JP2020050917A - Martensitic stainless steel for high hardness and high corrosion resistant applications, excellent in cold workability, and manufacturing method therefor - Google Patents

Abstract

To provide a martensitic stainless steel for high hardness and high corrosion resistant applications, with extremely improved softening property at low cost as a raw material for cold processing or cold forging with a complicated shape.SOLUTION: There is provided a martensitic stainless steel for high hardness and high corrosion resistant applications, excellent in cold workability, containing, by mass%, C:0.12 to 0.70%, Si:1.0% or less, Mn:1.5% or less, S:0.01% or less, P:0.05% or less, Ni:1.5% or less, Cr:10.5 to 16.0%, Mo:0.9 to 3.0%, N:0.14% or less, and Al:0.008 to 1.0, and having C+N/2:0.14 to 0.70%, 10 or more carbonitride with 1 μm or more existing in 1600 μm, and Hv hardness of 200 or less. Further preferably O:0.001 to 0.008% is contained and average Al concentration of oxide with diameter of 1 to 5 μm or less is 15 to 40 mass%.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a part for cold working and relates to a martensitic stainless steel for high hardness and high corrosion resistant parts which has excellent softening properties and can be subjected to strong cold working, and a method for producing the same.

  In recent years, there is a great need for high hardness and high corrosion resistant martensitic stainless steel, and it is used for many automobile parts and screw fastening parts (Patent Documents 1 to 4). These high hardness and high corrosion resistant martensitic stainless steel parts, particularly large automobile parts, are formed into a complex shape by cold working such as cold forging. For this reason, it is desired that the softening annealing is performed before the cold working and that the stainless steel after the softening annealing is in a softened state having an Hv hardness of 200 or less, preferably an Hv hardness of 180 or less. After cold working, quenching is performed to obtain high hardness and high corrosion resistance martensitic stainless steel.

  However, since high-hardness, high-corrosion-resistant martensitic stainless steel contains a large amount of alloying elements such as C, N, Mo, and Ni, it is sufficiently softened by softening annealing and has excellent cold workability (cold workability). It is difficult to secure forgeability). For example, Patent Document 5 proposes a component design and a soft annealing technique for improving cold forgeability, but fails to soften to a level required for the present invention.

  As described above, according to the conventional technique, it is not possible to sufficiently soften martensitic stainless steel for high hardness and high corrosion resistance by softening annealing and cold-work (cold forging) into a complex shape.

Japanese Patent No. 3340225 Japanese Patent No. 4252145 JP-A-2006-50320 Japanese Patent No. 3587330 Japanese Patent No. 3328791

  The problem to be solved by the present invention is to provide a martensitic stainless steel for high-hardness and high-corrosion-resistant use, which has significantly improved softening properties, as a material for cold working of a complex shape or a cold forged part, and a method for manufacturing the same. It is to provide to. In the stainless steel after softening and annealing, which is a target of the present invention, the steel structure is composed of ferrite and carbonitride, and is not a martensite structure. On the other hand, the stainless steel of the present invention is quenched after cold working, and the final product has a martensitic structure. Therefore, the stainless steel of the present invention is referred to as a martensitic stainless steel.

  The present inventors have conducted various studies to solve the above-described problems, and as a result, in a martensitic stainless steel for high corrosion resistance and high hardness applications in which components are adjusted, control the dispersion state of fine carbonitrides by softening annealing at high temperature. By doing so, it was found that the Hv hardness was softened to 200 or less and the cold workability was remarkably improved. Further, it is more preferable to control the composition of a fine deoxidation product that does not pin the dislocations and the grain boundaries to an Al-containing system.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) In mass%,
C: 0.12 to 0.70%,
Si: 1.0% or less,
Mn: 1.5% or less,
S: 0.01% or less,
P: 0.05% or less,
Ni: 1.5% or less,
Cr: 10.5 to 16.0%,
Mo: 0.9 to 3.0%,
N: 0.01 to 0.14%,
Al: contains 0.008 to 1.0%,
Having a chemical composition consisting of the balance Fe and unavoidable impurities,
C + N / 2: 0.14 to 0.70%,
A martensitic stainless steel characterized in that there are at least 10 carbonitrides of not less than 1.0 μm in 1600 μm ² and the Hv hardness is not more than 200.
(2) Further, in mass%,
O: contains 0.001 to 0.008% or less,
The martensitic stainless steel according to (1), wherein the oxide having a diameter of 1 to 5 μm has an average Al concentration of 15 to 40% by mass.
(3) Further in mass%,
Cu: 1.5% or less,
W: 1.5% or less,
Co: 1.5% or less,
B: 0.01% or less,
Sn: 0.3% or less,
Sb: The martensitic stainless steel according to the above (1) or (2), which contains one or more kinds of 0.3% or less.
(4) In mass%,
Nb: 0.1% or less,
Ti: 0.1% or less,
V: 0.2% or less,
The martensitic stainless steel according to any one of the above (1) to (3), wherein one or more of Ta: 0.2% or less are contained.
(5) Further, in mass%,
Mg: 0.01% or less,
Ca: 0.01% or less,
Hf: 0.01% or less,
REM: The martensitic stainless steel according to any one of the above (1) to (4), which contains one or more kinds of 0.01% or less.

(6) As the soft annealing treatment,
The heat treatment is performed at a temperature higher than 870 ° C. and a C concentration and a solid solution temperature of carbide represented by the following formula (a): 20 to 120 ° C. lower than T for 1 to 48 hours, and subsequently an average of 60 ° C./h or less. The method for producing martensitic stainless steel according to any one of the above (1) to (5), wherein cooling is performed at a cooling rate of 250 ° C. lower than T.
log (C) =-6100 / (T + 273) +4 (a)
In the formula (a), “C” means C concentration (% by mass), and “T” means solid solution temperature (° C.) of carbide.

  ADVANTAGE OF THE INVENTION According to this invention, strong cold forging (cold working) or near-net forming is possible for a complicated component shape, and the effect of a large reduction in component cost by cold forging (cold working) for automobiles can be exhibited. A martensitic stainless steel for high hardness and high corrosion resistance can be provided.

The metallographic structure of a 13Cr-2Mo-0.2C-0.1N system steel when softened and annealed by a known method is shown. The metallographic structure of 13Cr-2Mo-0.2C-0.1N system steel when softened and annealed by the method of the present invention is shown.

  Hereinafter, each requirement of the present invention will be described. In the following description, (%) is mass (%) unless otherwise specified.

  The present invention is directed to a martensitic stainless steel having a high hardness and high corrosion resistance, which is excellent in cold workability, is a stainless steel softened by softening and annealing the steel, and has a steel structure of ferrite and carbonitride. Consists of Cold working is performed using the softened stainless steel of the present invention as a raw material, followed by quenching treatment to increase the hardness to obtain a final product. In addition, by containing the component composition of the present invention described below, most of the steel has a martensite structure by quenching, and can be a martensitic stainless steel. Specifically, about 80% or more of the structure is a steel having a martensite structure by a quenching treatment from 1000 to 1200 ° C.

  The effect of improving the cold workability by the softness of the present invention is remarkably exhibited in a martensitic stainless steel having high hardness and high corrosion resistance of 500 Hv or more after quenching. For steels with a maximum quenching of less than 500 Hv, sufficient cold workability can be ensured by conventional techniques, and the effect of the present invention becomes unclear. Therefore, the content of C, N, C + N / 2, which controls the quenching hardness, is limited, and the range where the effect of the present invention is clear is defined.

  C is limited to 0.12% or more, and C + N / 2 is limited to 0.14% or more. However, if the content of C exceeds 0.70% and C + N / 2 exceeds 0.70%, coarse carbides and fine nitrides increase and the cold workability deteriorates. . The preferred range of C is 0.14 to 0.40%, and the preferred range of C + N / 2 is 0.18 to 0.45%.

  N is contained in an amount of 0.01% or more in order to secure the corrosion resistance of the product in addition to the quenching hardness described above. However, when N is contained in more than 0.14%, coarse carbonitrides are generated, and the cold workability is deteriorated, so the upper limit is made 0.14%. The preferred range is 0.02 to 0.11%. More preferably, it is 0.04 to 0.10%.

  Since Si is an element that solid-solution strengthens the ferrite structure during soft annealing and finely disperses carbon nitride to deteriorate cold workability, its content is limited to 1.0% or less. Preferably, it is 0.7% or less. Si does not have to be contained.

  Mn limits the content to 1.5% or less because it increases the strength after soft annealing and deteriorates cold workability. Preferably it is 1.2% or less. Mn may not be contained.

  S forms a sulfide and deteriorates the cold workability, so the content is limited to 0.01% or less. Preferably it is 0.008% or less.

  Since P segregates at the grain boundaries and deteriorates cold workability, the content of P is limited to 0.05% or less.

  Ni is an element that improves the toughness and corrosion resistance of the martensitic stainless steel product, and therefore may be included. However, when the content exceeds 1.5%, the strength after softening annealing is not softened to 200 or less in Hv hardness, and the cold workability deteriorates. Therefore, it is limited to 1.5% or less. Preferably, it is at most 1.3%. Ni may not be contained.

  Cr is a basic element for obtaining the function of high corrosion resistance of stainless steel, and contains 10.5% or more. However, when the content exceeds 16.0%, the product hardness of high hardness which is a feature of the present invention cannot be obtained, and the cold workability can be secured by the conventional technology. Therefore, it is limited to 16.0% or less. The preferable range of Cr is 11.0 to 15.0%.

  Mo is contained to obtain a high corrosion resistant martensitic stainless steel. In addition, it is an element which inhibits coarsening of carbonitride during softening annealing and makes it difficult to soften the material, and is limited to 0.9% or more, at which the effect of the soft and high cold workability of the present invention becomes clear. . If it is less than 0.9%, cold workability can be ensured by a known softening annealing method, and the effectiveness of the present invention is not clear. On the other hand, if the content is excessively more than 3.0%, the growth of carbonitrides described later is suppressed, so that even with the method of the present invention, carbonitrides are hardly coarsened and softened, and cold workability is reduced. to degrade. Therefore, it is limited to 3.0% or less. A preferred range is 1.0-2.5%.

  Al is an element effective for reducing deoxidation products by deoxidation, so that Al is contained in an amount of 0.008% or more. However, the addition of more than 1.0% not only saturates the deoxidizing effect, but also forms a coarse oxide and significantly deteriorates the cold workability. Therefore, the upper limit is limited to 1.0%. Preferably it is 0.01-0.2%.

The distribution of carbonitride in the martensitic stainless steel (after soft annealing) of the present invention affects the softening behavior (softening behavior after soft annealing) of the martensitic stainless steel, and after the soft annealing. If carbonitrides are finely dispersed in the steel, it is difficult to perform cold working by pinning the movement of dislocations and grain boundaries in cold working (after softening annealing). The larger the carbonitride size is, the better. If the number of carbonitrides of 1.0 μm or more in 1600 μm ² is 10 or more, the number of fine carbonitrides of less than 1.0 μm is reduced. Chemical properties are obtained. FIG. 1 shows a microstructure of a 13Cr-2Mo-0.16C-0.1N system steel when soft-annealed by a known method (low-temperature annealing at 650 ° C. for 4 hours). Submicron rod-shaped carbides are precipitated at the interface of the lath martensite structure, and have a Hv hardness of 305 even after softening annealing, which is inferior in cold workability. On the other hand, FIG. 2 shows an example of steel softened and annealed by the method of the present invention described later. In FIG. 2, there are 10 or more carbonitrides having a size of 1.0 μm or more in 1600 μm ² , and softened to an Hv hardness of 200 or less. The larger the carbonitride size is, the better, the softening property is obtained when the number of carbonitrides having a size of 1 μm or more in 1600 μm ² is 10 or more. Preferably, 1600 .mu.m carbonitride or more sizes 2μm in ² is 10 or more. Here, the carbonitride size refers to (long diameter + short diameter) / 2 of carbonitride.

  In order to achieve a remarkable effect by cold working to a complicated shape by softening compared to the known technology, the Hv hardness of the stainless steel (after soft annealing) of the present invention is limited to 200 or less. Further, when the Hv hardness is 180 or less, cold forging to a large-sized component having a complicated shape becomes possible, and the industrial and economical effect is greatly increased. is there.

Next, each requirement of the present invention described in claim 2 will be described.
Since the deoxidized product is decomposed and refined during hot rolling, it suppresses softening of the material during soft annealing. Therefore, the composition of the deoxidized product is controlled by the Al content and the O content of the deoxidizing element, the solidification conditions, and the like, and the deoxidized product does not pin dislocations or crystal grain boundaries and does not induce cold working cracks. Controlling the size is preferable because softening can be further promoted. The requirements are described below.

  O greatly affects the composition and size of fine deoxidation products (oxides) in Al-containing steel. The average diameter of the deoxidized product during solidification is reduced to 5 μm or less to make it substantially harmless to cold working cracks, and the deoxidized product is thermally stabilized to less than 1 μm during hot rolling. It is important to suppress decomposition and miniaturization into sizes. Therefore, it is preferable to limit O in the steel to about 0.014% or less because formation of coarse oxides can be suppressed. More preferably, the content of O in steel is set to 0.001 to 0.008%. O is T. Means O. The average Al concentration in the deoxidized product (oxide) is obtained by setting the O content range to the more preferable range shown on the left and setting the Al content to the preferable range of 0.01 to 0.2%. Is 15 to 40% by mass, and the deoxidized product is thermally stabilized to suppress decomposition and miniaturization. If O is less than 0.001%, it is difficult to carry out industrially, and if it exceeds 0.008%, the average Al concentration in the deoxidized product exceeds 40% and a coarse oxide is formed. Therefore, there is a concern that the cold workability may deteriorate. Preferably, it is 0.002 to 0.006%.

  Fine deoxidation products in steel are generated during solidification, and when thermodynamically unstable, decomposition and refinement progress by hot working such as hot rolling, and the movement of dislocations and grain boundaries during softening annealing. Pinning inhibits softening. In the case of martensitic stainless steel, when the average cooling rate of the cast slab between 1500 and 1300 ° C. is in the range of 1 to 500 ° C./s, the average size of the deoxidized product generated at the time of solidification becomes 5 μm or less. Softening can be achieved by suppressing the miniaturization. If the average cooling rate is lower than 1 ° C./s, the deoxidized product becomes coarser than 5 μm, and not only the softening effect of the present invention becomes unclear, but also the cold workability deteriorates. On the other hand, if the average cooling rate is higher than 500 ° C./s, the deoxidized product becomes finer than 1 μm, so that it becomes difficult to promote the softening of the material even when thermally stabilized. Therefore, it is preferable to limit the size of the fine oxide specified in the present invention to a size of 1 to 5 μm, which is a size generated when solidified at an average cooling rate of 1 to 500 ° C./s. Here, the size of the oxide is the average diameter of the oxide, and is (major axis + minor axis) / 2.

  Here, the average Al concentration of the oxide is evaluated for an oxide having a size of 1 to 5 μm. This is because the peak of the size distribution of the oxide is about 1 to 5 μm, and the size indicates a typical composition of the oxide (deoxidation product). If it is less than 1 μm, it is difficult to define it because of analytical accuracy, and if it is more than 5 μm, it may be an irregular oxide such as a slag-based oxide, so it was excluded. In addition, the average composition in the oxide is a value obtained by converting in terms of mass% including O, excluding S element in nonmetallic inclusions. The generation of Al-containing thermodynamically stable (not decomposed during hot rolling and not miniaturized) Al-containing deoxidation products that are difficult to pin the movement of dislocations and grain boundaries during soft annealing is softening during soft annealing. It is effective for promotion.

Next, each requirement of the present invention described in claim 3 will be described.
Cu may be contained as necessary to improve the corrosion resistance of the product. However, even if the content exceeds 1.5%, the effect is saturated and the cold workability is deteriorated. Therefore, the content is set to 1.5% or less. Preferably, it is at most 0.35%.

  Co and W may be contained as needed to improve the toughness and corrosion resistance of the product. However, even if the content exceeds 1.5%, the effect is saturated and the cold workability is deteriorated. Therefore, the content is set to 1.5% or less. Preferably, it is 1.0% or less.

  B may be contained as necessary to improve the toughness of the product. However, if the content exceeds 0.01%, the effect is saturated and conversely, coarse boride is generated to deteriorate the cold workability, so the content is set to 0.01% or less. Preferably, it is 0.006% or less.

  Sn and Sb may be contained as needed in order to improve the corrosion resistance of the product. However, if the content exceeds 0.3%, the effect is saturated and the hot workability is remarkably deteriorated. Therefore, the content is set to 0.3% or less. Preferably, it is 0.1% or less.

Next, each requirement of the present invention described in claim 4 will be described.
Nb and Ti may be contained as necessary to improve the toughness and corrosion resistance of the product. However, even if the content exceeds 0.1%, the effect is saturated, and conversely, coarse carbonitrides are generated to deteriorate the cold workability, so that the content is 0.1% or less. I do. Preferably, it is 0.06% or less.

  V and Ta may be contained as necessary in order to improve the toughness and corrosion resistance of the product. However, even if the content exceeds 0.2%, the effect is saturated, and conversely, coarse carbonitrides are generated and the cold workability is deteriorated, so that the content is 0.2% or less. I do. Preferably, it is 0.1% or less.

Next, each requirement of the present invention described in claim 5 will be described.
Mg, Ca, Hf, and REM increase the thermodynamic stability of the deoxidized product and are effective in softening during softening annealing. Therefore, Mg, Ca, Hf, and REM may be contained as necessary. However, even if added over 0.01%, the effect is saturated and conversely, a coarse oxide is generated to deteriorate the cold workability, so the content is made 0.01% or less. . Preferably, it is 0.005% or less.
REM (rare earth element) is a general definition of two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu) according to a general definition. They may be contained alone or in a mixture.

The stainless steel of the present invention is composed of a chemical component consisting of Fe and inevitable impurities, other than the above-mentioned elements.
Typical unavoidable impurities include P, S, Zn, Bi, Pb, Ge, Se, Ag, Se, Te, and the like. Is mixed in a range of about 1%.
In addition, typical optional elements are defined in the above (3) to (5), but elements that are not described in the present specification do not impair the effects of the present invention. Can be contained.

Next, each requirement of the present invention described in claim 6 will be described.
In order to make the stainless steel softened by the size and dispersion of the carbonitride described above, it is necessary to carry out the softening and annealing treatment at a high temperature of 870 ° C. or more and at the high temperature of carbide calculated by the following equation (a). Solid solution temperature: It is preferable to perform a holding heat treatment for 1 to 48 hours in a temperature range lower than T by 20 to 120 ° C., and then gradually cool at an average cooling rate of 60 ° C./h or less. If the retention heat treatment time is shorter than 1 hour, the size of the carbon nitride becomes fine and softening cannot be expected. Conversely, if it is longer than 48 hours, the effect is saturated and industrial economic rationality is lost. Therefore, the retention heat treatment time is limited to 1 to 48 h. A preferred range is from 2 to 10 h. Note that the solid solution temperature of the carbide based on the C content can be calculated by the equation (a).
log (C) =-6100 / (T + 273) +4 (a)
In the formula (a), “C” means C concentration (% by mass), and “T” means solid solution temperature (° C.) of carbide.

  When the temperature of the heat treatment is 870 ° C or lower than (T-120) ° C, the size of the carbonitride becomes finer and the softening cannot be expected. Conversely, the heat treatment is performed at a temperature higher than (T-20) ° C. Then, it becomes a film-like grain boundary carbide, and the cold workability deteriorates. The preferred range of the retention heat treatment temperature is 900 ° C. or higher and 30 to 100 ° C. lower than T.

  Regarding the cooling rate of the slow cooling from the retention heat treatment temperature, if the cooling rate is an average of more than 60 ° C./h, the carbonitride becomes fine and softening cannot be expected.

  Regarding the temperature of the end of slow cooling, if the temperature is not gradually cooled to (T-250) ° C., softening cannot be expected due to the refinement of carbonitrides and the formation of hard martensite structure. Therefore, it is preferable to gradually cool to a temperature lower than (T-250) ° C. At a temperature lower than (T-250) ° C., the cooling rate need not be particularly specified.

  Since the size of the carbide and the dispersion state are determined by the above-described softening annealing method of the present invention, the effect is continued even if the conventional annealing method is applied after the annealing method of the present invention, so that it may be combined with the conventional annealing method. .

  According to the present invention described above, a martensitic stainless steel having a softening property and capable of strong cold working into a complex shape for high hardness and high corrosion resistance can be provided at low cost.

<< Example 1 >>
Steel having the chemical composition shown in Tables 1 and 2 was melted in a 150 kg vacuum melting furnace and cast into a mold having a diameter of 200 mm. The amount of O was varied depending on the amount of the deoxidizing element added, such as Al, Si, and Mn, and the time from the introduction of the deoxidizing element to the molten steel to the tapping time to the mold. Then, after heating at 1200 ° C., hot forging was performed to process to a diameter of 70 mm. Next, it was annealed at 800 ° C. for 1 hour, peeled to a diameter of 66 mm, hot-extruded at 1150 ° C. as hot roll into steel bars having a diameter of 14 mm assuming hot rolling, and air-cooled to room temperature. Thereafter, as softening annealing, a holding heat treatment was performed for 5 hours at each temperature shown in Tables 1 and 2, and the temperature was gradually cooled to 650 ° C. at 20 ° C./h. Then, the effects of components on the state of softening, cold workability, and the state of carbonitrides and fine oxides were investigated. Tables 3 and 4 show the survey results.

  Regarding the state of softening, the bar of 20 mm length was embedded in the center section in the longitudinal direction and polished, and the Hv hardness at the center of the section was evaluated under a load of 1 kg.

  For cold workability, is it possible to make a compression test piece of φ8 mm and height 12 mm from the obtained steel bar, apply end face compression at a strain rate of 10 / s in the height direction, and perform cold compression without cracking? Judgment was made based on no. When cold compression was possible at a processing rate of 75%, it was evaluated as ○, when cracks occurred, ×, and when cold compression was possible at a processing rate of 80%, it was evaluated as ◎. The working ratio is (12−H) / 12 × 100 (%), and H is the thickness (mm) of the test piece after cold compression working.

The dispersed state of the carbonitride was evaluated by etching the buried polished surface with aqua regia and SEM / EDS. ○ a case where 1600 .mu.m carbonitride or more in diameter 1μm size during ² is 10 or more, carbonitride or more in diameter 2μm size in 1600 .mu.m ² were evaluated when there 10 or more in ◎. The size of the diameter is calculated by (major axis + minor axis) / 2. Carbonitride is a precipitate mainly composed of Cr, Fe, C, and N in EDS analysis.

  Regarding the oxide composition evaluation, a steel material whose surface layer was polished # 500 in a non-aqueous solution (3% maleic acid + 1% tetramethylammonium chloride) The mixture was electrolyzed (constant voltage of 100 mV) with methanol) to dissolve the matrix, and the solution was filtered with a filter to extract an oxide. After that, the oxide remaining on the filter was subjected to composition analysis by SEM / EDS by selecting 20 oxides having a diameter of 1 to 5 μm. In addition, the composition analysis of the oxide was similarly performed on the hot-pressed material, and it was confirmed that the state of the oxide did not change during the main quenching treatment. The diameter size is calculated by (major axis + minor axis) / 2. An oxide is a nonmetallic inclusion mainly composed of O, Al, Mn, Si, Fe, Cr, Ti, and the like by EDS analysis. Excluding S, the mass% of each element is calculated as 100%. Calculated.

  Regarding the evaluation of the coarse oxide, the embedded abrasive was observed with an optical microscope, and when there was a coarse oxide having a major axis of 30 μm or more, it was described in the remarks column.

The martensitic stainless steel of the present invention is used for high hardness and high corrosion resistance, and is required to have high hardness and high corrosion resistance at the stage of performing a quenching treatment after cold working to obtain a final product. You.
Regarding the high hardness characteristics, Hv hardness was evaluated by performing air cooling quenching from a temperature of T + 50 ° C. after cold working. If the Hv is 500 or more, the requirements of the present invention are satisfied. In the examples, when the Hv is less than 500 after quenching, "Insufficient quenching hardness" is described in the remarks column of Table 2.
Regarding the corrosion resistance property, after cold working, air cooling quenching was performed from a temperature of T + 50 ° C. After polishing the surface # 500, the corrosion resistance was evaluated by JIS neutral salt water spray test for 24 hours with salt water spray, and no red rust was generated. It has good corrosion resistance. In the examples, when red rust was generated (excluding the end portion), "Insufficient corrosion resistance" was described in the remarks column of Table 2.

  Examples 1 to 46 in Tables 3 and 4 are examples of the present invention. Regarding the hardness of the product, the steel of the present invention obtained an Hv hardness of 200 or less, and a preferable Hv hardness of 180 or less was obtained in most cases. Regarding the cold workability, the steels of the present invention were all ○ or ◎, indicating excellent cold workability. The dispersion state of carbonitrides was all ○ or ◎ in the steels of the present invention, indicating the dispersion state of carbonitrides contributing to excellent cold workability.

  In Examples 3 to 44 and 46, the Al content is in a preferable range of 0.01 to 0.2%, and the average Al concentration of the oxide of the present invention in the size of 1 to 5 μm is 15 to 40% by mass. , Indicating an oxide state contributing to softening.

  On the other hand, in Examples 47 to 61, which are comparative steels, it is understood that the component composition of the steel does not satisfy the specified range of the present invention and does not satisfy the required characteristics.

<< Example 2 >>
Next, the influence of the manufacturing method of the soft annealed material was investigated. The hot-extruded φ14 mm steel bar of the steel C of the present invention manufactured as described above was subjected to softening annealing under various conditions shown in Table 5, and the effect of the manufacturing method on softening, cold workability and carbonitride state. Was investigated. In addition, since the state of the fine oxide does not change during the soft annealing, the investigation is not performed in this section. Table 5 shows the method of producing the soft annealed material and the results of the investigation.

  In each of the examples of the present invention, the dispersion state of carbonitride contributing to excellent cold workability was exhibited, and the cold forgeability was excellent. In Example 72, as described in the remarks of Table 5, "Addition of softening annealing", after softening annealing of the present invention, after holding at 850 ° C for 2 hours in the past, gradually cooling down to 700 ° C at 30 ° C / h to remove Although the softening annealing of the furnace is given, the effect of the present invention is inherited.

  On the other hand, in Examples 73 to 77 which are comparative examples, the softening and annealing conditions did not satisfy the specified range of the present invention, and did not satisfy the dispersion state of carbonitride and the excellent cold forgeability of the present invention. I understand.

  As is clear from the above examples, the present invention stably provides a softened and annealed material of martensitic stainless steel for high hardness and high corrosion resistance, which is excellent in cold workability such as cold forgeability. By mass-producing by cold forging, the manufacturing cost of parts can be greatly reduced, which is extremely useful in industry.

Claims (6)

In mass%,
C: 0.12 to 0.70%,
Si: 1.0% or less,
Mn: 1.5% or less,
S: 0.01% or less,
P: 0.05% or less,
Ni: 1.5% or less,
Cr: 10.5 to 16.0%,
Mo: 0.9 to 3.0%,
N: 0.01 to 0.14%,
Al: contains 0.008 to 1.0%,
Having a chemical composition consisting of the balance Fe and unavoidable impurities,
C + N / 2: 0.14 to 0.70%,
A martensitic stainless steel characterized in that there are at least 10 carbonitrides of not less than 1.0 μm in 1600 μm ² and the Hv hardness is not more than 200. In mass%,
O: contains 0.001 to 0.008% or less,
The martensitic stainless steel according to claim 1, wherein the oxide having a diameter of 1 to 5 µm has an average Al concentration of 15 to 40% by mass. In mass%,
Cu: 1.5% or less,
W: 1.5% or less,
Co: 1.5% or less,
B: 0.01% or less,
Sn: 0.3% or less,
The martensitic stainless steel according to claim 1 or 2, wherein one or more of Sb: 0.3% or less are contained. In mass%,
Nb: 0.1% or less,
Ti: 0.1% or less,
V: 0.2% or less,
The martensitic stainless steel according to any one of claims 1 to 3, wherein one or more types are contained among Ta: 0.2% or less. In mass%,
Mg: 0.01% or less,
Ca: 0.01% or less,
Hf: 0.01% or less,
The martensitic stainless steel according to any one of claims 1 to 4, wherein at least one of REM: 0.01% or less is contained. As soft annealing treatment,
The heat treatment is performed at a temperature higher than 870 ° C. and a C concentration and a solid solution temperature of carbide represented by the following formula (a): 20 to 120 ° C. lower than T for 1 to 48 hours, and subsequently an average of 60 ° C./h or less. The method for producing martensitic stainless steel according to any one of claims 1 to 5, wherein the cooling is performed at a cooling rate of 250 ° C lower than T.
log (C) =-6100 / (T + 273) +4 (a)
In the formula (a), “C” means C concentration (% by mass), and “T” means solid solution temperature (° C.) of carbide.