JPS5967320A - Manufacture of high tension steel plate for deep drawing having two-phase structure - Google Patents

Abstract

PURPOSE:To obtain the titled nonaging steel plate having a low yield ratio as well as high burning hardenability, r-value, ductility and tensile strength by restricting the N and B contents of a dead soft steel having a specified composition, the coiling temp. after hot rolling, the annealing temp. after cold rolling, and the cooling rate. CONSTITUTION:A steel contg., by weight, <=0.009% C, 0.03-1.0% Si, 0.1-0.8% Mn, <=0.1% P, 0.01-0.1% Al, <=0.01% N, Nb and B satisfying Nb=(2-10)C and B>=0.3N is hot rolled and coiled at <=700 deg.C. The coil is cold rolled and annealed at a temp. at which alpha- andgamma-phases coexist, and cooling to 600 deg.C is carried out at >=4 deg.C/sec average cooling rate. The titled steel plate having about 35-50kg f/mm.<2> tensile strength is obtd.

Description

[Detailed Description of the Invention] The present invention relates to a method for producing a high-strength steel sheet with a two-phase structure for deep drawing, and particularly for steel sheets that are used for automobiles and require severe deep drawing workability. This paper presents the results of development aimed at achieving the same strength as conventional steel even if the height and thickness of the plate are made thinner.

The following three types of high tensile strength cold rolled steel sheets for conventional processing are known.

■ It is a so-called refusal steel made by adding P to low carbon aluminum killed steel, and if box annealed, a material with good ductility and drawability can be obtained, but the tensile strength is about 40 kgf/2. The drawback is that there are limits.

Note that when continuous annealing is applied to Lifous steel, it cannot exhibit sufficient workability because of its high yield strength and poor ductility and drawability.

■ Si, Mn and Qr in low carbon aluminum guild steel
It is a so-called low yield ratio two-phase structure high-strength steel sheet that has a ferrite + martenzite structure formed by controlling the cooling rate after continuous annealing. Although it is excellent, it has a drawback in that the Lankford value (r value) is as low as about 1.0 and the drawability is poor.

■ It is P-added ultra-low carbon steel with Nb or Ti added, and has a high r value and ductility of 1, but its tensile strength is as low as 40 kgf/rn, 2 or less, and Adding bake hardenability has the disadvantage that non-aging properties cannot be ensured.

Therefore, this invention has a low yield ratio and non-aging property, and
It has high bake hardenability (BH property), has excellent quotient R value, and has excellent ductility, and can exhibit a wide range of tensile strength of 35 to 50 kgf, especially +okgf/;
The purpose of the present invention is to develop an ideal manufacturing method for a commercial tensile strength steel sheet for processing that can exhibit a high tensile strength of 100 to 100%, and to solve the above-mentioned drawbacks.

Now, Nb + B, Si is added to ultra-low carbon aluminum killed steel.
When investigating the mechanical properties of steel sheets containing additive elements such as P and P after continuous annealing, we found that certain compositions had no yield elongation and had an iAr value, and we carefully examined their microstructures. When added, it was found that it was bainite-like, that is, the size was 0:0.
0007~o, oos%, N:0, OOl, 5~0.
0120%, A l 2 0.005
~0.15%.

si: 0.01~], 3% composition region, the yield elongation of ultra-low carbon cold rolled annealed sheet (hereinafter referred to as "FYE6", 2%
) to this] Nb, 10. Figure 1 shows the results of investigating the influence of B/N.

In this case, the side surface is also 870'0, 7" after soaking for 40 seconds.
(In 3 and 4, the results of continuous annealing under air cooling conditions and without skin bath are plotted.
b, /6≦]0 (○ mark), unless B/N≧0.8, it will not be less than 1%, while Nb/C〉10 (×
), it is always below 1% regardless of the ByH value.

Naoko\5i (0,08% () mark), 'AA<0.
01% (Mu mark) or N) When 0.01% (0 mark), even if Nb/C>10, it is 4 (YE regardless of the value)
Although N was 1% or more, the amount of Nb (Nl) was 0.1% or less in all cases, and the maximum value of 10 was 28.

Next, the NbZC value and the yield ratio YR (= YS/TS
) is shown in Figure 2, where b≧0.
．． 8, Si = 0.03-1.0%, A/ =
It was found that in the case of 0.01 to 0.1%, YR was 60% or less when Nb/10 was in the range of 2 to 1o.

As a result of the above, Nb//G considered to be the optimal composition: 2 to 10
, /N, 0.8 or more, Al: 0.01% or more, Si: o, oa% or more, N: 0.01% or less. FIG. 3 shows the results of investigating the effect of annealing on the bake hardenability (BH properties) of the steel sheet obtained after the annealing.

The BH property is determined by subtracting the work strain hardening due to the pre-strain from the yield strength increased by applying 8 to 15% pre-strain after the annealing and processing at 170'0x20'.

When the winding temperature is higher than 700°C, the BH property deteriorates.
It can be seen that the properties of the bake-hardened high-tensile steel sheet are lost when it becomes less than '/mm2.

Next, the C content is the Lankford value (r value) of the cold rolled annealed plate.
Figure 4 shows the results of investigating the relationship. C turtle is 0.
If it exceeds 0.009%, the r value will be less than 1.6! It is unsuitable for deep drawing. Here, Mn>0.8% (0 mark), Si>1. o% (△ mark), Al〉0.1% (
It was found that for the steels containing 0.1% (x) and 0.1% (x), the r value was 1.6 or less even when the content was 000()09% or more, making them unsuitable for deep drawing.

Based on the results of the above-mentioned experiments, this invention has C:
0.009% or less, S soil: 0.08-1.0%,
Mn: 0.1-(1.8%, P: 0.1%), A6: 0.01-0.1%, N: 0
．． 01% Nb content, 2 to 10 times the C content
, Ultra-low carbon steel with a composition containing 063 times or more of B than N content is hot rolled and rolled at a temperature of 700'0 or less, and then cold rolled to a temperature where α and γ coexist. The combination of increasing the average cooling rate to 600° C. after area annealing to 4'CJJ3/6 provides an effective solution to the problem mentioned at the beginning.

The reason for limiting the component composition of this invention is clear from the experimental results already mentioned and can be summarized as follows.

C Niji 0.009% C is a steel plate for deep drawing when cooled at a predetermined cooling rate or higher after annealing according to Figure 4. to U O,009
% or less.

si: 0.03-1.0% Sl needs to be 0.03% or more in order to keep the total yield elongation below 1.0% according to Figure 1, and according to Figure 4, In order to make the r value of the annealed steel sheet 6 or higher, it must be 1.0% or less, and the range of Sl should be 0.03 to 1.0%.

In: 0.1 to 0.8% Mn must be contained at 0.1% or more to ensure the required tensile strength, but according to Figure 4, the r value is 1.6 in total.
In order to achieve the above, it is necessary to set the content to 0.8% or less, and the range of Mn is 0.1 to 0.8%.

P: 0.1% or less P must be 0.1% or less according to I-j: Fig. 4 in order to ensure the strength of the steel and to make the r value 1.6 or more.

A/? : 0.0] to 0.1% A-1 is 0.0% in order to make YEI 1.0% or less according to FIG. In order to make the r value of the annealed steel sheet 1.6 or more as shown in Figure 4, it is necessary to contain more than 0%
．． It is necessary to keep it below 1%, and the range of A6 is 0.
．． 0] to 0.1%.

N: (1.01% or less In order to keep YFJ'k 1.0% or less, N according to Figure 1.
It is necessary to keep it below 0.01%.

Nb/6:2 to 10 YEI i ], In order to make it 0% or less, it is necessary to make Nb10 i 10 or less according to Figure 1, and to make YRf: 60% or less, According to figure 2, N
Since it is necessary to set b7a −t 2~]0,
Nb/C is in the range of 2 to ]0.

B/N, ≧0.3 In order to keep the yield elongation to 1.0% or less, K is set to B according to Figure 1.
/N i must be 0.8 or more. Next, in this invention, the coiling temperature after heating is required to be 700'0 or less, and in order to maintain the characteristics as a hardened hardening type high tensile strength steel sheet with the above chemical composition,
As shown in the figure, the BH property was not as high as the H property, and the influence of the winding temperature was shown.
In order to obtain a gfZ of 2 or higher and a low yield ratio, it is necessary to set the winding temperature of K to 700'00 or higher.

The cooling rate after breaking annealing must be 4°O/s or more, and in order to obtain all the necessary tensile strength with a yield ratio of 60% or more, the experimental results shown in Table 1 After cold rolling and annealing in the α and γ coexisting temperature range, the average cooling rate up to 600'O should be 4°C.

Table], when the cooling rate is slower than 4° C.4 as in Comparative Steels 5, 6 and 10, the yield ratio YR increases and the tensile strength also decreases. Therefore, it can be seen that in order to lower the yield ratio at a desired tensile strength, the cooling rate must be higher than 4°C.

It can be seen that if the hot rolling coiling temperature is too high, exceeding 700'', as in the case of Comparative Steel No. 11, the yield ratio becomes too high, and the steel is not suitable for the intended purpose.

In addition, the results divided into J in Table 1 are obtained by bath-making steel in a converter that has been adjusted to the composition according to the present invention, including Si, In, and p-R, in order to significantly control the tensile strength. After tapping this steel and making it into a slab by continuous rrOi casting or ingot-blowing method, it is hot-rolled at a heating temperature of 1250"0, rolled at each temperature listed in the same table, and then cold-rolled according to the usual method. They were obtained by rolling, followed by continuous annealing in the range of 790 to 870'C under the conditions listed in the same table, and by varying the subsequent cooling rate.

The thickness of each test steel plate was 0. The mechanical properties were measured without skin bath after annealing, and according to Table 1, it is clear that the present invention has achieved its intended purpose.

As described above, this invention is based on ultra-low carbon Ad-killed steel, and contains gasi, Mn, Nb, Bf.
x content 1r: Since it is controlled to the optimum value, the steel sheet has excellent surface properties such as plating properties, chemical conversion treatment properties, and corrosion resistance, and is also advantageous as a two-phase structure hypertonic hot-dip galvanized steel sheet for deep drawing. . Furthermore, since it is non-aging, it has the advantage that the material does not deteriorate even when subjected to various surface treatments that are baked at low temperatures.

[Brief explanation of the drawing]

Figure 1 is a graph showing the influence of BA on YEl of ultra-low carbon Ad-killed cold-rolled annealed steel sheet specimens with different S'i, AJ, N, and Nb10 values. ≧0.01%,
A graph showing the trivial influence on the yield ratio after continuous annealing of ultra-low carbon cold-rolled steel sheets with Si of 20.08% to ≦1. Shadow of hot rolling coiling temperature li! Graph showing 4I FIG. 4 is a graph showing the influence of C content on the r value of various cold rolled annealed sheets. Patent applicant: Kawasaki Steel Corporation Figure 1 Figure 2 Nb/C

Claims (1)

[Claims] LQ: 0.009% by weight or less, Si: o, 0
3-1.0 overlapping%, Mn: o, 1-o, sN
Fine %, P: 0.1% by weight or less, A6: 0.
Contains 01 to 0.1 heavy agitation%, N: 0. % or less of the old market amount, Nb that is 2 to JO times the C content, 0 of the N content
．． Ultra-low carbon steel with a composition of 3 times or more B' ,6
A method for producing a cylindrical tension steel plate with a two-phase structure for deep drawing, characterized in that the average cooling rate up to 00°C is faster than 11f [4C/S.