Laser-welded joints and laser welding methods

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 a wear-resistant steel plate used as various members of steel structures such as construction machinery, industrial machinery, shipbuilding, civil engineering, and architecture, and particularly relates to a laser welded joint of a wear-resistant steel plate joined by laser welding and a laser welding method therefor.

Background Art

[0002] Many wear-resistant steel plates are subjected to welding work. Gas shielded arc welding is used for welding conventional wear-resistant steel plates. In general gas shielded arc welding, problems have been that the hardness of the weld metal decreases and a region softened more than the base material is widely formed in the heat affected zone (hereinafter also referred to as "HAZ") of the base material and its periphery. In Patent Document 1, as a method for manufacturing a welded joint of a wear-resistant steel plate, a method of maintaining the hardness of the weld metal at the same level as that of the base material by using a flux-containing wire adjusted to specific components in gas shielded arc welding has been carried out. However, with this method, it is impossible to suppress the size of the softened region in the heat affected zone of the base material.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] As described above, in the gas shielded arc welded joint of a wear-resistant steel plate, there has been a problem that a softened region is widely formed in the welded portion and its periphery. In view of such a situation, the present invention has been made, and an object of the present invention is to provide a welded joint in which the softened region of the heat affected zone is reduced by using a laser welding method for a welded joint of a wear-resistant steel plate.

Means for Solving the Problems

[0005] The inventors diligently investigated methods for reducing the softened area in welded joints of wear-resistant steel. As a result, they found that by applying laser welding to the fabrication of welded joints under certain conditions and reducing the welding heat input, softening of the weld area can be reduced. Furthermore, they obtained the finding that by limiting the width of the weld bead to a predetermined range, the softened area in welded joints of wear-resistant steel can be reduced.

[0006] This invention was completed based on the aforementioned findings and further investigations, and the gist of this invention is as follows. [1] A laser-welded joint having weld metal formed by laser welding together steel plates having a plate thickness (t) of 3 to 15 mm and a Vickers hardness (HV) of 425 or more, wherein the chemical composition of the steel plates is, 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 remainder being Fe and unavoidable impurities. [2] A laser-welded joint characterized in that, in addition to the chemical composition of [1], it further contains one or more selected from Ti: 0.020% or less and B: 0.010% or less by mass%. [3] A laser-welded joint characterized in that, in [1] or [2] above, the ratio (t / W) of the plate thickness (t) to the weld bead width (W) of the weld metal is 1.0 or more. [4] A laser welding method characterized by obtaining a welded joint having weld metal obtained by laser welding 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 remainder being Fe and unavoidable impurities. [5] A laser welding method characterized in that, in addition to the chemical composition of [4], it further contains one or more selected from Ti: 0.020% or less and B: 0.010% or less by mass%. [6] A laser welding method characterized in that, in [4] or [5], the ratio (t / W) of the plate thickness (t) to the weld metal bead width (W) is 1.0 or more. [7] A laser welding method characterized in that, in any one of [4] to [6] above, the laser welding output in the laser welding method 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 area of ​​the welded part is reduced for wear-resistant steel plates, which has a significant industrial effect. [Brief explanation of the drawing]

[0008] [Figure 1] This is a schematic diagram showing the macroscopic cross-sectional shape of the laser-welded joint according to the present invention. [Figure 2] This is a schematic diagram showing the Vickers hardness distribution around the weld metal of a laser-welded joint according to the present invention. [Modes for carrying out 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 is a region (hereinafter referred to as the "softened region") where the hardness is lower compared to the original material of the base material (steel plate).

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

[0011] An example of the measurement results is shown in Figure 2. From this figure, it can be seen that the hardness of the steel plate (base material) is maintained at a high level, but the hardness decreases from the heat-affected zone, the hardness recovers slightly in the weld metal, but the hardness decreases again in the central part of the weld metal. In this invention, the average hardness of the base material is determined, and the region where the hardness is lower than that average is defined as the "softened region".

[0012] Therefore, in order to reduce the softened area in welded joints of wear-resistant steel plates, it is important to reduce the width of the heat-affected zone. Generally, the width of the heat-affected zone expands 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 this invention, by selecting laser welding, which has a low heat input, as an alternative to general gas shielded arc welding, and applying it to wear-resistant steel plates, it was found that the width of the heat-affected zone can be reduced, and as a result, the width of the softened region can be reduced.

[0014] Furthermore, 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 falls into the softened range. However, in this invention, the width of the weld metal is reduced by applying laser welding that enables welding with an I-groove.

[0015] Furthermore, as will be described later, it was found that 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 width W (mm) of the weld metal to 1.0 or greater.

[0016] The present invention, derived from the above findings, will be described in detail below.

[0017] [Steel plate] First, we will explain the plate thickness (t) and Vickers hardness (HV) of the wear-resistant steel plate that is the subject of this invention.

[0018] The plate thickness (t) of the steel plate was 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 welding defects occurring within the welding condition range of the present invention increases. Preferably, it is 3.2 to 14.5 mm.

[0019] Also, the Vickers hardness (HV) was limited to 425 or more. If HV is less than 425, it does not correspond to the wear-resistant steel plate targeted by the present invention. The upper limit is preferably 650 or less.

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

[0021] [Chemical composition of the steel plate] Next, the basic composition among the chemical compositions of the steel plate used for the laser welded joint according to the present invention will be described. Hereinafter, “%” in the chemical composition means “mass %”.

[0022] The basic composition of the steel plate contains 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 consists of Fe and inevitable impurities. Each composition will be described in detail below.

[0023] [C: 0.15 to 0.35%] C is an element that has the effect of increasing the hardness of the steel plate surface layer and improving the 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. Accordingly, the C content was limited to 0.15 to 0.35%. Preferably, it is 0.17 to 0.33%, and more preferably, it is 0.20 to 0.30%.

[0024] [Si: 0.20 to 0.55%] Si is an element that acts as a deoxidizing agent. Furthermore, Si forms a solid solution in steel, increasing its hardness through solid solution strengthening. To obtain these effects, the Si content should be 0.20% or higher. On the other hand, if the Si content exceeds 0.55%, the toughness decreases. Therefore, the Si content should be 0.55% or less. Consequently, the Si content must satisfy the range of 0.20 to 0.55%. Preferably, it is 0.22 to 0.55%, and more preferably, 0.24 to 0.53%.

[0025] [Mn: 0.50~1.60%] Mn is an element that increases the hardenability of steel, increases the hardness of the steel plate surface, and improves wear resistance. To obtain this effect, the Mn content should be 0.50% or more. On the other hand, if the Mn content exceeds 1.60%, toughness decreases, and the alloy cost becomes excessively high. Therefore, the Mn content should be 1.60% or less. Accordingly, the Mn content needs to satisfy the range of 0.50 to 1.60%. Preferably, it is 0.52 to 1.58%, and more preferably, 0.55 to 1.55%.

[0026] [P:0.050% or less] P is an element that is inevitably present as an 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 it is acceptable if it is 0.050% or less. Accordingly, the P content must satisfy the requirement of 0.050% or less. Preferably, it is 0.001 to 0.040%, and more preferably 0.001 to 0.030%.

[0027] [S:0.050% or less] S is an element that is inevitably present as an impurity, existing in steel as sulfide-based inclusions such as MnS, and adversely affecting the toughness of the base material. Therefore, it is desirable to keep the S content as low as possible, but it is acceptable if it is 0.050% or less. Accordingly, the S content must satisfy the requirement of 0.050% or less. Preferably, it is 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 plate surface and improves wear resistance. However, if the Cr content exceeds 0.80%, it leads to a decrease in toughness. Therefore, the Cr content should be 0.80% or less. Accordingly, the Cr content is limited to 0.80% or less. Preferably, it is 0.01 to 0.78%, and more preferably, 0.05 to 0.75%.

[0029] [Optional composition] The steel sheet used in the laser-welded joint according to the present invention has the above-described basic composition. In the present invention, in addition to this basic composition, one or more elements selected from Ti: 0.020% or less and B: 0.010% or less may be included as an optional composition as needed.

[0030] [Ti:0.020% or less] Ti has a strong tendency to form nitrides and has the effect of fixing nitrogen and reducing dissolved nitrogen. Therefore, the toughness of the base metal and weld metal can be improved by adding Ti. Furthermore, when both Ti and B are added, the precipitation of BN is suppressed by the fixing of N by Ti, and as a result, the hardenability-improving effect of B is enhanced. However, if the Ti content exceeds 0.020%, a large amount of TiC precipitates, reducing workability. Therefore, when Ti is included, it is preferable to keep the Ti content at 0.020% or less. More preferably, it is 0.001 to 0.018%.

[0031] [B:0.010% or less] B is an element that improves hardenability. Therefore, adding B can improve wear resistance. However, if the B content exceeds 0.010%, weldability decreases. For this reason, when adding B, it is preferable to keep the B content at 0.010% or less. More preferably, it is 0.001 to 0.008%.

[0032] [Remaining composition] The remaining composition, other than the composition described above, consists of Fe and unavoidable impurities. Examples of unavoidable impurities include 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] Furthermore, as long as the aforementioned basic composition and optional composition are satisfied, this does not preclude the inclusion of other unavoidable impurity elements, 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 that of the steel plate when no filler material is used. However, when a filler material is used during laser welding, some of the chemical composition of the filler material is mixed in.

[0036] [Chemical composition of weld metal] The basic chemical composition of 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 remainder being Fe and unavoidable impurities. In addition to the above basic composition, it may also contain one or more elements selected from Ti: 0.020% or less and B: 0.010% or less.

[0037] [C:0.15~0.35%] Carbon (C) in weld metal is an element that increases hardness. To obtain this effect, it is preferable to have a C content of 0.15% or more. On the other hand, if the C content exceeds 0.35%, toughness decreases. Therefore, it is preferable to have a C content of 0.35% or less. More preferably, it is between 0.17% and 0.33%.

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

[0039] [Mn: 0.50~1.60%] Mn is an element that increases the hardenability of steel, increases the hardness of weld metal, and improves wear resistance. To obtain this effect, it is preferable to have a Mn content of 0.50% or more. On the other hand, if the Mn content exceeds 1.60%, toughness decreases, and the alloy cost becomes excessively high. Therefore, it is preferable to have a Mn content of 1.60% or less. More preferably, it is between 0.52% and 1.58%.

[0040] [P:0.050% or less] P is an element that is inevitably present as an impurity and segregates at grain boundaries, adversely affecting resistance to high-temperature cracking. Therefore, it is desirable to keep the P content as low as possible, but a value of 0.050% or less is acceptable. Accordingly, it is preferable that the P content be 0.050% or less. More preferably, it is 0.001 to 0.030%.

[0041] [S:0.050% or less] S is an element that is inevitably present as an impurity and segregates at grain boundaries, adversely affecting resistance to high-temperature cracking. Therefore, it is desirable to keep the S content as low as possible, but a value of 0.050% or less is acceptable. Accordingly, it is preferable that the S content be 0.050% or less. More preferably, it is 0.001 to 0.030%.

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

[0043] [One or more types selected from Ti: 0.020% or less and B: 0.010% or less] Ti has a strong tendency to form nitrides and is an element that fixes nitrogen and reduces dissolved nitrogen. Therefore, the toughness of the weld metal can be improved by adding Ti. Furthermore, when both Ti and B are added, the precipitation of BN is suppressed by the fixation of N by Ti, and as a result, the hardenability-improving effect of B is enhanced. However, if the Ti content exceeds 0.020%, a large amount of TiC precipitates, reducing workability. Therefore, when Ti is included, it is preferable to keep the Ti content at 0.020% or less. More preferably, it is 0.001 to 0.018%.

[0044] B is an element that improves hardenability. Therefore, adding B can improve wear resistance. However, if the B content exceeds 0.010%, weldability decreases. For this reason, when adding B, it is preferable to keep the B content at 0.010% or less. More preferably, it is 0.001 to 0.008%.

[0045] [Remaining composition] The remaining composition, other than the composition described above, consists of Fe and unavoidable impurities. Examples of unavoidable impurities include 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] Furthermore, as long as the aforementioned basic composition and optional composition are satisfied, this does not preclude the inclusion of other unavoidable impurity elements, and such embodiments are also included within the technical scope of the present invention.

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

[0048] Regarding the chemical composition of the filler material, it is more preferable that the CI value shown in 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 that element in the filler material.

[0049] The above formula (1) is an index obtained by focusing on C, Si, and Mn in the chemical composition of the filler material. These C, Si, and Mn are all elements that increase the hardness of the weld metal. To obtain this effect, it is more preferable to use a filler material that satisfies the CI value obtained from the above formula (1) of 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. Even more preferably, it is 0.27% or more. Furthermore, from the viewpoint of preventing cracking during welding, it is preferable to have it be 0.40% or less.

[0050] The filler material is obtained by drawing wire from melted steel, and its 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 of the steel plate thickness t to the bead width W (t / W) be 1.0 or greater. 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 should be 1.5 or greater. Furthermore, from the viewpoint of preventing welding defects, it is preferable to have a ratio of 10.0 or less.

[0052] [Laser welding method] The laser welding method involves laser cutting the steel plate and joining the I-shaped ends together as a butt joint I-groove with a root gap of 0 to 0.5 mm, using a 10 kW fiber laser welding machine or a 20 kW CO2 laser welding machine. If filler material is used, the welding can be performed while supplying the filler material.

[0053] The preferred welding conditions in this case are as follows: • Laser welding output: 3-20kW • Welding speed: 1000-5000 mm / min • Shielding gas: 100% Ar or He, N2, etc. • Welding heat input: 800~5000 J / cm² [Examples]

[0054] An experiment was conducted to evaluate the size of the softened region by fabricating butt welded joints using abrasion-resistant steel plates (base material) with a laser welding method. The results of the present invention example using the laser welding method and the results of welding using the MAG welding method as a comparative example are summarized. The laser welding conditions were: laser output: 4-15 kW, welding speed: 2000-4000 mm / min, shielding gas: 100% Ar. The groove width (root gap) was set to 0-0.5 mm. Table 1 shows the chemical composition (mass%), plate thickness t (mm), Vickers hardness HV (mean value) of the base material used, and the welding conditions for laser welding and MAG welding. In the example where filler material was added, the chemical composition (mass%) and CI value of the filler material 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, for the evaluation of the softened region, a Vickers hardness test was performed on the cross-section of the weld at a depth of 0.5 mm from the surface, and the hardness distribution was obtained across the weld. As mentioned above, the region with a hardness lower than the average hardness of the base material was defined as the softened region, and a width of 10 mm or less was evaluated as good (○), while a width exceeding 10 mm was evaluated as unacceptable (×). The evaluation results are shown in Table 2.

[0057] [Table 2]

[0058] In the present invention example, it was confirmed that the softened area was kept small, at 10 mm or less. On the other hand, in Comparative Examples 1 and 2, it was confirmed that the softened area was formed to a wide extent, at 25 mm or more. Furthermore, in Comparative Example 3, welding defects occurred, and a desirable welded joint could not be obtained. In addition, in Comparative Example 4, it was confirmed that the Vickers hardness of the base material was insufficient, resulting in inadequate wear resistance despite a small softened area. [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 weld metal formed by laser welding together steel plates having a thickness (t) of 3 to 15 mm and a Vickers hardness (HV) of 425 or more, wherein the chemical composition of the steel plates is, 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 remainder being Fe and unavoidable impurities.

2. The laser welded joint according to claim 1, further characterized by containing, in addition to the above chemical composition, one or more selected from Ti: 0.020% or less and B: 0.010% or less by mass%.

3. The laser welded joint according to claim 1 or 2, characterized in that the ratio (t / W) of the plate thickness (t) to the weld metal bead width (W) is 1.0 or more.

4. A laser welding method characterized by obtaining a welded joint having weld metal obtained by laser welding together steel plates having a plate thickness (t) of 3 to 15 mm, a Vickers hardness (HV) of 425 or higher, 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 remainder being Fe and unavoidable impurities.

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

6. The laser welding method according to claim 4 or 5, characterized in that the ratio (t / W) of the plate thickness (t) to the weld bead width (W) of the weld metal is 1.0 or more.

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

8. The laser welding method according to claim 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.

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