EP4592414A1

EP4592414A1 - Coated ultrahigh-strength steel with low spot welding crack sensitivity and manufacturing method therefor

Info

Publication number: EP4592414A1
Application number: EP23867653.0A
Authority: EP
Inventors: Hua PAN; Ming Lei; Xinyan JIN; Tao Gong; Haomin JIANG
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2022-09-23
Filing date: 2023-09-25
Publication date: 2025-07-30
Also published as: US20260098327A1; MX2025003397A; CN117802400A; EP4592414A4; WO2024061368A1; AU2023345395A1; JP2025531930A; KR20250075594A

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

Technical field

The present disclosure relates to a high strength steel and a manufacturing method therefor, in particular to an ultra-high-strength steel having a coating and a manufacturing method therefor.

Background

At present, with the advancement of the lightweight automobile process, as well as the requirement of automotive industry for improved vehicle corrosion resistance, many car companies have become more and more demanding for ultra-high strength steel with a coating. For cold-stamped steel, the coating is mainly a zinc-containing coating, such as a coating of hot-dip pure zinc, hot-dip galvanized iron and electroplating pure zinc. Resistance spot welding has become the most important connection method in automobile welding production due to its advantages of fast welding speed and low cost, etc. Joints of many ultra-high-strength steel with a zinc-containing coating often show good quasi-static mechanical properties during spot welding, but a kind of welding cracks appears on the surface of the steel plate in contact with electrodes and the nearby area, and this kind of welding cracks is called spot welding cracks. Figure 1 schematically shows the spot-welding cracks of ultra-high-strength steel with a zinc coating. However, it is very difficult to suppress the formation of such cracks. Therefore, this kind of cracks has become an important obstacle and bottleneck for the application of ultra-high-strength steel with a zinc-containing coating in the automotive field.
Some technical solutions are known in the prior art to control welding cracks, but there are still many shortcomings:

For example, the Chinese patent document with the publication number of CN108015401A, published on May 11, 2018 , and titled "Resistance spot welding method of galvanized high-strength steel with good joint performance", discloses a method for inhibiting spot-welding surface cracks while ensuring that the performance of the solder joint is not reduced through spot welding process innovation.
For another example, the Chinese patent document with the publication number CN109385515A, published on February 26, 2019 , and titled "Multilayer steel and method for reducing liquid metal embrittlement", discloses a method for suppressing the generation of spot welding cracks in high-strength steel, which inhibits the occurrence of spot welding cracks by first decarburizing and then galvanizing the high-strength steel, so as to control the thickness of the decarburized layer between 10 and 50 microns.
For another example, the Chinese patent document with the publication number of CN110892087A, published on March 17, 2020 , and titled "Zinc-coated steel plate with high resistance spot weldability", discloses a technical solution for forming an internal oxide layer in the steel plate by increasing the dew point, so as to improve the distribution of surface components, thereby reducing the occurrence of spot-welding cracks. However, the technical solution has the problem that it is difficult to control the thickness and uniformity of the internal oxide layer and the uniformity of the distribution of the surface components of the steel plate.

Summary

One of the objects of the present disclosure is to provide an ultra-high-strength steel having a coating with low spot-welding crack sensitivity, which has very excellent quality and performance. It satisfies the user's requirements for the performance of coated high-strength steel and the mechanical properties of spot-welding joints, while having low spot-welding crack sensitivity, and it has a good application prospect.
In order to achieve the above purpose, the present disclosure provides an ultra-high-strength steel having a coating with low spot-welding crack sensitivity, which comprises a steel substrate and a zinc-containing coating on the surface of the steel substrate, wherein the steel substrate comprises Fe and unavoidable impurity elements, and the steel substrate further comprises the following chemical elements in a mass percentage:
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%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the steel substrate comprises each chemical element in a mass percentage as follows:
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%; with a balance of Fe and unavoidable impurity elements.
Further, in the ultra-high-strength steel having a coating of the present disclosure, each chemical element of the steel substrate further comprises Mo: 0.10~2.00%.
In the above technical solution of the present disclosure, the chemical composition design based on carbon, silicon, manganese, boron or boron-molybdenum composite is adopted, which makes full use of the role of carbon, silicon, manganese, boron, molybdenum elements in the phase transformation of the material, thereby realizing the unification of high mechanical properties and high spot welding performance in the ultra-high-strength steel of the present disclosure, and finally obtaining an ultra-high-strength steel product having a coating with low spot welding crack sensitivity.
The chemical elements in the ultra-high-strength steel having a coating of the present disclosure are designed according to the following principles:
C: In the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, the solubility of carbon in austenite is much higher than that in ferrite, which can prolong the gestation period before the austenite transition and reduce the Ms temperature. The higher the mass percentage of carbon in the steel, the higher the fraction of residual austenite, and the higher the enrichment degree of carbon in residual austenite during partitioning, which is conducive to enhancing the stability of residual austenite, producing the TRIP effect and improving the ductility of the material. In addition, carbon is also the most basic solid solution-strengthening element in steel. However, it should be noted that the carbon content in the steel should not be too high. When the mass percentage of carbon is too high, it will reduce the weldability of the steel, especially significantly increase the tendency of cracks on the surface of welding spot on the joint. Therefore, in order to give full play to the beneficial effect of the C element, in the present disclosure, the mass percentage of the 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 at 0.15~0.20%.
Si: In the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, the solubility of silicon in carbides is very small. Si can strongly inhibit the formation of cementite during the partitioning treatment, promote the enrichment of carbon into the residual austenite, and improve the stability of the residual austenite. However, it should be noted that excessive silicon should not be added to the steel, and an overly high mass percentage of silicon will reduce the high-temperature plasticity of the steel, and form a stable oxide on the surface of the steel plate to reduce the wettability of the steel plate. In particular, the inventors in this case found that silicon is an element that significantly increases the tendency of cracks on the surface of solder joint. Therefore, in the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, the mass percentage of the Si element is specifically controlled at 0.30~0.90%.
Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Si element can be further preferably controlled at 0.40~0.80%.
Mn: In the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, manganese can expand the austenite phase zone, reduce the Ac₃, M_s and M_f points, improve the austenite stability and hardenability of steel, and reduce the critical transition rate, which is conducive to the preservation of residual austenite at room temperature. At the same time, manganese can also play a solid solution strengthening effect in steel. When the content of Mn in steel is too low, the ferrite and pearlite band structure will be formed at low cooling velocity due to segregation. In addition, the mass percentage of manganese in the steel should not be too high. When the mass percentage of manganese in the steel is too high, it will aggravate the grain coarsening trend, reduce the plasticity and toughness of the steel, and deteriorate the corrosion resistance, especially will increase the enrichment of manganese in the superficial layer of the base metal under the coating, increase the sensitivity of cracks on the surface of welding spot on the joint, and deteriorate the welding performance. Therefore, in order to give play to the beneficial effect of the Mn element, in the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, the mass percentage of the Mn element is controlled at 1.00~2.20%.
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 at 1.00-2.00%.
B: In the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, boron can significantly improve the hardenability of steel. On the one hand, boron is easy to segregate at grain boundaries, which fills grain boundary defects and reduces grain boundary energy. It increases the difficulty of new phase nucleation on austenite grain boundaries where ferrite nucleation originally takes place, and enhances austenite stability, thereby improving hardenability. On the other hand, the addition of boron reduces the demand of steel for elements that increase hardenability, such as carbon and manganese.
In addition, the inventors in this case also found that boron will reduce the segregation of manganese to the grain boundary, thereby reducing the formation of manganese enrichment layer in the steel matrix under the coating, and significantly reducing the formation of spot-welding cracks. But more boron is not always better. When the grain boundary defects are filled, if there are still more non-equilibrium segregation of boron, the "boron phase" precipitation will be formed at the grain boundary, increasing the grain boundary energy. At the same time, the "boron phase" will be used as a core of new phase, which will increase the nucleation velocity, reduce the stability of austenite, and reduce the hardenability. That is, there is obvious "boron phase" precipitation which has a negative effect on hardenability, and a large amount of "boron phase" precipitation will make the steel brittle, which will have a bad impact on the mechanical properties of steel. Moreover, the embrittled grain boundaries will promote the occurrence and propagation of spot-welding cracks.
The inventors in this case found that the boron content of 0.0030% is the inflection point for the embrittlement of grain boundaries. Therefore, in order to give full play to the excellent effect of the B element, the mass percentage of the B element is controlled at 0.001~0.003% in the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure.
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 at 0.0015~0.003%.
Al: In the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, when aluminum exists in a solid solution state, it can increase the stacking fault energy, inhibit the precipitation of cementite and the transition from y to martensite, and improve the stability of austenite. Moreover, aluminum forms fine and diffusely distributed insoluble mass points with carbon and nitrogen that can refine the grains, but 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 continuous casting for steelmaking. Therefore, in the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, the mass percentage of the Al element is controlled at 0.30~1.00%.
Mo: In the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, molybdenum can reduce the M_s point while shifting the C curve to the right, thereby improving the hardenability of the steel, and increase the elongation of the steel while increasing the strength of the steel. In addition, molybdenum is a strong carbide forming element, which will form fine and diffusely distributed MoC particles during smelting, and will be distributed in martensite in a form of hard second phase during the subsequent phase transition, so as to improve the toughness of steel. More importantly, the inventors in this case found that the addition of molybdenum can significantly improve the spot-welding crack resistance of the steel. Therefore, comprehensively considering the above factors and the cost of adding molybdenum, in the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, the mass percentage of the Mo element is controlled at 0.10~2.00%. In some embodiments, the mass percentage of the Mo element is controlled at 0.15~2.00%.
Of course, in some preferred embodiments, in order to obtain better implementation effects, the mass percentage of the Mo element can be further preferably controlled at 0.10∼1.00%. In some embodiments, the mass percentage of the Mo element is controlled at 0.15~1.00% or 0.20~1.00%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the mass percentage 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: 0.10~1.00%.

Further, in the ultra-high-strength steel having a coating of the present disclosure, the mass percentage of the unavoidable impurity elements of the steel substrate satisfies: P≤0.01%, S≤0.005%.
In the above technical solution, P and S elements are the impurity elements in the ultra-high-strength steel having a coating of the present disclosure, and if technical conditions permit, in order to obtain the steel with better performance and better quality, the content of impurity elements in the ultra-high-strength steel having a coating should be reduced as much as possible.
In the present disclosure, P and S elements are impurity elements. Although P can play a role in solid solution strengthening, inhibit the formation of carbides and is conducive to improving the stability of residual austenite, an overly high mass percentage of P will weaken the grain boundaries, increase the brittleness of the material, and deteriorate the welding performance. That is to say, the positive effect of the P element is weaker than its negative effect. Therefore, the P mass percentage is preferably controlled at P≤0.01%.
Correspondingly, the S element in the steel is easy to form low-melt eutectic at the grain boundaries, and the plasticity of the material will be significantly deteriorated when its mass percentage is too high. Therefore, the mass percentage of the S element is controlled at S≤0.005%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the microstructure of the steel substrate is ferrite + martensite + residual austenite.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the volume fraction of ferrite is 25%~45%; and/or the volume fraction of martensite is 45%~65%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, in ferrite, the volume of grains having a size of 10 µm or less accounts for ≥85%, and the volume of grains having a size of 5 µm or less accounts for ≥55%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the residual austenite has an average grain size of ≤ 2 µm; and/or the residual austenite has an average C content of ≥1 .0%, such as 1.0-1.3%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the "low spot welding crack sensitivity" means that: no cracks on the surface of solder joint is produced when spot welding is adopted for welding, or, if cracks on the surface of solder joint are generated, the maximum length of the cracks on the surface of solder joint is less than 5% of the plate thickness, preferably less than 3% of the plate thickness, and preferably less than 1% of the plate thickness.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the mechanical properties satisfy: a yield strength of ≥600MPa, such as ≥700MPa, preferably ≥750MPa; a tensile strength of ≥980MPa, preferably≥1040MPa; a uniform elongation of ≥13%, preferably ≥15.5%; an elongation at break of ≥15%, preferably ≥20%, more preferably≥22.5%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the mechanical properties satisfy: a yield strength of 600MPa~850MPa, a tensile strength of 980MPa~1150MPa, a uniform elongation of no less than 13%, an elongation at break of no less than 15%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the mechanical properties satisfy: a yield strength of 755~845MPa, a tensile strength of 1040~1140MPa, a uniform elongation of 14.7~16.4%, an elongation at break of 22.6~25.4%.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the coating is a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating or an aluminum-zinc coating.
Further, in the ultra-high-strength steel having a coating of the present disclosure, the coating has a weight of 30~120g/m², preferably 50~100g/m².
Accordingly, another object of the present disclosure is to provide a method for manufacturing the ultra-high-strength steel having a coating, which is simple for production, and can provide high-strength steel having significantly improved spot-welding performance, especially the resistance of cracks on the surface of solder joint while having the same mechanical properties.
To achieve the above purpose, the present disclosure provides a manufacturing method for the ultra-high-strength steel having a coating, which comprises steps of:

(1) smelting and thin slab continuous casting;
(2) heating;
(3) hot-rolling: wherein the thickness of oxide scale on the surface of the strip steel after hot rolling is controlled at ≤4µm, and the mass percentage of FeO+ Fe₃O₄ in the oxide scale on the surface of the strip steel after hot rolling is ≤50wt%;
(4) pickling, or pickling + cold rolling;
(5) continuous annealing: wherein the steel is annealed at 800~920 °C, then slowly cooled to 700~770 °C at a cooling rate of 3~10 °C/s; then rapidly cooled to 200~300°C with a cooling rate of 50~500°C/s (such as 100~500°C/s, 200~500°C/s or 400~500°C/s); then re-heated to 360~460°C, held for 50~600s; and finally cooled to room temperature;
(6) plating a zinc-containing coating.

In the technical solution of the present disclosure, the manufacturing method adopts a thin slab continuous casting process matched with pickling or acid rolling process, and can obtain an ultra-high-strength steel having a coating with low spot-welding crack sensitivity after continuous annealing and coating manufacturing. The ultra-high-strength steel having a coating produced by this manufacturing method can be welded using the conventional spot-welding process of car factories, and the maximum length of the crack on the surface of the solder joint is less than 5% of the plate thickness, which has a very low spot-welding crack sensitivity and has very excellent quality.
In the above step (1) of the present disclosure, because a thin slab continuous casting is adopted, the rough rolling process can be omitted, and the hot rolling deformation rate can be reduced, so that the performance of the steel plate in the subsequent step (4) and step (5) is guaranteed. In addition, because the thin slab continuous casting is used in step (1), it can make full use of the heat of the slab and reduce the energy consumption required for heating, so as to obtain a more uniform structure of ferrite or ferrite+pearlite, which is conducive to maintaining a certain amount of fine grain ferrite in the microstructure of the substrate of the final product in step (6) and improving the uniformity of the structure.
In step (2), the thickness of oxide scale on the surface of the strip steel after hot rolling is controlled at ≤4µm, and the (FeO+ Fe₃O₄) in the oxide scale on the surface of the strip steel after hot rolling is ≤50wt%, which is conducive to the subsequent progress in step (4), and has an important influence on the properties of the steel plate obtained after continuous annealing. It is because that in the technical solution of the present disclosure, FeO, Fe₃O₄ are more difficult to be pickled than Fe₂O₃. Controlling the thickness of the oxide scale on the surface of the strip steel after hot rolling and the (FeO+Fe₃O₄) in the oxide scale on the surface of the strip steel after hot rolling prepared by the present disclosure to be ≤50wt%, can effectively improve the pickling effect, and obtain the surface of the pickled plate that can be used for direct continuous annealing. Because the pickled plate can be directly subjected to continuous annealing, the deformation rate of the hot-rolled microstructure is small, and the microstructure of the steel plate is dominated by pearlite and ferrite. Therefore, under the same continuous annealing conditions, the strength of the material can be reduced, so as to provide more uniform microstructure, thereby obtaining excellent ductility.
Correspondingly, in step (5), the inventors optimized the design for the continuous annealing process. A homogenized austenite or austenite + ferrite structure can be formed by controlling the annealing temperature of 800~920 °C for annealing. Then, the ferrite content in the structure can be further adjusted by slowly cooling the steel to 700~770°C at a cooling rate of 3~10°C/s to obtain a certain proportion of ferrite, so as to improve the plasticity of the material. After that, it is cooled to 200~300°C (that is, between M_s (the start temperature of martensite transition) and M_f (the end temperature of martensite transition)) at a rate of 50~500°C/s. At this time, a part of austenite is transformed into martensite, which can ensure the high strength of the steel. After that, it is re-heated to 360~460°C and held for 50~600s, which can partition carbon in martensite and austenite to form a certain amount of carbon-rich residual austenite that can be stably maintained to room temperature. Due to the TRIP effect, the work hardening ability and formability of the steel can be significantly improved, and the high-strength steel plate with excellent ductility can be obtained.
In addition, in view of the inventors' understanding of the influence of carbon, silicon, manganese, boron or boron-molybdenum on spot welding cracks, especially the understanding that it is easy to enrich silicon and manganese in the superficial layer of the steel plate to significantly increase the sensitivity of cracks on the surface of solder joint, the present disclosure not only strictly limits the content of carbon, silicon, manganese, boron or boron-molybdenum of the steel in the composition design, but also reduces the content of carbon, silicon and manganese elements compared with the same strength grade of steel, and adds boron and molybdenum elements to increase the hardenability of the steel, so as to ensure that the required microstructure components are obtained in step (5) and the ultra-high-strength steel having a zinc coating obtained after step (6) has low spot welding crack sensitivity, that is, the maximum length value of cracks on the surface of solder joint 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 not limited to hot dipping, electroplating and vacuum evaporating technology.
Based on the above, because the ultra-high strength steel having a coating designed in the present disclosure adopts a chemical composition design of carbon, silicon, manganese, boron, aluminum, molybdenum matched with ferrite grain refinement, during the continuous annealing process, the nucleation point of austenite reverse phase transition increases, and the grain size can be further refined, so that the average grain size of the residual austenite that can be stably maintained to room temperature is ≤ 2 µm and the average C content in the residual austenite is ≥ 1.0%.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, in step (1), the slab thickness at the outlet of thin slab continuous casting is controlled at 50~58mm.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, in step (1), the drawing speed of thin strip continuous casting is controlled at 2~5m/min.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, in step (2), the slab is heated to 1200~1250°C.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, in step (3), the rolling-end temperature is controlled at 860~930°C, the coiling temperature is controlled at 450~600°C.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, in step (4), when pickling + cold rolling is adopted, the cold rolling deformation rate is controlled at 40%~60%.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, in step (5), the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled at 10~15%.
Further, in the method for manufacturing the ultra-high-strength steel having a coating of the present disclosure, the annealing process parameters of step (5) satisfy at least one of the following items:

an annealing temperature of 820~870°C;
slowly cooled to 700~730°C at a cooling rate of 3~10°C/s;
rapidly cooled to 250~300°C;
re-heated to 400~430°C after rapid cooling, held for 180~300s.

Compared with the prior art, the ultra-high-strength steel having a coating with low spot-welding crack sensitivity of the present disclosure, and the manufacturing method thereof have the following advantages and beneficial effects:

In the ultra-high-strength steel having a coating designed in the present disclosure, its chemical element composition is based on C, Si, Mn, B, Al and Mo, and by optimizing the ratio of each element, the ultra-high-strength steel having a coating with low spot welding crack sensitivity can be obtained.
The ultra-high-strength steel having a coating designed in the present disclosure has excellent quality and performance, which meets the user's requirements for the performance of high-strength steel having a coating and the mechanical properties of spot welded joints, and also has low spot weld crack sensitivity.
The ultra-high-strength steel having a coating prepared by this technical solution of the present disclosure comprises a steel substrate and a zinc-containing coating on the surface of the steel substrate, and its mechanical properties satisfy: a yield strength of 600MPa~850MPa, a tensile strength of 980MPa~1150MPa, a uniform elongation of not less than 13%, and an elongation at break of not less than 15%. In addition, when the spot welding process is actually used for welding, if cracks on the surface of solder joint are generated, the maximum length of the cracks on the surface of solder joint is less than 5% of the plate thickness.

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

Description of the drawings

Fig. 1 schematically shows the spot-welding cracks in the ultra-high-strength steel having a zinc-containing coating layer.

Detailed Description

The ultra-high-strength steel having a coating with low spot-welding crack sensitivity and the manufacturing method thereof of the present disclosure will be further explained and interpreted below in conjunction with the specific examples of the description, but the explanation and interpretation do not constitute an undue limitation to the technical solution of the present disclosure.

Examples 1-28 and Comparative Examples 1-4

The ultra-high-strength steel having a coating of Examples 1-28 of the present disclosure and the comparative steel of Comparative Examples 1-4 were prepared by the following steps:

(1) smelting and thin slab continuous casting were performed according to the chemical compositions and ratios shown in Table 1, wherein the slab thickness at the outlet of thin strip continuous casting was controlled to be 50~58mm, and the drawing speed of thin strip continuous casting was controlled to be 2~5m/min to obtain the corresponding slab;
(2) heating: wherein the slab was heated to 1200~1250°C;
(3) hot-rolling: wherein the thickness of oxide scale on the surface of the strip steel after hot rolling was controlled at ≤4µm, (FeO+ Fe₃O₄) in the oxide scale on the surface of the strip steel after hot rolling was ≤50wt%, the rolling-end temperature was controlled at 860~930°C and the coiling temperature was controlled at 450~600°C;
(4) pickling, or pickling + cold rolling: when pickling + cold rolling was adopted, the cold rolling deformation rate was controlled to be 40%~60%, and when the steel was galvanized with zinc or zinc alloy directly after pickling, the cold rolling deformation rate in Table 2-2 was 0;
(5) continuous annealing: wherein the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace was controlled to be 10~15%; wherein the steel was annealed at 800~920 °C, preferably the annealing temperature being controlled at 820~870 °C, then slowly cooled to 700~770 °C at a cooling rate of 3~10 °C/s to obtain a certain percentage of ferrite, wherein the cooling-end temperature of slow cooling was preferably controlled at 700~730°C; then rapidly cooled to 200~300°C with a cooling rate of 50~500°C/s so as to convert part of austenite into martensite, wherein the cooling-end temperature of rapid cooling was preferably controlled at 250~300°C; then re-heated to 360~460°C, preferably 400~430°C, after rapid cooling, held for 50~600s, preferably 180~300°C; and finally cooled to room temperature;
(6) plating a coating of zinc or zinc alloy, wherein the coating may be specifically selected as a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating or an aluminum-zinc coating.

It should be noted that, in the present disclosure, the chemical composition and related process parameters adopted for the ultra-high-strength steel having a coating of Examples 1-28 all met the control requirements designed in the present disclosure. Correspondingly, there were parameters in the chemical composition and related process parameters adopted for the comparative steel of Comparative Examples 1-4 that did not meet the requirements designed in the present disclosure.

Table 1 lists the mass percentage of each chemical element of the ultra-high-strength steel having a coating of Examples 1-28 and the comparative steel of Comparative Examples 1-4.

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

Ex.	C	Si	Mn	B	Mo	Al	P	S
Ex. 1	0.30	0.60	1.40	0.0030	0.60	0.80	0.0100	0.0035
Ex. 2	0.10	0.30	2.20	0.0010	0.10	0.30	0.0075	0.0025
Ex. 3	0.24	0.50	1.00	0.0016	1.10	0.50	0.0045	0.0021
Ex. 4	0.20	0.40	2.00	0.0013	2.00	0.40	0.0055	0.0045
Ex. 5	0.15	0.70	1.80	0.0022	1.60	0.70	0.0051	0.0042
Ex. 6	0.16	0.80	1.20	0.0019	0.45	0.60	0.0065	0.0027
Ex. 7	0.18	0.90	1.60	0.0025	1.00	0.75	0.0085	0.0038
Ex. 8	0.11	0.90	1.10	0.0029	-	1.00	0.0095	0.0029
Ex. 9	0.13	0.80	1.30	0.0026	0.70	0.90	0.0053	0.0019
Ex. 10	0.15	0.70	1.50	0.0023	-	0.78	0.0068	0.0023
Ex. 11	0.17	0.60	1.70	0.0020	0.50	0.69	0.0057	0.0031
Ex. 12	0.19	0.50	1.90	0.0015	-	0.58	0.0092	0.0026
Ex. 13	0.20	0.40	2.10	0.0011	0.15	0.49	0.0088	0.0036
Ex. 14	0.195	0.35	1.05	0.0028	0.73	0.36	0.0045	0.0032
Ex. 15	0.175	0.45	1.25	0.0026	-	0.55	0.0082	0.0040
Ex. 16	0.155	0.55	1.45	0.0023	0.29	0.66	0.0063	0.0037
Ex. 17	0.135	0.65	1.65	0.0021	-	0.74	0.0057	0.0024
Ex. 18	0.115	0.75	1.85	0.0018	0.44	0.86	0.0076	0.0022
Ex. 19	0.15	0.80	1.00	0.0030	0.48	0.95	0.0082	0.0028
Ex. 20	0.20	0.79	1.15	0.0015	1.00	0.93	0.0061	0.0030
Ex. 21	0.165	0.72	1.35	0.0017	0.42	0.84	0.0059	0.0026
Ex. 22	0.145	0.63	1.55	0.0022	0.32	0.73	0.0067	0.0023
Ex. 23	0.125	0.67	1.75	0.0024	0.22	0.64	0.0071	0.0031
Ex. 24	0.105	0.51	1.95	0.0027	0.12	0.52	0.0059	0.0034
Ex. 25	0.125	0.31	2.18	0.0011	0.84	0.89	0.0043	0.0021
Ex. 26	0.150	0.40	1.81	0.0016	1.00	0.68	0.0051	0.0026
Ex. 27	0.165	0.43	1.48	0.0022	0.95	0.46	0.0047	0.0032
Ex. 28	0.200	0.80	1.03	0.0029	0.10	0.32	0.0056	0.0016
CEx. 1	0.10	*1.10*	2.20	*0.0005*	*0.01*	*0.01*	0.0035	0.0073
CEx. 2	0.18	0.65	*2.50*	*0.0009*	*0.005*	*0.20*	0.0095	0.0061
CEx. 3	*0.09*	0.35	1.75	*0.0002*	*0.008*	*0.15*	0.0089	0.0075
CEx. 4	0.20	*1.80*	*2.30*	*0.0007*	0.002	*0.25*	0.0041	0.0078

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

Table 2-1.

No.	Step (1)		Step (2)	Step (3)
No.	Slab thickness (mm)	Drawing speed of thin strip continuous casting (m/ min)	Slab heating temperature (°C)	Thickness of oxide scale on surface of strip steel after hot rolling (µm)	Proportion of (FeO+ Fe₃O₄) in oxide scale on surface of strip steel after hot rolling (%)	Rolling-end temperature of hot rolling (°C)	Coiling temperature of hot rolled coil (°C)
Ex. 1	55	2.0	1235	3.5	45.9	875	480
Ex. 2	58	5.0	1235	2.6	33.5	880	496
Ex. 3	52	3.2	1225	3.9	42.3	920	550
Ex. 4	53	4.1	1230	3.7	17.3	900	465
Ex. 5	56	2.6	1250	2.1	21.7	890	585
Ex. 6	55	4.9	1245	2.8	27.5	860	515
Ex. 7	52	4.3	1230	4.0	23.8	910	535
Ex. 8	57	3.7	1215	3.3	25.6	885	520
Ex. 9	54	2.9	1225	1.5	46.8	930	555
Ex. 10	57	2.1	1210	2.2	29.5	889	535
Ex. 11	58	2.2	1200	2.8	35.4	882	565
Ex. 12	51	2.7	1220	3.2	42.3	878	575
Ex. 13	54	3.3	1215	2.7	47.9	865	510
Ex. 14	57	3.8	1240	3.5	37.8	899	600
Ex. 15	51	4.4	1250	1.8	22.5	870	590
Ex. 16	58	4.8	1210	2.5	36.8	908	520
Ex. 17	56	4.0	1240	3.3	23.6	897	450
Ex. 18	50	3.4	1240	2.9	49.5	863	500
Ex. 19	56	2.6	1250	1.1	28.3	903	580
Ex. 20	55	2.1	1250	2.5	35.8	920	530
Ex. 21	57	2.6	1240	2.6	27.8	910	565
Ex. 22	53	3.0	1200	2.7	26.4	920	490
Ex. 23	54	3.7	1235	3.1	23.8	890	470
Ex. 24	53	4.5	1225	1.8	35.2	895	475
Ex. 25	56	4.9	1205	2.4	42.2	915	510
Ex. 26	55	3.6	1210	1.2	27.3	895	550
Ex. 27	58	4.6	1210	3.4	24.3	875	545
Ex. 28	56	2.3	1205	3.7	35.7	915	525
CEx. 1	56	2.0	1215	*4.6*	*59.1*	910	530
CEx. 2	56	3.0	1240	*5.5*	*53.6*	915	540
CEx. 3	50	4.0	1230	*5.2*	*61.5*	905	550
CEx. 4	57	5.0	1220	*6.8*	*58.7*	890	560

Note: in the above Table 2-1, "Proportion of (FeO+ Fe₃O₄)" refers to the mass percentage of FeO and Fe₃O₄ in the oxide scale on surface of the strip steel after hot rolling, based on the mass of the oxide scale.

Table 2-2.

No.	Step (4)	Step (5)
No.	Cold rolling deformatio n rate (%)	Hydrogen volume content of reducing atmospher e in annealing furnace (%)	Annealing temperatur e (°C)	Slow coolin g rate (°C/s)	Cooling-end temperatur e of slow cooling (°C)	Cooling-end temperatur e of fast cooling (°C)	Fast coolin g rate (°C/s)	Re-heating temperatur e (°C)	Holding time of re-heatin g (s)
Ex. 1	50	12.5	800	8	718	295	460	400	500
Ex. 2	50	10.0	820	5	704	280	475	430	260
Ex. 3	49	15.0	830	5	754	285	100	401	180
Ex. 4	55	14.2	835	7	730	290	350	430	290
Ex. 5	0	11.3	870	4	770	235	150	390	180
Ex. 6	58	14.9	840	3	736	218	350	383	180
Ex. 7	0	10.1	918	7	720	283	430	398	300
Ex. 8	44	11.2	864	7	770	200	380	440	350
Ex. 9	45	13.8	855	10	755	297	490	428	460
Ex. 10	0	12.6	910	5	750	275	480	405	340
Ex. 11	54	10.4	880	6	765	225	50	400	140
Ex. 12	0	11.4	810	7	725	293	485	425	270
Ex. 13	0	14.6	845	3	715	215	250	380	295
Ex. 14	46	13.6	860	8	720	257	495	413	260
Ex. 15	0	12.1	852	5	725	240	450	393	280
Ex. 16	57	10.2	915	7	705	265	425	363	400
Ex. 17	44	14.8	896	9	700	277	445	360	250
Ex. 18	0	11.5	830	4	749	245	460	368	380
Ex. 19	58	13.5	858	6	735	250	415	423	590
Ex. 20	0	12.7	843	6	715	270	470	403	600
Ex. 21	48	14.4	857	9	716	210	200	372	570
Ex. 22	0	10.6	846	9	745	205	410	415	225
Ex. 23	56	11.8	900	4	735	300	475	387	230
Ex. 24	60	13.2	905	5	728	265	435	460	540
Ex. 25	0	12.8	820	9	720	255	440	408	240
Ex. 26	40	13.7	838	8	755	280	420	378	420
Ex. 27	43	12.9	903	4	765	270	500	418	50
Ex. 28	52	11.9	855	6	728	230	460	450	290
CEx . 1	0	15.0	*790*	3	*600*	183	*390*	350	500
CEx . 2	45	13.8	*930*	7	*814*	353	300	*470*	300
CEx . 3	0	10.0	865	10	715	220	*485*	310	50
CEx. 4	55	11.3	810	5	745	255	*445*	*530*	*620*

It should be noted that, before the plating process of the above step (6), in order to verify that the ultra-high-strength steel having a coating of Examples 1-28 had very excellent properties. Before the plating process, the inventor sampled the ultra-high-strength steel substrate having a coating of Examples 1-28 obtained after the continuous annealing process of step (5) and the comparative steel substrate of Comparative Examples 1-4, and conducted microstructure observations on the substrate of each Example and Comparative Example. At the same time, the mechanical properties of the substrate of each Example and Comparative Example were tested. The results of microstructure observation of the substrate of each Example and Comparative Example were listed in Table 3 below, and the test results of mechanical properties of the substrate of each Example and Comparative Example were listed in Table 4 below.

Table 3 lists the result of microstructure observation of the ultra-high-strength steel substrate having a coating in Examples 1-28 and the comparative steel substrate in Comparative Examples 1-4. The steel substrate was tested according to GB/T 15749-2008 quantitative metallographic determination method.

Table 3.

No.	Fraction of ferrite (%)	Fraction of martensite (%)	Proportion of ferrite having a grain size of ≤10µm (%)	Proportion of ferrite having a grain size of ≤5µm (%)	Average grain size of residual austenite (µm)	C content in residual austenite (%)
Ex. 1	32.97	55.34	95.78	58.93	1.5	1.15
Ex. 2	34.23	54.98	85.23	59.87	0.7	1.26
Ex. 3	44.54	46.33	94.83	72.58	0.8	1.06
Ex. 4	34.87	55.11	85.98	57.65	1.7	1.21
Ex. 5	40.88	48.76	92.47	67.78	0.8	1.11
Ex. 6	39.68	48.34	92.48	57.12	0.9	1.09
Ex. 7	39.12	50.89	94.74	59.63	1.3	1.03
Ex. 8	36.85	52.45	94.52	69.24	0.7	1.20
Ex. 9	37.66	52.23	90.67	67.89	1.9	1.09
Ex. 10	36.64	51.69	86.76	69.38	2.0	1.18
Ex. 11	26.25	64.67	90.55	72.69	1.4	1.08
Ex. 12	28.27	61.34	94.67	66.34	1.7	1.18
Ex. 13	25.25	63.58	91.22	62.87	1.9	1.08
Ex. 14	26.31	63.79	87.33	68.54	1.0	1.16
Ex. 15	26.18	64.15	93.64	65.87	1.1	1.04
Ex. 16	35.78	54.21	90.12	65.67	1.8	1.28
Ex. 17	43.11	45.32	94.37	73.24	1.4	1.23
Ex. 18	41.25	46.45	92.43	73.87	1.2	1.17
Ex. 19	39.59	49.87	92.43	56.89	0.4	1.23
Ex. 20	30.76	60.64	88.45	69.75	1.5	1.27
Ex. 21	28.45	60.65	88.76	66.87	0.6	1.05
Ex. 22	30.22	59.25	94.56	63.77	1.8	1.02
Ex. 23	28.43	61.25	90.63	58.45	1.3	1.22
Ex. 24	30.56	59.11	92.56	57.43	0.6	1.21
Ex. 25	28.34	61.56	92.78	63.43	0.5	1.05
Ex. 26	28.11	61.23	87.35	66.18	0.3	1.14
Ex. 27	32.78	56.32	87.69	67.56	1.3	1.09
Ex. 28	34.56	55.68	92.65	62.95	1.6	1.26
CEx. 1	*51.32*	*37.12*	*80.35*	*49.43*	*2.3*	1.11
CEx. 2	23.95	68.18	87.36	58.54	0.8	*0.82*
CEx. 3	41.45	48.15	*81.34*	56.47	1.1	*0.91*
CEx. 4	41.85	47.36	*79.44*	*47.59*	1.5	*0.77*

Correspondingly, when the mechanical properties of the substrate of each Example and Comparative Example were tested, the relevant mechanical properties testing methods were as follows:
Tensile property test: tensile test was performed according to GB/T228.1-2010 Metallic materials-Tensile testing-Part 1: Method of test at room temperature, to test the yield strength, tensile strength, uniform elongation and elongation at break of the ultra-high-strength steel substrate having a coating obtained in Examples 1-28 and the comparative steel substrate in Comparative Examples 1-4.

Table 4 lists the observation results of mechanical properties of the ultra-high-strength steel substrate having a coating in Examples 1-28 and the comparative steel substrate in Comparative Examples 1-4.

Table 4.

No.	Yield strength (MPa)	Tensile strength (MPa)	Uniform elongation (%)	Elongation at break (%)
Ex. 1	770	1040	15.2	23.2
Ex. 2	815	1070	15.7	23.3
Ex. 3	780	1115	15.8	23.8
Ex. 4	800	1110	16.1	22.9
Ex. 5	765	1090	16.1	24.3
Ex. 6	845	1050	16.2	23.3
Ex. 7	800	1125	16.2	23.6
Ex. 8	755	1075	16.1	23.9
Ex. 9	785	1080	14.8	24.8
Ex. 10	820	1105	15.3	24.2
Ex. 11	765	1110	15.8	24.8
Ex. 12	780	1085	15.7	24.7
Ex. 13	825	1080	14.7	23.7
Ex. 14	775	1115	15.9	24.1
Ex. 15	790	1080	15.6	22.6
Ex. 16	765	1110	15.1	23.5
Ex. 17	795	1060	15.2	23.5
Ex. 18	835	1075	15.9	23.5
Ex. 19	770	1135	15.4	22.8
Ex. 20	795	1060	15.1	24.4
Ex. 21	805	1105	15.9	23.1
Ex. 22	790	1080	15.4	23.9
Ex. 23	830	1090	16.4	23.3
Ex. 24	775	1140	15.7	23.1
Ex. 25	820	1065	14.6	25.4
Ex. 26	810	1120	15.9	23.9
Ex. 27	810	1085	15.2	24.6
Ex. 28	780	1090	15.9	23.4
CEx. 1	*585*	1035	18.1	24.8
CEx. 2	*870*	1050	*11.4*	*14.3*
CEx. 3	*895*	1080	*10.7*	*13.8*
CEx. 4	*860*	1060	*11.2*	*13.9*

Correspondingly, in order to verify that the final product of the ultra-high-strength steel having a coating prepared in Examples 1-28 according to the above steps (1)-(6) has fairly excellent low spot welding crack sensitivity, the inventors took samples respectively for the final product of the ultra-high-strength steel having a coating of Examples 1-28 and the comparative steel plate of Comparative Examples 1-4, and conducted welding tests with spot welding on the final product of each Example and Comparative Example. The relevant spot-welding process parameters are listed in Table 5 below.

Table 5 lists the specific spot-welding process parameters of the ultra-high-strength steel having a coating of Examples 1-28 and the comparative steel plate of Comparative Example 1-4.

Table 5.

No.	Steel plate thickness (mm)	Electrode pressure (kN)	Number of pulses	Time for 1 pulse (ms)	Cooling time (ms)	Total welding time (ms)	Keeping time (ms)
Ex. 1	1.3	3.6	3	120	20	400	250
Ex. 2	1.4	3.6	3	120	20	400	250
Ex. 3	1.5	3.6	3	120	20	400	250
Ex. 4	1.5	3.6	3	120	20	400	250
Ex. 5	1.2	2.6	1	270	0	270	250
Ex. 6	2	4.0	4	120	40	600	250
Ex. 7	1.3	3.6	3	120	20	400	250
Ex. 8	1	2.6	1	230	0	230	250
Ex. 9	2	4.0	4	120	40	600	250
Ex. 10	1.8	3.6	3	130	40	470	250
Ex. 11	1.2	2.6	1	270	0	270	250
Ex. 12	1.85	3.6	3	130	40	470	250
Ex. 13	1.8	3.6	3	130	40	470	250
Ex. 14	1.2	2.6	1	270	0	270	250
Ex. 15	1.4	3.6	3	120	20	400	250
Ex. 16	1.2	2.6	1	270	0	270	250
Ex. 17	1.85	3.6	3	130	40	470	250
Ex. 18	1	2.6	1	230	0	230	250
Ex. 19	1.4	3.6	3	120	20	400	250
Ex. 20	1.85	3.6	3	130	40	470	250
Ex. 21	1.6	3.6	3	130	40	470	250
Ex. 22	1.4	3.6	3	120	20	400	250
Ex. 23	1.2	2.6	1	270	0	270	250
Ex. 24	1.2	2.6	1	270	0	270	250
Ex. 25	1.4	3.6	3	120	20	400	250
Ex. 26	1.6	3.6	3	130	40	470	250
Ex. 27	1.2	2.6	1	270	0	270	250
Ex. 28	1.4	3.6	3	120	20	400	250
CEx. 1	1.6	3.6	3	130	40	470	250
CEx. 2	1.4	3.6	3	120	20	400	250
CEx. 3	1.5	3.6	3	120	20	400	250
CEx. 4	1.85	3.6	3	130	40	470	250

In the actual spot-welding process, each of tensile shear (TSS) specimen, cross tensile (CTS) specimen, metallographic specimen was welded under each welding current, TSS, CTS can correspond to the joint bearing capacity of the sample after spot welding of the final product of the steel plate of each Example and Comparative Example measured by a tensile testing machine according to ISO 14273-2016 and ISO 14272-2016 standards. The results are shown in Table 6 below.
Correspondingly, for the obtained metallographic samples, the coating on the surface of the joint was removed with dilute hydrochloric acid first, and the distribution and direction of spot-welding cracks were observed under the microscope. The cross-section that passes through the nugget center where the most spot-welding cracks could be cut was selected as the metallographic section of the joint, and the cross-section was sampled by wire cutting, wherein the cross-section included all the welding characteristic areas of the spot-welding joint. The surface of the intercepted sample was washed to prevent the interference of foreign matter on the test results, and the washed sample was dried. The dried sample was mounted, ground and polished, and measured under a metallographic microscope, and the maximum crack length was filled in Table 6.

Table 6 lists the mechanical properties of the solder joints and the test results of the cracks of solder joints for the ultra-high-strength steel having a coating in Examples 1-28 and Comparative Examples 1-4 after spot welding.

Table 6.

No.	Steel plate thickne ss (mm)	Nominal weight of the coating (g/m²)	Coating type	Tensile shear force of the joint (kN)	Cross tensile force of the joint (kN)	No welding splash		Welding splash
No.	Steel plate thickne ss (mm)	Nominal weight of the coating (g/m²)	Coating type	Tensile shear force of the joint (kN)	Cross tensile force of the joint (kN)	Maximum crack length (µm)	Maximum crack length /steel plate thickness (%)	Maximum crack length (µm)	Maximum crack length /steel plate thickness (%)
Ex. 1	1.3	71/72	GI	≥14	≥10.5	0	0	34.0	2.62
Ex. 2	1.4	87/85	GI	≥15	≥11	0	0	26.8	1.91
Ex. 3	1.5	75/75	GI	≥16.5	≥11	0	0	69.0	4.60
Ex. 4	1.5	67/63	GI	≥16.5	≥11	0	0	30.8	2.05
Ex. 5	1.2	56/57	GA	≥12	≥8.2	0	0	25.7	2.14
Ex. 6	2	76/77	GI	≥24	≥13.8	0	0	40.7	2.04
Ex. 7	1.3	70/69	GI	≥14	≥10.5	0	0	26.1	2.01
Ex. 8	1	62/66	GI	≥10	≥7.8	0	0	39.3	3.93
Ex. 9	2	72/73	GI	≥24	≥13.8	0	0	5.4	0.27
Ex. 10	1.8	74/76	GI	≥21	≥13.8	0	0	4.4	0.24
Ex. 11	1.2	68/69	GI	≥12	≥8.2	0	0	24.4	2.03
Ex. 12	1.85	76/77	GI	≥22	≥13.8	0	0	38.6	2.09
Ex. 13	1.8	86/86	GI	≥21	≥13.8	0	0	4.2	0.23
Ex. 14	1.2	74/74	GI	≥12	≥8.2	0	0	47.4	3.95
Ex. 15	1.4	70/71	GI	≥15	≥11	0	0	46.1	3.29
Ex. 16	1.2	73/73	GI	≥12	≥8.2	0	0	33.3	2.78
Ex. 17	1.85	65/67	GI	≥22	≥13.8	0	0	0	0
Ex. 18	1	72/73	GI	≥12	≥7.8	0	0	34.4	3.44
Ex. 19	1.4	65/67	GI	≥15	≥11	0	0	0	0
Ex. 20	1.85	68/68	GI	≥22	≥13.8	0	0	33	1.78
Ex. 21	1.6	35/36	ZM	≥18	≥11.8	0	0	35.3	2.21
Ex. 22	1.4	68/69	GI	≥15	≥11	0	0	0	0
Ex. 23	1.2	40/40	ZM	≥12	≥8.2	0	0	29.8	2.48
Ex. 24	1.2	75/73	GI	≥12	≥8.2	0	0	0	0
Ex. 25	1.4	45/46	GA	≥15	≥11	0	0	66.4	4.74
Ex. 26	1.6	73/70	ZM	≥18	≥11.8	0	0	36.3	2.27
Ex. 27	1.2	71/73	AZ	≥12	≥8.2	0	0	54.1	4.51
Ex. 28	1.4	100/100	AZ	≥15	≥11	0	0	54.7	3.91
CEx. 1	1.6	73/74	GI	≥18	≥11.8	59.4	3.71	301.1	18.82
CEx. 2	1.4	70/72	GI	≥15	≥11	49.7	3.55	246.8	17.63
CEx. 3	1.5	78/79	GI	≥16.5	≥11	61.8	4.12	289.8	19.32
CEx. 4	1.85	72/73	GI	≥22	≥13.8	84.0	4.54	336.9	18.21

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

As can be seen from the above Table 3 of the present disclosure, the microstructure of the substrate of the steel designed by Examples 1-28 and Comparative Examples 1-4 of the present disclosure are: ferrite + martensite + residual austenite. Moreover, the microstructure of the substrate of each Example and Comparative Example satisfies the following indexes: the volume fraction of ferrite is 25%~45%, and the volume fraction of martensite is 45%~65%; wherein the volume of ferrite grains having a size of 10 µm or less accounts for ≥85%, and the volume of ferrite grains having a size of 5 µm or less accounts for ≥55%, the average grain size of the residual austenite was ≤2 µm, and the average C content in the residual austenite was ≥ 1.0%.
As can be seen from the above Table 4, the substrate of the steel of Examples 1-28 of the present disclosure and Comparative Examples 1-4 had very excellent mechanical properties, and the mechanical properties all met the design requirements of the present disclosure, and the substrate of the ultra-high-strength steel having a coating of Examples 1-28 had a yield strength of 755~845MPa, a tensile strength of 1040~1140MPa, an uniform elongation of 14.7-16.4%, and an elongation at break of 22.6-25.4%.
At the same time, it can be seen from Table 6 that for the final ultra-high-strength steel having a coating in Examples 1-28 in the present disclosure, when the welding current is less than the current when splash occurs, there is no spot-welding crack in the welded solder joint; when the welding current is larger than the current when splash occurs, the longest crack of the welded solder joint is less than 5% of the plate thickness. For the comparative steel plate designed in the present disclosure, regardless of whether the welding current is greater than the current when splash occurs, the welded solder joints will have cracks, and the cracks of the solder joints with welding splash are more serious than those of the solder joints without welding splash, and the ratio of the maximum crack length to the plate thickness is much greater than 5%. Thus, it shows that the ultra-high-strength steel having a coating in each Example of Examples 1-28 designed by adopting the technical solution of the present disclosure has excellent low spot-welding crack sensitivity while ensuring the performance of the plate.
It should be noted that combinations of the various technical features in this case are not limited to the combinations disclosed in the claims of this case or the combinations disclosed in the specific Examples. All technical features disclosed in this case can be combined freely or associated in any way unless a contradiction occurs.
It should also be noted that the Examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above Examples, and changes or modifications made thereto can be directly derived from the present disclosure or easily conceived of by those skilled in the art, all of which fall within the protection scope of the present disclosure.

Claims

An ultra-high-strength steel having a coating with low spot-welding crack sensitivity, which comprises a steel substrate and a zinc-containing coating on the surface of the steel substrate, wherein the steel substrate comprises Fe and unavoidable impurity elements, and the steel substrate further comprises the following chemical elements in a mass percentage:
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 ultra-high-strength steel having a coating of claim 1, wherein the steel substrate comprises each chemical element in a mass percentage as follows:
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%; with a balance of Fe and unavoidable impurity elements.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein the mass percentage 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%.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein the chemical element of the steel substrate further comprises Mo: 0.10~2.00%; preferably, the mass percentage of Mo is 0.15~2.00%.
The ultra-high-strength steel having a coating of claim 4, wherein the mass percentage of Mo of the steel substrate is 0.10~1.00%; preferably, the mass percentage of Mo is 0.15~1.00%.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein the mass percentage of the unavoidable impurity elements of the steel substrate satisfies: P≤0.01%, S≤0.005%.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein the microstructure of the steel substrate is ferrite + martensite + residual austenite.
The ultra-high-strength steel having a coating of claim 7, wherein the volume fraction of ferrite is 25%~45%; and/or the volume fraction of martensite is 45%~65%.
The ultra-high-strength steel having a coating of claim 7, wherein in ferrite, the volume of grains having a size of 10 µm or less accounts for ≥85%, and the volume of grains having a size of 5 µm or less accounts for ≥55%.
The ultra-high-strength steel having a coating of claim 7, wherein the residual austenite has an average grain size of ≤ 2 µm; and/or the residual austenite has an average C content of ≥1.0%.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein when spot welding is adopted for welding, if cracks on the surface of solder joint are generated, the maximum length of the cracks on the surface of solder joint is less than 5% of the plate thickness.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein the mechanical properties satisfy: a yield strength of 600MPa~850MPa, a tensile strength of 980MPa~1150MPa, a uniform elongation of no less than 13%, and an elongation at break of no less than 15%.
The ultra-high-strength steel having a coating of claim 1 or 2, wherein the coating is a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating or an aluminum-zinc coating.
A manufacturing method for the ultra-high-strength steel having a coating according to any one of claims 1-13, which comprises steps of:
(1) smelting and thin slab continuous casting;

(2) heating;

(3) hot-rolling: wherein the thickness of oxide scale on the surface of the strip steel after hot rolling is controlled at ≤4µm, and the mass percentage of FeO+ Fe₃O₄ in the oxide scale on the surface of the strip steel after hot rolling is ≤50wt%;

(4) pickling, or pickling + cold rolling;

(5) continuous annealing: wherein the steel is annealed at 800~920 °C, then slowly cooled to 700~770 °C at a cooling rate of 3~10 °C/s; then rapidly cooled to 200~300°C with a cooling rate of 50~500°C/s; then re-heated to 360~460°C, held for 50~600s; and finally cooled to room temperature;

(6) plating a zinc-containing coating.
The manufacturing method according to claim 14, wherein in step (1), the slab thickness at the outlet of thin strip continuous casting is controlled at 50~58mm.
The manufacturing method according to claim 14, wherein in step (1), the drawing speed of thin strip continuous casting is controlled at 2~5m/min.
The manufacturing method according to claim 14, wherein in step (2), the slab is heated to 1200~1250°C.
The manufacturing method according to claim 14, wherein in step (3), the rolling-end temperature is controlled at 860~930°C, the coiling temperature is controlled at 450~600°C.
The manufacturing method according to claim 14, wherein in step (4), when pickling + cold rolling is adopted, the cold rolling deformation rate is controlled at 40%~60%.
The manufacturing method according to claim 14, wherein in step (5), the volume content of hydrogen in the reducing atmosphere in the continuous annealing furnace is controlled at 10~15%.
The manufacturing method according to claim 14, wherein the annealing process parameters of step (5) satisfy at least one of the following items:
an annealing temperature of 820~870°C;

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

rapidly cooled to 250~300°C;

re-heated to 400~430°C after rapid cooling, held for 180~300s.