Cold rolled steel sheet

Abstract

Provided is a steel sheet having: ferrite and pearlite composing 80% or more, in area fraction, of the microstructure; yield strength of 60 ksi or more; elongation of at least 23%; an n-value of at least 0.14; incidental impurities; and, in weight percent: C: 0.03˜0.10 Si: 0˜0.6 Mn: 0.5˜1.5 Cu: 0˜1.0 Ni: 0˜1.0 Nb: 0˜0.06 Ti: 0˜0.1 Mo: 0˜0.5% Cr: 0˜1.0 Al: 0˜0.06 N: 0.0001˜0.006 Ca: 0˜0.006 P: 0˜0.02 S: 0˜0.005.

Claims

1. A cold rolled high strength steel sheet comprising, in weight percent: C: 0.03 to 0.10 Si: 0 to 0.6 Mn: 0.5 to 1.5 Cu: 0 to 1.0 Ni: 0 to 1.0 Nb: 0 to 0.06 Ti: 0 to 0.1 Mo: 0 to 0.5 Cr: 0 to 1.0 Al: 0 to 0.06 N: 0.0001 to 0.006 Ca: 0 to 0.006 P: 0 to 0.02 S: 0 to 0.005 and further comprising iron and incidental impurities, wherein the sheet has ferrite and pearlite composing 80% or more, in area fraction, of the microstructure has yield strength of 60 ksi or more, elongation of at least 23%, and an n-value of at least 0.14.

2. The steel sheet according to claim 1, wherein the weight percent of Mo is no more than 0.060.

3. The steel sheet according to claim 1, wherein pearlite content is greater than 0% and 20% or less based on the area fraction.

4. The steel sheet according to claim 2, wherein pearlite content is greater than 0% and 20% or less based on the area fraction.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: illustrates Nital Etch-Middle microstructure of a sheet material.

(2) FIG. 2: illustrates Nital Etch-Middle microstructure of a sheet material of a finished product according to some aspects as provided herein.

(3) FIG. 3: illustrates L-orientation YS Capability of a sheet material of a finished product according to some aspects as provided herein.

(4) FIG. 4: illustrates L-orientation TS Capability of a sheet material of a finished product according to some aspects as provided herein.

(5) FIG. 5: illustrates L-orientation Elongation Capability of a sheet material of a finished product according to some aspects as provided herein.

(6) FIG. 6: illustrates L-orientation n value Capability of a sheet material of a finished product according to some aspects as provided herein.

(7) FIG. 7: illustrates L-orientation Yield Strength vs. Gauge of a sheet material of a finished product according to some aspects as provided herein.

(8) FIG. 8: illustrates process parameters of a sheet material of a finished product according to some aspects as provided herein.

DESCRIPTION

(9) Non-limiting embodiments of the invention are hereinafter described.

(10) Sheet Material

(11) Sheet material according to a non-limiting embodiment of the invention comprises, in weight percent:

(12) TABLE-US-00002 C: 0.03~0.10 Ca: 0~0.006 Si: 0~0.6 P: 0~0.02 Mn: 0.5~1.5 S: 0~0.005 Cu: 0~1.0 Ni: 0~1.0 Nb: 0 to 0.06 Ti: 0 to 0.1 Mo: 0 to 0.06 Cr: 0~1.0 Al: 0~0.06

(13) N: 0.0001˜0.006

(14) and further comprising iron and incidental impurities, wherein the sheet

(15) has ferrite and pearlite composing 80% or more, in area fraction, of the microstructure has yield strength of 60 ksi or more elongation of at least 23% has an n-value of at least 0.14. has pearlite content no more than 20% based on area fraction.
Method

(16) The method is for use with a steel slab having, in weight percent: C: 0.03˜0.10

(17) Si: 0˜0.6

(18) Mn: 0.5˜1.5

(19) Cu: 0˜1.0

(20) Ni: 0˜1.0

(21) Nb: 0 to 0.06

(22) Ti: 0 to 0.1

(23) Mo: 0 to 0.06

(24) Cr: 0˜1.0

(25) Al: 0˜0.06

(26) Ca: 0˜0.006

(27) P: 0˜0.02

(28) S: 0˜0.005

(29) N: 0.0001˜0.006

(30) and further comprising iron and incidental impurities, the method comprising:

(31) heating the slab to a temperature of 1050° C. to 1150° C. to produce a heated slab; rolling the heated slab once at a rolling reduction rate of 20% to 80% in a temperature range above the austenite recrystallization temperature to produce a rolled slab; rolling the rolled slab two or more times at a rolling reduction ratio of 40% to 80% in a temperature range below austenite recrystallization temperature and above Ar3 to produce a rolled sheet; cooling the rolled sheet at a cooling rate of 20° C. to 50° C./sec to produce a cooled sheet; hot rolling the cooled sheet at a temperature of 300° C. to 690° C. to produce a hot rolled sheet; cold-working the hot rolled sheet at a reduction rate of 30% to 80% to produce a cold rolled sheet; and annealing the cold rolled sheet for 10 hours or more at 1300° F. or more.

Experimental

(32) Liquid metal having the following composition

(33) C: 0.03˜0.10

(34) Si: 0˜0.6

(35) Mn: 0.5 ˜1.5

(36) Cu: 0˜1.0

(37) Ni: 0˜1.0

(38) Nb: 0 to 0.06

(39) Ti: 0 to 0.1

(40) Mo: 0 to 0.06%

(41) Cr: 0˜1.0

(42) Al: 0˜0.06

(43) Ca: 0˜0.006

(44) P: 0˜.02

(45) S: 0˜0.005

(46) N: 0.0001 ˜0.006

(47) and further comprising iron and incidental impurities, was cast into 78 millimeter (mm) slab at Algoma Steel, Ontario. The slab was cleaned by pickling to remove the oxide layer, then used in the following method to produce fifty two (52) coils. Briefly, the slab is heated to a temperature of 1050° C. to 1150° C. to produce a heated slab; the heated slab if rolled once at a rolling reduction rate of 20% to 80% in a temperature range above the austenite recrystallization temperature to produce a rolled slab; the rolled slab is rolled two or more times at a rolling reduction ratio of 40% to 80% in a temperature range below austenite recrystallization temperature and above Ar3 to produce a rolled sheet; the rolled sheet is cooled at a cooling rate of 20° C. to 50° C. / sec to produce a cooled sheet; the cooled sheet is hot rolled at a temperature of 300° C. to 690° C. to produce a hot rolled sheet; the hot rolled sheet is cold-worked at a reduction rate of 30% to 80% to produce a cold rolled sheet; and the cold rolled sheet is annealed for 10 hours or more at 1300° F. or more.

(48) Each coil was tested for coil chemistry, and the process parameters were monitored during production; details of the same are provided in FIG. 8.

(49) Micros were cut in the longitudinal direction of the rolled slab [after cold reduction, preceding annealing] and mounted, ground, polished and Nital etched to reveal the microstructures, as shown in FIG. 1. The elongated grains visible in FIG. 1 clearly demonstrate that large amount of permanent deformation induced microstructure of full hard strip.

(50) The finished product was also inspected after Nital etching as shown in FIG. 2. The fully recovered and recrystallized uniaxial grains and partially recovered grains at centerline clearly demonstrate that ideal microstructure of post annealing.

(51) The fifty two (52) rolls produced were tested for mechanical properties against SAE J2340 420X; the results are provided in FIGS. 3-7. Persons of ordinary skill will appreciate that, surprisingly, notwithstanding the relatively low amounts of Niobium, the coils meet the standard.

(52) Whereas two specific embodiments are herein shown and described, persons of ordinary skill will readily appreciate that variations are possible. Accordingly, the invention should be understood to be limited only by the accompanying claims, purposively construed.