Hot rolled coated steel sheet having high strength, high formability, excellent bake hardenability and method of manufacturing same

Patent metadata

Field	Value
Publication number	US-11834727-B2
Application number	US-202318103718-A
Country	US
Kind code	B2
Filing date	Jan 31, 2023
Priority date	Jul 12, 2018
Publication date	Dec 5, 2023
Grant date	Dec 5, 2023

Abstract

The hot-rolled coated steel sheet comprising: in wt %, C: 0.05-0.14%, Si: 0.1-1.0%, Mn: 1.0-2.0%, P: 0.001-0.05%, S: 0.001-0.01%, AI: 0.01-0.1%, Cr: 0.005-1.0%, Ti: 0.005-0.13%, Nb: 0.005-0.03%, N: 0.001-0.01%, Fe residues, and other inevitable impurities; a mixed structure of ferrite and bainite as a main phase; and as a remaining structure, one or more selected from the group consisting of martensite, austenite, and phase martensite (MA), wherein a fraction of the ferrite and bainite is 95-99 area % and Equation 1 is satisfied. [Equation 1] FCO {110}<112> +FCO {112}<111> ≥10 where, FCO {110}<112> and FCO {112}<111> , each representing an area fraction occupied by a structure having ac crystal orientation of {110}<112> and {112}<111>.

First claim

The invention claimed is:

1. A method of manufacturing a hot-rolled coated steel sheet, comprising operations of:
reheating a steel slab including in wt %, C: 0.05 to 0.14%, Si: 0.1 to 1.0%, Mn: 1.0 to 2.0%, P: 0.001 to 0.05%, S: 0.001 to 0.01%, Al: 0.01 to 0.1%, Cr: 0.005 to 1.0%, Ti: 0.005 to 0.13%, Nb: 0.005 to 0.03%, N: 0.001 to 0.01%, and a balance of Fe and inevitable impurities,
hot rolling the reheated steel slab at a temperature of Ar3 or higher to 1000° C. to obtain a hot-rolled steel sheet;
primary cooling the hot-rolled steel sheet to a temperature of 550 to 700° C.;

ultra-slow cooling the primary cooled hot-rolled steel sheet to satisfy the following Equation 2;
secondary cooling the ultra-slow cooled hot-rolled steel sheet to a temperature of 300 to 500° C., and then winding the same;
charging the wound hot-rolled steel sheet into a heating table of 350 to 550° C., heating the hot-rolled steel sheet, and then extracting the same;
introducing the heated hot-rolled steel sheet into a hot-dip plating bath of 450 to 550° C. to form a plating layer on a surface of the hot-rolled steel sheet; and
temper rolling the hot-rolled steel sheet on which the plating layer is formed,
wherein the following Equation 3 is satisfied in the heating operation and the temper rolling operation,
| T−T R |≤2

T R =241+109[C]+16.9[Mn]+22.7[Cr]−11.1[Si]−5.4[Al]−0.87Temp+0.00068Temp 2   [Equation 2]

where T is an actual ultra-slow cooling time, T R is a theoretical ultra-slow cooling time, Temp is an intermediate temperature during ultra-slow cooling, and the [C], [Mn], [Cr], [Si], and [Al] is a content of each alloy element,
15≤(1000− TH )× El SPM ≤250  [Equation 3]

T H is an average temperature of a charging temperature of a heating table of the hot-rolled steel sheet before being charged into a hot-dip plating bath and an extraction temperature of the same, and El SPM is a difference in length of the hot-rolled steel sheet before temper rolling and immediately after temper rolling.

2. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein a reheating temperature of the steel slab is 1180 to 1300° C.

3. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein a cooling rate is 20° C./sec or higher, during the first cooling.

4. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein an ultra-slow cooling rate is 2.0° C./sec or lower, during the ultra-slow cooling.

5. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein an ultra-slow cooling maintaining time is 10 seconds or lower, excluding 0 seconds, during the ultra-slow cooling.

6. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein the Temp is 545 to 745° C.

7. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein a cooling rate is 20° C./sec or higher, during the second cooling.

8. The method of manufacturing the hot-rolled coated steel of claim 1 , further comprising an operation of pickling the wound hot-rolled steel sheet, after the winding.

9. The method of manufacturing the hot-rolled coated steel claim 8 , wherein the pickling is performed at 200° C. or lower.

10. The method of manufacturing the hot-rolled coated steel sheet of claim 8 , further comprising an operation of correcting the shape of the wound hot-rolled steel sheet, before the pickling.

11. The method of manufacturing the hot-rolled coated steel of claim 1 , wherein when forming a plating layer on a surface of the hot-rolled steel sheet, a hot-dip plating bath introduction speed of the hot-rolled steel sheet is 10 to 60 mpm (m/min).

12. The method of manufacturing the hot-rolled coated steel sheet of claim 1 , wherein the El SPM is 0.03 to 0.5%.

Assignees

• Posco Co Ltd

Inventors

• Na Hyun-Taek
• Kim Sung-Il

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D9/46 Primary

  for sheet metals · CPC title

• B32B15/013

  one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium · CPC title

• C21D6/002

  containing Cr (C21D6/004 takes precedence) · CPC title

• C21D6/005

  containing Mn · CPC title