JPH01162750A - Martensitic stainless steel for welding construction - Google Patents

Abstract

PURPOSE:To prevent cracking at the time of hot rolling and also to prevent the occurrence of weld crack at the time of welding by reducing respective contents of P, S, N, and O in a stainless steel and specifying Ca content based on S content. CONSTITUTION:A structural martensitic stainless steel has a composition consisting of, by weight, <=0.04% C, <=1.0% Si, <=2.0% Mn, 10-15% Cr, 3.6-8.0% Ni, <=0.015% N, <=0.01% O, <=0.03% P, <=0.01% S, Ca in an amount 1-10times S content, and the balance essentially Fe and satisfying the relationship in (%)C+(%)N<=0.050. Further, 0.1-1.0% Mo is incorporated, if necessary. The steel of the above composition has superior toughness and causes no cracking at the time of hot rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) Stainless steels can be broadly classified into five types: ausnitic, ferritic, martensitic, two-phase, and precipitation hardening.

When using these stainless steels for welded structures that require strength and corrosion resistance, such as building structures, food-related machinery, vehicles, ships, and steel towers, the strength level, ease of welding, and welded area must be determined. It is necessary to consider toughness as well as stress corrosion cracking induced by welding residual stress.

Martensitic stainless steel has excellent features such as high strength compared to other types of stainless steel, except for weldability and weld toughness, and low susceptibility to stress corrosion cracking. It is advantageous as a structural material.

In recent years, with advances in welding technology and higher functionality of structures, structures are becoming larger. This trend toward larger sizes is also seen in structures using martensitic stainless steel, and there is a demand for thicker steel plates. Therefore, there is currently an urgent need to develop a martensitic stainless steel with excellent manufacturability, toughness, and weldability, and martensitic stainless steels that meet these requirements will be described below.

(Prior art) Japanese Patent Publication No. 42-16870 proposes a method for producing steel with good strength, toughness, weldability, and corrosion resistance by applying heat treatment to finely disperse an appropriate amount of austenite in a martensite matrix. has been done.

(Problems to be Solved by the Invention) Ordinary martensitic stainless steels sometimes develop cracks on the surface or edges during the hot rolling process, resulting in a decrease in walking performance. Further, during welding, weld cracks occur in the welded portion, and particularly in the case of electron beam welding, blow balls also occur.

This invention advantageously solves the above-mentioned problems.
The purpose of the present invention is to propose a martensitic stainless steel for welded structures that does not cause cracks on the surface and edges during hot rolling, and does not cause weld cracks or blowholes in welded parts.

(Means for Solving the Problem) As a result of intensive research to achieve the stated purpose, the inventors found that cracks in the surface or edge portion during hot rolling can be prevented by 'Reducing J and containing Ca at least equal to the S content, P and Sl
We obtained the knowledge that reducing the amount of N and 0 is particularly effective.

This invention is based on the above knowledge.

That is, C: 0.04 wt% or less (hereinafter simply expressed as %), Si: 1.0% or less, Mn: 2.0% or less, Cr: 10 to 15%, Ni: 3.6 to 8.0%,
N: 0.015% or less, O: 0.01% or less, P
: 0.03% or less and S: 0.01% or less, and also contains Ca in an amount equivalent to 1 to 10 times the S content. The remaining part is martensitic stainless steel for welded structures (first invention) that satisfies the relationship (X) and has a substantially iron composition.

Further, C: 0.04% or less, Si: 1.0% or less, Mn: 2.0% or less, Cr: 10 to 15%, Ni:
3.6-8.0%, Mo: 0.1-1.0%, N
: 0.015% or less, O: 0.01% or less, P:
In addition to containing 0.03% or less and S: 0.01% or less, it contains Ca in an amount equivalent to 1 to 10 times the S content,
Ka<7. The martensitic stainless steel for welded structures (second invention) satisfies the relationship: >C+(X)N≦0.050(%), and the remainder has a substantially iron composition.

In order to investigate the effect of Ca content relative to S on hot workability and induction resistance, C: 0.03%, Si
: 0.3%, Mn: 0.55%, Cr: 1
2.7%, Ni: 4.2%, N: 0.01%, O:
0.0060%, P: 0.02%, S: 0.00
O~0.05 by changing the amount of Ca to 5~0.07% steel
%, and for hot workability, after rapid heating to 1250°C and cooling to 1000°C, the tensile speed was 100 m.
A tensile test was conducted at m/s to measure the aperture at break, and for resistance to induction, a salt spray test at 35°C was conducted to measure the trigger point.

These results are shown in FIG.

As is clear from the figure, when Ca/S is in the range of 1 to 10, both hot workability and induction resistance are excellent.

Next, in order to investigate the occurrence of cracks and blowholes in the weld when electron beam welding was performed, C: 0.02
%, Si: 0.25%, Mn: 0.45%, C
r: 13.1%, Ni: 4.5%, Ca: 0.
Electron beam welding was performed on steel plates with a thickness of 80 mm with various amounts of N, O, P, and S using 0.01% as the basic component steel under the conditions of an accelerating voltage of 80 kV, a beam current of 620 mA, and a welding speed of 300 mm /min.

These results are shown in Table 1.

Table 1 From Table 1, there is no blowhole when the value is 0180ppn+ or less, and P and S are each 0.025%,
It can be seen that if the content is less than 0.007%, weld cracking does not occur.

(for production) The reason for limiting the ingredients of this invention will be described.

C: 0.04% or less, it is necessary to suppress it to 0.04% or less in order to improve the toughness of the steel plate, reduce the cracking susceptibility of the weld heat affected area, and ensure toughness, and preferably 0.03%. The following are desirable.

Si: 1.0% or less, an indispensable component for deoxidizing; however, if added in excess, toughness decreases, so the upper limit was set at 1.0%.

Mn: 2.0% or less, it has the effect of fixing S during the day and widening the high-temperature austenite single phase region to improve hardenability, but the upper limit must be set to avoid reducing toughness if added in large amounts. It was set at 2.0%.

Cr: 10-15% It is necessary to add 10% or more to ensure corrosion resistance, but since adding a large amount causes a decrease in hot workability and toughness of the weld heat affected area, the upper limit is set to 15%. Limited.

Ni: 3.6-8.0% It is an effective component for expanding the high-temperature austenite single phase region and suppressing the formation of a second phase during cooling from high temperatures. Ensure high toughness. It also has the effect of increasing the amount of retained austenite produced, which has a remarkable effect on improving toughness. In this invention, since the content of C and H is reduced to a small amount, it is necessary to add 3.6% or more in order to widen the high-temperature austenite single phase region.However, addition of more than 8% is necessary. 8. will result in insufficient strength due to the increased amount of retained austenite.
It was set to 0% or less.

N: 0.015% or less N
It is necessary to reduce it as much as possible, and it is limited to 0.015% or less.

In addition, from the viewpoint of the toughness of the steel plate, C+N
≦0.050% is required.

0: 0.01% or less Like N, it is a component that causes blowholes in welded parts in electron beam welding, and since it is better to have as little as possible, the upper limit was set at 0.01%.

P: 0.03% or less P is a component that reduces hot workability and increases the cracking sensitivity of welded parts, so it was limited to 0.03% or less.

S: o, ot% or less Similarly to P, it was limited to 0.01% or less in order to prevent a decrease in hot workability and improve the cracking sensitivity of the welded part.

Reducing the S content is effective in improving hot workability by 1 to 10 times the Ca:S content and preventing weld cracking. Ca for the purpose of suppressing the harmfulness of S.
However, if it is less than the same amount of S content, there is no effect, and if it is more than 10 times, the rust resistance is impaired, so the amount is set to be 1 to 10 times the S content.

In the second invention, Mo is contained in an amount of 0.1 to 1.0%. Mo is a component that is effective in improving corrosion resistance and suppressing temper embrittlement, so it needs to be added in an amount of 0.1% or more, but addition of a large amount reduces toughness, so it should be added in an amount of 0.1% or more.
It was set as 1.0%.

Next, the method for manufacturing martensitic stainless steel for welded structures of the present invention is characterized by including a degassing refining process,
For example, there is a process of melting in an electric furnace or converter → degassing refining → ingot making or continuous casting → hot rolling → heat treatment.

(Example) After melting steel having the chemical composition η shown in Table 2, a part of the steel slab was sampled and rapidly heated to 1200'C, then cooled to 1000°C, and the tensile strength was 100 mm/s. Hot workability was investigated by high-speed tensioning. The remaining steel pieces were rolled into steel plates with a thickness of 80 mm, and then heated at 930°C1.
After 2 hours of solution treatment and 15 hours of tempering at 600°C, the acceleration voltage was 100kV and the beam current was 60°C.
Electron beam welding was performed under conditions of 0 mA and a welding speed of 350 mm/min to examine weldability and toughness.

The results thus obtained are also listed in Table 2.

As is clear from Table 2, Examples Nos. 1 to 5
The steel has no blowholes or weld cracks during electron beam welding and has excellent hot workability compared to steels A to G, which are comparative examples.

(Effects of the Invention) The martensitic stainless steel for welded structures of the present invention has excellent toughness and does not crack during hot rolling.
Furthermore, electron beam welding, which is a highly efficient welding method, can be used, and its effects are significant.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the effects of Ca addition on hot workability and induction resistance. Figure 1

Claims (1)

[Claims] 1. C: 0.04 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, Cr: 10 to 15 wt%, Ni: 3.6 to 8.0 wt%, In addition to containing N: 0.015 wt% or less, O: 0.01 wt% or less, P: 0.03 wt% or less, and S: 0.01 wt% or less, an amount of Ca equivalent to 1 to 10 times the S content A martensitic stainless steel for welded structures that satisfies the relationship (%)C+(%)N≦0.050 (wt%), with the remainder being substantially iron. 2, C: 0.04 wt% or less, Si: 1.0 wt% or less, Mn: 2.0 wt% or less, Cr: 10-15 wt%, Ni: 3.6-8.0 wt%, Mo: 0.1- In addition to containing 1.0 wt%, N: 0.015 wt% or less, O: 0.01 wt% or less, P: 0.03 wt% or less, and S: 0.01 wt% or less, Ca is 1 to 10 of the S content. Martensitic stainless steel for welded structures, containing an amount equivalent to twice as much, and satisfying the relationship (%)C + (%)N≦0.050 (wt%), with the remainder being substantially iron. steel.