780 mpa-class cold-rolled and annealed dual-phase steel and manufacturing method therefor

Abstract

Disclosed is a cold-rolled and annealed dual-phase steel having a tensile strength of greater than 780 MPa. A matrix structure thereof is fine and uniform martensite+ferrite. The cold-rolled and annealed dual-phase steel contains the following chemical elements in the following mass percentages: C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, and Ti: 0.01-0.03%. Furthermore, the cold-rolled annealed dual-phase steel does not contain the elements Cr or Mo. In addition, also disclosed is a method for manufacturing the cold-rolled and annealed dual-phase steel, comprising smelting and continuous casting, hot rolling, cold rolling, annealing, tempering and flattening. The cold-rolled and annealed dual-phase steel of the present invention is not only economical, but also has the characteristics of high strength, excellent elongation and cold bending properties.

1 . A cold-rolled and annealed dual-phase steel having a tensile strength of >780 MPa, wherein the cold-rolled and annealed dual-phase steel has a matrix structure of fine and uniform martensite+ferrite, wherein the cold-rolled and annealed dual-phase steel comprises the following chemical elements in mass percentages, in addition to Fe:
 C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%;   wherein the cold-rolled and annealed dual-phase steel is free of Cr and Mo elements.   
     
     
         2 . The cold-rolled and annealed dual-phase steel according to  claim 1 , wherein the chemical elements have the following mass percentages:
 C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and a balance of Fe and other unavoidable impurities.   
     
     
         3 . The cold-rolled and annealed dual-phase steel according to  claim 1 , wherein the chemical elements have mass percentage contents satisfying at least one of the following:
 C: 0.11-0.125%,   Si: 0.5%-0.7%,   Mn: 1.7%-1.8%,   Al: 0.015%-0.045%,   Nb: 0.015-0.025%,   Ti: 0.015-0.025%.   
     
     
         4 . The cold-rolled and annealed dual-phase steel according to  claim 2 , wherein the unavoidable impurities include P, S and N elements, and contents thereof are controlled to be at least one of the following: P≤0.015%, S≤0.003%, N≤0.005%. 
     
     
         5 . The cold-rolled and annealed dual-phase steel according to  claim 1 , wherein mass percentage contents of Nb and Ti further satisfy: Nb %+Ti %×3≥0.047%. 
     
     
         6 . The cold-rolled and annealed dual-phase steel according to  claim 1 , wherein the martensite has a phase proportion of >55%. 
     
     
         7 . The cold-rolled and annealed dual-phase steel according to  claim 1 , wherein the martensite has a grain diameter of not greater than 5 microns, and ferrite has a grain diameter of not greater than 5 microns. 
     
     
         8 . The cold-rolled and annealed dual-phase steel according to  claim 1 , wherein its performances satisfy at least one of the following: yield strength>420 MPa; tensile strength>780 MPa; elongation at break with A 50  gauge length 18%; 90 degree cold bending parameter R/t≤1, where R represents bending radius in mm, t represents plate thickness in mm. 
     
     
         9 . The cold-rolled and annealed dual-phase steel according to  claim 5 , wherein the mass percentage contents of Nb and Ti further satisfy: 0.047%≤Nb %+Ti %×3≤0.10%. 
     
     
         10 . A manufacturing method for the cold-rolled and annealed dual-phase steel according to  claim 1 , wherein the method comprises steps of:
 (1) Smelting and continuous casting;   (2) Hot rolling;   (3) Cold rolling;   (4) Annealing: annealing soaking temperature: 770-820° C.; annealing time: 40-200 s; cooling at a rate of 3-5° C./s to a starting temperature of rapid cooling; rapid cooling at a rate of 30-80° C./s, wherein the starting temperature of rapid cooling is 650-730° C., and the rapid cooling is ended at a temperature of 200-270° C.;   (5) Tempering; and   (6) Temper rolling.   
     
     
         11 . The manufacturing method according to  claim 10 , wherein in step (2), a slab is first heated to 1160-1220° C.; held for 0.6 hours or longer; hot rolled at a temperature of 850-900° C.; rapidly cooled at a rate of 30-80° C./s after rolling; coiled with the coiling temperature being controlled at 500-600° C.; and air cooled after the coiling. 
     
     
         12 . The manufacturing method according to  claim 10 , wherein in step (3), a cold rolling reduction rate is controlled at 50-70%. 
     
     
         13 . The manufacturing method according to  claim 10 , wherein in step (5), a tempering temperature is controlled at 200-270° C., and a tempering time is 100-400 s. 
     
     
         14 . The manufacturing method according to  claim 10 , wherein in step (6), a temper rolling reduction rate is controlled at 50.3%. 
     
     
         15 . The manufacturing method according to  claim 10 , wherein in step (4), the annealing soaking temperature is 790-810° C. 
     
     
         16 . The cold-rolled and annealed dual-phase steel according to  claim 2 , wherein the chemical elements have mass percentage contents satisfying at least one of the following:
 C: 0.11-0.125%,   Si: 0.5%-0.7%,   Mn: 1.7%-1.8%,   Al: 0.015%-0.045%,   Nb: 0.015-0.025%, and   Ti: 0.015-0.025%.   
     
     
         17 . The cold-rolled and annealed dual-phase steel according to  claim 2 , wherein mass percentage contents of Nb and Ti further satisfy: Nb %+Ti %×3≥0.047%. 
     
     
         18 . The cold-rolled and annealed dual-phase steel according to  claim 2 , wherein: the martensite has a phase proportion of >55%; and/or, the martensite has a grain diameter of not greater than 5 microns, and ferrite has a grain diameter of not greater than 5 microns; and/or its performances satisfy at least one of the following: yield strength≥420 MPa; tensile strength>780 MPa; elongation at break with A 50  gauge length 18%; 90 degree cold bending parameter R/t≤1, where R represents bending radius in mm, t represents plate thickness in mm. 
     
     
         19 . The manufacturing method according to  claim 10 , wherein the chemical elements of the cold-rolled and annealed dual-phase steel have the following mass percentages:
 C: 0.1%-0.13%, Si: 0.4%-0.8%, Mn: 1.65%-1.9%, Al: 0.01%-0.05%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and a balance of Fe and other unavoidable impurities.   
     
     
         20 . The manufacturing method according to  claim 19 , wherein the chemical elements of the cold-rolled and annealed dual-phase steel have mass percentage contents satisfying at least one of the following:
 C: 0.11-0.125%,   Si: 0.5%-0.7%,   Mn: 1.7%-1.8%,   Al: 0.015%-0.045%,   Nb: 0.015-0.025%, and   Ti: 0.015-0.025%.