Maraging steel, member, and method for producing same

Abstract

The invention provides high-timeliness maraging steel and a manufacturing method thereof. A maraging steel having: a component composition containing, in mass%, 0.02% or less of C, 0.1% or less of Si, 0.1% or less of Mn, 0.01% or less of P, 0.01% or less of S, 0.01% or less of N, 12-25% of Ni, 5-12% of Co, 2-7% of Mo, 0.5-1.5% of Ti, and 0.01-0.1% of Al, with the remainder comprising iron and unavoidable impurities; and a transformation-processed martensite phase in an area ratio of 90% or more.

Description

Technical Field

[0001] The present invention relates to a maraging steel having excellent aging properties and capable of reducing the treatment time in an aging treatment step, a component, and a method for producing the same. Background Art

[0002] At present, maraging steel is a precipitation-aged high-strength steel sheet that is widely used in commercial fields such as aerospace and golf club heads, and has recently been applied to three-dimensional laminated products. Maraging steel is used by precipitating intermetallic compounds such as Ni3Ti in the steel sheet structure after solution treatment and aging treatment at a temperature range of 400-550°C. However, in addition to the high alloy cost, the aging treatment generally takes about 3 hours, and the manufacturing efficiency is low, so it is classified as a steel with high material cost.

[0003] As a technology related to shortening the aging treatment time, for example, Patent Document 1 can be cited. Patent Document 1 discloses a gas nitriding treatment method for a maraging steel thin plate, wherein the decomposition rate of ammonia gas is set to 1 to 6%, and the gas nitriding treatment is performed at a temperature of 420 to 470°C for 600 to 3600 seconds. Patent Document 1 states that according to this method, a relatively short heat treatment can be performed to obtain a maraging steel that can be applied to a hoop of a steel belt for a CVT (Continuously Variable Transmission).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Publication No. 7-116585 Summary of the invention

[0007] Problems to be solved by the invention

[0008] However, although the above-mentioned Patent Document 1 discloses a technique for shortening the aging time during gas nitriding treatment, the manufacturing cost increases due to the gas nitriding treatment, and the invention cannot be applied to three-dimensional laminated molded products having cavities inside. Therefore, it is necessary to develop a maraging steel having excellent aging properties that can shorten the aging treatment time without requiring a high manufacturing cost process such as gas nitriding treatment.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-aging maraging steel, a member using the high-aging maraging steel, and a method for producing the same.

[0010] Here, high time efficiency means that, for k1 satisfying the following formula (1) and k2 satisfying the following formula (2), k shown in formula (3) is 1.05 or more.

[0011] YS1=k1logt+YS …Formula (1)

[0012] YS2=k2logt+YS …Formula (2)

[0013] k=k2 / k1 …Equation (3)

[0014] in,

[0015] YS: In the present invention, it refers to the yield strength (0.2% proof stress) of steel before aging treatment without performing treatment in the heating step (heating treatment to a heating temperature of Ac3 or higher and Ac3+50°C or lower and holding time at the heating temperature of 3000 seconds or lower) or treatment in the cooling step (cooling the steel after the heating step at an average cooling rate of 8°C / s or higher to a cooling stop temperature of 50°C or lower) after the solutionizing step.

[0016] YS1: In the present invention, it refers to the yield strength of steel after the solutionizing step without subjecting it to the treatment in the above-mentioned heating step, the treatment in the above-mentioned cooling step, and the aging treatment (aging treatment at 400° C. or higher and 550° C. or lower);

[0017] YS2: In the present invention, it refers to the yield strength of the steel after the solution treatment, the heating treatment, the cooling treatment and the aging treatment;

[0018] t: aging time of 1 to 10800 seconds;

[0019] k1: aging coefficient (aging coefficient when aging treatment is performed without performing treatment in the heating process or treatment in the cooling process after the solution treatment process);

[0020] k2: Aging coefficient (aging coefficient in the case where aging treatment is performed after the solution treatment process and the treatment in the cooling process).

[0021] It should be noted that the conditions (temperature conditions, cooling rate conditions, etc.) for the treatment (treatment in the solution treatment process) performed on the steel in order to measure the above-mentioned YS, YS1, and YS2 are all the same between YS, YS1, and YS2, and the conditions (temperature conditions, etc.) for the treatment (aging treatment in the aging treatment process) performed on the steel in order to measure YS1 and YS2 are also all the same between YS1 and YS2.

[0022] In addition, YS, YS1, and YS2 were measured by a tensile test in accordance with JIS Z 2241 (2011).

[0023] Here, since k=k2 / k1={(YS2-YS) / logt} / {(YS1-YS) / logt}, k≥1.05 means that when the aging time t is the same in formulas (1) and (2), YS2 of formula (2) adopting the conditions of the present invention is greater than YS1.

[0024] This means that the aging time required to make YS2 of formula (2) the same value as YS1 of formula (1) is shorter under the conditions of formula (2) than under the conditions of formula (1).

[0025] That is, compared with the condition of formula (1), the condition of formula (2) can obtain the target yield strength in a short time (short aging time), so in the present invention, when k ≥ 1.05, it is judged that the aging property is high.

[0026] In addition, the maraging steel referred to in the present invention refers to steel that is strengthened by precipitation of intermetallic compounds in the structure through aging treatment, and is steel containing Ni, Co, Mo, and Ti that precipitate intermetallic compounds.

[0027] Methods used to solve problems

[0028] In order to solve the above-mentioned problems, the present inventors have conducted intensive studies on the structure of maraging steel before aging treatment and its heat treatment conditions, and as a result, have obtained the following findings: by performing a short-time heating treatment of 3000 seconds or less in a temperature range of Ac3 or more and Ac3+50°C or less, reverse transformation from martensite to austenite accompanied by strain introduction occurs, and by rapidly cooling to room temperature to avoid recrystallization of austenite, transformation-processed martensite is formed in the steel sheet, and thus the aging treatment time can be significantly shortened.

[0029] The present invention has been accomplished based on the above findings, and according to the present invention, for example, it is possible to provide a maraging steel having a yield strength of 1800 MPa or more after aging treatment by significantly shortening the aging treatment time.

[0030] It should be noted that, unlike rolling, forging, etc., the present invention can achieve strain introduction using only heat treatment, and aging treatment can also be performed on products processed into component shapes.

[0031] The gist of the present invention is as follows.

[0032] [1] A maraging steel having:

[0033] A composition containing, in mass%, C: 0.02% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.01% or less, S: 0.01% or less, N: 0.01% or less, Ni: 12 to 25%, Co: 5 to 12%, Mo: 2 to 7%, Ti: 0.5 to 1.5%, Al: 0.01 to 0.1%, and the balance being iron and inevitable impurities; and

[0034] A steel structure having a transformed martensite phase with an area ratio of 90% or more.

[0035] [2] A member made of the maraging steel described in [1] above.

[0036] [3] A method for producing maraging steel, comprising:

[0037] A solution treatment step, wherein the steel having the composition described in [1] is solution treated at a temperature of 800° C. or higher and then cooled to room temperature;

[0038] A heating step, wherein the steel after the solutionizing step is subjected to a heating treatment to a heating temperature of Ac3 or higher and Ac3+50° C. or lower, and the holding time at the heating temperature is set to be 3000 seconds or less;

[0039] a cooling step of cooling the steel after the heating step at an average cooling rate of 8° C. / s or more to a cooling stop temperature of 50° C. or less; and

[0040] The aging treatment step is to perform an aging treatment at 400° C. or higher and 550° C. or lower on the steel after the cooling step to obtain maraging steel.

[0041] [4] A method for manufacturing a component, comprising the step of subjecting the maraging steel according to [1] above to at least one of forming and joining to produce the component.

[0042] Effects of the Invention

[0043] According to the present invention, a high-aging maraging steel, a component using the high-aging maraging steel, and a method for manufacturing the same are provided. By using the maraging steel having excellent aging properties of the present invention, the aging treatment time can be shortened, thereby reducing the manufacturing cost. In addition, the maraging steel and its component (product) of the present invention are not limited, and steel plates such as thin steel plates (including thin steel plates for automobiles, thin steel plates for home appliances, and thin steel plates for electrical appliances), steel for molds, three-dimensional laminated molded products, etc. can be used. DETAILED DESCRIPTION

[0044] Hereinafter, embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments.

[0045] (Martening steel)

[0046] The maraging steel of the present invention has a component composition containing, by mass%, C: 0.02% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.01% or less, S: 0.01% or less, N: 0.01% or less, and Ni: 12 to 25%, Co: 5 to 12%, Mo: 2 to 7%, Ti: 0.5 to 1.5%, Al: 0.01 to 0.1%, with the balance being iron and inevitable impurities, and has a steel structure having a transformation martensite phase of 90% or more by area ratio.

[0047] The maraging steel of the present invention can have k, which is the ratio of k1 satisfying the formula (1) to k2 satisfying the formula (2): k2 / k1, be 1.05 or more.

[0048] YS1=k1logt+YS …Formula (1)

[0049] YS2=k2logt+YS …Formula (2)

[0050] k=k2 / k1 …Equation (3)

[0051] in,

[0052] YS: In the present invention, it refers to the yield strength (0.2% yield strength) of steel before aging treatment after the solution treatment without performing the treatment in the heating step (heating treatment to a heating temperature of Ac3 or higher and Ac3+50°C or lower and holding time at the heating temperature of 3000 seconds or lower), the treatment in the cooling step (cooling the steel after the heating step at an average cooling rate of 8°C / s or higher to a cooling stop temperature of 50°C or lower);

[0053] YS1: In the present invention, it refers to the yield strength of steel after the solutionizing step without subjecting it to the treatment in the above-mentioned heating step, the treatment in the above-mentioned cooling step, and the aging treatment (aging treatment at 400° C. or higher and 550° C. or lower);

[0054] YS2: In the present invention, it refers to the yield strength of the steel after the solution treatment, the heating treatment, the cooling treatment and the aging treatment;

[0055] t: aging time of 1 to 10800 seconds;

[0056] k1: aging coefficient (aging coefficient when aging treatment is performed without performing treatment in the heating process or treatment in the cooling process after the solution treatment process);

[0057] k2: Aging coefficient (aging coefficient in the case where aging treatment is performed after the solution treatment process and the treatment in the cooling process).

[0058] It should be noted that the conditions (temperature conditions, cooling rate conditions, etc.) for the treatment (treatment in the solution treatment process) performed on the steel in order to measure the above-mentioned YS, YS1, and YS2 are all the same between YS, YS1, and YS2, and the conditions (temperature conditions, etc.) for the treatment (aging treatment in the aging treatment process) performed on the steel in order to measure YS1 and YS2 are also all the same between YS1 and YS2.

[0059] The maraging steel of the present invention and a member using the maraging steel can be obtained from a steel plate such as a thin steel plate, steel for a mold, or a laminated molded product such as a three-dimensional laminated molded product.

[0060] The component composition, steel structure, and manufacturing method of the maraging steel of the present invention are specifically described below. It should be noted that the present invention is not limited to the following embodiments. The manufacturing method of the maraging steel of the present invention may include a process of manufacturing a steel ingot by melting and casting a raw material containing the above-mentioned component composition, or a process of manufacturing a steel plate by hot rolling, pickling, cold rolling, etc. in a conventional manner after casting.

[0061] In addition, the maraging steel of the present invention can be obtained from a laminated molded article, and the method for producing the maraging steel may include a step of producing a laminated molded article obtained by a three-dimensional laminated molding method according to a conventional method.

[0062] The above-mentioned manufacturing method may include, for example, a step of heating the steel plate or ingot to 1100°C or higher and 1350°C or lower after melting and casting to homogenize the segregation generated during casting, and a step of hot rolling or forging the steel plate or ingot at a final temperature of 900°C or higher.

[0063] The above-mentioned manufacturing method may include a step of repeatedly performing a process (powder bed fusion method) of melting raw material powder of maraging steel using laser or electron beam as a heat source to manufacture a three-dimensional laminated molded product.

[0064] First, the composition of the maraging steel of the present invention will be described. In the present specification, all % indicating the composition of the maraging steel are mass %.

[0065] <C: 0.02% or less>

[0066] C is an element that reacts with Ti contained in steel to precipitate TiC. If TiC is formed, the formation of Ti-based intermetallic compounds, such as Ni3Ti, which contribute to high temperature strength, is suppressed. Therefore, the C content needs to be reduced. In the present invention, the effect of the invention can be obtained by setting the C content to 0.02% or less. Therefore, the C content is set to 0.02% or less. The C content is preferably 0.01% or less, and more preferably 0.005% or less.

[0067] The lower limit of the C content is not particularly limited, but from the viewpoint of production cost, the C content is preferably 0.001% or more, more preferably 0.002% or more.

[0068] <Si: 0.1% or less>

[0069] Si is an element that reacts with dissolved O in steel to form oxides, thereby reducing the toughness of the steel. Therefore, the Si content needs to be reduced. In the present invention, by setting the Si content to 0.1% or less, the effect of the invention can be obtained without impairing toughness after aging treatment. Therefore, the Si content is set to 0.1% or less. The Si content is preferably 0.08% or less, and more preferably 0.06% or less.

[0070] The lower limit of the Si content is not particularly limited, but from the viewpoint of production cost, the Si content is preferably 0.001% or more, more preferably 0.005% or more.

[0071] <Mn: 0.1% or less>

[0072] Mn is an element that can form oxides in steel, similar to Si. From the perspective of toughness, the Mn content needs to be reduced. In the present invention, by setting the Mn content to 0.1% or less, the effect of the invention can be obtained without damaging the toughness after aging treatment. Therefore, the Mn content is set to 0.1% or less. The Mn content is preferably 0.08% or less, and more preferably 0.06% or less.

[0073] The lower limit of the Mn content is not particularly limited, but from the viewpoint of production cost, the Mn content is preferably 0.001% or more, more preferably 0.002% or more, further preferably 0.005% or more, more preferably 0.01% or more.

[0074] <P: 0.01% or less>

[0075] P is an element that deteriorates the toughness of the steel plate when its content is high. Therefore, the P content is preferably reduced. In the present invention, by setting the P content to 0.01% or less, the effect of the invention can be obtained without impairing the toughness after aging treatment. Therefore, the P content is set to 0.01% or less. The P content is preferably 0.008% or less.

[0076] The lower limit of the P content is not particularly limited, but from the viewpoint of production cost, the P content is preferably 0.001% or more, more preferably 0.002% or more.

[0077] <S: 0.01% or less>

[0078] S is an element that deteriorates the toughness of the steel plate when its content is high. Therefore, the S content is preferably reduced. In the present invention, by setting the S content to 0.01% or less, the effect of the invention can be obtained without damaging the toughness after aging treatment. Therefore, the S content is set to 0.01% or less. The S content is preferably 0.006% or less, and more preferably 0.004% or less.

[0079] The lower limit of the S content is not particularly limited, but from the viewpoint of production cost, the S content is preferably 0.001% or more, more preferably 0.002% or more.

[0080] <N: 0.01% or less>

[0081] N is an element that reacts with Ti to precipitate TiN and hinder the formation of Ni3Ti, which contributes to high temperature strength, similarly to C. Therefore, in the present invention, the effect of the invention can be obtained by setting the N content to 0.01% or less. Therefore, the N content is set to 0.01% or less. The N content is preferably 0.006% or less, and more preferably 0.004% or less.

[0082] The lower limit of the N content is not particularly limited, but from the viewpoint of production cost, the N content is preferably 0.001% or more, more preferably 0.002% or more.

[0083] <Ni:12~25%>

[0084] Ni is an element required for the precipitation of Ni3Ti, which improves the toughness of maraging steel and contributes to high temperature strength. In addition, it is an essential element for forming the transformation processed martensite in the present invention. When the Ni content is less than 12%, when the solid solution is heated to a heating temperature above Ac3 and below Ac3+50°C, the reverse phase transformation from martensite to austenite accompanied by strain introduction does not occur, and the transformation processed martensite cannot be formed. As a result, the desired aging property cannot be obtained. Therefore, the Ni content is set to more than 12%. The Ni content is preferably more than 14%, and more preferably more than 16%.

[0085] On the other hand, when the Ni content exceeds 25%, the morphology of the martensite structure changes from lath to lens or thin plate, and the desired strength cannot be obtained. As a result, the desired aging property cannot be obtained. Therefore, the Ni content is set to 25% or less. The Ni content is preferably 24% or less, and more preferably 23% or less.

[0086] <Co:5~12%>

[0087] Co is an element that promotes the precipitation of intermetallic compounds such as Fe2Mo, and this effect can be obtained with a content of 5% or more. When the Co content is less than 5%, the increase in YS relative to the aging time cannot be fully achieved, and as a result, the k value cannot be ensured to be above 1.05. Therefore, the Co content is set to 5% or more. The Co content is preferably 6% or more, and more preferably 7% or more.

[0088] On the other hand, Co is an element that lowers the martensitic transformation temperature. When the content exceeds 12%, the transformation temperature becomes below room temperature depending on the Ni content, and the morphology of the martensitic structure changes from lath to lens or thin plate, and the desired strength cannot be obtained. As a result, the desired aging property cannot be obtained. Therefore, the Co content is set to 12% or less. The Co content is preferably 11% or less, and more preferably 10% or less.

[0089] <Mo:2~7%>

[0090] Mo is an element that reacts with Fe to precipitate intermetallic compounds such as Fe2Mo. By containing more than 2%, precipitation can be fully promoted and high-temperature strength can be improved. When the content of Mo is less than 2%, the increase in YS relative to the aging time cannot be fully achieved, and as a result, the k value cannot be ensured to be more than 1.05. Therefore, the Mo content is set to more than 2%. The Mo content is preferably more than 3%.

[0091] On the other hand, when the Mo content exceeds 7%, precipitation on the grain boundary proceeds excessively, resulting in a decrease in toughness. Therefore, the Mo content is set to 7% or less. The Mo content is preferably 6% or less, and more preferably 5% or less.

[0092] <Ti: 0.5~1.5%>

[0093] Ti is an element that reacts with Ni in steel, precipitates intermetallic compounds such as Ni3Ti, and improves high temperature strength. This effect can be fully achieved by setting the Ti content to 0.5% or more. When the Ti content is less than 0.5%, the increase in YS relative to the aging time cannot be fully achieved, and as a result, the k value cannot be ensured to be 1.05 or more.

[0094] Therefore, the Ti content is set to 0.5% or more, preferably 0.8% or more, and more preferably 1.1% or more.

[0095] On the other hand, when the Ti content exceeds 1.5%, precipitation on the grain boundary proceeds excessively, resulting in a decrease in toughness. Therefore, the Ti content is set to 1.5% or less. The Ti content is preferably 1.4% or less, and more preferably 1.3% or less.

[0096] <Al: 0.01~0.1%>

[0097] Al is an element required for deoxidation that reacts with O in molten steel to form oxides and removes them, thereby reducing the concentration of O in steel. In order to obtain this effect, it is necessary to contain 0.01% or more. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.03% or more, and more preferably 0.05% or more.

[0098] On the other hand, Al is used as an element that can achieve high strength through precipitation of Ti3Al in maraging steel, but when the content exceeds 0.1%, precipitation on the grain boundary proceeds excessively, resulting in a decrease in toughness. Therefore, the Al content is set to 0.1% or less. The Al content is preferably 0.09% or less.

[0099] The maraging steel of the present invention preferably has a composition containing the above-mentioned basic components and the balance being iron (Fe) and inevitable impurities.

[0100] Next, the structure and characteristics of the maraging steel according to the present invention will be described.

[0101] <Transformed martensite phase with an area ratio of 90% or more>

[0102] In the present invention, the martensitic steel obtained by cooling to room temperature after solution treatment is heated to a temperature of Ac3 or higher and Ac3+50° C. or lower to cause reverse austenite transformation.

[0103] The austenite reverse transformation of a general steel plate is classified as a diffusion type that produces a change from martensite to austenite phase by random rearrangement of atoms. In this regard, by setting the Ni content to more than 12%, the austenite reverse transformation produces a shear-type reverse transformation classified as produced by the coordinated movement of atoms, and through this phase transformation, a phase transformation strain is introduced into the austenite. If such austenite is cooled again, the phase transformation strain continues in the martensitic phase transformation, producing a phase transformation processed martensite with a high density of dislocations that can become precipitation sites for intermetallic compounds. By aging the phase transformation processed martensite, the aging time can be significantly shortened. When the area ratio of the phase transformation processed martensite is less than 90%, the above-mentioned effect of shortening the aging time cannot be obtained. In addition, when the area ratio of the phase transformation processed martensite is less than 90%, the precipitation amount varies depending on the position, and sometimes strength fluctuations occur in the organization, and toughness or high temperature strength decreases.

[0104] Therefore, the area ratio of the transformed martensite is set to 90% or more. The area ratio of the transformed martensite is preferably 93% or more, more preferably 95% or more, and may be 100%.

[0105] The method for measuring transformation processed martensite is described.

[0106] Regarding the area ratio of phase transformation martensite, the plate thickness section was cut out, mirror polished, and then etched with 5% by volume or more nitric acid ethanol etching solution. The scanning electron microscope (SEM: Scanning Electron Microscope) was used to observe and photograph three fields of view at 1000 times at the 1 / 4 position of the plate thickness, and the photographed tissue photos were analyzed for quantification. Phase transformation martensite temporarily reverses to austenite during its formation, so the intermetallic compounds precipitated during the heating process are dissolved in the austenite again, thus presenting a structure without intermetallic compounds, but the intermetallic compounds precipitated during the heating process in the area where the austenite reverse transformation did not occur are confirmed by black contrast. Therefore, the structure without this intermetallic compound is defined as phase transformation martensite, and its area ratio is calculated as a percentage.

[0107] In the maraging steel of the present invention, the remainder other than the transformed martensite phase is 10% or less in terms of area ratio. The remainder may contain recrystallized ferrite and austenite.

[0108] <Ratio of k1 satisfying equation (1) to k2 satisfying equation (2): k2 / k1=k: 1.05 or more>

[0109] YS1=k1logt+YS …Formula (1)

[0110] YS2=k2logt+YS …Formula (2)

[0111] k=k2 / k1 …Equation (3)

[0112] in,

[0113] YS: In the present invention, it refers to the yield strength (0.2% proof stress) of steel before aging treatment without performing treatment in the heating step (heating treatment to a heating temperature of Ac3 or higher and Ac3+50°C or lower and holding time at the heating temperature of 3000 seconds or lower) or treatment in the cooling step (cooling the steel after the heating step at a cooling rate of 8°C / s or higher to a cooling stop temperature of 50°C or lower) after the solutionizing step.

[0114] YS1: In the present invention, it refers to the yield strength of steel after the solutionizing step without subjecting it to the treatment in the above-mentioned heating step, the treatment in the above-mentioned cooling step, and the aging treatment (aging treatment at 400° C. or higher and 550° C. or lower);

[0115] YS2: In the present invention, it refers to the yield strength of the steel after the solution treatment, the heating treatment, the cooling treatment and the aging treatment;

[0116] t: aging time of 1 to 10800 seconds;

[0117] k1: aging coefficient (aging coefficient when aging treatment is performed without performing treatment in the heating process or treatment in the cooling process after the solution treatment process);

[0118] k2: Aging coefficient (aging coefficient in the case where aging treatment is performed after the solution treatment process and the treatment in the cooling process).

[0119] It should be noted that the conditions (temperature conditions, cooling rate conditions, etc.) for the treatment (treatment in the solution treatment process) performed on the steel in order to measure the above-mentioned YS, YS1, and YS2 are all the same between YS, YS1, and YS2, and the conditions (temperature conditions, etc.) for the treatment (aging treatment in the aging treatment process) performed on the steel in order to measure YS1 and YS2 are also all the same between YS1 and YS2.

[0120] As an index of aging property, the formulas shown in Formula (1), Formula (2) and Formula (3) are defined, and maraging steel having a k value of 1.05 or more is regarded as maraging steel having excellent aging property in the present invention.

[0121] The value of k is calculated by the method shown below.

[0122] The value of k can be determined by measuring the yield strength at a plurality of aging times t (at least 0 seconds and two points other than that) and substituting the values ​​into equations (1) and (2).

[0123] Specifically, first, the steel subjected to only the solution treatment specified in the present invention is subjected to aging treatment for various times at a constant temperature selected from a temperature range of 400°C to 550°C, and a JIS No. 5 tensile test piece is cut from the maraging steel subjected to the aging treatment, and a tensile test is performed with N=3 (according to JIS Z2241 (2011)), and k1 is calculated from the formula (1) based on the aging time t and YS1 obtained by the tensile test. YS is the yield strength when the aging time t=0 seconds, that is, when no aging treatment is performed (when no treatment in the heating process and no treatment in the cooling process are performed after the solution treatment process).

[0124] Next, for the steel which has undergone a heating process of heating to a heating temperature of not less than Ac3 and Ac3+50°C and a holding time at the heating temperature of not more than 3000 seconds after the solution treatment specified in the present invention, and a cooling process of cooling at an average cooling rate of not less than 8°C / s to a cooling stop temperature of not more than 50°C, aging treatment is performed for various times at an aging temperature selected from a temperature range of not less than 400°C and not more than 550°C and the same as when k1 is calculated, and the same tensile test is carried out. Based on the aging time t and YS2 obtained by the tensile test, k2 is calculated by formula (2).

[0125] Then, the obtained k1 and k2 are substituted into the formula (3) to obtain the value of k.

[0126] It should be noted that, when calculating k, the aging time t is limited to be greater than 1 second and less than 10800 seconds. When the time t exceeds 10800 seconds, there is almost no change in YS and TS caused by the aging treatment, and it deviates from the purpose of improving the aging property of the present invention. Therefore, the aging time t for calculating k is less than 10800 seconds.

[0127] Next, the method for producing the maraging steel of the present invention will be described.

[0128] The method for producing maraging steel of the present invention comprises: a solution treatment step, in which the steel having the above-mentioned component composition is solution treated at a temperature of 800° C. or higher, and then cooled to room temperature; a heating step, in which the steel after the solution treatment step is heated to a heating temperature of Ac3 or higher and Ac3+50° C. or lower, and the holding time at the heating temperature is set to 3000 seconds or lower; a cooling step, in which the steel after the heating step is cooled at an average cooling rate of 8° C. / s or higher to a cooling stop temperature of 50° C. or lower; and an aging treatment step, in which the steel after the cooling step is aged at 400° C. or higher and 550° C. or lower to obtain maraging steel.

[0129] <Solution treatment process: Solution treatment at a temperature above 800°C, followed by cooling to room temperature>

[0130] A solution treatment is applied to maraging steel, which is manufactured by a process of manufacturing a steel plate or a steel ingot by melting and casting a raw material having the above-mentioned component composition of the present invention, a process of manufacturing a thin steel plate by hot rolling, pickling, cold rolling, etc. by a conventional method after casting, or a process of manufacturing a laminated molded product obtained by a conventional three-dimensional laminated molding method. When the solution temperature in the solution treatment is lower than 800°C, the intermetallic compounds and other structures precipitated in the above-mentioned manufacturing process are not fully dissolved, resulting in deterioration of strength and toughness. Therefore, the solution temperature is set to be above 800°C.

[0131] The solution temperature is preferably 830° C. or higher, more preferably 860° C. or higher. The upper limit is not particularly limited, but from the viewpoint of ensuring strength, the solution temperature is preferably 980° C. or lower, more preferably 950° C. or lower.

[0132] In this step, after the solution treatment, the material is cooled to room temperature. Here, room temperature refers to 5 to 50°C.

[0133] <Heating step: performing heating treatment to a heating temperature of Ac3 or higher and Ac3+50° C. or lower, and setting the holding time at the heating temperature to 3000 seconds or less>

[0134] <Cooling step: Cooling at an average cooling rate of 8°C / s or more to a cooling stop temperature of 50°C or less>

[0135] In the present invention, in order to reversely transform all the martensite phase obtained by cooling after solution treatment into austenite and obtain transformation processed martensite by subsequent cooling, it is necessary to perform heating at a heating temperature of Ac3 or higher.

[0136] On the other hand, when the heating temperature is a temperature exceeding Ac3+50°C, the transformation strain obtained by the reverse transformation of austenite is lost due to recrystallization, and the area ratio of the transformation processed martensite obtained by the subsequent cooling is less than 90%. Therefore, the heating temperature in the heating step is set to be greater than Ac3 and less than Ac3+50°C. The heating temperature is preferably less than Ac3+40°C, and more preferably less than Ac3+30°C.

[0137] In addition, after reaching the heating temperature, when the holding time at the heating temperature exceeds 3000 seconds, recrystallization occurs, and the area ratio of the transformation processed martensite is less than 90%. Therefore, the holding time at the heating temperature is set to less than 3000 seconds. The holding time is preferably less than 1500 seconds, more preferably less than 500 seconds. That is, the time spent until the cooling in the cooling process after the heating process begins is set to within 3000 seconds, preferably within 1500 seconds, and more preferably within 500 seconds.

[0138] The lower limit of the holding time at the heating temperature is not particularly limited, but from the viewpoint of operability, the holding time at the heating temperature is preferably set to 1 second or longer, and more preferably set to 5 seconds or longer.

[0139] In addition, in the heating after the above-mentioned heating process and the cooling after the holding, by setting the average cooling rate to 8°C / s or more, recrystallization can be suppressed, and the martensitic phase transformation occurs sufficiently, ensuring that more than 90% of the phase transformation processed martensite is ensured in terms of area ratio. Therefore, the average cooling rate in the cooling process is set to 8°C / s or more. The average cooling rate is preferably 10°C / s or more, and more preferably 12°C / s or more. In addition, the upper limit of the average cooling rate is not particularly limited. In order not to produce poor shape during cooling, the average cooling rate is preferably 200°C / s or less, and more preferably 100°C / s or less.

[0140] In the above cooling, the cooling stop temperature is set to 50° C. or lower. The lower limit of the cooling stop temperature is not particularly limited, and may be set to room temperature.

[0141] In addition, the above-mentioned Ac3 can be determined by actual measurement based on thermal expansion measurement during heating or electrical resistance measurement using a Formaster test or the like.

[0142] The average cooling rate is obtained by dividing the difference (° C.) between the cooling start temperature (heating temperature) and the cooling stop temperature by the cooling time (s).

[0143] <Aging treatment process: Aging treatment at 400°C or higher and 550°C or lower>

[0144] Maraging steel can obtain high strength by forming fine intermetallic compounds through aging treatment. At an aging temperature (temperature during aging treatment) lower than 400°C, the aging treatment time for obtaining sufficient intermetallic compounds becomes long. Furthermore, at an aging temperature lower than 400°C, sufficient strength increase cannot be obtained, and as a result, the k value is less than 1.05.

[0145] On the other hand, when the aging temperature exceeds 550°C, the intermetallic compounds formed are coarsened, which does not contribute to the increase in strength, and not only that, but also reduces the toughness. Therefore, the aging temperature is set to be 400°C or higher and 550°C or lower. The aging temperature is preferably 420°C or higher, and more preferably 440°C or higher. In addition, the aging temperature is preferably 530°C or lower, and more preferably 510°C or lower.

[0146] As described above, the maraging steel of the present invention can be obtained.

[0147] (member)

[0148] Next, the member of the present invention and a method for producing the same will be described.

[0149] The component of the present invention is a component obtained by subjecting the maraging steel of the present invention to at least one of forming and joining. In addition, the method for manufacturing the component of the present invention includes the step of subjecting the maraging steel of the present invention to at least one of forming and joining to produce the component.

[0150] The maraging steel of the present invention has high aging properties. Therefore, the components obtained by using the maraging steel of the present invention also have high aging properties. In addition, if the components of the present invention are used, weight reduction can be achieved. Therefore, the components of the present invention can be suitably used for, for example, vehicle body frame parts. The components of the present invention also include welded joints.

[0151] The forming process can use general processing methods such as press working without limitation. In addition, the joining process can use general welding such as spot welding and arc welding, rivet joining, caulking joining, etc. without limitation.

[0152] Example

[0153] The present invention is further described in detail in the following examples.

[0154] Through a hot rolling process and a cold rolling process, a thin steel plate of the present invention example and a thin steel plate of a comparative example having the composition shown in Table 1 are manufactured, wherein the hot rolling process is: a steel ingot obtained by melting in a high-frequency vacuum melting furnace and then pouring into a mold is kept at 1250°C for 24 hours under an Ar atmosphere to perform a homogenization treatment, and then, after heating at 1250°C for 1 hour, hot rolling is performed to obtain a hot-rolled plate of 5 to 6 mm; the cold rolling process is: cold rolling is performed at a rolling rate of 50% to obtain a thin steel plate with a plate thickness of 1.4 mm.

[0155] The produced thin steel sheets were subjected to the heat treatments listed in Table 2, and the steel structure and k value measurement results of the obtained maraging steel (thin steel sheet) are shown in Table 2. The cooling stop temperature in the cooling step was set to 25°C.

[0156] k is a ratio shown in the following formula (3) which is related to k1 obtained by the formula (1) and k2 obtained by the formula (2).

[0157] YS1=k1logt+YS …Formula (1)

[0158] YS2=k2logt+YS …Formula (2)

[0159] k=k2 / k1 …Equation (3)

[0160] Here,

[0161] YS: Yield strength (0.2% proof stress) of steel before aging treatment after solution treatment without heating treatment, cooling treatment in cooling treatment, or aging treatment;

[0162] YS1: In the present invention, it refers to the yield strength of steel after the solution treatment process without performing the treatment in the heating process, the treatment in the cooling process, and the aging treatment;

[0163] YS2: In the present invention, it refers to the yield strength of steel after the solution treatment, the cooling treatment and the aging treatment.

[0164] t: aging time of 1 to 10800 seconds;

[0165] k1: aging coefficient (aging coefficient when aging treatment is performed without performing treatment in the heating process or treatment in the cooling process after the solution treatment process);

[0166] k2: Aging coefficient (aging coefficient in the case where aging treatment is performed after the solution treatment step, the treatment in the heating step and the treatment in the cooling step).

[0167] It should be noted that the conditions (temperature conditions, cooling rate conditions, etc.) for the treatment (treatment in the solution treatment process) performed on the steel in order to measure the above-mentioned YS, YS1, and YS2 are all the same between YS, YS1, and YS2, and the conditions (temperature conditions, etc.) for the treatment (aging treatment in the aging treatment process) performed on the steel in order to measure YS1 and YS2 are also all the same between YS1 and YS2.

[0168] YS, YS1, and YS2 were obtained by cutting JIS No. 5 tensile test pieces from steel and performing a tensile test at N=3 (in accordance with JIS Z 2241 (2011)).

[0169] It is understood that the steels of the examples of the present invention shown in Table 2 are maraging steels having k values ​​of 1.05 or more and excellent aging properties.

[0170] On the other hand, in the comparative examples, the values ​​of k were less than 1.05 except for No. 15 and No. 17. In addition, No. 15 and No. 17 had low toughness of steel, so they broke during the stretching and could not be evaluated.

[0171]

[0172]

Claims

1. A maraging steel having: A composition containing, in mass%, C: 0.02% or less, Si: 0.1% or less, Mn: 0.1% or less, P: 0.01% or less, S: 0.01% or less, N: 0.01% or less, Ni: 12 to 25%, Co: 5 to 12%, Mo: 2 to 7%, Ti: 0.5 to 1.5%, Al: 0.01 to 0.1%, and the balance being iron and inevitable impurities; and A steel structure having a transformed martensite phase with an area ratio of 90% or more.

2. A component comprising the maraging steel according to claim 1.

3. A method for producing maraging steel, comprising: A solution treatment step, wherein the steel having the composition described in claim 1 is solution treated at a temperature above 800° C. and then cooled to room temperature; A heating step, wherein the steel after the solutionizing step is subjected to a heating treatment to a heating temperature of Ac3 or higher and Ac3+50° C. or lower, and the holding time at the heating temperature is set to be 3000 seconds or less; a cooling step of cooling the steel after the heating step at an average cooling rate of 8° C. / s or more to a cooling stop temperature of 50° C. or less; and The aging treatment step is to perform an aging treatment at 400° C. or higher and 550° C. or lower on the steel after the cooling step to obtain maraging steel. 4 . A method for manufacturing a component, comprising the step of subjecting the maraging steel according to claim 1 to at least one of forming and joining to produce the component.

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