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

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%.

1 . 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%.   
     
     
         2 . 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.   
     
     
         3 . The ultra-high-strength steel having a coating of  claim 1 , 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%.   
     
     
         4 . The ultra-high-strength steel having a coating of  claim 1 , wherein the chemical element of the steel substrate further comprises Mo: 0.10˜2.00%; or, the mass percentage of Mo is 0.15˜2.00%. 
     
     
         5 . 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%; or, the mass percentage of Mo is 0.15˜1.00%. 
     
     
         6 . The ultra-high-strength steel having a coating of  claim 1 , wherein the mass percentage of the unavoidable impurity elements of the steel substrate satisfies: P≤0.01%, S≤0.005%. 
     
     
         7 . The ultra-high-strength steel having a coating of  claim 1 , wherein the microstructure of the steel substrate is ferrite+martensite+residual austenite. 
     
     
         8 . 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%. 
     
     
         9 . 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%. 
     
     
         10 . 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%. 
     
     
         11 . The ultra-high-strength steel having a coating of  claim 1 , 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. 
     
     
         12 . The ultra-high-strength steel having a coating of  claim 1 , wherein the mechanical properties satisfy: a yield strength of 600 MPa˜850 MPa, a tensile strength of 980 MPa˜1150 MPa, a uniform elongation of no less than 13%, and an elongation at break of no less than 15%. 
     
     
         13 . The ultra-high-strength steel having a coating of  claim 1 , wherein the coating is a pure zinc coating, a zinc-iron alloy coating, a zinc-aluminum-magnesium coating or an aluminum-zinc coating. 
     
     
         14 . A manufacturing method for the ultra-high-strength steel having a coating according to  claim 1 , 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 ≤4 μm, and the mass percentage of FeO+Fe 3 O 4  in the oxide scale on the surface of the strip steel after hot rolling is ≤50 wt %;   (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˜600 s; and finally cooled to room temperature;   (6) plating a zinc-containing coating.   
     
     
         15 . 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˜58 mm, and/or the drawing speed of thin strip continuous casting is controlled at 2˜5 m/min. 
     
     
         16 . (canceled) 
     
     
         17 . The manufacturing method according to  claim 14 , wherein in step (2), the slab is heated to 1200˜1250° C. 
     
     
         18 . 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. 
     
     
         19 . 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%. 
     
     
         20 . 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%. 
     
     
         21 . 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˜300 s.