JPS626616B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a method for producing steel products such as pipes, round bars, and shapes by hot extrusion using continuously cast materials of ferrite-austenite duplex stainless steel (hereinafter simply referred to as duplex stainless steel). It is related to. Materials subjected to hot extrusion are generally melt-produced.
Ingot-forming - A circular cross-section is achieved through the hot blooming and rolling process. However, with the development of continuous casting technology, it is now possible to manufacture continuously cast pieces with a circular cross section (hereinafter, the material produced by this process will be referred to as a continuous cast material). The development of continuous casting technology has reached a stage where even duplex stainless steel can benefit from the above-mentioned advantages of omitting the process, and by applying continuous casting materials to hot extrusion processing, a significant improvement in yield is expected. Ru. However, when continuously cast duplex stainless steel material is hot extruded, hot processing cracks and streak defects in the extrusion direction frequently occur on the surface of the extruded product. The present invention directly hot extrudes a continuously cast material without hot blooming rolling duplex stainless steel without causing hot work cracks, and suppresses the occurrence of streak defects after hot extrusion. The purpose is to The present invention limits the components of duplex stainless steel,
In particular, by reducing the amount of S and O in the steel, it is possible to maintain a two-phase structure and improve hot workability without compromising strength and stress corrosion cracking resistance. By limiting this, the occurrence of streak-like defects is suppressed. In the first invention, the components of the duplex stainless steel targeted by the present invention are C≦0.08 in weight%.
%, Si≦3.0%, S≦0.005%, Cr:20-35%,
Ni: 3-15%, Mo: 0.5-8%, Cu≦3.0%,
N: 0.03~0.35%, Al: 0.001~0.20%, O≦
0.005%, balance consisting of Fe and unavoidable impurities,
In the second invention, further, Ca: 0.001 to 0.10
%, Mg: 0.001-0.10%, Y: 0.001-0.10%,
REM: 0.005 to 0.10% alone or in any combination, including a total of 0.001 to 0.10%, and in the third invention, further includes Sn: 0.01 to 0.30%, Sb: 0.01 to 0.30
%, As: 0.01 to 0.30% alone or in any combination, including a total of 0.01 to 0.30%. The invention involves melting duplex stainless steel with such components, continuously casting it at a temperature range of +10 to 60°C above the liquidus line to obtain a continuous cast material, and subjecting the continuous cast material to heat treatment such as blooming. It is characterized by direct hot extrusion processing without rolling. The reasons for limiting the steel components are detailed below. Cr is an important element that improves the corrosion resistance of steel, and it is necessary to add 20% or more in order to withstand severe corrosive environments (for example, H ₂ S――Cl ^- -H ₂ O environment), but if it exceeds 35%, it will cause heat damage. The ductility and toughness at high temperatures and low temperatures decrease. Further, as the amount of Cr added increases, it becomes necessary to increase the amount of austenite-forming elements such as Ni and N in order to obtain a ferrite-austenite two-phase structure, which increases the cost, so the amount of Cr was set at 20 to 35%. Like Cr, Ni is an important element that improves corrosion resistance and ductility at low temperatures. In order to obtain a ferrite-austenite dual-phase structure when Cr is in the range of 20 to 35%, 3% or more Ni is required, but if the addition exceeds 15%, the corrosion resistance improvement effect approaches saturation, resulting in high costs. 3-15
%. Mo is the element that has the greatest effect on corrosion resistance in the Cr-Ni-Mo system. To improve corrosion resistance, it is necessary to add 0.5% or more, but if it exceeds 8%, hot workability will deteriorate significantly even if the measures described below are taken, and sigma embrittlement will become significant. ~8%. From the viewpoint of intergranular corrosion resistance, it is desirable that C be as low as possible, and if it exceeds 0.08%, the intergranular corrosion resistance will deteriorate rapidly, so it is limited to 0.08% or less. N is an austenite-forming element and also has the effect of improving corrosion resistance. However, if it is less than 0.03%, the effect of improving corrosion resistance is small, and if it exceeds 0.35%, blowholes will occur in the continuously cast material, so 0.03~
Limited to 0.35%. Al is an effective element as a deoxidizing agent. Even when improving hot workability by deoxidizing and desulfurizing with Ca, Mg, Y, etc., which will be described later, it is necessary to sufficiently lower the oxygen in the molten steel in advance. For this reason, although adding 0.001% or more of Al
If the amount exceeds 0.20%, nonmetallic inclusions will increase and corrosion resistance will deteriorate, so it was set to 0.001 to 0.20%. Si is an effective element for deoxidizing steel, but in the present invention, since deoxidation is performed with Al or the like, it does not need to be added. However, if one or more of strength, oxidation resistance, and resistance to stress corrosion cracking in a chloride environment is to be further improved, it may be added, but adding excessively reduces ductility and toughness, so the upper limit was set at 3%. Cu is added when it is necessary to improve corrosion resistance against non-oxidizing acids, but since excessive addition reduces hot workability, the upper limit was set at 3%. Sn, Sb, and As are all elements that improve corrosion resistance such as general corrosion and stress corrosion cracking, and are added as necessary. If the amount added is less than 0.01%, the effect will be small, but if it exceeds 0.30%, corrosion resistance will decrease and hot workability will also deteriorate, so it was set at 0.01 to 0.30%. A similar effect is also shown when two or more of Sn, Sb and As are added. When adding two or more kinds in combination, the total amount should be 0.01 to 0.30%. The reason why hot work cracking is likely to occur in duplex stainless steel is that impurities (S, O, etc.) tend to segregate at the ferrite and austenite phase boundaries, and furthermore, phases with different deformation resistance and deformability exist adjacent to each other. This is thought to be because stress tends to concentrate at the phase boundaries, easily leading to cracking. Therefore,
In the present invention, S and O are limited as follows. S significantly reduces not only hot workability but also cold workability and corrosion resistance. In particular, in the case of ferritic-austenitic duplex stainless steel, S is
If it exceeds 0.005%, hot workability will be significantly deteriorated, so it was limited to 0.005% or less. O significantly reduces hot workability. Particularly in component systems with low hot workability, such as a ferrite-austenite dual-phase structure, when O exceeds 0.005%, hot workability is significantly reduced. Therefore, O
was set at 0.005% or less. Recently, the refining technology of stainless steel has made remarkable progress, and it has become possible to manufacture high-purity steel with significantly reduced S and O content by adopting methods such as the AOD method. It is quite possible to do so. As mentioned above, duplex stainless steel is made of Cu, Sn, Sb, and As in order to improve corrosion resistance.
Even if hot workability deteriorates due to the addition of these elements, there is no problem with hot workability if the amounts of S and O in the steel are reduced as described above. Furthermore, by adding Ca, Mg, Y, and REM to perform strong desulfurization and strong deoxidation, S, O
The amount is further reduced and hot workability is further improved.
Therefore, these elements are added as necessary. Ca, Mg and Y can be desulfurized by adding 0.001% or more.
The deoxidizing effect is great, but if it exceeds 0.10%, nonmetallic inclusions will increase and the material will deteriorate.
It was limited to 0.001-0.10%. REM has similar effects to Ca, Mg and Y;
Addition of 0.005% or more will have a great effect, but if added in excess of 0.10%, nozzle throttling will occur during continuous casting, and surface defects will occur on the continuous casting material.
It was set at 0.005 to 0.10%. Preferably 0.010-0.050%
Good. Note that the same effect can be obtained by adding two or more of Ca, Mg, Y, and REM in combination. In that case, the total amount should be 0.001 to 0.10%. Although the present invention is directed to duplex stainless steel having the above-mentioned components, the component system is selected depending on the purpose and use. Next, we will discuss how to improve the streak-like defects that occur when continuously cast materials are hot extruded. It is well known that when continuously cast cast materials of ferrite single phase, austenite single phase, and ferrite-austenite duplex stainless steel are hot extruded, streak-like defects occur significantly in the longitudinal direction. The present inventors conducted various experiments and research to clarify the generation mechanism of this streak-like defect, and found that the streak-like defect is caused by the unevenness of the crystal grain itself, and that it depends on the size of the crystal grain. I figured out what he was doing. That is, the depth of the streak-like defects increases in proportion to the size of the crystal grains of the continuously cast slab. Focusing on this, we conducted various experiments to refine the casting structure of ferrite-austenitic duplex stainless steel, and found that the continuous casting temperature is the key to significantly reducing the streak defects in hot extruded products. It has been revealed that the most effective method is to keep the temperature below the liquidus line +60°C. However, at casting temperatures below the liquidus line +10℃,
Since nozzle throttling occurs during continuous casting, the continuous casting temperature was limited to liquidus +10 to 60°C. In addition, fine graining of the solidified structure can be achieved by increasing the Ni content, Mo content, and N content, but it is preferable to control the casting temperature since this increases the cost. In the present invention, such a continuously cast material is directly hot extruded without performing hot rolling such as blooming.
Note that "direct hot extrusion processing" means that the continuously cast material is hot extruded without going through a hot rolling process, and hot extrusion processing is a conventional processing. Examples of the present invention will be described below. Invention examples 1 to 12 and comparative steel with the components shown in Table 1
Steel No. 13 to 15 was melted, and by continuous casting, the casting temperature was controlled to the liquidus line + ΔT (℃) in Table 1, and the outer diameter was 175.
mm bloom. After skinning, cutting, and perforating this into a hollow billet with an outer diameter of 170 mm, an inner diameter of 58 mm, and a length of 600 mm, it was heated to 1200°C and hot extruded into a hollow billet with an outer diameter of 70.3 mm and a wall thickness of 7 mm. manufactures steel pipes,
We investigated hot working cracks and streak defects in steel pipes. Table 1 shows the results. Hot work crackability was evaluated based on the presence or absence of cracking by visual observation. For streak defects, the inner and outer circumferences of the steel pipe were measured, and the 10 deepest defects were selected and evaluated based on the average value. No hot working cracks were observed in the examples of the present invention, and the depth of streak defects was 1/10 or less compared to the comparative materials, which was a significant improvement. On the other hand, in the comparative example, hot working cracks occurred frequently and the depth of the streak-like defects was as deep as 340 to 450μ. As explained above, the method for manufacturing duplex stainless steel of the present invention ensures good corrosion resistance, surface texture, and hot workability, and also enables direct hot extrusion processing without rolling the continuously cast material. Therefore, it greatly contributes to omitting hot working steps and improving yield.

【table】

【table】

Claims (1)

[Claims] 1% by weight: C≦0.08%, Si≦3.0%, S≦
0.005%, Cr: 20-35%, Ni: 3-15%, Mo:
0.5-8%, Cu≦3.0%, N: 0.03-0.35%, Al:
0.001~0.20%, O≦0.005%, balance Fe and unavoidable impurities, and the continuous cast duplex stainless steel material is made by continuously casting molten steel in the temperature range of +10~60℃ above the liquidus line, and is directly heated without hot rolling. A method for producing duplex stainless steel material characterized by inter-extrusion processing. 2 In weight%, C≦0.08%, Si≦3.0%, S≦
0.005%, Cr: 20-35%, Ni: 3-15%, Mo:
0.5-8%, Cu≦3.0%, N: 0.03-0.35%, Al:
0.001~0.20%, O≦0.005%, and Ca: 0.001~
0.10%, Mg: 0.001~0.10%, Y: 0.001~0.10
%, REM: 0.005 to 0.10% alone or in any combination, containing a total of 0.001 to 0.10%, the balance consisting of Fe and unavoidable impurities, duplex stainless steel made by continuously casting molten steel at a temperature range of +10 to 60℃ above the liquidus line. A method for producing duplex stainless steel material, which is characterized by directly hot extruding a continuously cast steel material without hot rolling. 3 In weight%, C≦0.08%, Si≦3.0%, S≦
0.005%, Cr: 20-35%, Ni: 3-15%, Mo:
0.5-8%, Cu≦3.0%, N: 0.03-0.35%, Al:
0.001~0.20%, O≦0.005%, and Ca: 0.001~
0.10%, Mg: 0.001~0.10%, Y: 0.001~0.10
%, REM: 0.005-0.10% alone or in any combination for a total of 0.001-0.10%, Sn: 0.01-0.30%,
Contains Sb: 0.01~0.30%, As: 0.01~0.30% alone or in any combination, totaling 0.01~0.30%, the balance
A two-phase continuous cast stainless steel material consisting of Fe and inevitable impurities, which is produced by continuously casting molten steel at a temperature range of +10 to 60°C above the liquidus line, and is directly hot extruded without hot rolling. Manufacturing method for stainless steel materials.