JP2008075136A - Steel material composed of nitriding steel for cold-working excellent in weldability and strong characteristic, and nitriding steel part composed of this steel material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for nitriding and a nitriding steel part composed of this steel material excellent in cold-workability, weldability, tensile strength, bending fatigue strength, with a low alloy-based steel material drastically reducing the cost. <P>SOLUTION: The steel material composed of the nitriding steel for cold-working, excellent in the weldability, the strength characteristic is composed by mass% of 0.03-0.2% C, 0.01-1.0% Si, 0.3-2.0% Mn, 1.0-5.0% Ni, 0.2-4.5% Cr, 0.5-1.5% Cu, 0.5-1.5% Al and further, ≤0.03% S, ≤30ppm O, ≤100ppm N and satisfying the above component elements 32≤fn1≤50 and the balance Fe with inevitable impurities. Wherein, fn1=153.5C+8Mn+5.1Ni+5.2Cr. Respective component element shows the value shown with the mass%. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel material excellent in fatigue characteristics used for a member used for power transmission such as automobile parts, and in particular, formed by cold working, strengthened by nitriding treatment, and joined by welding for assembling. The present invention relates to a steel material used for manufacturing a part, and further relates to a nitrided steel part made of this steel material.

  Nitride steel is a special steel to which a suitable amount of nitride-forming elements such as Al, Cr and V is added in order to obtain high surface hardness by nitriding. By nitriding this nitrided steel, excellent performance in fatigue strength, heat resistance, wear resistance, corrosion resistance, and the like can be obtained.

  Nitriding is being studied for applications that also require wear resistance for stainless steel, but in the case of stainless steel, the presence of a passive film on the surface hinders the ingress of nitrogen, making nitriding difficult. there were. Today, nitriding of stainless steel can be performed by pretreatment to remove the passive film.

  Precipitation hardening type stainless steel is known as a material for nitriding such stainless steel, but in this case, a large amount of expensive elements that are excellent in cold workability, weldability and strength are added. Therefore, there was a problem of cost increase. Further, it is easy to form a hard compound layer on the surface during nitriding, and there is a problem that this compound layer peels off during loading and deteriorates fatigue characteristics.

  On the other hand, a soft nitriding steel material that has excellent fatigue properties, wear resistance, pitting resistance and spalling resistance, simply by soft nitriding after cold working, using steel with excellent machinability and cold workability. Has been proposed (see, for example, Patent Document 1). However, it has good fatigue strength at bending load where stress concentration in the core is small, but the tensile strength where the same stress acts on the outer periphery and core, the strength is insufficient because the core strength is low. there were.

Japanese Patent Application Laid-Open No. 11-302779

  The problem to be solved by the present invention is the development of a low-alloy steel material that is significantly reduced in cost compared to precipitation hardened stainless steel material, and includes cold workability, weldability, tensile strength, bending fatigue. An object of the present invention is to provide a nitriding steel material excellent in strength and a nitridated steel part made of this steel material.

  The means of the present invention for solving the above-mentioned problems is to add Ni, Cu, Al to the main components of C, Si, Mn, Cr and Fe of low alloy structural steel components, and to precipitate and harden the steel. It is a steel material made of nitrided steel. Further, the steel part is nitrided from this nitrided steel, has a core hardness of 350 HV or more, and a tensile strength of 1100 MPa or more.

  That is, in the invention of claim 1, the means of the present invention is, in mass%, C: 0.03-0.2%, Si: 0.01-1.0%, Mn: 0.3-2.0. %, Ni: 1.0-5.0%, Cr: 0.2-4.5%, Cu: 0.5-1.5%, Al: 0.5-1.5%, S: not more than 0.03%, O: not more than 30 ppm, N: not more than 100 ppm, satisfying 32 ≦ fn1 ≦ 50 in the above component elements, and consisting of the balance Fe and inevitable impurities, It is a steel material made of nitrided steel for cold working with excellent strength characteristics.

  However, fn1 = 153.5C + 8Mn + 5.1Ni + 5.2Cr, and each component element is a numerical value represented by mass%.

  In the invention of claim 2, in terms of% by mass, the components of the steel material of the means of claim 1 above are further added Mo: 0.1-2.0%, V: 0.05-0.5%, Ti: 0 0.005 to 0.5%, Nb: containing one or more selected from 0.005 to 0.5%, satisfying 32 ≦ fn1 ≦ 50 in the above component elements, remaining Fe and inevitable impurities It is a steel material made of nitrided steel for cold working and having excellent weldability and strength characteristics.

  However, fn1 = 153.5C + 8Mn + 5.1Ni + 5.2Cr, and each component element is a numerical value represented by mass%.

  In the invention of claim 3, the steel material made of nitrided steel for cold working according to the means of claim 1 or claim 2 is nitrided to have a core hardness of 350 HV or more and a tensile strength of 1100 MPa or more. It is a nitrided steel part for cold working with excellent weldability and strength characteristics.

  The reason for limiting the range of the steel component of the nitrided steel in the above-described means of the present invention will be described. In addition, a steel component shall be shown by the mass%.

C: 0.03-0.2%, desirably 0.03-0.15%
C is an element necessary for securing the core hardness, tensile strength and bending fatigue strength, and for this purpose, 0.03% or more is necessary. However, if it is more than 0.2%, weldability and cold workability are impaired. Therefore, C is 0.03 to 0.2%, preferably 0.03 to 0.15%.

Si: 0.01 to 1.0%, desirably 0.1 to 0.5%
Si is a deoxidizing material and a necessary element, and further an element necessary for improving the tensile strength. For this purpose, 0.01% or more is necessary. However, if it is more than 1.0%, the toughness is deteriorated and the cold workability is hindered. Therefore, Si is 0.01 to 1.0%, preferably 0.1 to 0.5%.

Mn: 0.3 to 2.0%, desirably 0.8 to 2.0%
Mn is an element necessary for ensuring core hardness, tensile strength and bending fatigue strength. For this purpose, Mn is required to be 0.3% or more. However, if it is more than 2.0%, weldability and cold workability are impaired. Therefore, Mn is 0.3 to 2.0%, preferably 0.8 to 2.0%.

P: 0.03% or less, desirably 0.01 to 0.03%
P is an element contained as an impurity by melting by electric furnace refining. In ordinary electric furnace refining, the lower limit is 0.01% and the upper limit is 0.03%. Therefore, P is 0.03% or less, preferably 0.01 to 0.03%.

S: 0 to 0.03%, desirably 0 to 0.02%
S is an element that decreases the bending fatigue strength when contained in an amount of more than 0.03%. Therefore, S is 0 to 0.03%, desirably 0 to 0.02%.

Ni: 1.0-5.0%, desirably 1.5-3.5%
Ni is an element necessary for securing the core hardness, tensile strength and bending fatigue strength, and is an element that causes strengthening by the Ni-Al intermetallic compound during aging treatment. For this purpose, 1.0% or more is necessary. However, if it is more than 5.0%, weldability and cold workability are impaired. Therefore, Ni is set to 1.0 to 5.0%, preferably 1.5 to 3.5%.

Cr: 0.2 to 4.5%, desirably 0.8 to 2.5%
Cr is an element necessary for securing the core hardness, tensile strength and bending fatigue strength, and contributes to an increase in surface hardness during nitriding, thereby improving wear resistance. For this purpose, 0.2% or more is necessary. However, if it is more than 4.5%, weldability and cold workability are impaired, and a harmful compound layer is formed during nitriding. Therefore, Cr is 0.2 to 4.5%, preferably 0.8 to 2.5%.

Cu: 0.5 to 1.5%, desirably 0.7 to 1.5%
Cu is an element that contributes to an increase in hardness during aging treatment and contributes to prevention of softening during nitriding, and for that purpose, 0.5% or more is necessary. However, if it is more than 1.5%, the generation of surface defects during hot rolling is promoted and the toughness is lowered. Therefore, Cu is 0.5 to 1.5%, preferably 0.7 to 1.5%.

Al: 0.5 to 1.5%, desirably 0.7 to 1.5%
Al forms a Ni—Al intermetallic compound during aging treatment and contributes to an increase in hardness. Further, at the time of nitriding, it is an element that forms a large number of AlN compounds in the nitrided layer to further increase the surface hardness and improve the wear resistance. However, if it is more than 1.5%, the surface hardness during nitriding becomes too high, and the bending fatigue strength decreases. Therefore, Al is 0.5 to 1.5%, preferably 0.7 to 1.5%.

O: ≦ 30 ppm, desirably ≦ 20 ppm
When O is contained in an amount of more than 30 ppm, a large number of oxides are generated to lower the bending fatigue strength. Therefore, O is ≦ 30 ppm, preferably ≦ 20 ppm.

N: ≦ 100 ppm, desirably ≦ 80 ppm
When N is contained in an amount of more than 100 ppm, a large number of coarse nitrides are generated during solidification of the steel, and this becomes the starting point of fatigue fracture, thereby lowering the bending fatigue strength. Therefore, N must be limited to ≦ 100 ppm, preferably ≦ 80 ppm.

Mo: 0.1-2.0%, desirably 0.3-2.0%
Mo is an element that improves strength and toughness, and for that purpose, 0.1% or more is required. However, if the amount is too high, the cost increases, so the upper limit is set to 2.0%. Therefore, Mo is 0.1 to 2.0%, preferably 0.3 to 2.0%.

V: 0.05 to 0.5%, desirably 0.1 to 0.35%
V is an element that is necessary for improving the strength by precipitation hardening of carbide and making the crystal grains finer, and is an element that contributes to improving the strength by forming a nitride during nitriding. For that purpose, V is 0.05% or more. . However, when V is added excessively, a hard compound layer is formed on the surface at the time of nitriding and the fatigue characteristics are lowered, so the addition amount is made 0.5% or less. Therefore, V is set to 0.05 to 0.5%, preferably 0.1 to 0.35%.

Ti: 0.005 to 0.5%, desirably 0.15 to 0.35%
Ti is an element necessary for improving the strength by precipitation hardening of carbides and refining crystal grains. For that purpose, Ti is made 0.005% or more. However, when Ti is more than 0.5%, a large number of nitrides are generated, and the bending fatigue strength is lowered. Therefore, Ti is 0.005 to 0.5%, preferably 0.15 to 0.35%.

Nb: 0.005 to 0.5%, desirably 0.03 to 0.35%
Nb is an element necessary for strength improvement and grain refinement by precipitation hardening of carbides. For that purpose, Nb is set to 0.005% or more. However, if the amount is too high, the cost increases, so the upper limit is made 0.5%. Therefore, Nb is 0.005 to 0.5%, preferably 0.03 to 0.35%.

Further, the reason why 32 ≦ fn1 ≦ 50 will be described.
fn1: 32 ≦ fn1 ≦ 50, preferably 36 ≦ fn1 ≦ 50
fn1 is composed of 153.5C + 8Mn + 5.1Ni + 5.2Cr, and when each component element is a numerical value represented by mass%, the value of fn1 is necessary for securing the weldability, cold workability, core hardness and strength of the steel material. Value, which requires 32 or more. However, if the value of fn1 exceeds 50, weldability and cold workability are hindered. Therefore, 32 ≦ fn1 ≦ 50, preferably 36 ≦ fn1 ≦ 50.

Furthermore, the reason why the core hardness of the nitrided steel part is 350 HV or higher will be described.
In order to ensure the tensile strength of the nitrided steel part to 1100 MPa or more, the core part strength of the steel part needs to be 350 HV or more. Therefore, the core hardness of the nitrided steel part is set to 350 HV or higher.

  The present invention provides a nitrided steel material that is excellent in cold workability and weldability, excellent in tensile strength and bending fatigue strength, and a component made of the steel material, by using the above means. This is an excellent effect.

The best mode of the present invention will be described with reference to the table.
Invention steel and comparative steel containing the components shown in Table 1 were melted in a 100 kg VIM furnace and cast into an ingot.

  Next, the above ingot was heated to 1250 ° C. and forged into a φ20 mm material. Further, the forged material was subjected to a solution treatment at 870 ° C. for 1 hour to produce a cold upsetting test piece having a diameter of 14 mm × length of 21 mm. The cold upsetting test piece was subjected to 60% cold upsetting using a universal testing machine, and then the cold workability was confirmed by the presence or absence of cracks. Table 2 shows the results.

  Furthermore, after producing the forged material as described above into a 0.5 mm-thick plate by cold rolling, solution treatment is performed at 870 ° C. for 1 hour in an atmosphere furnace or a vacuum furnace in which decarburization and oxidation are suppressed, Next, degreasing and descaling were performed to obtain two 0.5 mm thick × 20 mm wide plate materials, and these two plate materials were brought into contact with each other, and then TIG welding (conditions: electrode diameter 2 mm, current 100 A, Ar gas flow rate 10 liters) / Min, scanning speed 150 mm / min, no preheating), and the presence or absence of weld cracks on the surface and cross section of the joint was confirmed.

  Similarly to the above, after producing the forged material by cold rolling into a 0.5 mm thick plate, a tensile test piece of 0.5 mm thickness × 10 mm width × 150 mm length was prepared, and this test piece was Perform solution treatment at 870 ° C. for 1 hour in an atmosphere furnace or vacuum furnace that suppresses decarburization and oxidation, then perform aging treatment at 500 ° C. for 5 hours, and further gas nitriding treatment at 450 ° C. for 1 hour. did. Table 2 shows the core hardness after nitriding.

  Furthermore, using the above plate-like tensile test piece, a tensile test was performed as an index of strength, and the tensile strength was measured.

  Further, using the above-described plate-like tensile test piece, as a strength index, a bending fatigue test was performed with both ends fixed, and the bending fatigue strength was measured.

  As can be seen from Table 2, the nitrided steel of the present invention is excellent in cold workability and weldability without cracking, the core hardness after nitriding is 350 HV or more, and the fatigue strength is 1100 MPa or more. Further, the bending fatigue strength was 520 MPa or more. On the other hand, among the comparative steels, those that deviate from the component range of the present invention or whose fn1 value deviates from the range of the present invention have any characteristics as compared with the present invention as shown by shading in Table 2. It was inferior.

Claims (3)

In mass%, C: 0.03-0.2%, Si: 0.01-1.0%, Mn: 0.3-2.0%, Ni: 1.0-5.0%, Cr: 0.2 to 4.5%, Cu: 0.5 to 1.5%, Al: 0.5 to 1.5%, S: 0.03% or less, O: 30ppm or less, N: A steel material made of nitrided steel for cold working and having excellent weldability and strength characteristics, containing 100 ppm or less, satisfying 32 ≦ fn1 ≦ 50 in the above component elements, and comprising balance Fe and inevitable impurities .
However, fn1 = 153.5C + 8Mn + 5.1Ni + 5.2Cr, and each component element is a numerical value represented by mass%. In addition to the components of the above steel materials in terms of mass%, Mo: 0.1 to 2.0%, V: 0.05 to 0.5%, Ti: 0.005 to 0.5%, Nb: 0.005 1 to 2% or more selected from -0.5%, satisfying 32≤fn1≤50 in the above component elements, and comprising the balance Fe and inevitable impurities, with weldability and strength characteristics Steel material made of nitrided steel for excellent cold working.
However, fn1 = 153.5C + 8Mn + 5.1Ni + 5.2Cr, and each component element is a numerical value represented by mass%.   A steel part comprising the cold-worked nitrided steel according to claim 1 or 2 is nitrided with excellent weldability and strength characteristics, wherein the core has a hardness of 350 HV or more after nitriding. Steel parts.