JPH02163318A - Production of high-tension cold rolled steel sheet having excellent press formability - Google Patents

Abstract

PURPOSE:To produce the high-tension cold rolled steel sheet which can be formed to thinner thicknesses and has excellent press formability by subjecting an extra-low carbon steel ingot which is added with Ti and Nb in combination and is further added and incorporated with Mn, P, and B as solutionization intensifying elements to hot rolling and cold rolling under specific conditions then to continuous annealing. CONSTITUTION:The ingot or slab of the extra-low carbon steel contg., by weight%, 0.0020 to 0.0100% C, <=0.3% Si, 1.0 to 2.0% Mn, 0.04 to 0.095% P, <=0.01% S, 0.010 to 0.10% sol. Al, <=0.0070% N, 0.002 to 0.0010% B, (48/12%C+48/14%N+48/32%S) to 0.10% Ti, and 0.01 to 0.10% Nb is heated to 1150 to 1280 deg.C and thereafter, the hot rolling thereof is ended in the temp. range of (Ar3 point +50 deg.C) to 1000 deg.C to form the sheet material which is coiled at 400 to 600 deg.C. This sheet material is pickled and is descaled; thereafter, the sheet material is cold rolled at 70 to 90% draft to the final sheet thickness. The sheet is then continuously annealed in the temp. region of the recrystallization temp. to Ac3 point.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a high-strength cold-rolled steel sheet containing Ti and Nb1, which has excellent press formability.

(Prior art and its problems) Conventional cold-rolled steel sheets for general processing have low tensile strength (hereinafter T, S,
) is 28 to 40 kgf/mm",
After press forming, it is used in large quantities as interior and exterior steel sheets for automobiles. In recent years, in the automobile industry, demand for high-strength thin steel sheets has increased as the weight of vehicle bodies has been reduced, and there has also been a strong demand for improved press formability, especially drawability, and various types of non-aging, high-strength cold-rolled thin steel sheets have been developed. being developed.

In general, increasing the strength of steel sheets deteriorates press formability, and in particular, the Lankford value (
Although the r value (hereinafter abbreviated as r value) is effective as an index of drawability, it is extremely difficult to increase this r value.

However, among these methods, a method known to produce high-strength cold-rolled thin steel sheets with a high r value is a method that uses carbonitride-forming elements and S
This is a thin steel sheet to which substitutional solid solution strengthening elements such as i, Mn, and P are added.

As such prior art, Japanese Patent Publication No. 55-49147 discloses that precipitation-strengthening elements such as Ti and V are added, and although the r value and ductility are not sufficient, T, S, and 55 kgf/mm are added.
' grade high-strength steel plates were disclosed, and
204-, by adding Cu and P, T, 5.35~4
Steels that can obtain strength of 0 kgf/mm2 class and high deep drawability are disclosed.

However, the conventional techniques shown in the above examples all use T, S, and 4 steel sheets, which are required as automotive steel sheets that have good press formability that enables thinning of processed parts.
It was impossible to obtain a steel plate that had both 0 kgf/111112 or more and an r value of 1.7 or more.

Furthermore, in addition to the above-mentioned properties, there has recently been a demand for steel sheets that have small in-plane anisotropy of the r value and are excellent in secondary work resistance and marring properties.

(Knowledge related to problem solving) The present invention has a T of 5.40k, which is required as an automotive steel sheet with excellent press formability that enables thinning.
The objective is to manufacture a steel plate with an r value of 1.7 or more, a small in-plane anisotropy of the r value, and excellent secondary workability.

The present inventors conducted various studies on the composition and manufacturing method of high-strength cold-rolled steel sheets, and found that Ti, Nb%j [solid solution strengthening elements Mn, P, and B within predetermined limits were added to the combined steel] It was discovered that a high-strength steel plate having the above characteristics can be obtained by adding a composite material and controlling the various steps from hot rolling to annealing within a predetermined range of conditions.

(Structure of the invention) The above problem is as follows: 1% by weight Te, C: 0,0020 to 0.0
100%, S1≦0.30%, Mn: 1.2-2.
0%, P: 0.040-0.095%, S≦0.
01%, Sol, Al: 0.01.0 to 0.10
0%, N≦0, 0070%, B: 0.0002~
0.0010%, Ti: (48/12%C+48
/14%N + 48/32%S)~0.10%, N
b: After heating a Dj-Nb composite additive ultra-low carbon steel slab containing 0.01 to 0.10% and the balance Fe and unavoidable impurities to 1150 to 1280°C, (A
Hot rolling is completed at a temperature above 1.000°C (r1 transformation point + 50°C) and below 1.000°C, and winding is performed at a temperature above 400°C and below 600°C.
A press characterized in that after pickling, cold rolling is performed at a reduction rate of 70 to 90%, and then the obtained cold rolled sheet is continuously annealed in a temperature range from a recrystallization temperature to an AC3 transformation point. This is achieved by a method for manufacturing high-strength cold-rolled steel sheets with excellent formability.

Next, the reasons for limiting the composition in the present invention will be explained.

C: The lower the content of C, the more advantageous it is for drawability and ductility, and if it exceeds o, oio%, good drawability cannot be obtained even if the amount of Ti added, which will be described later, is increased. On the other hand, 0
．． If it is less than 0.020%, the fine carbonitrides that contribute to the strength T and S will decrease and the manufacturing cost will increase. Therefore, c xi is 0.0020~0
．． 010%.

Si: Si is effective for increasing the strength of steel sheets, but if it exceeds 0.3%, the in-plane anisotropy ratio of the r value increases and the elongation decreases, so the amount added should be 0.3% or less. shall be.

Mn: Mn is an alloying element that forms the basis of the present invention and is effective in increasing the strength of the steel sheet without deteriorating the deep drawability. In addition, unlike Si, Mn has a very small effect on the in-plane anisotropy of the r value, and while it is possible to reduce the amount of P added, which causes grain boundary embrittlement as described below, Mn is a solid solution strengthening element. It is an effective element as

However, if it exceeds 2.0%, the Ac transformation point will drop significantly and become close to the recrystallization temperature, and α→
γ metamorphosis is promoted. For this reason, the (]T1) plane cold rolling-recrystallization texture formed in the recovery-recrystallization process, which is advantageous for the r value, is damaged. Furthermore, a hardened structure may be produced during the cooling process during annealing. Therefore, if it exceeds 2.0%, the elongation and r value of the steel sheet will be significantly deteriorated, so the amount of An added should be 2.0% or less. In addition, if it is less than 1.0%, the target strength cannot be obtained even if the solid solution strengthening element P described below is changed within a predetermined range, so the lower limit is 1.0%.
The above shall apply.

Like Si and Mn, FDP increases the strength of steel sheets, but
．． If it exceeds 0.095%, not only will secondary work cracking resistance drop significantly, but also weldability will be inhibited, and the recrystallization temperature will become extremely high, causing repeated annealing in the line.
95% or less. Moreover, if it is less than 0.040%, the target high strength cannot be obtained even if the above-mentioned Mn is varied within a predetermined range, so the lower limit is set to 0.040%.

AlΔl is 0.0 due to deoxidation. (Although 110% or more is added, if it exceeds 0.100%, it will have an adverse effect on the surface quality, so the upper limit is set at 0.10%.

SO3 combines with Mn to form nonmetallic inclusions, which tends to cause defects such as cracks during press molding.

Furthermore, since Ti and TiS are generated, the amount of Ti added increases, which is not preferable in terms of cost, so the amount is set to 0.01% or less.

N: N is an impurity element in steel, but TiN
However, if the N content exceeds 0.0070%, the amount of T1 added required for stabilization increases, which is not preferable in terms of loss 1. Therefore, the upper limit is set to 0.0070%.

Ti: Ti fixes S, N and C, (11,
1) Face cold rolling - Added to develop recrystallized texture and texture, lower limit of (48/22%C + 48/14%N+
48/32%S). The reason why the upper limit is set to 0.10% is that the effect will be saturated even if it is added in excess of this amount.

Nb: Nb is generated during hot rolling with stable Ti carbonitride as a core, and forms coarse ingots and Nb composite carbonitride. For this reason, the (111) plane cold rolling-recrystallization texture is further developed, thereby improving the drawability and the in-plane anisotropy of the r value. However, if the addition amount is less than 0.01%, hundreds of in-plane anisotropy cannot be observed, so the upper limit is set to 0.01% or more, and the upper limit is set to 0.10%. This is because the effect saturates even if

Snake: B preferentially segregates at the grain boundaries in the steel sheet than the aforementioned P, and P
B suppresses the deterioration of press formability due to grain boundary brittleness, but if its content is too large, it inhibits grain growth and reduces the r value and elongation of the shoulder plate, so the upper limit of B addition is set to o, oo.
io%.

On the other hand, the lower limit effective for suppressing grain boundary segregation of P and preventing inferior press formability due to grain boundary brittleness is 0.0002%.
shall be.

In addition, the manufacturing conditions for the steel of the present invention are as follows: molten steel whose composition has been adjusted to the above-mentioned composition range is continuously cast, and this slab is sent directly without cooling, or - once cooled and reheated as a cold piece, it is then heated. Roll for a while. The heating temperature of this slab is set at a lower limit of 1150° C. in order to ensure a hot rolling finishing temperature of (Ar, transformation point +50° C.) or higher. The upper limit temperature is set to 1280'C in order to prevent precipitates from forming a solid solution in the heating furnace and producing very small precipitates during hot rolling. This is because these very small precipitates suppress the development of the (11,1) plane cold rolling-recrystallization texture and lower the r value.

The finishing temperature was Ar as shown in FIG.

Below the transformation point, an unfavorable hot-rolled-recrystallized texture or unrecrystallized texture is produced compared to the (I II I) plane cold-rolled-recrystallized texture, so it is usually set above the Ar point. However, by changing the Ti-Nb precipitate,
In order to further improve the in-plane anisotropy of the r value, the temperature is set to be at least (Ar3 transformation point +50°C). Furthermore, the reason for setting the upper limit temperature of 1000°C is that if the hot rolling heating temperature is higher than this, the grain size of the hot rolled sheet will increase and the development of (111) plane cold rolling-recrystallization aggregate weave will be suppressed. This is because the r value decreases.

Next, the upper limit of the winding temperature is set at 600'C because if the scale is too high, the cleaning properties of the scale deteriorate, and the lower limit is set at 400'C to prevent defects in the shape of the plate after winding.

During cold rolling, the surface of the steel sheet is cleaned by ordinary pickling and then rolling is performed. At that time, the rolling reduction ratio was 1.7
In order to ensure the above, the lower limit is set to 70% as shown in FIG. The upper limit is set at 90%.

Even if cold rolling is performed beyond this value, the effect will be saturated, so it is set to 90% or less.

The obtained cold-rolled sheet is continuously annealed in a temperature range of Ac above the recrystallization temperature and below the transformation point.

In this manner, according to the present invention, a high-strength cold-rolled steel sheet with excellent press formability, good resistance to secondary work cracking, and small in-plane anisotropy is manufactured.

(Specific disclosure of invention) The present invention will be explained by examples.

Table 1 shows the chemical compositions of the test materials used in the prototype experiments, and the steel of the present invention has ~a, l, 2.3.6.7.12.

Furthermore, the chemical compositions of comparative steels with the steels of the present invention are also shown in Table 1.

In the prototype experiment, the composition was adjusted in a converter, melted, and continuously cast into a slab with a thickness of 250 mm. Subsequently, these slabs were hot rolled to a thickness of 2.3.2.7.
．． 3.2. A naturally rolled sheet of 4.0 mm was prepared, pickled, cold rolled, and continuously annealed to produce a high tensile strength cold rolled steel sheet of 0.8 mm in thickness. No.], 2.3゜6, 7.12 in the same table are the steels of the present invention, low YP, 40kgf
/ mm2 or more high T, S, 1.7 or more high r value,
It is possible to obtain a tTy44 fabric having a high Vi-Q (elongation rate) of 35% or more and good resistance to secondary processing cracking.

Table 2 specifically shows manufacturing conditions such as hot rolling winding temperature, finishing temperature, cold rolling rate, and annealing degree U, and product characteristics.

As can be seen from the results in Table 2, No. of the steel of the present invention.

Items 1 to 3 have a high T, S, of 40 kgf/mm2 or more, an r value of 1.7 or more, and a high Eρ (elongation) of 35% or more, and a transition temperature of -40°C or less and secondary work cracking resistance. It can be seen that this is also a good high tensile strength cold rolled steel sheet.

N004 steel, which has a C content higher than the range of the present invention, has high values of T and S, but has a low r value of 1.26, and has a problem with workability.

No. 5 g where the amount of h is larger than the range of the present invention is No. 4
It is similar to steel and has problems with drawing formability.

No. 8 steels with cold rolling reductions lower than the range of the present invention are also T and S.
, but the elongation and r value are low and there are problems with press formability.

NO19 steel without the addition of B has good T, S, and r values, but has a high transition temperature of 110°C during press forming.

There is a problem of secondary processing cracks.

(Effects of the Invention) The method for producing a high-strength cold-rolled steel sheet with composite addition of Ti and Nb according to the present invention makes it possible to produce a high-strength cold-rolled steel sheet that has excellent press formability that enables thickening. , Effect of the invention on industrial use Figure 1 is a diagram showing the relationship between the amount of C added on T, S, and r values, and Figure 2 is a diagram showing the relationship between the amount of C added and the r value.
Figure 3 shows the relationship between hot rolling finishing temperature and r value. Figure 4 shows the relationship between cold rolling rolling rate and r value. It is a diagram.

Claims (1)

[Claims] In weight%, C: 0.0020 to 0.0100%, Si≦0.3%, Mn: 1.0 to 2.0%, P: 0.040 to 0.095%, S≦0.01%, Sol. Al: 0.010-0.100%, N≦0.0
070%, B: 0.0002 to 0.0010%, Ti: (48/12%C + 48/14%N + 48/32
%S) ~ 0.10%, Nb: 0.01 ~ 0.10%, and the balance is Fe and unavoidable impurities.
After heating to 80°C, hot rolling is completed at a temperature of (Ar_3 transformation point +50°C) or more and 1000°C or less, and then heated to 400°C or more and 60°C.
After rolling at 0℃ or below and pickling, the reduction rate is 70~
A high tensile strength cold rolled steel sheet with excellent press formability, characterized in that after cold rolling at 90%, the obtained cold rolled steel sheet is continuously annealed in a temperature range above the recrystallization temperature and below the Ac_3 transformation point. Production method.