Abstract: A non-heat treated steel for hot forging and for a connecting rod, containing, in mass percent: 0.3 to 0.8% of C; 0.1 to 2.0% of Si; 0.5 to 1.5% of Mn; 0.01 to 0.15% of P; 1.0% or less of Cr; 0.4% or less of V; 0.05% or less of Al; 0.005 to 0.03% of N; and the balance Fe and inevitable impurities. The non-heat treated steel has a value E of not larger than 150, the value E being calculated from the following equation: E=2804?1549×Ceq?8862×P?23.4×H where H is a value of a Rockwell hardness of the connecting rod made of the non-heat treated steel; P is a phosphorus content (%) in the connecting rod; and Ceq is a value of a carbon equivalent.

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 ?m 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[?m2]/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.

Abstract: Disclosed is a free-cutting steel containing Ti- and/or Zr-carbosulfide as inclusion and having good machinability both in turning and drilling. The free-cutting steel consists essentially of, in mass %, C: 0.05-0.80%, Si: 0.01-2.5%, Mn: 0.1-3.5%, Ti+0.52Zr: 0.03-1.2%, S:0.015-0.6% and the balance of Fe and inevitable impurities, and is characterized in that the contents of Ti, Zr and S meet the condition of (Ti+0.52Zr)/S<2. The free-cutting steel may contain, in addition to the above basic alloy composition, Ca: 0.0005-0.02% and/or Mg: 0.0003-0.02%. The latter steel exhibits much more improved machinability.

Abstract: Disclosed is a steel for machine structural use having good machinability and chip-breakability as well as a method of producing the steel. The steel consists essentially of, by wt. %, C: 0.05-0.8%, Si: 0.01-2.0%, Mn: 0.1-3.5%, S: 0.01-0.2%, Al: 0.001-0.020%, Ca: 0.0005-0.02%, O: 0.0005-0.01% and N: 0.001-0.04%, and further, one or both of Ti: 0.002-0.010% and Zr: 0.002-0.025%, the balance being Fe and inevitable impurities. At production of the steel controlled deoxidization is conducted by operation meeting certain conditions so that at least a certain amount of “duplex inclusion” having a specific chemical composition may be formed, and Ti and/or Zr is added to precipitate finely dispersed MnS inclusion particles with nuclei of Ti-oxide and/or Zr-oxide. The finely dispersed MnS inclusions must share a determined part of the total sulfide inclusions.

Abstract: Disclosed is a free-cutting steel which exhibits good machinability in turning and small roughness of the turned surfaces, and in which substantially no macro-streak-flaw occurs. The free-cutting steel contains, by weight %, C: 0.03-0.20%, Mn: 0.5-3.0%, P: 0.02-0.40%, S: more than 0.2% up to 1.0%, one or both of Ti and Zr (in case of both, the total amount): 0.01-3.0%, O: 0.0005-0.0050% and Pb: less than 0.01%, the balance being Fe and inevitable impurities. The steel is characterized in that it contains, as the inclusion therein, Ti-based and/or Zr-based carbosulfide compound or compounds. The steel may further contain at least one from the group of Bi: up to 0.4%, Se: up to 0.5% and Te: up to 0.1%.

Abstract: Disclosed is a free-cutting steel containing Ti- and/or Zr-carbosulfide as inclusion and having good machinability both in turning and drilling. The free-cutting steel consists essentially of, in mass %, C: 0.05-0.80%, Si: 0.01-2.5%, Mn: 0.1-3.5%, Ti+0.52Zr: 0.03-1.2%, S:0.015-0.6% and the balance of Fe and inevitable impurities, and is characterized in that the contents of Ti, Zr and S meet the condition of (Ti+0.52Zr)/S<2. The free-cutting steel may contain, in addition to the above basic alloy composition, Ca: 0.0005-0.02% and/or Mg: 0.0003-0.02%. The latter steel exhibits much more improved machinability.

Abstract: Disclosed is a free-cutting steel which exhibits good machinability in turning and small roughness of the turned surfaces, and in which substantially no macro-streak-flaw occurs. The free-cutting steel contains, by weight %, C: 0.03-0.20%, Mn: 0.5-3.0%, P: 0.02-0.40%, S: more than 0.2% up to 1.0%, one or both of Ti and Zr (in case of both, the total amount): 0.01-3.0%, O: 0.0005-0.0050% and Pb: less than 0.01%, the balance being Fe and inevitable impurities. The steel is characterized in that it contains, as the inclusion therein, Ti-based and/or Zr-based carbosulfide compound or compounds. The steel may further contain at least one from the group of Bi: up to 0.4%, Se: up to 0.5% and Te: up to 0.1%.