HIGH-TOUGHNESS HOT-ROLLING HIGH-STRENGTH STEEL WITH YIELD STRENGTH OF 800 MPA, AND PREPARATION METHOD THEREOF

Abstract

A high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa with its chemical components, in weight percentages, being C 0.02-0.05%, Si≦0.5%, Mn 1.5-2.5%, P≦0.015%, S≦0.005%, Al 0.02-0.10%, N≦0.006%, Nb 0.01-0.05%, Ti 0.01-0.03%, 0.03%≦Nb+Ti≦0.06%, Cr 0.1%-0.5%, Mo 0.1-0.5%, B 0.0005-0.0025%, and the balance of Fe and unavoidable impurities, and a preparation method thereof. The present invention acquires, via direct quenching, an ultra-low carbon martensite structure with a yield strength of 800 Mpa and an impact energy of more than 100J under a temperature of −80° C.

Claims

1. A high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa., the components thereof, in weight percentages, being: C 0.02-0.05%, Si≦0.5%, Mn 1.5-2.5%, P≦0.015%, S≦0.005%, Al 0.02-0.10%, N≦0.006%, Nb 0.01-0.05%, Ti 0.01-0.03%, 0.03%≦Nb+Ti≦0.06%, Cr 0.1%-0.05%, Mo 0.1-0.5%, B 0.0005-0.0025%, and the balance of Fe and unavailable impurities.

2. The high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa of claim 1, wherein the hot-rolled high-strength steel has a yield strength ≧800 MPa, a tensile strength ≧900 MPa, an elongation ≧13%, and an impact energy at −80° C. of 100J or higher.

3. The high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa of claim 1, wherein the hot-rolled high-strength steel has a microstructure of lath martensite.

4. A method of manufacturing the high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa of claim 1, comprising the following steps: Smelting and casting A composition of claim 1 is smelted in a converter or electrical furnace, subjected to secondary refining in a vacuum furnace, and cast to a cast blank or ingot; Heating The cast blank or ingot is heated, wherein the heating temperature is 1100-1200° C., and the hold time is 1-2 hours; Hot rolling The initial rolling temperature is 1000-1100° C.; multi-pass large reduction rolling is conducted at a temperature of 950° C. or higher with an accumulated deformation rate ≧50%; subsequently, an intermediate blank is held till 900-950° C.; and then, the last 3-5 paths of rolling are conducted with an accumulated deformation rate ≧70%; On-line quenching process Rapid on-line quenching is conducted at a cooling rate ≧5° C./s from a temperature that is 800-900° C. above the temperature at which ferrite begins to precipitate to a temperature below Ms or room temperature to obtain fine ultralow-carbon lath martensite.

5. The method of manufacturing the high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa according to claim 4, wherein the hot-rolled high-strength steel has a yield strength ≧800 MPa, a tensile strength ≧900 MPa, an elongation ≧13%, and an impact energy at −80° C. of 100J or higher.

6. The method of manufacturing the high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa according to claim 4, wherein the hot-rolled high-strength steel has a microstructure of lath martensite.

7. The method of manufacturing the high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa according to claim 5, wherein the hot-rolled high-strength steel has a microstructure of lath martensite.

8. The high-toughness hot-rolled high-strength steel with a yield strength of Grade 800 MPa of claim 2, wherein the hot-rolled high-strength steel has a microstructure of lath martensite.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0044] The specific features and performances of the disclosure will be set out with reference to the following examples and drawings.

[0045] FIG. 1 is a schematic view of the manufacture process of the disclosure;

[0046] FIG. 2 is a typical metallographical photo of the steel of Example 1 according to the disclosure;

[0047] FIG. 3 is a typical metallographical photo of the steel of Example 2 according to the disclosure;

[0048] FIG. 4 is a typical metallographical photo of the steel of Example 3 according to the disclosure;

[0049] FIG. 5 is a typical metallographical photo of the steel of Example 4 according to the disclosure;

[0050] FIG. 6 is a typical metallographical photo of the steel of Example 5 according to the disclosure.

BEST MODES FOR CARRYING OUT THE DISCLOSURE

[0051] The disclosure will be further illustrated with reference to the following Examples and accompanying drawings.

[0052] The steel compositions of the Examples according to the disclosure are listed in Table 1. Table 2 shows the process for manufacturing the steel of the Examples according to the disclosure. Table 3 shows the mechanical properties of the steel of the Examples according to the disclosure.

[0053] The process flow of the Examples according to the disclosure: smelting in a converter or electrical furnace.fwdarw.secondary refining in a vacuum furnace.fwdarw.casting blank (ingot).fwdarw.reheating the cast blank (ingot).fwdarw.hot rolling+on-line quenching.fwdarw.steel coiling, wherein the temperature for heating the cast blank (ingot) was 1100-1200° C.; the hold time was 1-2 hours; the initial rolling temperature was 1000-1100° C.; multi-pass large reduction rolling was conducted at temperatures of 950° C. and higher, and the accumulated deformation rate was ≧50%; subsequently, the intermediate blank was held till 900-950° C.; then, the last 3-5 paths of rolling were conducted, and the accumulated deformation rate was ≧70%; rapid on-line quenching was conducted at a cooling rate ≧5° C./s from a temperature that was 800-900° C. above the temperature at which ferrite began to precipitate to a temperature below Ms or room temperature to obtain fine ultralow-carbon lath martensite, as shown by FIG. 1.

TABLE-US-00001 TABLE 1 unit: weight % Ex. C Si Mn P S Al N O Nb Ti Cr Mo B 1 0.02 0.45 2.47 0.006 0.003 0.08 0.0032 0.0026 0.05 0.01 0.28 0.22 0.0025 2 0.03 0.28 1.91 0.007 0.004 0.09 0.0033 0.0024 0.02 0.02 0.41 0.13 0.0015 3 0.03 0.49 1.55 0.009 0.003 0.02 0.0046 0.0023 0.01 0.03 0.45 0.14 0.0020 4 0.04 0.35 1.74 0.010 0.005 0.04 0.0036 0.0028 0.04 0.01 0.23 0.35 0.0010 5 0.05 0.11 2.25 0.008 0.006 0.07 0.0040 0.0029 0.03 0.02 0.10 0.28 0.0005

TABLE-US-00002 TABLE 2 Finishing Critical End Cooling Heating rolling Steel Plate Cooling Temperature Temperature Temperature Thickness Rate In Quenching Ex. ° C. ° C. mm ° C./s ° C. 1 1150 900 3 5 Room Temperature 2 1100 880 6 13 350 3 1200 850 8 24 Room Temperature 4 1150 800 10 7 150 5 1200 830 12 13 250 Note: Steel blank thickness 120 mm.

TABLE-US-00003 TABLE 3 Mechanical Properties Of Steel Plates Tensile Yield Strength Strength Impact Rp0.2 Rm Elongation A Yield Ratio Energy Ex. MPa MPa % Rp0.2/Rm (−80° C.) 1 805 903 15.0 0.89 162 2 814 928 14.0 0.88 162 3 820 939 14.0 0.87 158 4 834 946 14.5 0.88 144 5 856 943 13.0 0.91 156

[0054] FIGS. 2-6 show the typical metallographical photos of the test steel of Examples 1-5.

[0055] As can be seen clearly from the metallographical photos, the structure of the steel plates is fine lath martensite. As can be seen clearly in the rolling direction, the prior austenite grain boundary has a tabular shape with a width of about 6-7 μm, having a fine prior austenite equivalent grain size. The finer the prior austenite grains, the smaller the lath after the steel plate is quenched, leading to higher strength and better low-temperature impact toughness. As can be discovered by observation through SEM, when the steel plate was quenched to room temperature, carbides have no time to form, and thus the structure is substantially free of carbides. When quenched to different temperatures such as 150° C., 250° C. and 350° C., the structure of the steel plate comprises a certain number of carbides. Since the alloy itself is designed to comprise ultralow carbon, the amount of the carbides precipitated is limited, and these carbides contribute little to the strength.

[0056] To sum up, the design concept of the disclosure is the use of ultralow-carbon martensite, wherein austenite grain size is reduced by combined addition of Nb and Ti; hardenability and temper softening resistance are improved by combined addition of Cr and Mo; a hot continuous rolling process is utilized to obtain ultralow-carbon martensite structure by direct quenching or low-temperature coiling, wherein in addition to high strength (yield strength ≧800 MPa), the resulting steel still exhibits excellent impact toughness (impact energy at −80° C.>100J, and in fact almost being 150J or higher for all the Examples) when kept at −80° C. These properties cannot be achieved by presently similar steel design concept based on ultralow-carbon bainite, wherein the strength is low while the impact toughness is close to that in the disclosure; or the strength is close to that in the disclosure, but the impact toughness is poorer. The disclosure combines these two advantages.