JP5151662B2 - Method of manufacturing steel for soft nitriding - Google Patents

Description

  The present invention relates to a method for producing a soft nitriding steel material. Specifically, crankshafts and connecting rods used in automobiles, industrial machinery, construction machinery, etc. are hot-forged into a predetermined shape and then annealed, and then subjected to soft nitriding. The present invention relates to a method for producing a soft nitriding steel material suitable as a material for machine structural parts.

  For mechanical structural parts such as crankshafts and connecting rods used in automobiles, industrial machines, construction machines and the like, fatigue strength is an important mechanical property to be provided.

  In order to improve the fatigue strength of the above-mentioned parts, hot forging the carbon steel material for machine structure or alloy steel material for machine structure into the desired shape and subjecting it to the forged condition as it is or after forging. In such a state, soft nitriding may be performed.

  Although the nitrocarburizing will certainly improve the fatigue strength of mechanical structural parts, for example, in the case of parts such as crankshafts, soft warping will cause “warping”, so that it is necessary to correct the warpage. . In addition, the ease of correcting the warp generated in the mechanical structural part is often referred to as a term such as “bend straightening” or “bendability”, and the mechanical structural part subjected to soft nitriding (hereinafter, “ This is one of the important characteristics that should be provided with fatigue strength.

  However, since the ease of warping correction (hereinafter referred to as “bend straightening”) and fatigue strength are generally in a trade-off relationship, high fatigue strength and excellent bending correction are required for machined structural parts after nitrocarburizing. There is an increasing demand for having sex.

Therefore, in order to meet the aforementioned requirements, to if baked at a temperature just above A _3-point, the optimization of the alloy components, techniques such as optimization of the forging process has been proposed.

  For example, Patent Document 1 states that “alloy element content is mass%, C: 0.15 to 0.40%, Si: 0.50% or less, Mn: 0.20 to 1.50%, Cr : 0.05 to 0.50%, and if necessary, <1> Ni: 0.50% or less, Mo: 0.5% or less of one or two, <2> N: 0.005 to 0.030%, V: 0.3% or less, Nb: 0.3% or less, Ti: 0.2% or less, Zr: 0.1% or less, Ta: 0.2% or less <3> S: 0.01 to 0.30%, <4> Pb: 0.3% or less, Ca: 0.05% or less, Bi: 0.2% or less, Te : Containing at least one element selected from the group of four elements of one or more of 0.05% or less, the balance consisting of Fe and inevitable impurities, A nitrided steel in which the structure after the cold working is substantially a ferrite-pearlite structure, the area ratio of ferrite is 30% or more, the grain size number of ferrite is 5 or more, and the average dimension of pearlite is 50 μm or less. Is disclosed.

  Patent Document 2 states that “in mass%, C: 0.30 to 0.45%, Si: 0.1 to 0.5%, Mn: 0.6 to 1.0%, Ti: 0.005 to 0.005%. 0.1% and N: 0.015 to 0.030%, if necessary, <1> Nb: 0.003 to 0.1%, Mo: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0% and B: one or more selected from 0.001 to 0.005%, <2> S: 0.01 to 0.1% and Ca: one or two elements out of 0.0001 to 0.005%, an element selected from at least one element group of two element groups, the balance being Fe and impurities, It has a mixed structure composed of bainite and ferrite or a mixed structure composed of bainite, ferrite and pearlite, and the mixed structure Bainite fraction is non-heat treated steel for soft-nitriding 5 to 90% "is disclosed in.

  Patent Document 3 states that “in weight percent, C: 0.20 to 0.50, Si: 0.20 to 0.40, Mn: 0.50 to 1.20, Cr: 1.50 or less, Mo: 1.00 or less, Al: 1.00 or less, Nb: 0.025 or less, V: 0.05 or less, and the steel composed of the remaining Fe and unavoidable impurities is heated to 400 to 700 ° C. before nitriding, By performing an annealing treatment under the conditions of a heating time of 0.5 to 10 hours, the nitride layer surface hardness is adjusted to Hv 350 to 500, and the nitride layer hardness gradient is adjusted to 300 to 500 Hv / mm. A method for producing nitrided steel "is disclosed.

Japanese Patent Laid-Open No. 9-291339 WO2005 / 021816 Japanese Patent Laid-Open No. 3-104816

  The technique proposed in the above-mentioned Patent Document 1 aims at achieving both fatigue strength and bend correction by mainly refining the structure. However, Cr that has an adverse effect on bend straightening is intentionally added to ensure fatigue strength, and bending straightness is ensured by reducing the amount of C added for those with a large Cr addition. Therefore, the fatigue strength according to the Ono-type rotating bending fatigue test with notches is 403 MPa at the maximum, and there is a possibility that it cannot be used for a member that requires high fatigue strength.

  Similarly, the technique proposed in Patent Document 2 aims to achieve both fatigue strength and bend correction by refining the structure. In this technique, Cr is not added because it causes bending straightening deterioration, and scrap with a high amount of Cr as an impurity cannot be used. Therefore, it is necessary to strictly control raw materials.

  In the technique proposed in Patent Document 3, Cr, Al and V are precipitated as nitrides by annealing before nitriding treatment, and the nitriding treatment is performed after adjusting the solid solution amount of these elements. Thus, by controlling the amount of new nitride generated and adjusting the surface hardness and hardness gradient of the nitride layer, both fatigue strength and bend straightening can be achieved. However, although the steel proposed in Patent Document 3 contains Nb for grain refinement, the effect of grain refinement during normalization at about 880 ° C. is not achieved with such steel. Even if it can be expected, it cannot be said to be sufficient for suppressing grain growth around 1200 ° C., which is the heating temperature for hot forging, and there is a concern about the coarsening of the structure after hot forging. And the structure | tissue in which the crystal grain coarsened deteriorated the bend correction property, Therefore For this reason, sufficient high bend correction property was not necessarily obtained.

  Accordingly, an object of the present invention is to provide a method for producing a soft nitriding steel material that is excellent in fatigue strength and bend straightening after soft nitriding and is suitable as a material for a soft nitriding mechanical structural component.

  Since mechanical structure parts such as crankshafts are required to ensure core strength and wear resistance, medium carbon steel is often used as the material. In general, the structure after hot forging of the medium carbon steel material is a mixed structure of ferrite and pearlite unless special controlled forging is performed. For this reason, when performing soft nitriding without performing special treatment such as heating to an austenite region and quenching further after hot forging, for example, in general, a mechanical structural component made of a medium carbon steel material ( Hereinafter, the structure at the time of soft nitriding of “medium carbon steel mechanical structural component”) is a mixed structure of ferrite and pearlite. In addition, the above-mentioned “ferrite” is a phase (structure) that looks like a so-called “block” when observed with an optical microscope. In the following description, in order to distinguish this ferrite from ferrite constituting pearlite, “ Polygonal ferrite ". On the other hand, the ferrite that forms pearlite together with cementite is referred to as “lamellar ferrite”.

  Therefore, the present inventors have made various studies in order to improve both the bendability and fatigue strength of a medium carbon steel mechanical structural component whose structure after soft nitriding is composed of polygonal ferrite and pearlite. As a result, the following findings (a) to (i) were obtained.

  (A) The bend straightening property of a medium carbon steel mechanical structural part whose structure after soft nitriding is composed of polygonal ferrite and pearlite is affected by both the ductility of polygonal ferrite and the ductility of pearlite. However, since cementite has almost no ductility, the ductility of pearlite is governed by the ductility of lamellar ferrite, and therefore the bend straightening of the above medium carbon steel machine structural parts after soft nitriding is the ductility of polygonal ferrite. And the ductility of lamellar ferrite.

  (B) An adverse effect of elements having high affinity with N (nitrogen), such as Cr and Al, on bending straightening properties can be mentioned, and among these elements, Cr is an element that is inevitably easily mixed into steel. If the nitriding is present in a solid solution state during soft nitriding, the bending straightness is greatly deteriorated.

  (C) Scrap and iron ore contain a small amount of Cr, and even if Cr is not added intentionally, the steel contains about 0.05 to 0.20% Cr in mass%. I often do. And the adverse effect of Cr on bend straightening is noticeable even at only about 0.05%, and as the content increases, the bad influence on bend straightening also increases, making it difficult to achieve both fatigue strength and bend straightening. I have to.

  (D) The mechanism of bending straightness deterioration due to Cr is due to the interaction between Cr and N. That is, when soft nitriding of a mechanical structural part in a state where Cr is dissolved in the matrix, Cr nitride or a cluster of Cr and N is generated in the vicinity of the surface layer, so that only the vicinity of the surface layer of the part is hardened. In addition, since the diffusion coefficient of N atoms decreases due to the presence of solid solution Cr, the diffusion of N in the direction of the core portion of the mechanical structural component is suppressed. Therefore, when solid solution Cr exists, the inclination of the hardness from the surface layer toward the core portion becomes steep, and the bending straightness deteriorates.

  (E) The above-mentioned deterioration of the bending straightening due to Cr occurs when Cr is in a solid solution state. Therefore, the solid solution Cr concentration in polygonal ferrite or lamellar ferrite, or the Cr concentration in both ferrites is determined. By reducing it, the bending straightness can be improved.

  (F) Cr is an element easily concentrated in cementite. Therefore, pearlite is formed in the mixed structure of polygonal ferrite and pearlite, particularly by fixing Cr to cementite remaining in the matrix by leaving cementite in the matrix and annealing at an appropriate temperature. The solute Cr concentration in lamellar ferrite can be reduced. If the Cr concentration in the lamellar ferrite decreases, even when soft nitriding is performed, only the surface layer of the mechanical structural component is not hardened by the interaction of Cr and N, and the lamellar ferrite after soft nitriding is not hardened. Therefore, the straightening property of the nitrocarburized machine structural component is improved.

(G) In order to leave cementite in the matrix before soft nitriding, the temperature of the hot forged steel is
A1 = 723-10.7 (Mn%) + 29.1 (Si%) − 16.9 (Ni%) + 16.9 (Cr%) (1)
It must not be raised to a temperature range exceeding A1 ° C. represented by the formula: Therefore, it is necessary to perform the annealing at a temperature of A1 ° C. or lower.

  In addition, (Mn%), (Si%), (Ni%), and (Cr%) in the above formula (1) represent the contents in the steel material in terms of mass% of Mn, Si, Ni, and Cr, respectively.

  (H) However, when the crystal grain after hot forging becomes coarse and the diameter of the pearlite colony exceeds 150 μm, even if annealing is performed at a temperature of A1 ° C. or lower, pearlite is formed. Since the lamellar spacing is also wide, it takes time to diffuse Cr during annealing, and the concentration of Cr to cementite is difficult to proceed.

  (I) In order not to coarsen the crystal grains after hot forging, Ti that stably forms particles having a pinning action even when heated to a temperature range of 1100 to 1300 ° C. during hot forging is included. Is effective.

  This invention is completed based on said knowledge, The summary exists in the manufacturing method of the steel material for soft nitriding shown to following (1)-(3).

(1) By mass%, C: 0.25 to 0.50%, Si: 0.1 to 0.5%, Mn: 0.3 to 1.5%, P: 0.05% or less, S: 0.1% or less, Ti: 0.005 to 0.05%, Cr: 0.40% or less, Al: 0.05% or less and N: 0.005 to 0.030%, the balance being Fe and Ri Do from impurities, the V is Ru der less than 0.05% the steel in impurities, in heating to 1100 to 1300 ° C., after hot forging finish temperature of 900 ° C. or higher, 570 ° C. or higher, and, A method for producing a steel material for soft nitriding, characterized by annealing at a temperature of A1 ° C. or less represented by the following formula (1).
A1 = 723-10.7 (Mn%) + 29.1 (Si%) − 16.9 (Ni%) + 16.9 (Cr%) (1)
However, (Mn%), (Si%), (Ni%) and (Cr%) in the formula (1) represent the contents in the steel material in terms of mass% of Mn, Si, Ni and Cr, respectively.

A1 = 723-10.7 (Mn%) + 29.1 (Si%) − 16.9 (Ni%) + 16.9 (Cr%) (1)
However, (Mn%), (Si%), (Ni%) and (Cr%) in the formula (1) represent the contents in the steel material in terms of mass% of Mn, Si, Ni and Cr, respectively.

  (2) The steel is further characterized by containing, in mass%, at least one of Mo: 0.50% or less, Cu: 0.60% or less, and Ni: 0.60% or less. The manufacturing method of the steel material for soft nitriding as described in said (1).

  (3) The method for producing a steel material for soft nitriding as described in (1) or (2) above, wherein the steel further contains, by mass%, Ca: 0.005% or less.

  Hereinafter, the inventions related to the method for producing a soft nitriding steel material (1) to (3) are referred to as “present invention (1)” to “present invention (3)”, respectively. Also, it may be collectively referred to as “the present invention”.

  According to the method of the present invention, a steel material for soft nitriding that is excellent in fatigue strength and bend straightening after soft nitriding and that is suitable as a material for a soft nitriding mechanical structural component can be obtained. If this steel material for soft nitriding is used, even if the fatigue strength is the same level, it is possible to obtain a soft nitriding mechanical structural component having an excellent bending straightness of 50% or more.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) Chemical composition C: 0.25 to 0.50%
C is an indispensable element for bearing the strength and toughness of nitrocarburized mechanical structural parts, stabilizing austenite during hot forging, ensuring wear resistance of products (soft nitriding mechanical structural parts), and fixing Cr In order to increase the amount of cementite for the purpose, a content of 0.25% or more is necessary. On the other hand, if the content exceeds 0.50%, the hardenability is excessively increased and the formation of hard martensite that impairs the machinability is easily caused. Therefore, the content of C is set to 0.25 to 0.50%. In addition, in order to fully exhibit the effect | action of C, it is preferable to make the minimum of C content into 0.30%. Moreover, in order to suppress the rise of hardenability and make it difficult to generate martensite, the upper limit of the C content is preferably set to 0.45%.

Si: 0.1 to 0.5%
Si is added as a deoxidizer in the steel making process, but it is also effective for strengthening the solid solution of ferrite, so a content of 0.1% or more is necessary. On the other hand, if the Si content exceeds 0.5%, the hot deformation resistance of steel is increased, and the toughness and machinability are deteriorated. Therefore, the Si content is set to 0.1 to 0.5%. In order to fully exhibit the effects of Si deoxidation and ferrite strengthening, the lower limit of the Si content is preferably 0.15%. In order to ensure hot workability and toughness, the upper limit of Si content is preferably 0.3%.

Mn: 0.3 to 1.5%
Mn is added in the steel making process as a deoxidizing agent similarly to Si. Moreover, austenite is stabilized and hardenability is improved. Furthermore, Mn combines with S in steel to form MnS, which is effective in improving machinability. For this purpose, a Mn content of 0.3% or more is necessary. On the other hand, if the Mn content exceeds 1.5%, the hardenability is excessively increased, and martensite that is harmful to the machinability is easily generated. Therefore, the Mn content is set to 0.3 to 1.5%. In order to fully exhibit the action of Mn, it is preferable to set the lower limit of the Mn content to 0.5%. Moreover, in order to suppress the rise of hardenability and make it difficult to generate martensite, the upper limit of the Mn content is preferably set to 1.2%.

P: 0.05% or less P is an impurity contained in steel and segregates at grain boundaries to promote grain boundary embrittlement cracking. In particular, when the content exceeds 0.05%, grain boundary embrittlement occurs. The occurrence of cracking becomes remarkable. Therefore, the content of P is set to 0.05% or less. The P content is preferably 0.03% or less.

S: 0.1% or less S is an impurity contained in steel, but is also an element having an effect of enhancing the machinability of steel. However, when S is contained excessively, segregation defects occur in the steel slab or hot workability is deteriorated. In particular, when the S content exceeds 0.1%, segregation in the steel slab is caused. Defects are generated and hot workability is significantly reduced. Therefore, the content of S is set to 0.1% or less. In order to suppress the occurrence of segregation defects in the steel slab and the decrease in hot workability, the upper limit of the S content is preferably 0.09%. On the other hand, since the machinability improvement effect of S can be reliably obtained with a content of 0.02% or more, when obtaining the machinability improvement effect, the lower limit of the S content should be 0.02%. Is preferred.

Ti: 0.005 to 0.05%
Ti is an essential element for forming pinning particles for suppressing crystal grain coarsening during hot forging. Pinning particles include Ti nitrides, carbides, and carbonitrides. In order to generate pinning particles having a sufficient distribution density, the content must be 0.005% or more. On the other hand, even if the Ti content exceeds 0.05%, the above effect is saturated, and in addition, the solid solution Ti that cannot be precipitated as pinning particles causes the detrimental effect that the bend straightening after soft nitriding deteriorates. . Therefore, the Ti content is set to 0.005 to 0.05%. In addition, in order to suppress the harmful effect due to solute Ti, the upper limit of the Ti content is preferably set to 0.035%.
Cr: 0.40% or less Cr is an element derived from scrap or iron ore and inevitably mixed in molten steel by about 0.05 to 0.20%. Further, Cr may be intentionally added to increase the surface layer hardness after soft nitriding and to increase the fatigue strength.

  However, even if the Cr content is so small that it is inevitably mixed, the bend straightening after soft nitriding will deteriorate. Specifically, when the Cr content is 0.05% or more, the bending straightness after soft nitriding tends to deteriorate. It is effective to perform the annealing of the present invention, which will be described later, against the deterioration of the bending straightness after soft nitriding. That is, although the effect of increasing the surface hardness of Cr is slightly reduced by annealing, a great effect of suppressing the deterioration of the bending straightening after soft nitriding by Cr is obtained. However, if the Cr content increases and exceeds 0.40%, the bending straightening after soft nitriding cannot be sufficiently prevented from being deteriorated even if the annealing of the present invention is performed. Therefore, the Cr content is set to 0.40% or less. In order to surely suppress the deterioration of the bending straightening after soft nitriding, the Cr content is preferably 0.20% or less.

Al: 0.05% or less Al is usually added at the time of melting as a deoxidizer, but remains in the steel as alumina particles or forms AlN by combining with N to increase the surface hardness. As a result, the bending straightening after soft nitriding is deteriorated. Alumina is an oxide-based inclusion with high hardness, and shortens the life of a tool used for cutting. Furthermore, Al has a small cementite / ferrite distribution coefficient, that is, the ratio of the concentration of solid solution in both phases of cementite and ferrite is small, and it can be almost concentrated in cementite even after annealing of the present invention described later. As a result, bending straightening after soft nitriding is deteriorated. In addition, since AlN dissolves in the matrix during hot forging, it cannot be expected to function as pinning particles, and is hardly useful for crystal grain refinement. Therefore, the content of Al is preferably low, and is 0.05% or less. The Al content is preferably 0.005% or less.

N: 0.005-0.030%
N stabilizes austenite, constitutes pinning particles to suppress grain coarsening, forms Fe nitride to contribute to precipitation strengthening, and further contributes to solid solution strengthening as solid solution nitrogen Thus, it has an effect of increasing the strength of the base material, so it is positively contained. In order to obtain the above effect, it is necessary to contain N in an amount of 0.005% or more. On the other hand, if the content of N exceeds 0.030%, bubble defects may be generated in the ingot and the material may be damaged. Therefore, the N content is set to 0.005 to 0.030%. In addition, in order to fully exhibit the effect | action of N, it is preferable to make the minimum of N content into 0.010%. Moreover, in order to suppress generation | occurrence | production of the bubble defect in an ingot, it is preferable that the upper limit of N content shall be 0.025%.

  For the reasons described above, the present invention (1) includes the production of a steel for soft nitriding using a steel whose chemical composition is in the above-mentioned range from C to N, and the balance is Fe and impurities. Stipulated.

  In addition, in the manufacturing method of the steel for soft nitriding of this invention, the steel containing 1 or more types of elements chosen from Mo, Cu, Ni, and Ca can further be used as needed.

  Hereinafter, the above optional elements will be described.

  Mo, Cu and Ni all have the effect of improving fatigue strength. For this reason, when it is desired to obtain a greater fatigue strength, it may be contained in the following range.

Mo: 0.50% or less Mo increases the hardenability of the steel and contributes to increasing the strength and improving the fatigue strength. Therefore, Mo may be included to obtain this effect. However, if the Mo content exceeds 0.50%, the hardenability becomes too high and the formation of martensite is promoted, resulting in a decrease in bending straightness and toughness after soft nitriding. Therefore, the Mo content is set to 0.50% or less. In order to suppress a decrease in bending straightening and toughness after soft nitriding, the upper limit of the Mo content is preferably 0.30%.

  On the other hand, in order to reliably obtain the effect of improving the fatigue strength of Mo, the lower limit of the Mo content is preferably 0.03%, and more preferably 0.05%.

Cu: 0.60% or less Since Cu strengthens ferrite and improves fatigue strength, Cu may be contained to obtain this effect. However, since Cu has a melting point as low as 1083 ° C., the time remaining as a liquid phase in the solidification process in the steelmaking process becomes long, and segregates at the grain boundaries of the steel to induce hot cracking. When the content exceeds 0.60%, the above tendency becomes remarkable. Therefore, the Cu content is set to 0.60% or less. In order to suppress the induction of cracking in the hot state, the upper limit of the Cu content is preferably 0.50%.

  On the other hand, in order to reliably obtain the effect of improving the fatigue strength of Cu, the lower limit of the Cu content is preferably 0.05%, and more preferably 0.10%.

Ni: 0.60% or less Ni has an effect of strengthening ferrite and improving fatigue strength. Moreover, Ni is an element effective in preventing the hot cracking resulting from Cu, when steel contains Cu. However, since these effects are saturated when the Ni content increases, the Ni content is set to 0.60% or less in order not to increase the steelmaking cost.

  In order to reliably obtain the effect of Ni, the lower limit of the Ni content is preferably 0.05%, and more preferably 0.10%.

  In addition, said Mo, Cu, and Ni can be contained only in any 1 type in them, or 2 or more types of composites.

Ca: 0.005% or less Ca has an effect of enhancing the machinability of the steel material, and therefore, Ca may be contained in order to obtain this effect. However, when Ca is contained excessively, segregation defects occur in the steel slab, or hot workability is deteriorated. In particular, when the Ca content exceeds 0.005%, The occurrence of segregation defects and the decrease in hot workability become significant. Therefore, the Ca content is set to 0.005% or less. In order to suppress the occurrence of segregation defects in the steel slab and the decrease in hot workability, the upper limit of the Ca content is preferably 0.003%.

  On the other hand, in order to reliably obtain the above-described effect of improving the machinability of Ca, the lower limit of the Ca content is preferably 0.0001%, and more preferably 0.0003%.

  For the reasons described above, the present invention (2) further includes one of Mo: 0.50% or less, Cu: 0.60% or less, and Ni: 0.60% or less. It was stipulated that steel for soft nitriding should be produced using steel containing more than seeds.

  Similarly, the present invention (3) further comprises producing a steel for soft nitriding using the steel of the present invention (1) or the present invention (2), further using a steel containing Ca: 0.005% or less. Stipulated.

  In the steel used in the present invention, elements other than the above-described elements from C to Ca are not intentionally added. Therefore, among the elements that may be mixed into the steel as an impurity element, the upper limit content will be described particularly regarding V that affects the bending straightening after soft nitriding.

V: Less than 0.05% When the V content increases, especially 0.05% or more, if the cooling rate of hot forging is about air cooling, phase interface precipitation does not occur completely during cooling. Part V remains up to room temperature in a state of solid solution in the matrix. And since the above-mentioned V in the solid solution is precipitated during soft nitriding and excessively hardens the vicinity of the surface layer, the bend straightening property is deteriorated. Therefore, it is not preferable that the impurity element contains a large amount of V, and the V content is preferably less than 0.05%. A more preferable V content is 0.02% or less.

(B) Hot forging conditions Even in the steel having the chemical composition described in the above (A), if the heating temperature during hot forging becomes too high, the crystal grains become coarse. In order to prevent coarsening of crystal grains, the upper limit of the heating temperature during hot forging needs to be 1300 ° C. The upper limit of the heating temperature is preferably 1250 ° C. On the other hand, the lower the heating temperature during hot forging in the austenite single-phase region, the finer the crystal grains, and the fatigue strength and bend straightening after the subsequent soft nitriding improve, but the temperature of the material during forging Since the burden on a mold | type will become large rapidly when it becomes low, it is necessary to make a minimum into 1100 degreeC.

  Similarly, the lower the austenite single-phase finish temperature for hot forging, the finer the crystal grains, and the fatigue strength and bend straightening after subsequent soft nitriding will improve, but the burden on the mold will be reduced. In order to reduce, the lower limit of finishing temperature needs to be 900 degreeC. Preferably it is 950 degreeC or more.

(C) Annealing conditions In the stage before soft nitriding, the steel having the chemical composition described in the item (A) is hot-forged on the condition described in the item (B).
A1 = 723-10.7 (Mn%) + 29.1 (Si%) − 16.9 (Ni%) + 16.9 (Cr%) (1)
When annealing is performed at a temperature of A1 ° C. or less represented by the formula: Cr can be fixed to the cementite remaining in the matrix, and the steel material has a pearlite colony diameter described later of 150 μm or less. Since the crystal grains are not coarsened, good bend straightening can be ensured after soft nitriding.

  In addition, (Mn%), (Si%), (Ni%), and (Cr%) in the above formula (1) represent the contents in the steel material in terms of mass% of Mn, Si, Ni, and Cr, respectively.

  In order to diffuse Cr by annealing and fix it to cementite, it is better to perform the treatment in a high temperature range where the diffusion coefficient of Cr is large, but at a temperature exceeding A1 ° C. expressed by the above formula (1). When annealed, the cementite is dissolved in the matrix, so that a sufficient Cr fixing effect by the cementite cannot be ensured. On the other hand, when the annealing temperature is below 570 ° C., it takes a long time for Cr to be fixed to cementite.

  Therefore, in the present invention, after hot forging the steel having the chemical composition described in the item (A) under the condition described in the item (B), the temperature is 570 ° C. or more and the formula (1) It was decided to anneal at a temperature below A1 ° C.

  In addition, since the diffusion of Cr becomes insufficient when the annealing time is short, the lower limit of the annealing time is preferably about 30 minutes. On the other hand, if the annealing time is too long, the cementite is spheroidized and the hardness of the material is drastically lowered. Therefore, the upper limit of the annealing time is preferably about 10 hours.

(D) Finishing to desired soft-nitrided part shape Finishing to the desired soft-nitrided part shape may be performed by an appropriate method such as cutting, for example, and may be appropriately selected according to the part shape. This finishing may be performed after hot forging and before the annealing described in the section (C), or after performing the annealing described in the section (C) after the hot forging. May be implemented.

(E) Soft nitriding treatment In the present invention, the soft nitriding method is not particularly limited, and a normal method such as gas soft nitriding, salt bath soft nitriding, or ion nitriding can be used.

  Whichever method is used, it is possible to uniformly form a compound layer (nitride layer) having a thickness of about 20 μm and a diffusion layer directly therebelow on the surface of the product (soft nitriding mechanical structural component).

  In order to obtain a desired soft nitriding machine structural component by gas soft nitriding, for example, in an atmosphere in which endothermic metamorphic gas (RX gas) and ammonia gas are mixed 1: 1, the soaking temperature is set to 560 to 620 ° C. And processing may be performed for 1 to 3 hours.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  After steels A to G having the chemical compositions shown in Table 1 were melted in a 180 kg vacuum melting furnace, the ingot was heated to 1250 ° C., hot forged to a finishing temperature of 1000 ° C., and then released in the atmosphere. It was cooled to form a round bar having a diameter of 50 mm.

  Steels A to F in Table 1 are steels of the present invention examples whose chemical compositions are within the range defined by the present invention, while Steel G is a comparative example whose chemical composition deviates from the conditions defined by the present invention. Of steel. In Table 1, the value of A1 represented by the above formula (1) is also shown.

  In order to quantitatively compare the roughness of the structure as it was allowed to cool in the air after the above hot forging, the pearlite colony diameter was measured by the following method.

  First, from the D / 4 part (“D” represents the diameter of the round bar) of each round bar having a diameter of 50 mm obtained as described above, a cross section parallel to the forging direction (forging axis) is formed on the test surface. A sample for microstructural observation was collected as described above, embedded in a resin, mirror-polished and then corroded with a nital corrosive solution, and an optical micrograph was taken at a magnification of 200 times.

  Next, the pearlite colony was visually determined on the photograph, surrounded by a closed curve, and the diameter of the circle corresponding to the area, that is, the equivalent circle diameter was defined as the pearlite colony diameter.

  Table 2 shows the pearlite colony diameter measured as described above. From Table 2, it can be seen that the steel G with no addition of Ti, which is a steel of the comparative example, has a coarser structure than other steels containing Ti.

  In addition, the round bar having a diameter of 50 mm obtained by cooling in the air after the hot forging was annealed at various temperatures and times. In addition, annealing was performed in the air, and after annealing, it was left to cool in the air to room temperature.

  Table 3 shows the detailed conditions of the annealing. In addition, "-" in the annealing column of Table 3 indicates that no annealing is performed.

  Collected Ono type rotating bending fatigue test piece for fatigue strength measurement and three-point bending test piece for bending straightness measurement from D / 4 part of each round bar after annealing in parallel to the forging direction (forging axis) did.

  The Ono-type rotating bending fatigue test piece is a round bar test piece with a notch having a depth of 1 mm and an R of 3 mm, and its shape is shown in FIG. The unit of the dimension in FIG. 1 is “mm”.

  The above Ono-type rotating bending fatigue test piece was subjected to soft nitriding which will be described later and subjected to a fatigue test.

The fatigue test was carried out under conditions of room temperature, air, and rotation speed of 3400 rpm, and the maximum stress that did not break when the stress was applied 10 ⁷ times was defined as the fatigue strength.

  The three-point bending test piece is a square test piece having a depth of 5 mm and a notch having a bottom curvature radius of 10 mm and a length of 100 mm, and the shape thereof is shown in FIG. The unit of dimension in FIG. 2 is also “mm”.

  The three-point bending test piece was subjected to soft nitriding described later and subjected to a three-point bending test.

  The three-point bending test was performed as shown in FIG. That is, two fulcrums were provided in the longitudinal direction of the surface including the notches so that the distance between the fulcrums was 70 mm with the notch as the center, and the pushing speed was 0.5 mm / min. In order to measure the amount of strain at the bottom of the notch, strain gauges are affixed to the two edges on the bottom of the notch in a direction parallel to the longitudinal direction of the test piece, and the indentation stroke is applied to one gauge. The bending straightness was evaluated using the strain value indicated by the other gauge when increasing until the wire was broken as the straightening limit strain amount.

  The Ono-type rotating bending fatigue test piece and the three-point bending test piece were subjected to gas soft nitriding under the condition of holding at 580 ° C. for 2 hours in an atmosphere in which RX gas and ammonia gas were mixed 1: 1. Cooled into 100 ° C. oil.

  Table 3 also shows the fatigue strength and correction limit strain amount obtained as described above. Further, the relationship between the correction limit strain amount and the fatigue strength, which is an index of bending straightness in Table 3, is summarized and shown in FIG.

  FIG. 4 shows that when the inventive example and the comparative example are compared at the same level of fatigue strength, the inventive example has a larger correction limit strain amount of about 60% to 340% than the comparative example. Therefore, it is clear that if the steel for soft nitriding manufactured under the conditions specified in the present invention is used, the nitrocarburized mechanical structural component can have both fatigue strength and bend straightening at a high level.

  According to the method of the present invention, a steel material for soft nitriding that is excellent in fatigue strength and bend straightening after soft nitriding and that is suitable as a material for a soft nitriding mechanical structural component can be obtained. If this steel material for soft nitriding is used, even if the fatigue strength is the same level, it is possible to obtain a soft nitriding mechanical structural component having an excellent bending straightness of 50% or more.

It is a figure which shows the shape of the Ono type | formula rotation bending fatigue test piece for the fatigue strength measurement used in the Example. It is a figure which shows the shape of the three-point bending test piece for a bending straightness measurement used in the Example. It is a figure explaining the method of the three-point bending test implemented in the Example. It is a figure which arrange | positions and shows the relationship between the amount of correction limit distortion which is a parameter | index of a bending straightness, and fatigue strength about the test numbers 1-17 of an Example.

Claims (3)

In mass%, C: 0.25 to 0.50%, Si: 0.1 to 0.5%, Mn: 0.3 to 1.5%, P: 0.05% or less, S: 0.1 %: Ti: 0.005 to 0.05%, Cr: 0.40% or less, Al: 0.05% or less, and N: 0.005 to 0.030%, the balance being Fe and impurities Do Ri, the V is Ru der less than 0.05% the steel in impurities, were heated to 1100 to 1300 ° C., after hot forging finish temperature of 900 ° C. or higher, at 570 ° C. or higher, and, following ( 1) A method for producing a soft nitriding steel material, characterized by annealing at a temperature of A1 ° C. or lower represented by the formula.
A1 = 723-10.7 (Mn%) + 29.1 (Si%) − 16.9 (Ni%) + 16.9 (Cr%) (1)
However, (Mn%), (Si%), (Ni%) and (Cr%) in the formula (1) represent the contents in the steel material in terms of mass% of Mn, Si, Ni and Cr, respectively.   The steel further contains at least one of Mo: 0.50% or less, Cu: 0.60% or less, and Ni: 0.60% or less in mass%. 2. A method for producing a soft nitriding steel material according to 1. The method for producing a steel material for soft nitriding according to claim 1 or 2, wherein the steel further contains, by mass%, Ca: 0.005% or less.

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