A laser welding method of 1200mpa grade cold rolled high strength steel hot rolled substrate

Abstract

The application relates to a laser welding method of a 1200MPa-grade cold-rolled high-strength steel hot-rolled base plate, adopts a laser butt welding process, and welding process parameters are as follows: an induction heating power before welding is 0-4kW; a laser power is 10-11kW; a welding speed is 6-8m / min; and an induction heating power after welding is 8-12kW. Through control of raw material characteristics, laser welding power, welding speed and front and rear induction heating process parameters, the welding stability is improved, welding can be completed without other auxiliary equipment and special welding environment, the production process is simplified, and the production cost is reduced; the welding seam welded by the method can meet the requirements of stable production of a 1200MPa-grade cold-rolled high-strength steel pickling and rolling unit.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of welding technology, and particularly relates to a laser welding method for 1200MPa-grade cold rolled high-strength steel hot-rolled base plate capable of improving the stability of weld rolling. BACKGROUND

[0002] In cold-rolled products for automobiles, high-strength steel plates aiming at weight reduction and energy saving are cold-rolled high-strength steels, and the application of cold-rolled high-strength steels is one of important means to realize automobile lightening. At present, 1200MPa-grade cold-rolled high-strength steels are mostly produced in batches by acid-rolling combined units, and laser welding is used to weld the front and rear coils to realize continuous production. However, in the actual production process, weld rolling breakage is one of the main factors affecting the production efficiency of 1200MPa-grade cold-rolled high-strength steels.

[0003] In order to achieve ideal target performance, the alloy content of 1200MPa-grade cold-rolled high-strength steels is often high, so that the carbon equivalent is also high, which leads to great welding difficulty, and in the actual production process, problems such as weld rolling breakage are prone to occur, which seriously affects the production stability of 1200MPa-grade cold-rolled high-strength steels, produces some waste products, and increases downtime. At present, the research on laser welding process of 1200MPa-grade cold-rolled high-strength steels mainly focuses on the application process of finished products in automobiles, and the welding research of 1200MPa-grade cold-rolled high-strength steel hot-rolled base plate is less, especially the weld breakage has become a key factor affecting the rolling stability, so designing a suitable laser welding process has great significance for the industrialized batch production of 1200MPa-grade cold-rolled high-strength steels.

[0004] Chinese patent application CN109909615A discloses "a laser welding equipment for plate-shaped defective steel strip and its welding method", the equipment includes a base, a welding trolley, a shearing device, a preheating device, a laser welding device, a post-weld heat treatment device and a post-weld weld rolling device, wherein the laser welding device includes a laser focusing head and a filler wire mechanism arranged on the inner wall of the upper part of the welding trolley; the laser focusing head is symmetrically provided with a pre-welding synchronous correction mechanism on both sides of the center line of the laser focusing head, and the pre-welding synchronous correction mechanism includes an upper flat wheel group and a lower flat wheel group arranged symmetrically above and below the welding trolley; the welding method is to shear the edge of the steel strip by using the shearing device on the welding trolley; after shearing, the seams are spliced; during welding, the welding trolley moves along the weld, and the pre-welding preheating, pre-welding synchronous correction, synchronous filler wire during laser welding, post-weld heat treatment and post-weld rolling are carried out in turn. The method improves the weld toughness and forming performance, and ensures the production of cold rolled high strength steel. From its process and examples, it can be seen that the use of pre-welding synchronous correction mechanism reduces the influence of cold rolled high strength steel welded joint plate shape on the quality of the weld, but the existing laser welder needs to be modified and new equipment needs to be added during implementation, and the process parameters are not improved to solve the problem of production stability fluctuation caused by poor weld quality of cold rolled high strength steel.

[0005] Chinese patent application CN113787098A discloses "a pickling and cold rolling method for a tensile strength of 780MPa grade high strength steel", which adopts laser welding process, preheating treatment before welding, preheating current 60-90A, joint form is butt joint, laser power 1-1.5kW, welding speed 3.5-8.0m / min, wire feeding speed 0-8.0m / min, post-weld annealing treatment, annealing current 120-180A. From the process and examples, it can be seen that the laser welding process parameters combined with the filler wire welding process realize the welding of 780MPa grade cold rolled high strength steel. But in the application process, welding wire needs to be added, and the strength level of the corresponding target steel and 1200MPa grade cold rolled high strength steel is quite different, the welding difficulty is not at the same level, and the welding machine model corresponding to the welding process parameters provided is relatively old, not suitable for the current mainstream welding machine, and the stability of the filler wire process is difficult to guarantee.

[0006] Chinese patent application CN111521495A discloses a "method for optimizing the toughness of the coarse-grained region of dual-phase high-strength steel welding", which analyzes the fracture mechanism of the coarse-grained region of dual-phase high-strength steel welding using thermal simulation method, then builds a response surface model between test parameters and impact energy by combining response surface method with quantum genetic algorithm, and solves the model to get the optimal value. The optimal value corresponds to the optimal welding process parameters. Before welding dual-phase high-strength steel, a reliable response surface model is established by using a limited number of thermal simulation tests to replace a large number of tests, and then the optimal welding process parameters for improving the brittle of the coarse-grained region of dual-phase high-strength steel welding joint and improving the toughness of the coarse-grained region are obtained, so as to improve the welding quality of dual-phase high-strength steel and minimize the risk of brittle fracture of dual-phase high-strength steel welding joint. As can be seen from the process and examples, the thermal simulation experiment combined with genetic algorithm is used to optimize the welding process, and the number of experiments required for optimizing the welding parameters is reduced. However, the target is the fracture of the coarse-grained region of dual-phase high-strength steel welding, and the application scenario is not a cold rolling combined unit, which is not suitable for the welding of 1200MPa cold rolled high-strength steel hot rolled substrate described in the present invention.

[0007] Chinese patent application CN114850675A discloses a "laser welding method for 1000MPa cold rolled high-strength steel hot rolled substrate", the strip steel is produced by an acid rolling combined unit, when the tail of the front coil strip steel is laser butt welded with the head of the rear coil strip steel, the laser welding process parameters are as follows: the pre-welding heating temperature is 350-550℃, the heating time is 3-4s; the laser power is 11-12kW; the welding speed is 2.6-3.4m / min; the post-welding heating temperature is 460-580℃, the heating time is 3-4s, and then the environment is cooled. It improves the transition stability of the weld performance by controlling the laser welding and the front and rear induction heating process parameters, without the need for other auxiliary equipment and special welding environment to complete the welding, simplifying the production process and reducing the production cost. However, it is matched with 1000MPa cold rolled high-strength steel, and the composition design and phase transition sensitivity of different strength levels of high-strength steel are quite different. The design concept for controlling the uniformity of phase transition is completely different from that of the present invention, and the process parameters are also quite different. SUMMARY

[0008] The present invention provides a laser welding method for 1200MPa cold rolled high-strength steel hot rolled substrate, which improves the stability of the weld rolling by controlling the raw material characteristics, laser welding power, welding speed and front and rear induction heating process parameters, without the need for other auxiliary equipment and special welding environment to complete the welding, simplifying the production process and reducing the production cost. The weld joint welded by this method can meet the requirements of stable production of 1200MPa cold rolled high-strength steel acid rolling unit.

[0009] To achieve the above object, the present application adopts the following technical solutions:

[0010] A laser welding method of a 1200MPa-grade cold-rolled high-strength steel hot-rolled base plate, the chemical composition of the hot-rolled base plate is as follows in terms of mass percentage: C 0.15% to 0.24%, Si 1.60% to 2.00%, Mn 2.55% to 2.95%, P ≤0.005%, S ≤0.0005%, and the balance being Fe and inevitable impurities; the laser butt welding process is adopted, and the welding process parameters are as follows: the induction heating power before welding is 0 to 4kW; the laser power is 10 to 11kW; the welding speed is 6 to 8m / min; and the induction heating power after welding is 8 to 12kW.

[0011] Further, the incoming material requirements of the hot-rolled base plate are as follows: the tensile strength difference between the head and tail of the strip and the middle part of the strip is ≤100MPa, the head and tail of the strip refer to the area within 10m from the two ends of the strip, and the rest is the middle part of the strip; the flatness of the hot-rolled base plate is ≤12I; the thickness deviation of the front and rear coils is ≤0.2mm; the strip is sheared by a downward cutting shear before welding, the shearing frequency is ≥2 times, the shearing cross section is at an angle of 90° with the longitudinal direction of the strip, and the sheared end surface is smooth and straight.

[0012] Further, the width of the strip is 800 to 1200mm, and the thickness is 2.3 to 2.8mm; the cold rolling compression ratio is ≤60%; the yield strength of the finished strip after annealing is 820 to 1100MPa, the tensile strength is ≥1200MPa, and the elongation is ≥14%.

[0013] Further, the laser butt welding process is as follows: the tail of the front coil and the head of the rear coil are pressed into an arc-shaped segment by the welding machine pinch roll respectively, and then positioning and centering are performed; the front coil and the rear coil after centering are sheared at the head and tail by a double cutting shear, the strip is clamped by a clamping guide plate, the welding wheel and the rolling wheel of the welding machine fall down, the protective gas is blown off, the welding machine moves from the operation side to the transmission side, the pre-welding heating device and the post-welding heating device move synchronously, and the butt welding of the strip is realized.

[0014] Further, the post-welding inspection requirements are as follows: the weld is full and free of inclusions; under the condition that the weld rolling wheel pressure is ≥17.5kN, the surface of the weld after rolling is smooth, free of cracks, free of lap joints, and free of holes.

[0015] Compared with the prior art, the present application has the following beneficial effects:

[0016] (1) The present application adopts laser welding combined with pre-welding and post-welding heat treatment, realizes butt welding of the same steel hot-rolled base plate without adding additional equipment, reduces the production cost and the process complexity, greatly improves the anti-deformation ability of the weld and the rolling stability, and realizes the purpose of efficient and stable production of 1200MPa-grade cold-rolled high-strength steel.

[0017] (2) The welding seam obtained by the method is full and free of inclusions, and the surface of the welding seam is smooth, free of cracks, free of overlaps and free of holes after rolling; the cup convex experiment does not show the phenomenon of cracking along the welding seam, and the fracture position is in the base material region after the tensile experiment, and the rolling breakage caused by the welding seam quality does not occur in the cold rolling production process, and the rolling stability of the welding seam is significantly improved. DETAILED DESCRIPTION

[0018] The laser butt welding process of the 1200MPa-grade cold-rolled high-strength steel hot-rolled base plate adopts the following process: the tail of the front coil strip steel and the head of the rear coil strip steel are pressed into an arc-shaped section by the welding machine pinch roll before and after the welding machine, which provides a margin for strip shearing or re-welding. After positioning and centering the head and tail of the strip, the double cutting shear is used to shear the head and tail of the strip to ensure the quality of the welding seam end face; the strip is clamped by the clamping guide plate, the welding wheel and the rolling wheel of the welding machine are lowered, the protective gas is blown, the laser welding machine moves from the operating side to the transmission side, and the pre-welding and post-welding heating devices move synchronously to realize the butt welding of the strip.

[0019] The incoming requirements of the hot-rolled base plate are as follows: the tensile strength difference between the head and tail and the middle part of the hot-rolled base plate is ≤100MPa, the part within 10m from both ends of the strip is defined as the head and tail region of the strip, and the remaining part is the middle region of the strip. The flatness of the strip is ≤12I; the thickness deviation of the front and rear coils is ≤0.2mm; the strip needs to be sheared by the lower cutting shear before welding, the shearing frequency is ≥2 times, the shearing cross section is 90° with the longitudinal direction of the strip, and the shearing end face is smooth and straight.

[0020] The welding process parameters are as follows: the induction heating power before welding is 0-4kW; the laser power is 10-11kW; the welding speed is 6-8m / min; and the induction heating power after welding is 8-12kW.

[0021] Post-welding inspection: the welding seam should be full and free of inclusions, and the surface of the welding seam should be smooth, free of cracks, free of overlaps and free of holes under the condition that the welding seam rolling wheel pressure is ≥17.5kN.

[0022] The chemical composition of the strip is as follows in terms of mass percentage: C 0.15%-0.24%, Si 1.60%-2.00%, Mn 2.55%-2.95%, P ≤0.005%, S ≤0.0005%, and the balance is Fe and unavoidable impurities.

[0023] The width of the strip is 800-1200mm, the thickness is 2.3-2.8mm, the cold rolling compression ratio is ≤60%, the yield strength of the finished strip after annealing is 820-1100MPa, the tensile strength is ≥1200MPa, and the elongation is ≥14%.

[0024] The process design reasons of the laser welding method are as follows:

[0025] The welding purpose of the 1200MPa grade cold-rolled high-strength steel hot-rolled base plate is to ensure the continuous and stable production of the strip steel in the tandem cold rolling mill, and to avoid the strip steel breakage in the stretch leveling machine and the cold rolling mill. By improving the plate shape requirement of the hot-rolled base plate welding joint, the effective control of the weld height difference and the weld penetration is realized; by controlling the welding process parameters, the microstructure regulation under the high carbon equivalent condition is completed, the reduction of the weld peak hardness and the improvement of the coordinated deformation ability of the microstructure are realized; by controlling the post-weld rolling wheel and the weld visual inspection standard, the effective control of the weld fullness and the surface defects is realized. After adopting the above measures, the weld quality is effectively improved, and the stable passing of the weld in the small deformation amount stretch leveling and the large deformation amount rolling of the cold rolling mill is realized.

[0026] Raw material requirement: in the welding process of the strip steel in the tandem cold rolling mill, if the plate shape value (flatness) or the thickness difference of the welding base material is large, the weld height difference and the significant increase of the internal stress are easy to appear, which seriously affects the weld quality; in addition, the 1200MPa grade cold-rolled high-strength steel is sensitive to the cooling speed in the hot-rolling coiling process due to its high alloy composition design, and the microstructure and performance are easy to be uneven, and the plate shape is poor, which is especially serious in the head and tail parts of the strip steel as the welding joint, therefore, the present application puts forward the requirements for the weld shearing times and the shearing surface to reduce the influence.

[0027] Welding process parameters: unreasonable laser power setting is easy to cause poor weld penetration and stress concentration leading to weld fracture; in order to ensure the weld penetration of the 1200MPa grade cold-rolled high-strength steel hot-rolled base plate, the laser power used in the present application is 10-11kW. The microstructure and internal stress of the weld are controlled by the welding speed and the front and rear induction heating power, and the essence is to control the temperature and time of the weld heat treatment. Due to the strong phase transformation sensitivity of the 1200MPa grade cold-rolled high-strength steel, the center of the weld is the coarse lath martensite, which can be reduced by reasonable post-weld heat treatment, and a small amount of martensite is easy to appear in the heat affected zone of the weld except ferrite, which can be reduced by reasonable post-weld heat treatment, and the martensite that has been produced is tempered to improve its deformation ability. However, the pre-weld heat treatment has little effect on the 1200MPa grade cold-rolled high-strength steel. Therefore, the present application designs the pre-weld induction heating power as 0-4kW, the laser power as 10-11kW, the welding speed as 6-8m / min, and the post-weld induction heating power as 8-12kW.

[0028] Post-weld inspection: due to the strong phase transformation sensitivity of the 1200MPa grade cold-rolled high-strength steel, and the easy appearance of surface defects, the post-weld rolling wheel pressure and surface inspection are required.

[0029] In order to more directly embody the present application, the embodiments of the present application are further described in conjunction with examples. The following examples are merely the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any skilled in the art can obtain the technical solutions within the technical range disclosed by the present application, including simple changes or equivalent replacements, which are within the protection scope of the present application.

[0030] [Examples]

[0031] In the present example, the chemical composition of the 1200MPa-grade cold-rolled high-strength steel hot-rolled substrate is as follows:

[0032]

[0033] It is calculated that the carbon equivalent and the crack sensitivity coefficient of the 1200MPa-grade cold-rolled high-strength steel hot-rolled substrate in the present example are 0.67% and 0.76, respectively, indicating a high risk of weld zone hardening and cold crack generation.

[0034] In the present example, the specification of the 1200MPa-grade cold-rolled high-strength steel hot-rolled substrate is 2.50mm (thick) x 1134mm (wide).

[0035] The strip steel is welded by a laser welding machine in an acid-rolling combined unit, and the strip steel head and tail are first cut by a double-cut shear to ensure the quality of the welded end face, and the clamping guide plate is lowered to clamp the strip steel to make it more flat.

[0036] The welding process parameters of the present example are shown in Table 1:

[0037] Table 1 Welding process parameters

[0038]

[0039] After the above welding process is implemented, the results of the cupping experiment of the welds obtained by each example are all qualified, and there is no cracking phenomenon along the weld or parallel to the weld; after the weld tensile experiment, the weld strength is higher than that of the base material area, and there is no phenomenon of fracture at the weld. After the cold rolling process of the bending and large deformation of the tension roll, the weld quality is good, and the weld rolling breakage rate is zero.

[0040] As can be seen from the above examples, the 1200MPa-grade cold-rolled high-strength steel hot-rolled substrate prepared by the welding method of the present application has good weld quality, no breakage phenomenon in cold rolling production, and can meet the production needs of enterprises.

[0041] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art, according to the technical solution and inventive concept of the present application, makes equivalent replacement or change within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A method of laser welding of a 1200 MPa grade cold rolled high strength steel hot rolled substrate, characterized in that, The chemical composition of the hot-rolled substrate is, by mass percent: C 0.15% to 0.20%, Si 1.80% to 2.00%, Mn 2.55% to 2.95%, P ≤ 0.005%, S ≤ 0.0005%, and the balance being Fe and unavoidable impurities; the incoming material requirement of the hot-rolled substrate is that the difference in tensile strength between the head and tail of the strip and the middle part of the strip is ≤ 100 MPa, the head and tail of the strip refers to the area within 10 m from the two ends of the strip, and the rest is the middle part of the strip; the flatness of the strip is ≤ 12I; the thickness deviation of the front and rear coils is ≤ 0.2 mm; the strip is sheared by a down shear before welding, the shearing frequency is ≥ 2 times, the shearing cross section is at an angle of 90° with the longitudinal direction of the strip, and the sheared end surface is smooth and flat; the width of the strip is 800 to 1200 mm, and the thickness is 2.3 to 2.5 mm; the cold rolling compression ratio is ≤ 60%; the yield strength of the finished strip after annealing is 820 to 1100 MPa, the tensile strength is ≥ 1200 MPa, and the elongation is ≥ 14%; The laser butt welding process is adopted, and the welding process parameters are: the induction heating power before welding is 0 to 4 kW; the laser power is 10 to 10.8 kW; the welding speed is 6 to 8 m / min; and the induction heating power after welding is 8 to 12 kW; The post-welding inspection requirement is that the weld is full and free of inclusions; under the condition that the weld rolling wheel pressure is ≥ 17.5 kN, the surface of the weld after rolling is smooth, free of cracks, free of lap joints, and free of holes.

2. A method of laser welding a hot-rolled substrate of a 1200 MPa grade cold-rolled high strength steel according to claim 1, characterized in that, The laser butt welding process is: the tail of the front coil and the head of the rear coil are pressed into an arc-shaped section by the welding machine pinch roll, and then positioning and centering are performed; the front coil and the rear coil after centering are sheared at the head and tail by a double shear, the strip is clamped by a clamping guide plate, the welding wheel and the rolling wheel of the welding machine are lowered, the protective gas is blown, the laser welding machine moves from the operating side to the transmission side, the pre-welding heating device and the post-welding heating device move synchronously, and the butt welding of the strip is realized.

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