JPS6219483B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a duplex stainless steel wire. Duplex stainless steel, which consists of about 60% austenite structure and about 40% ferrite structure, has low impurity sensitivity when made into a welding rod and can be applied to welding a wide range of materials, so it is suitable for welding for the production of clad steel sheets. It is often used for overlay repair, etc. Based on AWS (American Welding Society) standards
ER312 steel is a typical example. However, this material has the disadvantage that it often cracks during the process from steel ingot to wire rod product, which tends to hinder subsequent processing. The present inventors conducted research with the intention of realizing the production of duplex stainless steel wire rods with a high yield of non-defective products, and confirmed that the cracks described above are caused by the precipitation of the hard and brittle σ phase. The present invention was achieved by establishing conditions for avoiding the precipitation. The method for producing a duplex stainless steel wire rod of the present invention involves blooming a duplex stainless steel ingot, and then under conditions such that the cooling rate on the surface of the steel billet is 5°C/min or more to 550°C or less. rapidly cooled to a temperature of
The method is characterized in that wire rod rolling is performed thereafter. To explain the process leading to the establishment of the above cooling conditions, the present inventors first inspected a duplex stainless steel ingot (in the ascast state and after heating for blooming) and found that there were no defects. I asked. Next, when the steel slabs after blooming were cooled under different conditions, cracks were observed in the slowly cooled or air-cooled steel slabs, while no cracks were observed in the water-cooled ones. When we examined the distribution of σ in the cross section of each steel slab, we found that the water-cooled material had no σ phase at all, while the air-cooled material had a surface layer of approximately
10 mm, and the slowly cooled material has a σ phase in the surface layer of about 20 mm.
In both cases, no σ phase was observed in the center. This situation is shown in Figures 1 to 3. Each figure graphically represents the cooling method and the ratio of ferrite structure in the cross section of the steel billet after blooming and rolling in the blooming mill, and also schematically shows the structure of the cross section. Fig. 1 shows the case of slow cooling, Fig. 2 shows the case of air cooling, and Fig. 3 shows the case of water cooling. A small amount of ferrite structure means that transformation to the σ phase has occurred depending on the degree of ferrite structure. For each steel piece, the depth of the crack and the thickness of the layer where the σ phase exists are almost the same, and the fact that the crack was caused by the precipitation of the σ phase is confirmed by the microstructure of the crack.
EPMA test and SEM test of cracked fracture surface,
This was also supported by the EPMA test. The fact that the σ phase does not precipitate in the rapidly cooled material shows the importance of the cooling conditions, but the σ phase occurs in the surface layer where cooling is relatively fast and does not occur in the center where cooling is slow. This also means that the magnitude of processing strain applied during blooming greatly contributes to the generation of the σ phase. In addition, it was found that the higher the heating temperature for blooming, the slower the precipitation of the σ phase. The fact that cracks occurred in the surface layer where the σ phase occurred is
It is understood that the σ transformation accompanies volumetric contraction, and tensile stress acts on the surface of the steel piece, causing it to crack at the brittle σ phase portion. The correctness of this understanding has been demonstrated not only by micro-examination of the cross-section of the steel bill, but also by the results of surface residual stress measurements (tensile stress exists on the surface of the air-cooled material, while compressive stress acts on the surface of the water-cooled material). It was done. Precipitation of the σ phase can occur between 950°C and 550°C, so quenching of the steel billet should be carried out until a temperature of 550°C or less is reached. The above findings can be summarized as shown in FIGS. 4 and 5. In other words, the relationship between the cooling curve of the central part of the slab after blooming and the σ phase precipitation nose is shown in Figure 4, and that of the surface layer is shown in Figure 5. When is low, the σ phase precipitation nose moves to the position indicated by the chain line, and when processing strain is further applied, it advances to the position indicated by the broken line in FIG. Then, if the cooling rate is slow, the σ phase will precipitate and cause cracking. The only condition to avoid this is the above-mentioned rapid cooling at a cooling rate of 5° C./min or more in the surface layer of the steel piece. Such rapid cooling can be easily achieved by water-cooling the steel billet after blooming and rolling. A typical duplex stainless steel to which the present invention can be applied is the above-mentioned ER312 steel, whose alloy composition is as follows. C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, Ni: 8 to 10%, Cr: 28 to 32%, the balance being Fe and impurities.The wire rod rolling of the steel billet prepared as described above ,
According to techniques known to those skilled in the art, the process may be carried out after performing a process such as removing scratches using a grinder, if necessary. In order to prevent the formation of the σ phase, it is preferable to heat the steel billet to a high temperature and subject it to wire rolling.
Soaking at 1300℃ for 30 minutes is appropriate. If the temperature of the material decreases due to rolling conditions and there is a risk of precipitation of the σ phase, it is advisable to perform auxiliary heating using a hand burner or the like. In addition, there is a concern that a coil obtained by wire rolling may generate a σ phase when slowly cooled. According to experience, there are no particular problems with normal air cooling, but forced cooling with a fan or blower is more reliable. Example A duplex stainless steel ingot with the following alloy composition (wt%, balance Fe) was bloomed into a 110 mm
It was made into a square piece of steel.

[Table] This was soaked at 1300°C for 30 minutes and then rolled into a wire rod with a diameter of 50 mm. The cooling conditions of the steel slab after blooming rolling were varied, and the formation of σ phase in the surface layer of the steel slab was investigated, and cracks during rolling were recorded. The results are as follows.

[Table] From this result, it can be seen that the cooling of the steel billet is
It is confirmed that rapid cooling at a rate of ℃/min or more should be performed to reach a temperature of 550℃ or less.

[Brief explanation of the drawing]

Figures 1 to 3 are graphs and horizontal cross-sectional views showing the relationship between the cooling method and the ratio of ferrite structure in the cross section of a billet of duplex stainless steel after blooming. FIG. 1 shows the case of slow cooling, FIG. 2 shows the case of air cooling, and FIG. 3 shows the case of water cooling. Figures 4 and 5 are graphs showing the relationship between the cooling curve of a steel slab and the σ phase precipitation nose, with Figure 4 targeting the center of the slab and Figure 5 targeting the surface layer. It is something.

Claims (1)

[Claims] 1. After blooming a duplex stainless steel ingot, rapidly cooling it to a temperature of 550°C or less under conditions such that the cooling rate on the surface of the steel ingot is 5°C/min or more,
A method for producing a duplex stainless steel wire rod, which comprises subsequently performing wire rod rolling. 2 The duplex stainless steel has C: 0.15% or less,
The manufacturing method according to claim 1, having an alloy composition containing Si: 1.0% or less, Mn: 2.0% or less, Ni: 8 to 10%, and Cr: 28 to 32%, with the balance consisting of Fe and impurities. .