High-strength hot rolled steel plate and manufacturing method thereof

Abstract

The present invention provides a new high-strength and Si—Cr containing hot rolled steel plate provided with higher strength as well as excellent workability and a method for manufacturing the steel plate. The high-strength steel plate can be obtained by controlling the particle size of prior austenite to be 10 μm or less, and properly selecting the coiling temperature. The steel plate obtained includes a retained austenite phase in a volume fraction of 5% to 20%; a martensite phase in a volume fraction equal to or less than 10%; and a bainite phase in the remaining volume fraction. The particle size of the retained austenite particle is 1 μm or less and the retained austenite particles are dispersed uniformly.

1 . A high-strength hot rolled steel plate comprising: a retained austenite phase in a volume fraction of 5% to 20%; a martensite phase in a volume fraction of 0% to 10%; and a bainite phase in a remaining volume fraction, wherein a particle size of retained austenite particles is 1 μm or less, and the retained austenite particles are dispersed in a density of seven or more particles per 10 μm 2 . 
   
   
       2 . The high-strength hot rolled steel plate according to  claim 1 , wherein a particle size of prior austenite particles is 10 μm or less, and an average aspect ratio of the prior austenite particles is 2.0 or less. 
   
   
       3 . The high-strength hot rolled steel plate according to  claim 1 , wherein the steel plate has a composition comprising: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and remaining components including Fe and other inevitable impurities. 
   
   
       4 . The high-strength hot rolled steel plate according to  claim 1 , the steel plate has a plate thickness of 1.0 to 3.0 mm and a tensile strength of 1200 MPa or greater. 
   
   
       5 . A method for manufacturing a high-strength hot rolled steel plate, comprising the steps of:
 preparing a steel material having a composition containing: C (0.13 to 0.21 (% by weight)), Si (0.5 to 2.0), Mn (0.2 to 1.0), Cr (1.0 to 4.0), Ni (0.02 to 1.0), Mo (0.05 to 0.4), P (0 to 0.010), S (0 to 0.003), N (0.005 to 0.015), and remaining components including Fe and other inevitable impurities;   roughly rolling the steel material under conditions of: 1250° C. or higher of an extraction temperature of a reheating furnace; 1030° C. or higher of a discharging-side temperature of roughly rolling mills; and 30% or higher of a reduction ratio for each of roughly rolling final three passes;   finish rolling of the steel material under conditions of: 950° C. or higher of a discharging-side temperature of finish rolling mills; 40% or higher of a reduction ratio for each mill on a finish front-stage, and 0.5 or greater of accumulated strain due to a reduction by three rolling mills on a finish rear-stage; and   cooling the steel material by air-cooling for 2 to 6 seconds, followed by water-cooling, and coiling the steel material at a coiling temperature of 550 to 650° C.   
   
   
       6 . The method for manufacturing a high-strength hot rolled steel plate according to  claim 5 , wherein upon the finish rolling, a reduction amount of a topmost portion of the steel material is set greater than an expected reduction amount, in one or more rolling mills other than a mill on a final stage, wherein the reduction amount of the top portion is set at a value increased by less than 10% of the expected reduction amount of the rolling mill, and wherein a length to be rolled in an increased reduction amount is within 5 m as measured from a biting site of the topmost portion of the steel material, and thereafter the reduction amount is retuned to the expected reduction amount. 
   
   
       7 . The method for manufacturing a high-strength hot rolled steel plate according to  claim 5 , wherein a high-grip roll having micro-carbide particles dispersed on a surface of the high-grip roll is used as a working roll for each finish rear-stage rolling mill including a final rolling mill.