Thick steel plate with low cracking sensitivity and low yield ratio and manufacturing method thereof

Abstract

An ultra-heavy steel plate with low cracking sensitivity and low yield ratio, the mass percentages of chemical components of the steel plate are C 0.05-0.09; Si 0.2-0.4; Mn 1.3-1.6; Al 0.02-0.04; Nb 0.03-0.08; V 0.03-0.08; Cr 0.1-0.5; Ni 0.1-0.5; Mo 0.1-0.3; Cu 0.2-0.5; Ti 0.01-0.02; P0.015; S0.003; N0.007, the balance being Fe and inevitable impurities; the carbon equivalent is 0.43, the cold cracking sensitivity coefficient Pcm is 0.20. A low cracking sensitivity and low yield ratio steel plate with a thickness of 40-70 mm is manufactured by the process steps of KR molten iron pretreatment-converter smelting-LF refining-RH vacuum degassing-continuous casting-lid-covering slow cooling for the continuous casting slabs-casting slabs heating-controlled rolling-controlled cooling-hot straightening-air cooling and so on.

Claims

1. An steel plate with low cracking sensitivity and low yield ratio (i.e. ratio of yield strength to tensile strength), characterized in that the mass percentages of chemical components of the steel plate are C 0.05-0.09; Si 0.2-0.4; Mn 1.3-1.6; Al 0.02-0.04; Nb 0.03-0.08; V 0.03-0.08; Cr 0.1-0.5; Ni 0.1-0.5; Mo 0.1-0.3; Cu 0.2-0.5; Ti 0.01-0.02; P0.015; S0.003; N0.007, the balance being Fe and inevitable impurities, carbon equivalent C.sub.eq is 0.43, wherein C.sub.eq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15, cold cracking sensitivity coefficient P.sub.cm is 0.20, wherein P.sub.cm=C+Si/30+(Mn+Cu+Cr)/20Ni/60+Mo/15+V/10+5B, microstructure is the ferrite/bainite dual-phase structure over the entire thickness of the section, Charpy impact energy of 150 J when it is measured at thickness and thickness of the steel plates at the temperature of 60 C., and a method for manufacturing the steel plate with low cracking sensitivity and low yield ratio comprises the following process steps: (1) producing molten steel by KR molten iron pretreatment, converter smelting, LF refining, RH vacuum degassing treatment, then producing a continuous casting slab with a thickness of 150-450 mm by an ultra-heavy slab continuous casting process, thereafter treating the continuous casting slabs with covering by a lid, stacking for slow cooling and hydrogen diffusion, wherein the time of stacking for slow cooling is 120 hours; (2) beating the continuous casting slabs to 1130-1250 C. incubation for 150-180 min, and descaling the continuous casting slabs by water after exiting a furnace; (3) exerting two stages of rolling on the continuous casting slabs, the first stage is rough rolling with a rolling starting temperature of 1050-1150 C. and reduction ratio of each rolling pass is 15%; the second stage is finish rolling with a starting temperature of 840-900 C. and reduction ratio of accumulated rolling passes is 60%; (4) applying a controlled cooling after rolling, the controlled cooling comprising two stages, the first stage is an air-cooling stage with a cooling starting temperature of 800-860 C. and a final cooling temperature of 600-750 C.; the second stage is an accelerated cooling stage with a cooling rate of 13-17 C./s and a final cooling temperature of 300-450 C.; and (5) after the controlled cooling stage, applying a hot straightening treatment, and finally cooling by air to obtain the steel plate finished-products.

2. The steel plate with low cracking sensitivity and low yield ratio according to claim 1, characterized in that the steel plate has a thickness of 40-70 mm, a yield strength of 460 MPa, a tensile strength of 570-760 MPa, a yield ratio of 0.80, an elongation rate of 17%, a reduction of area in Z direction is 35%, which can serve at a temperature of 60 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a microstructure diagram positioned at thickness of the 70 mm thicksteel plate according to an embodiment of the invention;

(2) FIG. 2 is a microstructure diagram positioned at thickness of the 70 mm thicksteel plate according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(3) The invention is now described in further details with reference to embodiments shown in the drawings.

Examples 1-2

(4) The steel plate manufacturing method related to the two embodiments: KR molten iron pretreatment-converter smelting-LF refining-RH vacuum degassing-continuous casting-lid-covering slow cooling for the continuous casting slabs-detection and cleaning of the continuous casting slabs-heating of the casting slabs-descaling by high pressure water-controlled rolling-controlled cooling-hot straightening-air-cooling, to manufacture two sets of low cracking sensitivity, low yield ratio and high strength steel plates with a thickness of 70 mm and can be used at low temperature. It is also completely suitable for manufacturing the steel plates having a thickness less than 70 mm.

(5) Specific processes of the above heating, rolling and slow cooling include heating the continuous casting slabs with a thickness of 370 mm to 1180 C. and incubating for 180 min (Example 1), or heating to 1220 C. and incubating for 150 min (Example 2), descaling the continuous casting slabs by high pressure water after exiting the furnace; then applying two stages of rolling, the first stage is rough rolling, the rolling starting temperature is 1060 C. (Example 1) or 1100 C. (Example 2), the thickness of the immediate slab is 240 mm and reduction of the each rolling pass is 16%; the second stage has the rolling starting temperature of 860 C. with reduction of accumulated rolling passes 65% (Example 1), or the second stage has the rolling starting temperature of 840 C. with reduction of accumulated rolling passes of 65% (Example 2), the finished steel plates have a thickness of 70 mm (Example 1) and 70 mm (Example 2); after rolling, the steel plates are cooled by air to 680 C. (Example 1) and 650 C. (Example 2); thereafter, an accelerated cooling is applied, with a cooling rate of 13-17 C./s and a final cooling temperature of 400 C. (Example 1) and 430 C. (Example 2), and finally cooling by air to ambient temperature.

(6) The chemical components of the steel plates produced by example 1 and 2 are listed in table 1, the mechanical properties of the steel plates are listed in table 2, and the microstructures positioned at and thickness of the steel plates are shown in FIG. 1 and FIG. 2.

(7) TABLE-US-00001 TABLE 1 The chemical components (wt. %) of the ultra-heavy steel plates with low cracking sensitivity and low yield ratio of examples 1 and 2 Example C Si Mn P S Al Nb V Ti Cr Ni Mo Cu N Ceq Pcm 1 0.06 0.28 1.55 0.012 0.002 0.026 0.036 0.041 0.014 0.16 0.27 0.12 0.23 0.0028 0.42 0.19 2 0.07 0.23 1.54 0.011 0.002 0.030 0.034 0.036 0.015 0.16 0.27 0.11 0.23 0.0027 0.42 0.19 Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

(8) TABLE-US-00002 TABLE 2 The mechanical properties of the ultra-heavy steel plates with low cracking sensitivity and low yield ratio of examples 1 and 2 Reduction Yield Tensile Breaking of area in strength/ strength/ elongation Yield Impact energy Zdirection/ Thickness/mm Position MPa MPa rate/% ratio at 60 C./J % Example 1 70 mm 504 676 26.2 0.75 252, 236, 247 75, 70, 72 483 669 27.5 0.72 205, 196, 216 Example 2 70 mm 498 668 25.9 0.75 228, 232, 248 73, 75, 70 479 657 28.0 0.73 198, 185, 21

(9) The invention has filled the domestic blank by the components design of ultra-low carbon, low carbon equivalent and low cracking sensitivity and by the low cracking sensitivity, low yield ratio steel plates with a thickness of 40-70 mm which is successfully manufactured by the controlled rolling and controlled cooling process and can be used at the condition of 60 C.