Steel with Controlled Yield Ratio and Manufacturing Method therefor

Abstract

Disclosed are a steel with controlled steel ratio and a manufacturing method therefor. The steel comprises the following components in percentage by mass: C: 0.245-0.365%, Si: 0.10-0.80%, Mn: 0.20-2.00%, P:≤0.015%, S:≤0.003%, Cr: 0.20-2.50%, Mo: 0.10-0.90%, Nb: 0-0.08%, Ni: 2.30-4.20%, Cu: 0-0.30%, V: 0.01-0.13%, B: 0-0.0020%, Al: 0.01-0.06%, Ti: 0-0.05%, Ca:≤0.004%, H:≤0.0002%, N:≤0.013%, O:≤0.0020%, and the balance of Fe and inevitable impurities, wherein the components satisfy (8.57*C+1.12*Ni)≥4.8% and 1.2%≤(1.08*Mn+2.13*Cr)≤5.6%. The steel has excellent low-temperature impact toughness and aging impact toughness at −20° C. and −40° C., a rationally controlled yield ratio, and ultra-high strength, ultra-high toughness, and ultra-high plasticity, which can be used in applications such as offshore platform mooring chains, mechanical structures, and automobiles that require high strength and toughness of the steel.

Claims

1. A steel with controlled yield ratio, comprising the following components in percentage by mass: C: 0.245-0.365%, Si: 0.10-0.80%, Mn: 0.20-2.00%, P:≤0.015%, S:≤0.003%, Cr: 0.20-2.50%, Mo: 0.10-0.90%, Nb: 0-0.08%, Ni: 2.30-4.20%, Cu: 0-0.30%, V: 0.01-0.13%, B: 0-0.0020%, Al: 0.01-0.06%, Ti: 0-0.05%, Ca:≤0.004%, H:≤0.0002%, N:≤0.013%, O:≤0.0020%, and the balance of Fe and inevitable impurities, wherein the components satisfy (8.57*C+1.12*Ni)≥4.8% and 1.2%≤(1.08*Mn+2.13*Cr)≤5.6%; and the steel with controlled yield ratio has a yield ratio of 0.85-0.95, a tensile strength of 1,100 MPa or more, and a yield strength of 900 MPa or more.

2. The steel with controlled yield ratio of claim 1, wherein a microstructure of the steel with controlled yield ratio is tempered martensite+tempered bainite.

3. The steel with controlled yield ratio of claim 1, wherein the steel with controlled yield ratio has a Charpy impact energy A.sub.kv, at −20° C. of 90J or more, a Charpy impact energy A.sub.kv at −40° C. of 70J or more, a Charpy impact energy A.sub.kv, at −20° C. of 80J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a Charpy impact energy A.sub.kv, at −40° C. of 60J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a yield ratio of 0.85-0.95, a tensile strength of 1,100 MPa or more, a yield strength of 900 MPa or more, an elongation rate of 15% or more, an area reduction of 50% or more, a strength toughness product (Tensile Strength*Charpy Impact Energy A.sub.kv at −20° C.) of 115 GPa*J or more, and a strength plasticity product (Tensile Strength*Elongation Rate) of 16 Gpa*% or more.

4. A manufacturing method for a steel with controlled yield ratio, comprising the following steps: S1: smelting and casting, wherein the smelting and casting are carried out according to the components in claim 1 to form a casting billet; S2: heating, wherein the casting billet is heated at a heating temperature of 1,010-1,280° C.; S3: rolling or forging, wherein a final rolling temperature is 720° C. or more or a final forging temperature is 720° C. or more; and performing air cooling, water cooling or retarded cooling after the rolling; S4: quenching heat treatment, wherein the quenching is performed at a quenching temperature of 830-1,060° C. using water quenching or oil quenching, and a ratio of the quenching time to the thickness or diameter of the steel is 0.25 min/mm or more; and S5: tempering heat treatment, wherein a tempering temperature is 490-660° C., a ratio of the tempering time to the thickness or diameter of the steel is 0.25 min/mm or more, and performing air cooling, retarded cooling or water cooling after the tempering.

5. The manufacturing method for the steel with controlled yield ratio of claim 4, wherein a microstructure of the steel with controlled yield ratio is tempered martensite+tempered bainite.

6. The manufacturing method for the steel with controlled yield ratio of claim 4, wherein the steel with controlled yield ratio has a Charpy impact energy A.sub.kv at −20° C. of 90J or more, a Charpy impact energy A.sub.kv at −40° C. of 70J or more, a Charpy impact energy A.sub.kv at −20° C. of 80J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a Charpy impact energy A.sub.kv at −40° C. of 60J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a yield ratio of 0.85-0.95, a tensile strength of 1,100 MPa or more, a yield strength of 900 MPa or more, an elongation rate of 15% or more, an area reduction of 50% or more, a strength toughness product (Tensile Strength*Charpy Impact Energy A.sub.kv at −20° C.) of 115 GPa*J or more, and a strength plasticity product (Tensile Strength*Elongation Rate) of 16 Gpa*% or more.

7. The steel with controlled yield ratio of claim 2, wherein the steel with controlled yield ratio has a Charpy impact energy A.sub.kv at −20° C. of 90J or more, a Charpy impact energy A.sub.kv, at −40° C. of 70J or more, a Charpy impact energy A.sub.kv at −20° C. of 80J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a Charpy impact energy A.sub.kv at −40° C. of 60J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a yield ratio of 0.85-0.95, a tensile strength of 1,100 MPa or more, a yield strength of 900 MPa or more, an elongation rate of 15% or more, an area reduction of 50% or more, a strength toughness product (Tensile Strength*Charpy Impact Energy A.sub.kv at −20° C.) of 115 GPa*J or more, and a strength plasticity product (Tensile Strength*Elongation Rate) of 16 GPa*% or more.

8. The manufacturing method for the steel with controlled yield ratio of claim 5, wherein the steel with controlled yield ratio has a Charpy impact energy A.sub.kv at −20° C. of 90J or more, a Charpy impact energy A.sub.kv at −40° C. of 70J or more, a Charpy impact energy A.sub.kv at −20° C. of 80J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a Charpy impact energy A.sub.kv at −40° C. of 60J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a yield ratio of 0.85-0.95, a tensile strength of 1,100 MPa or more, a yield strength of 900 MPa or more, an elongation rate of 15% or more, an area reduction of 50% or more, a strength toughness product (Tensile Strength*Charpy Impact Energy A.sub.kv at −20° C.) of 115 GPa*J or more, and a strength plasticity product (Tensile Strength*Elongation Rate) of 16 GPa*% or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] FIG. 1 is an optical microscope image (500×) of the microstructure morphology of the steel rod according to Example 3 of the present invention; and

[0054] FIG. 2 is a scanning electron microscope image (10,000×) of the microstructure morphology of the steel rod according to Example 3 of the present invention.

DETAILED DESCRIPTION

[0055] The present invention will be further illustrated below in combination with examples and the accompanying drawings. Those examples are merely used for describing the optimal implementation modes of the present invention, but not intended to make any limitation to the scope of the present invention.

[0056] Compositions of the examples of the present invention are shown in Table 1. The manufacturing method according to the examples of the present invention comprises the following steps: smelting, casting, heating, forging or rolling, quenching treatment, and tempering treatment; in the casting process, die casting or continuous casting is adopted; in the heating process, the heating temperature is 1,010-1,280° C., and the final rolling temperature or the final forging temperature is 720° C. or more; and in the rolling process, a steel billet can be directly rolled to the final specification, or the steel billet is rolled to a specified intermediate billet size and then heated and rolled to the final finished product size. The quenching temperature is 830-1,060° C. using water quenching or oil quenching, while the ratio of the quenching heating time to the thickness or diameter of the steel is 0.25 min/mm or more. The tempering temperature is 490-660° C., and perform air cooling, retarded cooling or water cooling to the steel after tempering.

[0057] Test methods: 1. the tensile property is measured in accordance with the Chinese standard GB/T228 Metallic materials—Tensile testing at ambient temperature; 2. the impact performance is measured in accordance with GB/T229 Metallic materials—Charpy pendulum impact test method; and

[0058] 3. the strain aging measurement process is derived from the DNV Rules for Classification of Ships (Offshore mooring chain and accessories. Approval of manufacturers DNVGL-CP-0237 Edition July 2018).

[0059] The product according to the present invention can be used in applications such as offshore platform mooring chains and the like that require rods with high strength, and the size specification of the rods can reach a diameter of 200 mm (the diameter of the round steel in the Chinese patent CN103667953A is only 70-160 mm).

Example 1

[0060] Electric furnace or converter smelting is carried out in accordance with the compositions shown in Table 1, then casting is carried out to form a continuously casted billet or a steel ingot. The continuously casted billet or the steel ingot is heated to 1,280° C. and rolled with a final rolling temperature of 1,020° C., and the size of the intermediate billet is 260*260 mm; after rolling, retarded cooling is carried out; the intermediate billet is then heated to 1,010° C. and rolled with a final rolling temperature of 720° C. to obtain a finished product rod with a specification of .sub.φ20 mm; after rolling, air cooling is carried out; then the product rod is heated for quenching at 830° C. for 35 minutes and adopts water quenching treatment; then tempering is carried out at 490° C. for 35 minutes, and after tempering, air cooling is carried out.

Example 2

[0061] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,220° C.; the final rolling temperature is 980° C.; the size of an intermediate billet is 260*260 mm; and after rolling, retarded cooling is carried out; the intermediate billet is heated to 1,050° C.; the final rolling temperature is 770° C.; the specification of the finished product rod is .sub.φ60 mm, and after rolling, water cooling is carried out; the finished product rod is heated for quenching at 880° C. for 70 minutes and adopts oil quenching treatment; then tempering is carried out at 540° C. for 80 minutes, and after tempering, retarded cooling is carried out.

Example 3

[0062] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,180° C.; the final rolling temperature is 940° C.; the specification of the finished product rod is .sub.φ70 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 940° C. for 90 minutes and adopts oil quenching process; then tempering is carried out at 560° C. for 100 minutes, and after tempering, water cooling is carried out.

Example 4

[0063] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,110° C.; the final rolling temperature is 920° C., the specification of a finished product rod is .sub.φ110 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 960° C. for 120 minutes and adopts water quenching process; then tempering is carried out at 600° C. for 180 minutes, and after tempering, air cooling is carried out.

Example 5

[0064] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,080° C.; the final rolling temperature is 900° C.; the specification of the finished product rod is .sub.φ130 mm, and after rolling, retarded cooling is carried out; the finished product rod is heated for quenching at 980° C. for 170 minutes and adopts water quenching treatment; then tempering is carried out at 610° C. for 260 minutes, and after tempering, water cooling is carried out.

Example 6

[0065] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,010° C.; the final rolling temperature is 870° C.; the specification of the finished product rod is .sub.φ2.00 mm, and after rolling, retarded cooling is carried out; the finished product rod is heated for quenching at 1,060° C. for 350 minutes and adopts water quenching treatment; then tempering is carried out at 660° C. for 350 minutes, and after tempering, water cooling is carried out.

Example 7

[0066] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,230° C.; the final rolling temperature is 960° C.; the specification of the finished product rod is .sub.φ90 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 920° C. for 30 minutes and adopts water quenching treatment; then tempering is carried out at 620° C. for 60 minutes, and after tempering, water cooling is carried out.

Example 8

[0067] The implementation steps are the same as that in Example 1, wherein the heating temperature is 1,200° C.; the final rolling temperature is 980° C., the specification of the finished product rod is .sub.φ100 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 920° C. for 30 minutes and adopts water quenching treatment; then tempering is carried out at 600° C. for 60 minutes, and after tempering, water cooling is carried out.

Comparative Example 1

[0068] Then implementation steps are the same as that in Example 1, except that the heating temperature is 1,150° C.; the final rolling temperature is 960° C.; the specification of the finished product rod is .sub.φ110 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 920° C. for 35 minutes and adopts water quenching treatment; then tempering is carried out at 550° C. for 60 minutes, and after tempering, water cooling is carried out.

Comparative Example 2

[0069] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,120° C.; the final rolling temperature is 940° C.; the specification of the finished product rod is .sub.φ130 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 910° C. for 40 minutes and adopts water quenching treatment; then tempering is carried out at 530° C. for 70 minutes, and after tempering, water cooling is carried out.

Comparative Example 3

[0070] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,100° C.; the final rolling temperature is 900° C.; the specification of the finished product rod is .sub.φ100 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching at 870° C. for 50 minutes and adopts water quenching treatment; then tempering is carried out at 520° C. for 50 minutes, and after tempering, water cooling is carried out.

Comparative Example 4

[0071] The implementation steps are the same as that in Example 1, except that the heating temperature is 1,040° C.; the final rolling temperature is 880° C.; the specification of the finished product rod is .sub.φ80 mm, and after rolling, air cooling is carried out; the finished product rod is heated for quenching is at 930° C. for 30 minutes and adopts water quenching treatment; then tempering is carried out at 600° C. for 40 minutes, and after tempering, water cooling is carried out.

[0072] The mechanical properties of the steel with controlled yield ratio in Examples 1-8 and steel in Comparative examples 1-4 in the present invention are measured based on the test methods above, and the results are shown in Table 2.

[0073] It can be seen from Table 1 and Table 2 that C and B in Comparative example 1 do not satisfy the composition range of the present invention, therefore the refining effect of C on bainite and ferrite lamellar cannot be sufficiently utilized; and relatively high B content may cause segregation of B at the grain boundaries, which will deteriorate the low-temperature impact performance, resulting in low strength and low impact energy of the steel. In Comparative example 2, the steel does not satisfy 8.57*C+1.12*Ni≥4.8%; although the tensile strength of the steel reaches 1,100 MPa, as the effect of Ni in reducing the stacking fault energy cannot be sufficiently utilized and the refining effect of C on the bainite lamellar is not effectively imparted, the low-temperature impact energy of the steel is rather low. In Comparative example 3, content of Mn and Mo exceeds the composition range of the present invention; although the solid dissolution strengthening effect of Mn improves the strength of the steel and results in a tensile strength of over 1,200 MPa, as Mn will segregate towards the grain boundaries in the welding process and relatively large carbides of Mo tend to reduce the low-temperature toughness of the steel, the impact energy of the steel of Comparative example 3 is low. In Comparative example 4, the steel does not satisfy 1.2%≤1.08 Mn+2.13Cr≤5.6%, and the Nb content exceeds the desired composition range of the present invention, therefore the solid dissolution strengthening effect of Mn and Cr and the carbide precipitation strengthening effect of Cr cannot be sufficiently utilized, resulting in the formation of coarse NbC precipitate particles, therefore the steel of Comparative example 4 only have a yield strength of 890 MPa, a tensile strength which does not reach 1,100 MPa, a yield ratio of 0.84, and low impact energy.

[0074] The steel with controlled yield ratio provided by the present invention has a Charpy impact energy A.sub.kv at −20° C. of 90J or more, a Charpy impact energy A.sub.kv at −40° C. of 70J or more, a Charpy impact energy A.sub.kv at −20° C. of 80J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a Charpy impact energy A.sub.kv at −40° C. of 60J or more after holding at a temperature of 100° C. for 1 h after 5% strain, a yield ratio of 0.85-0.95, a tensile strength of 1,100 MPa or more, a yield strength of 900 MPa or more, an elongation rate of 15% or more, an area reduction of 50% or more, a strength toughness product (Tensile Strength*Charpy Impact Energy A.sub.kv at −20° C.) of 115 GPa*J or more, and a strength plasticity product (Tensile Strength*Elongation Rate) of 16 GPa*% or more

[0075] With reference to FIG. 1 and FIG. 2, it can be seen from FIG. 1 and FIG. 2 that the microstructure of the steel rod in Example 3 of the present invention is tempered martensite and tempered bainite. The width of the tempered bainite or tempered martensite lath is 0.3-2 μm. Nano-scaled carbide precipitates can be seen inside the lath, and fine lamellar-shaped cementite precipitates with a thickness of 50 nm and a length of about 0.2-2 μm along the interface of the lath.

TABLE-US-00001 TABLE 1 Unit: % C Si Mn P S Cr Mo Nb Ni Cu V B Al Ti Ca H N O Example 1  0.245 0.80 2.0 0.015 0.002 0.2 0.9 0.08 3.4 0.2 0.13 0.0010 0.03 0.050 0.0035 0.00020 0.010 0.0015 Example 2 0.26 0.35 0.2 0.012 0.001 2.5 0.4 0.05 2.6 0 0.03 0.0015 0.05 0.030 0.0020 0.00018 0.004 0.0020 Example 3 0.28 0.50 1.4 0.007 0.003 1.8 0.6 0.04 2.3 0.3 0.10 0.0020 0.01 0.010 0.0025 0.00010 0.006 0.0010 Example 4 0.30 0.10 0.8 0.006 0.002 1.0 0.5 0   3.8 0.1 0.06 0    0.06 0.005 0.0015 0.00007 0.007 0.0011 Example 5 0.32 0.25 0.5 0.011 0.001 1.5 0.2 0.01 3.2 0 0.01 0    0.03 0.002 0.0010 0.00011 0.013 0.0009 Example 6  0.365 0.60 1.6 0.01 0.003 0.8 0.1 0.02 4.2 0 0.05 0    0.04 0 0 0.00010 0.002 0.0015 Example 7  0.245 0.1 0.3 0.01 0.002 0.7  0.35 0   2.5 0 0.04 0.0003 0.015 0.003 0.0015 0.00013 0.006 0.0013 Example 8 0.33 0.8 0.8 0.015 0.003 1.6 0.8 0.01 3.5 0.3 0.11 0.001  0.05 0.004 0.0018 0.00010 0.008 0.0016 Comparative 0.23 0.60  1.10 0.015 0.003 0.9 0.7 0.03 2.9 0.2 0.03 0.005  0.04 0.008 0.0020 0.00010 0.008 0.0018 example 1 Comparative 0.25 0.7 0.3 0.015 0.003 0.8 0.4 0.02 2.3 0.1 0.04 0.001  0.02 0.02 0.0015 0.00015 0.006 0.002 example 2 Comparative 0.28 0.5 2.5 0.015 0.003 1.2 1   0.02 3.2 0 0.05 0    0.02 0.01 0.002 0.00017 0.009 0.0015 example 3 Comparative 0.3  0.8 0.6 0.012 0.002 0.25 0.6 0.10 2.9 0.2 0.08 0.0008 0.03 0.007 0.0018 0.00016 0.015 0.002 example 4

TABLE-US-00002 TABLE 2 5% Strain Strength Toughness Aged Charpy Product (Tensile Strength Plasticity Charpy Impact Impact Energy Yield Tensile Strength*Impact Product (Tensile Energy A.sub.kv J A.sub.kv J Strength Strength Yield Elongation Area Energy A.sub.kv Strength*Elongation −20° C. −40° C. −20° C. −40° C. MPa MPa Ratio rate % Reduction % at −20° C.) GPa*J rate) GPa*% Example 1 142 128 122 116 1147 1233 0.93 16 61 175 20 Example 2 138 121 115 108 1118 1215 0.92 16 64 168 19 Example 3 151 139 123 113 1057 1188 0.89 17 63 179 20 Example 4 133 115 117 115 1098 1220 0.90 15 62 162 18 Example 5 121 109 107 105 1078 1253 0.86 16 61 152 20 Example 6 116 104 108 98 1205 1282 0.94 15 63 149 19 Example 7 147 128 130 112 1092 1165 0.94 16 62 171 19 Example 8 132 111 122 103 1110 1190 0.93 16 61 157 19 Comparative 101 57 70 34 985 1055 0.92 16 60 107 17 example 1 Comparative 56 48 32 24 990 1110 0.89 15 58 62 17 example 2 Comparative 42 34 21 17 1096 1201 0.91 14 55 50 17 example 3 Comparative 51 37 32 18 890 1060 0.84 14 56 54 15 example 4