JP4952708B2 - Martensitic stainless steel and method for producing the same - Google Patents

Description

  INDUSTRIAL APPLICABILITY The present invention is suitable for steel materials such as oil wells for crude oil, natural gas, etc., gas wells (hereinafter collectively referred to simply as “oil wells”), or steel pipes for their transportation, and is resistant to corrosion and stress corrosion cracking. The present invention relates to martensitic stainless steel excellent in strength and toughness and a method for producing the same.

  In oil well environments containing carbon dioxide and trace amounts of hydrogen sulfide, 13% Cr martensitic stainless steel is often used because corrosion resistance, stress corrosion cracking resistance, weldability, toughness and strength are generally required. . Specifically, API-13% Cr steel (13% Cr-0.2% C) defined by API (American Petroleum Institute) is often used because it has good carbon dioxide corrosion resistance. However, this API-13% Cr steel has a relatively low toughness, and can sufficiently withstand the use as a yield stress of 552 to 655 MPa (80 to 95 ksi) class, which is required as a general oil well pipe strength. At a high strength of 759 MPa (110 ksi) or higher yield stress required for the development of deep oil wells, there is a problem that the toughness is lowered and cannot be used.

  In recent years, for the purpose of improving corrosion resistance, improved 13% Cr steel has been developed in which the C content is extremely low and Ni is added instead. This improved 13% Cr steel is used in a severer corrosive environment, and since it can secure good toughness even if it has a high strength, it has been used in an environment where high strength is required. However, if the C content is reduced, δ ferrite that is harmful to hot workability, corrosion resistance, toughness, etc. is likely to precipitate. Therefore, it is necessary to contain expensive Ni in an appropriate amount depending on the amount of added Cr, Mo, etc. There is a problem that the price will rise significantly.

In order to improve such a problem, several attempts to improve toughness with high strength in 13Cr steel have been proposed. For example, Patent Document 1 discloses an attempt to improve strength and toughness by using effective N that is not fixed to Al based on API-13% Cr steel. However, in this prior art, as shown in the examples, the fracture surface transition temperature of the Charpy impact test at yield stress 552 to 655 MPa (80 to 95 ksi) class is at most about -20 to -35 ° C. Therefore, it is not a means for ensuring toughness even at a high strength of 759 MPa (110 ksi) or higher.

On the other hand, many techniques for utilizing retained austenite for improving the properties of 13% Cr steel have been shown. Patent Document 2 discloses a martensitic structure having a high C concentration after performing segregation of C in the austenite phase newly formed at the prior austenite grain boundaries by performing two-phase heating before tempering. Discloses a technique of refining to low strength and high toughness by forming coarse carbides.

In Patent Document 3 , C and Ni are concentrated in austenite by two-phase region heating, the C and Ni concentrations in the martensite which is the parent phase are reduced, and the effect of solid solution strengthening is reduced. A technique for tempering strength and toughness is disclosed.

  However, all of the techniques disclosed in these documents are intended to stably temper low strength and high toughness, and are means for ensuring high strength and high toughness as a property improvement of 13% Cr steel. It is not.

Furthermore, a technique for obtaining high strength and high toughness steel by utilizing retained austenite in steel is also disclosed. In Patent Document 4 , in a 13Cr steel containing C, by performing two-phase region heating of Ac _{1 to} Ac ₃ , a reverse-transformed austenite remains in the martensite matrix, and then tempered at Ac ₁ or less. Techniques for performing processing to ensure high strength and high toughness are shown. However, this publication does not disclose a high-strength material having a yield stress of 759 MPa (110 ksi) or higher necessary for the development of a deep oil well.

Patent Document 5 discloses that a modified 13% Cr steel having a low C content causes austenite to remain in the martensite matrix by performing two-phase heating of Ac _{1 to} Ac ₃ , thereby increasing the strength. Techniques for ensuring high toughness are shown. However, although the steel shown in the publication can obtain high toughness, it is characterized by adding a considerable amount of expensive Ni and further controlling the heat treatment conditions to a narrow range to precipitate residual austenite. . For this reason, the steel of the above publication has a problem that the price is significantly higher than that of API-13% Cr steel.

JP-A-8-120415 Japanese Patent Laid-Open No. 5-112818 JP-A-8-260038 JP-A-11-310823 JP 2000-226614 A

As disclosed in Patent Document 2 and Patent Document 5 described above, means for improving the toughness of 13% Cr steel by making retained austenite present in the steel is known. However, it is also known that the presence of retained austenite is a means for obtaining low strength (for example, Patent Document 3 ). Thus, the presence of residual austenite means that the toughness of the steel is improved, but at the same time the strength is reduced.

Furthermore, as shown in Patent Document 4 and Patent Document 5 described above, methods for obtaining high strength and tough steel by utilizing retained austenite have been proposed. Steel materials that can be used even at a yield stress of 759 MPa (110 ksi) or higher, which is necessary for the development of the steel, are not shown.

  The present invention has been made in view of the above-mentioned problems of the prior art, has the corrosion resistance required for oil wells, and has the high strength, toughness, and low-cost martens required for the development of deep oil wells. The object is to provide a site-based stainless steel and a method for producing the same.

  In order to solve the above-mentioned problems, the present inventors have studied a steel that satisfies the high strength that can be applied even at a yield stress of 759 MPa or higher, and has excellent toughness and low cost. It has been found that by controlling the amount of precipitation appropriately, high strength and toughness can be secured even if the amount of Ni added is reduced.

The present invention has been completed based on such knowledge, and the gist of the following (1) martensitic stainless steel and (2) and (3) martensitic stainless steel manufacturing methods. It is.

(1) By mass%, C: 0.01 to 0.1%, Si: 0.05 to 1%, Mn: 0.05 to 1.5%, P: 0.03% or less, S: 0.0. 0.1% or less, Cr: 9~15%, Ni: 0.1~2.0%, Al: 0.05% or less and N: 0.1% or less of free Mutotomoni, Cu: 0.05~4%, Mo: 0.05-3%, V: 0.005-0.5%, Nb: 0.005-0.5%, B: 0.0002-0.005%, Ca: 0.0003-0. Steel containing at least one of 005%, Mg: 0.0003 to 0.005% and REM: 0.0003 to 0.005%, the balance being Fe and impurities, and measured by the following method The retained austenite phase has a thickness of 100 nm or less, and the X-ray integrated intensities 111γ and 110α satisfy the following formula (a): Nsite stainless steel.

0.005 ≦ 111γ / (111γ + 110α) ≦ 0.05 (a).
However, 111γ: X-ray integrated intensity of the austenite phase (111) plane,
110α: X-ray integrated intensity of the martensite phase (110) plane ,
Thickness of residual austenite phase: 10 regions of 1750 nm × 2250 nm were randomly selected from the thin film collected from the steel material, and a dark field image of residual austenite was taken with an electron microscope. The average value when the minor axis of the ellipse is calculated by approximating to the ellipse.

(2) After reheating the steel having the composition described in (1 ) above from room temperature to Ac ₃ point or higher, the steel is cooled at a cooling rate of 0.08 ° C./sec or higher from 800 ° C. to 400 ° C., A method for producing martensitic stainless steel, characterized by cooling to 150 ° C. at a cooling rate of 1 ° C./sec or less.

(3) A steel having the composition described in (1) above is designated as Ac. _Three After heating to a point or higher and hot working, it is cooled at a cooling rate of 0.08 ° C./sec or more from 800 ° C. to 400 ° C., and further cooled at a cooling rate of 1 ° C./sec or less to 150 ° C. A method for producing martensitic stainless steel.

The present invention as above mentioned are those developed in consideration circumstances and findings obtained therefrom will be described below. The background and knowledge are as follows.

First, in order to finely disperse the retained austenite, the heat treatment performed in the conventional two-phase region of Ac ₁ to Ac ₃ is performed while changing the temperature and time, and the retained austenite precipitation form, precipitation Quantity and mechanical properties were investigated.

  FIG. 1 shows an electron microscope of a metal structure obtained by subjecting a 12% Cr-6.2% Ni-2.5% Mo-0.007% C steel to a two-phase region heat treatment (640 ° C. × 1 h, allowed to cool). It is a figure which shows an example of a photograph. As shown in the figure, the retained austenite is precipitated relatively coarsely in the martensite matrix and in the vicinity of the prior austenite grain boundary. The thickness of the retained austenite was about 150 nm, and the yield stress at this time was as low as 607 MPa.

As shown in FIG. 1, relatively coarse retained austenite is formed when the two-phase heating of Ac _{1 to} Ac ₃ is carried out, and the reverse transformed austenite precipitates relatively coarsely. In addition, austenite forming elements such as C, N, Ni, Cu, and Mn are concentrated. For this reason, the temperature at which the martensitic transformation of the austenite part starts, the Ms point, the temperature at which the martensitic transformation is completed, and the Mf point are greatly lowered. Part remains relatively coarse.

  That is, the characteristic of the process of forming coarse retained austenite is that the concentration of elements in the reverse transformed austenite is large when held for a certain time in a two-phase region (high temperature) where sufficient diffusion of atoms occurs. As a result, both the Ms point and the Mf point are greatly reduced. As a result, the retained austenite generated in the steel becomes relatively coarse. Although such coarse austenite improves toughness, it also lowers strength at the same time. Therefore, it is difficult to obtain characteristics of high strength and high toughness by the method of precipitating retained austenite by two-phase heating.

  Next, using the same 12% Cr-6.2% Ni-2.5% Mo-0.007% C steel as described above, the remaining austenite is finely precipitated while being cooled, not in the two-phase region heat treatment. We investigated whether it was possible. As a result, retained austenite did not precipitate even when the cooling rate was changed, and high toughness was obtained, but the toughness was low.

However, the same experiment was performed using 11% Cr steel with varying C content. As a result, the steel containing 0.01% or more of C was heated to an austenite region of ₃ or more points of Ac, and then heated to high temperature. It was found that high strength and high toughness can be obtained when the zone is cooled relatively quickly and further, without cooling rapidly from the vicinity of the martensitic transformation point to room temperature.

FIG. 2 shows an average cooling rate of 0.8 ° C./sec from 800 ° C. to 400 ° C. after heating 11% Cr-0.5% Ni-0.25% Mo-0.03% C steel to Ac ₃ point or higher. It is a figure which shows an example of the electron micrograph of the metal structure | tissue obtained by cooling by 400 and cooling by 400 to 150 degreeC by the average cooling rate of 0.13 degree-C / sec.

  In the metal structure shown in the figure, very thin plate-like retained austenite is seen at the lath interface of martensite. It has been found that the steel having such a structure has a small decrease in strength and good toughness can be obtained. This is because the retained austenite is fine. That is, since the number of retained austenite is large, the effect of improving toughness becomes remarkable.

As a result of detailed examination of the process in which fine austenite remains, the present inventors have obtained the following findings [1] to [4] .

[1] When heated to Ac ₃ or higher and then cooled, martensitic transformation starts below the Ms point. From the Ms point to the Mf point, the two-phase structure of the already transformed and martensite part and the untransformed austenite Become.

  When the steel is not rapidly cooled, C gradually concentrates in the austenite region, so that the Mf point of the untransformed austenite portion decreases. As the temperature further decreases and martensitic transformation progresses, the concentration of C in the austenite region becomes more prominent, and finally a very small austenite remains at the lath interface where the Mf point is lower than room temperature. On the other hand, in particular, when the temperature range below the Ms point is rapidly cooled, no enrichment of C in the austenite occurs, so no retained austenite occurs.

[2] In the case of the above-described two-phase region heating, reverse-transformed austenite grows when held at a high temperature, and alloy elements such as Ni, Mn, and Cu as well as C and N are concentrated in the austenite. Due to the concentration of the alloy elements, the Ms point and the Mf point are lowered, and most of the grown reverse transformed austenite remains as retained austenite. For this reason, the retained austenite in steel becomes coarse.

On the other hand, in the process of gradually cooling from the vicinity of the Ms point after heating to the Ac ₃ point or higher, the alloy element is concentrated only at a low temperature after the start of martensitic transformation. Therefore, C and N can be concentrated in austenite, but Ni, Mn, Cu, and the like do not concentrate because they are difficult to diffuse at low temperatures. Further, the concentration becomes conspicuous only in a very small region left after the martensitic transformation proceeds. Due to such an action, the retained austenite becomes extremely fine.

[3] On the other hand, when the high temperature range of 800 to 400 ° C. is gradually increased, carbides are precipitated. Therefore, even if the low temperature range of 400 to 150 ° C. is slowly cooled, sufficient concentration of C does not occur. A sufficient amount of retained austenite cannot be secured. Therefore, in the high temperature range until the martensitic transformation starts, carbide is not precipitated, so that a certain cooling rate is required.

[4] Residual austenite in steel is present mainly at the lath interface of martensite and has a plate-like structure having a thickness of 100 nm or less. In addition, since this retained austenite is an extremely thin layer, even if an attempt is made to determine from the X-ray integrated intensity of 220γ, 200γ and 200α, 211α, which is a normal X-ray determination method, the X-ray integrated intensity of austenite is small. Is difficult. Therefore, paying attention to 111γ with the highest X-ray intensity,
111γ / (111γ + 110α)
Where 111γ: X-ray integrated intensity of the austenite phase (111) plane
110α: When the X-ray integrated intensity of the martensite phase (110) plane is used as an index for quantification, when the following formula (a) is satisfied, a decrease in strength is small and good toughness is obtained.

0.005 ≦ 111γ / (111γ + 110α) ≦ 0.05 (a)
Here, the lath interface is an interface newly formed by martensitic transformation and includes a packet / block interface which is an interface between laths having different orientations.

  In the present invention, the reason why the chemical composition, metal structure and manufacturing method of steel are defined as described above will be described. First, the reasons for defining the chemical composition of the martensitic stainless steel of the present invention will be described. In the following description, the chemical composition is indicated by mass%.

1. Chemical composition of steel C: 0.01 to 0.1%
C is an austenite-forming element and has the effect of concentrating to austenite during cooling to stabilize the austenite and leave it untransformed. In the steel of the present invention, C concentrates in the untransformed austenite portion of the martensite lath interface below the Ms point and stabilizes the austenite. In order to obtain such an effect, 0.01% or more is necessary.

On the other hand, when the content exceeds 0.1%, the strength of the steel is remarkably increased and the toughness is significantly decreased. Moreover, Cr carbide tends to precipitate at the grain boundaries, and the corrosion resistance and stress corrosion cracking resistance in a corrosive environment containing CO ₂ , H ₂ S and the like deteriorate. Therefore, the C content is set to 0.01 to 0.1%. Note that the C content is desirably 0.02% or more, a preferable range is 0.02 to 0.08%, and a more preferable range is 0.02 to 0.045%.

Cr: 9-15%
Cr is an essential element for ensuring the corrosion resistance of stainless steel, and is an important element for ensuring the corrosion resistance and the stress corrosion cracking resistance in a severe corrosive environment. By containing 9% or more, the corrosion rate can be reduced to a range where there is no practical problem under various environments. On the other hand, when the content exceeds 15%, δ ferrite is easily generated in the metal structure, the strength is lowered, and hot workability and toughness are deteriorated. Therefore, the Cr content is 9 to 15%. Furthermore, preferred ranges are Ru der less than 9-12%.

S i: 0.05 to 1%
Si is an element effective as a deoxidizer. However, if the content is less than 0.05%, deoxidation is insufficient. On the other hand, if the Si content exceeds 1%, the toughness decreases. Therefore, the Si content is 0.05 to 1%.

Mn: 0.05% to 1.5%
Mn is an effective element for increasing the strength of the steel material. Further, it is an austenite-forming element, and is an element that has the effect of suppressing the precipitation of δ ferrite during the quenching treatment of the steel material, and stably converting the metal structure of the steel material into martensite. However, the martensite effect is small when its content is less than 0.05%. On the other hand, if the Mn content exceeds 1.5%, the toughness and corrosion resistance deteriorate. Therefore, the content of Mn is set to 0.05 to 1.5%.

P: 0.03% or less P is contained as an impurity in steel, significantly adversely affects the toughness of the steel, and deteriorates the corrosion resistance in a corrosive environment containing CO _{2 and the} like. Therefore, the lower the content, the better. However, if it is 0.03%, there is no particular problem, so the upper limit is made 0.03%.

S: 0.01% or less S, like P, is contained as an impurity in steel and has a significant adverse effect on the hot workability of steel. Therefore, the lower the content, the better. However, if it is 0.01%, there is no particular problem, so the upper limit is made 0.01%.

Ni: 0.1 to 2.0%
Ni is an austenite-forming element, and is an element that has the effect of suppressing the precipitation of δ ferrite during the quenching treatment of the steel material and stably converting the metal structure of the steel material into martensite. For these purposes, it is necessary to contain 0.1% or more. On the other hand, if it exceeds 2.0%, the steel material becomes expensive and the retained austenite increases, and the strength may not be ensured. Therefore, the Ni content is 0.1 to 2.0%.

Al: 0.05% or less Al may not be contained. However, since Al is an element effective as a deoxidizer, when used as a deoxidizer, it is contained in an amount of 0.0005% or more. However, if its content exceeds 0.05%, the toughness of steel deteriorates. . Therefore, the Al content is 0.05% or less.

N: 0.1% or less N does not need to be contained because it reduces toughness, but it has the effect of suppressing the precipitation of δ ferrite during the quenching treatment of the steel material, and stably converting the metal structure of the steel material to martensite. Because it is an element, it is added as necessary. However, when the content exceeds 0.1%, the toughness is greatly deteriorated. Moreover, it becomes easy to generate | occur | produce a weld crack at the time of welding of steel materials. Therefore, the N content shall be the most 0.1%.

In addition to the above elements C to N , the martensitic stainless steel of the present invention further contains at least one of the following amounts of Cu, Mo, V, Nb, B, Ca, Mg, and REM. It contains.

Of the elements from Cu to REM, Cu to Nb (hereinafter referred to as “Group A elements”) are all H. ₂ It is an element that has the effect of improving the stress corrosion cracking resistance in a corrosive environment containing S, and from B to REM (hereinafter referred to as “elements of group B”) all have hot workability of steel. It is an element having an action to improve.

Group A elements:
Cu: 0.05 to 4%
Cu does not need to be contained, but when it is contained, it is an element that improves corrosion resistance and stress corrosion cracking resistance in a corrosive environment containing CO ₂ , Cl ⁻ , and H ₂ S. When it is desired to obtain the effect, 0.05% or more is contained. However, if the content exceeds 4%, the effect is saturated, and hot workability and toughness are reduced. Therefore, when it contains, it is good to set it as 0.05 to 4%.

Mo: 0.05-3%
Mo may not be contained. Like Mo, when Mo is contained, corrosion resistance and stress corrosion cracking resistance in a corrosive environment containing CO ₂ , Cl ⁻ and H ₂ S are improved. When it is desired to obtain the effect, 0.05% or more is contained. However, when Mo is contained exceeding 3%, the effect is saturated and toughness is reduced. Therefore, if necessary, not good that the inclusion of Mo 0.05~3%.

V : 0.005-0.5% and Nb: 0.005-0.5%
These elements are not necessarily added. However, all are elements that improve the stress corrosion cracking resistance against a corrosive environment containing H ₂ S. In order to obtain the effect, any one or more of them can be contained. The effect becomes significant when the content of both V and Nb is 0.005% or more. However, the content exceeding 0.5% deteriorates the toughness of the steel. Therefore, when it contains, content of V and Nb shall be 0.005-0.5%, respectively.

Group B elements:
B: 0.0002 to 0.005%, Ca: 0.0003 to 0.005%, Mg: 0.0003 to 0.005%, and REM: 0.0003 to 0.005%
These elements are all elements that improve the hot workability of steel. Therefore, when it is desired to particularly improve the hot working of steel, any element can be contained alone or in combination of two or more elements. The effect becomes remarkable when the content is 0.0002% or more in the case of B, and in the case of Ca, Mg and REM, the content is 0.0003% or more in all cases. However, if the content of any element exceeds 0.005%, the toughness of the steel is deteriorated and the corrosion resistance in a corrosive environment containing CO _{2 and the} like is deteriorated. Therefore, when B is contained, B is 0.0002 to 0.005%, and Ca, Mg, and REM are each 0.0003 to 0.005%.

2. Metal structure The martensitic stainless steel of the present invention is characterized by containing the following retained austenite in the matrix of the martensite structure.

  First, there must be a residual austenite phase with a thickness of 100 nm or less. When coarse residual austenite is present, the strength is significantly reduced. Therefore, fine residual austenite having a thickness of 100 nm or less must be present. When residual austenite is present in the prior austenite grain boundary, the concentration of the alloy element becomes particularly remarkable due to grain boundary diffusion, so that coarse austenite is generated and the strength is greatly reduced. Therefore, the main body of the retained austenite generation site targeted by the present invention is the martensite lath interface.

  In the present invention, the thickness of the retained austenite phase was quantified as follows. A dark field image of residual austenite in a thin film collected from the steel was taken with an electron microscope, and the length of the minor axis was measured. The quantification was performed by approximating each retained austenite to an ellipse by an image analysis method, and calculating the minor axis of the ellipse. Ten regions of 1750 nm × 2250 nm were randomly selected for each material, all the retained austenite in the field of view was measured, and the average value was defined as the thickness of the retained austenite phase.

  Next, the X-ray integrated intensities 111γ and 110α must satisfy the following formula (a).

0.005 ≦ 111γ / (111γ + 110α) ≦ 0.05 (a)
Where 111γ: X-ray integrated intensity of the austenite phase (111) plane
110α: X-ray integrated intensity of martensite phase (110) plane In the above formula (a), 111γ / (111γ + 110α) is a value proportional to the amount of retained austenite, and when this value is less than 0.005, retained austenite The amount is small and the toughness cannot be improved. On the other hand, if this value exceeds 0.05, the amount of retained austenite becomes too large, and it becomes difficult to ensure high strength.

X-ray diffraction definitive to the present invention, after removal of the chemical polishing to processed layer sample surface layer portion were measured at a scan speed 0.2 degrees / min. The integrated intensities of 111γ and 110α were calculated after performing background processing and peak dispersion processing using Windows version JADE (4.0) manufactured by Rigaku Corporation.

3. Production Method In the present invention, the following production method is employed in order to obtain the above retained austenite using a steel containing the chemical composition defined by the present invention as a raw material.

That is, heated pressurized the Material steel _three or more points A c, the steel plate by hot working, after the shape of such steel, from 800 ° C. to 400 ° C. and cooled at 0.08 ° C. / sec or more cooling rate Further, it is cooled to 150 ° C. at a cooling rate of 1 ° C./sec or less , or even after cooling to room temperature, as a final heat treatment, after heating to Ac ₃ point or higher, from 800 ° C. to 400 ° C. Is cooled at a cooling rate of 0.08 ° C./sec or more and further cooled to 150 ° C. at a cooling rate of 1 ° C./sec or less. Incidentally, material steel Ac ₃ point of the present invention are different but about 750 to 850 ° C. by its chemical composition.

  When the cooling rate is increased from 800 ° C. to 400 ° C. higher than 0.08 ° C./sec, the material steel has very good hardenability, but when this temperature range is cooled slower than 0.08 ° C./sec, coarse carbides This is because, even if the low temperature region of 400 ° C. to 150 ° C. is slowly cooled, sufficient concentration of C does not occur, and a sufficient amount of retained austenite cannot be ensured, resulting in a decrease in toughness.

  As described above, in the structure of the raw steel, C is concentrated in the untransformed austenite portion between the martensite lath below the Ms point, and austenite remains at the lath interface by stabilizing the austenite. At this time, if cooling from 400 ° C. to 150 ° C. at a rate faster than 1 ° C./sec, the martensite transformation is completed before C is concentrated in the austenite, so the amount of retained austenite becomes insufficient and the toughness is increased. Getting worse. Therefore, it is necessary to make the cooling rate from 400 ° C. to 150 ° C. slower than 1 ° C./sec.

  As is apparent from the above description of the chemical composition, metal structure and manufacturing method, the martensitic stainless steel and the manufacturing method of the present invention seek to obtain a predetermined metal structure by defining the chemical composition of the steel. The material steel having a predetermined chemical composition is not used, and an appropriate manufacturing method is employed to obtain a suitable metal structure, thereby ensuring high strength and excellent toughness. .

12 steel types having the chemical composition shown in Table 1 and 75 kg of each were melted in a vacuum melting furnace, and then cast into a steel ingot. The steel ingot was subjected to diffusion annealing at 1250 ° C. for 2 hours and then forged to produce a block having a thickness of 50 mm and a width of 120 mm.

And heating the resulting blocks to 1200 ° C., the thickness of 7mm, 15mm, 20mm, 25mm, and hot-rolled steel sheet of the 35mm and 45 mm, 8 00 ° C. from 400 ° C. high temperature range and 400 ° C. from 0.99 ° C. Cooling was performed by changing the cooling rate in the low temperature range. Some steel plates were cooled to room temperature and then reheated, and were similarly cooled by changing the cooling rate. Cooling rate after or after reheating the hot rolling, cooling, forced air cooling, mist cooling, water cooling, oil cooling, from the high temperature zone and 400 ° C. of 400 ° C. The cooling means insulation cover annealing or furnace cooling from 800 ° C. A detailed study was conducted by appropriately adopting in a low temperature region of 150 ° C. and changing the cooling conditions. For the mark 27 of the exemplary conditions, it was performed tempering. Table 2 shows the rolling completion temperature, reheating conditions, cooling rate, and tempering conditions.

  The tensile properties (yield stress: YS (MPa)), impact properties (fracture surface transition temperature: vTrs (° C.)) and residual austenite distribution of the produced steel plates were examined. The tensile test was performed using a round bar tensile test with a diameter of 4 mm taken from each steel plate after the heat treatment. The Charpy impact test was performed using a 2 mm V notch test piece having a sub-size of 5 mm × 10 mm × 55 mm, which was also taken from each steel plate after the heat treatment.

  As described above, the thickness of the retained austenite was obtained by taking a dark field image of the retained austenite in the thin film collected from the steel material with an electron microscope and quantifying the length of the minor axis. The quantification was performed by approximating each retained austenite to an ellipse by an image analysis method, and calculating the minor axis of the ellipse. Ten regions of 1750 nm × 2250 nm were randomly selected for each material, all the retained austenite in the field of view was observed, and the average value was defined as the thickness of the retained austenite. Those having a residual austenite thickness of 100 nm or less were evaluated as o.

  Regarding the amount of retained austenite, a steel plate sample having a thickness of 2 mm, a width and a length of 20 mm was cut out, the sample surface layer was chemically polished to remove the processed layer, and then X-ray diffraction was performed. The integrated intensity of 111γ and 110α was measured at a scan speed of 0.2 ° / min, and after performing background processing and peak separation processing using Windows version JADE (4.0) manufactured by Rigaku Corporation, 111γ / The value of (111γ + 110α) was calculated.

  Table 3 shows the measurement results of the thickness of retained austenite, the amount of retained austenite, yield stress, and impact properties.

  Based on Tables 1 to 3, the results of the examples are classified and analyzed as examples of the present invention and comparative examples. First, it explains from the result of a comparative example, and then mentions the result of the example of the present invention.

1. For the comparative examples (marks 13 to 21 and 24 to 27 ),
The mark 13 is an example using material steel having a Cr content exceeding the upper limit. Although the form (thickness and amount) of retained austenite satisfies the conditions of the present invention, a large amount of δ ferrite precipitates, and a predetermined high strength cannot be obtained.

Marks 14 and 15 are examples using material steel with a C content outside the range. The material steel of the mark 14 is an extremely low C steel, has low strength, and even if the temperature range from 400 ° C. to 150 ° C. is gradually cooled, no retained austenite precipitates and high toughness cannot be obtained. The steel of the mark 15 is the Example using the raw material steel in which C amount exceeded the upper limit. Although a structure containing retained austenite in a predetermined form is obtained, the strength is remarkably increased and the toughness is lowered.

The marks 16 to 21 and 24 to 26 use the raw steel defined in the present invention, but when the retained austenite having a predetermined shape cannot be obtained, or the retained austenite having a defined shape is obtained, the amount thereof is This is the case.

That is, since the mark 17 is gradually cooled in a high temperature range of 800 to 400 ° C. to precipitate carbide, even if the low temperature range of 400 to 150 ° C. is slowly cooled, sufficient concentration of C does not occur, and the prescribed retained austenite However, the toughness is inferior. Further, the marks 16, 18 to 21 and 24 to 26 can ensure solid solution C, but the retained austenite is hardly generated and high strength is obtained, but the toughness is inferior.

Mark 27, by tempering after rolling completion, the strength decreases, the residual austenite is decomposed and have a have a good toughness can be obtained.

2. Examples of the present invention (marks 1 to 6 and 9 to 11) Marks 1 to 6 and 9 to 11 are made of raw steel defined in the present invention, and after cooling is completed or after cooling once reheated, 800-400 ° C. is cooled at a cooling rate of 0.08 ° C./sec or more to suppress carbide precipitation, and the low temperature range of 400-150 ° C. is slowly cooled or gradually cooled to make fine residual austenite exist. This is an example in which the metal structure defined in the present invention was obtained. In either case, it can be seen that the strength is high and the toughness is remarkably good as compared with the comparative example.

  According to the martensitic stainless steel of the present invention and the method for producing the same, the C content is relatively high, and even if it is high in strength, it has high toughness and good corrosion resistance. It is extremely effective. Further, since it is not necessary to reduce the C content unlike the conventional improved 13% Cr steel, the expensive Ni content can be reduced and the cost can be reduced.

An example of an electron micrograph of a metal structure obtained by subjecting 12% Cr-6.2% Ni-2.5% Mo-0.007% C steel to a two-phase region heat treatment (640 ° C. × 1 h, allowed to cool). FIG. 11% Cr-0.5% Ni-2.5% Mo-0.03% C steel with a microstructure obtained by heating steel to Ac ₃ point or higher and then gradually cooling from the vicinity of the martensite transformation point to room temperature. It is a figure which shows an example of a microscope picture.

Claims (3)

In mass%, C: 0.01 to 0.1%, Si: 0.05 to 1%, Mn: 0.05 to 1.5%, P: 0.03% or less, S: 0.01% or less , Cr: 9~15%, Ni: 0.1~2.0%, Al: 0.05% or less and N: 0.1% or less containing Mutotomoni, Cu: 0.05~4%, Mo: 0 0.05-3%, V: 0.005-0.5%, Nb: 0.005-0.5%, B: 0.0002-0.005%, Ca: 0.0003-0.005%, Residue in steel containing one or more of Mg: 0.0003 to 0.005% and REM: 0.0003 to 0.005%, the balance being Fe and impurities , measured by the following method A martensite characterized in that the austenite phase has a thickness of 100 nm or less and the X-ray integrated intensities 111γ and 110α satisfy the following formula (a): Stainless steel.
0.005 ≦ 111γ / (111γ + 110α) ≦ 0.05 (a)
However,
111γ: X-ray integrated intensity of the austenite phase (111) plane 110α: X-ray integrated intensity of the martensite phase (110) plane
Thickness of residual austenite phase: 10 regions of 1750 nm × 2250 nm were randomly selected from the thin film collected from the steel material, and a dark field image of residual austenite was taken with an electron microscope. The average value when the minor axis of the ellipse is calculated by approximating The steel having the composition according to claim 1 is reheated from room temperature to Ac ₃ point or higher, and then cooled at a cooling rate of 0.08 ° C / sec or higher from 800 ° C to 400 ° C, and further to 150 ° C. The manufacturing method of the martensitic stainless steel characterized by cooling with the cooling rate of 1 degrees C / sec or less. Steel having the composition according to claim 1 is Ac. _Three After heating to a point or higher and hot working, it is cooled at a cooling rate of 0.08 ° C./sec or more from 800 ° C. to 400 ° C., and further cooled at a cooling rate of 1 ° C./sec or less to 150 ° C. A method for producing martensitic stainless steel.

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