High yield ratio hot rolled steel sheet which has excellent low temperature impact energy absorption and haz softening resistance and method of production of same

Abstract

Hot rolled steel sheet which has a maximum tensile strength of 600 MPa or more and has an excellent low temperature impact energy absorption and HAZ softening resistance and a method of production of the same are provided, that is, sheet which contains, by mass %, C: 0.04 to 0.09%, Si: 0.4% or less, Mn: 1.2 to 2.0%, P: 0.1% or less, S: 0.02% or less, Al: 1.0% or less, Nb: 0.02 to 0.09%, Ti: 0.02 to 0.07%, and N: 0.005% or less, where 2.0≦Mn+8[% Ti]+12[% Nb]≦2.6, has a balance of Fe and unavoidable impurities, has an area percentage of pearlite of 5% or less, has a total area percentage of martensite and retained austenite of 0.5% or less, has a balance of a metal structure of ferrite and / or bainite, has an average grain size of ferrite and bainite of 10 μm or less, has an average particle size of alloy carbonitrides with incoherent interfaces which contain Ti and Nb of 20 nm or less, and has a yield ratio of 0.85 or more.

Description

TECHNICAL FIELD[0001]The present invention relates to maximum tensile strength 600 MPa or more high yield ratio hot rolled steel sheet which has an excellent low temperature impact energy absorption and HAZ (heat affected zone) softening resistance and a method of production of the same. The steel sheet is suitable as a base material for booms and frames of construction machinery and as a base material for frames, members, etc. of trucks and cars which are shaped mainly by bending and, further, as a base material for line pipe.BACKGROUND ART[0002]The frames of construction machinery and trucks are assembled by shaping hot rolled steel sheet mainly by bending and arc welding the shaped parts. Therefore, the base material which is used for these parts is required to have excellent bendability and arc weldability. Further, construction machinery and trucks are sometimes used in low temperature environments, so in particular with frames for trucks etc., the properties of being resistant...

AI Technical Summary

Benefits of technology

The present invention relates to a hot rolled steel sheet with excellent properties of high yield ratio, maximum tensile strength of 600 MPa or more, and low temperature energy absorption and bendability. The hot rolled steel sheet can be used in cold regions, reduces thickness of products, and reduces weight of construction machinery, automobiles, and trucks. The invention also provides a method of production of the hot rolled steel sheet with excellent low temperature impact energy absorption and HAZ softening resistance. The excellent low temperature impact energy absorption means a Charpy impact test at minus 40 °C with a minimum value of 70 J / cm2, while the excellent HAZ softening resistance means a difference between the Vicious hardness of the weld heat affected zone and the base material of 40 or less. The "excellent bendability" means an rlim / t of 1.0 or less in a 90-degree V bending test with no cracks occurring.

Problems solved by technology

However, such structures of steel sheet had the problems of being low in yield stress and having issues in bendability.

However, this method suffered from large softening of the heat affected zone (HAZ) of the arc weld zone and the inability to obtain a sufficient weld joint strength and, further, was disadvantageous in terms of production costs.

However, steel sheet which utilizes such precipitation strengthening sometimes suffers from a large softening of the arc weld heat affected zone and a drop in joint strength.

Further, there were the problems that sometimes brittle fracture occurred at a low temperature and sometimes the amount of impact energy absorption became small.

However, with these methods, there were the problems that sometimes brittle fracture occurred at a low temperature and sometimes the impact energy absorption amount became small.

This method controls the recrystallization in the rough rolling and finish rolling of the steel slab to obtain a fine grain metal structure and obtain steel sheet which has excellent low temperature toughness, but does not intend control of the size or distribution of alloy carbonitrides.

As a result, these are not optimized, so there was the problem of a drop in the impact energy absorption.

The object of the optimization of the rough rolling process in this method is the promotion of the recrystallization of steel, but this does not intend control of the size or distribution of alloy precipitates.

As a result, these are not optimized, so there was the problem of a drop in the impact energy absorption.

Regarding the finish rolling conditions as well, with the method described in PLT 10, there was the problem that it is not possible to control the size or distribution of the alloy precipitates and excellent impact absorption energy cannot be obtained.

However, this art disperses fine precipitates in the HAZ of the steel sheet during arc welding, but the size of the precipitated particles in the steel is not optimized, so as a result there was the problem that the steel sheet was not excellent in impact energy absorption.

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