Hot-rolled steel sheet having excellent cold workability and excellent surface properties and hardness after working

Abstract

A hot-rolled steel sheet which has a thickness of 3-20 mm and contains specific amounts of C, Si, Mn, P, S, Al and N, with the balance made up of iron and unavoidable impurities. This hot-rolled steel sheet contains a specific amount of solid-solved N, and the contents of C and N satisfy the relation 10C + N <= 3.0. This hot-rolled steel sheet contains bainitic ferrite and pearlite, respectively in an area ratio of 5% or more and in an area ratio of less than 20% relative to the whole structure, with the balance made up of polygonal ferrite. The average crystal grain size of the bainitic ferrite is 3-50 [mu]m.

Description

technical field [0001] The present invention relates to a hot-rolled steel sheet that exhibits good cold workability during processing and exhibits predetermined surface properties (also referred to as "surface quality") and post-processing hardness after processing. Background technique [0002] In recent years, from the viewpoint of environmental protection, in order to improve the fuel efficiency of automobiles, there has been an increasing demand for the reduction of the weight of various parts of automobiles, such as transmission parts such as gears and casings, that is, higher strength. soaring. In order to meet such demands for weight reduction and high strength, generally used steel materials are steel materials obtained by hot forging bar steel (hot forging materials). In addition, in order to reduce CO in the parts manufacturing process 2 emissions, so far, the demand for cold forging of parts such as gears processed by hot forging is also increasing. [0003] T...

AI Technical Summary

Problems solved by technology

[0011] However, there is a problem with this technology that nitriding treatment is required after cold working, and sufficient cost reduction cannot be achieved.

[0013] However, in this hot-rolled steel sheet, N is limited to an extremely low content as a harmful element, and it is completely different from the hot-rolled steel sheet of the present invention in which N is actively used.

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