AU2019100570A4

AU2019100570A4 - Low yield strength ratio, high strength and ductility thick gauge steel plate and manufacturing method therefor

Info

Publication number: AU2019100570A4
Application number: AU2019100570A
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: 2019-05-29
Publication date: 2019-07-04
Anticipated expiration: 2025-08-08
Also published as: WO2018227740A1; JP2020506293A; CN107312981A; AU2017418679B2; AU2017418679A1; JP6798041B2

Abstract

The disclosure discloses 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% 5 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. The steel plate has high yield strength and low yield ratio as well as good low temperature impact toughness. The disclosure also discloses a manufacturing method of the above steel plate, comprising the following steps: heating, rolling and thermal treatment. This manufacturing method only needs primary heat treatment, and is simple in process and easy to produce and implement. il4

Description

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 9 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 5 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 9 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 25 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 30 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,

2019100570 29 May 2019 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 5 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 of high strength, high toughness and low yield ratio on the thick gauge steel plate via a simple 3 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 high-strength bridge steel and a manufacturing method thereof. The yield strength of the 3 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.

SUMMARY [0009] The objective of the disclosure: the objective of the disclosure is to provide 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.

[0010] Another objective of the disclosure is to provide a manufacturing method of a 30 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.

[0011] A technical solution is as follows: a low-yield-ratio high-toughness thick gauge steel plate comprises the following chemical components by mass percent: 0.060-0.080% of C,

2019100570 29 May 2019

5.5-6.0% of Μη, 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% ofNi, 0.01-0.03% of Ti, <0.006% of S, <0.010% of P and the balance of Fe and inevitable impurity elements.

[0012] 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:

[0013] 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 3 disclosure, the content of C is controlled to a relatively low level of 0.060-0.080%.

[0014] 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 5 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%.

[0015] Si is a deoxidized element in the process of steelmaking. Appropriate Si can inhibit segregation of Mn and P, while both too high O content and segregation of Mn and P can 3 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 content cannot be too high. Thus, in the disclosure, the content of Si is controlled to 0.10-0.30%.

[0016] Al is a deoxidized element in the process of steelmaking and can also reduce the 25 quantity of solid solution N atoms, thereby improving toughness and aging strain resistance.

Furthermore, formed AIN can also refine grains, thereby further reducing the ductile-brittle transition temperature. However, excess addition can form large-size AI3O2 and AIN, and damages the toughness. Thus, in the disclosure, the content of Al is controlled to 0.015-0.040%.

[0017] Mo can improve the strength of a martensite after tempering, and can also weaken a 30 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 controlled to 0.15-0.30%.

[0018] Cr can generate solid solution strengthening, but the too high Cr content reduces

2019100570 29 May 2019 weldability. Thus, in the disclosure, the content of Cr is controlled to 0.20-0.40%.

[0019] 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 5 disclosure, the content of Ni is controlled to 0.15-0.40%.

[0020] 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%.

[0021] S easily forms MnS with Μη, 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%.

[0022] 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. [0023] 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.

[0024] 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: 25 [0025] (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. 30 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

2019100570 29 May 2019 controlled to 90-150min.

[0026] (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 5 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 3 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.

[0027] (3) heat treatment: heating the steel plate to 605-645°C, carrying out heat preservation 5 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 605-645 °C 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 3 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 complex-phase structure of tempered martensite + return austensite.

[0028] The disclosure has the beneficial effects that the low-yield-ratio high-toughness thick 25 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 30 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

2019100570 29 May 2019 [0029] Fig.l 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 [0030] Example 1: a low-yield-ratio high-toughness steel plate having a thickness of 50mm 5 was manufactured by the following steps:

[0031 ] (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 3 and the balance of Fe and inevitable impurity elements;

[0032] (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.

[0033] Table 1 Reduction Schedule In Example 1

Passes	1	2	3	4	5	6	7	8
Thickness of fed material mm	200	176	155	132	112	89	72	59
Thickness of discharged material mm	176	155	132	112	89	72	59	50

[0034] Total deformation was 75%, water cooling was carried out after rolling was ended, and a final cooling temperature was 25 °C;

[0035] (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.

[0036] 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% ofNi, 0.010% of Ti, 0.003% of S, 0.006% of P and the balance of Fe and impurity elements. The structure of the steel plate was a complex-phase structure of tempered martensite + return austensite. Fig.l 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

2019100570 29 May 2019 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 155 J.

[0037] Example 2: a low-yield-ratio high-toughness steel plate having a thickness of 70mm 5 was manufactured by the following steps:

[0038] (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 llOmin, 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 3 and the balance of Fe and inevitable impurity elements;

[0039] (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.

[0040] Table 2 Reduction Schedule In Example 2

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

[0041] Total deformation was 65%, water cooling was carried out after rolling was ended, and a final cooling temperature was 68°C;

[0042] (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.

[0043] 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% ofNi,

0.026% of Ti, 0.006% of S, 0.010% of P and the balance of Fe and impurity elements. The micro structure of the steel plate was a complex-phase structure of tempered martensite + return 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.

[0044] Example 3: a low-yield-ratio high-toughness steel plate having a thickness of 80mm was manufactured by the following steps:

[0045] (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 5 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;

[0046] (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 3 formulated according to Table 3.

[0047] Table 3 Reduction Schedule In Example 3

2019100570 29 May 2019

Passes	1	2	3	4	5	6	7	8
Thickness of fed material mm	320	282	248	211	179	143	115	95
Thickness of discharged material mm	282	248	211	179	143	115	95	80

[0048] Total deformation was 75%, water cooling was carried out after rolling was ended, and a final cooling temperature was 72°C;

[0049] (3) heat treatment: the steel plate was put into a heating furnace to be heated to 620 °C 1 5 and subjected to heat preservation for 90min, and after discharging, the steel plate was air cooled to room temperature.

[0050] 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% ofNi,

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 + 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 93 J.

[0051] Example 4: a low-yield-ratio high-toughness steel plate having a thickness of 100mm was manufactured by the following steps:

[0052] (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% ofNi, 0.021% of Ti, 0.001% of S, 0.008% ofP and the balance of Fe and inevitable impurity elements;

[0053] (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.

[0054] Table 4 Reduction Schedule In Example 4

2019100570 29 May 2019

Passes	1	2	3	4	5	6	7	8	9
Thickness of fed material mm	320	282	248	218	192	169	149	131	114
Thickness of discharged material mm	282	248	218	192	169	149	131	114	100

[0055] Total deformation was 69%, water cooling was carried out after rolling was ended, and a final cooling temperature was 130°C;

[0056] (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.

[0057] 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% ofNi, 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 + return austensite and had a yield strength of 690MPa, a yield ratio of 0.74 and charpy impact 20 test side knock absorption energy at -60 °C of 60J.

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,

5 0.015-0.040% of Al, 0.15-0.30% of Mo, 0.20-0.40% of Cr, 0.15-0.40% ofNi, 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.
3 2. The low-yield-ratio high-toughness thick gauge steel plate according to claim 1, wherein, the steel plate has a thickness of 50-100mm.

3. The low-yield-ratio high-toughness thick gauge steel plate according to claim 1, wherein, the volume fraction of the return austenite in the microstructure of the steel plate is 5-15%.
4. A manufacturing method of the low-yield-ratio high-toughness thick gauge steel plate according to claims 1-3, comprising the following steps:

(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;

3 (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 25 cooling to a room temperature.
5. The manufacturing method according to claim 4, wherein, in the step (2), the final cooling temperature of water cooling after rolling is ended ranges from the room temperature to 130 °C.