JP4396561B2 - Induction hardening steel - Google Patents

Description

  The present invention relates to induction hardening steel. Specifically, the present invention is suitable for use as, for example, a crankshaft of a reciprocating engine, because delayed fracture is unlikely to occur even after induction forging and machining after being melted into a raw material, even after induction hardening. The present invention relates to a steel for induction hardening excellent in delayed fracture resistance.

  The crankshaft of a reciprocating engine such as a gasoline engine or diesel engine is a component for extracting the engine output as a continuous rotational force by converting the downward movement of the piston in the combustion process transmitted by the connecting rod into a rotational motion It is. Since this crankshaft rotates at a high speed while receiving a large load, high strength and rigidity are required. For this reason, the crankshaft is generally subjected to induction hardening and tempering after material forging made of high carbon steel, chrome molybdenum steel, nickel chrome steel, etc., and then machining such as grinding. Has been manufactured. In recent years, the crankshaft has been required to further improve the fatigue strength in order to cope with the higher speed of the engine. For this reason, the tempering temperature has been reduced or omitted.

  However, if the tempering is simply performed at a low temperature or omitted, there is a possibility that a phenomenon that abrupt destruction occurs, that is, a delayed fracture (time delayed fracture) occurs after a certain time has elapsed since the load is applied due to residual stress or the like.

  This delayed fracture broadly means embrittlement or fracture of steel by hydrogen. Static time-delayed fracture in a corrosive environment is usually called stress corrosion cracking, which is an anodic reaction type crack that dissolves iron and absorbs hydrogen and cracks along grain boundaries. There is a cathodic reaction-type crack formation.

  On the other hand, the delayed fracture in a narrow sense means a fracture that occurs in an atmospheric environment in a quenched steel material that has been induction-hardened. That is, the crankshaft manufactured by induction hardening or lowering the temperature of the tempering is expanded or the retained austenite continuously expands to martensite, and the residual compressive stress on the surface increases to increase the internal tensile stress. To increase its distribution. For this reason, in this crankshaft, concentration of hydrogen is promoted in a minute defect portion existing in the crankshaft and becomes brittle, so that delayed fracture is likely to occur.

  In Patent Document 1, C: 0.40 to 0.70% (in this specification, “%” means “mass%” unless otherwise specified), Si: 0.05 to 0.00. 8%, Mn: 0.5 to 2.0%, S: 0.01 to 0.15%, P: 0.01 to 0.07%, Cr: 0.1 to 1.5%, Ti: 0 0.005-0.05%, Nb: 0.005-0.05%, B: 0.0005-0.005%, Al: 0.005-0.05%, N: 0.005-0.02 An invention is disclosed in which a ferrite-pearlite structure is obtained by hot-forging a steel having a steel composition containing% after air forging or cooling.

Japanese Patent Laid-Open No. 9-78183

  However, as a result of intensive studies by the present inventors, the invention relating to the high toughness hot non-tempered steel for induction hardening described in Patent Document 1 is C: 0.40 to 0.70%, B: It has a steel composition of 0.0005 to 0.005%, P: 0.01 to 0.07%, N: 0.005 to 0.020%, and because of its high C content, it has delayed fracture susceptibility. When the P content and the N content are high, the P amount at the grain boundary becomes excessive with respect to the solid solution B amount in the steel, and the precipitation of BN is promoted at the grain boundary. The toughness of the steel is reduced, and delayed fracture is likely to occur. For this reason, even with this invention, it is not possible to provide a steel for induction hardening that can be suitably used, for example, for a crankshaft of a reciprocating engine by sufficiently suppressing the occurrence of delayed fracture even with induction hardening.

  The object of the present invention is that it is difficult to cause delayed fracture even after induction hardening after being machined into a product shape. Therefore, for example, induction hardening excellent in delayed fracture resistance that can be suitably used for a crankshaft of a reciprocating engine. Is to provide steel.

  The present invention sets the C content as low as less than 0.40% in order to prevent delayed fracture even when induction hardening is performed, and optimizes the P, N, and B contents in order to ensure induction hardening performance. In addition, it is possible to prevent delayed fracture even with induction hardening and to ensure induction hardening performance, thereby making it possible to improve both fatigue strength and productivity. It is.

The present invention is by mass%, C: 0.20 to 0.38% , Si: 0.35 to 2.0%, Mn: 0.05 to 2.0%, P: 0.030% or less, S : 0.0
4 to 0.1%, Cr: 0.05 to 2.0%, N: 0.010% or less, B: 0.0003 to 0.0030%, Al: 0.001 to 0.05% Solid solution B: (0.0061 × P-0.00001)% or more, steel for induction hardening excellent in delayed fracture resistance , characterized by having a steel composition composed of the balance Fe and inevitable impurities.

  In the steel for induction hardening according to the present invention, Ti: 0.01 to 0.15%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.10%, and V: 0.00. It is desirable to contain one or more selected from the group consisting of 01 to 0.20%.

In the steel for induction hardening according to the present invention, it is desirable to further contain Ca: 0.0003% or more and 0.020% or less.
These induction hardening steels according to the present invention are preferably used for a crankshaft of a reciprocating engine.

  According to the present invention, delayed fracture is less likely to occur even after induction hardening after being machined into a product shape. Therefore, for example, steel for induction hardening excellent in delayed fracture resistance that can be suitably used as a crankshaft for a reciprocating engine. Can be provided. For this reason, for example, the fatigue strength of the crankshaft can be improved without decreasing the production efficiency by improving the delayed fracture resistance.

Hereinafter, the best mode for carrying out the induction hardening steel according to the present invention will be described in detail. First, the reason for limiting the composition of the steel for induction hardening according to the present invention will be described.
C: 0.20% or more and less than 0.40% C increases the susceptibility to fracture through the morphological effect of carbide that increases strength and hardness and decreases toughness as the content increases in steel. . Since the hardness after induction hardening increases in proportion to the C content, delayed fracture is likely to occur, and the occurrence frequency of delayed fracture increases at 0.40% or more. On the other hand, if the C content is less than 0.20%, the desired mechanical properties cannot be ensured, for example, the surface hardness after induction hardening decreases. Therefore, in the present embodiment, the C content is limited to 0.20% or more and less than 0.40%. From the same viewpoint, the upper limit of the C content is desirably 0.35%, and the lower limit is desirably 0.25%.

Si: 0.35% or more and 2.0% or less Si contains 0.35% or more, so that it acts on steel as a deoxidizer during melting and contributes to improvement in core hardness as a strengthening element. . However, if the Si content exceeds 2.0%, not only the toughness of the core is deteriorated, but also the machinability and cold forgeability are adversely affected and the productivity is lowered. Therefore, in the present invention, the Si content is limited to 0.35% or more and 2.0% or less. From the same viewpoint, the lower limit of the Si content is desirably 0.5%, and the upper limit is desirably 1.5%.

Mn: 0.05% or more and 2.0% or less By containing 0.05% or more, Mn increases strength and toughness, improves hardenability and raises the eutectoid concentration of C. Suppresses precipitation. However, if the content exceeds 2.0%, the formation of a bainite structure is caused and the wear resistance and machinability are remarkably lowered. Therefore, in the present invention, the Mn content is limited to 0.05% or more and 2.0% or less. From the same viewpoint, the lower limit of the Mn content is desirably 0.5%, and the upper limit is desirably 1.5%.

P: 0.030% or less P is an unavoidable impurity in the present invention. When the P content exceeds 0.030%, grain boundary segregation occurs at the austenite grain boundary, and the grain boundary strength is lowered and delayed fracture occurs. It tends to occur. Therefore, in the present invention, the P content is limited to 0.030% or less. From the same viewpoint, the upper limit of the P content is preferably 0.025%.

S: 0.005% or more and 0.1% or less S increases in content and precipitates as a sulfide to improve machinability. However, if the S content is less than 0.005%, the effect is not sufficient. If the S content exceeds 0.1%, the number of sulfides in the steel increases, the frequency of occurrence of grinding cracks increases, and S increases. As a result, hot workability decreases. Therefore, in the present invention, the S content is limited to 0.005% or more and 0.1% or less. From the same viewpoint, the S content is preferably 0.04% or more and 0.1% or less, and more preferably 0.04% or more and 0.06% or less.

Cr: 0.05% or more and 2.0% or less Cr is contained in an amount of 0.05% or more, thereby reducing the ferrite ratio and lowering the Ac ₁ transformation point and Ac ₃ transformation point, as well as the Ac ₁ transformation point and Ac. By narrowing the interval between the _three transformation points and eliminating bainite precipitation at the martensite grain boundaries and residual undissolved ferrite, uniform martensite can be obtained. However, if the content exceeds 2.0%, a bainite structure is formed, and the wear resistance and machinability are significantly reduced. Therefore, in the present invention, the Cr content is limited to 0.05% to 2.0%. From the same viewpoint, the lower limit of the Cr content is desirably 0.1%, and the upper limit is desirably 1.5%.

N: 0.010% or less under <br/> N is an important role element in the present invention. If the N content is 0.010% or less, the amount of solid solution N at the grain boundary decreases, the formation of BN is prevented, the amount of solid solution B increases, the grain boundary strength is improved, and the occurrence of delayed fracture can be prevented. Therefore, in the present invention, the N content is limited to 0.010% or less. From the same viewpoint, the upper limit of the N content is preferably 0.008%.

B: 0.0003% to 0.0030% B is an element that plays an important role in the present invention. When the B content is 0.0003% or more, segregation occurs in the austenite grain boundaries in a solid solution state, and embrittlement elements such as P and Cu are excluded from the grain boundaries to increase the strength of the grain boundaries. However, if the B content exceeds 0.0030%, the grain boundary strength decreases conversely. Therefore, in the present invention, the B content is limited to 0.0003% or more and 0.0030% or less. From the same viewpoint, the lower limit of the B content is preferably 0.0005%, and the upper limit is preferably 0.0030%.

Al: 0.001% or more and 0.05% or less Al has a deoxidizing action and a grain refining action, and combined with N in the steel to become AlN, completely fixing the solid solution N, and BN precipitation To prevent. However, when the Al content exceeds 0.05%, such an effect is saturated, and rather the toughness is lowered. Therefore, in the present invention, the Al content is limited to 0.001% or more and 0.05% or less. From the same viewpoint, the lower limit of the Al content is desirably 0.01%.

Solid solution B: (0.0061 × P−0.00001)% or more B has the effect of causing segregation at the austenite grain boundary and removing embrittlement elements such as P and Cu from the grain boundary to increase the grain boundary strength. However, the amount of solid solution B in the solid solution state necessary for suppressing the occurrence of delayed fracture varies depending on the amount of P added. At the time of melting, the amount of solid solution B segregated at the grain boundaries can be adjusted by adjusting the Al content and the addition amount of one or more elements of Ti, Nb, Zr or V.

  Solid solution B was determined by measuring the total amount of B, extracting nitride by electrolytic extraction, and determining the difference between the two. If the solid solution B is (0.0061 × P−0.00001)% or more from the relationship with P, delayed fracture does not occur. Therefore, in the present invention, the solid solution B is limited to (0.0061 × P−0.00001)% or more. From the same viewpoint, the lower limit of the solid solution B is preferably 0.0002% or more.

Ti: 0.01% to 0.15%, Nb: 0.01% to 0.10%, Zr: 0.01% to 0.10%, and V: 0.01% to 0.1%. In the present invention, one, two or more selected from the group consisting of 20% or less , any of Ti, Nb, Zr or V is an optional additive element added as necessary. By adding one of these elements alone or in combination of two or more, solid solution N is completely fixed by bonding with N in the steel and precipitating carbonitride, thereby preventing precipitation of BN. Can be achieved.

  Therefore, when these elements are added, the necessary amount for forming carbonitride in steel may be added, Ti: 0.01% or more and 0.15% or less, V: 0.01% or more It is limited to 0.20% or less, Nb: 0.01% or more and 0.10% or less, Zr: 0.01% or more and 0.10% or less.

Ca: 0.0003% or more and 0.020% or less In the present invention, Ca is an optional additive element added as necessary. Ca improves the machinability by containing 0.0003% or more. However, even if the content exceeds 0.020%, such an effect is saturated and the cost is increased.
Therefore, when Ca is added, its content is desirably limited to 0.0003% or more and 0.020% or less.

The balance Fe and inevitable impurities Other compositions than the above are Fe and inevitable impurities. Inevitable impurities include Ni, Mo, and Cu in addition to P described above. Since Ni and Mo have no particular effect on delayed fracture, it is desirable that the Ni content is 0.30% or less and the Mo content is 0.15% or less. Also, Cu, like P, lowers the grain boundary strength and easily causes delayed fracture, so the Cu content is desirably 0.30% or less.

  The steel for induction hardening according to the present embodiment has such a steel composition. In particular, the C content is set to be as low as 0.20% or more and less than 0.40% and the P content is set to 0.030% or less. By restricting and limiting the solid solution B ≧ (0.0061 × P−0.00001)%, the austenite is stabilized and the transformation to martensite is suppressed and the grain boundary is strengthened by the solid solution B. The segregation of P to the grain boundary is prevented, whereby delayed fracture does not occur even with induction hardening, and induction hardening performance can be secured.

For this reason, according to the present invention, both fatigue resistance and productivity can be improved.
As described above, according to the present invention, delayed fracture is less likely to occur even after induction hardening after machining into a product shape, and thus, for example, excellent delayed fracture resistance that can be suitably used for a crankshaft of a reciprocating engine. It can provide steel for induction hardening.

  A steel material having a chemical composition shown in Table 1 was melted in a vacuum induction heating furnace having a capacity of 50 kg to form an ingot having a diameter of 210 mm. The ingot was heated to 1250 ° C. by a normal method and then hot forged to forge into a 65 mm round bar. The forging finishing temperature was 1000 ° C., and after hot forging, the product was allowed to cool to room temperature.

  After being allowed to cool to the atmosphere, the surface cut perpendicular to the surface of the test piece was polished to be a test surface, the test surface was corroded by an appropriate corrosion method, and the structure was confirmed with a microscope. All the samples had a ferrite-pearlite structure.

  Then, this round bar is turned by a normal method to produce a pipe having an outer diameter of 60 mm, an inner diameter of 50 mm, and a height of 80 mm, and the total thickness of 50 mm width becomes HV 450 at the center in the height direction. Was subjected to induction hardening. HV was measured under the condition of a test force of 49.03N.

  Then, for the evaluation of delayed fracture, an accelerated test for delayed fracture was conducted by immersing in 4.1% hydrochloric acid for 10 minutes to confirm the presence or absence of cracks. The results are shown in Table 1. In the column of “4.1% HCl (10 minutes) cracking” in Table 1, “O” indicates no cracking, and “X” indicates cracking.

  Steel No. 1 in Table 1 1 to 14 do not satisfy the solid solution B ≧ (0.0061 × P−0.00001)%, or do not satisfy the other conditions defined in the present invention, so that both delayed fracture resistance and hardness are compatible. I couldn't.

In contrast, steel No. 1 in Table 1 was used. Since all of Nos. 15 to 23 satisfy all the conditions defined in the present invention, both delayed fracture resistance and hardness could be achieved.

  Furthermore, the steel No. 1 in Table 1 was used. 2 satisfies the solid solution B ≧ (0.0061 × P−0.00001)%, but the C content exceeds 0.45% and the upper limit of the present invention. The transformation could not be suppressed, and the grain boundaries could not be strengthened by the solid solution B, and delayed fracture occurred. Steel No. 14 satisfies the solid solution B ≧ (0.0061 × P−0.00001)%, but the P content exceeds 0.035%, which exceeds the upper limit of the present invention. A large amount of element P was segregated at the grain boundaries, and delayed fracture occurred.

  FIG. 1 is a graph showing the relationship between P (%) and solute B (BE)% based on the results obtained in this example. As shown in the figure, in the steel of the present invention, if P: 0.030% or less and solid solution B ≧ (0.0061 × P−0.00001)% are satisfied, delayed fracture does not occur.

It is a graph which shows the relationship between P (%) and solid solution B (BE)% based on the result in an Example.

Claims (4)

By mass%, C: 0.20 to 0.38% , Si: 0.35 to 2.0%, Mn: 0.05 to 2.0%, P: 0.030% or less, S: 0.04 -0.1%, Cr: 0.05-2.0%, N: 0.010% or less, B: 0.0003-0.0030%, Al: 0.001-0.05%, Solid-solution B: Steel for induction hardening excellent in delayed fracture resistance , characterized by having a steel composition comprising (0.0061 × P-0.00001)% or more, the balance Fe and inevitable impurities. Further, in terms of mass%, Ti: 0.01 to 0.15%, Nb: 0.01 to 0.10%, Zr: 0.01 to 0.10% and V: 0.01 to 0.20% The steel for induction hardening excellent in delayed fracture resistance according to claim 1, comprising one or more selected from the group consisting of: Furthermore, the steel for induction hardening excellent in delayed fracture resistance according to claim 1 or 2 containing Ca: 0.0003% or more and 0.020% or less. The steel for induction hardening excellent in delayed fracture resistance according to any one of claims 1 to 3 , which is used for a crankshaft of a reciprocating engine.

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