JP7517508B2 - Manufacturing method of welded joint - Google Patents

Description

The present invention relates to a method for manufacturing welded joints using steel materials suitable for various welded structures such as ships, marine structures, bridges, and even construction machinery, buildings, and tanks, and in particular steel materials with excellent fatigue resistance at the welds, suitable for components that are subjected to repeated loads. The term "steel materials" as used here primarily includes thick steel plates, as well as shaped steel, thin steel plates, steel pipes, and base plates for steel pipes.

In recent years, high-strength steel has been increasingly used in the construction of welded structures to streamline design, reduce the weight of steel used, and reduce the labor required for welding by thinning. For this reason, the high-strength steel used is required to have excellent ductility and low-temperature toughness, as well as excellent weldability and excellent fatigue resistance to ensure structural safety. In many cases, fatigue cracks initiate at the weld toe of a welded structure, propagate through the steel material of the welded structure, and cause the structure to break (fatigue fracture). This is believed to be due to the fact that the weld toe is prone to become a stress concentration area due to its shape, and that tensile residual stress occurs after welding. For this reason, widely known techniques for suppressing crack initiation from the weld toe include a technique for reducing stress concentration by improving the shape of the weld toe by applying additional welding, and a technique for introducing compressive residual stress by shot peening, etc.

However, applying such technology to the many weld toes that exist on an industrial scale requires a great deal of effort and time, and is not realistic from the standpoint of productivity or economics. Therefore, even if a fatigue crack does occur, if the subsequent crack propagation rate in the steel material can be reduced, the fatigue life of a welded structure can be extended. For these reasons, there is a strong demand for improving the fatigue crack propagation resistance of steel materials.

In response to such demands, for example, Patent Document 1 proposes a "high fatigue strength steel plate for welded joints." The steel plate described in Patent Document 1 is a high strength steel plate in which the surface and back layers have a hardness that is at least 50 points or more harder on the Vickers hardness scale than the hardness of the inner part of the plate. The technology described in Patent Document 1 aims to significantly delay the early stage of fatigue crack propagation by making the surface layer structure harder than the hardness of the inner part of the plate.

Patent Document 2 also proposes a welded joint that reduces residual stress on the surface of a structure. The technology described in Patent Document 2 suggests that when welding by multi-layer welding, it is preferable to use a welding material used in the final layer that has a stress-strain relationship that results in a smaller stress for the same amount of strain than the welding material used in the material to be welded or the first layer. This makes it possible to reduce residual stress in the weld.

Patent Document 3 also proposes a method for increasing the fatigue strength of welds in high-tensile steel. The technology described in Patent Document 3 claims that by forming a soft surface layer with lower strength than the base material in advance on the surface of the high-tensile steel, at least in the area corresponding to the toe of the weld, the fatigue strength of the weld is increased compared to when no soft surface layer is formed.

Patent Document 4 also proposes a thick steel plate with a hardened region on the surface. The technology described in Patent Document 4 claims that the average Vickers hardness of the region from the front and back sides of the steel plate to 2 mm in the plate thickness direction is 1.20 times or more the average Vickers hardness of the region from 1/4 to 3/4 of the plate thickness, making the surface layer, which is the starting point of fatigue cracks, strong and improving the fatigue strength of the welded part.

Japanese Patent Application Publication No. 6-49587 Japanese Patent Application Publication No. 10-146688 Japanese Unexamined Patent Publication No. 56-50797 JP 2012-92419 A

However, the technology described in Patent Document 1 has little effect in suppressing the occurrence of fatigue cracks, the technology described in Patent Document 2 requires a different type of welding material to be used only for the final layer, making the welding work complicated, and the technology described in Patent Document 3 does not necessarily clarify the hardness and thickness range of the surface layer that is effective in improving the fatigue resistance properties of the welded part. Furthermore, the technology described in Patent Document 4 also increases the tensile residual stress of the welded part, which is thought to have little effect on the fatigue resistance properties.

The present invention aims to solve the problems of the conventional technology and propose a method for manufacturing welded joints using steel materials with excellent fatigue resistance.

In order to achieve the above-mentioned object, the inventors focused on the change in the structure of the steel material in the thickness direction. Then, in order to improve the fatigue resistance of the welded joint, they came up with the idea of making the surface layer of the steel material a soft layer with low hardness and making the center part (hereinafter also referred to as the bulk) a structure with excellent fatigue crack propagation resistance. By making the surface layer of the steel material a soft layer (soft layer), as shown in FIG. 1, at the weld toe where fatigue cracks occur, the soft layer first yields when a tensile load is applied, and when the load is removed, the soft layer is compressed, and compressive stress remains in the soft layer, and when repeated stress loads are applied, the repeated stress load is compression (σmin) - tension (σmax), and the occurrence of fatigue cracks is suppressed. Furthermore, if the center part is made of steel material with excellent fatigue crack propagation resistance, the progression (fatigue crack propagation) of the fatigue cracks that have occurred can be suppressed. It was found that these can dramatically improve the life until fracture of a welded structure.

The present invention was completed based on these findings and further investigations.
[1] When joining steel materials together to form a welded joint by welding,
The steel material has a surface layer portion on one or both sides and a central portion metallurgically joined to the surface layer portion,
The surface layer portion has a Vickers hardness of 90 HV or more and less than 150 HV and a thickness of 0.10 mm or more and 30% or less of the total thickness of the steel material,
the central portion is a steel material having excellent fatigue crack propagation resistance, that is, a Vickers hardness of 150 HV or more and a fatigue crack propagation rate of 1.75×10 ⁻⁸ m/cycle or less when the stress intensity factor range is 15 MPa√m;
This method for manufacturing a welded joint is characterized in that immediately after the welding, a heat treatment is performed near the welded portion until the temperature of the surface layer is 800°C or higher, and when the temperature of the surface layer has dropped to approximately 600°C after the heating is stopped, a quenching treatment is performed in which water, compressed air or liquid nitrogen is sprayed onto the welded portion at a pressure of 0.1 to 1 MPa until the surface layer of the welded portion reaches 100°C or lower, and then the welded portion is allowed to cool.
[2] The central portion is, in mass%,
C: 0.02-0.25%, Si: 0.01-0.60%,
Mn: 0.5 to 3.0%, P: 0.05% or less,
S: 0.02% or less; Al: 0.10% or less;
Alternatively, one or more selected from Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, and B: 0.005% or less,
The carbon equivalent Ceq defined by the following formula (1) is 0.45% or less,
and the weld crack susceptibility composition Pcm defined by the following formula (2) is 0.28% or less;
The method for producing a welded joint according to [1], characterized in that the composition satisfies the above, with the balance being Fe and unavoidable impurities.
Note Ceq (%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1)
Pcm (%)=C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+Cu/20+V/10+5B...(2)
Here, C, Si, Mn, Ni, Cr, Mo, Cu, V, B: Content of each element (mass%)

The present invention can improve the fatigue resistance of welded joints (welds), dramatically improving the lifespan of welded structures until fracture, providing significant benefits to the industry.

FIG. 1 is an explanatory diagram showing the concept of improving the fatigue life of a steel material having a soft surface layer. 1 is a graph showing an example of cross-sectional hardness of a steel plate. FIG. 2 is an explanatory diagram showing a method for welding steel materials. FIG. 2 is an explanatory diagram showing an outline of a fatigue test piece used in the examples.

[Steel material used in the manufacturing method of the welded joint according to the present invention]
First, a steel material used in the method for producing a welded joint according to the present invention (hereinafter, simply referred to as the "invention steel material") will be described.
The steel material of the present invention is a two- or three-layer steel material having surface layers on one or both sides and a central portion (hereinafter also referred to as bulk) metallurgically joined to the surface layers. An example of the cross-sectional hardness distribution of the steel material (steel plate) of the present invention is shown in Figure 2.

The surface layer has a Vickers hardness of 90 HV or more and less than 150 HV, and a thickness of 0.10 mm or more and 30% or less of the total thickness of the steel material.

If the hardness of the surface layer is less than 90 HV in Vickers hardness, the surface layer will be too soft and the steel surface will be easily damaged. On the other hand, if it exceeds 150 HV, the surface layer will be too hard and the effect of fatigue crack initiation resistance will decrease. For this reason, the hardness of the surface layer of the steel is limited to 90 HV or more and less than 150 HV in Vickers hardness. The hardness is preferably 95 to 145 HV. If the thickness of the softened surface layer is less than 0.10 mm, it will be too thin and the above-mentioned effect of the softened layer cannot be expected. On the other hand, if it is thicker than 30% of the total thickness, the required strength as a structure cannot be secured. For this reason, the thickness of the surface layer is limited to a range of 0.10 mm or more and 30% or less of the total thickness of the steel. The thickness of the surface layer is preferably 0.5 to 3 mm.

The method for forming the soft layer on the surface of the steel material includes the following:
(a) A method in which a soft steel material (mild steel) of a predetermined thickness is bonded to one or both entire surfaces of a steel material that will be the central portion (bulk) and then rolled (the so-called cladding method);
(b) A method in which soft weld metal is applied to the entire surface of one or both sides of the central (bulk) steel material to a specified thickness (multi-layer welding), and then rolled.
(c) A method in which the steel material to be the central part (bulk) is subjected to a high-temperature, long-term heat treatment in a decarburizing atmosphere (oxidizing atmosphere) to decarburize the surface layer;
(d) A method in which molten low-carbon steel (mild steel) is poured around the central part of the material placed in the mold, solidified, and rolled into a cast piece (cast-in clad method);
(e) A method in which the steel material to be the central part (bulk) is heated at a constant temperature and then cooled to cause ferrite transformation of the surface part (heat treatment method);
However, it goes without saying that the present invention is not limited to the above-mentioned methods.

The steel material of the present invention has the above-mentioned surface layer and a central portion (bulk) metallurgically joined via a boundary surface. The central portion (bulk) of the steel material of the present invention is a layer having a Vickers hardness of 150 HV or more and excellent fatigue crack propagation resistance such that the fatigue crack propagation rate is 1.75× ¹⁰⁻⁸ m/cycle or less when the stress intensity factor range is 15 MPa√m.

If the hardness of the central portion (bulk) is less than 150 HV in Vickers hardness, the desired high strength for a welded structure cannot be ensured. The hardness of the central portion is measured at multiple points (three or more points) in the thickness direction from the surface excluding the above-mentioned surface layer and at either 1/4 or 1/2 of the plate thickness, and the average value is used. There is no particular upper limit for the hardness of the central portion (bulk), but if it exceeds 350 HV, the strength increases significantly and low-temperature toughness decreases. For these reasons, the hardness of the central portion (bulk) is limited to 150 HV or more, and preferably 350 HV or less.

The central portion (bulk) is a layer having the above-mentioned hardness and excellent fatigue crack propagation resistance. If the fatigue crack propagation resistance satisfies a fatigue crack propagation rate of 1.75×10 ⁻⁸ m/cycle or less when the stress intensity factor range is 15 MPa√m, the propagation of an initiated fatigue crack can be delayed, and the life of the welded structure until fracture can be significantly extended. The fatigue crack propagation resistance is measured in accordance with ASTM E 647. Examples of such steel materials are given in, for example, JP 2014-95145 A and JP 2015-206112 A.

The preferred steel composition for the central portion (bulk) having the above-mentioned properties is as follows. The mass percentages in the composition are simply indicated as %.

In the steel material of the present invention, a steel material suitable for the central portion (bulk) has the above-mentioned hardness, contains C: 0.02-0.25%, Si: 0.01-0.60%, Mn: 0.5-3.0%, P: 0.05% or less, S: 0.02% or less, Al: 0.10% or less, or further contains one or more selected from Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, and B: 0.005% or less, and has a carbon equivalent Ceq of 0.45% or less and a weld crack susceptibility composition Pcm of 0.28% or less, with the balance being Fe and unavoidable impurities.

Here, the carbon equivalent Ceq is calculated by the following formula (1): Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1)
Here, C, Mn, Cu, Ni, Cr, Mo, V: Content of each element (mass%)
The weld crack susceptibility composition Pcm is defined as follows (2): Pcm(%)=C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+Cu/20+V/10+5B... (2)
Here, C, Si, Mn, Ni, Cr, Mo, Cu, V, B: Content of each element (mass%)
In addition, when the element described in each formula is not contained, it is regarded as 0%.

The reasons for limiting the composition of the steel material suitable for the central portion (bulk) are as follows.
C: 0.02-0.25%
C is an element that contributes to increasing strength, and in order to ensure the desired high strength, a content of 0.02% or more is required. On the other hand, a content of more than 0.25% reduces ductility, toughness, and weldability. For this reason, the C content is limited to the range of 0.02 to 0.25%, and preferably 0.04 to 0.20%.

Si: 0.01~0.60%
Silicon acts as a deoxidizer and is an element that contributes to increasing the strength of steel by forming a solid solution. In order to ensure a desired strength, a content of 0.01% or more is required. If the content exceeds 0.60%, it reduces weldability and toughness. For this reason, the Si content is limited to the range of 0.01 to 0.60%, and preferably 0.02 to 0.40%. be.

Mn: 0.5-3.0%
Mn is an element that contributes to increasing the strength and toughness of steel materials through increasing hardenability, and must be contained in an amount of 0.5% or more in order to ensure the desired strength and toughness. A content exceeding 0.0% leads to a decrease in weldability and toughness. For this reason, the Mn content is limited to the range of 0.5 to 3.0%, and preferably 0.5 to 2.0%. .

P: 0.05% or less, S: 0.02% or less P and S are present as impurities and are elements that adversely affect toughness and ductility, and it is desirable to reduce them as much as possible, but P is permissible if it is 0.05% or less and S is 0.02% or less. For this reason, P is limited to 0.05% or less and S is limited to 0.02% or less. Preferably, P is 0.035% or less and S is 0.015% or less.

Al: 0.10% or less Al acts as a deoxidizer and also forms the nitride AlN, which contributes to the refinement of crystal grains. In order to obtain such an effect, it is preferable to contain 0.005% or more of Al, but if it is contained in a large amount exceeding 0.10%, it will lead to a decrease in ductility and toughness. For this reason, Al is limited to 0.10% or less. The preferable range is 0.020 to 0.050%.

The above components are the basic components, but in addition to the above components, it may contain one or more selected from the following: Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less.

One or more selected from Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, B: 0.005% or less Cu, Ni, Cr, Mo, Nb, V, Ti and B are all elements which contribute to increasing the strength of the steel material, and one or more of them can be selected and contained as necessary.

Cu: 1.0% or less Cu is an element that contributes to increasing the strength of steel material by dissolving or precipitating, and also improves corrosion resistance. In order to obtain these effects, it is preferable to contain 0.05% or more. On the other hand, a content of more than 1.0% leads to a decrease in toughness and the occurrence of surface defects during the production of steel material. For this reason, when Cu is contained, it is preferable to limit the Cu content to the range of 1.0%. More preferably, it is 0.1 to 0.8%.

Ni: 2.0% or less Ni is an element that contributes to increasing the strength of steel material and also effectively contributes to improving low-temperature toughness, corrosion resistance, and hot brittleness caused by Cu. In order to obtain such effects, it is preferable to contain 0.2% or more. On the other hand, a content of more than 2.0% leads to an increase in manufacturing costs. For this reason, when Ni is contained, it is preferable to limit the Ni content to a range of 2.0% or less. The Ni content is more preferably 0.4 to 2.0%.

Cr: 1.0% or less Cr is an element that contributes to increasing the strength of steel materials, and in order to obtain this effect, it is preferable to contain 0.05% or more. On the other hand, a content of more than 1.0% leads to a decrease in weldability and toughness. Therefore, if Cr is contained, it is preferable to limit the Cr content to a range of 1.0% or less. The more preferable range is 0.05 to 1.0%.

Mo: 1.0% or less Mo is an element that effectively contributes to increasing the strength of steel materials, and in order to obtain this effect, a content of 0.1% or more is required. On the other hand, a content of more than 1.0% leads to a decrease in weldability and toughness. For this reason, when Mo is contained, it is preferable to limit the Mo content to a range of 1.0% or less. More preferably, it is 0.2 to 0.8%.

Nb: 0.1% or less Nb is an element that contributes to increasing the strength of steel materials and also to refining crystal grains by dissolving in solid solution or precipitating as carbides, nitrides, etc. In order to obtain such effects, a content of 0.005% or more is required. On the other hand, a large content exceeding 0.1% leads to a decrease in toughness. For this reason, when Nb is contained, it is preferable to limit the Nb content to a range of 0.1% or less. More preferably, it is 0.01 to 0.05%.

V: 0.1% or less Like Nb, V is an element that precipitates as carbides and the like, and contributes to increasing the strength of steel. In order to obtain such an effect, it is preferable to contain 0.02% or more. On the other hand, a content exceeding 0.1% leads to a decrease in weldability and toughness. For this reason, when V is contained, it is preferable to limit the V content to a range of 0.1% or less. More preferably, it is 0.02 to 0.8%.

Ti: 0.1% or less Ti is an element that contributes to increasing the strength of steel materials through the precipitation of precipitates such as carbides and nitrides, and also contributes to improving the toughness of welds. In order to obtain such effects, it is preferable to contain 0.02% or more. On the other hand, a content exceeding 0.1% leads to an increase in manufacturing costs. Therefore, when Ti is contained, it is preferable to limit the Ti content to a range of 0.1% or less. More preferably, it is 0.02 to 0.8%.

B: 0.005% or less B is an element that contributes to increasing the strength of steel material by improving hardenability. To obtain such an effect, it is preferable to contain 0.0001% or more. On the other hand, a large content of more than 0.005% leads to a decrease in weldability. For this reason, when B is contained, it is preferable to limit the B content to a range of 0.005% or less. More preferably, it is 0.0001 to 0.001%.

The central portion of the steel material of the present invention contains the above-mentioned components within the above-mentioned ranges, and satisfies the carbon equivalent Ceq: 0.45% or less and the weld crack susceptibility Pcm: 0.28% or less. Ceq and Pcm are expressed by the following formulas (1) and (2): Ceq(%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5... (1)
Pcm (%)=C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+Cu/20+V/10+5B...(2)
Here, C, Si, Mn, Ni, Cr, Mo, Cu, V, B: Content of each element (mass%)
are defined as follows, respectively.

If Ceq exceeds 0.45% and Pcm exceeds 0.28%, respectively, the weld hardenability and weld crack susceptibility become too high, the hardness of the weld increases, and the tendency for weld cracks to occur increases.

When the steel material of the present invention having the above-mentioned configuration is welded to form a welded structure, when repeated stress loads are applied, as shown in Figure 1, due to the presence of the soft surface layer, the soft surface layer will first yield when a tensile load is applied, and when the load is removed, the load changes to the compressive side, resulting in repeated compression-tension stress loads. Therefore, compared to when there is no soft surface layer, the occurrence of fatigue cracks is suppressed, and if the central part has excellent fatigue crack propagation resistance properties, the crack propagation speed will be slower, resulting in a welded structure with improved fatigue resistance properties.

[Method of manufacturing a welded joint according to the present invention]
Next, a preferred method for manufacturing a welded joint according to the present invention for producing a welded structure having improved fatigue resistance will be described.

When steel materials are joined together by welding to form a welded joint to manufacture a welded structure (welded joint), there is no need to limit the welding conditions for welding steel materials together, but it is desirable to use welding materials that correspond to the strength of the steel materials and to weld them under the standard welding conditions for the welding materials. Then, immediately after welding, the weld is quenched until the surface layer of the weld falls to 100°C or less, and then allowed to cool. When the weld is quenched immediately after welding, only the weld and the surface layer in its vicinity are cooled and contracted first, so that tensile residual stress temporarily occurs at the weld toe. Subsequent cooling causes the center to shrink as well, so that the tensile residual stress at the weld toe decreases or compressive residual stress occurs. This improves the fatigue crack initiation resistance of the weld toe, which is the crack initiation position. This situation is shown diagrammatically in Figure 3(a). As a method for rapidly cooling only the surface layer near the weld (near the weld toe) after welding, an example is to spray water, compressed air, liquid nitrogen, etc. through a nozzle, etc. When spraying compressed air, the pressure of the sprayed compressed air is preferably 0.1 to 1 MPa. If the pressure of the compressed air is less than 0.1 MPa, cooling will be insufficient, while if it exceeds 1 MPa, there will be a safety problem for the equipment.

In addition, after the above-mentioned welding and before the above-mentioned quenching treatment of the welded portion, the welded portion may be subjected to a heat treatment, and then the quenching treatment may be applied to the welded portion. The heat treatment of the welded portion causes the entire welded portion to expand, and the subsequent quenching treatment causes the center portion to shrink, which increases the amount of reduction in the tensile residual stress at the weld toe, and further improves the fatigue crack initiation resistance of the welded toe. This situation is shown diagrammatically in Figure 3(b). Note that a gas burner, high-frequency heating furnace, etc. can be used to heat the entire vicinity of the welded portion after welding. Note that the heating is preferably performed so that the surface layer temperature reaches 800°C or higher.

If this type of welding is applied to welding between steel materials that have a Vickers hardness of 150 HV or more and excellent fatigue crack propagation resistance, such as a fatigue crack propagation rate of 1.75 x 10 ^-8 m/cycle or less when the stress intensity factor range is 15 MPa√m, the fatigue crack initiation resistance of the weld toe will be further improved, and the propagation of any fatigue cracks that do occur will be slowed down, thereby extending the life of the welded structure until fracture.

Furthermore, in order to further improve the fatigue resistance of a welded structure, it is preferable to apply the above-mentioned welding to a steel material of the present invention, which has a surface layer portion on one or both sides and a central portion metallurgically joined to the surface layer portion, and wherein the surface layer portion has a Vickers hardness of 90 HV or more but less than 150 HV and a thickness of 0.10 mm or more and 30% or less of the total thickness of the steel material, and the central portion has a Vickers hardness of 150 HV or more and has excellent fatigue crack propagation resistance such that the fatigue crack propagation rate is 1.75×10 ^-8 m/cycle or less when the stress intensity factor is in the range of 15 MPa√m. The combination of the soft surface layer portion and the central portion having excellent fatigue crack propagation resistance improves the fatigue crack initiation resistance of the weld toe and also slows down the propagation rate of an initiated fatigue crack, thereby significantly extending the life of the welded structure as a whole.

The present invention will be further explained below based on examples.

Example 1
A weld metal was formed by multi-layer welding on both sides of steel plate A2 having the composition shown in Table 1 using a welding material for mild steel, hot-rolled, and then heat-treated in air (holding at 900°C for 48 hours) to decarburize the surface, followed by water-cooling to obtain steel plate A (plate thickness 25 mm) having soft layers on both surfaces. The surface layer A1 has the composition shown in Table 1 and is a soft layer with a hardness of 122 HV and an average thickness of 3.2 mm. In addition, steel plate A2 for the center (bulk) is a high-tensile steel plate of 590 MPa class, with an average hardness (measured near the center of the plate thickness) of 195 HV, and excellent fatigue crack propagation resistance, with a fatigue crack propagation rate da/dN of 1.27 x 10 ^-8 m/cycle, less than 1.75 x 10 ^-8 m/cycle, when the stress intensity factor range is 15 MPa√m, measured in accordance with the provisions of ASTM E 647.

In addition, the steel plate C1 having the composition shown in Table 1 was laminated on both sides of the steel plate C2 having the composition shown in Table 1, and then hot rolling was performed to obtain the steel plate C (plate thickness 23 mm). The steel plate C is a clad steel plate having a soft layer on the surface of both sides. The surface layer has the composition shown in Table 1, and is a soft layer having a hardness of 116 HV and an average thickness of 3.0 mm. The steel plate C1 for the center (bulk) is a high-tensile steel plate of 590 MPa class, and as shown in Table 1, the average hardness (measured near the center position of the plate thickness) is 220 HV, and the fatigue crack propagation rate da/dN at a stress intensity factor range of 15 MPa√m measured in accordance with the provisions of ASTM E 647 is 1.33×10 ⁻⁸ m/cycle, which is excellent in fatigue crack propagation resistance.

Test materials (width 80 mm x length 600 mm) were taken from the obtained steel plates A and C, and steel plates (plate thickness 10 mm x width 60 mm x length 100 mm) were joined to both sides of the test materials by box welding to produce out-of-plane gusset joints, as shown in Figure 4. The welding conditions for the box welding were, on average, current: 240 A, voltage: 31 V, and welding speed: 20 cm/min. The welding material used was flux-cored wire (equivalent to JIS Z3313 T 49J 0 T1-0C A-U). The shielding gas was 80% Ar-20% _CO2 .

A fatigue test was conducted on the obtained out-of-plane gusset joint to measure the life (cycle) until fracture. The fatigue test was conducted in the longitudinal direction of the test piece as the load direction, within the load stress range (stress ratio 0.1) shown in Table 2.

For steel plate B, which does not have a soft layer on the surface, an out-of-plane gusset joint similar to that described above was fabricated and subjected to a similar fatigue test to measure the life (cycles) until fracture, which was used as a comparative example. The results obtained are shown in Table 2.

The inventive examples (Test Nos. 1 to 3, 7 to 8) had improved fracture life compared to the welded joints (Comparative Examples: Test Nos. 4 to 6) using Steel Plate B whose surface layer was not soft.
Example 2
Test pieces (width 80 mm x length 600 mm) were taken from each of the steel plates A, B, D, and E shown in Table 1, and steel plates (plate thickness 10 mm x width 60 mm x length 100 mm) were joined to both sides of the test pieces by box welding as shown in Figure 4 to produce out-of-plane gusset joints, as in Example 1. The welding conditions for the box welding were, on average, current: 240 A, voltage: 31 V, and welding speed: 20 cm/min, as in Example 1. The welding material used was a flux-cored wire (equivalent to JIS Z3313 T 49J 0 T1-0C A-U). The shielding gas was 80% Ar-20% _CO2 .

Immediately after the above welding, the vicinity of the weld was heated with a gas burner until the surface temperature reached approximately 900°C. After the heating was stopped, when the surface temperature had dropped to approximately 600°C, a rapid cooling process was carried out by blowing compressed air (pressure: 0.9 MPa) around the weld. When the surface temperature had dropped to 97°C, the rapid cooling process was terminated and the weld was allowed to cool to room temperature.

Using the obtained out-of-plane gusset joint as a test piece, a fatigue test was performed in the same manner as in Example 1, and the life (cycle) until fracture was measured. The fatigue test conditions were the load stress range (stress ratio 0.1) shown in Table 3.

The results are shown in Table 3.

In the present invention examples (Tests No. 11 and No. 12) in which the welded joint (out-of-plane gusset joint) using steel plate B was heated and quenched after welding, the fracture life of the joint was improved compared to the examples in which the welded joint was not quenched (see Tests No. 5 and No. 6: Table 2). In the welded joints using steel plate A, which has a soft layer on the surface (Tests No. 9 and No. 10), the fracture life of the joint was also improved by quenching the welded joint after welding (see Tests No. 2 and No. 3). In the welded joints using steel plates D and E (Tests No. 13 to 16), the fracture life of the joint was also improved, resulting in welded joints with excellent fatigue resistance.

Claims (2)

When joining steel materials together to form a welded joint,
The steel material has a surface layer portion on one or both sides and a central portion metallurgically joined to the surface layer portion,
The surface layer portion has a Vickers hardness of 90 HV or more and less than 150 HV and a thickness of 0.10 mm or more and 30% or less of the total thickness of the steel material,
the central portion is a steel material having excellent fatigue crack propagation resistance, that is, a Vickers hardness of 150 HV or more and a fatigue crack propagation rate of 1.75×10 ⁻⁸ m/cycle or less when the stress intensity factor range is 15 MPa√m;
This method for manufacturing a welded joint is characterized in that immediately after the welding, a heat treatment is performed near the welded portion until the temperature of the surface layer is 800°C or higher, and when the temperature of the surface layer drops to 600°C after heating is stopped, a quenching treatment is performed in which water, compressed air or liquid nitrogen is sprayed onto the welded portion at a pressure of 0.1 to 1 MPa until the surface layer of the welded portion reaches 100°C or lower, and then the welded portion is allowed to cool. The central portion is, in mass%,
C: 0.02-0.25%, Si: 0.01-0.60%,
Mn: 0.5 to 3.0%, P: 0.05% or less,
S: 0.02% or less; Al: 0.10% or less;
Alternatively, one or more selected from Cu: 1.0% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: 0.1% or less, and B: 0.005% or less,
The carbon equivalent Ceq defined by the following formula (1) is 0.45% or less,
and the weld crack susceptibility composition Pcm defined by the following formula (2) is 0.28% or less;
2. The method for producing a welded joint according to claim 1, characterized in that the composition satisfies the above, with the balance consisting of Fe and unavoidable impurities.
Ceq (%)=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5...(1)
Pcm (%)=C+Si/30+Mn/20+Ni/60+Cr/20+Mo/15+Cu/20+V/10+5B...(2)
Here, C, Si, Mn, Ni, Cr, Mo, Cu, V, B: Content of each element (mass%)

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