface properties and excellent press formability was obtained.

Next, the present invention will be explained with reference to examples and in comparison with ratio axis examples.

Example 1 Steel having the composition shown in Table 1 was melted and made into a slab by continuous casting, which was then heated to 1150°C and finished at a finishing temperature of 88°C.
Hot rolling was carried out at 0℃9 coiling temperature of 600℃, and the thickness was 40℃.
m/m hot rolled steel plate.

Subsequently, the hot-rolled sheet was heated to a concentration of 50 to 90% at a concentration of 5 to 10%.
After pickling, the sheet was immersed in hydrochloric acid heated to 830°C and then cold rolled to a thickness of 08m/mK at a rolling rate of 80%, followed by continuous annealing at a temperature of 830°C for 1 minute or at a temperature of 700°C.
Box annealing was carried out for 5 hours at ℃, and after temper rolling with an elongation rate of 08%, a JISS No. tensile test was conducted.

Note that the T] equivalent and C equivalent in Table 1 and Table 2 both satisfy the above formula 0.

The results are shown in Table 2. The surface condition was evaluated based on the appearance of the steel plate after annealing.

As is clear from Table 2, steel sheets 1 and 2 of the present invention both have (C equivalent) --! -(T unit equivalent), Cu, etc. are within the range of the present invention, so it shows high strength and good r value,
In addition, the surface quality is good, but the CU of steel sheet 3 is outside the range of the present invention, so the 1" value is low, and the surface of the steel sheet after annealing has machine-like irregularities (seed blemishes) and the appearance is poor. is bad.

Example 2 Sl:0.0]-%, Mn, '0.18%, p:
o, oso%2Cu: 0. 10%, S
:0.006%, sot, AQ:
Contains 0.021 candy as a basic component and C. 001
Various steels were made into slabs by continuous casting, and the slabs were heated at 1200 IC and finishing temperature at 7.
Hot-rolled at 4oC and coiling temperature of 400'C to obtain a hot-rolled sheet with a thickness of 4.0m/m, followed by a rolling reduction of 80
A cold-rolled steel plate with a thickness of 08 m/m was obtained by cold rolling at a temperature of 100%, followed by continuous annealing at a temperature of 800°C and held for 90 seconds to obtain various ゛I'i equivalents and C. Cold-rolled steel sheets with different equivalent weights were produced.

The r value of this cold-rolled steel sheet was determined, and the results are shown in Figure 1 in relation to the amount of solid solute C in the cold-rolled sheet, that is, l (C equivalent) -- (Ti equivalent) shown as the above formula 0. Ta.

As shown in FIG. 1, it is clear that a high r value is exhibited when the value of the 0 formula is O,OO15 or less.

Example 3 After melting steel having the compositions shown in Table 3 and forming slabs by continuous casting, each slab was heated to a slab temperature of 1200°C, and then a finishing temperature of about 80°C.
After hot rolling at 0°C and a rolling roughness of approximately 600°C to obtain a hot rolled sheet with a thickness of 40 m/m, the sheet was pickled with hydrochloric acid, and then cold rolled at a rolling reduction rate of 80%. By performing continuous annealing under conditions of rolling and holding at a temperature of SOO°C for 90 seconds, the cold-rolled steel plates of the invention steel plates 1 to 15 and comparison steel plates 16 to 20, each having a thickness of 0.8 m to 7 m, were obtained. Manufactured.

In addition, some of the cold-rolled steel sheets of Comparative Steel Sheets 16 to 20 had compositions outside the range of the present invention.

(Applicable items are marked with * in Table 3.) Next, the tensile properties and r The measurement results of the values and the investigation results of the surface properties of the annealed plates are shown in Table 3.

As shown in Table 3, the cold-rolled steel sheets of the present invention steel sheets 1 to 15 have good surface properties and high strength and high r value, that is, good press formability, whereas the comparative steel sheets Cold-rolled steel sheets Nos. 16 and 17 have C4 outside the range of the present invention, so a kind of rash appears on the surface of the annealed sheets, resulting in poor surface quality and poor r-value.

In addition, since the cold-rolled steel sheet of Comparative Steel Sheet 18 contains too many carbonitride-forming elements, (C equivalent) -- (Ti equivalent) is outside the range of the present invention, so the r value is extremely low. The cold-rolled steel sheet has low strength because P is outside the limit of the range of the present invention.

In addition, in the cold-rolled steel sheet of comparative steel sheet 20, since the Cu content was outside the upper limit of the range of the present invention, hexagonal cracking occurred from the stage of hot-rolling, and patterns remained on the surface of the steel sheet after cold-rolling and annealing9. The situation is bad.

As mentioned above, the cold-rolled steel sheet according to the present invention has good press formability in terms of surface properties and high strength, and if this cold-rolled steel sheet is used in the bodies of automobiles and other vehicles, it will be possible to reduce weight and will be used in industrial applications. ” This has a dual effect.

[Brief explanation of drawings]

FIG. 1 is a curve diagram showing the relationship between the C equivalent (T1 equivalent) and the r value of a steel plate. Applicant: Sumitomo Metal Industries, Ltd.

Claims (2)

[Claims] (1) 'C: 0.001~0020%1Si:1
．． Less than 0%. Mn: 0.01-1.00%. P 004-020%. Cu: 0.05-0.18%. 5oL-All: 0.10% or less. N: 0.008% or less. Contains Ti: 0.20% or less. Nl): 0.20% or less. Zr: 0.20% or less. and the relationship between the T1 equivalent calculated by the above formula 0 and the C equivalent calculated by the above formula 0 satisfies the above formula 0, and Fe and unavoidable impurities remain. A high-strength cold-rolled steel sheet with good surface properties and having a composition (the above weight %) consisting of: (2) C: 0.001-0.020%. Si: 1.0% or less. Mn: 0.01-1.00%. P: 0.04-0.20%. Cu: 0.05-0.18%. 5at-AQ 010% or less. N: 0.008%. Contains Mo: 0.03-025%. REM: 0.03-020%. B, 0.0003-0.0050%. Contains one or more of the following, and further contains Ti2020% or less. Nb: 0.20% or less. Zr, 020% or less. and the relationship between the T1 equivalent calculated by the above formula 0 and the C equivalent calculated by the above formula 0 satisfies the above formula 0, Fe + unavoidable impurities: residual A high-strength cold-rolled steel sheet with good surface properties and a composition (weight %) of 9°.