Steel for machine structural use excellent in friability of chips

Abstract

Disclosed is a free cutting steel for machine structural use having excellent chip-breakability. The steel consists essentially of, by wt. %. C: 0.05-0.8%, Si: 0.01-2.5%, Mn: 0.1-3.5%, S: 0.01-0.2%, Ca or Ca+Mg; 0.0005-0.02%, Ti: 0.002-0.010% and / or Zr: 0.002-0.025%, O: 0.0005-0.010%, and the balance of impurities and Fe. At least five MnS inclusion particles having averaged particles sizes of 1.0 μm or more exists per mm2 per 0.01% of S-content in the steel. The steel satisfies the condition that, in the microscopic fields, (area[μm2] / aspect ratio)≧10, and that the the area percentage of Ca-containing sulfide inclusions containing at least 1.0 wt. % of Ca is in the range of 15-40% of the area of all the sulfide inclusions.

Description

TECHNICAL FIELD [0001] The present invention concerns a steel for machine structural use having excellent chip-breakability at machining with cemented carbide tools. The steel for machine structural use of the invention is characterized by configuration of sulfide inclusions in the steel. [0002] In the specification the term “Ca-containing sulfide inclusion” means the inclusion of the structure formed by a core inclusion mainly consisting of CaO, and another inclusion mainly consisting of sulfides and surrounding the core. In regard to the MnS inclusion the phrase “finely dispersed” means that the inclusion particles are finer than the MnS inclusion particles in the conventional steel, and that they are homogeneously dispersed throughout the steel without either coagulation or concentration. The “aspect ratio” is defined as the value given by dividing the longest diameter by the shortest diameter of the inclusion particles observed on the surface formed by cutting a steel sample alo...

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