Manufacturing method of high-strength-and-ductility high-carbon quenched-distributed-tempered steel laser tailored welded joint

Abstract

According to the laser splicing welding joint with the high product of strength and elongation and the manufacturing method of the laser splicing welding joint, a joint welding seam, a heat-affected coarse grain area and a heat-affected fine grain area are composed of tempered martensite, transition carbide, reversed austenite and stable nanometer precipitated carbide, the tensile strength of the joint is 1000-1950 MPa, and the ductility of the joint is 14-34%; the method comprises the following steps: i, butt-jointing high-carbon quenched, distributed and tempered steel, and then carrying out laser tailored welding; ii, at least one time of heat treatment is conducted on the splice welding joint, the workpiece is placed in the environment with the temperature T, the heat preservation time t meets the conditions that T is larger than or equal to 100 and smaller than or equal to 525-20 * C-16 * Mn + 12 * Si < 2 + > 10Cr-12Ni + 20Mo + 10Al + 15 * (Nb + V), t is larger than or equal to 15 + 10 * B, and B (mm) is the plate thickness; and iii, after the heat preservation time is up, the splice-welded joint is cooled to the room temperature at the cooling speed not exceeding 100 DEG C / s. By means of the method, the strength and the ductility of the high-carbon quenching-distribution-tempering steel laser splicing welding joint can be greatly improved.

Description

Technical Field

[0001] In the field of welding and post-weld heat treatment technology, specifically, it relates to a method for manufacturing a laser-welded joint of high-strength, high-ductility, high-carbon quenched-distributed-tempered steel. Background Technology

[0002] In recent years, global demands for reducing greenhouse gas emissions and lowering fuel consumption have been increasing, driving the development of lightweight automobiles. High-strength steel sheets can achieve weight reduction while ensuring vehicle safety, and are therefore widely used in automobile manufacturing. Advanced high-strength steels are divided into three generations based on their strength-ductility product. The first generation of advanced high-strength steels has a low strength-ductility product, below 30 GPa%. The second generation has a strength-ductility product above 50 GPa, but its application is limited due to high alloy element content and complex processes. The third generation of advanced high-strength steels has a strength-ductility product above 30 GPa% and lower cost. Therefore, the current trend in high-strength steel is to develop third-generation advanced high-strength steels that combine low cost and excellent performance. Representative third-generation advanced high-strength steels include medium-manganese steel, quenched & partitioned steel, and quenched-partitioned-tempered steel. In recent years, quenched-partitioned-tempered steel has achieved simultaneous improvement in strength and ductility through inexpensive carbon elements. In particular, high-carbon quenched-partitioned-tempered steel exhibits extremely high strength-ductility product and performance / price ratio. However, due to its high carbon content, high-carbon quenched-partitioned-tempered steel has poor weldability. How to achieve high-quality, high-performance welding of high-carbon quenched-partitioned-tempered steel through appropriate welding and post-weld treatment methods is an urgent problem to be solved for the industrial application of high-carbon quenched-partitioned-tempered steel.

[0003] In existing technical literature, the manufacturing methods for high-carbon quenched-partitioned-tempered steel are all about the base material, and there are no reports on its welding and post-weld treatment processes. Furthermore, it is difficult to effectively apply the welding post-treatment processes of other third-generation advanced high-strength steels to high-carbon quenched-partitioned-tempered steel.

[0004] Patent CN108356417B (hereinafter referred to as Patent Document 1) provides a heat treatment method to improve the plasticity of laser-welded joints of high-strength and high-ductility manganese steel. The welded joint is heat-treated at a temperature of 650-750℃, and the martensite in the weld zone is transformed into austenite and ferrite. This method is not applicable to high-carbon quenched-partitioned-tempered steel with a martensitic matrix.

[0005] Patent CN112222667B (hereinafter referred to as Patent Document 2) provides a post-weld treatment method to improve the performance of spot welded joints of medium manganese steel. However, this method is applicable to medium manganese steel (manganese content by mass percentage of 4%-12%), and good tensile and shear performance requires heat preservation at two-phase temperature. It is not applicable to high carbon quenched-distributed-tempered steel. Summary of the Invention

[0006] The purpose of this invention is to provide a manufacturing method for high-strength, high-density, high-carbon quenched-distributed-tempered steel laser-welded joints, which solves the problem of excessively brittle and hard joints and improves the comprehensive mechanical properties of high-carbon quenched-distributed-tempered steel laser-welded joints.

[0007] The solution provided by this invention is as follows:

[0008] A high-strength, high-volume laser-welded joint is provided, characterized in that: the microstructure of the melting zone, heat-affected coarse-grained zone, and heat-affected fine-grained zone of the laser-welded joint consists of tempered martensite, transition carbides, reversed austenite, and stable nano-precipitated carbides, wherein the content of tempered martensite is 60–98.8 vol.%, the content of transition carbides is 1–15 vol.%, the content of reversed austenite is 0.1–20 vol.%, and the content of nano-precipitated carbides is 0.1–5 vol.%; the tensile strength of the joint is 1000–1950 MPa, the elongation is 14–34%, and the carbon content is ≥0.35 wt.%.

[0009] Preferably, the contents of tempered martensite, transition carbides, reversed austenite, and stable nano-precipitated carbides are 77–96.8 vol.%, 2–10 vol.%, 1–10 vol.%, and 0.2–3 vol.%, respectively; and the carbon content of the joint is ≥0.45 wt.%.

[0010] A high-strength plastic volume laser-welded joint is provided, characterized in that: the hardness range of the joint base material, the subcritical heat-affected zone and the critical heat-affected zone is 350-550 HV, and the hardness range of the melting zone, the heat-affected coarse grain zone and the heat-affected fine grain zone is 450-800 HV.

[0011] A high-strength, high-volume laser-welded joint is provided, characterized in that: the microstructure of the base material is: martensite matrix content of 65-92%, retained austenite content of 5-40%, transition carbides of 1-3%, and stable nano-precipitated carbides of less than 1%; the mass fraction of the joint composition includes: C: 0.40-0.75, Mn: 1.0-2.0, Si: 1.0-2.0, Cr: 0-1.0, Ni: 0-2.0, Mo: 0-1.5, Al: 0-2.0, Nb+V: 0.02-0.06, with the remainder being iron.

[0012] A high-strength, high-volume laser-welded joint is provided, characterized in that: the martensitic matrix of the base material region consists of primary lath martensite and fine twinned secondary martensite, the retained austenite consists of fine lamellar or granular austenite, and is coherent with the martensitic matrix in a KS or NW relationship, the transition carbides are η or ε carbides, and the stable nano-precipitated carbides are niobium carbide and / or vanadium carbide and / or molybdenum carbide.

[0013] A high-strength plastic volume laser welding head is provided, characterized in that: the melting zone of the joint is X-shaped or Y-shaped weld morphology, and the upper and lower depressions of the weld do not exceed 10% of the plate thickness, where the plate thickness is the thickness of the thinnest workpiece in the joint.

[0014] In addition, a method for manufacturing high-strength plastic-polymer laser-welded joints is provided, characterized by comprising the following steps:

[0015] i. Provide a pair of steel workpieces for laser welding, and perform at least one laser welding operation by butt-jointing the steel workpieces;

[0016] ii. Perform at least one heat treatment on the welded joint, that is, hold the steel workpiece in an environment with an exothermic heat treatment temperature T for a holding time t;

[0017] iii. Remove the welded joint and cool it to room temperature at a cooling rate not exceeding 100℃ / s to obtain the final welded joint;

[0018] Among them, at least one of the steel workpieces is QPT steel manufactured using a quenching-distribution-tempering process;

[0019] The heat treatment temperature T (°C) satisfies: 100 ≤ T ≤ 525 - 20 × C - 16 × Mn + 12 × Si 2 +10Cr-12Ni+20Mo+10Al+15×(Nb+V);

[0020] The heat preservation time t(s) satisfies: t≥15+10×B, where B(mm) is the plate thickness.

[0021] A method for manufacturing high-strength, high-volume laser-welded joints is provided, characterized in that: the microstructure of the QPT steel is: martensite matrix content of 65-92%, retained austenite content of 5-40%, transition carbides of 1-3%, stable nano-precipitated carbides of less than 1%, its tensile strength is 1000-1900 MPa, and its elongation is 15-35%.

[0022] A method for manufacturing high-strength plastic volume laser-welded joints is provided, characterized in that: the butt joint gap of the workpieces is no more than 20% of the plate thickness B; and the plate thickness of the steel workpiece is B≤5mm.

[0023] Preferably, the cross-sectional shape of the welded joint is X-shaped or Y-shaped.

[0024] Preferably, the laser power is 1-10kW, the welding speed is 1-10m / min, the laser beam defocusing amount is -10-60mm, argon gas is used to protect the front side, and the gas flow rate is 5-30L / min.

[0025] Preferably, the laser beam is Gaussian or uniformly distributed, and the laser beam may be composed of one or more light spots.

[0026] Preferably, the heating treatment method involves heating the workpiece as a whole in a heating furnace using air as a medium.

[0027] Preferably, the heat treatment method involves immersing the entire workpiece or the welding area in oil or salt within the heating temperature range using a liquid medium; the oil is conventional heat treatment oil bath oil, and the salt is low-temperature salt bath salt.

[0028] Preferably, the cooling method can be air cooling, water cooling, oil cooling, or other gaseous or liquid cooling media.

[0029] The beneficial effects of this invention are as follows:

[0030] After laser welding of high-carbon quenched-partitioned-tempered steel, the weld and the coarse-grained and fine-grained regions of the heat-affected zone are all brittle and hard high-carbon martensite. In some areas, due to the high carbon content (around 1.3 wt.%) of the retained austenite in the high-carbon quenched-partitioned-tempered steel, even higher carbon martensite is formed. Using the manufacturing method of this invention, the high-carbon martensite in the weld and the coarse-grained and fine-grained regions of the heat-affected zone transforms into tempered martensite, transition carbides, and reversed austenite. The hardness of the weld and the coarse-grained and fine-grained regions of the heat-affected zone is reduced. Due to the precipitation of transition carbides, the high carbon content of the martensite is depleted, and the toughness is improved. Furthermore, the transition carbides can play a precipitation strengthening role. At the same time, the reversed austenite can generate dislocation crossing the martensite / austenite interface effect and a phase transformation-induced plasticity effect, thereby further improving the strength and toughness of the joint.

[0031] The manufacturing method of this invention has little impact on the microstructure of the base material. The high Si content in the high-carbon quenched-partitioned-tempered steel can suppress the precipitation of cementite during tempering, and the microstructure of the base material remains basically unchanged. After treatment, the microstructure of the base material is still martensitic matrix, retained austenite, transition carbides and a small amount of stable precipitates. Therefore, the welded joint has high strength and plasticity. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the different locations of existing high-strength plastic volume laser-welded joints.

[0033] Figure 2 This is a scanning electron microscope image of the high-carbon quenched-distributed-tempered steel base material from Example 1.

[0034] Figure 3 This is a scanning electron microscope image of the weld seam after laser welding of high-carbon quenched-partitioned-tempered steel in Example 1.

[0035] Figure 4This is a scanning electron microscope image of the weld seam of the laser-welded joint of high-carbon quenched-distributed-tempered steel in Example 1 after heat treatment.

[0036] Figure 5 This describes the manufacturing process of laser-welded joints for high-strength, high-ductility, high-carbon quenched, distributed, and tempered steel.

[0037] Figure 6 This is a schematic diagram of a high-strength steel butt joint workpiece.

[0038] In the attached diagram, 1-first steel workpiece, 2-second steel workpiece, b-assembly clearance, B-plate thickness. Detailed Implementation

[0039] The present invention will now be described in detail with reference to specific embodiments, and preferred embodiments of the present application are shown in the accompanying drawings. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0040] Please refer to Figure 1 This invention provides a high-strength, high-volume laser-welded joint; the microstructure of the molten zone, heat-affected coarse-grained zone, and heat-affected fine-grained zone of the laser-welded joint consists of tempered martensite, transition carbides, reversed austenite, and stable nano-precipitated carbides, wherein the tempered martensite content is 77–96.8 vol.%, the transition carbide content is 2–10 vol.%, the reversed austenite content is 1–10 vol.%, and the nano-precipitated carbide content is 0.2–3 vol.%; the carbon content is ≥0.45 wt.%. Figure 2 , 3Figure 4 shows the morphological characteristics of tempered martensite, retained austenite, and transition carbides. The hardness range of the base metal, subcritical heat-affected zone, and critical heat-affected zone is 350–550 HV, and the hardness range of the melting zone, coarse-grained heat-affected zone, and fine-grained heat-affected zone is 450–800 HV. The microstructure of the base metal is as follows: the martensite matrix content is 65–92%, consisting of primary lath martensite and fine twinned secondary martensite; the retained austenite content is 5–40%, consisting of fine lamellar or granular austenite, coherent with the martensite matrix using KS or NW; transition carbides: 1–3%, consisting of η or ε carbides; stable nano-precipitated carbides: less than 1%, consisting of niobium carbide and / or vanadium carbide and / or molybdenum carbide. The base metal composition (mass fraction, %) is: C: 0.40–0.75, Mn: 1.0–2.0, Si: 1.0–2.0, Cr: 0–1.0, Ni: 0–2.0, Mo: 0–1.5, Al: 0–2.0, Nb+V: 0.02–0.06, with the remainder being iron. The molten zone in the joint has an X-shaped or Y-shaped weld morphology, where the vertical concavity of the weld in the molten zone does not exceed 10% of the plate thickness, where plate thickness B is the thickness of the thinnest workpiece in the joint. The tensile strength of the joint is 1000–1950 MPa, and the elongation is 14–34%.

[0041] This invention also provides a method for manufacturing high-strength, high-density, high-volume laser-welded joints, as detailed in the following reference. Figure 5 :

[0042] i. Provide a pair of high-strength steel workpieces 1 and 2 for laser welding, wherein at least one workpiece is made of high-strength steel using the QPT process, the butt joint gap b of the workpieces is no more than 20% of the plate thickness B, and the workpiece plate thickness B ≤ 5mm. Figure 6As shown, workpieces 1 and 2 can be flat plates or stamped parts with a certain shape; the steel workpieces are butt-welded at least once to form a laser welded joint; the laser beam is generally emitted by a laser, which can include various types, such as solid-state laser beams or gas laser beams, specifically including fiber lasers, disk lasers, semiconductor diode lasers and Nd:YAG type solid-state lasers, or CO2 gas lasers. Other types are also possible, as long as they can generate a laser beam and create a keyhole and a molten weld pool. During laser welding, the laser beam can also consist of one or more beams, and the beam energy distribution can be Gaussian, average, or point-ring distribution, etc. During welding, the laser beam can travel along the welding direction in a fixed or synchronous high-speed motion, such as oscillation. The oscillation shape can include circular, broken line, figure-eight, ∞, etc., with an oscillation frequency generally between 50-500Hz and an oscillation amplitude between 0.2-1.5mm. During the welding process, various single or mixed shielding gases can be added, such as Ar, He, etc., or the welding can be performed without a shielding gas. In a more preferred embodiment, the laser power is 1 to 10 kW, the welding speed is 1 to 10 m / min, the laser beam defocusing distance is -10 to 60 mm, argon gas is used to protect the front side, the gas flow rate is 5 to 30 L / min, and the resulting laser welded joint cross-sectional morphology is X-shaped or Y-shaped.

[0043] ii. Perform at least one heat treatment on the above-mentioned welded joints, placing the workpiece in an environment at temperature T and holding it at that temperature for a certain time t, wherein the heat treatment temperature T (°C) satisfies:

[0044] 100≤T≤525-20×C-16×Mn+12×Si 2 +10Cr-12Ni+20Mo+10Al+15×(Nb+V)(1)

[0045] Wherein C, Mn, Si, Cr, Ni, Mo, Al, Nb, and V represent the weight percentages of carbon, manganese, silicon, chromium, nickel, molybdenum, aluminum, niobium, and vanadium in the billet.

[0046] The heat treatment time t(s) satisfies:

[0047] t ≥ 15 + 10×B (2)

[0048] Where B (mm) is the plate thickness.

[0049] There are various methods for heat treatment heating. It can be done by heating with a gaseous medium, in which the workpiece is placed in an air furnace, a vacuum heat treatment furnace, or other gas atmosphere heating furnaces. Alternatively, it can be done by heating with a liquid medium, in which the workpiece or the welded area is immersed in oil or salt within the heating temperature range. The oil is conventional heat treatment oil bath oil, and the salt is low-temperature salt bath salt.

[0050] iii. After the holding time is reached, the welded joint is removed and cooled to room temperature at a cooling rate not exceeding 100°C / s to obtain the final welded joint. In a more preferred embodiment, the cooling rate does not exceed 60°C / s, and the cooling method can be air cooling, water cooling, oil cooling, or other gaseous or liquid cooling media. After heat treatment, the weld microstructure changes from mainly martensite to tempered martensite, transition carbides, and reversed austenite. After the above heat treatment, the mechanical properties of the laser-welded joint reach: tensile strength 1000–1950 MPa, elongation 15–34%.

[0051] Example

[0052] Table 1 shows the specific composition, mechanical properties, laser welding process parameters, and heat treatment process of the high-carbon quenched-partitioned-tempered steel base material in the embodiments and comparative examples of the present invention. The plate thickness is 1.5 mm, and the tensile test is conducted according to GB / T228-2002 "Metallic Materials - Tensile Testing at Room Temperature". Table 2 shows the differences in the microstructure distribution and microhardness values ​​of the resulting final welded joints.

[0053]

[0054]

[0055]

[0056] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A high-strength, high-volume laser welding head, characterized in that: The microstructure of the molten zone, heat-affected coarse-grained zone, and heat-affected fine-grained zone of the laser-welded joint consists of tempered martensite, transition carbides, reversed austenite, and stable nano-precipitated carbides. The tempered martensite content is 60–98.8 vol.%, the transition carbide content is 1–15 vol.%, the reversed austenite content is 0.1–20 vol.%, and the nano-precipitated carbide content is 0.1–5 vol.%. The joint has a tensile strength of 1000–1950 MPa, an elongation of 14–34%, and a carbon content ≥0.35 wt.%.

2. The laser welding head as described in claim 1, characterized in that: The contents of the tempered martensite, transition carbides, reversed austenite, and stable nano-precipitated carbides are 77–96.8 vol.%, 2–10 vol.%, 1–10 vol.%, and 0.2–3 vol.%, respectively; the carbon content of the joint is ≥0.45 wt.%.

3. The laser welding joint as described in claim 1 or 2, characterized in that: The hardness range of the joint base material, subcritical heat-affected zone, and critical heat-affected zone is 350–550 HV, and the hardness range of the melting zone, heat-affected coarse-grained zone, and heat-affected fine-grained zone is 450–800 HV.

4. The laser welding head as described in claim 3, characterized in that: The microstructure of the base material is as follows: martensite matrix content is 65-92%, retained austenite content is 5-40%, transition carbides: 1-3%, and stable nano-precipitated carbides: less than 1%; the mass fraction of the joint composition includes: C: 0.40-0.75, Mn: 1.0-2.0, Si: 1.0-2.0, Cr: 0-1.0, Ni: 0-2.0, Mo: 0-1.5, Al: 0-2.0, Nb+V: 0.02-0.06, with the remainder being iron.

5. The laser welding head as described in claim 4, characterized in that: The martensitic matrix in this parent material region consists of primary lath martensite and fine twinned secondary martensite. The retained austenite consists of fine lamellar or granular austenite and is coherent with the martensitic matrix using the KS or NW method. The transition carbides are η or ε carbides, and the stable nano-precipitated carbides are niobium carbide and / or vanadium carbide and / or molybdenum carbide.

6. The laser welding head as described in claim 1, characterized in that: The melting zone of the joint has an X-shaped or Y-shaped weld morphology, and the upper and lower depressions of the weld do not exceed 10% of the plate thickness, which is the thickness of the thinnest workpiece in the joint.

7. A method for manufacturing a laser-welded joint as described in any one of claims 1-6, characterized in that, Includes the following steps: i. Provide a pair of steel workpieces for laser welding, and perform at least one laser welding operation by butt-jointing the steel workpieces; ii. Perform at least one heat treatment on the welded joint, that is, hold the steel workpiece in an environment with an exothermic heat treatment temperature T for a holding time t; iii. Remove the welded joint and cool it to room temperature at a cooling rate not exceeding 100℃ / s to obtain the final welded joint; Among them, at least one of the steel workpieces is QPT steel manufactured using a quenching-distribution-tempering process; The heat treatment temperature T (°C) satisfies: 100 ≤ T ≤ 525 - 20 × C - 16 × Mn + 12 × Si 2 +10Cr-12Ni+20Mo+10Al+15×(Nb+V); The heat preservation time t(s) satisfies: t≥15+10×B, where B(mm) is the plate thickness.

8. The method for manufacturing a laser-welded joint as described in claim 7, characterized in that, The microstructure of the QPT steel is as follows: martensite matrix content is 65-92%, retained austenite content is 5-40%, transition carbides are 1-3%, stable nano-precipitated carbides are less than 1%, its tensile strength is 1000-1900 MPa, and its elongation is 15-35%.

9. The method for manufacturing a laser-welded joint as described in claim 7, characterized in that, The butt joint gap b of the welded joint shall not exceed 20% of the plate thickness B, and B ≤ 5mm.

10. The method for manufacturing a laser-welded joint as described in claim 7, characterized in that, The laser power is 1-10kW, the welding speed is 1-10m / min, the laser beam defocusing is -10-60mm, and argon gas is used to protect the front side with a gas flow rate of 5-30L / min.

11. The method for manufacturing a laser-welded joint as described in claim 10, characterized in that, The laser beam is Gaussian or uniformly distributed, and the laser beam consists of one or more light spots.

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