JPS5852435A - Production of cold rolled steel plate of high ductility for deep drawing by continuous annealing - Google Patents

Abstract

PURPOSE:To produce a cold-rolled steel plate of high ductility for deep drawing by subjecting the aluminum-killed steel which is limited of a compsn. range of C, Mn, Al, N, P, etc., and a compsn. relation of Mn and S and is reduced of C, N, P to continuous annealing under adequate conditions. CONSTITUTION:After the low carbon aluminum-killed steel contg. 0.02-0.045% C, 0.10-0.35% Mn, >=0.7 S by weight ratio of Mn/S, 0.01-0.06% Al, <=0.0040% N, and <=0.010% P is hot-rolled at finishing temp. higher than the Ar3 point and is coiled at >=650 deg.C, the hot-rolled plate is cold-rolled to a prescribed thickness and is then subjected to the continuous annealing wherein the rolled plate is subjected to an overaging treatment for >=1min at 350-450 deg.C after it is soaked for >=10sec at temp. higher than the recrystallization temp. and lower than 850 deg.C, whereby a cold-rolled steel plate for deep drawing having excellent ductility is produced without application of skin pass rolling. The above-mentioned content of C is decreased to the extent of not requiring any vacuum degassing device, whereby costs are reduced.

Description

[Detailed Description of the Invention] The present invention relates to a method for manufacturing a cold-rolled steel sheet for deep drawing with high ductility by continuous annealing. The company manufactures cold-rolled steel sheets for deep drawing.

Several methods have been proposed so far for producing cold-rolled steel sheets for press working by continuous annealing using low carbon aluminum killed steel as a material. For example, reducing the Mn content in low carbon aluminum killed steel to 0.20% or less and heating the slab to 1100%
There has been an attempt to manufacture cold-rolled steel sheets that do not cause hot embrittlement and have excellent press formability by applying continuous annealing to reduce solid solution S by heating at a low temperature of ~1200°C.

In addition to this, there has been an attempt to manufacture a cold rolled steel sheet with excellent ductility through continuous annealing by specifying the Si and N contents of low carbon aluminum killed steel and adding Cr. However, the aluminum-killed cold-rolled steel sheets actually manufactured using these methods have inferior ductility (elongation) and deep drawability compared to conventional box-annealed aluminum guild steel, and are particularly susceptible to aging deterioration caused by solid solute C. As a result, the ductility further deteriorates after manufacture until use, and problems such as cracking and necking often occur during press forming.

In order to prevent these drawbacks and to produce cold-rolled steel sheets for deep drawing with excellent ductility by continuous annealing, it has been necessary to
A method has been adopted in which Ti and Nb are added to so-called ultra-low carbon aluminum killed steel, which has been decarburized to 1% or less, to fix nitrogen and carbon in the steel.

These steel sheets have both good press formability and non-aging properties, and have perfect material properties, but they require the installation of a vacuum degassing device and are expensive to manufacture. It has the following drawback.

Therefore, C: 0. which does not require vacuum degassing. [12~
We investigated a method for producing high-ductility cold-rolled steel sheets for deep drawing by continuous annealing with a low carbon aluminum killed steel composition of about 0.10%, and found that the C content could be reduced to 0.02 to 0.02%.
0.045%, P is o, oi% or less, N is 0.00
Both reduced to 40% or less, and 650℃ in hot rolling.
It has been found that when rolled at the above temperature, a cold-rolled steel sheet for deep drawing with excellent ductility can be produced by continuous annealing.

By the way, cold-rolled steel sheets for deep drawing may be used as outer plates or inner plates. When used for this inner plate, it is acceptable even if the surface quality is not so good as long as it can withstand press working.

Now, in continuously annealed low carbon aluminum killed steel, the deterioration of ductility, e.g. elongation, due to aging is caused by the skin pass applied after continuous annealing and the precipitation of solid solution C at mobile dislocations generated in the steel during the skin pass. Because it is caused by strain aging,
It was found that not applying a skin pass was more effective in preventing deterioration of elongation. Based on this knowledge, in the present invention, a cold rolled steel sheet for deep drawing is produced after continuous annealing of low carbon aluminum killed steel without skin pass.

The present invention will be explained in detail below. First, regarding the components in steel, it is effective to reduce C as much as possible within the range that can be obtained without requiring vacuum degassing, which is 0.02 to 0.
045%. (0.02% or less) C is effective for improving ductility, but in order to stably reduce C to 0.02% or less,
, a vacuum degassing device is required, which is undesirable because it is subject to restrictions on equipment and increases costs. On the other hand, C
If the content increases, ductility deteriorates, so the content should be 0.045% or less.

Although Mn has a small contribution to ductility, it is set to 035 or less in order to maintain the τ value at a high level. Moreover, if it is less than 0.10%, hot embrittlement occurs and troubles such as edge cracking are likely to occur during hot rolling.

In order to prevent hot brittleness, the amount of S is controlled by the weight ratio with Mn so that Mn/S≧07.

AI is necessary for deoxidation and has the effect of increasing the 7 value, but if its content is too low or too high, the 7 value will deteriorate as shown in Figure 1. Therefore, it is set to 0.01 or more and 0.06% or less. This Al refers to acid-soluble A7.

The sample in Figure 1 is C: 0.02~0,0
45%, Mn: 0.10~0.5°1%, P
: 0.003~0. ．． 010%, N: 0.0022~
This is a cold-rolled steel plate obtained by hot rolling aluminum killed steel containing 0.0040%, and then continuously annealing it at 860 to 865°C after cold rolling.

Since ductility is increased by reducing the N content, it is set to 0.0040% or less. This effect of improving ductility is further strengthened by the synergistic effect of the reduction in N and the reduction in P, which will be described later.

P plays an extremely important role in ductility, and reducing its content has the effect of improving ductility. In order to exhibit this effect, the content should be o, o io% or less.

The improvement in ductility due to the reduction of P and N will be described with reference to FIG. This figure 2 shows C: 0.02-0.04%.

Mn: 0.10-0.51%, Al: 0.01
A cold-rolled steel sheet made of aluminum killed steel with a base composition of 5 to 0.050% and varying N and P contents, hot rolled at a finishing temperature of 885°C or higher, and then continuously annealed at 850°C after cold rolling. Shows elongation. Note that the plate thickness is 0.8 mm. As is clear from this figure, steels in which the N content is reduced to 0.004% or less and the P is reduced to 0.0 IQ.% or less, like ordinary low carbon aluminum killed steel, have a P content of 0.004% or less. 015～
The elongation is significantly superior to that containing 20% O,'0.

The steel composition is as above, and the manufacturing conditions will be described next.

Steel having the above-mentioned components is obtained by melting in a converter, electric furnace, etc., and is made into a slab by continuous casting or an ingot-blooming method. This slab is heated immediately or after heating in a furnace.
Hot rolled. In this hot rolling, if the finish temperature is low, the 7 value will deteriorate, so finish rolling is performed at a temperature of Ar3 or higher. Further, if the winding temperature becomes too low, the ductility will deteriorate, so the winding is performed at a temperature of 650° C. or higher.

Next, it is cold rolled, and the rolling reduction is arbitrary as long as it is 60% or more. After cold rolling, continuous annealing is performed. The soaking temperature in this continuous annealing is higher than the recrystallization temperature and lower than 850°C. In the present invention, the lower limit is set as the recrystallization temperature.
, N are both reduced, so good ductility can be obtained if recrystallized. Further, the upper limit is set to 850° C. because when the soaking temperature becomes high, mixed grains may appear, leading to deterioration of ductility. The product is soaked at the soaking temperature for 10 seconds or more. The reason why the soaking time is set to 10 seconds or more is because if the soaking time is too short, the crystal grains become small and the ductility deteriorates.

After being soaked at the soaking temperature, an overaging treatment is performed at a temperature of 350 to 450° C. for 1 minute or more.

The reason why the overaging treatment is performed at 350 to 450°C is that this temperature is effective for reducing solid solution C. If the overaging treatment time is short, the amount of solid solution C precipitated is small, so the overaging treatment time is set to 1 minute or more. However, it is impossible to completely precipitate the solute carbon in the steel by this over-aging treatment, and this residual solid solute C sticks to the mobile dislocations generated by the skin pass, lowering the yield point and
The problem is that it significantly impedes press formability due to the deterioration of ductility. Strain aging due to solid solute carbon in low carbon aluminum killed steel is not normally present in box annealed materials at room temperature, but is a phenomenon specific to continuous annealing where the cooling rate is high, and is a major cause of inhibiting the press formability of continuously annealed materials. ing.

Through experiments, we have found that as a means to prevent this strain aging, it is extremely simple and effective to minimize the number of movable dislocations in the steel without applying any skin passes, as shown in Figure 6. . The steel in Fig. 3 has C: 0.0
22%, Mn: 0.18%.

P: 0.006chi, N: 0.0023%, Al: 0,
Hot rolled with low carbon aluminum killed steel containing 0.040%,
After cold rolling, it was continuously annealed at 800°C and over-aged at 400°C for 5 minutes. FIG. 3A shows the effect of skin pass rolling reduction on elongation (El) for a skin pass board and a case where the skin pass board was artificially aged at 100° C. for 60 minutes. Further, B in FIG. 3 shows the degree of deterioration of elongation CEl> due to artificial aging. Note that the plate thickness is 0.8 inch. From these figures, the skin path “
It is clear that there is no deterioration in elongation of the skin pass material not applied (rolling reduction: 0)/-.

Since this no skin pass material has elongation at yield point,
Although it is difficult to apply to parts where stretcher strain on the outer plate is a problem, it is fully applicable to deep-drawn inner plate parts where stretcher strain is not a problem and ductility is particularly required. Even if the skin pass is omitted, the heat flattening effect (heat f
Due to the lattening effect), the steel plate has excellent properties, and there are no practical problems. In addition, surface roughness can be adequately adjusted for inner plate parts by adjusting the roughness using cold rolling rolls.

Next, examples will be shown. Table 1 shows the chemical composition and manufacturing conditions of each sample. In addition, each sample was hot rolled to 2.5 mlK.
and cold rolled to 0.8 In. Table 1 also shows the mechanical property test results.

As can be seen from these results, the trap sample manufactured by the method of the present invention, %1.2.3, has better elongation than the others, and it is particularly clear that there is no deterioration in elongation due to aging.

As described above, a cold-rolled steel sheet for deep drawing with excellent ductility is manufactured by the present invention in which low-carbon aluminum-killed steel with reduced P and N content is continuously annealed and free of cracks and skin passes.

[Brief explanation of drawings]

Figure 1 shows the influence of Al content on 7 values, Figure 2
The figure shows the effect of P and N contents on elongation, and Figure 3 shows the effect of skin pass reduction on elongation and deterioration of elongation due to aging. Stem l Must-Kaya Eni FJ CZ)

Claims (1)

[Claims] C: 0.02-0.045%, Mn: 0.10
~0.55%, S to Mn weight ratio Mn/8 is 0.7
Above, A/: 0.01-0.06%, N 0.00
A low carbon aluminum killed steel containing 40% or less and 0.010% or less of P is hot rolled at a finishing temperature of 3 points or more of Ar,
After rolling at a temperature of 0°C or higher, it is cold-rolled to a predetermined thickness, then soaked at a temperature above the recrystallization temperature and below 850°C for 10 seconds or more, and then at a temperature of 350 to 450°C for 1 minute or more. A method for producing a highly ductile cold-rolled steel sheet for deep drawing by continuous annealing, which is characterized by carrying out continuous annealing with over-aging treatment and producing without skin pass.