Laser welded joint and laser welding process

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, and the like, and in particular 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. In general gas-shielded arc welding, there are problems in that the hardness of the weld metal decreases and a region that is softer than the base metal is formed widely in the heat-affected zone (hereinafter also referred to as "HAZ") of the base metal and its surroundings. In Patent Document 1, as a manufacturing method for a welded joint of wear-resistant steel plates, a flux-cored wire adjusted to a specific composition is used for 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 region 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, a problem with gas-shielded arc-welded joints of wear-resistant steel plates is that a wide softened region is formed at the weld and its periphery. The present invention has been made in consideration of the current situation, and aims to provide a welded joint of wear-resistant steel plates in which the softened region of the heat-affected zone is reduced by using a laser welding method. [Means for solving the problem]

[0005] The present inventors have intensively studied a method for reducing the softened region in a welded joint of wear-resistant steel. As a result, they have found that softening of the weld can be reduced by applying laser welding to the preparation of the welded joint under certain conditions and by using low welding heat input. They have also found that the softened region in a welded joint of wear-resistant steel can be reduced by limiting the width of the weld bead to a predetermined range.

[0006] The present invention was completed based on these findings and through further investigation, and the gist of the present invention is as follows. [1] A laser welded joint having a weld metal formed by butt-joining steel plates having a plate 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] The laser welded joint according to [1] above, further comprising, in addition to the chemical composition, one or more selected from Ti: 0.020% or less and B: 0.010% or less, in mass percent. [3] The laser welded joint according to [1] or [2], 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. [4] A laser welding method comprising the steps of 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, to obtain a welded joint having a weld metal joined by laser welding. [5] The laser welding method according to [4] above, further comprising, in addition to the chemical composition, one or more selected from Ti: 0.020% or less and B: 0.010% or less, in mass %. [6] The laser welding method according to [4] or [5], 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. [7] The laser welding method according to any one of [4] to [6], characterized in that the laser welding output is 3 to 20 kW and the welding speed is 1.0 to 5.0 m / min. Effect 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, and this has a significant industrial effect. [Brief description 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. [Diagram 2] FIG. 2 is a schematic diagram showing a Vickers hardness distribution around a weld metal of a laser welded joint according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] [Background to the 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") in which 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 macro 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 parts of steel plate 1 on both sides that are in contact with weld metal 2 are heat-affected zone (HAZ) 3. A bead of raised weld metal is formed on the upper part (welded side) of weld metal 2, and the width of the surface bead formed 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 layer of steel plate 1. The measurement intervals were 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 high, 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 with a hardness 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. In general, the width of the heat-affected zone increases with an increase in the 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 small welding heat input, is selected as an alternative to conventional gas-shielded arc welding, and it has been found that when this is applied to wear-resistant steel plate, 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, since 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 by normal welding, and the weld metal also becomes a softened region. However, in the present invention, the width of the weld metal is reduced by applying laser welding, which allows welding with an I-shaped groove.

[0015] Furthermore, as described below, 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 plate 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 now be described in detail.

[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, the possibility of occurrence of welding defects increases under the welding condition range of the present invention. The thickness (t) is preferably 3.2 to 14.5 mm.

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

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

[0021] [Chemical composition of steel plate] Next, the basic composition of the 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 "mass %".

[0022] The basic composition of the steel sheet 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, 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 has the effect of increasing the hardness of the surface layer of a steel sheet and improving wear resistance. To obtain this effect, the C content is set to 0.15% or more. On the other hand, if the C content exceeds 0.35%, the 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. In addition, Si dissolves in steel and has the effect of increasing hardness through solid solution strengthening. In order 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%, the toughness decreases. Therefore, the Si content is set to 0.55% or less. Therefore, the Si content needs to 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 acts to increase the hardenability of steel, increase the hardness of the surface layer of the steel sheet, and improve the wear resistance. To obtain 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 the toughness decrease, but the alloy cost will also become excessively high. For this reason, the Mn content is set to 1.60% or less. Therefore, the Mn content needs to 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 inevitable impurity, and has adverse effects such as reducing the toughness of the base material by segregating at grain boundaries. Therefore, it is desirable to reduce the P content as much as possible, but 0.050% or less is acceptable. Therefore, the P content must satisfy 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 inevitable impurity, and exists in steel as sulfide-based inclusions such as MnS, which has adverse effects such as reducing the toughness of the base material. Therefore, it is desirable to reduce the S content as much as possible, but 0.050% or less is acceptable. Therefore, the S content must satisfy 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 steel sheet surface layer and improves wear resistance. However, if the Cr content exceeds 0.80%, it leads to a decrease in toughness. Therefore, the Cr content is set to 0.80% or less. 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 in the laser welded joint according to the present invention has the above-mentioned basic composition. In addition to this basic composition, the present invention may further contain one or more elements selected from Ti: 0.020% or less and B: 0.010% or less as optional compositions, if 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, the addition of Ti can improve the toughness of the base material and the weld metal. In addition, when both Ti and B are added, Ti fixes N, thereby suppressing the precipitation of BN, and as a result, the hardenability improving effect of B is promoted. However, if the Ti content exceeds 0.020%, a large amount of TiC precipitates, which reduces the 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, the addition of B can improve wear resistance. However, if the B content exceeds 0.010%, the weldability decreases. Therefore, when B is added, the B content is preferably 0.010% or less. More preferably, it is 0.001 to 0.008%.

[0032] [Remaining composition] The remaining composition other than the above composition is composed of Fe and inevitable impurities. Examples of inevitable impurities include N, O (oxygen), Nb, Mo, V, W, Sn, Sb, As, Pb, Bi, and REM. The total content of inevitable impurities is permissible if it is 0.10% or less.

[0033] As long as the above-mentioned 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 obtain this effect, the C content is preferably 0.15% or more. On the other hand, if the C content exceeds 0.35%, the toughness decreases. Therefore, the C content is preferably 0.35% or less. More preferably, it is 0.17 to 0.33%.

[0038] [Si: 0.20~0.55%] Si is an element that acts as a deoxidizer. In addition, Si dissolves in steel and has the effect of increasing hardness through solid solution strengthening. In order to obtain these effects, the Si content is preferably 0.20% or more. On the other hand, if the Si content exceeds 0.55%, the 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 acts to increase the hardenability of steel, increase the hardness of weld metal, and improve wear resistance. To obtain 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 become excessively high. Therefore, the Mn content is preferably 1.60% or less. More preferably, it is 0.52 to 1.58%.

[0040] [P:0.050% or less] P is an element contained as an inevitable impurity, and since it segregates at grain boundaries and adversely affects hot cracking resistance, it is desirable to make the P content as low as possible, but it is permissible if it is 0.050% or less. Therefore, the P content is preferably 0.050% or less. Furthermore, it is more preferably 0.001 to 0.030%.

[0041] [S:0.050% or less] S is an element contained as an inevitable impurity, and since it segregates at grain boundaries and adversely affects hot cracking resistance, it is desirable to make the S content as low as possible, but it is permissible if it is 0.050% or less. Therefore, the S content is preferably 0.050% or less. In addition, it is more preferably 0.001 to 0.030%.

[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 leads to a decrease in toughness. Therefore, the Cr content is preferably 0.80% or less. More preferably, it is 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, the addition of Ti can improve the toughness of the weld metal. In addition, when both Ti and B are added, Ti fixes N, thereby suppressing the precipitation of BN, and as a result, the hardenability improving effect of B is promoted. However, if the Ti content exceeds 0.020%, a large amount of TiC precipitates, which reduces the 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, the addition of B can improve wear resistance. However, if the B content exceeds 0.010%, the weldability decreases. Therefore, when B is added, the B content is preferably 0.010% or less. More preferably, it is 0.001 to 0.008%.

[0045] [Remaining composition] The remaining composition other than the above composition is composed of Fe and inevitable impurities. Examples of inevitable impurities include N, O (oxygen), Nb, Mo, V, W, Sn, Sb, As, Pb, Bi, and REM. The total content of inevitable impurities is permissible if it is 0.10% or less.

[0046] As long as the above-mentioned 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 a 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. All of these C, Si, and Mn are elements that increase the hardness of the weld metal. In order to obtain 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. 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 cracks during welding, it is preferably 0.40% or less.

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

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

[0052] [Laser welding method] The laser welding method is to cut the steel plate with a laser and join the I-shaped end faces together to form an I-shaped butt groove with a groove width (root gap): 0 to 0.5 mm, using a 10 kW fiber laser welder or a 20 kW CO 2 This is done using a laser welding machine. If a filler metal is used, this can be done while the filler metal is being fed.

[0053] The preferable welding conditions in this case are as follows. Laser welding power: 3~20kW Welding speed: 1000~5000mm / min Shielding gas: 100% Ar or He, N 2 etc. Welding heat input: 800~5000J / cm EXAMPLES

[0054] A butt welded joint was prepared by a laser welding method using wear-resistant steel plate (base material), and an experiment was conducted to evaluate the size of the softened region. The results of the present invention using the laser welding method and the results of welding using the MAG welding method as a comparative example were summarized. The laser welding conditions were laser power: 4 to 15 kW, welding speed: 2000 to 4000 mm / min, and shielding gas: 100% Ar. The groove width (root gap) at this time 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. In addition, in the examples in which a filler metal was added, the chemical composition (mass%) and CI value of the filler metal were listed. Furthermore, the bead width W (mm) of the weld metal obtained after welding was measured, and the ratio (t / W) to the plate thickness t was calculated.

[0055] [Table 1]

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

[0057] [Table 2]

[0058] In the present invention, it was confirmed that the softened region was small, 10 mm or less. On the other hand, in Comparative Examples 1 and 2, it was confirmed that the softened region was formed widely, 25 mm or more. In Comparative Example 3, welding defects occurred, and a preferable welded joint was not obtained. Furthermore, in Comparative Example 4, it was confirmed that the Vickers hardness of the base material was insufficient, and the wear resistance was insufficient even though the softened region was small. [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 welded joint having a weld metal formed by butt-joining steel plates having a plate thickness (t) of 3 to 15 mm and a Vickers hardness (HV) of 425 or more by laser welding, wherein 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. The laser welded joint according to claim 1, characterized in that in addition to the above chemical composition, it further contains, by mass%, one or more selected from Ti: 0.020% or less and B: 0.010% or less.

3. 3. The laser welded joint 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. A laser welding method comprising the steps of 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 having a weld metal joined by laser welding.

5. The laser welding method according to claim 4, characterized in that in addition to the above chemical composition, the steel further contains, by mass%, one or more selected from Ti: 0.020% or less and B: 0.010% or less.

6. 6. The laser welding method according to claim 4, wherein a ratio (t / W) of the plate thickness (t) to the bead width (W) of the weld metal is 1.0 or more.

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

8. 7. The laser welding method according to claim 6, 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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