JP2006152445A - High strength case hardened steel and parts thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve strength of steel parts consisting of a case hardened steel by positively ultra fining austenitic grains according to rehardening after carburization. <P>SOLUTION: The high-strength case hardened steel includes, by mass%, 0.10 to 0.35% C, 0.03 to 0.50% Si, 2.0% or less Mn, 0.020% or less N and 0.005 to 0.05% Al, and further includes one or two kinds selected from 0.02 to 0.10% V and 0.02 to 0.1% Nb, and the balance Fe with inevitable impurities, and has a martensitic structure in which old austenitic grains are fined to No.11 prescribed in JIS G0551 or finer. The manufacturing method for the parts comprises employing the above high strength case hardened steel, making the parts from the steel, carburizing and rehardening them to form the martensitic structure with fined grains to No.11 of JIS G0551 or finer therein. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to parts made of machine structural steel, for example, case-hardened steel for automobiles that require high strength such as gears and CVJ, and carburized parts thereof.

  Automotive parts such as gears and CVJs are required to have high strength as they are reduced in size and weight. Therefore, 1) alloying elements are added to increase the strength like Ni steel such as SNCM. However, when alloying elements are added in this way to increase the strength, the material cost increases, and cold workability cannot be achieved due to poor cold workability. There is a problem that a heat treatment such as annealing is required.

  2) Further, after carburizing, re-quenching is performed to refine crystal grains and increase the strength. However, also in this case, for example, when general steel such as SCM is used, there is a problem in that sufficient refinement cannot be achieved because there is a limit to refinement of crystal grains.

  3) Alternatively, the crystal grains are refined by processing to increase the strength. However, in this case as well, there is a problem that the shape of the part is limited because it cannot be applied to a difficult-to-mold product because the molding and heat treatment are combined.

  Further, an application for preventing coarsening of γ grains during carburizing has been filed and disclosed (for example, see Patent Document 1).

Japanese Patent Laid-Open No. 10-81938

  The problem to be solved by the present invention is to solve the above-mentioned conventional problems, and an object is to positively refine ultrafine austenite grains and improve the strength by re-quenching after carburizing.

  The means of the present invention for solving the problem is that, in the invention of claim 1, in mass%, C: 0.10 to 0.35%, Si: 0.03 to 0.50%, Mn: 2.0 %: N: 0.020% or less, Al: 0.005-0.05%, V: 0.02-0.10%, Nb: 1 out of 0.02-0.1% No. 1 containing two or more seeds, the balance being Fe and inevitable impurities, and the prior austenite grain size specified in JIS G0551. It is a high-strength case-hardened steel comprising a martensite structure refined to 11 or more.

  In the invention of claim 2, the steel component of the means of claim 1 is further added in mass%, Cr: 2.0% or less, Mo: 2.0% or less, Ni: 3.0% or less. Or containing two or more types, the balance of Fe and unavoidable impurities, and the old austenite grain size specified in JIS G0551. It is a high-strength case-hardened steel comprising a martensite structure refined to 11 or more.

  In the invention of claim 3, the steel component of the means of claim 1 or 2 further contains, in mass%, B: 0.0005 to 0.005%, and consists of the balance Fe and inevitable impurities, and comprises a prior austenite crystal No. in which the particle size is defined in JIS G0551. It is a high-strength case-hardened steel comprising a martensite structure refined to 11 or more.

  In the invention of claim 4, in order to improve machinability, the steel component of the means of any one of claims 1 to 3 is further mass%, S: 0.01 to 0.2%, Pb: 0.01-0.3%, Bi: 0.01-0.3%, Te: 0.01-0.2%, Se: 0.01-0.2%, Ca: 0.0003-0. No. 1 containing one or more selected from 01%, the balance being Fe and unavoidable impurities, and the prior austenite grain size specified in JIS G0551. It is a high-strength case-hardened steel comprising a martensite structure refined to 11 or more.

  In the invention of claim 5, steel parts made of high strength case-hardened steel with martensite structure according to any one of claims 1 to 4 are carburized and then re-quenched, and JIS G0551 No. It is a high-strength carburized part characterized by comprising a martensite structure refined to 11 or more.

  The reasons for limiting the steel components contained in the steel of the present invention will be described. In addition,% shows the mass%.

  C: C is an element necessary for securing the core strength after carburizing treatment as a machine structural component. If it is less than 0.10%, the effect cannot be sufficiently obtained, and conversely exceeds 0.35%. And reduce the toughness of the core. Therefore, the content was made 0.10 to 0.35%.

  Si: If Si is less than 0.03%, a sufficient deoxidation effect cannot be obtained. If it is contained in excess of 0.50%, the workability is lowered and the formation of a grain boundary oxide layer during carburizing is promoted. The characteristics are also lowered. Therefore, the content is set to 0.03 to 0.50%.

  Mn: Mn is an element necessary for ensuring hardenability, but if it exceeds 2.0%, workability is lowered. Therefore, the content is set to 2.0% or less.

  V: V forms carbides and has the effect of suppressing coarsening of the austenite grain size. In particular, nano-order VC finely dispersed in steel suppresses the growth of crystal grains. If V is less than 0.02%, the effect cannot be obtained, and if it exceeds 0.10%, the amount of precipitates becomes excessive and the workability is lowered. Therefore, the content is set to 0.02 to 0.10%.

  Nb: Nb forms carbides or nitrides and has the effect of suppressing coarsening of the austenite crystal grain size. In particular, nano-order NbC finely dispersed in steel suppresses the growth of crystal grains. If Nb is less than 0.02%, the effect cannot be obtained. If the content exceeds 0.10%, the amount of precipitates becomes excessive and the workability is lowered. Therefore, the content is set to 0.02 to 0.10%. The means of claim 1 can contain one or two of V and Nb.

  Cr: Cr is an element that improves hardenability and carburization. However, if it is contained in an amount exceeding 2.0%, coarse carbides are formed in the carburized layer, and mechanical properties and fatigue characteristics are deteriorated. Therefore, the content is made 2.0% or less.

  Mo: Mo is an element that improves hardenability and toughness. However, if it is contained in excess of 2.0%, it becomes a bainite or martensite structure after rolling or forging, which significantly reduces workability and leads to an increase in cost. Therefore, the content is made 2.0% or less.

  Ni: Ni is an element that improves hardenability and toughness. However, if it is contained in an amount exceeding 3.0%, it becomes a bainite or martensite structure after rolling or forging, and the workability is remarkably reduced, leading to an increase in cost. Therefore, the content is made 3.0% or less. In addition, Cr, Mo, and Ni can contain 1 type, or 2 or more types among them.

  If N: N is contained in an amount exceeding 0.020%, the amount of precipitates that are nitrides of Nb becomes excessive, and the workability is lowered. Therefore, in the invention of claim 1, the content is set to 0.020% or less.

  Al: Al is an element used as a deoxidizer, and if it is less than 0.005%, its effect is not sufficient, and if it exceeds 0.05%, alumina-based oxides increase and fatigue characteristics and workability deteriorate. . Therefore, the content is set to 0.005 to 0.05%.

  B: B is an element that remarkably improves the hardenability of steel by addition of a very small amount, and is an element that can be selectively added arbitrarily. However, if it is less than 0.0005%, the effect is not sufficient, and if it exceeds 0.005%, the hardenability is lowered. Therefore, the content is set to 0.0005 to 0.005%.

  As an element for improving machinability, S: 0.01 to 0.2%, Pb: 0.01 to 0.3%, Bi: 0.01 to 0.3%, Te: 0.01 to 0 0.2%, Se: 0.01 to 0.2%, Ca: 0.0003 to 0.01% selected from one or more. These elements do not have a sufficient machinability improvement effect below the lower limit value, and when the upper limit value is exceeded, the toughness decreases conversely.

  Describing the operation of the present invention, in order to improve the strength of the steel, the present invention is the No. 1 in which prior austenite grains are defined in JIS G0551. It is a steel with a martensite structure that is extremely refined to 11 or more. As a result, the fracture start point of steel shifts from intergranular fracture to intragranular fracture, and the bending strength and impact characteristics are improved.

  It is particularly important for the steel part made of the above steel of the present invention to be re-quenched after carburizing, and it does not become ultra-fine if it is subjected to normal carburizing and quenching. That is, when carburizing and quenching and quenching in a state where the previous structure is martensite (or bainite), it becomes ultrafine.

  Here, martensite (or bainite) once becomes austenite at the time of re-quenching heating, but since there are many austenite generation sites, austenite grains are formed everywhere. The nano-order V carbide, Nb carbide or nitride in which the austenite grains are finely dispersed in the steel stops the growth of crystal grains. Since it is quenched from that state, an ultrafine martensite structure is formed.

  By the way, general steel (such as SCM) is refined by re-quenching. Less than 11 is the limit, and mixed grains are likely to occur.

  In the steel added with Ti developed by the applicant, that is, the invention disclosed in Japanese Patent Application Laid-Open No. 10-81938, as in the above SCM, the coarsening of the crystal grains is prevented and the strength is high. The particle size is No. It is less than 11. On the other hand, in the present invention, the old austenite grain size is No. defined in JIS G0551. Steel made of martensite structure refined to 11 or more, and steel parts made of steel added with Nb or V are carburized and then re-quenched, so that nano-order V carbides or Nb carbides and nitrides Is finely dispersed to stop crystal grain growth (pinning effect). As a result, the grain size No. in the region that could not be achieved with general steel. It is possible to achieve ultrafineness exceeding 11, and it is possible to further increase the strength by making this crystal grain ultrafine.

  Thereby, mechanical properties (bending strength, toughness, etc.) can be greatly improved without using expensive elements such as Ni. Since it is not a thermomechanical treatment, it can be applied to all parts.

  As described above, the present invention relates to No. 1 in which the prior austenite grain size is defined by JIS G0551. A high-strength case-hardened steel having a martensite structure refined to 11 or more, and further steel steel parts made from this steel, and re-quenched after carburizing, the grain size of JIS G0551 No. What consists of the martensite structure refined | miniaturized to 11 or more is obtained, and the high intensity | strength case-hardened steel part which is not conventionally used as an automotive part is obtained.

  The steels according to the first to fourth embodiments of the present invention are No. 9-No. No. 18 with the steel composition shown in FIG. It consists of a steel with a martensite structure that is extremely refined to 11 or more. The carburized part according to the embodiment of claim 5 is a carburized part of the above steel and then re-quenched to obtain crystal grains according to JIS G0551 No. It consists of a martensite structure refined to 11 or more.

Steel pieces formed by melting the test materials of comparative steel and invention steel shown in Table 1 in a 100 kg vacuum melting furnace were heated to 1250 ° C. and forged into a φ20 mm bar steel. In this case, the steel slab is rolled at 1250 to 1400 ° C. in order to precipitate fine Nb and V carbides. Further when rolled into bars is rolled with Ac ₃ to 1050 ° C.. Next, after finishing and forging at 950 ° C., and processing into a cylindrical specimen of φ14 mm × 21 mmH, holding at 930 ° C. for 6 hours and carburizing and quenching, holding at 850 ° C. for 0.5 hours and re-quenching And tempering at 180 ° C. for 1.5 hours.

  The crystal grains of the test piece obtained above were measured according to JIS GO551, and the results are shown in Tables 2 and 3. No. of the steel of the present invention. Nos. 9 to 18 are comparative steel Nos. In comparison with 1-8, the particle size No. No. It can be seen that the grain size is 11 or more.

  Furthermore, in order to evaluate the crack generation load and the fracture load, a static three-point bending test was performed. The ratio to 1 is shown in Table 2. The static three-point bending test was performed at a distance of 50 mm between spans by providing a 2 mmV notch on a test piece of 10 mm square × 70 mm length that was re-quenched after carburizing. No. of the test material of the steel of the present invention. Nos. 9 to 18 are Nos. Of specimens for comparison steel. The initial crack initiation load and crack initiation load are superior to those of Nos. 1-8. It can be seen that the initial crack generation load is more than twice that of 1.

  Further, in order to further evaluate the impact value, a Charpy impact test was performed by providing a 10RC 2 mm notch on a 10 mm square × 55 mm long test piece which had been re-quenched after carburizing. The ratio to 1 is shown in Table 3. No. of the test material of the steel of the present invention. Nos. 9 to 18 are Nos. Of specimens for comparison steel. Compared to 1-8, the impact value is far superior. It can be seen that the impact value is more than double that of 1.

Claims (5)

C: 0.10 to 0.35%, Si: 0.03 to 0.50%, Mn: 2.0% or less, N: 0.020% or less, Al: 0.005 to 0. Containing 05%, further containing one or two of V: 0.02 to 0.10%, Nb: 0.02 to 0.1%, the balance being Fe and unavoidable impurities, and a prior austenite crystal No. in which the particle size is defined in JIS G0551. A high-strength case-hardened steel comprising a martensite structure refined to 11 or more. The steel component according to claim 1 further contains, in mass%, one or more of Cr: 2.0% or less, Mo: 2.0% or less, Ni: 3.0% or less, It consists of the remaining Fe and inevitable impurities, and the former austenite grain size is No. defined in JIS G0551. A high-strength case-hardened steel comprising a martensite structure refined to 11 or more. The steel component according to claim 1 or 2 further contains B: 0.0005 to 0.005% by mass%, and consists of the balance Fe and inevitable impurities, and the prior austenite grain size is defined by JIS G0551. No. A high-strength case-hardened steel comprising a martensite structure refined to 11 or more. In order to improve machinability, the steel component according to any one of claims 1 to 3 is further mass%, S: 0.01 to 0.2%, Pb: 0.01 to 0.3. %, Bi: 0.01-0.3%, Te: 0.01-0.2%, Se: 0.01-0.2%, Ca: 0.0003-0.01% Or a mixture of two or more types, the balance of Fe and inevitable impurities, and the former austenite grain size specified in JIS G0551. A high-strength case-hardened steel comprising a martensite structure refined to 11 or more. A steel part made of the high-strength case-hardened steel having a martensite structure according to any one of claims 1 to 4 is carburized and then re-quenched, and JIS G0551 No. 5 is used. A high-strength carburized part characterized by comprising a martensite structure refined to 11 or more.