JP2921324B2 - High-strength and high-toughness martensitic stainless steel for welded structures and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, high-toughness stainless steel for a welded structure which can be used as a material for a hydrofoil of a high-speed ship, and a method for producing the same.

[0002]

2. Description of the Related Art In a high-speed flowing water environment such as a runner for hydroelectric power generation, conventionally, cavitation erosion resistance (hereinafter, referred to as "cavitation erosion resistance") has been used.
13% Cr-4, such as SCS6, which has excellent CE resistance
６6% Ni cast steel has been used. By containing Ni, this steel has almost no δ ferrite even when C is reduced to about 0.03 to 0.05%, and several tens% of retained austenite is formed by high temperature tempering at _one or more Ac points. Therefore, the martensitic stainless steel has relatively good toughness and weldability.

However, in recent years, with the increase in speed and speed of high-speed ships, weight reduction of components such as hydrofoils and CE resistance have been required.
There is a need for improved sex. Lightweight and CE resistant
In all cases, increasing the strength is the most effective means for improving the properties, and high strength stainless steel having a 0.2% proof stress of 800 MPa or more is currently required. The 0.2% proof stress of the 13Cr-3 to 6Ni steel is about 500 to 700 MPa. As a high-strength stainless steel having a 0.2% proof stress of 800 MPa or more,
A precipitation hardening type martensitic stainless steel such as SUS630 (17-4PH) can be used. However, this steel, which has been significantly strengthened by precipitation with Cu or the like, has poor ductility and is inferior in hot workability since it contains δ ferrite.
Furthermore, since the Cu precipitate re-dissolves under the influence of heat during welding, the strength of the welded joint is significantly reduced. For this reason, it is necessary to perform aging treatment after welding, and it has been difficult to apply the method to large structures.

[0004] On the other hand, Japanese Patent Publication No. 1-28827 discloses fine Ni by adding a small amount of V and Al to the aforementioned Ni-containing low C martensitic stainless steel and performing low temperature tempering. Acicular martensite structure, SUS63
It is disclosed that it has zero strength and excellent ductility. According to Japanese Patent Application Laid-Open Nos. 62-124218 and 3-188215, Ni-containing low C martensitic stainless steel to which such a trace element is added can be obtained at a high temperature of ₁ point or more of Ac. By performing tempering, in the former, fine and stable reverse transformed austenite is formed, and in the latter, reverse transformed austenite is transformed into finer martensite, so that not only strength and toughness, but also softening resistance of the weld heat affected zone Is also disclosed to be excellent.

[0005]

Problems to be solved by the Invention
No. 8827, JP-A-62-224218 and JP-A-3-188215, the ductility of steel is C
Despite being superior to precipitation hardened stainless steel significantly strengthened by precipitation of u, etc., each has an upper limit of about 20% in elongation and about 200 J in Charpy absorbed energy at 0 ° C. Considering strong working and low temperature use Then there was still a problem. In addition, Japanese Patent Publication No. 1-28827 and Japanese Patent Application Laid-Open No. Sho 62-124218 require a long tempering treatment of 1 hr or more, like precipitation hardening stainless steel, to improve production efficiency and reduce costs. From the viewpoint of reduction, simplification of heat treatment is required. The present invention has been made to solve the above problems, and has a 0.2% proof stress of 800 MPa or more, and can be used as a material for a hydrofoil of a high-speed ship, and has ductility, workability, and the like. An object of the present invention is to obtain inexpensively stainless steel excellent in weldability and softening resistance of a heat affected zone.

[0006]

Means for Solving the Problems The present inventors have examined the factors of deterioration of the ductility of the Ni-containing low C martensitic stainless steel described above, and found that carbon nitrides or intermetallics precipitated by the addition of trace elements were added. It has been found that the compound significantly deteriorates ductility. Also, Japanese Patent Application Laid-Open No. 62-12421
No. 8 also discloses a steel type containing no trace element, but in this case, it is necessary to increase the content of C, N, or Si in order to increase the strength, and similarly, the toughness deteriorates. did. Furthermore, when tempering was performed in a low temperature region near 500 ° C. where the highest strength was obtained, the reduction in ductility was most remarkable.

The inventors of the present invention have studied in detail the components and heat treatment conditions of a Ni-containing martensitic stainless steel to which a trace alloying element that forms a precipitate is not added. To reduce δ ferrite,
And when a small amount of retained austenite is contained, 500
It has been found that good ductility can be obtained even when tempering is performed in a temperature range around ℃. Furthermore, the extremely low C and N contents were also effective in improving workability and suppressing low-temperature cracking during welding. Based on the above findings, the above-mentioned problem is solved by the following component limitation and production method.

According to the first invention, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, Cu:
0.05-1.0%, Ni: 5.0-7.0%, Cr:
13.0 to 17.0%, Mo: 2.0% or less, N: 0.
0% or less, and the content of the element is (1) to
0.2 not less than 800 MPa, which satisfies the expression (3) and the balance substantially consists of Fe and unavoidable impurities.
It is a high toughness martensitic stainless steel for welded structures having a% proof stress. C (%) + N (%) ≦ 0.03 (1) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) − 9 × Ni (%) − 2 × Cu (%) −190 × C (%) −160 × N (%) ≦ 154 (2) 27.6 ≦ Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni ( %) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%) ≦ 31.6 (3)

[0009] A second invention provides a method for producing steel satisfying the requirements of the first invention, wherein after hot working, 850-1000 ° C.
After heating to a temperature range of M _f
Point is cooled to below the temperature range, furthermore, 450 ° C. to Ac ₁
800, characterized by cooling after heating to a point temperature range
This is a method for producing a high toughness martensitic stainless steel for welded structures having a 0.2% proof stress of not less than MPa.

In a third aspect of the invention, in the production of steel which also satisfies the requirements of the first aspect of the invention, after hot-heating to a temperature range of T.degree. 800 M, characterized by immediately cooling to a temperature range below the M _f point at a cooling rate of 3 ° C./min or more, further heating to a temperature range of 450 ° C. to ₁ point of Ac, and then cooling.
This is a method for producing a high-toughness martensitic stainless steel for welded structures having a 0.2% proof stress of not less than Pa. T = 450 x Ni (%) + 100 x Cu (%) + 9500 x C (%) + 8000 x N (%)-600 x Cr (%)-750 x Mo (%)-1000 x Si (%) + 8750 ...・ (4)

[0011]

The reasons for limiting the components of the stainless steel of the present invention will be described below. C is a component effective for increasing the strength, and can reduce the amount of Ni because it is an austenite-forming element. However, if the content is increased, toughness, weldability and workability are deteriorated. The value is 0.03%. Preferably it is 0.02% or less. Although Si is a component having a deoxidizing action, it is the same ferrite-forming element depending on the amount of Si added because it is a ferrite-forming element.
The amount of Cr added is limited. That is, the details will be described later.
However, the content of each element is in addition to the limitation, (2) and
It is a range satisfying the expression (3), and the amount of added Si increases.
And the amount of Cr, an element effective for corrosion resistance, are limited
Therefore, the upper limit is set to 1.0%. In addition, observation of deterioration of ductility
From the viewpoint, the upper limit is set to 1.0%.

Mn is a component having a deoxidizing effect like Si, and is known as an austenite forming element.
However, in this steel, the effect as an austenite-forming element was not recognized, and it was revealed that if the content exceeds 1.0%, the corrosion resistance is reduced. Therefore, the upper limit of the Mn content is set to 1.0% or less. Cu
Is an effective component for improving corrosion resistance and is also an austenite-forming element. However, the effect on corrosion resistance is Cu
When the content is less than 0.05%, the effect is not exhibited, and when the content exceeds 1.0%, the effect is saturated. Further, since the addition of a large amount deteriorates the ductility, the Cu content is set to 0.05 to 1.0%.

Ni is an austenite forming element,
It is an essential component for obtaining martensite excellent in ductility and weldability. However, as will be described in detail later, if the Ni content is less than 5.0%, even if other components are adjusted, residual austenite is not generated, and strength and toughness are reduced. Also, 7.
Additions exceeding 0% not only increase the cost, but also increase the residual austenite and decrease the strength. Therefore, the Ni content is set to 5.0 to 7.0%. Cr is the most important element for corrosion resistance, and its effect becomes remarkable when added at 13.0% or more. However, since it is also a ferrite forming element, if its content exceeds 17.0%, δ
As ferrite increases, retained austenite also becomes excessive, and strength and toughness are significantly reduced. Therefore, the Cr content is set to 13.0 to 17.0%.

Mo, like Cr, is an element effective for improving corrosion resistance and also a ferrite-forming element, so that it can be substituted. However, since the addition exceeding 2.0% lowers the toughness, the upper limit is set to 2.0%. N is an austenite-forming element like C and can reduce the Ni content, but hardly contributes to the increase in strength. Further, if its content increases, ductility and weldability deteriorate, so the upper limit is made 0.02%.

In general, it is known that the low-temperature cracking susceptibility during welding is reduced by reducing the amounts of C and N in the martensitic stainless steel. The inventors have found that, by satisfying the above, not only the cold cracking susceptibility is significantly reduced, but also the workability is improved. Therefore, the (C + N) amount is in a range that satisfies the expression (1) in addition to the above limitation. C (%) + N (%) ≦ 0.03 (1)

Furthermore, the present inventors have results of examining the relationship between the volume ratio [delta] _f and volume fraction gamma _f with components of the residual austenite of [delta] ferrite in the steel in detail, [delta] _f (%) and gamma _f (% ) Can be expressed by the equations (5) and (6), respectively. This steel reduces [delta] ferrite, a small amount of martensite structure and extending toughness satisfactory subjected to return following low temperature co-Ac ₁ point by having residual austenite is obtained, is in particular [delta] _f If it exceeds 5%, not only the toughness deteriorates, but also the strength decreases. On the other hand, γ _f
If less than 1%, the effect of improving toughness is not exhibited, and if more than 10%, the softening resistance of the heat affected zone decreases. Therefore, the content of the element is in the range satisfying the expressions (2) and (3) in addition to the above-mentioned limitation. _{δ f = 12 × Cr (%} ) + 15 × Mo (%) + 20 × Si (%) -9 × Ni (%) - 2 × Cu (%) - 190 × C (%) -160 × N (%) - 149 (5) logγ _f = [Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68 × C (% ) + 54 × N (%)] / 4-6.9 (6) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) − 9 × Ni (%) − 2 × Cu ( %) − 190 × C (%) −160 × N (%) ≦ 154 (2) 27.6 ≦ Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%) ≦ 31.6 (3)

Next, the reasons for the limitation of the production method will be described. In this steel, Cr carbide precipitates at the grain boundary in the temperature range of 600 to 700 ° C., and the toughness and the corrosion resistance are reduced. The quenching treatment is performed for the purpose of obtaining a solid solution of Cr carbide precipitated by hot working and a martensite structure excellent in toughness. However, if the quenching temperature after hot working is less than 850 ° C.,
If the carbide is not formed into a solid solution and the temperature is higher than 1000 ° C., the crystal grains become coarse and the toughness is deteriorated. Therefore,
The quenching temperature after hot working is in the range of 850 to 1000 ° C. Further, as a result of studying cooling conditions at the time of quenching, when the cooling rate is less than 3 ° C./min, although sintering is sufficiently performed, the above-mentioned Cr carbide reprecipitates during cooling.
Further, when the cooling stop temperature exceeds the M _f point, γ _f becomes significantly larger than the value represented by the equation (6), and the softening resistance of the joint decreases. Therefore, cooling at the time of quenching is performed at a cooling rate of 3 ° C./min or more to a temperature range of _Mf point or less.

The tempering treatment performed after quenching is generally performed to improve ductility. However, in this steel, when high-temperature tempering exceeding the Ac ₁ point is performed, γ _f is significantly increased as compared with the value represented by the equation (6), so that the tempering must be performed in a low temperature region below the Ac ₁ point. Further, as described above, it is possible to increase the strength of this steel by low-temperature tempering for an extremely short time (to the extent that the entire product is soaked), and heat treatment is easier than that of precipitation hardening stainless steel. . However, if the tempering temperature is lower than 450 ° C. or exceeds the Ac ₁ point, the effect of improving the strength is not exhibited. Therefore, the tempering temperature is 4
50 ° C. to Ac ₁ point. logγ _f = [Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%) ] /4-6.9 (6)

Further, by utilizing the characteristics of this steel having excellent hardenability, a manufacturing method in which the above-mentioned hardening treatment by reheating was omitted was examined. Usually, heating before hot working is 1100-1
It is performed in a temperature range of 300 ° C.
When the temperature exceeds 050 ° C., δ _f increases from the value represented by the equation (5). The heating time of the [delta] _f becomes substantially the same value as [delta] _f after hot working, it was revealed that not changed by subsequent tempering process. Therefore, the strength of the product by increasing the inventors [delta] _f, in order to prevent a decrease in toughness, the heating temperature and components and result relationship was examined in detail in the [delta] _f in the temperature range of 1300 ° C. or less, the value 5 % Maximum temperature T below
(° C.) was found to be expressed by equation (4). Therefore, the heating temperature before hot working is set to a temperature range of T ° C. or lower which can be expressed by the equation (4). However, if the heating temperature exceeds 1300 ° C., the surface properties of the product deteriorate, and such heating is not desirable.

Further, in order to obtain a martensitic structure having excellent toughness and corrosion resistance, the end temperature of the hot working needs to be in a temperature range in which Cr carbide is not precipitated. Cooling after hot working is performed at a cooling rate of 3 ° C./min or more to a temperature range of _Mf point or less, similarly to the quenching treatment. Since the above-described hot working-direct quenching treatment has the same effect as the reheating quenching treatment, the subsequent tempering treatment is also performed under the same conditions as the reheating quenching material. _{δ f = 12 × Cr (%} ) + 15 × Mo (%) + 20 × Si (%) -9 × Ni (%) - 2 × Cu (%) - 190 × C (%) -160 × N (%) - 149 (5) T = 450 × Ni (%) + 100 × Cu (%) + 9500 × C (%) + 8000 × N (%) − 600 × Cr (%) − 750 × Mo (%) − 1000 × Si (%) + 8750 (4)

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described as follows.

Example 1 A 50 kg ingot having the chemical components shown in Table 1 was heated at 1250 ° C., finished at 800 ° C., and then hot-rolled by air cooling (cooling rate: 50 ° C./min) to produce a steel sheet having a thickness of 15 mm. . After heating the steel sheet at 900 ° C for 5 minutes, quenching is performed by air cooling to 50 ° C below the M _f point.
Further, after heating at 550 ° C. × 5 min below Ac ₁ point, a tempering treatment of air cooling was performed, and then a sample for microstructure observation,
X-ray diffraction sample, tensile test specimen, bending test specimen, 2 mmV
A notched Charpy impact test specimen and a sample for measuring pitting corrosion potential were collected. [delta] _f 20% micro sample Na
OH electrolytic etching and γ _f were measured by an X-ray diffraction method. The results are shown in Table 1. Workability: bending radius: 0.5t, bending angle: 180 °
Was evaluated by the presence or absence of cracks in the table bending test. The pitting corrosion potential is 100 μm according to JIS G0577.
A / cm ² potential was determined. Further, in order to evaluate the weldability, a test piece (JIS Z3) was prepared from the heat-treated steel sheet.
158), and a commercially available coated arc welding rod (0.04
(C-5Ni-12Cr steel) at 10 ° C., RH: 60
%, Heat input: main welding was performed under the conditions of 10 KJ / cm, and the presence or absence of root cracks was examined by a cross-sectional microscopic method. On the other hand, the softening resistance of the weld heat-affected zone was determined by using a commercially available MIG wire (0.02C-5Ni-12Cr steel) for a welded joint with an X-groove, and measuring the fracture position in the joint tensile test (JIS Z3121). evaluated. The MIG welding was a multi-layer welding with a heat input of 20 KJ / cm.

FIG. 1 shows the relationship between δ _f and the absorbed energy at 0 ° C. and -50 ° C. and the components. According to FIG. 1, δ _f of this steel shows a good correspondence with the value of index B representing this,
It can be seen that if the value of B exceeds 154, the toughness is remarkably deteriorated even when the contents of C and N are low. FIG. 2 shows the relationship between γ _f , the absorbed energy at 0 ° C. and −50 ° C., and the fracture position and the component of the joint tension. That is, according to FIG. 2, γ _f of this steel shows a good correspondence with the value of the index C representing this,
It is understood that when the value of C is less than 27.6, the toughness decreases, and when it exceeds 31.6, the weld heat affected zone softens.

Table 2 shows invention steels 1 to 10 and comparative steels 11 to 11.
The tensile properties, bending workability, absorbed energy at 0 ° C. and −50 ° C., the Y-test results, the fracture position of the joint tensile strength, and the pitting potential of No. 19 were shown together. Table 2 shows that the steel of the present invention has excellent strength, ductility, workability, weldability, joint properties and corrosion resistance. In particular, in order to perform welding without strong working or preheating, the (C + N) content is set to 0.03.
It is understood that it needs to be less than%. Next, Table 1,
Table 2 is shown. Note that the underline in Table 1 indicates that the condition is outside the limited conditions of the steel of the present invention. Also, 1) in Table 1,
2), 3), δ _f and γ _f are as follows: 1) A = C (%) + N (%) 2) B = 12 × Cr (%) + 15 × Mo (%) + 20 ×
Si (%)-9 × Ni (%)-2 × Cu (%)-190
× C (%)-160 × N (%) 3) C = Cr (%) + 1.3 × Mo (%) + 1.5 × S
i (%) + 2 × Ni (%) + 0.7 × Cu (%) + 68
× C (%) + 54 × N (%) δ _f and γ _f are measured values.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

Example 2 A 50 kg ingot having the chemical composition shown in Table 3 was subjected to hot rolling and heat treatment under the conditions shown in Table 4 to a size of 15 mm.
Made of thick steel plate. After hot rolling of the direct quenched material or during the quenching process of the reheated quenched material, the material is cooled to 50 ° C. below the M _f point at the cooling rate shown in Table 4, and all other processes are air cooled. (About 50 ° C./min). As samples, a sample for micro observation, a sample for X-ray diffraction, a tensile test piece, a Charpy impact test piece with a 2 mm V notch, and a sample for pitting potential measurement were collected. Further, a welded joint was prepared by the same MIG welding as in Example 1, and the joint was pulled together. Various tests were performed in the same manner as in Example 1. Tensile properties of the steel sheet manufactured by the method of Table 4
Table 5 shows the absorption energy at -50 ° C, the breaking position of the joint tensile strength and the pitting potential. According to Table 5, the 0.2% proof stress of the steel manufactured by the method of the present invention has a value of 800 MPa or more, and the elongation: 25% or more, the absorbed energy at −50 ° C .: 230 J or more. Are better. Also,
It is understood that the softening resistance and corrosion resistance of the weld are also sufficient.

[0027]

[Table 3]

[0028]

[Table 4]

[0029]

[Table 5]

[0030]

As described above, according to the present invention, 800
Since the 0.2% proof stress of not less than MPa can be achieved by simple heat treatment without adding a trace element, etc., it is excellent in ductility, workability and weldability, and also has good softening resistance and corrosion resistance of the weld heat affected zone. This has the effect of obtaining a site-based stainless steel. Therefore, it is possible to provide an inexpensive stainless steel that can be used as a material for the hydrofoil of a high-speed ship.

[Brief description of the drawings]

FIG. 1 shows δ _f of steel according to Example 1 of the present invention and 0 ° C., −
The figure which shows the relationship between the absorption energy at 50 degreeC, and a component.

FIG. 2 shows γ _f of the steel according to Example 1 at 0 ° C. and −50.
The figure which shows the relationship between the absorption energy in ° C, the fracture | rupture position of joint tension, and a component.

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int. Cl. ⁶ , DB name) C22C 38/00-38/60 C21D 6/00 102 C21D 8/00

Claims (3)

(57) [Claims] 1. The method according to claim 1, wherein C: 0.03% or less, Si:
1.0% or less, Mn: 1.0% or less, Cu: 0.05 to
1.0%, Ni: 5.0-7.0%, Cr: 13.0-
17.0%, Mo: 2.0% or less, N: 0.02% or less, and the content of the element satisfies the formulas (1) to (3), and the balance is substantially Fe and inevitable. A high-toughness martensitic stainless steel for welded structures having a 0.2% proof stress of 800 MPa or more, comprising an impurity. C (%) + N (%) ≦ 0.03 (1) 12 × Cr (%) + 15 × Mo (%) + 20 × Si (%) − 9 × Ni (%) − 2 × Cu (%) −190 × C (%) −160 × N (%) ≦ 154 (2) 27.6 ≦ Cr (%) + 1.3 × Mo (%) + 1.5 × Si (%) + 2 × Ni ( %) + 0.7 × Cu (%) + 68 × C (%) + 54 × N (%) ≦ 31.6 (3) 2. In the production of the steel according to claim 1, after hot working, heating to a temperature range of 850 to 1000 ° C., and then at a cooling rate of 3 ° C./min or more to a temperature range of the _Mf point or less. A method for producing a high toughness martensitic stainless steel for a welded structure having a 0.2% proof stress of 800 MPa or more, which is cooled, further heated to a temperature range of 450 ° C. to _one point Ac, and then cooled. 3. In the production of steel according to claim 1, after heating to a temperature range of T.degree.
Perform hot working to finish in a temperature range of 0 ° C or more and immediately
After cooling to a temperature range below the M _f point at a cooling rate of at least C ° C./min, and further heating to a temperature range of 450 ° C. to Ac ₁ point,
0.2% of 800MPa or more characterized by cooling
Method for producing high toughness martensitic stainless steel for welded structures having proof stress. T = 450 x Ni (%) + 100 x Cu (%) + 9500 x C (%) + 8000 x N (%)-600 x Cr (%)-750 x Mo (%)-1000 x Si (%) + 8750 ...・ (4)