JPS61276930A - Production of cold rolled dead soft steel sheet having good elongation and deep drawability - Google Patents

Abstract

PURPOSE:To obtain a cold rolled sheet having the better elongation and deep drawability by limiting the contents of C, N, S, Ti and Nb in a specifically composed dead soft steel added with Ti and Nb in combination and limiting strictly hot rolling and subsequent cooling conditions as well as the heating and cooling conditions for continuous annealing. CONSTITUTION:The hot rolling of the steel contg., by weight, <=0.0050% C, <=1.0% Si, <=1.0% Mn, formula I Ti, formula II Nb, 0.005-0.10% Al, <=0.15% P, <=0.0050% N and <=0.015% S is completed at the finish rolling temp. above the Ar3 point. The cooling is started within 0.5sec right thereafter and the steel sheet is cooled at >=10 deg.C/s average cooling rate till coiling. The steel sheet is coiled at <=710 deg.C. After such steel is subjected to cold rolling at >=50% draft, the sheet is subjected to the continuous annealing at the heat cycle consisting of heating at >=5 deg.C/s heating rate up to 400-600 deg.C and holding for >=1sec in the temp. region of 700 deg.C - Ac3 point. The cold rolled dead soft steel sheet having the substantial effect of the combined addition of Ti and Nb and the better elongation and deep drawability is thus obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention relates to the production of cold-rolled steel sheets suitable for press working of automobile bodies, especially in areas where elongation and deep drawability are required. The specification states that the continuous annealing method is useful for producing materials with less anisotropy, excellent elongation and deep drawability, and excellent aging resistance and secondary processing brittleness. This paper describes the results of research and development on an appropriate manufacturing method for cold-rolled steel sheets.

<Conventional technology> Conventionally, steel sheets for press working have a low carbon content (C: 0.02 to 0
．． Q7wt% (hereinafter referred to simply as %) Δβ killed steel was generally manufactured by box annealing, but recently, in order to further improve pressability and high productivity, C<0
．． It is manufactured using a continuous annealing method using 0.1% ultra-low carbon steel.

In ultra-low carbon steel, solid solution in the steel improves the ductility, drawability, and
In order to fix C and N that deteriorate aging resistance, T
Carbonitride forming elements such as i, Nb, V, Zr and Ta are added. Conventionally, these elements are often added alone because they are expensive, and the properties of T1 and Nb, which are most commonly used, are compared as follows.

Compared to Nb-added steel, T1-added steel has a lower recrystallization temperature and has good descaling properties such as pickling. It has advantages such as good mechanical properties such as (r value).

On the other hand, compared to T1-added steel, Nb-added steel has characteristics such as less anisotropy in r value and good chemical conversion treatment properties as a pre-painting treatment.

Japanese Patent Publication No. Sho 58-107414 discloses how to exhibit the advantages of both Ti and Nb at the same time.

In this case, the upper limit of the content of T1 is set so that most of the content is preferentially Ti.
The remaining effective Ti (
By fixing with total Ti-TiasTiN) and Nb, it ensures non-aging properties and deep drawability.
In fact, it has characteristics such as good elongation, low value of 6, low anisotropy, and low recrystallization temperature, so it has been highly evaluated as a cold-rolled steel sheet so far.

(Problem to be Solved by the Invention) Actually, when experimenting within the range of effective T1 according to the above disclosure,
It has been found that C in steel is not effectively bonded at T1, sometimes resulting in significant deterioration of drawability and deterioration of aging property due to residual solid solution C.

In addition, α-grain refinement and complete precipitation of Nb, Ti, etc. in hot-rolled sheets are necessary to maximize the improvement of elongation and deep drawability, so we will deal with this by high-temperature coiling as usual. When doing so, the disadvantages of coarsening of α and deterioration of descaling properties are unavoidable.

Therefore, it is an object of the present invention to establish a method for manufacturing an ultra-low carbon steel cold-rolled sheet that further exhibits the effects of the combined addition of Ti and Nb and has good elongation and deep drawability.

(Unfortunate means to solve the problem) In view of this actual situation, the inventors developed the above-mentioned ultra-low carbon Ti,
Without sacrificing the advantages of steel with NN and 1 alloys, it has good workability, especially both elongation and deep drawability, and has little anisotropy in the material, as well as good aging resistance and secondary processing brittleness. We considered ways to improve this.

As a result of a more detailed side investigation on the combined addition effect of T1 and Nb, the inventors found that TiS and TiN preferentially precipitate during slab heating or during rough rolling, which is a stage prior to hot finish rolling. However, for solid solution C, the remaining effective T1 and Nb
It turned out to be fixed.

In other words, the effective T1 is (total Ti-Ti a
It was found that s TiN-Tias Ti5) should be used.

In addition, cooling after finish rolling is particularly effective for α grain refinement, Nb,
It has the most important effect on the precipitation of Ti, etc., and the effect is particularly significant as the time from finish rolling to the start of cooling is short.

As a result of the experiment, by starting cooling within 0.5 seconds, R
Both β and the lower price are positive.1. It has been confirmed that the rapid cooling immediately after hot rolling reduces the degree of supersaturation for ferrite formation and precipitation of Nb and T1, promoting α grain refinement and precipitation of Nll and T1.

Although the effects of the present invention can be achieved by uniformly cooling the material to the coiling temperature after starting rapid cooling within 0.5 seconds, it is also possible to provide air cooling for several seconds in the temperature range of 700 to 800°C during cooling. Alternatively, increasing the winding temperature also helps to increase the effects of this invention.

In this way, by limiting the amounts of C, N, S, Ti, and Nb in ultra-low carbon steel, and by strictly limiting the hot rolling and subsequent cooling conditions, and the heating and cooling conditions of continuous annealing, we were able to create a steel sheet for press working for the first time. I was able to get something that I was completely satisfied with.

This invention has C: 0.0050% or less, Si: 1.0
% or less, Mn: 1.0% or less.

Eight! -O, []05-0.10%, P: 0.15% or less.

N: 0.0050% or less, S: 0.015
%below.

Hot rolling of steel with a composition containing
Finish rolling is completed at a temperature of
It was cooled at a temperature of 710°C or lower, and then cold-rolled at a reduction rate of 50% or less.
A method of elongation and deep drawability characterized by continuous annealing in a heat cycle in which heating is performed at a heating rate of 5°C/S or more up to ~600°C and soaked for 1 second or more in the temperature range of 700°C to Ac 3 points. This is a method for producing a good ultra-low carbon steel cold-rolled plate.

As is clear, in this invention, elucidation of the effectiveness of Ti and Nb is more important than limiting the starting material components.
From the circumstances that led to this elucidation, we will sequentially explain the effects of this invention.
Proceed with the explanation.

(Function) First, the results of a laboratory experiment conducted by the inventors will be explained.

Chemical components: Si: tr -0.02%, Mn:
0.10-0.12%, P: 0.007-0.01
0%, Aj! : 0.02 to 0.04% at the same level, and furthermore, N: 0.0027%.

C: 0.0020%, S: 0.006%, 0
．． 0.013% and 0.01.8%, and Ti
:0.015%.

3 levels of 0.025% and 0.034% and N
b: Two levels of 0.008% and 0.020% convenience 1
Eight steel types were melted in the laboratory, bloomed into a sheet bar with a thickness of 30 mm, and then hot rolled in 7 passes to form a sheet bar of 2.8 mm.
mm thick and finished at 900±5°C.

Two seconds after the end of rolling, this steel plate was
It was cooled down to 550°C at °C/S.

Then, it was immediately charged into a furnace at 550°C, maintained at 511r, and then subjected to furnace cooling treatment. This process results in a winding temperature of 5
A simulation was performed at 50°C.

Then, after pickling, cold rolling was performed at a rolling ratio of 75%.

Subsequently, continuous annealing treatment is performed at 700℃ using a resistance heating device.
Heating at 12°C/S until 78°C, then at a heating rate of 3°C/S.
It was heated to 0°C, held at 780°C for 25 seconds, and then cooled to room temperature at a rate of 5°C/S.

Next, the steel plate was subjected to 0.75% temper rolling and then subjected to a tensile test.

As a test item, the lower value (Rank't value) was used as a measure of deep drawability, and AI (aging index) was used as a measure of aging resistance.

As the results are shown in FIGS. 1 and 2, the materials of each experimental steel varied greatly with respect to the amounts of Ti, S, and Nb.

The following materials are required for press working steel plates: ≧1.
6. AI≦3. It can be seen that if 7 mm2 is used as a guideline, this is the region (N = 0.0027%) and the Nb content is 0.008%.

It can be seen that even with the same amount of -C and -Nb of Zunawaji, the drawability and aging resistance deteriorate due to an increase in S, and it is necessary to increase the amount of Ti to match the increase in S.

On the other hand, regarding the effect of Nt4, even if the amount of TI is small and the amount of S is large by increasing the amount of Nb, it is possible to lower the aging resistance, that is, improve the aging resistance, but it is also known that there is almost no positive effect on the lower value. .

C: In order to improve the total elongation (Eβ) and Lankford value (r), which are the most important values for a steel plate for processing, the less C the better, preferably C50,0 from 50%.
035% is good. When C increases, a large amount of TI is required to fix it as carbide.

Nb is required, and not only the workability deteriorates due to precipitation strengthening of TiC, NbC, etc., which are generated, but also adverse effects such as an increase in the recrystallization temperature during continuous annealing occur.

Sl: It may be added to increase the strength of high-strength steel sheets for deep drawing, but excessive addition causes deterioration of secondary processing brittleness and chemical conversion treatment properties, so the upper limit should be set to 1. 0
%.

Mn: For the same reason as Si, Mn is set to 1.0%.

N: Like S, N is fixed at T1 before hot rolling, so it is not harmful by itself during N. However, since TiN formed by adding a large amount reduces the total elongation and lower value, the upper limit is set at 0.0050%, but the more preferable range is:
It is 0.0035% or less.

Also, if T1 is so small that N cannot be fixed, N becomes A[
Fixed as N. In this case, the hot rolling coiling temperature is 710℃
Below, the agglomeration of 8βN does not proceed, and as a result, the product becomes hard after continuous annealing, resulting in poor press workability.

S: In this invention, S is the most important element in relation to the amount of T1. S is rendered harmless as TiS during heating, for example, in a slab before hot rolling, but in the first
As shown in the results in Figure 2, excess S increases the amount of Tl needed to fix it, causing material deterioration.
The limit is set to 0.015%.

Ti: Ti is the most important element among the chemical components of this invention. Ti is A, i! S and N are fixed prior to hot rolling. Af+<, which was discussed in detail in FIGS. 1 and 2, the lower limit of Ti is equal to the amount that fixes S and N.

For the first limit of TI, the effective Ti (=total T
Considering that a part of i-Ti as TiN-Ti as Ti5) forms TiC, the precipitated Ti
C and T1 present in a solid solution state should be limited to a range that does not cause deterioration in material quality, increase in alloy cost, productivity, or decrease in productivity due to increase in recrystallization temperature. Considering these, the upper limit of Ti is as follows.

Nb: Nb is important for fixing C when the amount of Ti is small, and in relation to C, the lowest amount of Nh means that if TiC cannot fix C, only 20% of the solid solution C will be fixed by Nb. Although it may seem impossible, in our experience, most of the remaining 80% solid solution C forms a special atmosphere around the precipitated NbC, which seems to be in the pre-precipitation stage, resulting in decreased aging and ductility. It was confirmed that there was no adverse effect on the

By adding Nb in combination with Ti, the lower value and anisotropy of IEIl, which are the drawbacks of steel with only T1 added, are reduced. For example, in a T1 single steel with an average lower value of about T7, the rolling direction (ro) and the rolling direction (r9o) are about 2.1, but the diagonal direction (4.) is about 1.3. On the other hand, in the steel to which Nb is added according to the present invention, △r is about 0.2 to 0.4, and the anisotropy is extremely small, making it difficult to crack during pressing. However, excessive addition of Nb not only causes material deterioration during hot-rolling and low-temperature coiling as shown in Figures 1 and 2, but also causes a significant increase in recrystallization temperature and cost increase. Therefore, the upper limit is equivalent to C, that is, Al: Al is required to be at least 0.005% in order to fix 0 in molten steel and improve the yield of Ti and Nb.

- As mentioned above, most of the N in force-dropped steel is fixed at T1, so adding a large amount of Al2 increases the cost.
Therefore, the upper limit is set to 0.10%.

P: P is the most effective element for increasing strength without reducing the r value, but excessive addition is not desirable for secondary processing brittleness, and the first limit is 0.15%. do.

Next, regarding hot rolling conditions, the heating temperature of the slab before hot rolling is not particularly limited, but in order to fix S and N at T1, it is preferably 1280°C or lower, preferably 1230°C or lower, and more preferably 1150°C or lower. desirable.

Note that similar effects can be expected by performing so-called direct slab rolling or hot rolling as it is cast as a slab having a thickness of about 30 mm.

During hot rolling, if the temperature is not above Ar 3, the material will deteriorate.

After this finish rolling, the ferrite (α) grain size of the hot-rolled steel sheet changes significantly due to changes in the cooling pattern until coiling. In general, if the cooling rate from completion of rolling to coiling is slow, the α grains become coarse, and the TI of the present invention.

This tendency is particularly noticeable in Nb composite added steel. As the α grains become coarser, the grain boundary area decreases and the (11,1) texture does not develop after annealing, resulting in an inferior r value, and the grain size after annealing also increases, resulting in poor secondary processing resistance. It is also inferior in quality.

For this reason, it is necessary to start rapid cooling as soon as possible after finishing the second rolling, specifically within 0.5 seconds, and to make the average cooling rate from the start of cooling to coiling to be 10° C./S or more. The precipitation of Ti and Nb is also promoted by starting rapid cooling within 0.5 seconds after the completion of secondary rolling.

Although the material quality is good even if the winding temperature is as low as 600° C. or lower, the material quality is further improved if the winding is performed at a high temperature of 600° C. or higher.

If the winding temperature exceeds 710°C, not only the effect of improving the material quality will be saturated, but also the descaling property will deteriorate significantly, so the upper limit is set at 71θ°C.

Next, regarding the cold rolling conditions, in order to improve drawability, the cold rolling rate after descaling must be 50% or more, more preferably 70% to 90%.

Furthermore, as already mentioned, the continuous annealing conditions are C,
By limiting the amounts of Ti and Nb in accordance with N and S@, it is possible to produce steel sheets that are extremely easy to deep draw and have good aging resistance and anisotropy, but limiting these elements alone will result in poor secondary processing resistance. Improvement in vulnerability is not sufficient.

In particular, since the processed steel sheets treated with the present invention are often used in heavily processed parts such as automobile high roofs and engine oil pans, it is essential to improve the resistance to secondary processing. This is because if the secondary processing brittleness resistance is poor, the steel plate will be brittle and destroyed by a strong impact after strong press working, which is not good for the safety of the vehicle body.

Addition of B (boron), addition of sb < antimony, etc. can be considered as a method of improving the resistance to secondary processing brittleness. However, in the former case, there is a problem that the recrystallization temperature rises significantly to -, and in the latter case, the cost increases.

In this invention, by combining the cooling control during hot rolling described above and the heating control during continuous annealing described here,
This problem has been resolved.

Specifically, the heating rate from 400 to 600°C during heating is limited to 5°C/S or higher.

These temperature ranges are temperature ranges in which P dissolved in steel is extremely likely to segregate at grain boundaries, and rapid heating in this temperature range suppresses grain boundary segregation of P and increases grain boundary strength. It is necessary to limit the amount because it improves the resistance to secondary processing brittleness. In the temperature range of 600 to 400°C during cooling, the secondary processing embrittlement resistance is good even without special restrictions as in the case of heating, but if the temperature range is rapidly cooled at 10°C/S or more, Further improvement.

The maximum heating temperature during continuous annealing is set to ensure deep drawability.
Soaking at 700°C or higher for 1 second or more is required. On the other hand, A
If the temperature exceeds the C3 point (approximately 920 to 930°C), the deep drawability decreases rapidly, so the heating temperature is set to 700 to Ac3.

(Example) C: 0.0025%, Si: 0.01%, Mn: 0.
16%, P: 0.010%, S: 0.005%, l:o
, 035%, -=, 0.019%) The steel containing other unavoidable impurities was tapped from a converter and made into a slab by continuous casting. The slab was then reheated to 1250°C and then heated to 890°C for 3.
Finished to a thickness of 2mm, then immediately after 0.3 to 1 second
Rapid cooling was started at a temperature of 520 to 650°C, and winding was performed at each temperature of 520 to 650°C.

After pickling, cold rolling was performed at a rolling reduction of 75% to obtain a cold rolled sheet with a thickness of 0.8 mm.

Continuous annealing was then performed at 800°C. In addition, 400℃
The heating rate is 15℃/S until 600-800℃, and the heating rate is 4℃/S from 600 to 800℃.
Cool from 0℃ to 600℃ at 1.5℃/S, and then cool to 600℃
The following temperature range was cooled at approximately 5° C./S.

Table 1 shows the results after 0.5% temper rolling.

In Table 1, in addition to the results of the test steel (A>), the results are shown in comparison with the results of the comparative steel listed in Table 2, which has an inappropriate amount of C, Ti, or Nb.

Here, steel (B) is C1, (C) and (D) are T1, and steel (ε) and (F) are comparative steels with Nb removed.
Regarding sample steel (8), the material quality was inferior as well as when the start of quenching after finish rolling was delayed.

(Effects of the Invention) According to the present invention, it is possible to stably produce an ultra-low carbon steel cold-rolled sheet that satisfies the elongation and deep drawability required for steel sheets for press forming of automobile bodies, etc., and the effects thereof are tremendous.

[Brief explanation of the drawing]

FIG. 1 is a chart showing the effect of the amounts of Ti, S, and Nb on the r value of a steel sheet, and FIG. 2 is a chart showing the effect of the amounts of Ti, S, and Nb on the r value of a steel sheet.

Claims (1)

[Claims] 1. C: 0.0050wt% or less, Si: 1.0wt%
Hereinafter, Mn: 1.0 wt% or less, Ti: [(48/14)N (%) + (48/32)S(
%)] ~ [3・(48/12)C(%)+(48/14
)N(%)+(48/32)S(%)]wt%Nb:[
0.2・(93/12)C(%)]～[(93/12)
C (%)] wt% Al: 0.005 to 0.10 wt%,
Immediately after completing hot rolling of steel with a composition containing P: 0.15 wt% or less, N: 0.0050 wt% or less, S: 0.015 wt% or less at a finishing rolling temperature of Ar_3 points or higher, Start cooling within 0.5 seconds and cool at an average cooling rate of 10 ° C / s or more until winding, 710
It is rolled up at a temperature below ℃, then cold rolled with a reduction rate of 50% or more, and then heated at a heating rate of 5℃/s or higher from 400 to 600℃, and then rolled in a temperature range of 700℃ to Ac_3 points. A method for producing an ultra-low carbon cold-rolled steel sheet with good elongation and deep drawability, characterized by carrying out continuous annealing in a heat cycle where it is soaked for at least 1 second.