EP4578985A1

EP4578985A1 - 60-kilogram-grade cold-rolled low-alloy annealed dual-phase steel and manufacturing method therefor

Info

Publication number: EP4578985A1
Application number: EP23856629.3A
Authority: EP
Inventors: Wei Li
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2022-08-23
Filing date: 2023-08-22
Publication date: 2025-07-02
Also published as: JP2025528255A; MX2025002101A; EP4578985A4; WO2024041540A1; CN117660829A

Abstract

Disclosed in the present invention is 60-kilogram-grade cold-rolled low-alloy annealed dual-phase steel, comprising Fe and inevitable impurity elements, and further comprising the following chemical elements in percentage by mass: 0.08%-0.1% of C, 0.1%-0.2% of Si, 1.2%-1.4% of Mn, 0.01%-0.02% of Al, and 0.0020%-0.0030% of B. The chemical elements of the steel do not comprise Mo and Cr. The microstructure of the 60-kilogram-grade cold-rolled low-alloy annealed dual-phase steel comprises martensite + ferrite. Correspondingly, further disclosed in the present invention is a manufacturing method for the 60-kilogram-grade cold-rolled low-alloy annealed dual-phase steel. According to the present invention, the 60-kilogram-grade cold-rolled low-alloy annealed dual-phase steel obtained by means of reasonable chemical component design in combination with optimization processes has the characteristics of low costs and excellent mechanical properties, and has good popularization prospects and application value.

Description

Technical Field

The present disclosure relates to a metallic material and a manufacturing method therefor, in particular to a dual-phase steel and a manufacturing method therefor.

Background Art

In recent years, as the global energy crisis and environmental problems are getting more and more serious, energy conservation and safety have become the main targets that direct the development of the automobile manufacturing industry. Reducing vehicle weight is one of the measures to save energy and reduce emissions. In practical applications, it may be effective to use high-strength dual-phase steel for manufacturing vehicle structural parts due to its good mechanical performances and usability.
At present, with the development of ultra-high strength steel and the current market changes, it is desired by the market and users widely that ultra-high strength steel is economical and has high performances. With the continuous development of the trend of weight reduction and energy conservation in the automotive industry, and the rapid advancement of the technologies of global, particularly domestic, steel mills, the future development of dual-phase steel will inevitably be dominated by a combination of low cost and high performances.
With regard to the current existing technologies, researchers have conducted a lot of research on high-strength dual-phase steel and have achieved certain research results.
For example, Chinese Patent Application CN109112433A, published on January 1, 2019 and titled "590MPa-grade Cold-rolled Dual-phase Steel Without Streak Surface Defects And Production Method", discloses a 590MPa-grade cold-rolled dual-phase steel without streak surface defects, comprising the following components by weight percentage: C: 0.050% - 0.100%, Si≤0.25%, Mn: 1.20% - 2.00%, P≤0.012%, S≤0.008%, Al: 0.05% - 0.10%, Sb: 0.015 - 0.050%, and a balance of Fe and some other unavoidable impurities. The production method adopted in this technical solution comprises: smelting, continuous casting, hot charge heating, high-pressure water descaling and rough rolling, finishing rolling, laminar cooling, coiling, hot-rolled sheet pickling and continuous cold rolling, continuous annealing, air cooling, temper rolling and coiling. The 590MPa-grade cold-rolled dual-phase steel produced by this technical solution has a bright surface, no streak defects, good corrosion resistance and coating adhesion, and the mechanical performances of the steel sheet are excellent. However, the research has revealed that in the design of the chemical composition according to this technical solution, the Sb element is added to the steel, and the Mn content is rather high.
For another example, Chinese Patent Application CN109943778A, published on June 28, 2019 , and titled "590MPa-grade Cold-rolled Dual-phase Steel With Excellent Hole Expandability And Production Method Therefor", discloses a 590MPa-grade cold-rolled dual-phase steel with excellent hole expandability and a production method therefor, comprising the following chemical components: C: 0.06-0.09%, Si+Mn: 1.4-2.1%, Nb: 0.01-0.02%, Al: 0.03-0.06%, P≤0.020%, S≤0.010%, N: ≤0.006%, and a balance of Fe and unavoidable impurities. The main problem with this steel is the high Mn content and the presence of precious alloying element Nb which results in a high alloying cost.
For still another example, Chinese Patent Application CN103088258A, published on May 8, 2013 and titled "590MPa-grade Dual-phase Steel And Production Method Therefor", discloses a 590MPa-grade dual-phase steel, comprising the following components in mass percentage: C≤0.20%, Si≤1.80%, Mn≤2.00%, P≤0.050%, S≤0.015%, Nb≤0.10%, and Ti≤0.10%. The production method adopted in this technical solution comprises: molten iron pretreatment - converter smelting - alloy fine-tuning station - LF - continuous casting - continuous hot rolling, wherein the continuous hot rolling process includes: a heating temperature of 1150-1250°C, a rolling-end temperature of 800-900°C, rapid cooling to 650-750°C after rolling, air cooling for 6-11 seconds, and then rapid cooling again to the target coiling temperature which is 400-500°C. With appropriate control of the composition and process, this technical solution can achieve precise control and proportioning of the ferrite and bainite phases, which can effectively reduce the strength difference between the soft and hard phases in the material and eliminate the interface between the soft and hard phases. The product prepared has excellent strength and well-matched plasticity and toughness, and has good elongation and flanging performances. However, the steel designed by this technical solution has a high carbon equivalent and high contents of alloying elements Si and Mn, which is not conducive to welding or surface quality, and the steel does not have balanced performances.
As it can be seen, although some of the existing patent technologies for designing 590MPa dual-phase steel involve better forming performance, these technical solutions either use high C and Si contents, or include relatively high contents of alloying elements such as Cr and Mn, which is not only detrimental to the weldability, surface quality or phosphatability of the steel, but also leads to increased cost.
Therefore, in order to meet the current market demand, the present disclosure is intended to develop a new 60 kg-grade cold-rolled low-alloy annealed dual-phase steel with both low cost and excellent mechanical performances.

Summary

One of the objects of the present disclosure is to provide a new 60 kg-grade cold-rolled low-alloy annealed dual-phase steel. Owing to the reasonable design of the chemical composition, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel can acquire excellent mechanical performances while ensuring low cost. It has a yield strength of ≥340MPa, a tensile strength of ≥600MPa, and an A₅₀-gauge-length elongation at break of ≥25%. It has the characteristics of high strength and excellent elongation, and has very good promotion prospects and application value.
In order to achieve the above object, the present disclosure provides a 60 kg-grade cold-rolled low-alloy annealed dual-phase steel, comprising Fe and unavoidable impurity elements, as well as the following chemical elements in the following mass percentages:
C: 0.08% - 0.1%, Si: 0.1% - 0.2%, Mn: 1.2% - 1.4%, Al: 0.01% - 0.02%, B: 0.0020 - 0.0030%;
wherein the chemical elements of the steel are free of Mo and Cr.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel has a microstructure of martensite+ferrite.
Further, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the mass percentages of the chemical elements are:
C: 0.08% - 0.1%, Si: 0.1% - 0.2%, Mn: 1.2% - 1.4%, Al: 0.01% - 0.02%, B: 0.0020 - 0.0030%, and a balance of Fe and unavoidable impurity elements.
In the present disclosure, when designing the chemical composition of the dual-phase steel, the inventors have adopted a composition system based on C-Si-Mn, and simultaneously added a trace amount of high hardenability element B to further reduce the Mn content. Owing to the above appropriate composition design, the present disclosure enables the acquisition of a dual-phase steel with a strength of 60 kg grade, having both low cost and excellent mechanical performances, without addition of precious alloying elements such as Mo and Cr.
In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the chemical elements are designed according to the following principles:

C: In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the addition of the C element can improve the strength of the steel and the hardness of martensite in the steel. If the C content in the steel is lower than 0.08%, the strength of the steel sheet will be affected, and it's also detrimental to the formation of austenite in a desired amount and the stability thereof. If the C content in the steel is higher than 0.1%, the martensite hardness will be too high, and the grain will be coarse, which will be detrimental to the formability of the steel sheet. At the same time, the carbon equivalent will be too high, which will be detrimental to welding of the steel in use. Therefore, in order to guarantee the performances of the steel, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the mass percentage of the C element is specifically controlled in the range of 0.08% - 0.1%.
Si: In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the addition of the Si element to the steel can improve the hardenability of the steel, and the solid-dissolved Si in the steel can influence the interaction of dislocations, thereby increasing the work hardening rate and appropriately improving the elongation of the dual-phase steel, which is beneficial to acquisition of better formability. However, it should be noted that the content of the Si element in the steel should not be too high. If the content of the Si element in the steel is too high, it will be detrimental to the control of the surface quality of the steel sheet. Therefore, in order to bring into play the beneficial effects of the Si element, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the mass percentage of the Si element is controlled in the range of 0.1% - 0.2%.
Mn: In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the addition of the Mn element is not only beneficial to improving the hardenability of the steel, but also can effectively improve the strength of the steel sheet. If the content of the Mn element in the steel is lower than 1.2%, the strength of the steel sheet will be insufficient; and if the content of the Mn element in the steel is too high, that is, higher than 1.4%, the strength of the steel sheet will be so high that its formability will be degraded. In view of this, with the beneficial effects of the Mn element taken into account, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the mass percentage of the Mn element is controlled in the range of 1.2% - 1.4%.
Al: In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the Al element added to the steel can play a role in deoxidation and grain refinement. On the other hand, the lower the content of the Al element in the steel, the more favorable it is to the castability of the steel. Therefore, in order to bring into play the beneficial effects of the Al element, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the mass percentage of the Al element is controlled in the range of 0.01% - 0.02%.
B: In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the addition of the B element is not only beneficial to improving the hardenability of the steel, but also can effectively improve the strength of the steel sheet. If the content the B element in the steel is lower than 0.0020%, the strength of the steel sheet will be insufficient; if the B content in the steel is higher than 0.0030%, the strength of the steel sheet will be so high that its formability will be degraded. Therefore, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the mass percentage of the B element is controlled in the range of 0.0020% - 0.0030%.

Owing to the above chemical composition design of the dual-phase steel, the present disclosure obviates the addition of precious alloying elements such as Mo and Cr, thereby ensuring the economic efficiency. However, it should be noted that when designing the chemical composition, it is necessary to ensure that the alloying elements C, Mn and B are added in sufficient amounts to provide the dual-phase steel with sufficient hardenability, and ensure that the dual-phase steel acquires a tensile strength of 60 kg grade at a gas cooling rate of 40 - 100°C/s in continuous annealing. However, the contents of the alloying elements C, Mn and B in the dual-phase steel should not be too high; otherwise, it will be difficult to ensure that the final dual-phase steel has excellent weldability and formability.
Further, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, among the unavoidable impurities, P≤0.015%, S≤0.005%, and N≤0.005%.
In the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the P, S and N elements are all impurity elements in the steel. The lower the contents of the P, N and S elements in the steel, the better the performances of the steel. Specifically, MnS formed from the S element by compounding will seriously affect the formability, while the N element will easily cause cracks or bubbles on the surface of the slab. Therefore, when the technical conditions permit, in order to obtain a steel having better performances and higher quality, the contents of the impurity elements in the steel should be minimized, and the P, S and N elements in the steel should be specifically controlled to meet the following requirements: P≤0.015%, S≤0.005%, N≤0.005%.
Further, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the volume percentage content of martensite is >40% and ≤70%.
Further, the hardenability factor Y_Q of the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure satisfies: 1.6≤Y_Q≤2.0, wherein Y_Q=Mn+200×B, wherein the chemical elements each represent the numerical value before the mass percentage sign.
In the above technical solution of the present disclosure, while controlling the mass percentages of the chemical elements individually, the present disclosure may further control the ratios of the elements in the steel, that is, control the hardenability factor Y_Q calculated from the Mn and B elements to satisfy: 1.6≤Y_Q≤2.0, wherein Y_Q=Mn+200×B. In some embodiments, the hardenability factor Y_Q satisfies: 1.7≤Y_Q≤1.95.
In the present disclosure, the hardenability factor Y_Q can reflect the combined effect of the B and Mn elements in the steel. By controlling the hardenability factor Y_Q within the above numerical range, the mechanical performances, especially the strength, of the dual-phase steel can be further improved while the cost is reduced.
It should be noted that in the alloy design, the Mn content is the parameter that has the largest influence on the overall cost. Therefore, the addition of an appropriate amount of B according to the present disclosure can further reduce the Mn content, which is beneficial to reducing the cost. At the same time, the combined effect of Mn and B on hardenability is utilized to further improve the mechanical performances of the dual-phase steel, and it's also beneficial to improving the processing performance in on-site production, including the rolling stability of hot rolling and cold rolling.
Further, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the particle sizes of both martensite and ferrite are not greater than 15 µm.
In the above technical solution of the present disclosure, the particle sizes of both martensite and ferrite in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel are not greater than 15 microns, which will help to improve the strength and processing performance of the steel.
Further, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the particle size of martensite is in the range of 9.6-14.2 µm, and the particle size of ferrite is in the range of 10.7-14.5 µm.
Further, in the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure, the volume percentage content of martensite is >40% and ≤70%; preferably, the volume percentage content of martensite is in the range of 42-65%.
Further, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure has a yield strength of ≥340MPa, preferably ≥350MPa, a tensile strength of ≥600MPa, preferably ≥620MPa, and an A₅₀-gauge-length elongation at break of ≥25%, preferably ≥27%.
In some embodiments, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure has a yield strength of ≥360 MPa, a tensile strength of ≥630 MPa, and an A₅₀-gauge-length elongation at break is ≥27%.
In some embodiments, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure has a yield strength in the range of 352-390MPa, a tensile strength in the range of 603-683MPa, and an A₅₀-gauge-length elongation at break in the range of 25.8-30.3%.
In some embodiments, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure is able to withstand a force of at least 60 kg per square centimeter, preferably a force of 60-70 kg per square centimeter, and more preferably a force of 63-70 kg per square centimeter.
Accordingly, another object of the present disclosure is to provide a method for manufacturing the above 60 kg-grade cold-rolled low-alloy annealed dual-phase steel. This manufacturing method is easy and simple to implement, and it can be used to effectively prepare the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure.
In order to achieve the above object, the present disclosure proposes a method for manufacturing the above 60 kg-grade cold-rolled low-alloy annealed dual-phase steel, comprising the following steps:

(1) Smelting and casting;
(2) Hot rolling: firstly heating a continuously cast product to 1160-1190°C, holding for 150 min or longer (e.g., 150-200 minutes), then hot rolling with a rolling-end temperature being 850-890°C, then rapidly cooling at a rate of 30-80°C/s after the rolling; subsequently coiling with a coiling temperature being 500-540°C, and then air-cooling after the coiling;
(3) Cold rolling;
(4) Annealing: controlling an annealing soaking temperature to be 825-855°C and an annealing time to be 40-200s; then, reducing the temperature to a rapid cooling start temperature of 735-760°C at a rate of 3-5°C/s, and then performing rapid cooling at a rate of 40-100°C/s, with a rapid cooling end temperature being controlled to be 220-260°C;
(5) Tempering;
(6) Temper rolling.

In the above technical solution of the present disclosure, the inventors have optimized the design of the annealing process. Specifically, the annealing soaking temperature is controlled in the range of 825 - 855°C, preferably in the range of 830 - 840°C. Controlling the annealing soaking temperature within the above numerical range can make the grain size of the resulting dual-phase steel finer and the mechanical performances and formability better.
Further, in the manufacturing method of the present disclosure, in step (4), the annealing soaking temperature is 830-840°C.
Further, in the manufacturing method of the present disclosure, in step (2), the continuously cast product is first heated to 1160-1190°C, held for 150 min or longer, then hot rolled with the rolling-end temperature being 850-890°C, then rapidly cooled at a rate of 30-80°C/s after the rolling; then coiled at a coiling temperature of 500-540°C, and then air-cooled after the coiling.
Further, in the manufacturing method of the present disclosure, in step (3), the cold rolling reduction ratio is controlled to be 50 - 70%.
Further, in the manufacturing method of the present disclosure, in step (5), the tempering temperature is controlled to be 220-260° C, and the tempering time is controlled to be 100-400 s.
Further, in the manufacturing method of the present disclosure, in step (6), the temper rolling reduction ratio is controlled to be ≤0.5%.
Compared with the prior art, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure and the manufacturing method therefor have the following advantages and beneficial effects:
A new 60 kg-grade cold-rolled low-alloy annealed dual-phase steel is developed according to the present disclosure. The use of a reasonable chemical composition design in cooperation with an optimized manufacturing process allows for the acquisition of a dual-phase steel with both low cost and excellent performances (especially high strength and excellent elongation) with no addition of the Mo and Cr elements.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel designed according to the present disclosure exhibits very high strength and excellent elongation. Particularly, it has a yield strength of ≥340 MPa, a tensile strength of ≥600 MPa, and an A₅₀-gauge-length elongation at break of ≥25%. It can meet the needs of the market and users effectively, and has very good promotion prospects and application value.

Detailed Description

The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of the present disclosure and the manufacturing method therefor will be further explained and illustrated with reference to the following specific Examples. However, such explanation and illustration do not constitute any improper limitation on the technical solution of the present disclosure.

Examples 1-6 and Comparative Examples 1-14

Table 1-1 lists the mass percentages of the various chemical elements designed for the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 and the comparative steels of Comparative Examples 1-14.

Table 1. (wt%, the balance is Fe and other unavoidable impurities except P, S and N)

No.	Chemical elements
No.	C	Si	Mn	B	Al	P	S	N
Ex. 1	0.082	0.12	1.38	0.0022	0.012	0.011	0.0017	0.0045
Ex. 2	0.085	0.16	1.25	0.0027	0.015	0.009	0.0024	0.0038
Ex. 3	0.091	0.15	1.29	0.0025	0.018	0.013	0.0044	0.0031
Ex. 4	0.092	0.13	1.22	0.0024	0.017	0.014	0.0028	0.0042
Ex. 5	0.099	0.18	1.33	0.0029	0.011	0.012	0.0037	0.0033
Ex. 6	0.095	0.16	1.30	0.0025	0.014	0.015	0.0048	0.0044
Comp. Ex. 1	0.078	0.13	1.27	0.0025	0.014	0.011	0.0022	0.0037
Comp. Ex. 2	0.011	0.11	1.35	0.0028	0.018	0.009	0.0045	0.0033
Comp. Ex. 3	0.088	0.19	1.14	0.0024	0.013	0.012	0.0033	0.0028
Comp. Ex. 4	0.095	0.14	1.56	0.0024	0.019	0.013	0.0015	0.0035
Comp. Ex. 5	0.084	0.16	1.21	0.0014	0.016	0.011	0.0042	0.0034
Comp. Ex. 6	0.093	0.17	1.45	0.0035	0.012	0.008	0.0029	0.0029
Comp. Exs. 7-14	0.084	0.15	1.36	0.0024	0.017	0.014	0.0037	0.0036

Table 1-2 lists the values of the hardenability factor Y_Q of the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 and the comparative steels of Comparative Examples 1-14.

Table 1-2.

No.	Hardenability factor Y_Q
Ex. 1	1.82
Ex. 2	1.79
Ex. 3	1.79
Ex. 4	1.7
Ex. 5	1.91
Ex. 6	1.80
Comp. Ex. 1	1.77
Comp. Ex. 2	1.91
Comp. Ex. 3	1.62
Comp. Ex. 4	2.04
Comp. Ex. 5	1.49
Comp. Ex. 6	2.15
Comp. Exs. 7-14	1.84

Note: In Table 1-2 above, the hardenability factor Y_Q=Mn+200×B, wherein the chemical elements each represent the numerical value before the mass percentage sign.

The 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 according to the present disclosure and the comparative steels of Comparative Examples 1-14 were prepared by the following steps:

(1) Smelting and casting were carried out according to the chemical composition design shown in Table 1-1 and Table 1-2 to produce a continuously cast product.
(2) Hot rolling: The continuously cast product was subjected to hot rolling, wherein the continuously cast product was first heated to 1160-1190°C, held for 150 min or longer, then hot rolled with the rolling-end temperature being 850-890°C, then rapidly cooled at a rate of 30-80°C/s after the rolling; then coiled with the coiling temperature being controlled to be 500-540°C, and then air-cooled after the coiling.
(3) Cold rolling: The steel coil was subjected to cold rolling, and the cold rolling reduction ratio was controlled to be 50-70%.
(4) Annealing: The annealing soaking temperature was controlled to be 825-855°C, preferably 830-840°C; the annealing time was controlled to be 40-200s; then, the temperature was reduced to the rapid cooling start temperature of 735-760°C at a rate of 3-5°C/s; and then rapid cooling was performed at a rate of 40-100°C/s, with the rapid cooling end temperature being controlled to be 220-260°C.
(5) Tempering: The tempering temperature was controlled to be 220-260°C, and the tempering time was controlled to be 100-400 s.
(6) Temper rolling: The temper rolling reduction ratio was controlled to be ≤ 0.5% to obtain the finished dual-phase steel product..

In the technical solution designed according to the present disclosure, the chemical compositions designed for the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 prepared according to the present disclosure and the related processes all met the design specification requirements of the present disclosure.
Accordingly, although the comparative steels of Comparative Examples 1-14 were also prepared using the formulations shown in Tables 1-1 and 1-2 in combination with the above process flow, in order to highlight the superiority of the technical solution according to the present disclosure, the comparative steels of Comparative Examples 1-14 were designed to include parameters in chemical composition and/or related manufacturing process that didn't meet the design requirements of the present disclosure.
Specifically, the chemical compositions of the comparative steels of Comparative Examples 1-6 all included parameters that failed to meet the design requirements of the present disclosure; and although the chemical compositions of the steels corresponding to Comparative Examples 7-14 met the design requirements of the present disclosure, the related process parameters all included parameters that failed to meet the design specifications of the present disclosure.

Table 2-1 and Table 2-2 list the specific process parameters in the above process steps (1) to (6) for the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 and the comparative steels of Comparative Examples 1-14.

Table 2-1.

No.	Step (2)					Step (3)
No.	Heating temperature (°C)	Holding time (min)	Rolling-end temperature of hot rolling (°C)	Cooling rate (°C/s)	Coiling temperature (°C)	Cold rolling reduction ratio (%)
Ex. 1	1185	155	865	60	525	50
Ex. 2	1167	168	872	35	509	58
Ex. 3	1188	174	890	45	533	55
Ex. 4	1172	163	888	54	527	60
Ex. 5	1176	152	864	64	519	54
Ex. 6	1184	192	855	77	538	68
Comp. Ex. 1	1166	159	865	74	528	67
Comp. Ex. 2	1175	164	885	44	505	58
Comp. Ex. 3	1187	181	875	50	511	50
Comp. Ex. 4	1175	176	850	62	522	52
Comp. Ex. 5	1163	166	880	30	535	58
Comp. Ex. 6	1188	167	867	76	508	52
Comp. Ex. 7	1155	184	879	46	526	64
Comp. Ex. 8	1199	176	890	36	533	50
Comp. Ex. 9	1163	173	866	55	485	62
Comp. Ex. 10	1177	160	885	60	555	50
Comp. Ex. 11	1190	158	856	70	528	55
Comp. Ex. 12	1166	164	869	38	533	63
Comp. Ex. 13	1185	159	854	49	534	55
Comp. Ex. 14	1170	162	865	55	540	60

Table 2-2.

No.	Step (4)						Step (5)		Step (6)
No.	Annealing soaking temperature (°C)	Annealing time (s)	Cooling rate (°C/s)	Rapid cooling start temperature (°C)	Rapid cooling rate (°C/s)	Rapid cooling end temperature (°C)	Tempering temperature (°C)	Tempering time (s)	Temper rolling reduction ratio (%)
Ex. 1	826	160	3	736	95	250	250	150	0.3
Ex. 2	834	80	4	755	44	225	225	160	0.1
Ex. 3	833	115	5	760	75	245	245	200	0.4
Ex. 4	828	180	3	748	70	260	260	280	0.2
Ex. 5	844	200	5	745	60	235	235	320	0.5
Ex. 6	853	55	4	742	82	240	240	220	0.3
Comp. Ex. 1	850	195	5	740	68	242	242	240	0.3
Comp. Ex. 2	826	85	4	758	55	246	246	290	0.1
Comp. Ex. 3	842	150	4	744	48	221	221	310	0.4
Comp. Ex. 4	833	90	3	753	58	238	238	360	0.3
Comp. Ex. 5	826	110	4	755	45	252	252	220	0.2
Comp. Ex. 6	839	180	5	746	62	225	225	260	0.1
Comp. Ex. 7	845	50	4	738	58	238	238	340	0.5
Comp. Ex. 8	839	75	5	740	70	252	252	190	0.4
Comp. Ex. 9	847	140	4	749	82	227	227	250	0.2
Comp. Ex. 10	828	135	3	742	90	234	234	350	0.3
Comp. Ex. 11	850	85	4	752	66	278	278	370	0.1
Comp. Ex. 12	843	165	5	757	80	200	200	140	0.4
Comp. Ex. 13	815	120	4	746	45	246	246	270	0.1
Comp. Ex. 14	860	80	4	735	56	242	242	190	0.3

It should be noted that in Table 2-2 above, the rapid cooling end temperature was the same as the tempering temperature in each of the Examples and Comparative Examples. This is because the tempering operation was performed right after the rapid cooling operation was completed in the actual process operations.
Accordingly, after the above manufacturing process was completed, for the finished dual-phase steel products of Examples 1-6 and Comparative Examples 1-14, the inventors sampled the dual-phase steels of the Examples and Comparative Examples to obtain the corresponding steel sheet samples, and observed and analyzed the microstructures of the steel sheet samples of the Examples and Comparative Examples. It was found by the observation that the microstructure of each of the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 and the comparative steels of Comparative Examples 1-14 was martensite + ferrite.

To this end, the inventors further analyzed the microstructure of the steel sheet of each of the Examples and Comparative Examples to obtain the fraction of the martensite phase, martensite particle size, and ferrite particle size in the microstructure of the steel sheet of each of Examples 1-6 and Comparative Examples 1-14. The relevant test results are specifically listed in Table 3 below.

Table 3.

No.	Fraction of martensite phase (%)	Martensite particle size (µm)	Ferrite particle size (µm)
Ex. 1	52	12.4	13.5
Ex. 2	44	13.3	14.5
Ex. 3	49	14.2	13.5
Ex. 4	42	11.8	12.4
Ex. 5	59	9.6	12.5
Ex. 6	65	11.1	10.7
Comp. Ex. 1	36	15.6	16.2
Comp. Ex. 2	70	12.6	12.5
Comp. Ex. 3	37	14.8	15.3
Comp. Ex. 4	73	10.2	9.9
Comp. Ex. 5	32	15.5	16.3
Comp. Ex. 6	72	10.8	11.5
Comp. Ex. 7	34	14.4	15.5
Comp. Ex. 8	74	11.5	12.2
Comp. Ex. 9	72	9.6	10.8
Comp. Ex. 10	36	15.8	16.6
Comp. Ex. 11	37	14.6	15.2
Comp. Ex. 12	75	10.6	11.7
Comp. Ex. 13	74	11.8	12.3
Comp. Ex. 14	38	14.8	15.3

As it can be seen from Table 3 above, in the present disclosure, the microstructure of the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel prepared in each of Examples 1-6 was martensite + ferrite. Further, the volume percentage content (phase fraction) of martensite was in the range of 42-65%, the martensite particle size was in the range of 9.6-14.2 µm, and the ferrite particle size was in the range of 10.7-14.5 µm.
In the present disclosure, in order to identify the performances of the steel of each of the Examples and Comparative Examples, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels prepared in Examples 1-6 and the comparative steels prepared in Comparative Examples 1-14 were sampled to obtain the corresponding steel sheet samples. Tensile test was performed on the resulting steel sheet samples of Examples 1-6 and Comparative Examples 1-14 to obtain performance data of the steels of Examples 1-6 and Comparative Examples 1-14. The relevant test results are listed in Table 3 below.
The method for testing the relevant mechanical performances is as follows:
Tensile test: The tensile test was conducted according to GB/T228-2010: Metallic materials - Tensile test - Method of test at room temperature, so as to obtain the yield strength, tensile strength and A₅₀-gauge-length elongation at break, and kilogram force was obtained by calculation with the test results. The A₅₀-gauge-length elongation at break means that the parallel length * width of the tensile sample is 50 mm × 25 mm.

Table 3 lists the mechanical performance test results of the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 and the comparative steels of Comparative Examples 1-14.

Table 3.

No.	Yield strength (MPa)	Tensile strength (MPa)	A₅₀-gauge-length elongation at break (%)	Kgf (kg/cm²)
Ex. 1	384	655	27.6	66
Ex. 2	358	611	28.5	61
Ex. 3	362	629	27.5	63
Ex. 4	352	603	30.3	60
Ex. 5	390	666	26.5	67
Ex. 6	385	683	25.8	65
Comp. Ex. 1	333	585	32.6	59
Comp. Ex. 2	452	706	24.4	71
Comp. Ex. 3	345	578	33.3	58
Comp. Ex. 4	466	722	23.6	72
Comp. Ex. 5	323	566	34.3	57
Comp. Ex. 6	420	711	24.3	71
Comp. Ex. 7	342	577	29.6	58
Comp. Ex. 8	448	728	23.1	73
Comp. Ex. 9	450	725	24.2	73
Comp. Ex. 10	338	569	32.6	57
Comp. Ex. 11	343	577	31.8	58
Comp. Ex. 12	454	724	24.5	72
Comp. Ex. 13	444	722	24.5	72
Comp. Ex. 14	337	575	33.4	58

Note: Kg force, namely kilogram-force, is a commonly used unit of force. The international unit of force is Newton. 1 kgf refers to the gravity exerted on an object of 1 kilogram (i.e. 9.8N). So 1 kgf = 9.8 N.

As it can be seen from Table 3, in the present disclosure, the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6 prepared by the technical solution designed according to the present disclosure had quite excellent mechanical performances, including a yield strength in the range of 352-390 MPa, a tensile strength in the range of 603-683 MPa, and an A₅₀-gauge-length elongation at break in the range of 25.8-30.3%. The dual-phase steels of the Examples each acquired a tensile strength of greater than 600 MPa with no addition of precious alloying elements such as Mo and Cr. They were all 60 kg-grade cold-rolled low-alloy annealed dual-phase steels and exhibited good elongation.
Compared with the 60 kg-grade cold-rolled low-alloy annealed dual-phase steels of Examples 1-6, the overall performances of the comparative steels of Comparative Examples 1-14 were obviously inferior, because they included parameters in chemical composition and/or related manufacturing process that didn't meet the requirements of the present disclosure.
In summary, it can be seen that a dual-phase steel with both low cost and excellent performances is obtained in the present disclosure by designing a reasonable chemical composition in combination with an optimized process according to the present disclosure. The dual-phase steel exhibits excellent mechanical performances, including a yield strength of ≥340MPa, a tensile strength of ≥600MPa, and an A₅₀-gauge-length elongation at break of ≥25%. It has good application prospects.
It should be noted that combinations of the various technical features in this case are not limited to the combinations disclosed in the claims of this case or the combinations disclosed in the specific Examples. All technical features disclosed in this case can be combined freely or associated in any way unless a contradiction occurs.
It should also be noted that the Examples listed above are only specific embodiments of the present disclosure. Obviously, the present disclosure is not limited to the above Examples, and changes or modifications made thereto can be directly derived from the present disclosure or easily conceived of by those skilled in the art, all of which fall within the protection scope of the present disclosure.

Claims

A 60 kg-grade cold-rolled low-alloy annealed dual-phase steel, comprising Fe and unavoidable impurity elements, as well as the following chemical elements in the following mass percentages:
C: 0.08% - 0.1%, Si: 0.1% - 0.2%, Mn: 1.2% - 1.4%, Al: 0.01% - 0.02%, B: 0.0020 - 0.0030%;

wherein the chemical elements of the steel are free of Mo and Cr;

the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel has a microstructure of martensite+ferrite.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1, wherein the mass percentages of the chemical elements are:
C: 0.08% - 0.1%, Si: 0.1% - 0.2%, Mn: 1.2% - 1.4%, Al: 0.01% - 0.02%, B: 0.0020 - 0.0030%, and a balance of Fe and unavoidable impurity elements.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1 or 2, wherein among the unavoidable impurity elements, P≤0.015%, S≤0.005%, N≤0.005%.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1 or 2, wherein the martensite has a volume percentage content of >40% and ≤70%; preferably, the martensite has a volume percentage content of 42-65%.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1 or 2, wherein the steel has a hardenability factor Y_Q that satisfies: 1.6≤Y_Q≤2.0, wherein Y_Q=Mn+200×B, wherein chemical elements each represent a numerical value before a mass percentage sign.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1 or 2, wherein the martensite and the ferrite each have a particle size of no greater than 5 µm.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1 or 2, wherein it has a yield strength of ≥340MPa, a tensile strength of ≥600MPa, and an A₅₀-gauge-length elongation at break of ≥25%.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 7, wherein it has a yield strength of ≥360 MPa, a tensile strength of ≥630 MPa, and an A₅₀-gauge-length elongation at break of ≥27%.
The 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of claim 1 or 2, wherein the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel is able to withstand a force of at least 60 kg per square centimeter, preferably a force of 60-70 kg per square centimeter.
A method for manufacturing the 60 kg-grade cold-rolled low-alloy annealed dual-phase steel of any one of claims 1-9, wherein the method includes the following steps:
(1) Smelting and casting;

(2) Hot rolling: firstly heating a continuously cast product to 1160-1190°C, holding for 150 min or longer, then hot rolling with a rolling-end temperature being 850-890°C, then rapidly cooling at a rate of 30-80°C/s after the rolling; subsequently coiling with a coiling temperature being 500-540°C, and then air-cooling after the coiling;

(3) Cold rolling;

(4) Annealing: controlling an annealing soaking temperature to be 825-855°C and an annealing time to be 40-200s; then, reducing the temperature to a rapid cooling start temperature of 735-760°C at a rate of 3-5°C/s, and then performing rapid cooling at a rate of 40-100°C/s, with a rapid cooling end temperature being controlled to be 220-260°C;

(5) Tempering;

(6) Temper rolling.
The method of claim 10, wherein in step (4), the annealing soaking temperature is 830-840°C.
The method of claim 10, wherein in step (3), a cold rolling reduction ratio is controlled to be 50 - 70%.
The method of claim 10, wherein in step (5), the tempering temperature is controlled to be 220-260°C, and the tempering time is controlled to be 100-400 s.
The method of claim 10, wherein in step (6), a temper rolling reduction ratio is controlled to be ≤0.5%.