AU2017418679B2

AU2017418679B2 - Low yield strength ratio, high strength and ductility thick gauge steel plate and manufacturing method therefor

Info

Publication number: AU2017418679B2
Application number: AU2017418679A
Authority: AU
Inventors: Jun DANG; Qiang Li; Yigang SHAN; Chao Sun; Chongdao WANG
Original assignee: Nanjing Iron and Steel Co Ltd
Current assignee: Nanjing Iron and Steel Co Ltd
Priority date: 2017-06-13
Filing date: 2017-08-08
Publication date: 2020-09-03
Anticipated expiration: 2037-08-08
Also published as: AU2019100570A4; WO2018227740A1; CN107312981A; JP2020506293A; AU2017418679A1; JP6798041B2

Abstract

A low yield strength ratio, high strength and ductility thick gauge steel plate, the content of the chemical components thereof in terms of percentage by mass being: C: 0.060-0.080%, Mn: 5.5-6.0%, Si: 0.10-0.30%, Al: 0.015-0.040%, Mo: 0.15-0.30%, Cr: 0.20-0.40%, Ni: 0.15-0.40%, Ti: 0.01-0.03%, S ≤ 0.006%, P ≤ 0.010%, the remainder being Fe and unavoidable impurity elements. The steel plate is provided with increased yield strength, reduced yield strength ratio, and great low-temperature impact toughness. A manufacturing method for the steel plate comprises: heating, rolling, and heat treatment. The method requires only one heat treatment and has a simple process.

Description

LOW-YIELD-RATIO HIGH-TOUGHNESS THICK GAUGE STEEL PLATE AND MANUFACTURING METHOD THEREOF TECHNICAL FIELD

[0001] The disclosure pertains to a steel plate and a manufacturing methods thereof, and specifically pertains to a low-yield-ratio high-toughness thick gauge steel plate and a manufacturing method thereof. BACKGROUND

[0002] With the continuous increasing of requirements of a ship body, a bridge, a building, a J pressure container and an ocean platform on a structural material, development of a high-strength high-toughness thick gauge steel plate has received widespread attention. However, the high-strength steel plate has a remarkable problem that a yield ratio is difficultly reduced. The yield ratio is a ratio of yield strength to tensile strength, reflecting the work hardening capability of the material. The higher the yield ratio is, the more local stress concentration or local large deformation occurs in the process of steel plate deformation, and the steel structure absorbing a few amount of energy can lead to the breakage of the material or instability of the structure; the lower the yield ratio is, the larger the deformation capacity undergone by the steel plate starting from plastic deformation to final breakage is, the more the absorbed energy is, the better the earthquake resistant behavior of the steel structure is. Hence, a J low-yield-ratio steel plate should be applied in an occasion where the requirement on the stability of the steel structure is high. However, the yield ratio of the high-strength steel plate produced by using the existing quenching and tempering technology is generally not less than 0.92. The higher yield ratio restricts application scope of the steel plate.

[0003] Usually, steel having a single-structure type, such as a bainite and a martensite, easily reaches high yield strength and high tensile strength, but there is little difference between the numerical values of the yield strength and the tensile strength, and therefore the yield strength is higher. Acquisition of a complex-phase structure, including ferrite + martensite, ferrite + bainite and bainite + martensite, through modification of a process, is an effective method for achieving high strength and low yield ratio. When the complex-phase structure is deformed, a soft phase is firstly yielded, a hard phase provides tensile strength in the process of further deformation, and therefore the yield ratio is reduced. In the prior art, a process for obtaining a low-yield-ratio complex-phase structure is typically based on subcritical quenching, for example, reheating and quenching-subcritical quenching-tempering, normalizing-subcritical quenching-tempering, direct quenching-subcritical quenching-tempering, TMCP-subcritical quenching-tempering, direct quenching-tempering in a sub-temperature region, etc. But, these kinds of processes have a defect of long production period. Compared with a process based on subcritical quenching, a process based on quick heating on-line heat treatment can flexibly regulate and control the complex-phase structure, and is short in production period and high in efficiency, but is high in requirements on production equipment and difficult to generalize.

[0004] In addition, apart from a conflict between low yield ratio and high strength, high strength and high toughness are difficult to simultaneously obtain. Furthermore, it is greatly difficult to obtain high strength under a thick gauge condition. Thus, simultaneous achievement J of high strength, high toughness and low yield ratio on the thick gauge steel plate via a simple process is an urgent problem to be solved.

[0005] The patent with publication number CN104789892A discloses a low-yield-ratio high-toughness thick steel plate having excellent low-temperature impact toughness and a manufacturing method thereof. The chemical components of the low-yield-ratio high-toughness thick steel plate contain more than 3.6% of Ni, and thus cost is expensive.

[0006] The patent with publication number CN106399840A discloses a low-cost low-yield-ratio tempering type Q690E steel plate and a production method thereof. The low-yield-ratio tempering type Q690E steel plate is only 8-40mm in thickness.

[0007] The patent with publication number CN103352167A discloses a low-yield-ratio J high-strength bridge steel and a manufacturing method thereof. The yield strength of the low-yield-ratio high-strength bridge steel is not more than 600MPa, and can only ensure impact

toughness at -40°C.

[0008] The patent with publication number CN102277539A discloses a low-yield-ratio high-plasticity superfine grain high-strength steel and a manufacturing method thereof, and the structure of the low-yield-ratio high-plasticity superfine grain high-strength steel is a bainite.

[0009] Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.

SUMMARY

[0010] It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.

[0011] The objective of the disclosure: the objective of the disclosure is to provide in some embodiments a low-yield-ratio high-toughness thick gauge steel plate which has outstanding features of high strength, high toughness, thick gauge and low yield ratio.

[0012] Another objective of the disclosure is to provide a manufacturing method of a low-yield-ratio high-toughness thick gauge steel plate. The low-yield-ratio high-toughness thick gauge steel plate can be prepared by this method.

[0013] A technical solution is as follows: a low-yield-ratio high-toughness thick gauge steel plate, comprising the following chemical components by mass percent: 0.060-0.080% of C, 5.5-6.0% of Mn, 0.10-0.30% of Si, 0.015-0.040% of Al, 0.15-0.30% of Mo, 0.20-0.40% of Cr, 0.15-0.40% of Ni, 0.01-0.03% of Ti, <0.006% of S, < 0.010% of P and the balance of Fe and inevitable impurity elements; J wherein, the microstructure of the steel plate comprises a tempered martensite and a return austenite, and wherein the low-yield-ratio high-toughness thick gauge steel plate is prepared by the following method: (1) heating: controlling a heating temperature of a blank to 1070-1150°C, and carrying out

heat preservation for 90-150min after the center of the blank reaches this temperature; (2) rolling: controlling an initial rolling temperature to <1020°C, a final rolling temperature

to >820°C and total deformation to >65%, and carrying out water cooling after rolling is ended,

wherein, a final cooling temperature is <130°C; and

(3) heat treatment: heating the steel plate to 605-645°C, carrying out heat preservation for

50-120min after the center of the steel plate reaches this temperature, and then carrying out air J cooling to a room temperature.

[0014] Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

[0015] The definition reasons of mass percentage contents of various chemical components in the low-yield-ratio high-toughness thick gauge steel of the disclosure are as follows:

[0016] C element can significantly increase the strength of a matrix through solid solution strengthening and meanwhile stabilize an austenite phase, but the content of C should be reduced to the greatest extent in order to reduce a ductile-brittle transition temperature of a material. In addition, C is not beneficial to the welding property of the material. Thus, in the disclosure, the content of C is controlled to a relatively low level of 0 .0 6 0 - 0 .0 8 0 %.

[0017] Mn, as a main alloy element of the steel plate of the disclosure, is both a ferrite strengthening element and an austenite stabilizing element. In the aspect of improving the low-temperature toughness of the material, improvement of a Mn/C ratio can significantly reduce the ductile-brittle transition temperature, and thus Mn can replace high-price Ni to a certain extent, but the too high Mn content will aggravate a segregation degree, enlarge smelting difficulty and increase material cost. Thus, in the disclosure, the content of Mn is controlled to 5.5-6.0%.

[0018] Si is a deoxidized element in the process of steelmaking. Appropriate Si can inhibit segregation of Mn and P, while both too high 0 content and segregation of Mn and P can damage toughness. Si can also generate solid solution strengthening, but when its content exceeds 0.3%, the increment of the ductile-brittle transition temperature is caused, and thus the J content cannot be too high. Thus, in the disclosure, the content of Si is controlled to 0.10-0.30%.

[0019] Al is a deoxidized element in the process of steelmaking and can also reduce the quantity of solid solution N atoms, thereby improving toughness and aging strain resistance. Furthermore, formed AlN can also refine grains, thereby further reducing the ductile-brittle transition temperature. However, excess addition can form large-size A1 3 0 2 and AlN, and damages the toughness. Thus, in the disclosure, the content of Al is controlled to 0.015-0.040%.

[0020] Mo can improve the strength of a martensite after tempering, and can also weaken a grain boundary of Mn so as to improve the toughness. The too high Mo content will worsen welding property and increase material cost. Thus, in the disclosure, the content of Mo is J controlled to 0.15-0.30%.

[0021] Cr can generate solid solution strengthening, but the too high Cr content reduces weldability. Thus, in the disclosure, the content of Cr is controlled to 0 .2 0 - 0 .4 0 %.

[0022] Ni can stabilize an austenite phase, improve hardenability, reduce the ductile-brittle transition temperature and can improve deformation property, in addition, is also beneficial to weldability. However, excess addition of Ni element can significantly increase cost. Thus, in the disclosure, the content of Ni is controlled to 0.15-0.40%.

[0023] Ti can refine high-temperature austenite grains, is beneficial to improving the strength and toughness. Micro-scale addition can exert an effect, excess addition will lead to increment of occluded foreign substances. Thus, in the disclosure, the content of Ti is controlled to 0.010-0.030%.

[0024] S easily forms MnS with Mn, P is easily segregated in the grain boundary and reduces the crack growth resistance capability of the grain boundary. In order to improve the toughness of the material, it is needed to control S and P to the lowest extent. Thus, the disclosure requires S<0.006% and P<0.010%.

[0025] Where, the microstructure of the steel plate under the microscopic structure is a complex-phase structure of a tempered martensite and a return austenite. The tempered martensite is a matrix structure, and determines the yield strength of the material. The return austenite, as a diffused second phase, on the one hand, can improve the toughness of the material, and on the other hand, can also generate phase transformation in the process of deformation and improve the tensile strength, thereby reducing the yield ratio. Where, the volume fraction of the return austenite is measured to 5-15% through an X-ray diffractometer.

[0026] The steel plate has a thickness of 50-100mm, a yield strength of >690MPa, a yield ratio of <0.80 and charpy impact test side knock absorption energy at -60°C of >60J.

[10027] The manufacturing method of the low-yield-ratio high-toughness thick gauge steel plate of the disclosure comprises the following steps of heating, rolling and thermal treatment:

[0028] (1) heating: heating a blank having the same chemical components as those of the above low-yield-ratio high-strength thick gauge steel plate, controlling a heating temperature of

the blank to 1070-1150°C, and carrying out heat preservation for 90-150min after the center of

the blank reaches this temperature, wherein, a high-temperature austenite structure is obtained when the blank is heated, and meanwhile alloy elements are homogenized in a diffusion manner. Too high heating temperature or too long heat preservation time will result in too thick high-temperature austenite grains, while too low heating temperature or too short heat preservation time is not beneficial to homogenization of alloy elements. Thus, in the disclosure, the heating temperature is controlled to 1070-1150°C, and the heat preservation time is

controlled to 90-150min.

[0029] (2) rolling: rolling the heated blank, controlling an initial rolling temperature to <1020°C, a final rolling temperature to >820°C and total deformation to >65%, carrying out

water cooling after rolling is ended, wherein, a final cooling temperature is <130°C, a rolling

temperature interval is in an austenite phase region, too high initial rolling temperature is not beneficial to grain refinement, too low final rolling temperature makes deformation difficult, and therefore in the disclosure, the initial rolling temperature is controlled to <1020'C, the final

rolling temperature is controlled to >820°C and the total deformation is controlled to >65%,

ensuring enough strain accumulation and refining the austensite structure; water cooling is

carried out after rolling, the final cooling temperature <130°C is lower than a martensite

transition end point, the austenite is turned into lath martensite, and the structure is further fined, wherein after rolling is ended, the final cooling temperature of water cooling ranges from room temperature to 130°C.

[0030] (3) heat treatment: heating the steel plate to 605-645°C, carrying out heat preservation for 50-120min after the center of the steel plate reaches this temperature, and then carrying out air cooling to a room temperature, wherein, the heat treatment 605-645C is in a ferrite-austenite

two-phase region and can form the return austenite having a volume fraction of 5-15%, the return austenite enriches alloy elements such as C and Mn in the processes of temperature rising and heat preservation for 50-120min to obtain enough heat stability, and can still maintain a face-centered cubic structure when being cooled to -60°C; in addition, the martensite is

moderately rebounded at high temperature, and its strength is reduced and plastic toughness is improved; air cooling is carried out to room temperature after heat preservation to obtain a J complex-phase structure of tempered martensite + return austensite.

[0031] The disclosure has the beneficial effects that the low-yield-ratio high-toughness thick gauge steel plate of the disclosure has high yield strength and low yield ratio, and has the yield strength of >690MPa and the yield ratio of <0.80; due to charpy impact test side knock

absorption energy at -60°C of >60J, the steel plate has good low-temperature impact toughness;

furthermore, the thick gauge of the steel plate reaches 50-100mm. The high-strength high-toughness low-yield-ratio thick gauge steel plate can be produced through the manufacturing method of the low-yield-ratio high-toughness thick gauge steel plate of the disclosure; and the manufacturing process only needs primary heat treatment, and is simple and easy to produce and implement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Fig.1 is a transmission electron microscope picture of a low-yield-ratio high-toughness thick gauge steel plate structure in example 1.

DESCRIPTION OF THE EMBODIMENTS

[0033] Example 1: a low-yield-ratio high-toughness steel plate having a thickness of 50mm was manufactured by the following steps:

[0034] (1) heating: a blank having a thickness of 200mm was put into a heating furnace to be heated to 1110°C and subjected to heat preservation for 120min, wherein, the blank comprised

the following chemical components by mass percent: 0.060% of C, 5.5% of Mn, 0.22% of Si, 0.030% of Al, 0.15% of Mo, 0.20% of Cr, 0.15% of Ni, 0.010% of Ti, 0.003% of S, 0.006% of P and the balance of Fe and inevitable impurity elements;

[0035] (2) rolling: the heated blank was rolled, wherein, an initial rolling temperature was 1020°C, and a final rolling temperature was 845°C. A reduction schedule of a rolling mill was

formulated according to Table 1.

[00361 Table 1 Reduction Schedule In Example 1 Passes 1 2 3 4 5 6 7 8 Thickness of fed material 200 176 155 132 112 89 72 59 mm Thickness of discharged 176 155 132 112 89 72 59 50 material mm

[0037] Total deformation was 75%, water cooling was carried out after rolling was ended, and a final cooling temperature was 25°C;

[0038] (3) heat treatment: the steel plate was put into a heating furnace to be heated to 645°C and subjected to heat preservation for 50min, and after discharging, the steel plate was to air cooled to room temperature.

[0039] The steel plate comprised the following chemical components by mass percent: 0.060% of C, 5.5% of Mn, 0.22% of Si, 0.030% of Al, 0.15% of Mo, 0.20% of Cr, 0.15% of Ni, 0.010% of Ti, 0.003% of S, 0.006% of P and the balance of Fe and impurity elements. The J structure of the steel plate was a complex-phase structure of tempered martensite + return austensite. Fig.1 showed a transmission electron microscope picture of this steel plate structure, from which the tempered martensite and the return austensite that were distributed at intervals could be observed, wherein, the light color part represented the tempered martensite, and the dark color part represented the return austensite. The steel plate had a yield strength of 752MPa, a yield ratio of 0.80 and charpy impact test side knock absorption energy at -60°C of 155J.

[0040] Example 2: a low-yield-ratio high-toughness steel plate having a thickness of 70mm was manufactured by the following steps:

[0041] (1) heating: a blank having a thickness of 200mm was put into a heating furnace to be heated to 1115°C and subjected to heat preservation for 110mmin, wherein, the blank comprised

the following chemical components by mass percent: 0.065% of C, 5.6% of Mn, 0.20% of Si, 0.027% of Al, 0.18% of Mo, 0.22% of Cr, 0.24% of Ni, 0.026% of Ti, 0.006% of S, 0.010% of P and the balance of Fe and inevitable impurity elements;

[0042] (2) rolling: the heated blank was rolled, wherein, an initial rolling temperature was 1006°C, and a final rolling temperature was 827°C. A reduction schedule of a rolling mill was

formulated according to Table 2.

[0043] Table 2 Reduction Schedule In Example 2

Passes 1 2 3 4 5 6 7 8 Thickness of fed material 200 181 162 143 122 101 86 77 mm Thickness of discharged 181 162 143 122 101 86 77 70 material mm

[0044] Total deformation was 65%, water cooling was carried out after rolling was ended, and a final cooling temperature was 68°C;

[0045] (3) heat treatment: the steel plate was put into a heating furnace to be heated to 625°C and subjected to heat preservation for 90min, and after discharging, the steel plate was air cooled to room temperature.

[0046] The steel plate comprised the following chemical components by mass percent: 0.065% of C, 5.6% of Mn, 0.20% of Si, 0.027% of Al, 0.18% of Mo, 0.22% of Cr, 0.24% of Ni, 0.026% of Ti, 0.006% of S, 0.010% of P and the balance of Fe and impurity elements. The microstructure of the steel plate was a complex-phase structure of tempered martensite + return J austensite and had a yield strength of 743MPa, a yield ratio of 0.75 and charpy impact test side knock absorption energy at -60°C of 102J.

[0047] Example 3: a low-yield-ratio high-toughness steel plate having a thickness of 80mm was manufactured by the following steps:

[0048] (1) heating: a blank having a thickness of 320mm was put into a heating furnace to be heated to 1115°C and subjected to heat preservation for 90min, wherein, the blank comprised

the following chemical components by mass percent: 0.073% of C, 5.8% of Mn, 0.10% of Si, 0.040% of Al, 0.22% of Mo, 0.27% of Cr, 0.40% of Ni, 0.030% of Ti, 0.002% of S, 0.008% of P and the balance of Fe and inevitable impurity elements;

[0049] (2) rolling: the heated blank was rolled, wherein, an initial rolling temperature was 1005°C, and a final rolling temperature was 820°C. A reduction schedule of a rolling mill was

formulated according to Table 3.

[0050] Table 3 Reduction Schedule In Example 3 Passes 1 2 3 4 5 6 7 8 Thickness of fed material 320 282 248 211 179 143 115 95 mm

Thickness of discharged 282 248 211 179 143 115 95 80 material mm

[0051] Total deformation was 75%, water cooling was carried out after rolling was ended, and a final cooling temperature was 72°C;

[0052] (3) heat treatment: the steel plate was put into a heating furnace to be heated to 620°C and subjected to heat preservation for 90min, and after discharging, the steel plate was air cooled to room temperature.

[0053] The steel plate comprised the following chemical components by mass percent: 0.073% of C, 5.8% of Mn, 0.10% of Si, 0.040% of Al, 0.22% of Mo, 0.27% of Cr, 0.40% of Ni, 0.030% of Ti, 0.002% of S, 0.008% of P and the balance of Fe and inevitable impurity elements. The microstructure of the steel plate was a complex-phase structure of tempered martensite

+ J return austensite and had a yield strength of 708MPa, a yield ratio of 0.71 and charpy impact test side knock absorption energy at -60°C of 93J.

[0054] Example 4: a low-yield-ratio high-toughness steel plate having a thickness of 100mm was manufactured by the following steps:

[0055] (1) heating: a blank having a thickness of 320mm was put into a heating furnace to be heated to 1070°C and subjected to heat preservation for 150min, wherein, the blank comprised

the following chemical components by mass percent: 0.080% of C, 6.0% of Mn, 0.30% of Si, 0.015% of Al, 0.30% of Mo, 0.40% of Cr, 0.31% of Ni, 0.021% of Ti, 0.001% of S, 0.008% of P and the balance of Fe and inevitable impurity elements;

[0056] (2) rolling: the heated blank was rolled, wherein, an initial rolling temperature was 1002°C, and a final rolling temperature was 837°C. A reduction schedule of a rolling mill was

formulated according to Table 4.

[00571 Table 4 Reduction Schedule In Example 4 Passes 1 2 3 4 5 6 7 8 9 Thickness of 320 282 248 218 192 169 149 131 114 fed material mm Thickness of 282 248 218 192 169 149 131 114 100 discharged material mm

[0058] Total deformation was 69%, water cooling was carried out after rolling was ended, and a final cooling temperature was 130°C;

[0059] (3) heat treatment: the steel plate was put into a heating furnace to be heated to 605°C and subjected to heat preservation for 120min, and after discharging, the steel plate was air cooled to room temperature.

[0060] The steel plate comprised the following chemical components by mass percent: 0.080% of C, 6.0% of Mn, 0.30% of Si, 0.015% of Al, 0.30% of Mo, 0.40% of Cr, 0.31% of Ni, 0.021% of Ti, 0.001% of S, 0.008% of P and the balance of Fe and inevitable impurity elements. The microstructure of the steel plate was a complex-phase structure of tempered martensite

+ J return austensite and had a yield strength of 690MPa, a yield ratio of 0.74 and charpy impact test side knock absorption energy at -60°C of 60J.

Claims

Claims WHAT IS CLAIMED IS: 1. A low-yield-ratio high-toughness thick gauge steel plate, comprising the following chemical components by mass percent: 0.060-0.080% of C, 5.5-6.0% of Mn, 0.10-0.30% of Si, 0.015-0.040% of Al, 0.15-0.30% of Mo, 0.20-0.40% of Cr, 0.15-0.40% of Ni, 0.01-0.03% of Ti, <0.006% of S, < 0.010% of P and the balance of Fe and inevitable impurity elements; wherein, the microstructure of the steel plate comprises a tempered martensite and a return austenite, and wherein the low-yield-ratio high-toughness thick gauge steel plate is prepared by the following method: (1) heating: controlling a heating temperature of a blank to 1070-1150°C, and carrying out

heat preservation for 90-150min after the center of the blank reaches this temperature; (2) rolling: controlling an initial rolling temperature to <1020°C, a final rolling temperature

to >820°C and total deformation to >65%, and carrying out water cooling after rolling is ended,

wherein, a final cooling temperature is <130°C; and

(3) heat treatment: heating the steel plate to 605-645°C, carrying out heat preservation for

50-120min after the center of the steel plate reaches this temperature, and then carrying out air cooling to a room temperature.
2. The low-yield-ratio high-toughness thick gauge steel plate according to claim 1, wherein, J the steel plate has a thickness of 50-100mm.
3. The low-yield-ratio high-toughness thick gauge steel plate according to claim 1 or claim 2, wherein, the volume fraction of the return austenite in the microstructure of the steel plate is 5-15%.
4. The low-yield-ratio high-toughness thick gauge steel plate according to any one of the preceding claims wherein, in the step (2), the final cooling temperature of water cooling after

rolling is ended ranges from the room temperature to 130°C.