JP2010007120A - Method for manufacturing high-strength carburized component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To manufacture a carburized component wherein crystal grains are made fine to have excellent impact strength and bending strength that can not be expected until now. <P>SOLUTION: A steel for machine structure containing, by mass, 0.10-0.45% C, 0.05-2.00% Si, 0.10-2.00% Mn, ≤0.030% P, ≤0.20% S, 0.30-3.0% Cr, ≤0.30% Cu, 0.001-0.10% Al, 0.001-0.10% Al, 0.01-0.03% N and the balance Fe with inevitable impurities, is formed into the component-shape with a machining or a forging, and then a vacuum-carburization-quenching is performed and thereafter high-frequency induction hardening is applied one or more times, and then the component is tempered to manufacture the carbonized component, thus the carburized component excellent in the impact strength and the bending strength is manufactured. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  This invention manufactures carburized parts by carburizing and tempering from hardened steel, for example, carburizing from carburized steel of gears, automobiles, construction machines, machine tools, etc., CVJ and shafts. The present invention relates to a method for manufacturing formed carburized parts.

  In recent years, with higher output and smaller size and weight of automobiles, car parts that are carburized and tempered, such as gears, constant velocity joint parts, and shafts, are required to have higher strength and longer life. . Therefore, alloying elements such as Ni, Cr, and Mo are added to increase the strength as in nickel chromium molybdenum steel such as JIS standard SNCM. However, when alloying elements are added in this way to increase the strength, the material cost is high and cold workability is inferior, so cold forging cannot be performed. The cutting tool life is shortened. For this reason, these steel materials have a problem that requires heat treatment such as annealing.

  On the other hand, it is known that the strength of steel is improved by the refinement of crystal grains. This method by refining crystal grains can increase the strength of a steel material without adding an expensive alloy element. That is, it can be said that this is an extremely effective method because the workability such as forgeability and machinability of the material is not lowered and the strength can be increased without impairing the ductility and toughness.

  As a method for refining crystal grains, there is a method based on thermomechanical processing. However, in this case, since the forming process and the heat treatment are combined, it cannot be applied to a shape that is difficult to form. For this reason, there is a problem that the shape of the parts is limited and it is difficult to apply to automobile gears, CVJs, shafts, and the like.

  In order to solve these problems, a high strength hardened steel that can actively refine the prior austenite grains and improve the strength by repeatedly quenching after carburizing has been proposed (for example, (See Patent Documents 1 and 2).

  However, in the method of Patent Document 1, the case-hardened steel before carburizing is No. defined in JIS G0551. There is a problem that the martensite structure must be refined to 11 or more and the cost is high.

  In the methods of Patent Documents 1 and 2, the prior austenite grain size is refined by repeatedly quenching after carburizing and quenching. However, these methods have a problem that the strength cannot be improved sufficiently.

JP 2003-34843 A JP-A-8-92690

  As a result of investigating a method for further strengthening the steel material with respect to the technologies of the conventional patent documents 1 and 2, even if the crystal grain size is reduced in the conventional gas carburized steel, the strength is saturated at a certain grain size. The inventor has found out. This is because an abnormal carburization layer or grain boundary oxide layer is formed on the part surface during gas carburizing, and even if the prior austenite grain size is refined beyond that depth, the carburization abnormal layer and grain boundary oxide layer act as initial defects. It was estimated that the refinement effect of the prior austenite grains disappeared. Therefore, when the carburizing abnormal layer was eliminated by vacuum carburizing, no strength saturation phenomenon was observed, and it was found that the strength improved as the crystal grain size decreased.

  Therefore, the problem to be solved by the present invention is that the carburized steel has been made ultrafine grain compared to the conventional case and the carburizing abnormal layer acting as a surface defect is prevented by performing vacuum carburizing. It is to provide a method for producing a carburized part having a superior impact strength and bending strength than before by synergistic action.

  The means of the present invention for solving the above-mentioned problems is that, in the invention of claim 1, after mechanical structural steel is formed into a part shape by machining or forging, vacuum carburizing and quenching is performed, and then one or more times. This is a method for manufacturing a carburized part excellent in impact strength and bending strength, which comprises manufacturing a carburized part by performing induction hardening and then tempering it.

  In the invention of claim 2, the mechanical structural steel forming the tempered carburized parts is in mass%, C: 0.10 to 0.45%, Si: 0.05 to 2.00%, Mn: 0.00. 10-2.00%, P: 0.030% or less, S: 0.20% or less, Cr: 0.30-3.0%, Cu: 0.30% or less, Al: 0.001-0. The production of a carburized part having excellent impact strength and bending strength according to claim 1, characterized in that the steel contains 10%, N: 0.01 to 0.03%, and the balance is Fe and inevitable impurities. Is the method.

  In the invention of claim 3, the steel for machine structural forming the tempered carburized part is in addition to the steel component of claim 2 and further in mass%, Ni: 0.20-5.0%, Mo: 0.05 The production of a carburized part excellent in impact strength and bending strength according to claim 1, characterized in that it is a steel containing any one or two of ˜3.0% and the balance being Fe and inevitable impurities. Is the method.

  In the invention of claim 4, the steel for machine structure forming the tempered carburized part is Ti: 0.01 to 0.50%, V: 0.02 in addition to the steel components of claim 2 and in mass%. -0.50%, Nb: 0.02-0.50%, B: 0.0010-0.0050% of any one or two or more types of steel, the balance Fe and unavoidable impurities It is a manufacturing method of the carburized components excellent in impact strength and bending strength as described in any one of Claims 1-3 characterized by the above-mentioned. However, when Ti or B is added, 3.4 N [%] <Ti [%] is satisfied.

  In the invention of claim 5, the steel for machine structural forming the tempered carburized part is Ti: 0.01 to 0.50%, V: 0.02 in addition to the steel components of claim 3 and in mass%. -0.50%, Nb: 0.02-0.50%, B: 0.0010-0.0050% of any one or two or more types of steel, the balance Fe and unavoidable impurities It is a manufacturing method of the carburized components excellent in impact strength and bending strength as described in any one of Claims 1-3 characterized by the above-mentioned. However, when Ti or B is added, 3.4 N [%] <Ti [%] is satisfied.

  The reason why the mechanical structural steel components are limited by the methods of claims 2 to 5 in the above method will be described. In addition,% shows the mass%.

C: 0.10 to 0.45%, desirably C: 0.10 to 0.25%
C is an element necessary for securing the core strength after carburizing as a machine structural component. If C is less than 0.10%, the effect is not sufficiently obtained, and if it exceeds 0.45%, workability is lowered and toughness is lowered. Therefore, C is 0.10 to 0.45%, preferably 0.10 to 0.25%.

Si: 0.05-2.00%
Si is an element necessary for deoxidation. If it is less than 0.05%, sufficient deoxidation cannot be obtained, and if it exceeds 2.00%, workability is lowered. Therefore, Si is set to 0.05 to 2.00%.

Mn: 0.10 to 2.00%
Mn is an element necessary for ensuring hardenability, but if it is less than 0.10%, the effect cannot be sufficiently obtained, and if it exceeds 2.00%, workability is lowered. Therefore, Mn is set to 0.10 to 2.00%.

P: 0.030% or less P is an unavoidable element contained in scrap, but segregates at the austenite grain boundaries to reduce toughness such as impact strength and bending strength, so the upper limit of content is 0.030%. And

S: 0.20% or less S is an element that improves machinability, but produces MnS, which is a non-metallic inclusion, and lowers the toughness and fatigue strength in the transverse direction. Therefore, S is set to 0.20% or less. S may be omitted, but when machinability is required, S is added in a range of 0.001 to 0.20%.

Ni: 0.20 to 5.0%
Ni is an element that improves hardenability and toughness, but if it is less than 0.20%, the effect is not sufficient, and if it exceeds 5.0%, the workability is remarkably lowered and the cost is increased. . Therefore, Ni is set to 0.20 to 5.0%.

Cr: 0.30 to 3.0%
Cr is an element that improves hardenability and carburization, but if it is less than 0.30%, its effect is not sufficient, and if it exceeds 3.0%, the workability decreases. Therefore, Cr is set to 0.30 to 3.0%.

Mo: 0.05-3.0%
Mo is an element that improves hardenability and toughness, but if it is less than 0.05%, its effect is not sufficient, and if it exceeds 3.0%, workability is lowered. Therefore, Mo is set to 0.05 to 3.0%.

Cu: 0.30% or less Cu is an unavoidable element contained in scrap, and has aging properties and increases strength. However, when it exceeds 0.30%, hot workability decreases. Therefore, Cu is made 0.30% or less.

Al: 0.001 to 0.10%, desirably 0.02 to 0.05%
Al is an element used as a deoxidizing material, and if it is less than 0.02%, the deoxidation effect is insufficient, and if it exceeds 0.05%, alumina-based oxides increase and fatigue characteristics and workability deteriorate. . Therefore, Al is 0.001 to 0.10%, preferably 0.02 to 0.05%.

Ti: 0.05 to 0.50%, desirably Ti: 0.10 to 0.20%
Ti fixes free-N in steel, improves the hardenability effect of B, and finely precipitates Ti carbide, composite carbide containing Ti, and Ti nitride, thereby austenite crystals during carburizing It is an element necessary for suppressing the coarsening of the particle size. In particular, nano-order TiC finely dispersed in steel suppresses the growth of crystal grains. If Ti is less than 0.05%, the effect of preventing coarsening of crystal grains is not sufficient, and 0.1% or more is desirable. However, if it exceeds 0.5%, the amount of precipitate becomes excessive and the workability is lowered. Therefore, Ti is made 0.05 to 0.50%, preferably 0.10 to 0.20%.

N: 0.01 to 0.05%, desirably 0.01 to 0.03%
N is an element which forms Al, Nb, V and nitrides and carbonitrides, and has an effect of preventing grain coarsening. However, when N is less than 0.01%, the effect of crystal grain refinement is small. If it exceeds 0.05%, nitrides increase and fatigue strength and workability deteriorate. Therefore, N is set to 0.01 to 0.05%, preferably 0.01 to 0.03%. However, in the steel material having B, when free-N is present in the steel, BN is generated, solute B is reduced, and the effect of improving the hardenability and the strength of carburized parts is hindered. Therefore, before adding B, N must be TiN and fixed. Therefore, it is necessary to satisfy 3.4N [%] <Ti [%].

B: 0.0010 to 0.0050%
B is selectively contained as an element that remarkably improves the hardenability of the steel by containing a minimum amount and improves the strength of the carburized part. However, if it is less than 0.0010%, the effect of improving hardenability and strength is small, and if it exceeds 0.0050%, the strength is lowered. Therefore, B is 0.0010 to 0.0050%.

V: 0.02-0.50%
V forms carbides or carbonitrides, and has the effect of suppressing coarsening of the austenite crystal grain size, similar to Ti. If V is less than 0.02%, the effect cannot be obtained sufficiently, and if it exceeds 0.50%, the amount of precipitates becomes excessive and the workability deteriorates. Therefore, V is 0.02 to 0.50%.

Nb: 0.02 to 0.50%
Nb forms carbides or carbonitrides, and has the effect of suppressing coarsening of the austenite grain size, similar to Ti. If Nb is less than 0.02%, the effect cannot be sufficiently obtained, and if it exceeds 0.50%, the amount of precipitates becomes excessive and workability is lowered. Therefore, Nb is made 0.02 to 0.50%.

  The reason for limiting the process of the combination of crystal grain refinement and vacuum carburization by repeated induction hardening will be described.

About Repeated Induction Hardening The point of repeated induction hardening in the process of the method of the present invention will be described. In the present invention, a high-frequency repetitive quenching method is used as a method for refining crystal grains. However, the effect of the two-time quenching is greater than the one-time quenching. However, depending on the type of steel, there is a problem that, when the quenching is repeated three times or more, mixed grains are generated and the strength is lowered.

Necessity of the combination of grain refinement and vacuum carburizing When performing gas carburizing treatment, oxygen contained in the atmosphere enters from the surface of the steel material and combines with Si, Mn, Cr near the grain boundary to form oxides. Form. In the vicinity where these solid solution alloys are reduced, the hardenability is lowered, martensite is not generated during quenching, and troostite and bainite are generated. In particular, oxygen easily enters along the grain boundaries, and an abnormal carburization layer is generated along the grain boundaries. This carburized abnormal layer is particularly called a grain boundary oxide layer. When a grain boundary oxide layer is generated on the steel material surface, the grain boundary oxide layer acts as a defect. Therefore, the strength decreases as the depth of the grain boundary oxide layer increases.

By the way, when the crystal grain size of the gas carburized material is reduced, the strength is improved as the crystal grain size is reduced to a certain extent. do not do. It is estimated that this is because the grain boundary oxide layer has an influence. That is, when the crystal grain size is larger than the grain boundary oxide layer, the strength improves as the crystal grain size becomes smaller. However, when the crystal grain size becomes smaller than the grain boundary oxide layer, the grain boundary oxide layer becomes a defect. It becomes large and it is thought that the effect of crystal grain refinement cannot be obtained. Therefore, it is essential to reduce or prevent the grain boundary oxide layer in order to maximize the effect of crystal grain refinement. On the other hand, a method for improving the strength by reducing the grain boundary oxide layer is also known, but even if this method is used, if the crystal grains are large, the effect of reducing the grain boundary oxide layer cannot be sufficiently obtained, and the strength is not greatly improved. . In this way, either the grain refinement or the reduction of the grain boundary oxide layer has a small strength improvement effect. To increase the strength improvement effect, "crystal grain refinement" and "grain boundary oxide layer reduction The combination of both “prevention” and “prevention” is considered necessary.
In addition, vacuum carburization is a method in which a hydrocarbon-based gas is supplied into a furnace heated to a high temperature under reduced pressure and the workpiece inserted into the furnace is carburized, so that an abnormal carburizing layer such as grain boundary oxidation is not generated. There are features. That is, the impact strength and the bending strength can be greatly improved by not generating the grain boundary oxide layer by vacuum carburization and then refining the crystal grains by repeated quenching.

  The present invention prevents a carburizing abnormal layer such as grain boundary oxidation by vacuum carburizing and quenching, and after performing induction hardening at least once after vacuum carburizing and quenching, the crystal grains are refined by tempering this. With both of these measures, high-strength carburized parts made of carburized steel for machine parts such as gears and shafts of automobiles, construction machines, machine tools, etc. can be manufactured at low cost without reducing workability compared to conventional steel materials. The present invention, such as making it possible to produce, has an excellent effect that has not been achieved in the past.

  The best mode for carrying out the present invention will be described with reference to tables and drawings. First, the No. of the embodiment of the present invention shown in Table 1 is shown. Each steel containing 1 to 17 chemical components was melted in a 100 kg vacuum induction melting furnace and cast into an ingot. In these steels, in order to once precipitate Al, Nb, V, Ti precipitates and then finely precipitate them by heat treatment, this ingot is heated to 1250 ° C. and held for 5 hours for solution treatment. Thus, a steel material on which precipitates were finely precipitated was obtained.

  The solution-treated steel material was forged into a 40 mm square material. This material was heated to 900 ° C., held for 1 hour, and then air cooled to normalize. Further, a Charpy impact test piece 1 having a 2 mm 10 RC notch 2 shown in FIG. 1 and a static bending test piece 3 having a 2 mm V notch 4 shown in FIG. 2 were produced from this material. As shown in FIG. 3, each of these test pieces was heated to 950 ° C., preheated for 0.5 hours, vacuum carburized for 1 hour, held for 2.5 hours and diffused, and lowered to 850 ° C. to 0.5 Hold for a period of time, then quench into oil at 20 ° C. and temper to 180 ° C. Further, in order to compare and confirm the effect of vacuum carburizing, as shown in FIG. 4, it is heated to 930 ° C., preheated for 0.5 hours, gas carburized for 3 hours, held for 2.5 hours and diffused, 830 ° C. A test piece was also produced which was held for 0.5 hour and then gas-carburized and quenched to 60 ° C. oil at 180 ° C. under quenching conditions. Further, in the case of repeated quenching, after each carburizing and quenching by vacuum carburizing or gas carburizing, one or more induction quenching was performed without performing the above tempering, and then tempering to 180 ° C. These are shown in Table 2 as “gas carburized as-quenched”, “vacuum carburized as-quenched”, “induction-quenched once” or “induction-quenched twice”. In this way, three types of quenching and tempering were carried out for each carburizing method. The induction hardening was performed by water cooling at a maximum reaching point temperature of 850 to 950 ° C. and a heating time of 2 seconds.

  As described above, by changing the quenching and tempering conditions into three types for each carburizing method, specimens with different crystal grains were prepared, and the impact strength and static bending strength, and the crystal grain size effected on them were determined. The impact was investigated.

  The Charpy impact test piece produced as described above was subjected to an impact test using a Charpy impact tester, and the impact value was evaluated by the crack generation energy. This evaluation was made based on the average grain size of the carburized layer surface of the Charpy impact test piece. Together with the diameters, the impact test results are shown in Table 2. In Table 2, the average grain size of the carburized layer surface of the Charpy impact test piece is shown in μm, and the impact value is No. of the comparative example. The crack initiation energy of the gas carburized quenching / tempering test piece No. 1 was set to 1.0, and the crack initiation energy was compared with this value as a reference. The impact test was performed at room temperature.

  As shown in Table 2, when the steels of the examples were subjected to induction hardening after carburizing and quenching, all of the steels had a smaller prior austenite grain size, and the number of induction hardenings was smaller than twice than once.

  In the steels of the examples, both the gas carburized steel and the vacuum carburized steel are repeatedly subjected to induction hardening after carburizing and quenching, whereby the prior austenite grain size is reduced. However, the impact strength is not greatly improved even when the gas austenite particle size is reduced in the case of gas carburization, whereas the impact strength is greatly improved due to the refinement of the particle size of the prior austenite in the case of vacuum carburization. As described above, the impact value was significantly improved by performing induction hardening at least once after vacuum carburizing and quenching using the steel of the example.

  Further, as shown in FIG. 5, the static bending test piece 3 subjected to the heat treatment as described above is subjected to three-point bending with a fulcrum distance of 50 mm, and the central crosshead is moved at a moving speed of 2 mm / min, that is, the static bending test piece. The both ends of 3 were supported from below, and the center 5 was pushed by applying a load in the downward arrow direction, and a static bending test was performed. By this test, the static bending strength was evaluated using the load at the time when a crack was generated on the surface layer of the static bending test piece 3 as a crack generation load. The result of a bending test is shown. In Table 3, No. of the comparative example. The crack initiation load of 1 as the gas carburizing and quenching was set to 1.0, and each crack initiation load was shown as a value compared with this value. This static bending test was performed at room temperature.

  As shown in Table 3, in the steel of the example, the prior austenite grains were reduced by induction hardening after carburizing and quenching, as in the case of the impact test piece, and the number of induction hardening was smaller twice than once. In the steels of the examples, both the gas carburized steel and the vacuum carburized steel are repeatedly subjected to induction hardening after carburizing and quenching, whereby the prior austenite grain size is reduced. However, the gas-carburized one does not significantly improve the static bending strength even when the prior austenite grain size decreases, whereas the one that is vacuum carburized greatly improves the static bending strength due to the refinement of the former austenite grain size. did. As described above, the static bending strength was greatly improved by performing induction hardening at least once after vacuum carburizing and quenching using the steel of the example.

  As described above, carburized parts excellent in fine impact strength and static bending strength of crystal grains could be manufactured by performing induction hardening at least once after vacuum carburizing and quenching by the method of the present invention. .

It is a figure which shows the shape and magnitude | size of a Charpy impact test piece. It is a figure which shows the shape and magnitude | size of a static bending test piece. It is a figure which shows vacuum carburizing quenching tempering conditions. . It is a figure which shows gas carburizing quenching tempering conditions. It is a figure which shows the static bending test method to a test piece.

Explanation of symbols

1 Charpy impact test piece 2 10R2mmC notch 3 Static bending test piece 4 2mmV notch 5 Center

Claims (5)

  Machine structural steel is formed into parts by machining or forging, vacuum carburizing and quenching is performed, and then induction hardening is performed one or more times, followed by tempering to manufacture carburized parts. A method for producing a carburized part having excellent impact strength and bending strength.   Machine structural steel forming tempered carburized parts is in mass%, C: 0.10 to 0.45%, Si: 0.05 to 2.00%, Mn: 0.10 to 2.00%, P: 0.030% or less, S: 0.20% or less, Cr: 0.30 to 3.0%, Cu: 0.30% or less, Al: 0.001 to 0.10%, N: 0.00. A method for producing a carburized part excellent in impact strength and bending strength, characterized by being a steel containing 01 to 0.03% and the balance being Fe and inevitable impurities.   In addition to the steel components of claim 2, the steel for machine structural forming the tempered carburized part is further mass%, Ni: 0.20-5.0%, Mo: 0.05-3.0% The production of a carburized part excellent in impact strength and bending strength according to any one of claims 1 to 3, characterized in that it is a steel containing one or two kinds and the balance being Fe and inevitable impurities. Method.   In addition to the steel components of claim 2, the steel for mechanical structure forming the tempered carburized part is further in mass%, Ti: 0.05 to 0.50%, V: 0.02 to 0.50%, Nb : A steel comprising 0.02 to 0.50%, B: 0.0010 to 0.0050%, one or more, and the balance being Fe and inevitable impurities. The manufacturing method of the carburized component excellent in impact strength and bending strength as described in any one of 1-3. However, when Ti or B is added, 3.4 N [%] <Ti [%] is satisfied.   In addition to the steel components of claim 3, the steel for mechanical structure forming the tempered carburized part is further in mass%, Ti: 0.05 to 0.50%, V: 0.02 to 0.50%, Nb : A steel comprising 0.02 to 0.50%, B: 0.0010 to 0.0050%, one or more, and the balance being Fe and inevitable impurities. The manufacturing method of the carburized component excellent in impact strength and bending strength as described in any one of 1-3. However, when Ti or B is added, 3.4 N [%] <Ti [%] is satisfied.

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