Multiphase, cold-rolled ultra-high strength steel

Abstract

A cold-rolled ultra-high strength, multi-phase structured steel is disclosed with improved stamping and forming performance during manufacturing processes, while possessing one or more of the following properties: excellent castability and rollability, excellent galvanizability and/or coatability, excellent formability, excellent fracture resistance, excellent stretch formability and stretch flangeability, excellent dent resistance, excellent durability, excellent impact performance and structural performance, excellent intrusion and crash resistance, and excellent weldability, without the purposeful addition of boron.

1 . A galvannealed cold rolled, complex metallographic structured steel comprising:
 a composition comprising the following elements by weight:   carbon in a range from 0.02% to 0.2%,   manganese in a range from 1.0% to 3.5%,   phosphorous less than or equal to 0.1%,   silicon less than or equal to 1.2%,   aluminum in a range from 0.01% to less than 0.10%,   nitrogen less than or equal to 0.02%,   copper less than or equal to 0.6%,   vanadium less than or equal to 0.12%,   the composition having no purposeful addition of boron,   and the balance of the composition comprising iron and incidental ingredients   a multi-phase microstructure consisting of in combination, martensite, bainite, and ferrite, martensite between 35% and 65% by volume, bainite between 30% and 45% by volume, the remainder volume being essentially ferrite with ferrite present, and essentially no retained austenite.   
     
     
         2 . The cold rolled, complex metallographic structured steel of  claim 1 , further comprising:
 at least one chemical element chosen from molybdenum, chromium, nickel, and a combination thereof, in a range between about 0.05% and about 3.5%; or, molybdenum (Mo) present with chromium (Cr) satisfying a relationship Mo+Cr greater than or equal to 0.05% and less than or equal to 2.0%; or,   nickel (Ni) present with copper (Cu) satisfying a relationship Ni+Cu less than or equal to 0.8%.   
     
     
         3 . The cold rolled, complex metallographic structured steel of  claim 1 , further comprising at least one chemical element chosen from titanium, niobium and a combination thereof, in a range between 0.005% and 0.8%. 
     
     
         4 . The cold rolled, complex metallographic structured steel of  claim 1 , wherein tensile strength is greater than 1000 megapascals. 
     
     
         5 . The cold rolled, complex metallographic structured steel of  claim 1 , comprising at least one of the following properties: elongation greater than 10% in accordance with ASTM E8; and yield/tensile ratio greater than 60%. 
     
     
         6 . The cold rolled, complex metallographic structured steel of  claim 1 , wherein tensile strength is greater than 1000 megapascals, elongation is greater than 10%, and yield/tensile ratio is greater than 60%. 
     
     
         7 . The cold rolled, complex metallographic structured steel of  claim 1 , comprising, after continuously lap welding, stitch welding, gas metal arc welding, or laser beam welding the steel, a weld with weldability properties, of absent pores, shrinkage voids, surface cracks, splash, spatter and undercut. 
     
     
         8 . The cold rolled, complex metallographic structured steel of  claim 1 , wherein the martensite is uniformly distributed fine hard martensite islands/particles. 
     
     
         9 . A galvannealed cold rolled, complex metallographic structured steel comprising:
 a composition comprising the following elements by weight:   carbon in a range from 0.02% to 0.1%,   manganese in a range from 1.0% to 3.5%,   phosphorous less than or equal to 0.1%,   silicon less than or equal to 1.2%,   aluminum in a range from 0.01% to less than 0.10%,   nitrogen less than or equal to 0.02%,   copper less than or equal to 0.6%,   vanadium less than or equal to 0.12%,   the composition having no purposeful addition of boron,   and the balance of the composition comprising iron and incidental ingredients;   a multi-phase microstructure consisting of in combination, martensite, bainite, and ferrite, martensite between 30% and 70% by volume, bainite between 25% and 50% by volume, the remainder volume being essentially ferrite with ferrite present, and essentially no retained austenite.   
     
     
         10 . The cold rolled, complex metallographic structured steel of  claim 9 , wherein the martensite is uniformly distributed fine hard martensite islands/particles. 
     
     
         11 . The cold rolled, complex metallographic structured steel of  claim 9 , comprising, after continuously lap welding, stitch welding, gas metal arc welding, or laser beam welding the steel, a weld with weldability properties, of absent pores, shrinkage voids, surface cracks, splash, spatter and undercut. 
     
     
         12 . A method of making a complex metallographic structured cold rolled steel, the method comprising:
 a) continuously casting a molten steel into a slab, the molten steel having a composition comprising the following elements by weight:   carbon in a range from about 0.02% to about 0.2%,   manganese in a range from about 1.0% to about 3.5%,   phosphorous less than or equal to about 0.1%,   silicon less than or equal to about 1.2%,   aluminum in a range from about 0.01% to about 0.10%,   nitrogen less than or equal to about 0.02%,   copper less than or equal to about 0.5%,   vanadium less than or equal to about 0.12%,   the composition having no purposeful addition of boron,   and the balance of the composition comprising iron and incidental ingredients;   b) hot rolling the steel slab to form a sheet, wherein the steel sheet has a finishing exit temperature in a range between (Ar3-30) ° C. and 1000° C. (1832° F.);   c) cooling the sheet or the band at a mean cooling rate of at least 3° C./s (5.4° F./s) with coiling of the sheet or the band at a temperature between 425° C. and 825° C. and subsequently, optionally pickling; and   d) cold rolling the sheet or the band to a total cold thickness reduction greater than 30%; and   e) galvanizing and/or galvannealing a metal alloy coating on the sheet or band of step d);   wherein the cold rolled sheet of step e) has a multiphase microstructure consisting of, in combination, martensite, bainite, and ferrite, martensite between of 35% and 65% by volume, bainite between of 30% and 45% by volume, the remainder volume being essentially ferrite with essentially no retained austenite and a tensile strength greater than 1000 megapascals.   
     
     
         13 . The method of  claim 12 , wherein the chemical composition comprises at least one chemical element chosen from molybdenum, chromium, nickel, or a combination thereof, in a range between about 0.05% by weight and about 3.5% by weight, wherein, if present, molybdenum (Mo) is present with chromium (Cr) satisfying a relationship Mo+Cr greater than or equal to about 0.05% and less than or equal to about 2.0%, and, wherein, if present, nickel (Ni) is present with copper (Cu) satisfying a relationship Ni+Cu being less than or equal to about 0.8%, 
     
     
         14 . The method of  claim 12 , wherein the chemical composition comprises at least one chemical element chosen from titanium, niobium and a combination thereof, in a range between about 0.005% and about 0.8%. 
     
     
         15 . The method of  claim 12 , further comprising coiling the steel at a temperature between about 400° C. (about 752° F.) and about 800° C. (about 1472° F.) 
     
     
         16 . The method of  claim 12 , wherein the steel slab has an exit temperature in a range between about (Ar3-30)° C. and about 1000° C. (about 1832° F.) prior to hot rolling. 
     
     
         17 . The method of  claim 12 , wherein the steel slab is cooled at a mean cooling rate of at least about 3° C./s (about 37.4° F./s). 
     
     
         18 . An article made by the method of  claim 12 . 
     
     
         19 . The method of  claim 12 , further comprising:
 continuously lap welding, stitch welding, gas metal arc welding, or laser beam welding; and   providing a weld absent pores, shrinkage voids, surface cracks, splash, spatter and undercut.