High-grade duplex stainless steel with much suppressed formation of intermetallic phases and having an excellent corrosion resistance, embrittlement resistance castability and hot workability

Abstract

Formation of intermetallic phases such as sigma (σ) and khi (χ) shows detrimental effects on the corrosion and mechanical properties of high-grade duplex stainless steel. The present invention provides high-grade duplex stainless steel with much suppressed formation of intermetallic phases, of which the chemical composition consists essentially, on a weight basis, of: Cr: 21.0%˜38.0%, Ni: 3.0%˜12.0%, Mo: 1.5%˜6.5%, W: 6.5% or less, Si: 3.0% or less, Mn: 8.0% or less, N: 0.2%˜0.7%, C: 0.1% or less, at least one element selected from the group consisting of Ba: 0.0001˜0.6% and one or more elements of Mischmetal (MM) and Y: 0.0001˜1.0% in total, and a balance of Fe and incidental impurities. The pitting resistance equivalent has a value of 40≦PREW≦67 defined by the following formula (1): PREW=wt. % Cr+3.3(wt. % Mo+0.5 wt. % W)+30 wt. % N  ((1)The present high-grade duplex stainless steel exhibits an excellent corrosion resistance, embrittlement resistance, castability and hot workability.

Description

TECHNICAL FIELD [0001] The present invention relates to duplex stainless steel having excellent corrosion resistance, and more particularly to, high-grade duplex stainless steel having excellent corrosion resistance, embrittlement resistance, castability and hot workability by suppressing formation of intermetallic phases, such as sigma (θ) and khi (χ), formed during the production (casting, hot rolling or welding). BACKGROUND ART [0002] Duplex stainless steel where austenite (γ) phase providing high workability and ferrite (α) phase providing high corrosion resistance are minutely combined has higher strength than austenitic stainless steel by at least 1.7 times, and also shows high pitting resistance and high stress corrosion cracking (SCC) resistance. Commercial high-grade duplex stainless steels having a pitting resistance equivalent (PREW=wt % Cr+3.3(wt % Mo+0.5wt % W)+30wt % N) of about 46, such as SAF 2507 (UNS S32750), UR 52N+ (UNS 32550) and ZERON 100 (UNS 32760) have been ...

AI Technical Summary

Benefits of technology

[0014] The main object of the present invention is to remove brittleness and improve corrosion resistance by reducing a precipitation speed and amount of brittle intermetallic phases, by delaying diffusion and precipitation of intermetallic phases by adding appropriate amounts of Ba, Y, Ce, La, Nd, Pr, Ta, Zr and Ti atoms having a large atomic diameter, and additionally blocking diffusion of Cr, Mo, Si and W by using minute Rare Earth compounds or Ba oxides.

[0015] Another object of the invention is to prevent individual formation of Al2O3 and MnS inclusions which have detrimental effects on properties of steel by performing proper preliminary deoxidation according to a common method using Ti, Mg, Ca, Al and Ca+Al, as well as adding MM (Mischmetal: rare-earth metallic mixtures consisting of atoms with atomic numbers from 57 to 71, containing at least 50% or more of Ce, a certain amount of La, Nd and Pr, minute amounts of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Sc, and 1% or less of Fe. Hereinafter, the detailed description and embodiment of the present invention uses MM containing major elements of 51% Ce-26% La-15.5% Nd-5.5% Pr, minute amounts of Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Sc, and 1% or less of Fe) and / or Y.

[0016] Yet another object of the invention is to control solubility products in molten steel of rare-earth metal elements of MM and / or Y (REM, hereinafter, referred as ‘RE’ in compound formulae) within a certain range, supplying heterogeneous nucleation sites to make fine and minute structure during dendrite formation of the solidification, and controlling segregation of the solute elements such as Cr, Mo, W, Ni, Mn and Si by forming a rare-earth metallic compound mixture (RExOy or (RE,Al)xOy+RExOyS+RExSy) having a diameter below 5 μm in molten steel, resulting in improving mechanical properties, physical properties and corrosion resistance.

[0017] Yet another object of the invention is to remarkably suppress formation of intermetallic phases such as sigma in duplex stainless steel by adding new alloying elements, and to improve the production yield during mass production.

[0018] Yet another object of the invention is to considerably increase the production yield in casting and hot working, by improving embrittlement resistance and preventing cracks by lowering a precipitation speed of intermetallic phases such as sigma.

[0019] Yet another object of the invention is to improve corrosion resistance and mechanical properties and upgrade durability of equipments, by suppressing precipitation of sigma and khi phases deteriorating corrosion resistance and mechanical properties in a casting state, and controlling precipitation of such phases in heat-affected zone after welding when equipment components are necessarily welded in various application fields.

Problems solved by technology

However, as compared with commercial PREW 38-level duplex stainless steel such as SAF 2205, the PREW 46-level high-grade duplex stainless steel contains a large amount of Cr, Mo and W which are major elements of sigma and khi phases deteriorating mechanical properties and corrosion resistance, and thus easily forms precipitation phases thereof during the production or application.

Actually, embrittlement by the precipitation phases has been observed in cooling after continuous casting of duplex stainless steel, slow cooling after hot rolling, slow cooling of a heat affected zone after welding, and slow cooling of an ingot center unit after casting.

In addition, Mo for improving local corrosion and SCC resistances among the added alloying elements is a high-priced element facilitating formation of sigma phases and 475° C. brittleness, and thus restrictively used.

But, the conventional research and development have the following problems.

However, it is not relevant to duplex stainless steel containing austenite and ferrite.

The non-metallic inclusions such as oxides and sulfides are operated as the pitting point, which reduces corrosion resistance.

However, as confirmed in PREW formula that will later be described, effects of Mo for improving pitting resistance are twice as many as W. It is thus inefficient to decrease the content of Mo.

When intermetallic phases are precipitated at a high speed in the slow cooling, duplex stainless steel becomes embrittled and also shows low corrosion resistance.

However, when a cast product is manufactured using high-grade duplex stainless steel where intermetallic phases are easily precipitated, some ferritic phase is transformed into sigma and austenitic phases during the cooling process to the room temperature after casting, and thus sigma phase includes embrittlement.

That is, when sigma phase is precipitated during the cooling process, stainless steel is embrittled and also shows low corrosion resistance.

However, the above-described methods for adding the third alloying elements or controlling the cooling process during the heat-treatment cannot sufficiently suppress sigma phase in high-grade duplex stainless steel.

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