Steel for forging having excellent hydrogen embrittlement resistance and crankshaft

Abstract

The invention provides a hammered steel with good resistant property for hydrogen brittleness and crank made of said hammered steel, said steel comprises following components: C 0.2-0.6% (by weight percent, the same with following ones); Si 0.1-0.4%; Mn 0.7-1.5%; Ni less than 1.0%; Cr 1.2-3.5%; Mo 0.1-0.6%, Al 0.01-0.05%, and mixture containing at least one of Ti, Zr, Hf larger than 0.005%; the average circular degree of foreign material with maximum chord length being larger than 1 um is larger than 0.5; the number of foreign material with maximum chord length being larger than 20 um (100mm2) is less than 100, and the average circular degree is larger than 0.25, and the number of foreign material with maximum chord length being 1-10 um (100mm2) is larger than 100.

Description

technical field [0001] The present invention relates to forging steel used for large crankshafts for power transmission of ships, generators, etc., particularly high-purification steel excellent in high fatigue strength and hydrogen embrittlement resistance. Background technique [0002] So-called Cr-Mo steel represented by ISO standard 36CrNiMo6, DIN standard 32CrMO12, and ISO standard 42CrMo4 has been used as forging steel for large crankshafts for power transmission used in ships and generators, etc. . [0003] Since crankshafts are exposed to severe repetitive stress during use, even if they are manufactured using these Cr-Mo steels, high-purity steels with very little inclusions such as MnS that become the starting point of fatigue fracture are required. On the other hand, the crankshaft naturally has increasing requirements for products with high performance and high fatigue strength that will become lighter in weight in the future. [0004] However, in the case of C...

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Problems solved by technology

[0006] As in the above-mentioned dehydrogenation treatment, there is a limit to the reduction of the amount of hydrogen in terms of time and processing costs, and generally, production management is performed at the level of 1 to several ppm, but because hydrogen embrittlement occurs due to a trace amount of hydrogen, this level management cannot completely prevent hydrogen embrittlement

In addition, the above-mentioned method of adjusting the S concentration and forming sulfides, on the contrary, leads to deterioration of the degree of purification, mechanical properties, and fatigue strength of the finished steel, and it is difficult to expect a higher degree of purification and higher fatigue strength.

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