JPS60262946A - Two-phase stainless steel with superior hot workability - Google Patents

Abstract

PURPOSE:To obtain the titled two-phase stainless steel which reduces cracking to a minimum during cogging or hot rolling and gives a considerably improved yield by reducing the amount of S while restricting the amount of Al and by adding Ca according to the S content. CONSTITUTION:This two-phase stainless steel with superior hot workability contains, by weight, <=0.08% C, <=2.0% Si, <=5.0% Mn, 18.0-30.0% Cr, 1.0-9.0% Ni, 0.5-5.0% Mo, <=8.0% Cu, 0.05-0.80% N, <=0.04% P, 0.005-0.10% Al, <=0.005% S and (1-10)[%S] Ca in accordance with S content. When the austenitic-ferritic two-phase stainless steel is hot worked, cracking can be considerably reduced and the yield can be improved.

Description

[Detailed Description of the Invention] Technical Field The technical content described in this specification regarding duplex stainless steel is related to improving hot workability by modifying its component composition〇Background Technology Duplex stainless steel has been widely used as a structural material in recent years because it has superior corrosion resistance, stress corrosion cracking resistance, and weldability compared to austenitic stainless steel and ferritic stainless steel. Recently, it has been attracting attention as a steel for pipelines in environments containing COB, H, and S.However, since duplex stainless steel consists of a ferrite phase and an austenite phase, it is difficult to use during hot working. , cracks are likely to occur at their interfaces, resulting in extremely low yields during blooming and hot rolling.Purpose of the InventionThe present invention advantageously solves the above problems. The purpose of the present invention is to propose a duplex stainless steel with excellent hot workability 1, which makes it possible to improve the yield by minimizing the occurrence of cracks during blooming and hot rolling.

Elucidation of the solution A large number of studies have been conducted on the hot workability of stainless steel, which exhibits a two-phase structure of ferrite and austenite phases at high temperatures, and general measures to improve it include: It is said that reducing S and further adding REM are effective.

Therefore, the inventors first tried to improve the S content in the steel by reducing the S content to less than 0.0016% by weight (hereinafter referred to simply as "Substance"), and then produced a duplex stainless steel slab by continuous casting.
After sizing with a blooming mill and hot rolling, cracks with a length of about 705+ in the width direction of the hot-rolled sheet appeared everywhere on both ends of the hot-rolled sheet.

Therefore, in order to examine the efficacy of RIM, which has been conventionally said, we decided to
An ingot of duplex stainless steel containing 15% was made and subjected to the same experiment as described above. As a result, the occurrence of edge cracking in the hot-rolled sheet was slight, but the surface quality was worse than that of the REM-free sheet.
It has also been found that the improvement in yield as expected by the present invention cannot be expected much because the continuous casting method cannot be applied to additive steel and it is necessary to make it into an ingot material.

This invention advantageously solves the above-mentioned problems, and even when using continuous casting materials that are prone to cracking, the occurrence of edge cracks is minimized, resulting in a significant improvement in yield.・With regard to the composition of duplex stainless steel, especially AI! This invention is based on new knowledge that it is extremely effective to limit S to a certain range, reduce S, and add Oa in accordance with this Si.0 Invention composition, that is, inventiveness, a: o, oa% or less, S to s
, o% or less, In: 5.094 or less, Or:
18.0~B0.0-1Ni: 1.0~9.09&
, MO: 0.6-5.0%, QuS 8.0%
Hereinafter, N: 0.06-0.80%, P: 0.
04% or less, together with 0.006 to 0.10 inches of Ar and S with the mixed amount suppressed to less than o, ooa degrees, and Oa
A duplex stainless steel with excellent hot workability, containing Qa: (1 to 1O) [%S] depending on the S content, with the remainder consisting of Fe and unavoidable impurities. be.

This invention will be explained in detail below. First, the reason why the component composition is limited to the above range in this invention will be described.

0: 0.08% or less C is an element that is unavoidably contained in steel, but 0.08%
If the content exceeds 0.08%, the corrosion resistance and intergranular corrosion resistance will deteriorate, so the upper limit was set at 0.08%.

si: s, o or less Si is an element A useful as a dispersing agent, but the content is
If the O coefficient exceeds , the σ phase will occur and the corrosion resistance and toughness will deteriorate significantly, so the upper limit was set at 2.0%.O In: 6°0% or less Although it is useful as an adjusting element, if the content exceeds B, the corrosion resistance deteriorates, so the upper limit was set at 5.0.

Or: 18.0-80.0% Qr is an essential element for improving corrosion resistance and forming an austenite-ferrite S phase structure,
Considering resistance to pitting corrosion and crevice corrosion, is, Oq
If it is less than b, the effect is poor, while if it exceeds 80.0%, the σ phase tends to precipitate and the toughness tends to deteriorate, so the Qr content is limited to a range of 18.0 to 80.0 h. Ni: 1.0 to 9.0 Ni Vi is an essential element from the viewpoint of increasing resistance to general corrosion and forming a two-phase structure, but the content is 1.01
If the content is less than 9.0 tres, sufficient corrosion resistance cannot be obtained, while if the content exceeds 9.0 tres, the improvement effect not only reaches saturation but is also expensive, so it is limited to the range of 1.0 to 9.0 tres. Mo: 0.5 to 6.096 MoVi is an element useful for increasing resistance to local corrosion that occurs in a corrosive environment containing chlorine ions, but
If the content is less than 0.5%, the addition effect will be poor,
On the other hand, even if it is added in an amount exceeding 5.0%, the improvement effect will be extremely small, and since it is an expensive element,
．． Ocu limited to the range of 0%: s,o% Cu effectively contributes to improving corrosion resistance against non-oxidizing acids, but when added in excess of 8.0%, it deteriorates hot workability. Therefore, ON with the upper limit set to 8.0%: 0.0
In addition to being an important element for forming a two-phase structure,
Although it effectively contributes to corrosion resistance, if it is less than 0.059b, its addition effect is poor, while if it exceeds 0.80b, hot workability will be significantly deteriorated, so the content should be 0.0b.
OP limited to the range of ~0.80 modulus: 0.04 modulus or less P is an impurity that is inevitably mixed in and is an element harmful to improving hot workability, but at 0.04% t. Acceptable◎ AI! : 0.005~O,lO% AI! is an element useful for reducing oxygen, which deteriorates the hot workability of the steel of the present invention, but if it is less than 0.005%, the effect of its addition is poor, and the hot workability cannot be improved to a satisfactory degree. In order to suppress the occurrence of cracks during rolling, it is necessary to contain 0.0064 or more. Figure 1 shows the results of investigating the effect of HP in steel on hot workability. . In the experiment, C: about 0.02%, Si
: 1% or less, Mn: 0.5% to 1%,
Or: 22.0-28.0%, Ni: 5.0
~7.0%, M.

: 1.0 to 4.0%, Cu: 8% or less, N:
(1,10~0.20chi, P: 0.002~0.
04 Chi, S: 0.0005 to 0.008, and a composition with an Oa/S ratio of 8 to 5, and a sample with a ferrite amount of 80 to 60 at 960°C, was subjected to high-temperature, high-speed tensile testing at 950°C. A test was conducted to find out the cross-sectional reduction rate.The limit of the cross-sectional reduction rate at which zero-edge cracking does not occur is 70%, but AI! If the amount is less than 0.005%, it is below the critical section ratio.

However, AI! If the amount exceeds 0.10%, surface defects due to alumina clusters will increase, so the A/O content is limited to a range of 0.005 to 0.10%.
, ooa% or less S is an impurity that inevitably mixes into steel, and is an element that is particularly harmful to the hot workability of the steel according to the present invention, and if its content exceeds 0.008%. Even if Ca, which will be described later, is added, the amount of sulfide increases, which not only reduces the effect of Ca addition, but also deteriorates weldability, so the S content is suppressed to a range of 0.008% or less. It was decided to.

Ca: (1 to 10) (%5) Ca is a strong sulfide-forming element, reduces solid solution S by forming sulfides, and effectively contributes to improving hot workability. However, even if it is simply added, if the amount added is less than the amount required to fix solid solution S, the addition effect will be poor, and on the other hand, if the amount added is too large compared to the amount of solid solution S, weldability will deteriorate. becomes worse 0 Therefore, Qa is
It is important to add an appropriate amount in balance with the amount of S.The second factor is 0: 0.02%, Si: 0.4'
Iy, Mn: 1.5%, Or: zs4, Ni: 5
．． 5%, M□: 89bsN: 0.15ch, P
≦0.04chi, AI! ≧Q, 005% and S≦0.0
The results of an investigation of the hot workability of a steel whose basic component is 0.08% Ca and 0.08% Ca were added in various types were compared with the relationship between the 0% ratio and the cross-section reduction rate in a high-temperature, high-speed tensile test. show.

As is clear from the figure, when Ca/8 is less than 1, the area reduction rate is less than 70%, so the lower limit of 0a/s is set to 1.On the other hand, when 0a/s exceeds 10, Not only does the effect of adding Oa reach saturation, but also there is a risk that surface defects due to oxides and sulfides will occur, and if the amount of Ca added increases, weldability will also be inhibited as described above.
. The upper limit of a/s was set at lO.
EI) was made into a slab with a thickness of SOOrnm by a continuous casting method, and then hot rolled at 18 (blush rolling to lsmH, then reheated and then 4 cycles). The results of investigating the occurrence of ear cracks are shown in Table 1.0 If the length of the ear crack in the width direction is less than l Q mm, the actual damage will be small;
Test materials A1 and 8 whose O A/content is below the appropriate range of this invention, and test materials 4B and 6 whose Ca content is below the appropriate range, are acceptable as acceptable products.
, and further contains the amount of N and S exceeding the upper limit 4
In all cases of Machining 5, edge cracks of 20 mm or more occurred.On the other hand, in all of the examples (sample materials/467 to 18) that satisfied the appropriate range of this invention, the degree of edge cracks occurred. It felt extremely small, and the largest one was only 111 mm at most.

Effects of the Invention Thus, according to the present invention, it is possible to significantly reduce the occurrence of cracking during hot working, which was a concern in the past in austenitic-7elite dual-phase stainless steel, and therefore, it is also possible to improve the yield.

[Brief explanation of the drawing]

Figure wJ1 shows that Aj in steel does not affect hot workability and Al
Figure 2 is a graph showing the relationship between 0a/s and area reduction rate.0 Patent applicant: Kawasaki Steel Corporation Figure 1 Al amount ( Urn volume %) cross section reduction,! ! (crawl

Claims (1)

[Claims] LG: 0.08% by weight or less, Si: 2.0% by weight or less, Mn: 5.0% by weight or less, Or: 18.0-8o, O weight ratio, Ni: 1.
0 to 9.0 weight ratio, MO: 0.5 to 5.0 weight-1 Ou: 13.0 weight% or less, N: 0.05 to 0.80 weight%, and P: 0.04
% by weight or less, 0.005 to 0.10% by weight of Al and mixing amount o, oo
Contains S with S suppressed to s·wt% or less, and Qa,
A duplex stainless steel with excellent hot workability, characterized in that the S content is in the range of (3a: (1 to 10) (SS), with the remainder consisting of F8 and inevitable impurities.