JPH05156409A - High-strength martensitic stainless steel with excellent seawater resistance and its manufacturing method - Google Patents

Abstract

(57) [Summary] [Object] The present invention relates to a high-strength martensitic stainless steel having excellent seawater resistance and good weldability, and a method for producing the same. [Composition] C: 0.03% or less by weight%, Si: 1.0
% Or less, Mn: 2.0% or less, Cr: 11 to 15%, N
i: 2.0 to 7.0%, Mo: more than 2.0% to 4.0%,
N: 0.02% or less, and the sum of Creq and Nieq calculated from the above components is 23.0 or less, Creq
-High-strength martensitic stainless steel excellent in seawater resistance, characterized in that Nieq satisfies 11.0 or less and does not contain a ferrite phase. The above-mentioned invention steel in which one or two kinds of Nb and V and / or one or two kinds of Al and Ca are added in appropriate amounts, if necessary, to further improve strength, toughness and weldability. After the hot rolling of the above-mentioned invention steel is completed,
Quenched from 00 to 1100 ℃ to 100 ℃ or below, 500
A method for producing a high-strength martensitic stainless steel excellent in seawater resistance, characterized by tempering to ~ 700 ° C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength martensitic stainless steel having excellent seawater resistance and good weldability which can be used, for example, in ship structures and the like, and a method for producing the same.

[0002]

2. Description of the Prior Art Martensitic stainless steel is widely used as a blade and a spring material because its strength can be easily increased by quenching heat treatment. However, a large amount of Cr could not be added to maintain the metal structure in the martensite phase, and sufficient corrosion resistance to seawater could not be obtained. As proposed in JP-A-1-162750 and the like, the addition of Mo improves the corrosion resistance. Moreover, the effect is expected to be higher than Cr, but like Cr, when the addition amount is large, a ferrite phase is formed, and toughness, fatigue strength, and weldability are deteriorated, so that it cannot be used as a structural steel. On the other hand, the addition of Ni is effective in suppressing the formation of the ferrite phase, and is also effective in improving the toughness and weldability. However, Ni also suppresses the formation of martensite phase, so when the addition amount increases, an austenite phase is formed and the strength sharply decreases. For the above reasons, the amount of Mo added in conventional high-strength martensitic stainless steels is at most 2
%, It could not be said to have sufficient seawater resistance.

[0003]

In order to realize a martensitic stainless steel having excellent corrosion resistance that can be used in a seawater environment, the Mo content effective for improving the corrosion resistance is increased as compared with the conventional martensitic stainless steel, and the strength, It is necessary to suppress the formation of a ferrite phase which deteriorates the toughness and weldability or an austenite phase which causes a decrease in strength. In addition, reduction of non-metallic inclusions that are harmful to fatigue strength and weldability and shape control are important issues when applied to welded structures. The present invention controls the metallographic structure from the balance of the components so that the amount of M is sufficient to ensure seawater resistance.
o High-strength martensitic stainless steel with excellent corrosion resistance can be added, and non-metallic inclusions can be reduced.
It can be applied to welded structures by controlling the shape.

[0004]

In order to solve the above problems and realize a high-strength martensitic stainless steel excellent in seawater resistance, the present invention limits the components and the content thereof, and further the range of the components. Is the most effective manufacturing method. In other words, the first object of the present invention is the weight%
C 0.03% or less, Si 1.0% or less, Mn
2.0% or less, Cr 11-15% Ni 2.0-
7.0%, Mo over 2.0% to 4.0%, N 0.02%
It contains the following, and further contains Nb 0.01 to 0.
5%, 1 or 2 kinds of V 0.01 to 0.5% and / or Al 0.001 to 0.05%, Ca 0.0
Creq represented by the following equation (1) and Nieq represented by the following equation (2) contain 005 to 0.005% of one or two types, and When satisfied, the high-strength martensitic stainless steel excellent in seawater resistance is characterized in that its metallic structure is composed of a martensite phase containing no ferrite phase.

Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) (3) The formula is a conditional formula for preventing the delta / ferrite phase harmful to toughness from remaining in the metal structure after hot rolling,
Expression (4) is a conditional expression for transforming the structure into a martensite phase by quenching heat treatment and realizing high strength. Addition of Nb or V is effective in suppressing softening during tempering or deterioration of intergranular corrosion resistance. Furthermore, addition of Al and Ca is effective for reducing non-metallic inclusions and controlling the shape. Al and Ca reduce the contents of S and O, which are harmful to toughness and weldability.

A second object of the present invention is to subject a steel material limited to the above components to a quenching treatment from a temperature range of 900 to 1100 ° C. to 100 ° C. or lower after completion of hot rolling, and then to 500 to 700 ° C. A method for producing a high-strength martensitic stainless steel having excellent seawater resistance, which is characterized by performing a tempering treatment.

[0007]

The reason for limiting the components of the present invention will be described in detail. C: An element effective in hardening the martensite phase and increasing the strength, but its content was made 0.03% or less because it markedly deteriorates toughness and weldability.

Si: Inevitably contained in steel as a deoxidizing element, but if added in excess, it deteriorates toughness and weldability, so its content was made 1.0% or less. Mn: It has the effect of suppressing the delta-ferrite phase and also fixing S in the steel, but if it is added excessively, the toughness decreases, so its content was made 2.0% or less. Cr: It is a basic element of stainless steel, and at least 11% or more is necessary for obtaining excellent corrosion resistance. However, if added in excess of 15%, the delta ferrite phase remains in the martensite phase and deteriorates toughness and weldability.

Ni: An important additive element that improves the toughness of the martensitic phase and improves the weldability. In order to secure sufficient toughness and maintain good weldability of thick-walled materials, a content of 2.0% or more is necessary. However, if added in excess of 7.0%, the retained austenite phase will rapidly increase and the strength will decrease. Mo: An additive element important for improving corrosion resistance and ensuring seawater resistance. It is also effective in improving the strength and toughness after tempering. Sufficient seawater resistance can be obtained by adding over 2.0%, but if added excessively, the delta ferrite phase will not disappear, so the upper limit was made 4.0%.

Nb: an effective element for suppressing the decrease in intergranular corrosion resistance and the decrease in strength during tempering. In order to exert the effect, the content is required to be 0.01% or more, but if added in excess of 0.5%, cracking tends to occur during welding or hot rolling. Similar to V: Nb, it is an effective element that suppresses a decrease in intergranular corrosion resistance during tempering. In order to exert its effect, a content of 0.01% or more is necessary, but 0.5%
If it is added over the range, cracking tends to occur during welding or hot rolling.

As with N: C, the martensite phase is hardened.
Is an element effective in increasing the strength, but toughness and
And its weldability is significantly deteriorated, its content is set to 0.
It was set to 02% or less. Al: S and O harmful to toughness are reduced in the molten steel stage, and steel
Remaining O in the Al ₂O₃As an inclusion, N is AlN
To fix as. For that purpose, the content of 0.001% or more
The amount is necessary, but if the content exceeds 0.05%, Al
₂O₃Inclusions are distributed in clusters and fatigue strength or
Reduces toughness.

Similar to Ca: Al, S and O are reduced in the molten steel stage. Also, the addition of Ca changes S and O, which are harmful to toughness, into C
It is fixed as an a-Al-OS-s inclusion, and this inclusion itself does not extend by rolling like MnS, and Al ₂
Since it does not tend to be distributed in clusters like O ₃ , it is more effective in improving toughness. In order to exert such an effect of controlling the shape of inclusions, at least 0.
A content of 0005% or more is required, but 0.005%
If it is contained in excess, the inclusions become coarse and, conversely, the toughness and weldability are reduced. Further, in order to reduce the S and O contents and effectively control the shape of inclusions, it is most desirable to simultaneously add Al and Ca in the appropriate amounts. By this composite addition, inclusions remaining in the steel can be made minute and dispersed, which is particularly effective in improving toughness and fatigue properties.

Cu as another element has the effect of increasing the strength after tempering treatment, but if added in excess, the weld heat affected zone becomes extremely hard and weld cracking tends to occur, so the upper limit is 2%. It is desirable to do. P, S, and O inevitably contained in the steel lower the toughness of the steel material and deteriorate the weldability, so that the respective contents are 0.03% or less, 0.005% or less, 0.005% or less. % Or less is desirable. The addition of lanthanoid rare earth elements such as La and Ce is also effective for fixing S and O, but the oxides or sulfides thereof have a large specific gravity and are hard to float during refining, so they tend to remain in the steel. Therefore, it is desirable to suppress the addition amount to 0.05% or less.

Next, an optimum method for manufacturing a steel material limited to the above range of components will be described. If the quenching temperature after hot rolling is increased, a delta ferrite phase is generated, and if the quenching temperature is too low, carbides or nitrides are precipitated, and in any case, toughness and weldability are deteriorated. Therefore, the quenching temperature is 900 ° C or higher and 1100 ° C or lower. It is desirable that the cooling rate during quenching be 10 ° C. or more per minute. Further, the quenching treatment is effective either immediately after the hot rolling is completed or after being once cooled and then reheated to the above temperature range.

Since the martensite phase is unstable in the as-quenched state and residual stress due to thermal strain is introduced during the quenching process, sufficient toughness cannot be obtained and it causes delayed fracture of the welded part. .. Therefore, tempering heat treatment is necessary. If the tempering temperature is less than 500 ° C, the residual stress is not sufficiently removed, and if it exceeds 700 ° C, an unstable martensite phase is formed again during cooling, so that the tempering effect is reduced. It is desirable to carry out this tempering heat treatment even after welding.

In the present invention, in order to improve the toughness and weldability, the components that do not form the delta ferrite phase are limited and the production conditions are limited. However, the component segregation during melting and solidification, especially the segregation of Mo. When a delta-ferrite phase is partially formed by the above method, it is desirable to carry out homogenizing annealing before or after hot rolling. Further, it is necessary to prevent the mixing of hydrogen, which significantly impairs weldability, at the time of manufacturing the steel sheet of the invention steel or performing welding. When there is an influence of hydrogen, it is desirable to perform diffusion annealing at 600 ° C. or lower.

[0017]

[Examples] Table 1 shows the chemical composition of the test steel. The test steels were melted in a vacuum melting furnace and then hot rolled to 30 mm. Quenching and tempering treatment of the test steels A to J were performed under the same conditions. In other words, after the hot rolling is finished, it is reheated to 100
After keeping at 0 ℃ for 1 hour, water-cool to room temperature, then 600
The temperature was maintained at 4 ° C. for 4 hours, and then air cooling was performed.

[0018]

[Table 1]

Tensile test pieces, Charpy test pieces and fatigue test pieces were cut out from the center of the plate thickness of the base material perpendicularly to the rolling direction, and their mechanical properties and corrosion fatigue strength were investigated. For the corrosion fatigue test, a round bar test piece with a diameter of 40 mm in the parallel part was used, and 3.5% N
Immersed in an aCl aqueous solution and subjected to a unilateral tensile stress at a repetition rate of 1 Hz and a maximum number of repetitions of 2 × 10 ⁶ ,
The fatigue strength at that number of times was determined.

5 mm thick x 20 mm from the surface layer of the same material
A test piece having a width of 40 mm was cut out and a salt spray test was carried out. For salt spraying, a 3.5% NaCl aqueous solution was used, and after testing at room temperature for 24 hours, the presence or absence of rusting was confirmed. Further, in order to examine the weldability of the test steel, an H-shaped narrow groove (width 15 mm) was cut out from the material and TIG welding was performed. Welding uses a welding rod with the same composition as the sample steel H, welding current 25
A, 10 layers were formed on each side at a welding speed of 5 cm / min.
The preheating before welding was not carried out, and the cross section was cut out 24 hours after the completion of welding, and the presence or absence of cracking was confirmed.

0.2% proof stress, tensile strength, elongation at break, Charpy absorbed energy at 0 ° C., obtained from the above tests,
Table 2 shows the presence / absence of pitting corrosion in salt spray, the corrosion fatigue strength, and the presence / absence of cracking during welding. It can be seen from Table 2 that the steel of the present invention has high strength and excellent corrosion resistance in seawater, as well as excellent toughness, corrosion fatigue strength and weldability.

[0022]

[Table 2]

The sample steel K was quenched at various temperatures shown in Table 3,
A tempering heat treatment was performed. The holding time and the cooling method were the same as the conditions carried out for the sample steels A to J. afterwards,
A test piece was cut out in the same manner as above and examined for mechanical properties, corrosion resistance and weldability. The results are also shown in Table 3. From this table, it can be seen that if quenching or tempering is performed outside the temperature range of the present invention, one of strength, corrosion resistance, corrosion fatigue properties, and weldability deteriorates. It is clear that it has to be manufactured according to

[0024]

[Table 3]

From the above examples, it is understood that the steel of the present invention has high strength and excellent seawater resistance, high toughness and corrosion fatigue strength, and good weldability even in thick-walled materials.

[0026]

EFFECTS OF THE INVENTION The high-strength martensitic stainless steel of the present invention is excellent in corrosion resistance and exhibits excellent weldability even in thick-walled materials. Therefore, the industrial contribution is extremely large.

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[Procedure amendment]

[Submission date] April 3, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] 0019

[Correction method] Change

[Correction content]

Tensile test pieces, Charpy test pieces and fatigue test pieces were cut out from the center of the plate thickness of the base material perpendicularly to the rolling direction, and their mechanical properties and corrosion fatigue strength were investigated. For the corrosion fatigue test, a round bar test piece with a diameter of 10 mm in the parallel part was used, and 3.5% N
Immersed in an aCl aqueous solution and subjected to unilateral tensile stress at a repetition rate of 1 Hz and a maximum number of repetitions of 2 × 10 ⁶ ,
The fatigue strength at that number of times was determined.

Front Page Continuation (72) Inventor Hiroshi Tadokoro 20-1 Shintomi, Futtsu City, Chiba Shin-Nippon Steel Co., Ltd. Technology Development Division (72) Inventor Kazuhiro Suetsugu No. 1 Tobata-cho, Tobata-ku, Kitakyushu, Japan Shin-Nippon Steelworks Yawata Works

Claims (8)

[Claims] 1. C 0.03% or less by weight%, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, the balance consisting of Fe and inevitable impurity elements, and Creq represented by the following formula (1) and Nieq represented by the formula (2)
A high-strength martensitic stainless steel excellent in seawater resistance, characterized by satisfying the expressions (3) and (4) and comprising a martensite phase containing no ferrite phase in its metal structure. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 2. C 0.03% or less by weight%, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, and Nb 0.
01-0.5%, V 0.01-0.5% of 1 type or 2 types, the balance consists of Fe and unavoidable impurity elements, and Creq represented by the following formula (1): Nieq represented by the formula (2) satisfies the formulas (3) and (4), and is composed of a martensite phase containing no ferrite phase in its metal structure, which is excellent in seawater resistance. High strength martensitic stainless steel. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 3. C in an amount of 0.03% or less by weight, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, Al 0.
001-0.05%, Ca 0.0005-0.005
% Of 1 or 2 and the balance Fe and unavoidable impurity elements, and Creq represented by the following formula (1) and Nieq represented by the formula (2) below are represented by the formula (3): In addition, a high-strength martensitic stainless steel excellent in seawater resistance, characterized by satisfying the formula (4) and consisting of a martensite phase containing no ferrite phase in its metal structure. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 4. C by weight% is 0.03% or less, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, and Nb 0.0
1-0.5%, V 0.01-0.5% of 1 type or 2
Containing seeds, Al 0.001-0.05%, C
a 0.0005 to 0.005% of 1 type or 2 types, the balance consisting of Fe and unavoidable impurity elements,
Furthermore, when Creq represented by the following formula (1) and Nieq represented by the formula (2) satisfy the formulas (3) and (4), martensite containing no ferrite phase in its metal structure High-strength martensitic stainless steel with excellent seawater resistance, which consists of two phases. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 5. A weight percent of C 0.03% or less, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, the balance consisting of Fe and inevitable impurity elements, and Creq represented by the following formula (1) and Nieq represented by the formula (2)
By satisfying the expressions (3) and (4), a steel material containing no ferrite phase in its metal structure is subjected to quenching treatment from a temperature range of 900 to 1100 ° C. to 100 ° C. or less after hot rolling is completed, After that, 500-700 ° C
A method for producing high-strength martensitic stainless steel with excellent seawater resistance, which comprises subjecting a steel to a tempering treatment. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 6. A weight percent of C 0.03% or less, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, and Nb 0.
01-0.5%, V 0.01-0.5% of 1 type or 2 types, the balance consists of Fe and unavoidable impurity elements, and Creq represented by the following formula (1): When Nieq represented by the formula (2) satisfies the formulas (3) and (4), a steel material containing no ferrite phase in its metal structure is heated to 900 to 1 after completion of hot rolling.
A method for producing a high-strength martensitic stainless steel excellent in seawater resistance, which comprises performing quenching treatment from a temperature range of 100 ° C. to 100 ° C. or lower, and then tempering treatment at 500 to 700 ° C. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 7. A weight percent of C 0.03% or less, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, Al 0.
001-0.05%, Ca 0.0005-0.005
% Of 1 or 2 and the balance Fe and unavoidable impurity elements, and Creq represented by the following formula (1) and Nieq represented by the formula (2) below are represented by the formula (3): Also, by satisfying the expression (4), a steel material containing no ferrite phase in its metallographic structure should be
A method for producing a high-strength martensitic stainless steel excellent in seawater resistance, which comprises performing quenching treatment from a temperature range of 00 to 1100 ° C. to 100 ° C. or lower, and then tempering treatment to 500 to 700 ° C. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel. 8. A weight percentage of C 0.03% or less, Si
1.0% or less, Mn 2.0% or less, Cr 11 to 15
%, Ni 2.0-7.0%, Mo over 2.0% -4.
0%, N 0.02% or less, and Nb 0.0
1-0.5%, V 0.01-0.5% of 1 type or 2
Containing seeds, Al 0.001-0.05%, C
a 0.0005 to 0.005% of 1 type or 2 types, the balance consisting of Fe and unavoidable impurity elements,
Further, Creq represented by the following formula (1) and Nieq represented by the formula (2) satisfy the formulas (3) and (4), so that a steel material containing no ferrite phase in its metal structure can be obtained. After the hot rolling, the high-strength martensite excellent in seawater resistance is characterized by being subjected to quenching treatment from a temperature range of 900 to 1100 ° C. to 100 ° C. or lower, and then tempering treatment to 500 to 700 ° C. Manufacturing method of stainless steel. Nieq = Ni + 0.5 [Mn] +30 [C + N] (1) Creq = Cr + Mo + 1.5 [Si] (2) Creq-Nieq ≦ 11.0 (3) Creq + Nieq ≦ 23.0 (4) [] in the formula Indicates the content (% by weight) of each component in steel.