JP7474077B2 - Martensitic Stainless Steels for Laser Welding - Google Patents

Description

The present invention relates to high-hardness, high-corrosion-resistant materials to be laser welded, and to martensitic stainless steel for laser welding that is free of weld cracks and has high hardness and high corrosion resistance.

High-hardness, high-corrosion-resistant martensitic stainless steels with a hardness of 500 Hv or more have excellent durability properties, such as wear resistance, fatigue strength, and corrosion resistance, and are therefore widely used in parts used in severe corrosive environments such as infrastructure and automobiles, where chlorides and inferior fuels are present. Until now, they have been used in parts that have been cold-formed or hot-formed, but in recent years, their use in welded parts has also been considered. In particular, laser welding, which has high productivity, has come to be used frequently in the processing of steel parts in recent years, and there is a high demand for laser welding in high-hardness, high-corrosion-resistant martensitic stainless steels. However, it is difficult to maintain the corrosion resistance and high hardness of welds and weld HAZ parts while avoiding weld cracking.

With regard to laser welding of martensitic stainless steel, technology has been proposed for optimizing the laser welding method and cutting after welding with the aim of reducing defects in the weld and improving the toughness (Patent Document 1). However, no technology has been disclosed regarding the durability of the weld.

Regarding the corrosion resistance of laser welds of martensitic stainless steel, it has been disclosed that by optimizing the laser welding conditions by setting the C content of approximately 13% Cr martensitic stainless steel to 0.05% or less, it is possible to ensure corrosion resistance of the welds without weld cracks during laser welding (Patent Documents 2 and 3). However, the C content 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 high hardness and high corrosion resistance can be achieved by controlling the structure of martensitic stainless steel by adjusting the components and suppressing δ-ferrite, thereby suppressing the precipitation of Cr carbides at the interface (Patent Document 4). However, suppressing δ-ferrite in laser welding causes weld cracks.

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

The inventors have found that the known techniques or combinations described in the Background Art above are unable to suppress weld cracking and achieve both high hardness and corrosion resistance in laser-welded martensitic stainless steel components with a hardness of 500 Hv or more that are used in severe corrosive environments in which chlorides and acids are present.

The problem to be solved by the present invention is to provide a martensitic stainless steel for laser welding that can be processed into a part of a specified shape by laser welding without weld cracks, and that can improve durability by imparting high hardness and high corrosion resistance, including to the vicinity of the weld, for martensitic stainless steel parts used in severe corrosive environments where chlorides and inferior fuels are present.

In order to solve the above problems, the inventors conducted various studies on martensitic stainless steels that can achieve a high hardness of 500 Hv or more. As a result, they discovered that by adjusting the composition so that a specified amount of δ ferrite is generated during laser welding, even high-hardness materials can be joined without cracking during laser welding, and that by adding appropriate amounts of Mo, W, Nb, and V, softening of the weld HAZ is suppressed to ensure high hardness, and precipitation of Cr carbonitrides in the weld is suppressed, significantly improving high corrosion resistance in the vicinity of the laser weld. In other words, the laser weld satisfies all of the following requirements: resistance to weld cracking, high hardness of 500 Hv or more, and corrosion resistance in chloride environments.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
(1) 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%,
Cr: 10.5 to 16.0%,
N: 0.01 to 0.15%,
Al: 0.002 to 1.0%;
Mo: 0.05 to 3.0%,
W: 0.05 to 3.0%
Contains one or more of the following:
V: 0.01 to 1.0%,
Nb: 0.01 to 0.45%
Contains one or more of the following:
The balance has a chemical composition of Fe and impurities,
(a) the D value represented by the formula is 0 to 10;
A martensitic stainless steel for laser welding, characterized in that the M value represented by 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 indicates the content (mass %) of the corresponding element.
(2) Part of the Fe is replaced with further 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 (1) above, characterized in that it contains one or more of the following: Sb: 0.3% or less.
(3) Part of the Fe is replaced with further mass%:
Ti: 0.45% or less,
The martensitic stainless steel for laser welding according to (1) or (2), characterized in that it contains one or more of the following: Ta: 0.45% or less.
(4) Part of the Fe is replaced with further mass%:
Mg: 0.01% or less,
Ca: 0.01% or less,
Hf: 0.01% or less,
The martensitic stainless steel for laser welding according to any one of (1) to (3) above, characterized in that it contains one or more of the following: REM: 0.05% or less.

According to the present invention, by adjusting the composition of martensitic stainless steel appropriately to control the metal structure and precipitates, it is possible to suppress weld cracks during laser welding and significantly improve the high hardness and high corrosion resistance near the weld, providing a material suitable for durable parts processed by laser welding.

Each of the requirements of the present invention will be explained below. In the following explanation, (%) means mass (%) unless otherwise specified.

<Essential composition of steel>
The publicly known and disclosed low-C martensitic stainless steels cannot ensure industrially sufficient wear resistance and fatigue strength. The present invention is intended for parts with a hardness of 500 Hv or more, which is generally effective for wear resistance and fatigue strength, and is based on a high-hardness martensitic stainless steel that exhibits at least 500 Hv in a quenched state.

C is added at 0.15% or more to make the hardness of the base material 500Hv or more after hardening, or in laser welds and weld HAZs. However, adding more than 0.60% produces coarse Cr carbides, which deteriorates weld crack resistance 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 at 0.01% or more to suppress softening of the laser weld HAZ and ensure corrosion resistance. However, if it is contained at more than 0.15%, weld cracks (weld defects) will occur due to blowholes. Therefore, it is limited to 0.15% or less. The preferred range is 0.02 to 0.12%.

Si is contained at 0.1% or more to promote deoxidation in the molten zone of laser welds and suppress weld cracks (weld defects). However, if it is contained at more than 3.0%, it becomes embrittled and weld cracks (weld defects) occur. Therefore, it is limited to 3.0% or less. The preferred range is 0.2 to 2.0%.

Mn is added at 0.1% or more to promote deoxidation in the molten zone of laser welds and suppress weld cracks (weld defects). However, if added in excess of 5.0%, residual austenite is generated and it becomes impossible to ensure a high hardness of 500Hv or more in the welds. Therefore, it is limited to 5.0% or less. Preferably, it is 0.2 to 3.0%.

Since S promotes weld cracking and deteriorates corrosion resistance, its content is limited to 0.01% or less. Preferably, it is 0.007% or less.

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

Ni is added at 0.1% or more to improve the toughness of martensitic stainless steel and suppress weld cracking. However, if it is added in excess of 5.0%, retained austenite is generated and it becomes impossible to ensure a high hardness of 500Hv or more at the weld. Therefore, it is limited to 5.0% or less. The preferred range is 0.2 to 3.0%.

Cr is a basic element for imparting corrosion resistance to stainless steel, and is contained at 10.5% or more. However, if it is contained at more than 16.0%, it will not be possible to ensure a high hardness of 500Hv or more at the weld. Therefore, it is limited to 16.0% or less. The preferred range is 11.0 to 15.0%.

Al is contained at 0.002% or more to promote deoxidation of the molten zone of laser welds and suppress weld cracks (weld defects). However, if it is contained at more than 1.0%, coarse inclusions will form, causing weld cracks (weld defects). Therefore, it is limited to 1.0% or less. The preferred range is 0.005 to 0.2.

Mo and W are elements that improve corrosion resistance, and in order to suppress the formation of Cr carbonitrides after laser welding and ensure corrosion resistance, at least one of Mo and W is contained at 0.05% each. However, if the content exceeds 3.0%, toughness deteriorates and weld cracks are promoted. Therefore, the content is limited to 3.0% or less.

At least one of V and Nb is contained at 0.01% each in order to suppress the formation of Cr carbonitrides after laser welding and ensure corrosion resistance. However, if V exceeds 1.0% and Nb is contained at 0.45%, coarse precipitates will form and promote weld cracking. Therefore, V is limited to 1.0% or less and Nb to 0.45% or less.

The D value expressed by the formula (a) affects the amount of δ ferrite generated in the melted and rapidly solidified parts during laser welding, and is set to 0 or more in order to generate δ ferrite that traps impurities such as P and S and prevents weld cracks. However, if it exceeds 10, it will not be possible to ensure a high hardness of 500 Hv or more in the weld. Therefore, it is limited to 10 or less. It is preferably in the range of 2.0 to 7.0.

The M value expressed by the formula (b) above affects the amount of Cr carbonitrides formed during laser welding. In order to suppress the formation of Cr carbonitrides and ensure corrosion resistance, the M value is set to 0.3 or more. However, if it exceeds 4.0, toughness deteriorates and weld cracks are promoted. Therefore, it is limited to 5.5 or less. The preferred range is 0.5 to 4.0.

Optional Ingredients
The stainless steel of the present invention is composed of chemical components consisting of Fe and impurities other than the elements mentioned above. Furthermore, in addition to the above-mentioned composition, the following elements may be selectively contained in place of a portion of Fe.

Cu may be added as necessary to improve the corrosion resistance of the product. However, if it is added in excess of 3.0%, the effect saturates and residual austenite is generated, making it impossible to ensure a high hardness of 500 Hv or more at the weld, and also causing deterioration in weld cracking. Therefore, the content is set to 3.0% or less. Preferably, it is 1.5% or less.

Co may be added as necessary to improve the toughness and corrosion resistance of the product. However, if the content exceeds 3.0%, the effect saturates and residual austenite and ferrite are generated, making it impossible to ensure a high hardness of 500 Hv or more in the weld. Therefore, the content is set to 3.0% or less. Preferably, it is 2.0% or less.

B may be added as necessary to improve the toughness of the product and welds. However, if it is added in excess of 0.01%, the effect saturates and, conversely, it generates coarse borides that promote weld cracking and deteriorate corrosion resistance, so the content should be 0.01% or less. Preferably, it is 0.006% or less.

Sn and Sb may be added as necessary to improve the corrosion resistance of the product. However, if each is added in excess of 0.3%, the effect saturates and they promote weld cracking, so the content should be 0.3% or less. Preferably, it should be 0.1% or less.

Ti may be added as necessary to improve toughness and corrosion resistance. However, if it is added in excess of 0.45%, the effect will saturate and, conversely, it will generate coarse carbonitrides that promote weld cracking, so the content should be 0.45% or less. Preferably, it should be 0.3% or less.

Ta may be added as necessary to improve the toughness and corrosion resistance of the product. However, if it is added in excess of 0.45%, the effect will saturate and, conversely, it will generate coarse carbonitrides that promote weld cracking, so the content should be 0.45% or less. It is preferably 0.3% or less, and more preferably 0.1% or less.

Mg, Ca, and Hf may be added as necessary, as they increase the thermodynamic stability of the deoxidation products and are effective in softening the steel during softening annealing. However, if each of these elements is added in excess of 0.01%, the effect saturates and, conversely, they generate coarse oxides that promote weld cracking, so the content is set to 0.01% or less. Preferably, it is 0.005% or less.

REM increases the thermodynamic stability of deoxidation products and is effective in softening during softening annealing, so it may be added as needed. However, if added in excess of 0.05%, the effect saturates and, conversely, coarse oxides are generated, promoting weld cracking, so the content is set to 0.05% or less. Preferably, it is 0.005% or less. REM (rare earth elements) refers to the general term for two elements, scandium (Sc) and yttrium (Y), and 15 elements (lanthanoids) from lanthanum (La) to lutetium (Lu), as generally defined. They may be added alone or as a mixture.

Representative impurities contained in the stainless steel of the present invention include Zn, Bi, Pb, Ge, Se, Ag, Se, Te, etc., and these are usually mixed in at about 0.1% as impurities during the steel manufacturing process.
The impurity oxygen exists in steel mainly as inclusions, and the oxygen content of stainless steel produced by conventional refining is between 0.001 and 0.015%.
In addition, while representative optional elements are specified in (2) to (5) above, elements not listed in this specification may also be included within a range that does not impair the effects of the present invention.

According to the present invention as described above, it is possible to provide a martensitic stainless steel for use in high-hardness parts that are joined and processed by laser welding, which can prevent cracking during laser welding and achieve both high hardness and corrosion resistance at the welded part.
In the present invention, martensitic stainless steel refers to a steel that hardens by martensitic transformation during quenching. In the present invention, this means, for example, a steel in which 70% or more of the metal structure exhibits a martensite structure when quenched from 1,050°C by air cooling, and which hardens to 500 Hv or more.

Steel with 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. After heating to 1200°C, it was hot rolled into a 3 mm thick plate, softened annealed at 750°C for 3 hours, and cold rolled to a 2 mm thick plate. Test pieces measuring 40 x 40 x 2 mm were cut out and finished with surface polishing #500, and laser welding was performed. For laser welding, two 40 mm pieces of the test pieces were butted together and a fiber laser (spot diameter 0.6 mm, 2.0 kW, Ar shield) was irradiated to the contact surface at 1 m/min to join a 40 mm length. The presence or absence of weld cracks, hardness, and corrosion resistance were then evaluated. The evaluation results are shown in Table 2.

Weld cracks were evaluated by observing the area near the weld with a magnifying glass and measuring the crack length. If the total crack length was less than 1 mm, it was marked as ◯, if it was less than 0.5 mm, it was marked as ◎, and if it was 1 mm or more, it was marked as ×.

To evaluate the hardness near the weld, a cross section perpendicular to the longitudinal direction of the weld was embedded and polished as the inspection surface, and the Hv hardness (load 200 g) of the fusion zone and weld HAZ (0.2 mm, 0.4 mm, and 0.6 mm from the fusion line) was measured. The hardness of the weld HAZ was taken as the average value of the three measurement points.

Corrosion resistance was evaluated by polishing the surface of the laser welded part with #500 polishing and conducting a 48-hour salt spray test in accordance with JIS Z 2371 to evaluate the rusting condition. No rusting was evaluated as ◎, spot rusting was evaluated as ◯, and flow rusting was evaluated as ×.

To evaluate coarse oxides, the embedded and polished test surface was observed under an optical microscope, and if coarse oxides with a major axis of 20 μm or more were present, they were noted as "coarse oxides" in the remarks column of Table 2.

All of the invention examples 1 to 34 in Table 2 have high weld hardness of 500 Hv or more, and exhibit excellent weld crack resistance and corrosion resistance.

On the other hand, Comparative Examples 1 to 37 are outside the composition range of the present invention and are unable to satisfy all of the high hardness characteristics of 500 Hv or more, weld crack resistance, and corrosion resistance.

As is clear from the above examples, the present invention can provide martensitic stainless steel suitable for laser welding, which has excellent resistance to weld cracking during laser welding, high hardness of 500 Hv or more at the laser weld, and excellent corrosion resistance, and can significantly improve the durability of high-hardness parts used in severely corrosive environments, making it extremely useful in industry.

Claims (4)

In mass percent,
C: 0.15 to 0.60%,
Si: more than 0.1% and not more than 0.6%;
Mn: 0.1 to 5.0%,
S: less than 0.01%,
P: 0.05% or less,
Ni: 0.2 to 5.0%,
Cr: 10.7 to 16.0%,
N: 0.02 to 0.15%,
Al: 0.002 to 1.0%;
Mo: 0.05 to 3.0%,
W: 0.05 to 3.0%
Contains one or more of the following:
V: more than 0.01%, 1.0% or less ,
Nb: 0.01 to 0.45%
Contains one or more of the following:
The balance has a chemical composition of Fe and impurities,
(a) the D value represented by the formula is 0 or more and less than 10;
A martensitic stainless steel for laser welding, characterized in that the M value represented by 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 indicates the content (mass %) of the corresponding element. A part of the Fe is replaced with further mass%
Cu: 3.0% or less,
Co: 3.0% or less,
B: 0.01% or less,
Sn: 0.3% or less,
2. The martensitic stainless steel for laser welding according to claim 1, further comprising at least one of the following: Sb: 0.3% or less. A part of the Fe is replaced with further mass%
Ti: 0.45% or less,
3. The martensitic stainless steel for laser welding according to claim 1, further comprising at least one of the following: Ta: 0.45% or less. A part of the Fe is replaced with further mass%
Mg: 0.01% or less,
Ca: less than 0.01%,
Hf: 0.01% or less,
The martensitic stainless steel for laser welding according to any one of claims 1 to 3, characterized in that it contains one or more of the following: REM: 0.05% or less.