JP5965117B2 - Machine structural steel for carburized parts with excellent grain coarsening resistance, workability and toughness - Google Patents

Description

  The present invention relates to a steel for machine structure excellent in grain coarsening resistance and toughness used for carburizing parts of power transmission devices such as automobiles.

  Cold working methods such as cold forging and cold working are advantageous methods for reducing the manufacturing cost of parts such as automobile drive system parts. By the way, when parts are manufactured by carburizing directly after cold working, the fine austenite grains are formed at the initial stage of carburizing due to the influence of cold working, so that the crystal grains tend to be coarsened instead of carburizing. Have a problem. Since the strength of a part may decrease when the crystal grains become coarse, it is essential to suppress the coarsening of the crystal grains. Due to this problem, the cost merit of the cold work method cannot be fully utilized. By the way, in the process of heating the parts to the carburizing temperature after cold working, the ferrite is transformed into austenite once through the stage of fine recrystallization due to the influence of strain during cold working. It encourages the formation of grains. Therefore, as a conventional technique, annealing is performed in the process of heating to the carburizing temperature after cold working, and the strain energy that becomes the driving force of the above-mentioned ferrite recrystallization is released, thereby suppressing grain coarsening during carburizing. There is a method (for example, refer nonpatent literature 1). However, since a new process is added by this, it is hard to use from a viewpoint of cost reduction of components.

  On the other hand, the inventors have a non-uniform microstructure with respect to spheroidizing annealing performed for the purpose of improving workability prior to cold working for carburizing steel for structural use as a component material. Has been found to promote grain coarsening during carburizing. Usually, when spheroidizing annealing is applied to machine structural steel for carburizing treatment, partial lamellar pearlite is inevitably generated, and the resulting spheroidizing annealing structure is non-uniform. The part of pearlite in which carbides exist in lamellar form in the steel is hard, and is harder to deform than the parent phase steel, so locally uneven strain is generated around the lamellar pearlite in the process of cold forging the parts. Easy to concentrate. As a result, a process in which ferrite is recrystallized particularly finely around the lamellar pearlite when cold forging or cold working and cutting to the required shape by heating to the carburizing temperature. Therefore, austenite grains at the initial stage of carburizing are formed particularly finely. Due to this influence, crystal grains grow and become coarser during carburizing.

  Therefore, the applicant of the present invention added a limitation of the chemical composition, a limitation of the lamellar pearlite area ratio after spheroidizing annealing, a limitation of the spheroidizing annealing conditions, and cold forging or cold working, and as required. There has already been proposed a steel for machine structural use and a method for manufacturing the same that are less likely to cause crystal grain coarsening when carburizing is performed after cutting into a predetermined shape (see, for example, Patent Document 1). ). This proposed technique is based on the effect of suppressing grain coarsening by fine precipitates, such as AlN, NbC, and Nb (CN), which pin the grain boundaries contained in the machine structural steel for carburizing treatment. It is a steel for machine structural use that exhibits excellent grain coarsening resistance.

JP 2010-242209 A

K. C. Evanson, G.M. Krauss and D.C. K. Matlock: Grain Growth in Polycrystalline Materials III, ed. by H.M. Weiland, B.M. L. Adams and A.M. D. Rollet, TMS, Warrendale, PA (1993), 599. J. et al. JPN. Soc. Technol. Plast. 22 (1981), p. 139. Cold Forging Subcommittee Materials Research Group “Forging Subcommittee Standards”

  The problem to be solved by the present invention is that spheroidizing annealing is not performed by changing the amount of a part of chemical components from the invention described in Patent Document 1 in which a part of the inventor is the same as the present application. And after the heat treatment method (normalization or annealing) to become a ferrite and pearlite structure in which bainite does not precipitate, cold forging, which is one of the cold work, is performed, and further cut to obtain a predetermined shape. When carburizing treatment is performed after processing, it is difficult to cause crystal grain coarsening, and it has excellent resistance to crystal grain coarsening by AlN, NbC, Nb (CN) that pin the grain boundaries. It is to provide a machine structural steel for carburized parts having excellent grain coarsening characteristics and toughness.

In the means for solving the above-mentioned problems, the invention of claim 1 is, in mass%, C: 0.10 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.10 0.50%, P: 0.030% or less, S: 0.030% or less, Cr: 1.80 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.02 to 0 containing .10%, further, N: containing 0.0300% or less, and the balance Fe and Ni: less than 0.25%, Mo: a ing steel incidental impurities containing less than 0.05%, Machine structural steel for carburized parts with excellent grain coarsening characteristics and toughness, characterized in that the normalized structure or annealed structure is a ferrite-pearlite structure and the average value of the ferrite grain size is 15 μm or more. It is.

The invention of claim 2 is the chemical composition of the steel of the means of claim 1 in mass%, N: 0.0300% or less, N: less than 0.010%, further, of the means of claim 1 in addition to the chemical components of the steel, Ti: less than 0.050% B: containing from 0.0010 to 0.0050%, the balance being Fe and Ni: less than 0.25% Mo: less than 0.05% a ing steel incidental impurities containing, normalizing tissue or annealed structure is ferrite-pearlite structure, resistance to grain growth characteristics and toughness average of the ferrite grain size is equal to or is 15μm or more It is an excellent steel for machine structural use for carburized parts.

  Next, the reasons for limiting the chemical components of the mechanical structural steel of the present invention and the heat treatment conditions of the mechanical structural steel will be described. In addition,% of a component is the mass%.

C: 0.10 to 0.30%
C is an element necessary for ensuring the core strength after carburizing treatment of steel as a machine structural component. However, if C is less than 0.10%, the effect cannot be sufficiently obtained. On the other hand, when C exceeds 0.30%, workability is lowered and toughness is lowered. Therefore, C is set to 0.10 to 0.30%.

Si: 0.05-2.0%, desirably 0.05-1.0%
Si is an element necessary for deoxidation. However, if Si 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 2.0%, preferably 0.05 to 1.0%.

Mn: 0.10 to 0.50%, desirably 0.20 to 0.50%
Mn is an element necessary for ensuring hardenability. However, if Mn is less than 0.10%, the effect of hardenability cannot be obtained sufficiently. Further, when Mn exceeds 0.50%, the crystal grains of the ferrite / pearlite structure appearing by normalization or annealing become smaller than 15 μm. Therefore, Mn is 0.10 to 0.50%, more preferably 0.20 to 0.50%.

P: 0.030% or less P is an unavoidable element contained in scrap, and segregates at the austenite grain boundary to lower 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. Therefore, S is set to 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. Moreover, when Cr is added excessively, not only the workability is impaired, but also the carburizing property is inhibited. Therefore, Cr is 1.80 to 3.0%.

Ni: Less than 0.25% Ni is an element that increases the hardenability and toughness of steel. However, when Ni is added in an amount of 0.25% or more, bainite that is disadvantageous to the grain size characteristics is precipitated in the present invention. Crystal grains become coarse as nuclei. For this reason, the toughness is greatly reduced when the toughness is reduced by the coarsening of the crystal grains rather than the effect of improving the toughness by adding the alloy. Therefore, in order to avoid the precipitation of bainite, Ni is set to an inevitable impurity amount level of less than 0.25%.

Mo: Less than 0.05% Mo is an element that increases the hardenability and toughness of steel. However, when Mo is added in an amount of 0.05% or more, bainite that is disadvantageous to the grain size characteristics is precipitated in the present invention. Crystal grains become coarse as nuclei. For this reason, the toughness is greatly reduced when the toughness is reduced by the coarsening of the crystal grains rather than the effect of improving the toughness by adding the alloy. Therefore, in order to avoid precipitation of bainite, Mo is set to an inevitable impurity amount level of less than 0.05%.

Al: 0.005 to 0.050%, desirably 0.015 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 brings about an effect of suppressing grain coarsening by a pinning effect. In order to obtain this effect, it is necessary to add 0.005% or more of Al. On the other hand, when the Al content exceeds 0.050%, the alumina-based oxide increases, and the fatigue characteristics and workability deteriorate. Therefore, Al is made 0.005 to 0.050%, preferably 0.015 to 0.050%.

N: 0.0300% or less, desirably N: 0.0250% or less, but in steel containing one or two of Ti and B, N: less than 0.0100% N is AlN or Nb nitride in steel Or it precipitates finely as Nb carbonitride and brings about the effect which prevents a crystal grain coarsening by the pinning effect. However, if it exceeds 0.030%, nitrides increase, and fatigue strength and workability decrease. Therefore, in the invention of claim steel 1, N is 0.0300% or less, preferably 0.0250% or less. However, in the steel containing Ti or B as in the means of claim 2, when N is contained in an amount of 0.0100% or more, TiN is excessively generated and the workability and fatigue strength are impaired. Moreover, although B has the effect of improving the hardenability of steel by being present as solid solution B, when N is contained in an amount of 0.0100% or more, BN of the compound is generated and solid solution B is reduced. The effect of improving hardenability by B is hindered. Therefore, when Ti and B of the invention of claim 2 are included, N is made less than 0.0100%.

Nb: 0.02 to 0.10%, desirably 0.02 to 0.08%
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. In particular, nano-order NbC or Nb (CN) finely dispersed in steel suppresses the growth of crystal grains. If Nb 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 deteriorates. Therefore, Nb is 0.02 to 0.10%, preferably 0.02 to 0.08%.

Ti: Less than 0.050% Ti can contribute to improving hardenability by fixing free-N in steel and suppressing B from becoming compound BN. In order to obtain the effect, it is necessary to add Ti less than 0.050%.

B: 0.0010 to 0.0050%
B is selectively contained as an element that remarkably improves the hardenability of the steel when contained in a very small amount. In order to obtain the effect, the effect of improving the hardenability is small if it is less than 0.0010%, while the strength is lowered if it exceeds 0.0050%. Therefore, B is 0.0010 to 0.0050%.

  In the steel of the invention according to the claims of the present application, the Mn content is as low as 0.10 to 0.50% as described above. Therefore, after rolling, by performing normal normalization treatment or annealing treatment, a steel with a large microstructure composed of ferrite and pearlite crystal grains is obtained, and this steel is subjected to cold forging or other cold work, Further, when carburizing treatment is performed, refinement of the austenite grain size at the initial stage of carburizing, which becomes a cause of grain coarsening during carburizing, can be suppressed. Therefore, as described above, the Mn content of the chemical component of this steel is 0.10 to 0.50%, preferably 0.20 to 0.50%.

Steel of the present invention is the temperature of the _three or more points A, for example, sufficient time to 925 ° C., after holding for example 1 hour, or air to carry out the normalizing, or in place of the normalizing, as the annealing conditions After heating to 880 to 930 ° C. and holding for a predetermined time, annealing is performed to obtain a ferrite and pearlite structure. Thus, in addition to the steel component of the present invention, by carrying out normalization or annealing under the above conditions, a microstructure with large crystal grains composed of homogeneous ferrite and pearlite can be obtained. As a result, after cold forging or cold working, when carburizing treatment is performed after cutting into parts with the required shape, minimization of the austenite grain size at the beginning of carburizing is suppressed. Thus, the crystal grains do not become coarse.

  In the above-mentioned means of the present invention, the cold forging or other cold work and the cutting as required are carried out following the normalization or annealing by limiting the steel components, normalizing or annealing heat treatment conditions. When the carburizing process is performed after processing into the shape of the steel, the pinning effect of the compound consisting of AlN, NbC or Nb (CN) has finer grain size characteristics than conventional steel, so use the cold work method Therefore, it is possible to reduce the manufacturing cost of drive system parts such as gears and shafts of automobiles, construction machines, machine tools, etc., and the present invention has an unprecedented excellent effect.

It is a figure which shows the heat processing conditions of this invention, (a) shows the temperature pattern of a normalizing process, (b) is a figure which shows the temperature pattern of a simulated carburizing process. (A) is a figure which shows the ferrite pearlite grain of the microstructure of the micrograph of the normalization process of invention steel, (b) is the ferrite pearlite grain of the microstructure of the micrograph of the normalization process of the comparative steel. FIG. (A) shows the microstructure of the inventive steel pseudo-carburized at 950 ° C., and (b) shows the microstructure of the comparative steel pseudo-carburized at 950 ° C. (A) shows the microstructure of the invention steel pseudo-carburized at 975 ° C., and (b) shows the microstructure of the comparative steel pseudo-carburized at 975 ° C. (A) shows carburizing and quenching conditions, and (b) shows s tempering conditions. It is a figure which shows the magnitude | size of a Charpy impact test piece, (a) is a front view, (b) is a side view.

  EMBODIMENT OF THE INVENTION The form for implementing this invention is demonstrated below with reference to a table | surface and drawing. First, the steel material which consists of steel which has a chemical component shown in Table 1 of this invention consists of a ferrite pearlite structure | tissue. This steel material is subjected to cold forging and other cold working to form a part. Thereafter, the shaped part is carburized to make it excellent in grain characteristics. In the steel material forming this part, the Mn of the chemical component of this steel is 0.50% or less by mass, so that the ferrite pearlite grains are larger than 15 μm, and the crystal grains are pinned. By using particles such as AlN and Nb (CN), the steel material made of this steel is subjected to cold forging and other cold work, and the parts made of this steel material that are subsequently carburized are excellent. It has crystal grain size characteristics.

  The invention steel and comparative steel of the present invention shown in Table 1 were melted in a vacuum induction melting furnace to form a 100 kg steel ingot. The ingot was heated at 1250 ° C. for 5 hours and then forged into a steel bar having a diameter of 65 mm. Next, according to the pattern shown in FIG. 1A, the steel bar was heated and held at 925 ° C. for 1 hour, that is, 3.6 ks, and then air-cooled and normalized. A specimen was prepared from this normalized steel bar, mirror-polished, corroded with 5% nital, and then the microstructure was observed with an optical microscope to measure the size of ferrite / pearlite grains in the field of view. .

  FIG. 2 shows representative micrographs obtained by normalizing the inventive steel and the comparative steel obtained by the above treatment. The normalized structure of the invention steel of FIG. 2A is ferrite pearlite larger than 15 μm. On the other hand, the normalized structure of the comparative steel shown in FIG. 2B is ferrite pearlite having a size of 15 μm or less.

  Next, a cylindrical test piece having a diameter of 14 mm and a length of 21 mm was produced by cutting from the vicinity of the middle periphery of the above-described normalized steel bar having a diameter of 30 mm. The length direction of the specimen was matched with the forging direction of the base material. Using a universal testing machine, the test piece was cold upset 70% in height ratio. In the present invention, the cold working rate is not particularly limited to 70%. By the way, the shape of the test piece and the cold upsetting method were performed in accordance with the standards of the cold forging subcommittee of the Japan Society for Technology of Plasticity in Non-Patent Document 2.

  Next, a pseudo carburizing test was performed in order to confirm the grain coarsening temperature during carburizing. This test is a conventional pseudo carburizing method that simulates only the temperature pattern of the carburizing process and observes the grain size without actually carburizing. Specimens obtained by cold-working each of an inventive steel having a Mn content of 0.50% or less by mass and a comparative steel having a Mn content exceeding 0.50% by mass. The test piece was divided into four parts, and the one piece was heated to 950 ° C. at 300 ° C./Hr, held at 1950 ks at 950 ° C., and then water-cooled, as shown in FIG. The pseudo carburizing process shown was performed. After this pseudo carburizing treatment, the cross section of the test piece was mirror-polished, and this was corroded with a saturated picric acid solution to reveal prior austenite grain boundaries. This test piece was observed with an optical microscope, and the structure of the crystal grains is shown in FIG. 3A shows the crystal grains of the inventive steel, and FIG. 3B shows the crystal grains of the comparative steel. The crystal grains of the invention steel shown in FIG. 3 (a) are coarsened even when subjected to pseudo carburizing at 950 ° C. due to AlN, NbC and Nb (CN) which are crystal pinning particles. First, the crystal grains were smaller than that of the comparative steel shown in FIG. 3B, and the inventive steel shown in FIG. 3A had excellent crystal grain characteristics.

  In addition, the conventional steel carburizing method is used in the same manner as described above, and the Mn content is less than 0.50% by mass and the Mn content exceeds 0.50% by mass. Each of the steels was cold upset to form a test piece, this test piece was divided into four pieces, and one piece was heated to 975 ° C. instead of the above 950 ° C. at 300 ° C./Hr, After maintaining at 975 ° C. for 10.8 ks, it was cooled with water, and a pseudo carburizing process shown in FIG. After this pseudo carburizing treatment, the cross section of the test piece was mirror-polished, and this was corroded with a saturated picric acid solution to reveal prior austenite grain boundaries. This test piece was observed with an optical microscope, and the structure of the crystal grain is shown in FIG. 4A shows the crystal grains of the inventive steel, and FIG. 4B shows the crystal grains of the comparative steel. The crystal grains of the invention steel shown in FIG. 4 (a) are coarsened even when subjected to pseudo carburizing at 975 ° C. due to AlN, NbC and Nb (CN) which are crystal pinning particles. First, the crystal grains were smaller than that of the comparative steel shown in FIG. 4B, and the inventive steel shown in FIG. 4A had excellent crystal grain characteristics.

  Table 2 shows the measurement results of the grain coarsening temperature due to the appearance of the prior austenite grain boundaries in the simulated carburization test. The ferrite grain size of the steel of the present invention is all 15 μm or more. On the other hand, the ferrite grain size of the comparative steel for the developed steel is all smaller than 15 μm. Furthermore, No. of comparative steel. 16 and no. No. 17 was added with Ni. 18-No. Since Mo is added to No. 20, as shown in Table 2, a bainite structure is precipitated. In addition, No. of this invention steel. The amount of Ni of 1 to 10 and No. of comparative steel. 11-15, no. 18 to 20 are inevitable impurity amounts. 1-10 and comparative steel o. 11 to 17 are inevitable impurity amounts. No. of the steel of the present invention. 1-4, no. 7, no. 8, 1No. 0 uses the crystal pinning particles AlN, NbC and Nb (CN) in addition to Mn of 0.5% or less, and has the best crystal grain size characteristics. Since other invention steels cannot use AlN as pinning particles, they are slightly inferior to the above-mentioned invention steels, but even in this case, since Mn is 0.5% or less, grain size characteristics superior to those of comparative steels are achieved. Have. No. of comparative steel. 12, no. 13, no. The grain coarsening temperature of 15 is higher than that of other comparative steels because Nb is added. However, since Mn exceeds 0.5%, the result is inferior to the steel of the present invention. It has become. Moreover, No. of comparative steel. 16-20, although Mn is 0.5% or less, the grain coarsening temperature does not reach the steel of the present invention due to precipitation of bainite by addition of Ni or Mo.

  In order to investigate the toughness of the steels of the present invention and the comparative steels having different grain size characteristics, rough processing (width 14 mm, height 12 mm, length 55 mm, 10 RC notches) was performed on the Charpy specimen, and FIG. Carburizing and quenching was performed under the conditions shown in Fig. 5, followed by tempering as shown in Fig. 5B. The carburizing conditions in FIG. 5 are general conditions that are usually performed, and are temperatures at which crystal grains become coarse in the comparative steel. Then, the Charpy test piece was finished as shown in FIGS. 6A and 6B, and the Charpy test was performed. The test was performed at room temperature. Table 3 shows the Charpy impact test results. The Charpy impact value of the steel of the present invention is superior to that of the comparative steel, and the newly developed steel having excellent crystal grain size characteristics has excellent toughness.

Claims (2)

In mass%, C: 0.10 to 0.30%, Si: 0.05 to 2.0%, Mn: 0.10 to 0.50%, P: 0.030% or less, S: 0.030 %: Cr: 1.80 to 3.00%, Al: 0.005 to 0.050%, Nb: 0.02 to 0.10%, N: 0.0300% or less, and the balance Fe and Ni: less than 0.25% Mo: a ing steel incidental impurities containing less than 0.05%, normalizing tissue or annealed structure is ferrite-pearlite structure, 15 [mu] m is the average value of the ferrite grain size A machine structural steel for carburized parts with excellent grain coarsening characteristics and toughness characterized by the above. In the chemical component according to claim 1, N: 0.0300% or less instead of N: 0.0300% or less, and in addition to the chemical component according to claim 1, , Ti: less than 0.050% B: containing 0.0010 to 0.0050%, balance Fe and Ni: less than 0.25% Mo: ing incidental impurities containing less than 0.05% steel A machine for carburized parts excellent in crystal grain coarsening characteristics and toughness, characterized in that the normalized structure or the annealed structure is a ferrite / pearlite structure and the average value of the ferrite grain size is 15 μm or more. Structural steel.