JP2015074803A - Method for manufacturing steel component made of steel for machine structural use excellent in crystal grain size characteristic and impact characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a steel component such as a gear and a speed reducer which requires long life or high strength by fining prior austenite grain diameter of a steel material thereby improving impact characteristic thereof.SOLUTION: A steel containing, by mass%, C:0.25 to 0.75%, Si:0.05 to 2.00%, Mn:0.10 to 0.50%, P:0.030% or less, S:0.030% or less, Cr:1.80 to 3.00%, Al:0.010 to 0.050%, Nb:0.02 to 0.10%, N:0.0300% or less, and the balance Fe with inevitable impurities is used, quenched at 810 to 880°C after annealed at 750 to 900°C. Then immersion quenching or high frequency quenching is conducted at 810 to 880°C as repeat quenching and tempering is conducted to manufacture a steel component containing a steel for machine structural use excellent in crystal grain size characteristic and impact characteristic.

Description

  The present invention relates to a steel part made of steel for machine structural use, which has excellent crystal grain size characteristics and impact characteristics, which are used in the manufacture of parts such as automobiles, such as gears and reduction gears that require a longer life and higher strength. It relates to a manufacturing method.

  Automotive parts such as gears and CVJs are required to have high strength as they are reduced in size and weight. Therefore, generally, alloying elements such as Ni, Cr, and Mo are added to increase the strength as in the case of nickel, chromium, molybdenum steel in JIS standards. However, materials that have been strengthened by adding alloying elements in this way have high material costs, high deformation resistance, and poor cold workability, so cold forging is not possible. There is a problem that cutting is not possible because the strength becomes too high.

  Further, it has been found that refinement of the prior austenite grain size of the material is effective for increasing the strength, and the strength is increased by refining crystal grains by a combination of processing and heat treatment. However, in this case as well, since molding and heat treatment are combined, it cannot be applied to those that are difficult to mold, and there is a problem that the part shape is limited.

  Alternatively, as a means for refining the prior austenite crystal grain size, there is a material obtained by repeatedly quenching a material added with an element that works effectively as a pinning particle such as Ti or Nb. The former austenite crystal grain size is JIS G0551. The specified particle size No. It is said that a crystal grain size of 11 or more is stably obtained and the strength is improved (see, for example, Patent Document 1 and Patent Document 2).

JP 2003-034839 A JP 2008-293141 A

  However, in the proposed method described above, the addition of Ti as pinning particles deteriorates the impact characteristics, and the grain size No. 1 in which the prior austenite grain size is defined by JIS G0551. Toughness cannot be secured after a crystal grain size of 12 or more is stably obtained.

  Therefore, the problem to be solved by the present invention is made of mechanical structural steel that has excellent crystal grain size characteristics and impact characteristics that can respond to longer life and higher strength of parts such as automobiles, such as gears and reduction gears. It is to provide a method for manufacturing steel parts.

  The means of the present invention for solving the above-mentioned problems is that, in the first means, as a chemical component of steel, in mass%, C: 0.25 to 0.75%, Si: 0.05 to 2.00 %, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.030% or less, Cr: 1.80 to 3.00%, Al: 0.010 to 0.050% , Nb: 0.02 to 0.10%, N: 0.0300% or less, and using steel consisting of the remainder Fe and inevitable impurities, the steel part made of this steel is 750 to 900 ° C. for spheroidizing carbide After annealing, the steel is cold worked or hot forged and quenched at 810 to 880 ° C. with the aim of quenching from a carbide and austenite two-phase state. Then, it is a method for producing a steel part made of steel for machine structural use, which is excellent in crystal grain size characteristics and impact characteristics, and further comprising repeated quenching or induction quenching at 810 to 880 ° C. and then tempering.

The chemical composition of the steel material and the heat treatment method for steel parts made of this steel in the production method of the present invention will be described.
First, the chemical components of steel in the present invention will be described. The steel part in the method of the present invention refines crystal grains by repeated quenching. First, spheroidized carbides are precipitated by annealing. These carbides and elements with high pinning power such as Al and Nb added suppress the growth of very fine austenite initial grains generated during the first quenching heating, and further completely eliminate the carbides in the steel during heating during quenching. This carbide is used as pinning particles by not dissolving in solid solution.

  Next, the repetitive quenching will be described. The repetitive quenching method is used as a method for refining crystal grains. The effect of repeated quenching is greater when twice than when once, but depending on the component, mixed grains may be generated and the strength may be reduced.

  The reason for limiting the chemical composition of the steel material in the method of the present invention will be described in more detail. In addition, a chemical component is shown by the mass%. However, Ni and Mo are inevitable impurities and contained elements.

C: 0.025 to 0.075%
C is an element necessary for ensuring hardness in the quenching and tempering processes. For this purpose, 0.025% or more is necessary. Further, carbide is generated as pinning particles, and is effective for refining crystal grains. However, when C is 0.065% or more, impact characteristics are deteriorated. Therefore, the C content is 0.025 to 0.075%, preferably 0.035 to 0.065%.

Si: 0.05-2.00%
Si is an element necessary for deoxidation, and if it is less than 0.05%, deoxidation is not sufficiently performed. On the other hand, if Si exceeds 2.0%, workability is lowered. Therefore, Si is set to 0.05 to 2.00%, preferably 0.05 to 1.00%.

Mn: 0.10 to 2.00%
Mn is an element necessary for securing a hardened raw material. If Mn is less than 0.10%, the effect of hardenability cannot be sufficiently obtained. Moreover, when Mn exceeds 2.00%, workability will deteriorate. Therefore, Mn is set to 0.10 to 2.00%, preferably 0.20 to 0.50%.

P: 0.030% or less P is an unavoidable element contained from scraps and the like, and segregates at austenite grain boundaries to deteriorate toughness such as impact strength and bending strength. Therefore, P is set to 0.030% or less.

S: 0.030% or less S is an element that improves machinability. However, S combines with Mn to form MnS, which is a non-metallic inclusion, and lowers the toughness and fatigue strength in the transverse direction, where S is made 0.030% or less.

Cr: 1.80 to 3.00%
Cr is an element that increases the hardenability of steel, but if it is less than 1.80%, the effect cannot be sufficiently obtained. On the other hand, Cr produces carbides, and plays the role of pinning particles during repeated quenching, thereby refining crystal grains. However, if Cr is added in excess of 3.00%, workability is impaired. Therefore, Cr is 1.80 to 3.00%.

Ni: 0.25% or less Ni is an element that improves the hardenability and toughness of steel. However, when Ni is added in an amount of more than 0.25%, bainite, which is disadvantageous to the grain size characteristics, precipitates in the present invention. Grain coarsening occurs at the core. Therefore, the toughness reduction due to the coarsening of the crystal grains is larger than the toughness improving effect by adding the alloy element, and the toughness is reduced. Therefore, in order to avoid precipitation of bainite, Ni is set to 0.25% or less which is an inevitable impurity amount level.

Mo: 0.05% or less Mo is an element that improves the hardenability and toughness of steel. However, when Mo is added in an amount of more than 0.05%, bainite, which is disadvantageous to the grain size characteristics, precipitates in the present invention. Grain coarsening occurs at the core. Therefore, the toughness reduction due to the coarsening of the crystal grains is larger than the toughness improving effect by adding the alloy element, and the toughness is reduced. Therefore, in order to avoid precipitation of bainite, Ni is set to 0.05% or less, which is an unavoidable impurity level.

Al: 0.010 to 0.050%
Al is an element used as a deoxidizing material, and also binds to N to precipitate AlN as will be described later, and suppresses coarsening of crystal grains by a pinning effect. Moreover, at the time of repeated hardening, it becomes a pinning particle | grain and refines | miniaturizes a crystal grain. In order to obtain this effect, Al needs to be added by 0.010% or more. On the other hand, when Al is added in an amount of more than 0.050%, the alumina-based oxide increases, and the fatigue characteristics and workability deteriorate. Therefore, Al is made 0.010 to 0.050%.

Nb: 0.02-0.10%
Nb forms Nb carbide, Nb nitride, or Nb carbonitride, and brings about the effect of preventing the coarsening of crystal grains by their pinning effect. Moreover, at the time of repeated hardening, it becomes a pinning particle | grain and refines | miniaturizes a crystal grain. In particular, nano-order Nb carbide or Nb carbonitride finely dispersed in steel suppresses the growth of crystal grains. If Nb is less than 0.02%, those effects cannot be obtained, and if it exceeds 0.10%, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, Nb is 0.02 to 0.10%, preferably 0.02 to 0.08%.

N: 0.0050 to 0.0300%
N is finely precipitated as AlN, Nb nitride, and Nb carbonitride in the steel, and its pinning effect brings about the effect of preventing grain coarsening and the effect of grain refinement during repeated quenching. Add at least%. However, if the amount of N exceeds 0.0300%, nitride increases and fatigue strength and workability decrease. For this reason, N is set to 0.0050 to 0.0300%, preferably 0.0050 to 0.00. 0250%.

Grain size No. 12 or more Grain refinement improves impact value and improves material life. 12 or more, preferably 12.5 or more.

  The effect of the present invention will be described. It is particularly important for steel parts made of medium carbon steel of the present invention to be subjected to two quenching treatments at 810 ° C to 880 ° C after annealing at 750 ° C to 900 ° C. The crystal grains are not refined. When steel that has been annealed and spheroidized into carbides is quenched once under the condition that the carbides are not completely dissolved, and the previous structure is quenched in the state of martensite (or bainite), the prior austenite grain size becomes JIS G0551. The particle size no. It becomes 12 or more extremely fine structures.

  Here, the martensite (or bainite) generated by the first quenching is once austenitized during the re-quenching heating, but since there are many austenite generation sites, austenite grains can be produced everywhere, and these austenite grains are fine. Pinning by dispersed pinning particles (various carbides, nitrides, and carbonitrides) stops growth and quenches from that state, so that a fine martensite structure is formed.

  By adopting the above-mentioned means, the present invention allows the grain size of the steel part to be no. A mechanical structural steel having high impact characteristics can be obtained without using expensive elements such as Ni and Mo at the same time as fine as 12 or more.

(A) of the test piece after hardening is a side view, (b) is a front view. The detailed shape of a Charpy impact test piece is shown. (A) is a side view of the test piece, and (b) is a front view of the test piece.

  The steels of Comparative Examples and Examples having chemical components shown in Table 1 were melted in a 100 kg vacuum melting furnace, cast into an ingot, heated to 1200 ° C. and held for 4 hours to form a solution. Forged into bars. Furthermore, after annealing the material made of the forged bar material at 775 ° C., the material was first heated by oil quenching (60 ° C.) after heating at 850 ° C. for 20 minutes, and further heated at 830 ° C. for 20 minutes. Later, the second oil quenching (60 ° C.) was performed, and the crystal grain size was cut along the length direction as shown in FIG. 1 after the first quenching and immediately after the second quenching. Then, it was corroded with a saturated picric acid solution, and the former austenite grain boundaries appeared. The test piece was observed with an optical microscope, and the particle size number was determined from the middle part of the material.

Table 2 shows the prior austenite grain size No. 1 after quenching and twice quenching in Comparative Examples and Examples. And the impact value of the Charpy impact test result after 2 times quenching is shown. After the single quenching, the prior austenite grain size was the same as the grain size no. 12 or less. On the other hand, in all the examples after quenching twice, the prior austenite grain size is no. The impact value was finer than 12, and an impact value of 100 J / cm ² or more was better than that of the comparative example.
No. In comparative examples other than 9, the grain size No., which is the target value of the crystal grain size after quenching twice. 12 was not reached. Comparative Example No. 9 also had a fine grain size, but the amounts of C and Al were very high, and the impact value was worse than that of the examples. Comparative Example No. No. 3 had a large amount of Ni and Mo and showed a good impact value, but the amount of pinning particles was insufficient, and the crystal grain size was not refined by two-time quenching.

In the present invention, it is also an important factor to add annealing before the repeated quenching treatment. Therefore, for comparison with the present invention, the results of repeated quenching without annealing are shown in Table 3 below as comparative example Nos. Shown as 1-24.
The comparative example of Table 3 is a No. made up of chemical components shown in Table 1. Forging materials 1 to 24, heating at 850 ° C. for 20 minutes, followed by first quenching with oil quenching (60 ° C.), and further heating at 830 ° C. for 20 minutes followed by second oil quenching (60 ° C.) Carried out. After that, after the first quenching and the second quenching, a 20 mm diameter rod is cut along the length direction as shown in FIG. 1, and this section is mirror-polished and corroded with a saturated picric acid solution. The former austenite grain boundary was revealed. The test piece was observed with an optical microscope, and the particle size number was determined from the middle part of the material. The results are shown in Table 3. In any steel type, grain size No. It did not satisfy 12 or more.

1 Test piece after quenching 2 Grain size measurement position 3 Charpy impact test piece

Claims (1)

  In mass%, C: 0.25 to 0.75%, Si: 0.05 to 2.00%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.030 %: Cr: 1.80 to 3.00%, Al: 0.010 to 0.050%, Nb: 0.02 to 0.10%, N: 0.0300% or less, and the balance Fe and Using steel consisting of inevitable impurities, after annealing this steel material between 750 ° C. and 900 ° C., quenching at 810 to 880 ° C., and then repeatedly quenching or induction quenching at 810 to 880 ° C. And then tempering, a method for producing a steel part made of mechanical structural steel having excellent grain size characteristics and impact characteristics.

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