Laser welding method

Abstract

To provide a welded joint for a wear-resistant steel plate, minimizing the softened region in the heat-affected zone by utilizing a laser welding process.SOLUTION: A laser welded joint has welded metal, formed by butt-welding steel plates with a plate thickness t of 3-15 mm and a Vickers hardness HV of 425 or more, utilizing a laser welding process. The steel plate comprises, in mass%, C: 0.15-0.35%, Si: 0.20-0.55%, Mn: 0.50-1.60%, P: 0.050% or less, S: 0.050% or less, and Cr: 0.80% or less, and optionally comprising at least one selected from Ti: 0.020% or less and B: 0.010% or less, with the balance being Fe and inevitable impurities.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a welded joint of abrasion-resistant steel plates used as various members of steel structures for construction machinery, industrial machinery, shipbuilding, civil engineering, architecture, etc., and more particularly to a laser-welded joint of abrasion-resistant steel plates joined by laser welding and a laser welding method thereof. [Background technology]

[0002] Many wear-resistant steel plates are subjected to welding. Gas-shielded arc welding has been used for conventional welding of wear-resistant steel plates. Problems with conventional gas-shielded arc welding include a decrease in the hardness of the weld metal and the formation of a large area in the heat-affected zone (hereinafter also referred to as "HAZ") of the base metal and its surrounding area that is softer than the base metal. Patent Document 1 describes a method for manufacturing welded joints of wear-resistant steel plates, in which a flux-cored wire adjusted to a specific composition is used in gas-shielded arc welding to maintain the hardness of the weld metal at the same level as the base metal. However, this method cannot suppress the size of the softened area in the heat-affected zone of the base metal. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5696824 Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, gas-shielded arc-welded joints of wear-resistant steel plates have a problem in that a wide softened region is formed in the weld and its periphery. The present invention has been made in view of this current situation, and aims to provide a welded joint of wear-resistant steel plates in which the softened region in the heat-affected zone is reduced by using a laser welding method. [Means for solving the problem]

[0005] The present inventors have conducted extensive research into methods for reducing softened regions in welded joints of wear-resistant steel. As a result, they have found that applying laser welding to the fabrication of welded joints under certain conditions and using low welding heat input can reduce softening of the weld. Furthermore, they have discovered that limiting the width of the weld bead to a predetermined range can reduce the softened region in welded joints of wear-resistant steel.

[0006] The present invention was completed based on these findings and further investigations, and the gist of the present invention is as follows. [1] A laser welded joint formed by butt-joining steel plates having a thickness (t) of 3 to 15 mm and a Vickers hardness (HV) of 425 or more by laser welding, characterized in that the chemical composition of the steel plates contains, in mass %, C: 0.15 to 0.35%, Si: 0.20 to 0.55%, Mn: 0.50 to 1.60%, P: 0.050% or less, S: 0.050% or less, Cr: 0.80% or less, with the balance being Fe and unavoidable impurities. [2] A laser welded joint according to [1], characterized in that in addition to the chemical composition, it further contains, by mass%, one or more selected from Ti: 0.020% or less and B: 0.010% or less. [3] In the above [1] or [2], the ratio (t / W) of the plate thickness (t) to the bead width (W) of the weld metal is 1.0 or more. [4] A laser welding method comprising butting together steel plates having a plate thickness (t) of 3 to 15 mm, a Vickers hardness (HV) of 425 or more, and a chemical composition, in mass %, containing C: 0.15 to 0.35%, Si: 0.20 to 0.55%, Mn: 0.50 to 1.60%, P: 0.050% or less, S: 0.050% or less, Cr: 0.80% or less, with the balance being Fe and unavoidable impurities, to obtain a welded joint by laser welding. [5] The laser welding method according to [4], characterized in that in addition to the chemical composition, the material further contains, by mass%, one or more selected from Ti: 0.020% or less and B: 0.010% or less. [6] The laser welding method according to [4] or [5], wherein the filler metal has a chemical composition in which the Cl value of the following formula (1) is 0.25 mass% or more. CI=[C]+[Si] / 24+[Mn] / 6 ··· (1) Here, [element] indicates the content (mass %) of the element in the filler metal. [7] In any one of [4] to [6], the laser welding method is characterized in that the ratio (t / W) of the plate thickness (t) to the bead width (W) of the weld metal is 1.0 or more. [8] A laser welding method according to any one of [4] to [7], characterized in that the laser welding output is 3 to 20 kW and the welding speed is 1.0 to 5.0 m / min. [Effects of the Invention]

[0007] According to the present invention, it is possible to provide a laser welded joint in which the softened region of the weld is reduced in a wear-resistant steel plate, which has a significant industrial effect. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram showing a cross-sectional macro shape of a laser welded joint according to the present invention. FIG. [Figure 2] FIG. 2 is a schematic diagram showing the Vickers hardness distribution around the weld metal of the laser welded joint according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0009] [Background to the present invention] In the weld metal and heat affected zone (HAZ) of a welded joint formed by welding, there exists a region (hereinafter referred to as a "softened region") where the hardness is reduced compared to the original zone of the base material (steel plate).

[0010] Figure 1 shows a schematic diagram of the cross-sectional macroscopic shape around the weld metal. Steel plate 1 of thickness t is joined by laser welding to form weld metal 2 in the center. The portions of steel plate 1 on both sides that come into contact with weld metal 2 are the heat-affected zone (HAZ) 3. A bead of raised weld metal is formed on the upper part (welding side) of weld metal 2, and the width of the formed surface bead is bead width W. When measuring Vickers hardness, the Vickers hardness of the weld metal cross section is measured parallel to the steel plate surface at a depth of 0.5 mm from the surface of steel plate 1. The measurement interval was 0.2 to 0.3 mm.

[0011] An example of the measurement results is shown in Figure 2. As can be seen from this figure, the hardness of the steel plate (base metal) is maintained at a high level, but the hardness decreases from the heat-affected zone, and the hardness returns slightly to the weld metal, but decreases again in the center of the weld metal. In the present invention, the average hardness of the base metal is calculated, and the region where the hardness is lower than this average is defined as the "softened region."

[0012] Therefore, in order to reduce the softened region in the welded joint of the wear-resistant steel plate, it is important to reduce the width of the heat-affected zone. Generally, the width of the heat-affected zone increases with increasing welding heat input, so in order to reduce the width of the heat-affected zone, it is necessary to reduce the welding heat input.

[0013] In the present invention, laser welding, which has a low welding heat input, was selected as an alternative to conventional gas-shielded arc welding, and it was found that when applied to wear-resistant steel plates, the width of the heat-affected zone can be reduced, and as a result, the width of the softened region can be reduced.

[0014] In addition, because wear-resistant steel plates have high hardness, it is difficult to make the hardness of the weld metal equal to or greater than that of the base material using conventional welding, and the weld metal also becomes softened. However, in the present invention, the width of the weld metal is reduced by applying laser welding, which allows welding with an I-groove.

[0015] Furthermore, as will be described later, it has been found that in order to reduce the width of the softened region, it is more preferable to keep the ratio (t / W) of the thickness t (mm) of the wear-resistant steel plate to the bead width W (mm) of the weld metal at 1.0 or more.

[0016] The present invention, which was developed based on the above findings, will be described in detail below.

[0017] [Steel plate] First, the plate thickness (t) and Vickers hardness (HV) of the wear-resistant steel plate to which the present invention is applied will be described.

[0018] The thickness (t) of the steel plate is specified to be 3 to 15 mm. If it is less than 3 mm, the productivity of the wear-resistant steel plate deteriorates, and if it exceeds 15 mm, there is a high possibility of weld defects occurring under the welding condition range of the present invention. Preferably, it is 3.2 to 14.5 mm.

[0019] The Vickers hardness (HV) is limited to 425 or more because a steel plate with an HV of less than 425 does not qualify as a wear-resistant steel plate of the present invention. The upper limit is preferably 650 or less.

[0020] The Vickers hardness test was carried out in accordance with JIS Z 2244-1. In the present invention, the test load (test force F) was set to 4.903 N (hardness symbol: HV0.5). The test results were expressed as HV.

[0021] [Chemical composition of steel plate] Next, the basic chemical composition of the steel plate used in the laser welded joint according to the present invention will be described. Note that hereinafter, "%" in the chemical composition means "% by mass."

[0022] The basic composition of the steel sheet is C: 0.15 to 0.35%, Si: 0.20 to 0.55%, Mn: 0.50 to 1.60%, P: 0.050% or less, S: 0.050% or less, Cr: 0.80% or less, and the balance being Fe and unavoidable impurities. Each composition is described in detail below.

[0023] [C: 0.15~0.35%] C is an element that increases the hardness of the surface layer of a steel sheet and improves wear resistance. To achieve this effect, the C content is set to 0.15% or more. On the other hand, if the C content exceeds 0.35%, toughness decreases. Therefore, the C content is set to 0.35% or less. Therefore, the C content is limited to 0.15 to 0.35%. The C content is preferably 0.17 to 0.33%, and more preferably 0.20 to 0.30%.

[0024] [Si: 0.20~0.55%] Si is an element that acts as a deoxidizer. Furthermore, Si dissolves in steel and has the effect of increasing hardness through solid solution strengthening. To obtain these effects, the Si content is set to 0.20% or more. On the other hand, if the Si content exceeds 0.55%, toughness decreases. Therefore, the Si content is set to 0.55% or less. Therefore, the Si content must be within the range of 0.20 to 0.55%. The Si content is preferably 0.22 to 0.55%, and more preferably 0.24 to 0.53%.

[0025] [Mn: 0.50~1.60%] Mn is an element that has the effect of increasing the hardenability of steel, increasing the hardness of the surface layer of the steel sheet, and improving wear resistance. To achieve this effect, the Mn content is set to 0.50% or more. On the other hand, if the Mn content exceeds 1.60%, not only will toughness decrease, but the alloy cost will also become excessively high. Therefore, the Mn content is set to 1.60% or less. Therefore, the Mn content must be within the range of 0.50 to 1.60%. The Mn content is preferably 0.52 to 1.58%, and more preferably 0.55 to 1.55%.

[0026] [P:0.050% or less] P is an element contained as an unavoidable impurity, and has adverse effects such as reducing the toughness of the base material by segregating at grain boundaries. Therefore, it is desirable to keep the P content as low as possible, but a P content of 0.050% or less is acceptable. Therefore, the P content must satisfy the limit of 0.050% or less. The P content is preferably 0.001 to 0.040%, and more preferably 0.001 to 0.030%.

[0027] [S:0.050% or less] S is an element contained as an unavoidable impurity, and exists in steel as sulfide-based inclusions such as MnS, which have adverse effects such as reducing the toughness of the base material. Therefore, it is desirable to keep the S content as low as possible, but a content of 0.050% or less is acceptable. Therefore, the S content must satisfy the limit of 0.050% or less. The S content is preferably 0.001 to 0.040%, and more preferably 0.001 to 0.030%.

[0028] [Cr:0.80% or less] Cr is an element that increases the hardness of the surface layer of a steel sheet and improves wear resistance. However, if the Cr content exceeds 0.80%, toughness will decrease. Therefore, the Cr content is limited to 0.80% or less. The Cr content is preferably 0.01 to 0.78%, and more preferably 0.05 to 0.75%.

[0029] [Optional composition] The steel plate used for the laser welded joint according to the present invention has the basic composition described above. In addition to this basic composition, the present invention may further contain, as optional compositions, one or more elements selected from Ti: 0.020% or less and B: 0.010% or less, as necessary.

[0030] [Ti:0.020% or less] Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing the amount of solute N. Therefore, adding Ti can improve the toughness of the base metal and weld metal. Furthermore, when both Ti and B are added, Ti fixes N, thereby suppressing the precipitation of BN, thereby promoting the hardenability-improving effect of B. However, if the Ti content exceeds 0.020%, a large amount of TiC precipitates, reducing workability. Therefore, when Ti is contained, the Ti content is preferably 0.020% or less. More preferably, it is 0.001 to 0.018%.

[0031] [B:0.010% or less] B is an element that has the effect of improving hardenability. Therefore, adding B can improve wear resistance. However, if the B content exceeds 0.010%, weldability decreases. Therefore, when B is added, the B content is preferably 0.010% or less, and more preferably 0.001 to 0.008%.

[0032] [Remaining composition] The remainder of the composition other than the above consists of Fe and unavoidable impurities, such as N, O (oxygen), Nb, Mo, V, W, Sn, Sb, As, Pb, Bi, and REM. The total content of unavoidable impurities is acceptable if it is 0.10% or less.

[0033] As long as the basic composition and optional selected composition are satisfied, the inclusion of other unavoidable impurity elements is not precluded, and such embodiments are also included within the technical scope of the present invention.

[0034] [Chemical composition of weld metal] Next, the chemical composition of the weld metal formed by laser welding according to the present invention will be described.

[0035] The chemical composition of the weld metal is basically the same as the composition of the steel plate when no filler metal is used, but when a filler metal is used during laser welding, the chemical composition of the filler metal is partially mixed in.

[0036] [Chemical composition of weld metal] The basic chemical composition of the weld metal is C: 0.15-0.35%, Si: 0.20-0.55%, Mn: 0.50-1.60%, P: 0.050% or less, S: 0.050% or less, Cr: 0.80% or less, with the balance being Fe and unavoidable impurities. In addition to the above basic composition, the weld metal may contain one or more elements selected from Ti: 0.020% or less and B: 0.010% or less.

[0037] [C: 0.15~0.35%] C in the weld metal is an element that has the effect of increasing hardness. To achieve this effect, the C content is preferably 0.15% or more. On the other hand, if the C content exceeds 0.35%, toughness decreases. Therefore, the C content is preferably 0.35% or less. Furthermore, a C content of 0.17 to 0.33% is more preferable.

[0038] [Si: 0.20~0.55%] Si is an element that acts as a deoxidizer. Furthermore, Si dissolves in steel and has the effect of increasing hardness through solid solution strengthening. To obtain these effects, the Si content is preferably 0.20% or more. On the other hand, if the Si content exceeds 0.55%, toughness decreases. Therefore, the Si content is preferably 0.55% or less. More preferably, it is 0.22 to 0.55%.

[0039] [Mn: 0.50~1.60%] Mn is an element that has the effect of increasing the hardenability of steel, increasing the hardness of weld metal, and improving wear resistance. To achieve this effect, the Mn content is preferably 0.50% or more. On the other hand, if the Mn content exceeds 1.60%, not only will the toughness decrease, but the alloy cost will also become excessively high. Therefore, the Mn content is preferably 1.60% or less. A more preferred range is 0.52 to 1.58%.

[0040] [P:0.050% or less] P is an element contained as an unavoidable impurity, and since it segregates at grain boundaries and adversely affects hot cracking resistance, it is desirable to keep the P content as low as possible, but a P content of 0.050% or less is acceptable. Therefore, the P content is preferably 0.050% or less. More preferably, it is 0.001 to 0.030%.

[0041] [S:0.050% or less] S is an element contained as an unavoidable impurity, and since it segregates at grain boundaries and adversely affects hot cracking resistance, it is desirable to keep the S content as low as possible, but a content of 0.050% or less is acceptable. Therefore, the S content is preferably 0.050% or less. Furthermore, a content of 0.001 to 0.030% is more preferable.

[0042] [Cr:0.80% or less] Cr is an element that increases the strength of the weld metal. However, if the Cr content exceeds 0.80%, it will result in a decrease in toughness. Therefore, the Cr content is preferably 0.80% or less, and more preferably 0.01 to 0.78%.

[0043] [One or more selected from Ti: 0.020% or less and B: 0.010% or less] Ti is an element that has a strong tendency to form nitrides and has the effect of fixing N and reducing the amount of solute N. Therefore, adding Ti can improve the toughness of the weld metal. Furthermore, when both Ti and B are added, Ti fixes N, thereby suppressing the precipitation of BN, thereby promoting the effect of B in improving hardenability. However, if the Ti content exceeds 0.020%, a large amount of TiC precipitates, reducing workability. Therefore, when Ti is contained, the Ti content is preferably 0.020% or less. More preferably, it is 0.001 to 0.018%.

[0044] B is an element that has the effect of improving hardenability. Therefore, adding B can improve wear resistance. However, if the B content exceeds 0.010%, weldability decreases. Therefore, when B is added, the B content is preferably 0.010% or less, and more preferably 0.001 to 0.008%.

[0045] [Remaining composition] The remainder of the composition other than the above consists of Fe and unavoidable impurities, such as N, O (oxygen), Nb, Mo, V, W, Sn, Sb, As, Pb, Bi, and REM. The total content of unavoidable impurities is acceptable if it is 0.10% or less.

[0046] As long as the basic composition and optional selected composition are satisfied, the inclusion of other unavoidable impurity elements is not precluded, and such embodiments are also included within the technical scope of the present invention.

[0047] [Filler metal and CI value] In laser welding, when the thickness of the base material (steel plate) is large or when the distance between the base materials (hereinafter also referred to as "gap") is large, it is preferable to perform laser welding while supplying filler metal to the groove in order to fill the groove with weld metal and prevent welding defects.

[0048] With regard to the chemical composition of the filler metal, it is more preferable that the value of CI shown in the following formula (1) is 0.25 mass % or more. CI=[C]+[Si] / 24+[Mn] / 6 ··· (1) Here, [element] indicates the content (mass %) of the element in the filler metal.

[0049] The above formula (1) is an index determined by focusing on C, Si, and Mn among the chemical composition of the filler metal. These C, Si, and Mn are all elements that increase the hardness of the weld metal. To achieve this effect, it is more preferable to use a filler metal whose CI value determined from the above formula (1) satisfies 0.25% or more. This is because if it is less than 0.25%, the effect of ensuring the hardness of the weld metal is small. More preferably, it is 0.27% or more. From the viewpoint of preventing cracking during welding, it is desirable to set it to 0.40% or less.

[0050] The filler metal is obtained by wire drawing a molten steel material, and the wire diameter is preferably 0.9 to 1.6 mmφ.

[0051] [Relationship between steel plate thickness t and bead width W] As mentioned above, in order to reduce the width of the softened region around the weld metal, it is preferable that the ratio (t / W) of the steel plate thickness t to the bead width W is 1.0 or more. If it is less than 1.0, the width of the weld metal will be large, and the softened region itself will be large. More preferably, it is 1.5 or more. From the viewpoint of preventing welding defects, it is desirable that the ratio be 10.0 or less.

[0052] [Laser welding method] The laser welding method involves laser cutting the steel plate and joining the I-shaped end faces to form a butt I-shaped groove with a groove width (root gap): 0 to 0.5 mm, using a 10 kW fiber laser welder or a 20 kW CO2 laser welder. When a filler metal is used, the welding can be performed while the filler metal is being supplied.

[0053] The preferable welding conditions at this time are as follows. Laser welding power: 3~20kW Welding speed: 1000~5000mm / min Shielding gas: 100% Ar or He, N2, etc. Welding heat input: 800~5000J / cm [Example]

[0054] Experiments were conducted to evaluate the size of the softened region by fabricating butt-welded joints using a wear-resistant steel plate (base material) using a laser welding method. The results of the laser welding method for the present invention and the results of welding using a MAG welding method as a comparative example are summarized. The laser welding conditions were a laser power of 4 to 15 kW, a welding speed of 2000 to 4000 mm / min, and a shielding gas of 100% Ar. The groove width (root gap) was 0 to 0.5 mm. Table 1 shows the chemical composition (mass%) of the base material used, the plate thickness (t) (mm), the Vickers hardness (HV) (average value) of the base material, and the welding conditions for laser welding and MAG welding. For examples in which a filler metal was added, the chemical composition (mass%) and CI value of the filler metal are also listed. Furthermore, the bead width (W) (mm) of the weld metal obtained after welding was measured, and the ratio to the plate thickness (t) (t / W) was calculated.

[0055] [Table 1]

[0056] Next, to evaluate the softened region, a Vickers hardness test was performed on the cross section of the weld 0.5 mm below the surface, and the hardness distribution across the weld was obtained. As mentioned above, the softened region was defined as an area with a lower hardness than the average hardness of the base material, and a softened region with a width of 10 mm or less was evaluated as good (○), and a softened region with a width of more than 10 mm was evaluated as poor (×). The evaluation results are shown in Table 2.

[0057] [Table 2]

[0058] In the inventive examples, it was confirmed that the softened region was small, at 10 mm or less. On the other hand, in comparative examples 1 and 2, it was confirmed that the softened region was formed widely, at 25 mm or more. In comparative example 3, welding defects occurred, and a satisfactory welded joint was not obtained. Furthermore, in comparative example 4, it was confirmed that the Vickers hardness of the base material was insufficient, and even though the softened region was small, the wear resistance was insufficient. [Explanation of symbols]

[0059] 1 steel plate 2. Weld metal 3 Heat affected zone (HAZ) t Plate thickness W bead width

Claims

1. A laser welding method comprising butting together steel plates having a plate thickness (t) of 3 to 15 mm, a Vickers hardness (HV) of 425 or more, and a chemical composition, in mass %, of C: 0.15 to 0.35%, Si: 0.20 to 0.55%, Mn: 0.50 to 1.60%, P: 0.050% or less, S: 0.050% or less, Cr: 0.80% or less, with the balance being Fe and unavoidable impurities, and obtaining a welded joint having weld metal joined by laser welding using a filler metal having a chemical composition such that the CI value of the following formula (1) is 0.25% by mass or more. CI=[C]+[Si] / 24+[Mn] / 6... (1) Here, [element] indicates the content (mass %) of the element in the filler metal.

2. 2. The laser welding method according to claim 1, characterized in that, in addition to the chemical composition of the steel plate, the steel plate further contains, by mass%, one or more elements selected from Ti: 0.020% or less and B: 0.010% or less.

3. 3. The laser welding method according to claim 1, wherein a ratio (t / W) of the plate thickness (t) to the bead width (W) of the weld metal is 1.0 or more.

4. 3. The laser welding method according to claim 1, wherein the laser welding output is 3 to 20 kW and the welding speed is 1.0 to 5.0 m / min.

5. 4. The laser welding method according to claim 3, wherein the laser welding output is 3 to 20 kW and the welding speed is 1.0 to 5.0 m / min.

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