JP2010280950A - Heat resistant steel for exhaust valve and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heat resistant steel for an exhaust valve, which has high high-temperature mechanical strength and satisfactory hot workability, and to provide a method for producing the heat resistant steel for an exhaust valve. <P>SOLUTION: The heat resistant steel for an exhaust valve is composed of a high nitrogen-high Cr austenitic steel which contains 0.50 to 0.90% C and 15.0 to 25.0% Cr as additional elements comprising essential additional elements and optional additional elements which can be optionally contained, wherein fine carbides and/or fine carbonitrides are at least precipitated into crystal grains in a dispersed manner. As the essential additional elements, C, Cr, N, Mn, Ni and P are contained, and, as the optional additional elements, Nb, Ti, Si, W, Mo, V, Co, B, Zr, Mg, Ca and Cu are contained in an amount of prescribed mass%. The heat resistant steel for an exhaust valve is produced through: a forging step; a solid solution heat treatment step where, after holding in the temperature range of 1,000 to 1,200°C, oil cooling is performed; and a step where aging treatment is performed in the temperature range of 700 to 800°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a heat resistant steel for an exhaust valve used for an exhaust valve of an automobile engine or the like and a method for manufacturing the same, and more particularly to a heat resistant steel for an exhaust valve made of high nitrogen high Cr austenitic steel and a method for manufacturing the same.

  Ni-based alloys such as NCF751 have been used as alloys for exhaust valves of automobile engines and the like. Such a Ni-based alloy exhibits high high-temperature corrosion resistance due to Ni, and high high-temperature mechanical strength can be achieved by precipitating fine γ ′ phase particles made of Ni-based intermetallic compounds in the matrix.

  By the way, in recent years, lead-free gasoline has progressed due to considerations for environmental problems, and the problem of corrosion of hot gases such as sulfide-based gases has been greatly reduced in exhaust valves. Along with this, many alloy steels for exhaust valves have been developed that ensure high-temperature mechanical strength while reducing the amount of expensive Ni. For example, Patent Document 1 discloses a heat-resistant steel for exhaust valves made of austenitic heat-resistant steel in which the amount of Ni is reduced to 30 to 35%. However, if the amount of Ni is further reduced, the amount of precipitation of γ ′ phase particles composed of Ni-based intermetallic compounds is significantly reduced, and sufficient high-temperature mechanical strength as a heat-resistant steel for exhaust valves cannot be achieved. It is said.

  On the other hand, in an exhaust valve used in a low-power engine that burns at a relatively low temperature, the demand for high-temperature mechanical strength is low, the amount of Ni can be further reduced, and instead of particles made of Ni-based intermetallic compounds, carbides and / or Alternatively, a cheaper austenitic heat resistant steel on which carbonitride particles are precipitated can be used. For example, Patent Document 2 includes approximately 20% Cr and approximately 0.4% N with a weight percent equivalent to SUH35 (21-4N), suppressing Ni to a few percent, and includes carbides and / or charcoal. A heat resistant steel for exhaust valves made of high nitrogen high Cr austenitic heat resistant steel in which nitride particles are finely precipitated is disclosed.

JP-A-9-279309 JP 2001-323323 A

  Even in the austenitic heat resistant steel as described above, when the precipitation amount of carbide and / or carbonitride is increased, higher mechanical strength can be obtained. On the other hand, austenitic heat-resistant steel having such a high high-temperature mechanical strength is likely to be cracked in a hot working process such as a forging process in an exhaust valve manufacturing process, and is often difficult to process.

  The present invention has been made in view of such a situation, and the object thereof is a high nitrogen high Cr austenitic steel having high high temperature mechanical strength and good hot workability. The object is to provide heat-resistant steel for exhaust valves and a method for producing the same.

  The heat-resisting steel for exhaust valves according to the present invention is 0.5% to 0.90% C and 15.0 to 25.0 in terms of mass% including an essential additive element and an optional additive element that can be optionally contained. % Heat resistant steel for exhaust valves made of high nitrogen high Cr austenitic steel in which fine carbides and / or fine carbonitrides are dispersed and precipitated at least in crystal grains, the essential additive elements being C, Cr , N, Mn, Ni and P, the optional additional elements as Nb, Ti, Si, W, Mo, V, Co, B, Zr, Mg, Ca and Cu. N is included so that 0.90 ≦ C + N ≦ 1.3 within a range of 0.40 to 0.60%, Mn is within a range of 6.0 to 13.0%, and Ni is 4.0. In the range of ~ 8.0%, P in the range of 0.03 to 0.30% In the optional additive element, Nb and / or Ti are added in mass% so as not to exceed at least 1.0%, respectively, and 0.05 ≦ Nb + Ti ≦ 1.0, and Si is 1 Within 6%, W within 8.0%, Mo within 6.0%, V within 1.0%, Co within 5.0%, B within 0.03%, Zr within 0.1% %, Mg within 0.01%, Ca within 0.01%, and Cu within 5.0%.

  According to this invention, the essential additive element having a composition within a predetermined range enhances the substrate, reduces the amount of coarse primary carbides and / or carbonitrides precipitated at the grain boundaries, and makes fine secondary particles. Carbide and / or carbonitride can be mainly dispersed and precipitated in the crystal grains. Therefore, it is possible to prevent cracking in a hot working process such as forging in the manufacturing process of an exhaust valve, and a high nitrogen high Cr austenitic heat resistant steel in which conventional carbide and / or carbonitride particles are finely precipitated. Even when compared, high mechanical strength can be obtained. The above-described alloy may contain inevitable impurities in a range that does not significantly affect the structure and mechanical strength.

  In the above invention, Si may be added in an amount of 0.5% or more by mass%. That is, by adding at least a predetermined amount of Si, which is an optional additive element, below the upper limit of the above-described addition amount, high temperature mechanical strength and acid resistance required as heat resistant steel for exhaust valves without impairing hot workability It is possible to increase the chemical properties.

  In the above invention, W and Mo may be added in a mass% of 0.01% or more and 0.01% or more, respectively, so as to satisfy 3.0 ≦ Mo + 1 / 2W ≦ 6.0. Good. According to this invention, the high temperature mechanical strength required as a heat resistant steel for exhaust valves can be further increased without impairing hot workability.

  In the above invention, V may be added in mass% so as to satisfy 0.05 ≦ Nb + V + Ti ≦ 1.0. According to this invention, the coarsening of crystal grains can be prevented, and the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves can be further increased without impairing hot workability.

  In the above invention, 0.01% or more of Co may be added in mass%. According to this invention, it is possible to mainly reinforce the base without impairing hot workability, and to further increase the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves.

  In the above invention, B may be added in 0.001% or more by mass%. According to this invention, it is possible to enhance the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves, mainly by strengthening the grain boundaries without impairing hot workability.

  In the above invention, 0.001% or more of Zr may be added in mass%. According to this invention, it is possible to enhance the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves, mainly by strengthening the grain boundaries without impairing hot workability.

  In the above invention, 0.001% or more of Mg may be added in mass%. According to this invention, deoxidation and desulfurization can be performed more reliably at the time of melting, and hot workability can be further enhanced without impairing the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves.

  In the above invention, Ca may be added in 0.001% or more by mass%. According to this invention, deoxidation and desulfurization can be performed more reliably at the time of melting, and hot workability can be further enhanced without impairing the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves.

  In the above invention, Cu may be added at 0.1% or more by mass%. According to this invention, the cold workability required as a heat-resistant steel for exhaust valves can be enhanced without impairing the high-temperature mechanical strength.

  An exhaust valve manufacturing method using the above heat-resistant steel for exhaust valves, which includes a forging step, a solution heat treatment step in which oil cooling is performed after holding in a temperature range of 1000 to 1200 ° C, and a temperature range of 700 to 800 ° C. And an aging treatment step. According to this invention, it is possible to reduce the amount of coarse primary carbide and / or carbonitride precipitated in the crystal grain boundaries, so that cracking in the forging process can be prevented. Further, since fine secondary carbides and / or carbonitrides can be mainly dispersed and precipitated in crystal grains, conventional high nitrogen high Cr austenitic heat-resisting steel finely precipitated carbides and / or carbonitride particles and Even if compared, high mechanical strength can be obtained.

The composition about this invention and a comparative example is shown. A list of mechanical properties for the present invention and comparative examples is shown. The typical structure of the alloy of a comparative example is shown. 2 shows a typical structure of an example alloy.

  In the present inventor, a specific solid solution strengthening element is added in an addition amount within a specific range to enhance the solid solution of the substrate, and a specific element is added in an addition amount within a specific range, so that the conventional exhaust valve can be used. Compared to high nitrogen high Cr austenitic heat resistant steel precipitated with carbide and / or carbonitride which is heat resistant steel, the amount of primary carbide and / or carbonitride precipitated at grain boundaries is reduced to a predetermined amount, The inventors have found that a large amount of fine secondary carbides and / or carbonitrides can be precipitated in the crystal grains, and have reached the present invention. In other words, by balancing the former solid solution strengthening elements that can affect each other and the latter elements and obtaining a characteristic structure, high high-temperature mechanical strength corresponding to the use as an exhaust valve can be achieved. While having it, it is possible to provide a heat-resistant steel for exhaust valves that can provide good hot workability in the manufacturing process.

  The details will be described below.

  About each alloy shown in FIG. 1, 50 kg ingots were melted in a high frequency induction furnace. The ingot was held at 1180 ° C. for 16 hours and subjected to homogenization heat treatment, and then forged into a φ18 round bar. The round bar was held at 1050 ° C. (in part, 1150 ° C., details will be described later) for 30 minutes, then cooled with oil and subjected to a solution heat treatment. Further, the round bar was kept at 750 ° C. for 4 hours and then air-cooled and aged. The round bar after the aging treatment was machined into a test piece shape corresponding to each of the following tests.

  First, in the hardness measurement at room temperature (ordinary temperature), the Rockwell hardness C scale (HRC) was obtained with a Rockwell hardness meter. Moreover, in the hardness measurement in high temperature of 800 degreeC, the Vickers hardness (Hv) was calculated | required with the measurement load of 5 kg with the Vickers hardness meter.

  Next, in the tensile strength measurement, a tensile test was performed by performing a tensile test at room temperature and 800 ° C. using a round bar test piece having a diameter of 8 mm and a length of 90 mm in the parallel part.

  The above test results are shown in FIG.

  No. 1 in FIG. 1 to 12, a series of alloy compositions of the heat-resistant steel for exhaust valves according to the first embodiment of the present invention is 21-4N (JIS SUH35), which is a high nitrogen high Cr austenitic steel containing 0.4 mass% N. ) In a high nitrogen high Cr austenitic steel containing 0.50-0.90% C and 15.0-25.0% Cr, carbides such as C, Cr, N, Nb and Ti and Modifying the composition amount of elements involved in the formation of carbonitrides, elements that can provide solid solution strengthening such as W and Mo, and elements that stabilize the structure such as Mn and Ni that are austenite stabilizing elements . In particular, it has been found that P can greatly change the precipitation form of secondary carbides that can be precipitated in crystal grains, and the composition amount is adjusted.

As shown in FIG. 3, in the conventional high nitrogen high Cr austenitic steel such as 21-4N (JIS SUH35), many coarse carbides, particularly carbides such as Cr ₂₃ C ₆ , exist along the grain boundaries. It was precipitated and caused intergranular cracking during hot working such as a forging process. In addition, stringer-like MC-type carbides are formed in the grains, causing precipitation of dislocations during high-temperature deformation, resulting in a stress concentration source and causing a yield phenomenon, improving high-temperature mechanical strength. It was an obstacle.

  On the other hand, as shown in FIG. 4, the heat resistant steel for exhaust valves of the present invention can suppress the amount and size of carbides and / or carbonitrides along the grain boundaries, and is heat resistant for exhaust valves. Intergranular cracking during hot working as steel can be reduced. Further, fine granular carbides in the grains, in particular, MC type carbides and / or MX type carbonitrides in which M is made of Cr, Nb and / or Ti can be dispersed and deposited finely within a few μm, and an exhaust valve Therefore, the high-temperature mechanical strength as a heat-resistant steel can be increased.

  For example, as shown in FIG. 2, in the steel having a series of alloy compositions of the heat-resistant steel for exhaust valves that is the first embodiment according to the present invention, forging cracks are caused even if forging is performed from at least an ingot to a φ18 round bar. It was never generated. That is, it provides good hot workability. Moreover, in the high temperature hardness test at 800 ° C., a value of 190 HV or more is obtained in all cases, and in the high temperature tensile test at 800 ° C., a value of 370 MPa or more is obtained. That is, it has high high temperature mechanical strength. The comparison with the conventional steel will be described later.

  Further, in FIG. The series of alloy compositions of the heat-resistant steel for exhaust valves according to the second embodiment of the present invention shown in 13 to 15 is V in the series of alloy compositions of the heat-resistant steel for exhaust valves according to the first embodiment described above. It has an added composition. In addition, since the MC type carbide composed of VC has a high temperature required for its solid solution, the temperature of the solution heat treatment was set to 1150 ° C., which is 100 ° C. higher than that of the first example.

  Even in this case, as shown in FIG. 4, carbides and / or carbonitrides along the grain boundaries can be suppressed, and fine granular carbides in the grains, in particular, M is Cr, V, Nb. In addition, MC type carbide and / or MX type carbonitride composed of Ti and / or Ti can be finely dispersed and precipitated to increase the high-temperature mechanical strength. That is, as shown in FIG. 2, forging cracks did not occur even when forging at least a φ18 round bar from the ingot. That is, it provides good hot workability. Moreover, in the high temperature hardness test at 800 ° C., a value of 190 HV or more is obtained in all cases, and in the high temperature tensile test at 800 ° C., a value of 370 MPa or more is obtained. That is, it has high high temperature mechanical strength. In addition, No. with less addition amount of V. Compared with No. 15, No. 15 having a larger addition amount. High values of both hardness and tensile strength were obtained at 13 and 14.

  Further, in FIG. The alloy composition of the heat resistant steel for exhaust valves according to the third embodiment of the present invention shown in 16 and 17 is the same as that of the heat resistant steel for exhaust valves according to the first embodiment described above. It has an added composition.

  Further, in FIG. The alloy composition of the heat resistant steel for exhaust valves according to the fourth embodiment of the present invention shown in 18 and 19 is B and / or Zr in the alloy composition of the heat resistant steel for exhaust valves according to the first embodiment described above. It has an added composition.

  Further, in FIG. The alloy composition of the heat resistant steel for exhaust valves according to the fifth embodiment of the present invention shown in FIG. 20 has a composition in which Co is added to the alloy composition of the heat resistant steel for exhaust valves according to the first embodiment described above.

  Further, in FIG. The alloy composition of the heat resistant steel for exhaust valves according to the sixth embodiment of the present invention shown in FIG. 21 has a composition in which Cu is added to the alloy composition of the heat resistant steel for exhaust valves according to the first embodiment described above.

  No. 1 in FIG. Even in the case of 16 to 21, as shown in FIG. 4, carbides and / or carbonitrides along the grain boundaries can be suppressed, and fine granular carbides in the grains, in particular, M is Cr, V MC type carbide and / or MX type carbonitride composed of Nb and / or Ti can be finely dispersed and precipitated, and the high-temperature mechanical strength can be increased. That is, as shown in FIG. 2, forging cracks did not occur even when forging at least a φ18 round bar from the ingot. That is, it provides good hot workability. Moreover, in the high temperature hardness test at 800 ° C., a value of 190 HV or more is obtained in all cases, and in the high temperature tensile test at 800 ° C., a value of 370 MPa or more is obtained. That is, it has high high temperature mechanical strength.

  In FIG. 101-105 shows a series of alloy compositions of heat-resistant steel for exhaust valves as a comparative example. No. The composition shown in 101 is equivalent to 21-4N (JIS SUH35), which is a high nitrogen high Cr austenitic steel containing 0.4 mass% N, which is a typical exhaust valve steel. No. The composition shown in 102 is a composition in which W and Nb are further added to the above-described composition corresponding to 21-4N, but the amount of Si and the amount of W are small compared to the alloy composition in the first embodiment described above. The composition has a large amount. No. The composition shown in No. 104 is No. 104. Although it is a composition in which the amount of Si is increased from the composition shown in 102, C is lower.

  In this case, as shown in FIG. 3, many coarse carbides and / or carbonitrides are precipitated along the crystal grain boundaries, and stringer-like carbides that are continuous in a string form are precipitated in the grains. That is, as shown in FIG. In 103 and 105, forging cracks occurred when forging from an ingot into a round bar of φ18. Similarly, no. In 102, cracks occurred on the surface during the same forging process. That is, it turns out that it is inferior to the steel of the above-mentioned Example at least about hot workability. Moreover, in the high-temperature hardness test at 800 ° C., the hardness of each comparative example is significantly lower than that of the examples. Similarly, in the high-temperature tensile test at 800 ° C., the mechanical strength is significantly lower in all the comparative examples than in the examples.

  In addition, the guideline which calculates | requires each component range of the alloy composition of the heat resistant steel for exhaust valves by this invention from the above-mentioned Example and comparative example is as follows.

C is an austenite stabilizing element, stabilizes the austenite structure, suppresses the formation of σ phase and Laves phase that cause embrittlement of steel, and strengthens the substrate. Moreover, it can combine with Nb, V, and Ti preferentially to produce MC type carbide, thereby suppressing the coarsening of crystal grains in the solution heat treatment, and as a result, the mechanical strength can be improved. Furthermore, MC type carbides can work as a hardened phase to improve wear resistance. In particular, Cr can generate Cr ₂₃ C ₆ carbide to improve mechanical strength. On the other hand, if added excessively, carbides are excessively generated and workability required as heat resistant steel for exhaust valves is reduced. Therefore, C is mass%, and is within the range of 0.50 to 0.90%.

  N is an austenite stabilizing element and stabilizes the austenite structure. Further, N is an interstitial solid solution strengthening element that gives solid solution strengthening of the substrate and can work in combination with substitutional solid solution strengthening elements such as Mo and W to increase the mechanical strength. Therefore, N is mass%, and is within the range of 0.40 to 0.60%.

  As described above, C and N are strong austenite generating elements, which strengthen the substrate and serve as substitute elements for Ni and Mn. In particular, the ability to replace relatively expensive Ni with these inexpensive elements is effective for cost reduction. Here, N substitutes for the C site of the MC type carbide to form an MX type nitride, and C and N are in a complementary relationship. Therefore, the total amount of C and N can be defined, and 0.90 ≦ C + N ≦ 1.30%.

Cr can significantly improve the corrosion resistance and oxidation resistance for exhaust valve applications by forming an oxidation protective film of Cr ₂ O _{3 on} the alloy surface. In combination with C, Cr ₂₃ C ₆ carbide can be generated to improve mechanical strength. On the other hand, it is a ferrite stabilizing element that can destabilize the austenite structure, promote the formation of σ phase and Laves phase, which are embrittled phases, and heat required as heat resistant steel for exhaust valves. Interworkability and mechanical strength can be reduced. Therefore, Cr is mass% and can be in the range of 15.0 to 25.0%, preferably in the range of 17.0 to 23.5%.

  Furthermore, in the high nitrogen high Cr austenitic steel, optional additional elements can be added to the alloy composition as Nb, Ti, Si, W, Mo, V, Co, B, Zr, Mg, Ca and Cu, with the following guidelines. .

  Si is a deoxidizing material at the time of melting and can give oxidation resistance at high temperatures. Moreover, the solid solution strengthening of the substrate is given as a solid solution strengthening element. On the other hand, excessive addition reduces the mechanical strength and lowers lead corrosion resistance, which is a particular problem in exhaust valve steel. In addition, about the latter, since the necessity of the lead corrosion resistance in unleaded gasoline is reducing, the addition amount should be determined in relation to the fall of the former mechanical strength. Therefore, Si is mass% and is in the range of 0.5 to 1.6%, preferably in the range of 0.6 to 1.6%.

  Mn and Ni are austenite stabilizing elements, and can stabilize the austenite structure and strengthen the substrate. Further, Mn acts as an alternative element for expensive Ni and has an effect of increasing the solubility of N. Therefore, by mass%, Ni is in the range of 4.0 to 8.0% and Mn is in the range of 6.0 to 13.0%, preferably Ni: 4.5 to 7.5%, Mn: 7.0 to It is in the range of 11.0%.

  Mo and W are solid solution strengthening elements for the austenite phase, which is a parent phase, and provide strengthening of the substrate. It also improves high temperature mechanical strength. On the other hand, excessive addition increases deformation resistance at high temperatures and promotes the formation of σ phase and Laves phase, which are embrittled phases, and thus reduces hot workability. Therefore, in mass%, Mo is in the range of 1.0 to 6.0%, W is in the range of 1.0 to 8.0% or less, and Mo + 1 / 2W is set to 3. from the difference in contribution of Mo and W to the above effect. 0 to 6.0%. Preferably, Mo is in the range of 1.5 to 5.5% and W is in the range of 1.0 to 7.5% or less.

  Nb and Ti combine with C to generate MC type carbides. By dispersing and precipitating the fine MC type carbide, it is possible to suppress the coarsening of crystal grains during the solution heat treatment, and to further improve the high-temperature mechanical strength required as a heat-resistant steel for exhaust valves. However, excessive addition promotes the formation of ferrite and can generate coarse carbides, and conversely, the hot workability required as a heat-resistant steel for exhaust valves can be reduced. Therefore, Nb and Ti are each in mass%, and are in the range of 1.0% or less, and from the contribution to the above-described effects, 0.05% ≦ Nb + Ti ≦ 1.0%, preferably 0.1% ≦ Nb ≦ 0.9%, 0.15% ≦ Ti ≦ 0.8%.

  P may form a characteristic structure of carbide and / or carbonitride as described above. In particular, the refinement of carbides and / or carbonitrides is promoted to improve the high-temperature mechanical strength required for heat-resistant steel for exhaust valves. However, excessive addition can significantly lower the melting point of the alloy, reduce the high-temperature mechanical strength and hot workability required as heat-resistant steel for exhaust valves, and also reduce oxidation resistance. Therefore, P is mass% and is in the range of 0.03 to 0.30%, preferably in the range of 0.04 to 0.25%.

V combines with C to produce MC type carbides, and can suppress coarsening of crystal grains during solution heat treatment. In particular, high-temperature mechanical strength required as a heat-resistant steel for exhaust valves can be improved by adding it in combination with Nb and / or Ti. However, excessive addition promotes ferrite formation and reduces the stability of the substrate. If the amount of MC carbide produced is too large, the amount of MC-type carbide, particularly M ₂₃ C _6- type carbide, decreases, and the high-temperature mechanical strength required for heat-resistant steel for exhaust valves can be reduced. Therefore, V is mass% and is 1.0% or less, preferably 0.85% or less. Furthermore, 0.05% ≦ Nb + V + Ti ≦ 1.0% from the contribution of Nb, V and Ti to the above-described effect.

  Co is an austenite stabilizing element that stabilizes the austenite structure and can be substituted as an alternative element for Ni. However, since the addition of a large amount increases the cost, Co is 5% by mass or less.

  B and Zr segregate at the grain boundaries and strengthen the grain boundaries. Such an effect is obtained when each content is 0.001% or more. However, the hot workability required as heat-resistant steel for exhaust valves deteriorates when B is contained in an amount exceeding 0.03% and Zr exceeds 0.1%. Therefore, B is mass%, 0.001 to 0.03%, and Zr is 0.001 to 0.1%.

  Mg and Ca act as a deoxidizing agent and a desulfurizing agent when the alloy is melted, and contribute to the improvement of hot workability required as a heat-resistant steel for exhaust valves. Such an effect is recognized even with the addition of a very small amount of 0.001%. On the other hand, if it exceeds 0.01%, the workability required as a heat-resistant steel for exhaust valves is impaired. Therefore, it is in the range of 0.001% ≦ Mg + Ca ≦ 0.01% by mass%.

  Cu forms a solid solution in austenite, increases stacking fault energy, suppresses work hardening, and can improve cold workability during processing required as exhaust valve steel. Further, Cu can enhance the adhesion of the oxide film on the alloy surface and improve the oxidation resistance as an exhaust valve application. Such an effect is hardly obtained when the content is 0.1% or less, and the oxidation resistance hardly changes even if the content exceeds 5%. Therefore, Cu is mass% and is in the range of 0.1 to 5.0%.

  As described above, in this embodiment, a specific solid solution strengthening element is added in an addition amount within a specific range to enhance the solid solution of the substrate, and a specific element is added in an addition amount within a specific range, so that conventional exhaust gas is added. Compared with high nitrogen high Cr austenitic heat resistant steel with precipitated carbide and / or carbonitride, which is heat resistant steel for valves, the amount of primary carbide and / or carbonitride precipitated at grain boundaries is up to a predetermined amount. Along with the decrease, a lot of fine secondary carbides and / or carbonitrides are precipitated in the crystal grains. In other words, by balancing the former solid solution strengthening elements that can affect each other and the latter elements and obtaining a characteristic structure, high high-temperature mechanical strength corresponding to the use as an exhaust valve can be achieved. While having it, it is possible to provide a heat-resistant steel for exhaust valves that can provide good hot workability in the manufacturing process.

  So far, representative embodiments and modifications based on the embodiments have been described. However, the present invention is not necessarily limited thereto, and those skilled in the art will not depart from the scope of the appended claims. Various alternative embodiments and modifications may be found. For example, the amount of the essential additive element can be changed within a predetermined range without impairing the typical structure and mechanical properties of the steel according to the present invention, and the amount of the optional additive element, including the presence or absence of the addition, The typical structure and mechanical properties of the steel according to the invention can be changed without impairing it.

Claims (11)

The mass% of the additive element is an essential additive element and an optional additive element that can be optionally contained, and contains 0.50 to 0.90% C and 15.0 to 25.0% Cr. Or a heat resistant steel for exhaust valves made of high nitrogen high Cr austenitic steel in which fine carbonitride is dispersed and precipitated in at least crystal grains,
The essential additive elements are C, Cr, N, Mn, Ni and P,
The optional additive elements are Nb, Ti, Si, W, Mo, V, Co, B, Zr, Mg, Ca and Cu,
In the essential additive element, in mass%,
N within the range of 0.40 to 0.60%, 0.90 ≦ C + N ≦ 1.3
Including,
Mn within a range of 6.0 to 13.0%,
Ni in the range of 4.0 to 8.0%,
P is included in the range of 0.03 to 0.30%,
In the optional additive element, in mass%,
Nb and / or Ti are added so as not to exceed at least 1.0%, respectively, and 0.05 ≦ Nb + Ti ≦ 1.0.
Si within 1.6%,
W is within 8.0%,
Mo is within 6.0%,
V within 1.0%,
Co within 5.0%,
B within 0.03%,
Zr within 0.1%,
Mg within 0.01%,
Ca within 0.01%, and
Heat-resistant steel for exhaust valves, characterized by containing Cu arbitrarily within 5.0%.   The heat resistant steel for exhaust valves according to claim 1, wherein Si is added in an amount of 0.5% by mass or more.   The W or Mo is added by mass%, 0.01% or more and 0.01% or more, respectively, and 3.0 ≦ Mo + 1 / 2W ≦ 6.0. 2. Heat-resistant steel for exhaust valves as described in 2.   The heat resistant steel for exhaust valves according to any one of claims 1 to 3, wherein V is added in mass% so as to satisfy 0.05 ≦ Nb + V + Ti ≦ 1.0.   The heat resistant steel for exhaust valves according to any one of claims 1 to 4, wherein Co is added in an amount of 0.01% or more by mass%.   The heat resistant steel for exhaust valves according to any one of claims 1 to 5, wherein B is added in an amount of 0.001% or more by mass%.   The heat resistant steel for exhaust valves according to any one of claims 1 to 6, wherein Zr is added by 0.001% or more by mass%.   The heat resistant steel for exhaust valves according to claim 1, wherein 0.001% or more of Mg is added in mass%.   The heat resistant steel for exhaust valves according to claim 1, wherein 0.001% or more of Ca is added in mass%.   The heat-resistant steel for exhaust valves according to any one of claims 1 to 9, wherein Cu is added in an amount of 0.1% or more by mass%.   It is a manufacturing method of the exhaust valve using the heat-resistant steel for exhaust valves according to claim 1 to 10, comprising a forging step, a solution heat treatment step of oil cooling after being held in a temperature range of 1000 to 1200 ° C, 700 And a step of aging treatment in a temperature range of ˜800 ° C.