JP2995524B2 - High strength martensitic stainless steel and its manufacturing method - 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 martensitic stainless steel excellent in weldability and applicable to welded structures, such as architectural structures and marine structures, which can be used for parts requiring welding. It is. In particular, it is an object of the present invention to supply a thick high-strength steel material applicable to a large structure.

[0002]

2. Description of the Related Art Martensitic stainless steel is widely used as a cutting tool or a spring material because its strength can be easily increased by quenching heat treatment. However, these martensitic stainless steels cannot be used for welding structures because their toughness is lower than other stainless steels and their weldability is extremely poor. In recent years, steel materials in which the contents of C and N are reduced and the toughness and weldability are improved by further adding Ni are proposed in Japanese Patent Publication No. 51-13463 and Japanese Patent Application Laid-Open No. 61-136661. .

[0003]

When a thick martensitic stainless steel is applied to a welded structure, the toughness of the base metal and the weld heat affected zone becomes a problem. Insufficient toughness not only lowers the reliability of the structure, but also causes low-temperature cracking during the cooling process after heat treatment or during welding. In order to ensure the toughness and ductility of the base material and to prevent low-temperature cracking in the weld, merely reducing the contents of C and N and adding Ni is not sufficient. That is, it is necessary to control the delta-ferrite phase and inclusions that affect toughness, and to suppress the hydrogen content in steel to a certain value or less. The delta-ferrite phase, the cluster-like inclusions, and the hydrogen in the steel above a certain value all significantly deteriorate the toughness and weldability of the thick martensitic stainless steel.

The present invention provides a thick high-strength martensitic stainless steel applicable to a large welded structure by controlling the delta-ferrite phase and inclusions to reduce the amount of hydrogen contained in the steel to a certain value or less. An object of the present invention is to provide steel and a method for producing the steel.

[0005]

SUMMARY OF THE INVENTION The present invention overcomes the above-mentioned problems of the prior art and provides a component for realizing a high-strength martensitic stainless steel excellent in toughness of a heat affected zone and excellent in weldability. In particular, the present invention has found an effective production method for limiting the hydrogen content and controlling the hydrogen content in the component system of the present invention within a necessary range.

That is, the invention of claims 1 and 2 is based on
C: 0.03% or less, Si: 1.0% or less, Mn:
2.0% or less, Cr: 11 to 17%, Ni: 3.5 to
7.0%, N: 0.02% or less, H: 0.0005% or less, Al: 0.001 to 0.05%, Ca: 0.00
0.05 to 0.005%, and if necessary, Mo: 0.1 to 4.0% or Nb:
It contains one or two of 0.01 to 0.5%, and further, Nieq represented by the following formula (1) and Creq represented by the following formula (2) are obtained by formulas (3) and (4). By satisfying the formula, the gist of the invention is a high-strength martensitic stainless steel characterized in that its metal 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 expression to prevent 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. H unavoidably contained in the steel not only significantly deteriorates the ductility and toughness of the steel of the component system of the present invention having high strength, but also causes low-temperature cracking and delayed fracture of the welded portion.
This material deterioration due to H is remarkable in a thick material. In order to solve the above-mentioned problems caused by H, it is necessary to suppress the content to a certain value or less. Because the steel of the present invention has high strength,
The H content must be strictly limited to 0.0005% or less. Furthermore, Al and Ca stably reduce S and O, which are harmful to toughness, and are effective in controlling the shape of nonmetallic inclusions.
If these elements are not added, coarse inclusions remain in the steel, deteriorating the ductility and toughness of the steel material. In particular, the toughness in the thickness direction is significantly reduced. By satisfying the above component range, a high-strength martensitic stainless steel having a tensile strength of at least 90 kg / mm ² , high ductility and toughness, and excellent low-temperature cracking resistance or delayed fracture resistance during welding can be obtained. Realize.

The invention according to claims 3 and 4 is characterized in that after the steel material limited to the above components is subjected to hot rolling or quenching heat treatment,
After cooling to 200 ° C. or less, the structure is mostly transformed from the austenite phase to the martensite phase, and then Ht represented by the formula (5) is brought to a temperature range of 400 to 650 ° C.
A gist of the present invention is a method for producing a high-strength martensitic stainless steel, characterized in that H, which is inevitably contained in steel, is reduced to 0.0005% or less by performing a dehydrogenation heat treatment for a period of time.

[0009]

The reasons for limiting the components of the present invention will be described in detail. C: an element effective for increasing the strength by hardening the martensite phase, but significantly deteriorates the toughness and weldability and increases the sensitivity to hydrogen in the steel. % Or less.

Si: Inevitably contained in steel as a deoxidizing element, but if added excessively, it deteriorates toughness and weldability, so its content was made 1.0% or less. Mn: Suppresses the delta ferrite phase and also has the effect of fixing S in the steel. However, if added excessively, the toughness is reduced, so the content is set to 2.0% or less. Cr: a basic element of stainless steel, a content of at least 11% or more is required to obtain excellent corrosion resistance. However, if it exceeds 17%, a delta ferrite phase remains in the martensite phase, deteriorating toughness and weldability.

Ni: An important additive element for improving the toughness of the martensite phase and improving the weldability. In order to ensure sufficient toughness and maintain good weldability of thick-walled materials, a content of 3.5% or more is required. However, when added in excess of 7.0%, the retained austenite phase sharply increases and the strength decreases. Mo: While improving corrosion resistance, strength after tempering,
It is an effective additive element for improving toughness. 0.1%
The effect is sufficiently exhibited when the above addition is performed. However, when the addition is excessive, the ferrite phase hardly disappears even when Ni is added to the upper limit of the above-mentioned component range, so that it can be added to 4.0%.

Nb: It can be added as appropriate to suppress the reduction of corrosion resistance and strength during tempering and to improve the intergranular corrosion resistance of the heat affected zone by welding. In order to exert the effect, the content is required to be 0.01% or more.
If added in excess of 5%, cracks tend to occur during welding or hot rolling. N: Similar to C, it is an element effective for hardening the martensite phase and increasing the strength, but significantly deteriorates the toughness and weldability and increases the sensitivity to hydrogen in steel. Was set to 0.02% or less.

H: A harmful element that is inevitably present in steel, but impairs the ductility, toughness, and weldability of high-strength martensitic stainless steel. When the H content is high, delayed fracture may occur due to thermal stress generated during heat treatment or cooling after welding. The H content is such that the above problem does not occur within the component range of the present invention, that is, 0.0005.
It must be strictly limited to below%. The steel of the present invention is an austenite phase having a large amount of hydrogen solid solution at a high temperature side, and has a low temperature of transformation to a martensite phase of about 300 ° C., so that hydrogen easily remains in the steel. Therefore, in order to stably maintain the H content at 0.0005% or less, it is necessary to suppress the intrusion of H from a refractory or the like at the melting stage and to cool to 200 ° C. after hot rolling or quenching heat treatment. Then, after the structure is mostly changed to the martensite phase, the dehydrogenation heat treatment must be performed in a temperature range that does not reversely transform into the austenite phase.

Al, Ca: S and O, which are harmful to toughness, are reduced in the molten steel stage, and O remaining in the steel is fixed as inclusions. However, excessive addition will cause coarse inclusions,
Conversely, it deteriorates toughness and deteriorates manufacturability. Therefore, the addition amount of Al is 0.001 to 0.05%,
To 0.0005% to 0.005%. Cu as another element has the effect of increasing the strength after tempering, but if added excessively, the heat-affected zone becomes significantly harder and weld cracks are more likely to occur, so the upper limit is preferably 2%. . P inevitably contained in steel,
Since S and O decrease the toughness of the steel material and deteriorate the weldability, their contents are 0.003% or less and 0.005% or less, respectively.
% Or less, preferably 0.01% or less. La,
Addition of a lanthanoid-based rare earth element such as Ce is also effective for fixing S and O, but its oxide or sulfide has a large specific gravity and is difficult to float during refining, so that it tends to remain in steel. Therefore, it is desirable that the amount of addition be suppressed to 0.05% or less.

Next, a production method satisfying the above H content will be described. First, it is necessary to minimize the mixing of H in the smelting stage. For that purpose, it is effective to sufficiently dry the furnace material, the runner and the like to remove moisture. If a large amount of H is mixed at this stage, a long time is required for dehydrogenation heat treatment described later, which is industrially disadvantageous.

Next, after hot rolling or quenching heat treatment, 2
It must be cooled to below 00 ° C. to transform most of the metal structure into a martensitic phase. If this cooling is insufficient and a large amount of the austenite phase remains, the hydrogen content does not decrease over a long period of time in the subsequent dehydrogenation heat treatment. This is because the amount of solid solution of hydrogen in the austenite phase is large. That is, cooling is performed to 200 ° C. or lower to make most of the metal structure into a martensite phase having a small hydrogen solid solution amount, so that hydrogen is easily released from steel.

Thereafter, in order to promote the diffusion of hydrogen, it is necessary to heat to a temperature range in which the martensite phase does not reversely transform into the austenite phase. Its temperature range is 4
00 ° C to 650 ° C. If the temperature is lower than 400 ° C., the diffusion rate of hydrogen is low, and it takes a long time to release hydrogen. When heated above 650 ° C., the martensite phase undergoes a reverse transformation to the austenite phase, the amount of solid solution of hydrogen increases, and the hydrogen content does not decrease.

In this temperature range, the holding time required for dehydrogenation is related to the thickness of the steel material, and must be a time that satisfies the Ht time shown in equation (5). If the condition of the retention time is not satisfied, the hydrogen release is insufficient and the hydrogen content does not reach 0.0005% or less. This dehydrogenation heat treatment can also be used as a tempering heat treatment for adjusting mechanical properties. When hydrogen is mixed during welding, it is also effective to perform dehydrogenation heat treatment under the above conditions even after welding heat treatment.

In the present invention, in order to improve toughness and weldability, the components that do not form a ferrite phase are limited and the production conditions are limited. However, the ferrite phase is partially formed due to segregation of components during melting and solidification. When it is formed, it is desirable to perform homogenization annealing before or after hot rolling.
The quenching heat treatment does not produce a ferrite phase.
It is desirable that the temperature be 0 ° C. or lower.

[0020]

EXAMPLES Table 1 shows the chemical components of the test steel.

[0021]

[Table 1]

The test steels denoted by the symbols A to K in the table were prepared by melting in a laboratory and then hot rolling to 40 mm. The test steel is
Immediately after the hot rolling, it was kept at 900 ° C. for 1 hour and then air-cooled to room temperature. Then, Charpy test pieces and tensile test pieces were cut out from the central part of the base material at right angles to the rolling direction, and the mechanical properties thereof were examined. Further, a weld test piece having an H-shaped narrow groove (15 mm in width) was cut out from the same steel sheet, and weld cracking susceptibility was examined. The welding is TIG welding, using a welding rod made from test steel C, welding current 250A, welding speed 5c / min.
m and 10 layers per side. No preheating was performed. Twenty-four hours after the completion of welding, five sections were cut out to check for the occurrence of weld cracks. 0.2% proof stress, tensile strength, elongation at break,
Table 2 shows the Charpy absorbed energy at 0 ° C. and the number of cross sections where cracks occurred in the above welding test.

[0023]

[Table 2]

Next, a test steel L having the chemical composition shown in Table 3 was melted in a large electric furnace and cast into an ingot.
A slab with a thickness of 0 mm was produced and subsequently hot rolled to 40 mm. The hydrogen content shown in Table 3 is a value measured by collecting a sample after hot rolling. 9
After performing a quenching heat treatment at 50 ° C. for 1 hour, it was cooled to the temperature shown in Table 4, and a dehydrogenation heat treatment was performed while changing the holding temperature and the holding time. Thereafter, a Charpy test piece and a tensile test piece were cut out in the same manner as described above, and the mechanical properties of the base material were examined, and a welding test was performed under the same conditions as above to confirm whether or not cracking occurred. Table 4 shows the results and the residual hydrogen content after the dehydrogenation heat treatment.

[0025]

[Table 3]

[0026]

[Table 4]

From the above examples, the steel of the present invention has high strength,
The toughness is also excellent, and it can be seen that cracking does not occur even when welding with a thick material. Further, the steel of the present invention has excellent corrosion resistance and fatigue properties in addition to strength and toughness, and has a stress ratio of 0.1 and a repetition rate of 5 × 10 ⁶ in a fatigue test. ^Two or more fatigue strengths were obtained.

[0028]

The high-strength martensitic stainless steel of the present invention is excellent in toughness and exhibits excellent weldability even in thick-walled materials. Very large.

Continued on the front page (72) Inventor Akio Yamamoto 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Tetsuya Shimada 1-1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka New Nippon Steel Corporation Yawata Works (72) Inventor Kazuhiro Kazuhiro 1-1-1, Tobata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Nippon Steel Corporation Yawata Works (72) Inventor Hideo Sakurai Shintomi, Futtsu-shi, Chiba 20-1 Inside the Technology Development Division of Nippon Steel Corporation (56) References JP-A-5-156409 (JP, A) JP-A-1-162750 (JP, A) (58) Fields studied (Int. Cl. ⁶ , DB name) C22C 38/00-38/58 C21D 3/06-8/00

Claims (4)

(57) [Claims] 1. C: 0.03% or less by weight, Si:
1.0% or less, Mn: 2.0% or less, Cr: 11 to 17
%, Ni: 3.5 to 7.0%, N: 0.02% or less,
H: 0.0005% or less, Al: 0.001 to 0.0
5%, Ca: 0.0005 to 0.005%, containing one or two kinds, the balance being Fe and unavoidable impurity elements, and Nieq represented by the following formula (1).
And the high-strength martensite characterized in that Creq represented by the formula (2) satisfies the formulas (3) and (4), whereby the metal structure is composed of a martensite phase containing no ferrite phase. 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) ] Indicates the content (% by weight) of each component in steel. 2. Mo: 0.1 to 4.0%, Nb: 0.0
The high-strength martensitic stainless steel according to claim 1, wherein one or two kinds are contained in 1 to 0.5%. 3. C: 0.03% or less by weight, Si:
1.0% or less, Mn: 2.0% or less, Cr: 11 to 17
%, Ni: 3.5 to 7.0%, N: 0.02% or less,
Al: 0.001-0.05%, Ca: 0.0005-
One or two of 0.005% is contained, and the balance is F
e and unavoidable impurity elements, and furthermore, Nieq represented by the following formula (1) and Cr represented by the following formula (2).
When the eq satisfies the formulas (3) and (4), the steel material having no ferrite phase in its metal structure is cooled to 200 ° C. or less after hot rolling or quenching heat treatment, and then 400 to 650. By maintaining the Ht time in the temperature range of equation (5) in the temperature range of formula (5), H in the steel is reduced to 0.
A method for producing high-strength martensitic stainless steel, characterized by being at most 0005%. 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 expression Indicates the content (% by weight) of each component in steel. ^{Ht ≧ t 2/1000 (5} ) t is the steel thickness of formula (mm), Ht denotes the retention time (hours). 4. C: 0.03% or less by weight, Si:
1.0% or less, Mn: 2.0% or less, Cr: 11 to 17
%, Ni: 3.5 to 7.0%, N: 0.02% or less,
Al: 0.001-0.05%, Ca: 0.0005-
One or two of 0.005%,
o: 0.1 to 4.0%, Nb: one or two of 0.01% to 0.5%, the balance being Fe and unavoidable impurity elements, and the following formula (1) When Nieq represented by the formula and Creq represented by the formula (2) satisfy the formulas (3) and (4), the steel material having no ferrite phase in its metal structure is subjected to hot rolling or quenching heat treatment. After completion, cool to below 200 ° C,
A method for producing a high-strength martensitic stainless steel, wherein the H in the steel is reduced to 0.0005% or less by maintaining the temperature in the temperature range of 00 to 650 ° C. for the Ht time shown in equation (5). 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 expression Indicates the content (% by weight) of each component in steel. ^{Ht ≧ t 2/1000 (5} ) t is the steel thickness of formula (mm), Ht denotes the retention time (hours).