JPH03169455A - Production of continuously cast slab having excellent high temperature deformability - Google Patents

Abstract

PURPOSE:To produce a steel slab without surface defect by controlling S and P contents inevitably mingled into the steel to a specific range and specifying cooling velocity in a specific temp. range. CONSTITUTION:At the time of producing the boron containing high tensile strength steel containing 0.0005-0.0020wt.% B, 0.010-0.10% Al and <=0.0050% N with continuous casting, S and P contents inevitably mingled into the steel are restrained to <=0.0015% S, <=0.020% P and in the range satisfying 0.3P+S <=0.060%. Further, the cooling velocity in the range from the m.p. to 850 deg.C is controlled to <=0.5 deg.C/s. By this method, the development of slab defect, particularly, surface crack, is prevented.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a method for manufacturing a continuously cast slab having excellent high-temperature deformability, and in particular to advantageously prevents the occurrence of slab defects, including surface cracks, during continuous casting. This is what I am trying to do.

(Prior technology) Regarding the prevention of slab defects, especially surface cracks, during continuous casting of boron-added high-strength steel,
No. 73 (1987) No. I P. 115-122
”, to reduce free N, Ti is calculated using the following formula (wtχN
r o t s L O . 2 X w tχTi)
JP-A No. 56-8036 discloses a method in which the secondary cooling zone (1000 to 700°C) of continuous casting is slowly cooled at a rate of 0.5°C/s or less.
Publication No. 7 states that for boron-added steel that falls outside the range of the above formula, the average cooling rate (within 20 mm of the slab surface layer) from the melting temperature to 900''C is 0.01 to 1℃/s.
Various cooling methods have been proposed.

(Problem to be solved by the invention) However, when Ti is added to reduce free N, the toughness of the cast slough deteriorates significantly as shown in Table 1, and internal cracks due to internal stress occur. (in the center), and the toughness of the steel material after rolling is also extremely poor.

Table 1 This invention advantageously solves the above problems, and it goes without saying that surface cracks do not occur during slab production.
The purpose of the present invention is to propose an advantageous method for manufacturing a continuously cast slab with excellent deformability, which does not cause deterioration in toughness even when the cast slab is further rolled.

(Means for Solving the Problems) As a result of intensive research to solve the above problems, the inventors have succeeded in suppressing the contamination of impurities in steel, especially P and S, to a predetermined amount or less, and It has been found that suppressing the cooling rate below a certain level during the cooling process is extremely effective in achieving the intended purpose.

This invention is based on the above knowledge.

That is, in this invention, B: 0.0005 to 0. 0020wt% (hereinafter simply expressed as %) AI: 0.010-0. 10%
and N: When producing boron-added high-strength steel containing 0.0050% or less by continuous casting, (a) S and Pg that are unavoidably mixed in the steel are removed from S:
0.0015% or less, P: 0.020% or less, and the range that satisfies 0.3P+S≦0.0060%, and (0) the cooling rate from the melting temperature to 850°C is 0. This is a method for manufacturing a continuous casting slab having excellent deformability, which comprises controlling the deformability to .5° C./S or less.

This invention will be specifically explained below.

First, the experimental results that led to this invention will be explained.

C: 0.12%, Si: 0.20%, Mn
: 0.86%, Cu: 0.23%, Ni:
0.77%, Cr: 0.51% + Mo: 0
．． 45%, V: 0.03%, and B:O.
OO14%, AI. : 0.045% and N :
O. FIG. 1 shows the results of an investigation into the relationship between the amount of P and S mixed in and the high-temperature ductility in steel containing 45% QO when the mixed amounts of P and S were varied.

As is clear from the figure, good ductility with RA2 (an index representing high-temperature ductility) of 50% or more is exhibited when S≦0.0015% and P≦0.020%, as indicated by the circle in the figure. Big, 0.3
It was found that the range of P + S≦0.0060% was satisfied.

Furthermore, regarding steel with the above composition, especially B, AI
and N respectively, B: 0.0005 to 0.00
20%, Al: 0.010-0.10%, N
: When the influence of P and S was similarly investigated for steel types with various changes within the range of 0.0050% or less, it was found that
Almost the same results as in Figure 1 above were obtained.

The reason why good high-temperature ductility can be obtained by restricting P and S in the steel to the above range is that grain boundaries are strengthened by reducing P and S, and this strengthening is subsequently caused by the addition of BN, etc. This is thought to be due to the fact that it does not deteriorate much even when subjected to deterioration due to the precipitation of carbonitrides.

However, it has become clear that the desired effects may not necessarily be obtained solely by adjusting the precepts mentioned above.

However, it has been found that this problem can be solved by slowly cooling the cooling rate during continuous casting, especially in the embrittled region of B-added steel.

Therefore, when we examined the temperature range and cooling rate in which such controlled cooling should be performed, we found that at least 8
It has been found that it is necessary to slowly cool the temperature range of 50°C at a cooling rate of 0.5°C/S or less.

By performing such slow cooling, it is possible to change the precipitation sites of carbonitrides such as BN, and to reduce the difference in strength between grain boundaries and grains, so it is believed that the above problem will be resolved. available.

(Function) The object of this invention is boron-added high tensile strength steel having a tensile strength of 60 kg or more, and its typical composition is shown in Table 2.

Table 2 (z) Here, for Mo, Cr+ Ni, Cu, gate 0
It may be added alone, or in a combination of Mo + Cr, Ni + Cu, or Mo + Cr + Ni + Cu.

In this invention, it is particularly important to regulate B, ^1, N, and PS within the above ranges.

B: 0.0005-0.0020% B effectively contributes to improving the strength of steel, but its content is 0.0005%
If the content is less than 0.0020%, the effect cannot be expected, whereas if the content exceeds 0.0020%, it will have a negative effect on the adhesion properties, so it was limited to the above range.

AI: 0.010 to 0.10%, AI is an element that should be included from the viewpoint of deoxidizing and suppressing the growth of BN in order to ensure the hardenability of B.

In this case, if it is less than 0.010%, the effect of deoxidizing and suppressing BN generation will be weak, while if it exceeds 0.10%, the cleanliness of the steel will be significantly deteriorated, so it is limited to the above range.

N≦0.0050% Since N is unavoidably mixed into molten steel and deteriorates the toughness of the steel, the upper limit is set to 0.0050%.

S≦0.0015% In order to improve the toughness of the base metal and weld, the lower the better, but in this case, 0.0 from the viewpoint of surface defects in combination with P.
The upper limit was set at 0.015%.

P≦0.020% As with S, a lower level is preferable, but in this case, from the viewpoint of surface defects in combination with S, the upper limit was set at 0.020%.

However, it is not enough for S and P to simply satisfy the above ranges; in order to achieve the intended purpose, as shown in Figure 1 above, after satisfying the above ranges, the following formula must be used: It is also necessary to satisfy the following relationship: 0.3P+S≦0.0060%.

(Example) Molten steel having the various precepts shown in Table 3 was continuously cast under the following conditions. Cooling was performed at various rates shown in 3.

Table 3 also shows the results of examining the surface properties of each slab thus obtained.

(Effects of the Invention) Thus, according to the present invention, steel slabs can be manufactured without surface defects during continuous casting of boron-added high-strength steel slabs, and the resulting slabs have good deformability. Because of this, internal cracks may occur at the slab stage,
No deterioration of toughness occurs after rolling.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of P and S on high temperature ductility. O / O P (x7θ-3%) 20 30

Claims (1)

[Claims] 1. B: 0.0005 to 0.0020 wt%, Al: 0
．． 010-0.10wt% and N: 0.0050wt
(a) When producing boron-added high-strength steel containing less than 0% boron by continuous casting, (a) the amount of S and P that inevitably mixes in the steel is reduced to S: 0.
0015wt% or less, P: 0.020wt% or less, and suppressing the cooling rate to a range that satisfies 0.3P+S≦0.0060wt%, and (b) the cooling rate from the melting temperature to 850°C by 0.5°C. A method for manufacturing a continuous casting slab having excellent deformability, characterized by controlling the deformability to less than /s.