JP2021143354A - Martensitic stainless steel for laser welding - Google Patents

Abstract

To provide a martensitic stainless steel that has excellent weld crack resistance, corrosion resistance, and high hardness property of a laser weld-zone.SOLUTION: Provided is a martensitic stainless steel for laser welding that has a chemical composition containing, in mass%, C: 0.15 to 0.60%, Si: 0.1 to 3.0%, Mn: 0.1 to 5.0%, S: 0.01% or less, P: 0.05% or less, Ni: 0.1 to 5.0% or less, Cr: 10.5 to 16.0%, N: 0.01 to 0.15%, and Al: 0.002 to 1.0%, containing one or more of Mo: 0.05 to 3.0% and W: 0.05 to 3.0%, containing one or more of V: 0.01 to 1.0% and Nb: 0.01 to 0.45%, and the balance being Fe and unavoidable impurities, and in which the D value is 0 to 10 and the M value is 0.3 to 5.5.SELECTED DRAWING: None

Description

The present invention relates to a martensitic stainless steel for laser welding, which has high hardness and high corrosion resistance without welding cracks with respect to a high hardness and high corrosion resistance material to be laser welded.

Martensitic stainless steel with high hardness and high corrosion resistance of 500 Hv or more is used in severe corrosion environments where chlorides and inferior fuels such as infrastructure and automobiles are present due to its excellent durability characteristics such as wear resistance, fatigue strength, and corrosion resistance. It has come to be widely used in various parts. Until now, it has been used for parts in cold molding and hot molding, but in recent years, its application to welded parts has also been studied. In particular, in recent years, laser welding with high productivity has been widely used in the processing of steel parts, and there is a high need for laser welding even for martensitic stainless steel with high hardness and high corrosion resistance. However, it is difficult to maintain the corrosion resistance and high hardness of the welded portion and the welded HAZ portion while avoiding weld cracks.

Regarding laser welding of martensitic stainless steel, a technique of optimizing the laser welding method and cutting after welding has been proposed for the purpose of reducing defects and improving toughness of the welded portion (Patent Document 1). However, there is no disclosure of technology regarding the durability of welds.

Regarding the corrosion resistance of the laser welded part of martensite-based stainless steel, by optimizing the laser welding conditions with the C content of about 13% Cr martensite-based stainless steel set to 0.05% or less, welding cracks will not occur during laser welding. , It is disclosed that the corrosion resistance of the welded portion can be ensured (Patent Documents 2 and 3). However, the amount of C is low, and it is not possible to foresee ensuring corrosion resistance in a high hardness state of 500 Hv or more.

On the other hand, it has been proposed that both high hardness and high corrosion resistance can be achieved by adjusting the composition of martensitic stainless steel and controlling the structure by suppressing δ-ferrite and suppressing the precipitation of Cr carbides deposited on the interface (Patent Document 4). However, if δ-ferrite is suppressed by laser welding, welding cracks occur.

Japanese Patent No. 3797105 Japanese Patent No. 3033483 Japanese Patent No. 3064851 Japanese Patent No. 3340225

In the known techniques or combinations described in the above background techniques, the present inventor has laser welded high hardness martensitic stainless steel parts of 500 Hv or higher used in a severe corrosive environment in the presence of chlorides and acids. It was found that it is not possible to suppress welding cracks and achieve both high hardness and corrosion resistance.

The problem to be solved by the present invention is that a martensite-based stainless steel part used in a harsh environment in which chloride and inferior fuel are present can be processed into a part having a predetermined shape by laser welding without welding cracks. An object of the present invention is to provide a martensite-based stainless steel for laser welding, which can impart high hardness and high corrosion resistance including the vicinity of a welded portion to improve durability.

As a result of various studies on martensite-based stainless steel capable of obtaining a high hardness of 500 Hv or more in order to solve the above problems, the present inventors have adjusted the components so that a predetermined amount of δ ferrite is generated during laser welding. Even hard materials can be joined without cracking during laser welding, and by containing appropriate amounts of Mo, W, Nb, and V, softening of the welded HAZ portion is suppressed to ensure high hardness, and Cr coal in the welded portion. We obtained the findings that the precipitation of nitrides is suppressed and the high corrosion resistance near the laser weld is significantly improved. That is, the laser welded portion satisfies all of the welding crack resistance, the high hardness of 500 Hv or more, and the corrosion resistance in a chloride resistant environment.

The present invention has been made based on the above findings, and the gist thereof is as follows.
(1) By mass%
C: 0.15 to 0.60%,
Si: 0.1 to 3.0%,
Mn: 0.1 to 5.0%,
S: 0.01% or less,
P: 0.05% or less,
Ni: 0.1 to 5.0%,
Cr: 10.5 to 16.0%,
N: 0.01-0.15%,
Al: Containing 0.002 to 1.0%,
Mo: 0.05-3.0%,
W: 0.05 to 3.0%
Contains one or more of
V: 0.01-1.0%,
Nb: 0.01 to 0.45%
Contains one or more of
It has a chemical component consisting of the balance Fe and impurities,
The D value represented by the equation (a) is 0 to 10, and the value is 0 to 10.
A martensitic stainless steel for laser welding, characterized in that the M value represented by the formula (b) is 0.3 to 5.5.
D = (Cr + 0.5Si + 2.5Al)-(25C + 18N + Ni + 0.1Mn) + (1.2Mo + 2W + 3V + 3Nb) -6 ... (a)
M = Mo + W + 10 (V + Nb) ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (B)
In the above formula, the element symbol means the content (mass%) of the element.
(2) By changing to a part of the Fe and further by mass%,
Cu: 3.0% or less,
Co: 3.0% or less B: 0.01% or less,
Sn: 0.3% or less,
Sb: The martensitic stainless steel for laser welding according to (1) above, which contains one or more of 0.3% or less.
(3) By changing to a part of the Fe and further by mass%,
Ti: 0.45% or less,
Ta: The martensitic stainless steel for laser welding according to (1) or (2) above, which contains one or more of 0.45% or less.
(4) By changing to a part of the Fe and further by mass%,
Mg: 0.01% or less,
Ca: 0.01% or less,
Hf: 0.01% or less,
REM: The martensitic stainless steel for laser welding according to (1) to (3) above, which contains one or more of 0.05% or less.

According to the present invention, in martensite-based stainless steel, by appropriately adjusting the components for controlling the metallographic structure and deposits, welding cracks during laser welding are suppressed, and high hardness and high vicinity of the welded portion are achieved. A material that can significantly improve corrosion resistance and is suitable for durable parts processed by laser welding is provided.

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

<< Essential composition of steel >>
The publicly disclosed low-C martensitic stainless steel cannot industrially secure wear resistance and fatigue resistance. In the present invention, parts having 500 Hv or more, which are generally effective in abrasion resistance and fatigue resistance, are targeted, and high-hardness martensitic stainless steel exhibiting at least 500 Hv or more in a hardened state is used as a base.

C is contained in an amount of 0.15% or more after quenching of the base metal, or in order to make the hardness of the laser welded portion and the welded HAZ portion 500 Hv or more. However, if it is added in excess of 0.60%, coarse Cr carbides are generated, which deteriorates weld crackability and corrosion resistance. Therefore, it is limited to 0.60% or less. Preferably, it is more than 0.20% and 0.50% or less.

N is contained in an amount of 0.01% or more in order to suppress softening of the laser welded HAZ portion and ensure corrosion resistance. However, if it is contained in excess of 0.15%, welding cracks (welding defects) occur due to blow holes. Therefore, it is limited to 0.15% or less. It is preferably in the range of 0.02 to 0.12%.

Si is contained in an amount of 0.1% or more in order to promote deoxidation in the molten portion of the laser welded portion and suppress welding cracks (welding defects). However, if it is contained in excess of 3.0%, it becomes brittle and weld cracks (welding defects) occur. Therefore, it is limited to 3.0% or less. It is preferably in the range of 0.2 to 2.0%.

Mn is contained in an amount of 0.1% or more in order to promote deoxidation in the molten portion of the laser welded portion and suppress welding cracks (welding defects). However, if it is added in excess of 5.0%, retained austenite is generated and it becomes impossible to secure a high hardness of 500 Hv or more in the welded portion. Therefore, it is limited to 5.0% or less. Preferably, it is 0.2 to 3.0%.

Since S is an element that promotes welding cracks and deteriorates corrosion resistance, the content is limited to 0.01% or less. Preferably, it is 0.007% or less.

Since P is an element that segregates grain boundaries and promotes weld crackability, its content is limited to 0.05% or less. Preferably, it is 0.035% or less.

Ni is contained in an amount of 0.1% or more in order to improve the toughness of martensitic stainless steel and suppress welding cracks. However, if it is added in excess of 5.0%, retained austenite is generated and it becomes impossible to secure a high hardness of 500 Hv or more in the welded portion. Therefore, it is limited to 5.0% or less. Preferably, it is 0.2 to 3.0%.

Cr is a basic element for obtaining the function of corrosion resistance of stainless steel, and contains 10.5% or more. However, if it is contained in excess of 16.0%, it becomes impossible to secure a high hardness of 500 Hv or more in the welded portion. Therefore, it is limited to 16.0% or less. Preferably, it is 11.0 to 15.0%.

Al is contained in an amount of 0.002% or more in order to promote deoxidation of the molten portion of the laser welded portion and suppress welding cracks (welding defects). However, if the content exceeds 1.0%, welding cracks (welding defects) occur due to the formation of coarse inclusions. Therefore, it is limited to 1.0% or less. It is preferably in the range of 0.005 to 0.2.

Mo and W are elements that improve corrosion resistance, and in order to suppress the formation of Cr carbonitride after laser welding and ensure corrosion resistance, one or more types of Mo and W are each 0.05% or more. Incorporate. However, if it is contained in excess of 3.0%, the toughness deteriorates and weld cracking is promoted. Therefore, it is limited to 3.0% or less.

V and Nb contain one or more of V and Nb in an amount of 0.01% or more, respectively, in order to suppress the formation of Cr carbonitride after laser welding and ensure corrosion resistance. However, when V exceeds 1.0% and Nb contains 0.45%, coarse precipitates are formed and weld cracks are promoted. Therefore, V is limited to 1.0% or less, and Nb is limited to 0.45% or less.

The D value represented by the above equation (a) affects the amount of δ-ferrite produced in the molten / quenching solidified portion during laser welding, and δ-ferrite that traps impurities such as P and S is generated to generate weld cracks. Set to 0 or more to prevent. However, if it exceeds 10, it becomes impossible to secure a high hardness of 500 Hv or more of the welded portion. Therefore, it is limited to 10 or less. It is preferably in the range of 2.0 to 7.0.

The M value represented by the above equation (b) affects the amount of Cr carbonitride produced during laser welding. The M value is set to 0.3 or more in order to suppress the formation of Cr carbonitride and secure the corrosion resistance. However, if it exceeds 4.0, the toughness deteriorates and weld cracking is promoted. Therefore, it is limited to 5.5 or less. Preferably, it is in the range of 0.5 to 4.0.

<< Selective ingredients >>
The stainless steel of the present invention is composed of chemical components composed of Fe and impurities other than the elements described above. Further, in addition to the above-mentioned component composition, the following elements may be selectively contained in place of a part of Fe.

Cu may be contained if necessary in order to improve the corrosion resistance of the product. However, even if the content exceeds 3.0%, the effect is saturated, retained austenite is generated, and not only the high hardness of 500 Hv or more of the welded portion cannot be secured, but also the weld crack deteriorates. Therefore, the content is set to 3.0% or less. Preferably, it is 1.5% or less.

Co may be contained if necessary in order to improve the toughness and corrosion resistance of the product. However, even if each of them is contained in excess of 3.0%, the effect is saturated and retained austenite and ferrite are generated, and it becomes impossible to secure a high hardness of 500 Hv or more in the welded portion. Therefore, the content is set to 3.0% or less. Preferably, it is 2.0% or less.

B may be contained if necessary in order to improve the toughness of the product and the welded portion. However, even if it is contained in excess of 0.01%, the effect is saturated, and conversely, coarse bolide is generated, welding cracks are promoted, and corrosion resistance is deteriorated. Therefore, the content is 0.01% or less. And. Preferably, it is 0.006% or less.

Sn and Sb may be contained as necessary in order to improve the corrosion resistance of the product. However, even if each content exceeds 0.3%, the effect is saturated and welding cracks are promoted, so the content is set to 0.3% or less. Preferably, it is 0.1% or less.

Ti may be contained if necessary in order to improve toughness and corrosion resistance. However, even if the content exceeds 0.45%, the effect is saturated, and conversely, coarse carbonitride is generated to promote weld cracking, so the content is set to 0.45% or less. Preferably, it is 0.3% or less.

Ta may be contained if necessary in order to improve the toughness and corrosion resistance of the product. However, even if the content exceeds 0.45%, the effect is saturated, and conversely, coarse carbonitride is generated to promote weld cracking, so the content is set to 0.45% or less. It is preferably 0.3% or less, more preferably 0.1% or less.

Mg, Ca, and Hf may be contained as necessary because they increase the thermodynamic stability of the deoxidized product and are effective in softening during softening and annealing. However, even if each is added in excess of 0.01%, the effect is saturated, and conversely, coarse oxides are generated to promote weld cracking, so the content is set to 0.01% or less. Preferably, it is 0.005% or less.

REM may be contained as needed because it increases the thermodynamic stability of the deoxidized product and is effective in softening during softening and annealing. However, even if it is added in excess of 0.05%, the effect is saturated, and conversely, coarse oxides are generated to promote weld cracking, so the content is set to 0.05% or less. Preferably, it is 0.005% or less. REM (rare earth element) is a general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), according to a general definition. It may be contained alone or as a mixture.

Regarding the impurities contained in the stainless steel of the present invention, typical impurities include Zn, Bi, Pb, Ge, Se, Ag, Se, Te, etc., and usually, as impurities in the steel manufacturing process, 0. Mix in the range of about 1%.
Oxygen, which is an impurity, is mainly present as an inclusion in steel, but the oxygen content level of stainless steel produced by ordinary refining is 0.001 to 0.015%.
In addition, although typical optional additive elements are specified in (2) to (5) above, even elements not described in the present specification do not impair the effects of the present invention. Can be contained in.

According to the present invention described above, it is possible to provide a martensitic stainless steel for high-hardness parts joined and processed by laser welding, which can prevent cracking during laser welding and can achieve both high hardness and corrosion resistance of the welded portion. ..
In the present invention, the martensite-based stainless steel is a steel that is hardened by martensite transformation during quenching. In the present invention, for example, when quenching from 1050 ° C. by air cooling, 70% or more of the metal structure is martensite. It shows the structure and means that it is a steel that hardens to 500 Hv or more.

The steel having the chemical composition shown in Table 1 was melted at 1600 ° C. in a 50 kg vacuum melting furnace and then cast into a mold. Then, after heating at 1200 ° C., a plate having a thickness of 1200 ° C. is hot-rolled and softened and annealed at 750 ° C. for 3 hours, and then cold-rolled to a thickness of 2 mm. Finished by polishing and laser welded. In laser welding, two 40 mm pieces of test pieces were butted against each other, and a fiber laser (spot diameter 0.6 mm, 2.0 kW, Ar shield) was irradiated on the contact surface under the condition of 1 m / min to join a 40 mm length. .. After that, the presence or absence of weld cracks, hardness, and corrosion resistance were evaluated. Table 2 shows the evaluation results.

Weld cracks were evaluated by observing the vicinity of the weld with a magnifying glass and measuring the crack length. When the total length of the crack was less than 1 mm, it was judged as ◯, when it was less than 0.5 mm, it was judged as ⊚, and when it was 1 mm or more, it was judged as x.

To evaluate the hardness near the weld, the cross section perpendicular to the longitudinal direction of the weld is embedded and polished as an inspection surface, and the Hv hardness of the melt and the weld HAZ (0.2, 0.4 mm, 0.6 mm from the weld line) is hardened. (Load 200 g) was measured. The hardness of the welded HAZ portion was taken as the average value of the three measurement points.

The corrosion resistance was evaluated in the rusting condition by polishing the surface layer of the laser welded portion with # 500 and performing a salt spray test of JIS Z 2371 for 48 hours. The case of no rust was evaluated as ⊚, the case of spot rust was evaluated as 〇, and the case of flowing rust was evaluated as ×.

Regarding the evaluation of coarse oxides, the above-mentioned embedded and polished inspection surface was observed with an optical microscope, and when there was a coarse oxide having a major axis of 20 μm or more, it was described as “coarse oxide” in the remarks column of Table 2. ..

In Examples 1 to 34 of the present invention in Table 2, the hardness of the welded portion is as high as 500 Hv or more, and excellent weld cracking resistance and corrosion resistance are exhibited.

On the other hand, Comparative Examples 1 to 37 are out of the component range of the present invention, and cannot satisfy all of the high hardness characteristics of 500 Hv or more, weld cracking property, and corrosion resistance.

As is clear from each of the above examples, the present invention provides a martensitic stainless steel suitable for laser welding, which has excellent weld crack resistance during laser welding, high hardness characteristics of 500 Hv or more of the laser welded portion, and corrosion resistance. It is extremely useful industrially because it can significantly improve the durability of high-hardness parts used in severely corroded environments.

Claims (4)

By mass%
C: 0.15 to 0.60%,
Si: 0.1 to 3.0%,
Mn: 0.1 to 5.0%,
S: 0.01% or less,
P: 0.05% or less,
Ni: 0.1 to 5.0%,
Cr: 10.5 to 16.0%,
N: 0.01-0.15%,
Al: Containing 0.002 to 1.0%,
Mo: 0.05-3.0%,
W: 0.05 to 3.0%
Contains one or more of
V: 0.01-1.0%,
Nb: 0.01 to 0.45%
Contains one or more of
It has a chemical component consisting of the balance Fe and impurities,
The D value represented by the equation (a) is 0 to 10, and the value is 0 to 10.
A martensitic stainless steel for laser welding, characterized in that the M value represented by the formula (b) is 0.3 to 5.5.
D = (Cr + 0.5Si + 2.5Al)-(25C + 18N + Ni + 0.1Mn) + (1.2Mo + 2W + 3V + 3Nb) -6 ... (a)
M = Mo + W + 10 (V + Nb) ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ ・ (B)
In the above formula, the element symbol means the content (mass%) of the element. By changing to a part of the Fe, and further by mass%,
Cu: 3.0% or less,
Co: 3.0% or less B: 0.01% or less,
Sn: 0.3% or less,
The martensitic stainless steel for laser welding according to claim 1, wherein Sb: contains one or more of 0.3% or less. By changing to a part of the Fe, and further by mass%,
Ti: 0.45% or less,
The martensitic stainless steel for laser welding according to claim 1 or 2, wherein Ta: contains one or more of 0.45% or less. By changing to a part of the Fe, and further by mass%,
Mg: 0.01% or less,
Ca: 0.01% or less,
Hf: 0.01% or less,
REM: The martensitic stainless steel for laser welding according to any one of claims 1 to 3, wherein the martensitic stainless steel contains one or more of 0.05% or less.