Welding method of low-cost 780MPa-grade hot-dip galvanized dual-phase steel in acid rolling process

Abstract

The invention discloses low-cost 780MPa-grade hot-dip galvanized dual-phase steel and a welding method for an acid rolling process of the low-cost 780MPa-grade hot-dip galvanized dual-phase steel. The dual-phase steel comprises the following chemical components: C, Si, Mn, Cr, Nb, P, S, Als, N and the like, and the balance of Fe and inevitable impurities. According to the welding method, after a connecting material selected for welding and a raw material steel coil are sheared, welding faces of the connecting material and the raw material steel coil are aligned and located on the same horizontal plane, then welding wires are added for welding, after welding, welding lines are subjected to post-welding annealing and polishing treatment, and the quality of the welding lines is checked by measuring the thickness of the welding lines and conducting a cup convex test. According to the method, the good welding seam quality can be obtained, the subsequent loop control method, the pickling process requirement and the like are clearly required, meanwhile, the cold rolling reduction rate is adjusted according to the thickness of the strip steel, and stable and efficient production of the pickling rolling procedure is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of cold-rolled strip production, and in particular relates to a low-cost 780MPa grade hot-dip galvanized duplex steel product produced by adopting a combined acid rolling unit and a hot-dip galvanizing unit. Background Art

[0002] 780MPa grade duplex steel is a duplex steel with a yield strength of about 780MPa. The microstructure of duplex steel mainly consists of two phases: ferrite and martensite. The ferrite phase has good toughness and ductility, while the martensite phase provides high strength. This unique organizational structure makes the steel have both strength and toughness. With the increasing prominence of energy and environmental issues, energy conservation and environmental protection have become inevitable issues in the development of the automotive industry. Reducing the thickness of automobile steel through high strengthening to achieve weight reduction is considered to be the most scientific and effective way to promote the sustainable development of the automotive industry. Duplex steel is currently the most widely used advanced high-strength steel for automobiles, and 780MPa grade duplex steel has become one of the most widely used products. At the same time, with the continuous improvement of corrosion resistance requirements, the proportion of galvanized sheets is increasing. Hot-dip galvanized products need to be treated with balanced galvanizing at about 460℃ (in the bainite transformation temperature zone). A certain amount of alloy elements need to be added in the composition design to ensure hardenability, and the increase in alloy content brings certain difficulties to the production of the acid rolling process (welding quality, rolling load, etc.).

[0003] Therefore, how to obtain a low-cost welding method for 780MPa grade hot-dip galvanized dual-phase steel that ensures a stable and efficient pickling process has become a difficult problem that urgently needs to be solved. Summary of the invention

[0004] Based on this, in view of the deficiencies of the above-mentioned prior art, a low-cost 780MPa grade hot-dip galvanized dual-phase steel and a welding method for its acid rolling process are provided.

[0005] In order to achieve the above purpose, the following technical solutions are adopted: The invention provides a low-cost 780MPa grade hot-dip galvanized dual-phase steel. The chemical composition of the dual-phase steel comprises, by weight percentage, C: 0.05%-0.11%, Si: 0.20%-0.40%, Mn: 1.70%-2.30%, P≤0.017%, S≤0.008%, Als: 0.010%-0.080%, N≤0.008%, Cr: 0.35%-0.75%, Nb: 0.01%-0.05%, Ni≤0.03%, V≤0.02%, Mo≤0.01%, and the balance is Fe and unavoidable impurities.

[0006] In some embodiments, the chemical composition of the dual-phase steel includes, by weight percentage, C: 0.06%~0.10%, Si: 0.25~0.35%, Mn: 1.80%~2.20%, P≤0.015%, S≤0.006%, Als: 0.020%~0.070%, N≤0.006%, Cr: 0.40%~0.70%, Nb: 0.02~0.04%, Ni≤0.03%, V≤0.02%, Mo≤0.01%, and the balance is Fe and unavoidable impurities.

[0007] The present invention also provides a welding method for the above-mentioned low-cost 780MPa grade hot-dip galvanized dual-phase steel acid rolling process, wherein the welding material and the raw steel coil are sheared, the welding surfaces of the welding material and the raw steel coil are aligned and on the same horizontal plane, and then welding wire is added for welding, and the weld is annealed and polished after welding, and the weld quality is checked by measuring the weld thickness and cupping test; The carbon equivalent of the selected connection material and raw steel coil needs to be controlled by the carbon equivalent formula, which is: Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14(%); The difference in yield strength between the connecting material and the raw steel coil before and after welding is within 1.7 times.

[0008] In some embodiments, the carbon equivalent of the selected connecting material is controlled in the range of 0.42% to 0.90%.

[0009] In some embodiments, the weld is annealed after welding, with a power of 43 kW, a frequency of 32 kHz, and a weld heat treatment current compensation of 0-15%. During the heating process of the weld heat treatment, the heating rate is required to be ≤20°C / s, and the cooling rate is ≤15°C / s to below 300°C by air cooling.

[0010] In some embodiments, the step of "cutting the welding selected connecting material and the raw steel coil" includes: when cutting, the cutting length of the steel coil is controlled according to the following requirements, and welding is performed after ensuring the plate shape and surface quality; Use cross-cutting shears to shear the connecting materials and raw steel coils. Before welding, the tail of the previous steel coil and the head of the next steel coil need to be cut off by double cutting shears to ensure the plate shape and surface quality before welding. When the strip thickness t is less than 3.25mm, the cutting amount is 8.5m, and when the strip thickness t is 3.25mm≤less than 5.25mm, the cutting amount is 6.8m. It is also ensured that the defects at the head and tail of the strip can be completely removed without deteriorating the welding quality.

[0011] In some embodiments, the welding process parameters are: laser power 13±0.5KW, welding speed 2.9~4.5±0.4m / min; wherein, when the raw material thickness is ≤3.25mm, the welding speed is 4.5±0.4m / min, and the speed decreases by 0.4m / min for every 0.5mm increase in thickness; the welding speed compensation range is 0~15%; after welding, the weld needs to be cleaned with a rolling wheel, the rolling wheel pressure is ≤65KN, and the rolling wheel height is 0~6.0mm.

[0012] In some embodiments, after welding is completed, the strip shape is straightened and the strip is placed in a "tight" state, and the position of the strip is adjusted so as to keep the strip in the center of the equipment.

[0013] In some embodiments, the welded steel is subsequently pickled and cold rolled; wherein the pickling step is: The i-Box type acid tank is used to clean the iron oxide scale on the surface of the strip steel. The acid concentration of the 1# acid tank is 45~70g / L, the acid concentration of the 2# acid tank is 75~100g / L, and the acid concentration of the 3# acid tank is 105~140 g / L. The acid temperature is 60℃~80℃, and the pickling speed is 55m / min~220m / min. The pickling speed is 220m / min when the strip steel thickness is 2.50mm, and the pickling speed decreases by 15m / min for every 0.25mm increase in thickness. After that, the residue on the surface of the strip steel is rinsed and the pH value of the rinse agent is ≥5.0. Finally, it is dried at a drying temperature of 80~150℃.

[0014] In some embodiments, the cold rolling step includes: pickling the hot rolled coil and then cold rolling it into a thin strip steel of 0.70-2.30 mm, with a cold rolling reduction ratio of 54%-72%.

[0015] The present invention has the following beneficial technical effects: This solution provides a low-cost 780MPa grade hot-dip galvanized duplex steel pickling process welding method and low-cost 780MPa grade hot-dip galvanized duplex steel products. It describes in detail the straightening process, material, plate shape, surface quality of the raw materials, the shearing effect of the strip head and tail, the setting of key welding parameters (wire selection, laser power, welding speed, etc.), the evaluation of welding effect, etc. It also makes clear requirements for the subsequent loop control method and pickling process requirements, and adjusts the reduction rate according to the thickness of the strip, thereby realizing the batch, continuous and stable production of low-cost 780MPa grade hot-dip galvanized duplex steel products. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 A schematic diagram of a steel strip after pickling according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a cold-rolled steel strip being ground and inspected according to an embodiment of the present invention. DETAILED DESCRIPTION

[0018] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the embodiments of the present invention are further described in detail below in combination with specific embodiments and with reference to the accompanying drawings.

[0019] It should be understood that the embodiments of the present invention shown in the exemplary embodiments are only illustrative. Although only a few embodiments are described in detail in the present invention, it is easy for those skilled in the art to appreciate that multiple modifications are feasible without actually departing from the teaching of the subject matter of the present invention. Accordingly, all such modifications should be included within the scope of the present invention. Without departing from the gist of the present invention, other replacements, modifications, changes and deletions may be made to the design, operating conditions and parameters of the following exemplary embodiments.

[0020] The object of the present invention is to provide a low-cost 780MPa grade hot-dip galvanized dual-phase steel pickling process welding method, by regulating the welding process to obtain good weld quality, while the reduction rate is adjusted according to the thickness of the strip, to ensure stable and efficient production of the pickling process.

[0021] Based on the above purpose, in a first aspect of an embodiment of the present invention, a low-cost 780 MPa grade hot-dip galvanized dual-phase steel comprises, by weight percentage, the chemical composition of the dual-phase steel comprising: C: 0.05%~0.11%, Si: 0.20~0.40%, Mn: 1.70%~2.30%, P≤0.017%, S≤0.008%, Als: 0.010%~0.080%, N≤0.008%, Cr: 0.35%~0.75%, Nb: 0.01~0.05%, Ni≤0.03%, V≤0.02%, Mo≤0.01%, and the balance is Fe and unavoidable impurities.

[0022] The preferred low-cost 780MPa grade hot-dip galvanized dual-phase steel has the following chemical compositions, in weight percentage: C: 0.06%-0.10%, Si: 0.25-0.35%, Mn: 1.80%-2.20%, P≤0.015%, S≤0.006%, Als: 0.020%-0.070%, N≤0.006%, Cr: 0.40%-0.70%, Nb: 0.02-0.04%, Ni≤0.03%, V≤0.02%, Mo≤0.01%, and the balance is Fe and unavoidable impurities.

[0023] According to a second aspect of an embodiment of the present invention, a low-cost 780MPa grade hot-dip galvanized duplex steel acid rolling process welding method is provided, wherein the connecting material and the raw steel coil are selected for welding and the welding surfaces of the connecting material and the raw steel coil are sheared, and then welding wire is added to weld them so that the welding surfaces are aligned and on the same horizontal plane. After welding, the weld is subjected to post-weld annealing and grinding treatment, and the weld quality is inspected by measuring the weld thickness and performing a cupping test.

[0024] Among them, the carbon equivalent of the selected connecting material needs to be controlled by the carbon equivalent formula, which is: Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14(%). After calculation, the carbon equivalent of the selected connecting material and the raw steel coil is controlled in the range of 0.42%~0.90%. The difference in yield strength between the connecting material and the raw steel coil before and after welding is within 1.7 times, that is, the yield strength of the higher strength strip in the two steel coils before and after welding should not be higher than 1.7 times the yield strength of the lower strength strip. The surface of the hot-rolled raw material is required to be visually normal hot-rolled iron oxide scale, without excessive defects and no water accumulation, and also needs to have a good plate shape. The thickness range of the hot-rolled raw material for welding is 2.0~6.0mm, and the width range is 850~1650mm.

[0025] The step of "cutting the welding selected connecting material and the raw steel coil" includes: when cutting, the cutting length of the steel coil is controlled according to the following requirements, and welding is performed after ensuring the plate shape and surface quality; Use cross-cutting shears to shear the connecting materials and raw steel coils. Before welding, the tail of the previous steel coil and the head of the next steel coil need to be cut off with a double-cutting shear to ensure the plate shape and surface quality before welding. When the strip thickness t is less than 3.25mm, the cutting amount is 8.5m, and when the strip thickness 3.25mm≤t is less than 5.25mm, the cutting amount is 6.8m. At the same time, the operator needs to observe the actual situation of each coil to ensure that the defects at the head and tail of the strip can be completely removed and will not deteriorate the welding quality.

[0026] The welding process parameters are: laser power 13±0.5KW, welding speed 2.9~4.5±0.4m / min; among them, when the raw material thickness is ≤3.25mm, the welding speed is 4.5±0.4m / min, and the speed decreases by 0.4m / min for every 0.5mm increase in thickness; the welding speed compensation range is 0~15%; after welding, the weld needs to be cleaned with a rolling wheel, the rolling wheel pressure is ≤65KN, the rolling wheel height is 0~6.0mm, and due to the high alloy content of 780MPa grade hot-dip galvanized duplex steel, the weld needs to be heat treated to improve its weld quality.

[0027] After post-weld annealing and grinding, the weld surface must be smooth and free of cracks, and the weld area must not have defects such as inclusions, incomplete penetration, and misalignment. In addition, the weld quality must be checked through weld thickness measurement, cupping and other tests.

[0028] After welding, the weld is heat treated through a thermal cycle with a power of 43kW, a frequency of 32kHz, and a weld heat treatment current compensation of 0~15%. During the heating process of the weld heat treatment, the heating rate is required to be ≤20℃ / s, and the cooling rate is ≤15℃ / s to cool to below 300℃ by air cooling.

[0029] The thickness of the weld (t1) is 1.08~1.25 times the thickness of the raw material (1.08t≤t1≤1.25t, where t is the thickness of the raw material); when taking samples of the weld for the cupping test, at least 3 points should be inspected. When making a width jump connection, a crescent bend test is required. Only after the test is found to be problem-free can the transition of the width jump steel coil be carried out.

[0030] Use the straightening machine to straighten the strip shape, use the tension machine to make the strip "tight", and use the correcting roller to adjust the strip position (keep the strip in the center of the equipment) to avoid equipment damage and shutdown accidents caused by plate shape problems such as warping and strip deviation and collision with equipment.

[0031] The strip steel is adjusted by using the looper before pickling to ensure the smooth progress of the pickling process.

[0032] The mechanism of action of pre-pickling looper in the pickling process of strip steel 1) Buffer and storage functions The pre-pickling looper is a key equipment in the strip pickling production line. It plays a buffering role in the production process. Since the production speed of each process on the strip production line may fluctuate, for example, the previous hot rolling process or the strip unwinding speed may be unstable due to minor equipment failures, differences in the characteristics of different batches of strip steel, etc. The pre-pickling looper can store a certain length of strip steel. When the strip steel supply speed at the entrance section is fast, the excess strip steel can be stored in the looper to avoid the accumulation of strip steel at the entrance of the pickling process, causing production stagnation.

[0033] On the contrary, when the supply speed of the inlet section slows down, the strip steel stored in the looper can continue to be supplied to the pickling process, ensuring that the pickling process will not be interrupted due to insufficient strip steel supply. This buffering and storage function enables the pickling process to be carried out in a relatively stable strip steel supply state, just like a reservoir regulating the flow of a river, making the entire production process more stable.

[0034] 2) Tension regulation During the pickling process of strip steel, appropriate tension is an important factor to ensure the quality of pickling. The looper can adjust the tension of the strip steel before pickling. The looper can sense the change of the tension of the strip steel through its unique mechanical structure and control system. When the tension of the strip steel is too large, the looper can release the strip steel appropriately to reduce the tension; when the tension is too small, the looper tightens the strip steel to increase the tension.

[0035] Appropriate tension helps the strip to run smoothly in the pickling tank. If the tension is too high, the strip may deform or even break in the pickling tank, affecting the pickling effect and strip quality. If the tension is too low, the strip may become loose or deviate in the pickling tank, resulting in uneven pickling, and partial over-pickling or under-pickling.

[0036] 3) Guarantee of strip surface quality The smooth conveying and tension adjustment of the strip by the looper can reduce the scratches and damage on the strip surface before entering the pickling process. Without the buffering and adjustment of the looper, the strip may rub and collide with the edge of the equipment, guide rollers and other parts during the unstable conveying process, causing surface scratches. These scratches may become more obvious after pickling, reducing the surface quality of the strip.

[0037] In addition, the stable operation of the looper can make the strip enter the pickling tank in a uniform state, ensuring that the pickling solution can evenly contact the surface of the strip. This can avoid the problem of uneven pickling caused by the unstable posture of the strip entering the pickling tank, and help improve the cleanliness and quality of the strip surface after pickling.

[0038] 4) Ensure the coordination between the pickling process and the previous and subsequent processes The pre-pickling looper is an important link connecting the front and rear processes of the strip production line. It enables the pickling process to work better with the previous uncoiling, straightening and other processes and the subsequent pickling processes such as cleaning and drying. Through the adjustment of the looper, the pickling process can be unaffected by the production fluctuations of the previous process and can run stably according to the process parameters set by itself.

[0039] At the same time, the looper also provides strip steel of stable quality for the subsequent pickling process, enabling the entire strip steel pickling production line to produce efficiently and stably, improving production efficiency and consistency of product quality.

[0040] The welded steel is subsequently pickled and cold rolled; wherein the pickling step is: The i-Box type acid tank is used to clean the iron oxide scale on the surface of the strip steel. The acid concentration of the 1# acid tank is 45~70g / L, the acid concentration of the 2# acid tank is 75~100g / L, and the acid concentration of the 3# acid tank is 105~140 g / L. The acid temperature is 60℃~80℃, and the pickling speed is 55m / min~220m / min. The pickling speed is 220m / min when the strip steel thickness is 2.50mm, and the pickling speed decreases by 15m / min for every 0.25mm increase in thickness. Afterwards, the residue on the surface of the strip steel is rinsed and the pH value of the rinse agent is ≥5.0. Finally, it is dried at a drying temperature of 80~150℃ In some embodiments, the cold rolling step includes: pickling the hot rolled coil and then cold rolling it into a thin strip steel of 0.70-2.30 mm, with a cold rolling reduction ratio of 54%-72%. Example 1 (1) Low-cost 780MPa grade hot-dip galvanized duplex steel (chemical composition: 0.085%C, 0.32%Si, 2.05%Mn, 0.008%P, 0.003%S, 0.036%Als, 0.62%Cr, 0.026%Nb, 0.0028%N, 0%Ni, 0%V, 0% Mo, its carbon equivalent is 0.632) steel coil tail sheared 6.8mm, and then with the temperature of 49 ℃, specification 3.60×1180 low-cost 780MPa grade hot-dip galvanized duplex steel (0.078%C, 0.30%Si, 2.18%Mn, 0.010%P, 0.004%S, 0.033%Als, 0.55%Cr, 0.021%Nb, 0.0030%N, 0%Ni, 0%V, 0%Mo, its carbon equivalent is 0.637) steel coil head sheared 6.8mm, the welding current was 13.2kW, the wire was fed at a speed of 4.1m / min and the welding speed was 4.0m / min. The welds welded by the above welding process are smooth, without nodules, weld scars, or weld penetration. The cupping test shows no cracking along the welds, and the crescent bend is good. No belt breakage caused by the welds occurs during pickling and cold rolling. The acid concentrations of the acid tank are 60g / l, 85g / l, and 120g / l, respectively, the acid temperature is 75°C, the pickling speed is 150m / min, and the drying temperature is 90°C. The cold rolling reduction rates are 68% and 67%, respectively, and the plate shape is good after cold rolling.

[0041] Example 2 The low-cost 780MPa grade hot-dip galvanized duplex steel (chemical composition: 0.0.92%C, 0.28%Si, 2.22%Mn, 0.011%P, 0.002%S, 0.043%Als, 0.026%Nb, 0.61%Cr, 0.0025%N, 0%Ni, 0%V, 0%) with a coil temperature of 45°C and a specification of 4.20mm×1060mm was used. Mo, its carbon equivalent is 0.670) steel coil tail was sheared 6.8mm, and then it was cut 5.5mm with the head of hot-dip galvanized 590DP (0.080%C, 0.20%Si, 1.73%Mn, 0.010%P, 0.003%S, 0.031%Als, 0.32%Cr, 0.13%Mo, 0.0029%N, 0%Ni, 0%V, its carbon equivalent is 0.531) steel coil with a temperature of 42℃ and a specification of 4.00×1275. The welding current was 12.8kW, the wire was fed at a wire speed of 2.8m / min, and the welding speed was 3.8m / min. The welds welded by the above welding process are smooth, without nodules, weld scars, or weld penetration. The cupping test shows no cracking along the welds, and the crescent bend is good. No belt breakage caused by the welds occurs during pickling and cold rolling. The acid concentrations of the acid tank are 53g / l, 88g / l, and 113g / l, respectively, the acid temperature is 72°C, the pickling speed is 118m / min, and the drying temperature is 92°C. The cold rolling reductions are 64% and 68%, respectively, and the plate shape is good after cold rolling.

[0042] Comparative Example The low-cost 780MPa grade hot-dip galvanized duplex steel (chemical composition: 0.093%C, 0.33%Si, 2.18%Mn, 0.006%P, 0.003%S, 0.044%Als, 0.028%Nb, 0.62%Cr, 0.0027%N, 0%Ni, 0%V, 0% Mo, whose carbon equivalent is 0.667) steel coil tail was cut 6.8mm, and the DC05 (chemical composition is 0.0018%C, 0.01%Si, 0.04%Mn, 0.005%P, 0.002%S, 0.033%Als, 0.072%Ti, 0.0026%N, 0%Ni, 0%V, whose carbon equivalent is 0.010) steel coil with a temperature of 62℃ and a specification of 5.5mm×1550mm was cut 1.5mm mm, and the welding current was 13.1kW with a wire feeding speed of 3.5m / min and the welding wire was fed at a speed of 5.5m / min. The weld quality after welding by the above welding process was not good, and no cracking along the weld appeared in the cupping test. The production was carried out after changing the connecting material.

[0043] The above are exemplary embodiments disclosed in the present invention, but it should be noted that various changes and modifications may be made without departing from the scope of the embodiments disclosed in the claims. Although the elements disclosed in the embodiments of the present invention may be described or required in individual form, they may also be understood as multiple unless explicitly limited to the singular.

[0044] A person skilled in the art should understand that the discussion of any of the above embodiments is only exemplary and is not intended to imply that the scope of the disclosure of the embodiments of the present invention (including the claims) is limited to these examples; under the concept of the embodiments of the present invention, the technical features in the above embodiments or different embodiments can also be combined, and there are many other changes in different aspects of the above embodiments of the present invention, which are not provided in detail for the sake of simplicity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the embodiments of the present invention should be included in the protection scope of the embodiments of the present invention.

Claims

1. A low-cost 780MPa grade hot-dip galvanized dual-phase steel, characterized in that: The chemical composition of the dual-phase steel includes, by weight percentage, C: 0.05%-0.11%, Si: 0.20%-0.40%, Mn: 1.70%-2.30%, P≤0.017%, S≤0.008%, Als: 0.010%-0.080%, N≤0.008%, Cr: 0.35%-0.75%, Nb: 0.01-0.05%, Ni≤0.03%, V≤0.02%, Mo≤0.01%, and the balance is Fe and unavoidable impurities.

2. The low-cost 780MPa grade hot-dip galvanized dual-phase steel according to claim 1, characterized in that: The chemical composition of the dual-phase steel includes, by weight percentage, C: 0.06%-0.10%, Si: 0.25-0.35%, Mn: 1.80%-2.20%, P≤0.015%, S≤0.006%, Als: 0.020%-0.070%, N≤0.006%, Cr: 0.40%-0.70%, Nb: 0.02-0.04%, Ni≤0.03%, V≤0.02%, Mo≤0.01%, and the balance is Fe and unavoidable impurities.

3. A welding method for the low-cost 780MPa grade hot-dip galvanized dual-phase steel pickling process as claimed in claim 1 or 2, characterized in that: After the welding connection material and the raw steel coil are sheared, the welding surfaces of the connection material and the raw steel coil are aligned and on the same horizontal plane, and then welding wire is added for welding, and the weld is annealed and polished after welding, and the weld quality is checked by measuring the weld thickness and cupping test; The carbon equivalent of the selected connection material and raw steel coil needs to be controlled by the carbon equivalent formula, which is: Ceq=C+Mn / 6+Si / 24+Ni / 40+Cr / 5+Mo / 4+V / 14(%); The difference in yield strength between the connecting material and the raw steel coil before and after welding is within 1.7 times.

4. The welding method according to claim 3, characterized in that: The carbon equivalent of the selected connecting material is controlled in the range of 0.42%~0.90%.

5. The welding method according to claim 3, characterized in that: After welding, the weld is annealed with a power of 43kW, a frequency of 32kHz, and a weld heat treatment current compensation of 0~15%. During the heating process of the weld heat treatment, the heating rate is required to be ≤20℃ / s, and the cooling rate is ≤15℃ / s to cool to below 300℃ by air cooling.

6. The welding method according to claim 3, characterized in that: The step of "cutting the welding selected connecting material and the raw steel coil" includes: when cutting, the cutting length of the steel coil is controlled according to the following requirements, and welding is performed after ensuring the plate shape and surface quality; Use cross-cutting shears to shear the connecting materials and raw steel coils. Before welding, the tail of the previous steel coil and the head of the next steel coil need to be cut off by double cutting shears to ensure the plate shape and surface quality before welding. When the strip thickness t is less than 3.25mm, the cutting amount is 8.5m, and when the strip thickness t is 3.25mm≤less than 5.25mm, the cutting amount is 6.8m. It is also ensured that the defects at the head and tail of the strip can be completely removed without deteriorating the welding quality.

7. The welding method according to claim 3, characterized in that: The welding process parameters are: laser power 13±0.5KW, welding speed 2.9~4.5±0.4m / min; among them, when the raw material thickness is ≤3.25mm, the welding speed is 4.5±0.4m / min, and the speed decreases by 0.4m / min for every 0.5mm increase in thickness; the welding speed compensation range is 0~15%; after welding, the weld needs to be cleaned with a rolling wheel, the rolling wheel pressure is ≤65KN, and the rolling wheel height is 0~6.0mm.

8. The welding method according to claim 3, characterized in that: After welding, the strip shape is straightened and the strip is kept in a "tight" state, and the position of the strip is adjusted to keep it in the center of the equipment.

9. The welding method according to claim 3, characterized in that: The welded steel is subsequently pickled and cold rolled; wherein the pickling step is: The i-Box type acid tank is used to clean the iron oxide scale on the surface of the strip steel. The acid concentration of the 1# acid tank is 45~70g / L, the acid concentration of the 2# acid tank is 75~100g / L, and the acid concentration of the 3# acid tank is 105~140 g / L. The acid temperature is 60℃~80℃, and the pickling speed is 55m / min~220m / min. The pickling speed is 220m / min when the strip steel thickness is 2.50mm, and the pickling speed decreases by 15m / min for every 0.25mm increase in thickness. After that, the residue on the surface of the strip steel is rinsed and the pH value of the rinse agent is ≥5.

0. Finally, it is dried at a drying temperature of 80~150℃.

10. The welding method according to claim 9, characterized in that The cold rolling step is: after pickling the hot rolled coil, cold rolling it into a thin strip steel of 0.70-2.30 mm, and the cold rolling reduction rate is 54%-72%.

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