Austenite-type stainless steel hot-rolling steel material with excellent corrosion resistance, proof-stress, and low-temperature toughness and production method thereof

Abstract

An austenitic stainless steel hot-rolled steel material can be provided which has sea-water resistance and strength superior to conventional steel. Low-temperature toughness can be maintained, which is preferable in a structural member of speedy craft. The steel material can include an austenitic stainless steel hot-rolled steel material which excels in the properties of corrosion resistance, proof stress, and low-temperature toughness. In such austenitic stainless steel hot-rolling steel material, e.g., PI[=Cr+3.3(Mo+0.5W)+16N] ranges from 35 to 40, δ cal [=2.9 (Cr+0.3Si+Mo+0.5W)−2.6 (Ni+0.3Mn+0.25Cu+35C+20N)−18] ranges from −6 to +2, and a 0.2% proof stress at room temperature is not less than 550 MPa, Charpy impact value measured using a V-notch test piece at −40° C. is not less than 100 J / cm2, and the pitting potential measured in a deaerated aqueous solution of 10% NaCl at 50° C. (Vc′100) is not less than 500 mV (as it relates to saturated Ag / AgCl).

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a structural steel material which excels in corrosion resistance and is used in a marine (chloride) environment, for example; an austenite-type stainless steel hot-rolling steel material, as a hull-structural material which excels in strength as well as seawater resistance, and low-temperature toughness, upon being used as a material for an outer shell, a bulkhead, an frame, a hydrofoil, etc.; and a production method thereof. [0003] Priority is claimed on Japanese Patent Application No. 2005-26177, filed Feb. 2, 2005, Japanese Patent Application No. 2005-26176, filed Feb. 2, 2005, and Japanese Patent Application No. 2006-012569, filed Jan. 20, 2006, the contents of which are incorporated herein by reference. [0004] 2. Background Art [0005] Hitherto, coated steel sheets to which a heavy corrosive protection was applied were used for hull structures. The demand for speedy craft equippe...

AI Technical Summary

Benefits of technology

[0018] That is, an austenitic stainless hot-rolled steel material having excellent corrosion resistance, proof stress, and low-temperature toughness, including: 0.001 to 0.03 mass % of C, 0.1 to 1.5 mass % of Si, 0.1 to 3.0 mass % of Mn, 0.005 to 0.05 mass % of P, 0.0001 to 0.003 mass % of S, 15.0 to 21.0 mass % of Ni, 22.0 to 28.0 mass % of Cr, 1.5 to 3.5 mass % of Mo, 0.15 to 0.35 mass % of N and 0.0005 to 0.007 mass % of o, in which the PI value expressed by the following formula (1) ranges from 35 to 40, δ cal value expressed by the following formula (2) ranges from −6 to +2, the remnant consists of Fe and inevitable impurities, the content of intermetallic compounds contained in the steel material is not more than 0.5 mass %, a 0.2% proof stress at room temperature is not less than 550 MPa, the Charpy impact value measured using a V-notch test piece at −40° C. is not less than 100 J / cm2, and the pitting potential measured in a deaerated aqueous solution of 10% NaCl at 50° C. (Vc′100) is not less than 500 mV(vs saturated Ag / AgCl). PI=Cr+3.3(Mo+0.5W)+16N   (1) δcal=2.9(Cr+0.3Si+Mo+0.5W)−2.6(Ni+0.3Mn+0.25Cu+32C+20N)−18  (2)

[0031] The present invention can provide austenitic stainless steel having excellent sea water resistance, proof stress, and low-temperature toughness, by restricting the component and performing a specific heat treatment processing.

[0032] The present invention realizes an austenitic stainless steel suitable for hull structures having a high level of sea water resistance and proof stress and low-temperature toughness, which are required as components for structures of high-speed ships, and contributes to industry significantly.

[0035] A ninth aspect of the present invention is the austenitic stainless hot-rolled steel material having excellent corrosion resistance, and low-temperature toughness, as set forth in the eighth aspect of the present invention, further including one or more selected from the group consisting of 0.1 to 2.0 mass % of Cu, 0.003 to 0.03 mass % of Ti, 0.02 to 0.20 mass % of Nb, 0.05 to 0.5 mass % of V, 0.3 to 3.0 mass % of W, 0.0003 to 0.0060 mass % of B, 0.0005 to 0.0050 mass % of Ca, 0.0005 to 0.0050 mass % of Mg, and 0.005 to 0.10 of REM.

[0036] A tenth aspect of the present invention is a process for producing the austenitic stainless hot-rolled steel material having excellent corrosion resistance, and low-temperature toughness, as set forth in the eighth or ninth aspect of the present invention, including: performing homogenizing-heat treatment on a cast steel or a semi-finished product after a rough heat-rolling processing at a temperature of 1200 to 1300° C. for 1 hour or more, in order to reduce the content of the intermetallic compound in the steel material.

[0037] The present invention realizes an austenitic stainless steel suitable for hull structures having a high level of sea water resistance and proof stress, which are required as components for structures of high-speed ships, and low-temperature toughness, and contributes to industry significantly.

Problems solved by technology

Among these, although Patent document 2 discloses a production method of an austenitic stainless steel having a high proof stress while maintaining a low-temperature toughness, the sea water resistance is not taken into consideration in this austenitic stainless steel while maintaining low-temperature toughness.

On the other hand, a high strength steel material of an austenitic-type high corrosion resistance stainless steel having a PI value over 35 has not yet been put in practical use.

Stainless steel is more susceptible to crevice corrosion when it is shaped into a crevice form than when it is not shaped i.e. flat.

Among Cr, Mo and N, which increase corrosion resistance, as for Mo and Cr, it is not sufficient to simply add, because processability in hot-rolling will decrease remarkably due to the influence of delta ferrite contained in cast steel or semi-finished products; in addition, in the case of a high Cr and Mo steel, in general, the toughness of the steel deteriorates remarkably due to the influence of an intermetallic compound known as a a phase, and hence it is necessary to add a large amount of Ni in order to suppress the influence of both.

It should be noted that, two-phase steel cannot be adopted because of its low-temperature toughness.

On the other hand, as for adding N as is disclosed in Patent document 3, it is indeed effective for maintaining the strength, however, excessive N causes the generation of bubbles at a welded part, thereby it may decrease the bonding strength and reliability of the welded part, to the contrary.