WO2024061368A1 - Coated ultrahigh-strength steel with low spot welding crack sensitivity and manufacturing method therefor - Google Patents

Abstract

The present invention discloses coated ultrahigh-strength steel with low spot welding crack sensitivity, comprising a steel substrate and a zinc-containing coating on the surface of the steel substrate. The steel substrate contains Fe and inevitable impurity elements. The steel substrate further contains the following chemical elements in percentage by mass: 0.10-0.30% of C, 0.30-0.90% of Si, 1.00-2.20% of Mn, 0.001-0.003% of B, and 0.30-1.00% of Al. Correspondingly, the present invention further discloses a manufacturing method for the coated ultrahigh-strength steel. The coated ultrahigh-strength steel in the present invention can be effectively prepared by using the manufacturing method, the final yield strength is 600-850 MPa, the tensile strength is 980-1150 MPa, the uniform elongation is not less than 13%, and the elongation at break is not less than 15%.

Description

Coated ultra-high-strength steel with low spot welding crack sensitivity and manufacturing method thereof Technical field

The invention relates to a high-strength steel and a manufacturing method thereof, in particular to a coated ultra-high-strength steel and a manufacturing method thereof.

Background technique

At present, with the advancement of the lightweight process of automobiles and the improvement of the automotive industry's requirements for vehicle corrosion resistance, many automobile companies have an increasing demand for coated ultra-high-strength steel. For cold stamped steel, The coating is mainly zinc-containing coating, such as hot-dip pure zinc, hot-dip galvanized iron and electroplated pure zinc. Resistance spot welding has become the most important connection method in automotive welding production due to its advantages such as fast welding speed and low cost. When many zinc-coated ultra-high-strength steels are spot welded, the joints often show good quasi-static mechanical properties, but a welding crack appears on the surface of the steel plate in contact with the electrode and its nearby area, and this welding crack is called spot welding. Cracks, Figure 1 schematically shows spot welding cracks in zinc-coated ultra-high-strength steel. However, it is very difficult to suppress the formation of such cracks. Therefore, this crack has become an important obstacle and bottleneck for the application of zinc-coated ultra-high-strength steel in the automotive field.

Some technical solutions are known in the prior art to control welding cracks, but there are still many disadvantages:

For example: the publication number is CN108015401A, the publication date is May 11, 2018, and the Chinese patent document titled "Resistance spot welding method of galvanized high-strength steel with good joint performance" discloses an innovative spot welding process. This method ensures that the performance of the solder joint is not reduced while suppressing cracks on the spot welding surface.

Another example: The publication number is CN109385515A, the publication date is February 26, 2019, and the Chinese patent document titled "Multilayer Steel and Method for Reducing Liquid Metal Embrittlement" discloses a method to suppress spot welding cracks in high-strength steel. This method suppresses the occurrence of spot welding cracks by first decarburizing high-strength steel and then galvanizing it, so that the thickness of the decarburized layer is between 10 and 50 microns.

Another example: the publication number is CN110892087A, the publication date is March 17, 2020, and the Chinese patent document titled "Zinc-coated steel plate with high resistance spot weldability" discloses a A technical solution to increase the dew point and form an internal oxide layer on the steel plate to improve the distribution of surface components and thereby reduce the occurrence of spot welding cracks. However, this technical solution has problems such as the thickness and uniformity of the internal oxide layer and the uniformity of component distribution on the surface of the steel plate that are difficult to control.

Summary of the invention

One of the purposes of the present invention is to provide a coated ultra-high-strength steel with low spot welding crack sensitivity. The coated ultra-high-strength steel has very excellent quality and performance. While meeting the mechanical performance requirements of the head, it also has low spot welding crack sensitivity, which has good prospects for promotion and application.

In order to achieve the above object, the present invention provides a coated ultra-high-strength steel with low spot welding crack sensitivity, which includes a steel substrate and a zinc-containing coating on the surface of the steel substrate. The steel substrate contains Fe and inevitable impurity elements, The steel substrate also contains the following chemical elements in the following mass percentages:

C: 0.10～0.30%; Si: 0.30～0.90%; Mn: 1.00～2.20%; B: 0.001～0.003%; Al: 0.30～1.00%.

Furthermore, in the coated ultra-high strength steel of the present invention, the mass percentage of each chemical element of the steel substrate is:

C: 0.10~0.30%; Si: 0.30~0.90%; Mn: 1.00~2.20%; B: 0.001~0.003%; Al: 0.30~1.00%; the balance is Fe and inevitable impurity elements.

Furthermore, in the coated ultra-high-strength steel of the present invention, each chemical element of the steel substrate further contains Mo: 0.10 to 2.00%.

In the above-mentioned technical solution of the present invention, a chemical composition design based on carbon, silicon, manganese, boron or boron-molybdenum composite is adopted, making full use of the effects of carbon, silicon, manganese, boron and molybdenum elements in the material phase change. function, thereby realizing the unification of high mechanical properties and high spot welding performance of the ultra-high-strength steel of the present invention, and finally obtaining a coated ultra-high-strength steel product with low spot welding crack sensitivity.

In the coated ultra-high-strength steel of the present invention, the design principles of each chemical element are as follows:

C: In the coated ultra-high-strength steel with low spot weld crack sensitivity according to the present invention, the solubility of carbon in austenite is much higher than its solubility in ferrite, which can extend the time before austenite transformation. During the gestation period, lower Ms temperature. The higher the mass percentage of carbon in the steel, the greater the fraction of retained austenite. The higher the concentration of carbon in the retained austenite during distribution, which is beneficial to enhancing the stability of the retained austenite, producing the TRIP effect, and improving Material ductility. In addition, carbon is also the most basic solid solution strengthening element in steel. but It should be noted that the carbon content in steel should not be too high. When the mass percentage of carbon is too high, the weldability of the steel will be reduced, especially the tendency of surface cracks on the solder joints of the joint will be significantly increased. Based on this, in order to exert the beneficial effects of C element, in the present invention, the mass percentage content of C element is controlled at 0.10-0.30%.

Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the C element can be further preferably controlled between 0.15% and 0.20%.

Si: In the coated ultra-high-strength steel with low spot welding crack sensitivity according to the present invention, the solubility of silicon in carbides is extremely small, which can strongly inhibit the formation of cementite during the distribution process and promote the transfer of carbon to the residual It is enriched in austenite and improves the stability of retained austenite. However, it should be noted that excessive silicon should not be added to steel. Too high a mass percentage of silicon will reduce the high-temperature plasticity of the steel, form stable oxides on the surface of the steel plate, and reduce the wettability of the steel plate. In particular, the inventor of this case discovered that silicon It is an element that significantly increases the tendency of cracks on the surface of solder joints. Based on this, in the coated ultra-high-strength steel with low spot welding crack sensitivity according to the present invention, the mass percentage content of Si element is specifically controlled at 0.30 to 0.90%.

Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of Si element can be further preferably controlled between 0.40% and 0.80%.

Mn: In the coated ultra-high-strength steel with low spot welding crack sensitivity described in the present invention, manganese can expand the austenite phase area, reduce the Ac ₃ , M _s and M _f points, and improve the austenite stability and steel The hardenability of the steel reduces the critical transformation rate, which is beneficial to preserving the retained austenite to room temperature. At the same time, manganese can also play a solid solution strengthening effect in the steel. When the Mn element content in the steel is too low, it will cause segregation. Ferrite and pearlite band structures are produced at low cooling rates. In addition, the mass percentage of manganese in steel should not be too high. When the mass percentage of manganese in steel is too high, it will intensify the grain coarsening trend, reduce the plasticity and toughness of the steel, and worsen the corrosion resistance. In particular, it will increase the enrichment of manganese in the superficial layer of the base metal under the coating, increase the surface crack sensitivity of the joint's solder joints, and worsen the welding performance. Based on this, in order to exert the beneficial effects of the Mn element, the mass percentage of the Mn element in the coated ultra-high-strength steel with low spot weld crack sensitivity according to the present invention is controlled at 1.00 to 2.20%.

Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Mn element can be further preferably controlled between 1.00% and 2.00%.

B: In the coated ultra-high-strength steel with low spot weld crack sensitivity according to the present invention, boron can significantly improve the hardenability of the steel. On the one hand, boron easily segregates at grain boundaries, fills grain boundary defects, and reduces grain boundary energy, making it more difficult to nucleate new phases on austenite grain boundaries that are originally ferrite nucleation sites and increasing the stability of austenite. Strong, thus improving the hardenability; on the other hand, the addition of boron reduces the need for carbon, manganese and other elements that increase the hardenability of the steel.

In addition, the inventor of this case also found that boron element will reduce the segregation of manganese element to the grain boundary, thereby reducing the formation of manganese enrichment layer on the steel matrix under the coating, thereby significantly reducing the formation of spot welding cracks. However, the more boron content, the better. When the grain boundary defects are filled, if there is still more non-equilibrium segregation of boron, a "boron phase" precipitation will be formed at the grain boundary, increasing the grain boundary energy. The "boron phase" will serve as the core of the new phase, which will increase the nucleation speed, decrease the stability of austenite, and decrease the hardenability. That is, there is obvious "boron phase" precipitation that has a negative impact on the hardenability, and a large amount of "boron phase" precipitation will make the steel brittle and have a bad impact on the mechanical properties of the steel; and the brittle grain boundaries will Promote the occurrence and expansion of spot welding cracks.

The inventors of this case have found that 0.0030% boron content is the inflection point that causes grain boundary embrittlement. Based on this, in order to give full play to the excellent effect of element B, the mass percentage of element B in the coated ultra-high strength steel with low spot welding crack sensitivity described in the present invention is controlled to be 0.001-0.003%.

Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the B element can be further preferably controlled between 0.0015% and 0.003%.

Al: In the coated ultra-high-strength steel with low spot welding crack sensitivity according to the present invention, when aluminum exists in a solid solution state, it can increase stacking fault energy, inhibit cementite precipitation and γ to martensite transformation, and improve Austenite stability. In addition, aluminum, carbon and nitrogen form fine dispersed refractory points, which can refine the grains. However, the strengthening effect of aluminum is weaker than that of silicon, and its ability to stabilize austenite is also weaker than that of silicon. In addition, when the mass percentage of aluminum in steel is too high, it is easy to form a large number of oxide inclusions, which is not conducive to steelmaking and continuous casting. Therefore, in the coated ultra-high-strength steel with low spot weld crack sensitivity according to the present invention, the mass percentage content of the Al element is controlled between 0.30 and 1.00%.

Mo: In the coated ultra-high-strength steel with low spot weld crack sensitivity described in the present invention, molybdenum element can shift the C curve to the right and at the same time lower the Ms point, thereby improving the hardenability of the steel and improving the steel's hardenability. It not only improves the strength but also increases the elongation of steel; in addition, molybdenum is a strong carbide-forming element, which will form fine and dispersed MoC particles during smelting, and will be distributed in the form of a hard second phase during subsequent phase transformation. martensite, thus significantly improving the toughness of steel. More importantly, the inventor of this case found that the addition of molybdenum can significantly improve the resistance of steel to spot welding cracks. Based on this, taking into account the above factors and the cost of adding molybdenum, in the coated ultra-high-strength steel with low spot weld crack sensitivity according to the present invention, the mass percentage of the Mo element is controlled between 0.10 and 2.00%. In some embodiments, the mass percentage of Mo element is controlled between 0.15% and 2.00%.

Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of Mo element can be further preferably controlled between 0.10% and 1.00%. In some embodiments, the mass percentage of the Mo element is controlled between 0.15 and 1.00% or between 0.20 and 1.00%.

Further, in the coated ultra-high-strength steel of the present invention, the mass percentage content of each chemical element of the steel substrate satisfies at least one of the following items:

C: 0.15～0.20%;

Si: 0.40～0.80%;

Mn: 1.00～2.00%;

B: 0.0015～0.003%;

Mo: 0.10 to 1.00%.

Further, in the coated ultra-high-strength steel of the present invention, the mass percentage content of unavoidable impurity elements in the steel substrate satisfies: P≤0.01%, S≤0.005%.

In the above technical solution, P and S elements are both impurity elements in the ultra-high strength steel with coating described in the present invention. When technical conditions permit, in order to obtain steel with better performance and higher quality, the content of impurity elements in the ultra-high strength steel with coating should be reduced as much as possible.

In the present invention, P and S elements are both impurity elements. Although P can play a solid solution strengthening role, inhibit the formation of carbides, and is beneficial to improving the stability of retained austenite, if the mass percentage of P is too high, it will Weakening grain boundaries, increasing material brittleness, and deteriorating welding performance, that is to say, the positive effect of P element is weaker than its negative effect. Therefore, it is preferable to control the mass percentage of P to P≤0.01%.

Correspondingly, the S element in steel easily forms a low-melting eutectic at the grain boundary. When its mass percentage is too high, it will significantly deteriorate the plasticity of the material. Therefore, the mass percentage of the S element is controlled to S≤0.005%. .

Further, in the coated ultra-high-strength steel of the present invention, the microstructure of the steel substrate is ferrite+martensite+retained austenite.

Further, in the coated ultra-high strength steel of the present invention, the volume proportion of ferrite is 25% to 45%; and/or the volume proportion of martensite is 45% to 65%.

Further, in the coated ultra-high-strength steel of the present invention, in the ferrite, the volume proportion of crystal grains with a size of 10 μm or less is ≥85%, and the volume proportion of crystal grains with a size of 5 μm or less is ≥ 55%.

Further, in the coated ultra-high strength steel of the present invention, the average grain size of the retained austenite is ≤2 μm; and/or the average C content in the retained austenite is ≥1.0%, such as 1.0-1.3%. .

Furthermore, in the coated ultra-high-strength steel of the present invention, the "low spot welding crack sensitivity" means that no cracks on the surface of the solder joint are produced when the spot welding process is used, or if cracks on the surface of the solder joint are produced, crack, the maximum length of the crack on the surface of the solder joint is less than 5% of the plate thickness, preferably less than 3% of the plate thickness, and more preferably less than 1% of the plate thickness.

Further, in the coated ultra-high-strength steel of the present invention, its mechanical properties satisfy: its yield strength ≥ 600MPa, such as ≥ 700MPa, preferably ≥ 750MPa; tensile strength ≥ 980MPa, preferably ≥ 1040MPa; uniform elongation ≥ 13 %, preferably ≥15.5%; elongation at break ≥15%, preferably ≥20%, more preferably ≥22.5%.

Further, in the coated ultra-high-strength steel of the present invention, its mechanical properties meet the following requirements: its yield strength is 600MPa~850MPa, its tensile strength is 980MPa~1150MPa, the uniform elongation is not less than 13%, and the fracture elongation is not less than 15%.

Further, in the coated ultra-high-strength steel of the present invention, its mechanical properties meet: its yield strength is 755-845MPa, the tensile strength is 1040-1140MPa, the uniform elongation is 14.7-16.4%, and the fracture elongation is between Between 22.6~25.4%.

Further, in the coated ultra-high strength steel of the present invention, the coating is pure zinc coating, zinc-iron alloy coating, zinc-aluminum-magnesium coating or aluminum-zinc coating.

Furthermore, in the coated ultra-high strength steel of the present invention, the coating weight is 30 to 120 g/m ² , preferably 50 to 100 g/m ² .

Accordingly, another object of the present invention is to provide a method for manufacturing coated ultra-high-strength steel. The manufacturing method is simple to produce, and the obtained high-strength steel has excellent spot welding performance, especially anti-solder spot surface, under the same mechanical properties. The ability to crack is significantly improved.

In order to achieve the above objectives, the present invention proposes a manufacturing method of coated ultra-high-strength steel, which specifically includes the steps:

(1) Smelting and thin plate continuous casting;

(2) Heating;

(3) Hot rolling: Control the thickness of the oxide scale on the surface of the steel strip after hot rolling to ≤ 4 μm, and the mass percentage of FeO + Fe ₃ O ₄ in the oxide scale on the surface of the steel strip after hot rolling is ≤ 50wt%;

(4) Pickling, or pickling + cold rolling;

(5) Continuous annealing: annealing at 800-920°C, then slowly cooling to 700-770°C at a cooling rate of 3-10°C/s; then cooling at 50-500°C/s (e.g., 100-500°C/s, 200-500°C/s, or 400-500°C/s) Cool quickly to 200-300°C; then heat to 360-460°C, keep warm for 50-600s; finally cool to room temperature;

(6) Plating with zinc coating.

In the technical solution of the present invention, the manufacturing method adopts a thin slab continuous casting process combined with a pickling or acid rolling process, and after continuous annealing and coating manufacturing, coated ultra-high-strength steel with low spot welding crack sensitivity can be obtained . The coated ultra-high-strength steel produced by this manufacturing method can be welded using the conventional spot welding process of the car factory, and the maximum length of the crack on the surface of the welding point is less than 5% of the plate thickness. It has very low spot welding crack sensitivity and is very Excellent quality.

In the above step (1) of the present invention, since thin slab continuous casting is used, the rough rolling process can be omitted and the amount of hot rolling deformation can be reduced, thereby ensuring the steel plate performance in the subsequent steps (4) and (5). . In addition, since thin slab continuous casting is used in step (1), it can fully utilize the heat of the slab and reduce the energy consumption required for heating, thereby obtaining a more uniform ferrite or ferrite + pearlite structure, which in turn has It is beneficial to maintain a certain amount of fine-grained ferrite in the microstructure of the substrate of the finished product in step (6) and improve the uniformity of the structure.

In step (2), the thickness of the oxide scale on the surface of the hot-rolled steel strip is controlled to ≤ 4 μm, and the (FeO + Fe ₃ O ₄ ) in the oxide scale on the surface of the hot-rolled steel strip is ≤ 50wt%, which is beneficial to the subsequent step ( 4), and has an important impact on the properties of the steel plate obtained after continuous annealing. This is because: in the technical solution of the present invention, FeO and Fe ₃ O ₄ are more difficult to pickle than Fe ₂ O ₃ , and Controlling the oxide scale thickness on the surface of the hot-rolled steel strip prepared by the present invention and the (FeO+Fe ₃ O ₄ ) ≤ 50wt% in the oxide scale on the surface of the hot-rolled steel strip can effectively improve the pickling effect and obtain a product that can be used for direct continuous The surface of the annealed pickled plate, and because the pickled plate can be directly annealed continuously, the deformation of the hot-rolled structure is small, and the steel plate structure is mainly pearlite and ferrite; therefore, the material strength can be reduced under the same continuous annealing conditions , making the structure more uniform and achieving excellent ductility.

Correspondingly, in step (5), the inventor optimized the design of the continuous annealing process, which can form a homogenized austenite structure or austenite + ferrite by controlling the annealing temperature of 800 to 920°C for annealing. The structure is then slowly cooled to 700-770°C at a cooling rate of 3-10°C/s to further adjust the ferrite content in the structure and obtain a certain proportion of ferrite, thereby improving the shaping of the material; and then using Cooling to 200~300℃ at a speed of 50~500℃/s (that is, between M _s (the starting temperature of martensite transformation) and M _f (the end temperature of martensitic transformation)), at this time, the austenitic The body part is transformed into martensite, which can ensure that the steel obtains higher strength; then, it is heated to 360~460℃ and kept for 50~600s. It can make carbon distribute in martensite and austenite to form a certain amount of carbon-rich retained austenite, which can be stably maintained at room temperature. Due to the TRIP effect, it can significantly improve the work hardening ability and formability of steel and obtain ductility. Excellent high strength steel plate.

In addition, in view of the inventor's understanding of the influence of carbon, silicon, manganese, boron or boron molybdenum on spot welding cracks, especially the belief that silicon and manganese are easily enriched in the shallow surface layer of the steel plate and significantly increase the crack sensitivity of the solder joint surface. It is known that in addition to strictly limiting the content of carbon, silicon, manganese, boron or boron molybdenum in the steel during component design, the present invention also reduces the content of carbon, silicon and manganese elements compared with steel types of the same strength level. , and compound elements of boron and molybdenum are added to increase the hardenability of the steel, thereby ensuring that the required microstructure composition is obtained in step (5), and ensuring the ultra-high quality of the zinc-containing coating obtained after step (6). High-strength steel has low spot welding crack sensitivity, that is, the maximum length of cracks on the surface of the welding spot is less than 5% of the plate thickness.

It should be noted that when the zinc-containing coating is plated in step (6), the zinc-containing coating can be produced by, but is not limited to, hot-dip, electroplating, or vacuum plating techniques.

To sum up, the coated ultra-high-strength steel designed by the present invention adopts the chemical composition design of carbon, silicon, manganese, boron, aluminum, and molybdenum and cooperates with the ferrite grain refinement. Therefore, during the continuous annealing process, while the nucleation points of austenite reverse transformation increase, the grain size can be further refined, so that the average grain size of the retained austenite stably maintained at room temperature is ≤ 2 μm; the residual austenite can be stably maintained at room temperature. The average C content in austenite is ≥1.0%.

Further, in the manufacturing method of coated ultra-high-strength steel according to the present invention, in step (1), the thickness of the slab at the exit end of the thin strip continuous casting is controlled to be 50 to 58 mm.

Furthermore, in the manufacturing method of coated ultra-high-strength steel according to the present invention, in step (1), the casting speed of the thin strip is controlled to be 2 to 5 m/min.

Further, in the manufacturing method of coated ultra-high-strength steel according to the present invention, in step (2), the slab is heated to 1200-1250°C.

Further, in the manufacturing method of coated ultra-high-strength steel according to the present invention, in step (3), the final rolling temperature is controlled to be 860-930°C, and the coiling temperature is controlled to be 450-600°C.

Further, in the manufacturing method of coated ultra-high-strength steel according to the present invention, in step (4), when pickling + cold rolling is used, the cold rolling deformation is controlled to be 40% to 60%.

Further, in the manufacturing method of coated ultra-high-strength steel according to the present invention, in step (5), the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled to 10-15%.

Further, in the manufacturing method of coated ultra-high-strength steel according to the present invention, the step (5) The annealing process parameters meet at least one of the following:

Annealing temperature is 820~870℃;

Slowly cool to 700~730°C at a cooling rate of 3~10°C/s;

Quickly cool to 250~300℃;

After rapid cooling, heat again to 400-430°C and keep warm for 180-300 seconds.

Compared with the existing technology, the coated ultra-high-strength steel with low spot weld crack sensitivity and its manufacturing method according to the present invention have the following advantages and beneficial effects:

In the coated ultra-high-strength steel designed by the present invention, its chemical element composition is based on C, Si, Mn, B, Al and Mo. By optimizing the ratio of each element, low spot welding crack susceptibility can be obtained. Durable coated ultra-high-strength steel.

The coated ultra-high-strength steel with low spot weld crack sensitivity described in the present invention has very excellent quality and performance. While meeting the user's requirements for the performance of the coated high-strength steel and the mechanical properties of the spot welded joint, it also has low points. Weld crack susceptibility.

The coated ultra-high-strength steel prepared using the technical solution of the present invention includes a steel substrate and a zinc-containing coating on the surface of the steel substrate. Its mechanical properties meet: its yield strength is 600MPa~850MPa, its tensile strength is 980MPa~1150MPa, and it extends uniformly. The rate is not less than 13%, and the elongation at break is not less than 15%; and, when spot welding is actually used for welding, if cracks on the surface of the solder joint occur, the maximum length of the crack on the surface of the solder joint is less than 5% of the plate thickness.

The manufacturing method designed by the present invention has a simple production process, and the obtained high-strength steel has significantly improved resistance to spot welding cracks under the same mechanical properties, and will have good application prospects in the production of safety structural parts for downstream users.

Description of the drawings

Figure 1 schematically shows spot welding cracks in zinc-coated ultra-high-strength steel.

Detailed ways

The coated ultra-high-strength steel with low spot weld crack sensitivity and its manufacturing method according to the present invention will be further explained and described below with reference to specific examples. However, this explanation and description do not constitute an undue limitation on the technical solution of the present invention. .

Examples 1-28 and Comparative Examples 1-4

The coated ultra-high-strength steels of Examples 1-28 and the comparative steels of Comparative Examples 1-4 according to the present invention are prepared by the following steps:

(1) Smelting and thin-slab continuous casting are carried out according to the chemical composition ratio designed in Table 1, wherein the slab thickness at the outlet end of the thin-strip continuous casting is controlled to be 50-58 mm, and the pulling speed of the thin-strip continuous casting is controlled to be 2-5 m/min to obtain the corresponding slab.

(2) Heating: Heat the slab to 1200~1250℃.

(3) Hot rolling: Control the oxide scale thickness on the surface of the steel strip after hot rolling to ≤ 4 μm, and (FeO + Fe ₃ O ₄ ) in the oxide scale on the surface of the steel strip after hot rolling ≤ 50wt%, and control the final rolling temperature to 860 ~ 930 ℃, the coiling temperature is 450~600℃.

(4) Pickling, or pickling + cold rolling. When pickling + cold rolling is used, the cold rolling deformation should be controlled to 40% to 60%. When galvanizing or zinc alloy is directly plated after pickling, Table 2- The amount of cold rolling deformation in 2 is 0.

(5) Continuous annealing: Control the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace to 10 to 15%; anneal at 800 to 920°C, preferably the annealing temperature can be controlled to 820 to 870°C, and then 3 to 10 Slowly cool to 700-770℃ at a cooling rate of ℃/s to obtain a certain proportion of ferrite. The final cooling temperature of slow cooling can preferably be controlled between 700-730℃; and then cool at a cooling rate of 50-500℃/s. Cool quickly to 200~300℃ to partially transform austenite into martensite. The final cooling temperature of rapid cooling can preferably be controlled between 250~300℃; then heat to 360~460℃ after rapid cooling , preferably 400 to 430°C, holding for 50 to 600s, preferably holding for 180 to 300s; and finally cooled to room temperature.

(6) Zinc or zinc alloy coating. The coating can be specifically selected as pure zinc coating, zinc-iron alloy coating, zinc-aluminum-magnesium coating or aluminum-zinc coating.

It should be noted that in the present invention, the chemical composition design and related manufacturing processes used in the coated ultra-high-strength steels of Examples 1-28 meet the specification requirements designed by the present invention. Correspondingly, there are process parameters in the chemical composition design and related manufacturing processes used in the comparative steel materials of Comparative Examples 1-4 that do not meet the design requirements of the present invention.

Table 1 lists the mass percentages of each chemical element in the steel plate substrate of the coated ultra-high-strength steel of Examples 1-28 and the comparative steel of Comparative Examples 1-4.

Table 1. (wt%, the balance is Fe and other unavoidable impurities except P and S)

Correspondingly, Table 2-1 and Table 2-2 list the specific processes in the above process steps (1)-(6) for the coated ultra-high-strength steel of Examples 1-28 and the comparative steel of Comparative Examples 1-4. parameter.

table 2-1.


Note: In the above Table 2-1, "(FeO + Fe ₃ O ₄ ) proportion" represents the mass percentage of FeO and Fe ₃ O ₄ in the oxide scale on the surface of the strip after hot rolling.

Table 2-2.

It should be noted that before performing the plating process of step (6) above, in order to verify that the coated ultra-high-strength steel of Examples 1-28 has very excellent performance, the inventor will go through steps before performing the plating process. The coated ultra-high-strength steel base materials of Examples 1-28 and the comparative steel base materials of Comparative Examples 1-4 obtained after the continuous annealing process of (5) were sampled respectively, and the base materials of each Example and Comparative Example were sampled. The microstructure of the materials was observed, and the mechanical properties of the base materials of each example and comparative example were tested. Among them, the microstructure observation results of the base materials of each embodiment and comparative example are listed in the following Table 3, and the mechanical property testing results of the base materials of each embodiment and the comparative example are listed in the following Table 4.

Table 3 lists the coated ultra-high strength steel substrates of Examples 1-28 and the comparative steel substrates of Comparative Examples 1-4. Observation results of the microstructure of the material. Detection was carried out according to GB/T 15749-2008 quantitative metallographic determination method.

table 3.

Correspondingly, when testing the mechanical properties of the base materials of each example and comparative example, the relevant mechanical property testing methods are as follows:

Tensile property test: GB/T228.1-2010 Metal Materials Tensile Test Part 1: Room Temperature Test Method was used to conduct a tensile test to test the coated ultra-high-strength steel base materials of Examples 1-28 and Comparative Example 1. Comparative yield strength, tensile strength, uniform elongation and elongation at break of -4 steel base materials.

Table 4 lists the observed results of mechanical properties of the coated ultra-high strength steel substrates of Examples 1-28 and the comparative steel substrates of Comparative Examples 1-4.

Table 4.

Accordingly, in order to verify that the finished products of Examples 1-28 with coated ultra-high strength steel prepared through the above steps (1)-(6) have excellent low spot welding crack sensitivity, the inventors took samples of the finished products of Examples 1-28 with coated ultra-high strength steel and the comparative steel plates of Comparative Examples 1-4, and conducted spot welding tests on the finished steel plate samples of each embodiment and comparative example. The relevant spot welding process parameters are shown in Table 5 below.

Table 5 lists the specific spot welding process parameters of the coated ultra-high strength steel of Examples 1-28 and the comparative steel plates of Comparative Examples 1-4.

table 5.

In the actual spot welding process, one tensile shear (TSS) specimen, one cross tensile specimen (CTS) specimen, and one metallographic specimen are welded under each welding current. TSS and CTS can be correspondingly pressed using a tensile testing machine. The ISO 14273-2016 and ISO 14272-2016 standards measured the joint bearing capacity of the finished steel plate samples of each example and comparative example after spot welding. The results are shown in Table 6 below.

Correspondingly, for the obtained metallographic samples, first use dilute hydrochloric acid to remove the coating on the joint surface, and observe the distribution and direction of spot welding cracks under a microscope. Select the section that passes through the center of the nugget and can cut the most spot welding cracks as the The metallographic section of the joint was sampled using wire cutting. The section includes all the welding characteristic areas of the spot welding joint. The surface of the intercepted sample was rinsed to prevent foreign matter from interfering with the test results. The rinsed sample was dried; The dried samples were mounted, ground and polished, and measured under a metallographic microscope. The maximum crack length was filled in Table 6.

Table 6 lists the mechanical properties and solder joint crack test results of the coated ultra-high-strength steel of Examples 1-28 and Comparative Examples 1-4 after spot welding.

Table 6.


Note: The two values of the nominal coating weight in Table 6 refer to the coating weight on the front and back of the steel plate. GI means pure zinc coating, GA
It means zinc-iron alloy coating, ZM means zinc-aluminum-magnesium coating, and AZ means aluminum-zinc coating.

It can be seen from the above-mentioned Table 3 of the present invention that the microstructure of the substrates of the steel materials of Examples 1-28 and Comparative Examples 1-4 designed by the present invention are both: ferrite + martensite + retained austenite. Moreover, the microstructure of the substrates of each embodiment and comparative example meets the following indicators: the volume proportion of ferrite is 25% to 45%, and the volume proportion of martensite is 45% to 65%; among them, the size is less than 10 μm The volume proportion of ferrite grains is ≥85%, and the volume proportion of ferrite grains with a size below 5 μm is ≥55%; the average grain size of retained austenite is ≤2 μm, and the volume proportion of retained austenite is ≤2 μm. The average C content in the body is ≥1.0%.

It can be seen from the above Table 4 that the steel base plates of Examples 1-28 and Comparative Examples 1-4 designed by the invention all have very excellent mechanical properties, and their mechanical properties all meet the design requirements of the invention, and the Examples The yield strength of the coated ultra-high-strength steel substrate of 1-28 is specifically between 755 and 845MPa, its tensile strength is between 1040 and 1140MPa, its uniform elongation is between 14.7-16.4%, and its fracture elongation is between Between 22.6-25.4%.

At the same time, it can be seen from Table 6 that for the finished coated ultra-high-strength steel in Examples 1-28 of this case, when the welding current is less than the spatter current, the welded joints have no spot welding cracks; when the welding current is greater than the spatter current, the welded joints have the smallest The long cracks are also less than 5% of the plate thickness; and for the comparative example steel plates of Comparative Examples 1-4 designed in this case, no matter whether the welding current is greater than the spatter current, the welded joints will have cracks, and the cracks in the spattered solder joints will The crack is more serious than that of the spatter-free solder joint, and the ratio of the maximum crack length to the plate thickness is much greater than 5%. This illustrates that the coated ultra-high-strength steel of each of Examples 1-28 designed using the technical solution of the present invention has excellent low spot welding crack sensitivity while ensuring the performance of the plate.

It should be noted that the combination of each technical feature in this case is not limited to the combination described in the claims of this case or the combination described in the specific embodiments. All the technical features recorded in this case can be freely combined in any way or combination, unless there is a conflict between them.

It should also be noted that the embodiments listed above are only specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments, and subsequent similar changes or deformations that those skilled in the art can directly derive from the disclosed content of the present invention or can easily associate them should all fall within the protection scope of the present invention. .

Claims (21)

1. A kind of coated ultra-high-strength steel with low spot welding crack sensitivity, which includes a steel substrate and a zinc-containing coating on the surface of the steel substrate. The steel substrate contains Fe and inevitable impurity elements. It is characterized in that the steel substrate also Contains the following chemical elements in the following mass percentages:

   C: 0.10～0.30%; Si: 0.30～0.90%; Mn: 1.00～2.20%; B: 0.001～0.003%; Al: 0.30～1.00%.
2. The coated ultra-high-strength steel according to claim 1, wherein the mass percentage of each chemical element of the steel substrate is:

   C: 0.10~0.30%; Si: 0.30~0.90%; Mn: 1.00~2.20%; B: 0.001~0.003%; Al: 0.30~1.00%; the balance is Fe and inevitable impurity elements.
3. The coated ultra-high-strength steel according to claim 1 or 2, characterized in that the mass percentage content of each chemical element of the steel substrate satisfies at least one of the following items:
   C: 0.15～0.20%;
   Si: 0.40～0.80%;
   Mn: 1.00～2.00%;
   B: 0.0015～0.003%.
4. The coated ultra-high-strength steel according to claim 1 or 2, characterized in that the chemical element of the steel substrate also contains Mo: 0.10-2.00%; preferably, the mass percentage of Mo is 0.15-2.00%.
5. The coated ultra-high-strength steel according to claim 4, wherein the mass percentage of Mo in the steel substrate is 0.10-1.00%; preferably, the mass percentage of Mo is 0.15-1.00%.
6. The coated ultra-high-strength steel according to claim 1 or 2, characterized in that the mass percentage content of unavoidable impurity elements in the steel substrate satisfies: P≤0.01%, S≤0.005%.
7. The coated ultra-high-strength steel according to claim 1 or 2, wherein the microstructure of the steel substrate is ferrite+martensite+retained austenite.
8. The coated ultra-high strength steel as claimed in claim 7, characterized in that the volume proportion of ferrite is 25% to 45%; and/or the volume proportion of martensite is 45% to 65%.
9. The coated ultra-high strength steel as claimed in claim 7, wherein in ferrite, The volume proportion of crystal grains with a size of 10 μm or less is ≥85%, and the volume proportion of crystal grains with a size of 5 μm or less is ≥55%.
10. The coated ultra-high strength steel according to claim 7, wherein the average grain size of the retained austenite is ≤2 μm; and/or the average C content in the retained austenite is ≥1.0%.
11. The coated ultra-high-strength steel as claimed in claim 1 or 2, characterized in that when spot welding is used for welding, if cracks on the surface of the solder joint occur, the maximum length of the crack on the surface of the solder joint is less than 5% of the plate thickness.
12. The coated ultra-high-strength steel according to claim 1 or 2, characterized in that its mechanical properties satisfy: its yield strength is 600MPa～850MPa, its tensile strength is 980MPa～1150MPa, its uniform elongation is not less than 13%, and its fracture elongation The rate is not less than 15%.
13. The coated ultra-high-strength steel according to claim 1 or 2, characterized in that the coating is pure zinc coating, zinc-iron alloy coating, zinc-aluminum-magnesium coating or aluminum-zinc coating.
14. The manufacturing method of coated ultra-high-strength steel according to any one of claims 1 to 13, characterized in that it includes the steps:

    (1) Smelting and thin plate continuous casting;

    (2) Heating;

    (3) Hot rolling: Control the thickness of the oxide scale on the surface of the steel strip after hot rolling to ≤ 4 μm, and the mass percentage of FeO + Fe ₃ O ₄ in the oxide scale on the surface of the steel strip after hot rolling is ≤ 50wt%;

    (4) Pickling, or pickling + cold rolling;

    (5) Continuous annealing: anneal at 800~920℃, then slowly cool to 700~770℃ at a cooling rate of 3~10℃/s; then quickly cool to 200~300℃ at a cooling rate of 50~500℃/s ;Then reheat to 360~460℃ and keep warm for 50~600s; finally cool to room temperature;

    (6) Plating with zinc coating.
15. The manufacturing method as claimed in claim 14, characterized in that, in step (1), the thickness of the slab at the outlet end of the thin strip continuous casting is controlled to be 50-58 mm.
16. The manufacturing method as claimed in claim 14, characterized in that in step (1), the casting speed of the thin strip is controlled to be 2 to 5 m/min.
17. The manufacturing method as claimed in claim 14, characterized in that in step (2), the slab is heated to 1200-1250°C.
18. The manufacturing method as claimed in claim 14, characterized in that in step (3), the control terminal The rolling temperature is 860~930℃, and the coiling temperature is 450~600℃.
19. The manufacturing method as claimed in claim 14, characterized in that in step (4), when pickling + cold rolling is adopted, the cold rolling deformation amount is controlled to be 40% to 60%.
20. The manufacturing method as claimed in claim 14, characterized in that, in step (5), the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled to 10-15%.
21. The manufacturing method as claimed in claim 14, characterized in that the annealing process parameters of step (5) satisfy at least one of the following items:

    Annealing temperature is 820~870℃;

    Slowly cool to 700~730°C at a cooling rate of 3~10°C/s;

    Quickly cool to 250~300℃;

    After rapid cooling, heat again to 400-430°C and keep warm for 180-300 seconds.