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Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, and method for manufacturing the same

a technology hydrogen embrittlement, which is applied in the field of ultra-high strength steel sheets, can solve the problems of steel brittleness, high-strength steel, and triple steel sheets that suffer a newly emerging problem of delayed fracture, and achieve high ductility, high strength, and improved hydrogen embrittlement resistance.

Inactive Publication Date: 2005-07-14
KOBE STEEL LTD
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0011] The present invention has been made with the background described above, and has an object of providing a novel TRIP steel sheet having a bainitic ferrite structure as the matrix phase, that is an ultra-high strength steel sheet having a tensile strength of 1180 MPa or higher and improved hydrogen embrittlement resistance, while maintaining the high ductility characteristic of the TRIP steel, and a method for manufacturing the same.
[0015] According to the present invention, the ultra-high strength steel sheet having tensile strength of 1180 MPa or higher and improved hydrogen embrittlement resistance can be manufactured with high productivity.

Problems solved by technology

In the realm of ultra-high strength of 1180 MPa upward, however, the TRIP steel sheet is known to suffer a newly emerging problem of delayed fracture (crack, etc.) caused by hydrogen embrittlement, similarly to the conventional high strength steel.
Delayed fracture refers to the failure of high-strength steel under stress, that occurs as hydrogen originating in corrosive environment or the atmosphere infiltrates and diffuses in microstructural defects such as dislocation, void and boundary, and makes the steel brittle.
This results in decrease in ductility and in toughness of the metallic material.
Moreover, since the heat treatment conditions reported in the literatures described above involve heating temperature being set higher, there are such problems as low efficiency of practical manufacturing process.

Method used

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  • Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, and method for manufacturing the same
  • Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, and method for manufacturing the same
  • Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, and method for manufacturing the same

Examples

Experimental program
Comparison scheme
Effect test

example 1

Investigation On Composition

[0079] In this example, steel specimens A through P having the compositions shown in Table 1 (rest of the composition consists of iron and impurities, and the concentrations in the table being given in mass percentage) was made by vacuum melting to obtain an experimental slab that was subjected to the process described below (hot rolling→cold rolling→continuous annealing) to turn into a hot rolled steel sheet having thickness of 3.2 mm (thickness:2.5mm in No. Q to T), that was then pickled to remove scales from the surface and was cold rolled to a thickness of 1.2 mm.

[0080] Hot rolling process: Starting temperature (SRT) 1150° C., finishing temperature (FDT) 850° C., cooling rate of 40° C. / s and take-up temperature 550° C.

[0081] Cold rolling process: Rolling ratio 50%.

[0082] Continuous annealing process: Each steel specimen was kept at a temperature (A3 point+15° C.) (T1 in Table 1) for 120 seconds (t1 in Table 1), then cooled (water cooling) at a mea...

example 2

Investigation of Manufacturing Conditions

[0103] In this example, an experimental slab made by using the steel of type B shown in Table 1 (steel that satisfies the conditions of the present invention) was subjected to hot rolling and cold rolling under the same conditions as those of the Example 1, followed by continuous annealing under various conditions shown in Table 6, thereby to obtain the cold rolled steel sheets Nos.1 through 11, all 1.2 mm in thickness.

[0104] Then structures and various properties of these steel sheets were investigated similarly to the Example 1. The results are shown in Table 6.

TABLE 6Continuous annealingor plating processStructurePropertiest1CR1T2t2Zn-GAAreal ratio (%)BainiteTSELCathode CHNo.T1 (° C.)(S)(° C. / s)(° C.)(s)(° C.)BainiteFerriteResidual γOtherblock (μm)(MPa)(%)life (Seconds)181012025420120—821080101198151519293012025420120—821080301198151119376012025420120—1075150 598018—481030025420120—8210802511982211855810125420120—8910101111981111656810...

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Abstract

The present invention provides an ultra-high strength steel sheet having excellent hydrogen embrittlement resistance, which includes: 0.06 to 0.6% of C; 0.5 to 3% of Si+Al; 0.5 to 3% of Mn; 0.15% or lower of P; and 0.02% or lower of S in terms of mass percentage, and also includes 3% or higher of residual austenite structure, 30% or higher of bainitic ferrite structure, and preferably 50% or lower of polygonal ferrite in terms of an areal ratio to the entire structure, wherein a mean grain size of bainite blocks is smaller than 20 μm as determined by comparing observations of the same region of the bainitic ferrite structure by EBSP (electron back scatter diffraction pattern) and SEM.

Description

BACKGROUND OF THE INVENTION [0001] 1. Technical Field [0002] The present invention relates to an ultra-high strength steel sheet having strength of at least 1180 MPa class and excellent hydrogen embrittlement resistance, and a method for efficiently manufacturing the ultra-high strength steel sheet. [0003] 2. Background Art [0004] There are increasing demands for steel sheets, that are pressed into forms to be used in such applications as automobiles and industrial machines, to have both high strength and high ductility at the same time. Recently needs are increasing for ultra-high strength steel sheets having strength of at least 1180 MPa class. A type of steel sheet that is regarded as promising one to satisfy these needs is TRIP (transformation induced plasticity) steel sheet. [0005] The TRIP steel sheet includes residual austenite structure and, when processed to deform at a temperature higher than the martensitic transformation start point (Ms point), undergoes considerable elo...

Claims

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Application Information

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IPC IPC(8): C21D1/20C21D8/02C22C38/00C22C38/02C22C38/04C22C38/06
CPCC21D1/20C21D8/0247C21D8/0278C22C38/06C21D2211/005C22C38/02C22C38/04C21D2211/002
Inventor AKAMIZU, HIROSHIIKEDA, SHUSHIMAKII, KOICHIMUKAI, YOICHISUGIMOTO, KOICHI
Owner KOBE STEEL LTD
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