EP4578984A1

EP4578984A1 - Dual-phase steel and manufacturing method therefor

Info

Publication number: EP4578984A1
Application number: EP23856628.5A
Authority: EP
Inventors: Wei Li; Xiaodong Zhu
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: EP4578984A4; CN117660831A; JP2025528243A; US20260055491A1; MX2025002100A; WO2024041539A1

Abstract

Disclosed in the present invention is dual-phase steel, comprising, in addition to 90% or more of Fe and inevitable impurities, the following components in percentages by mass: C: 0.09-0.11%, Si: 0.1-0.3%, Mn: 1.4-1.6%, Al: 0.01-0.03%, Nb: 0.01-0.03%, Ti: 0.01-0.03%, and B: 0.0020-0.0030%. In the present invention, by rationally controlling the chemical components of steel, dual-phase steel with both low cost and high mechanical properties is obtained. Further disclosed in the present invention is a manufacturing method for the dual-phase steel.

Description

Technical Field

The present disclosure pertains to the field of metallurgy, and in particular to a dual-phase steel and a method for manufacturing the same.

Background Art

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. High-strength dual-phase steel has good mechanical performances and usability, and is widely used in the manufacturing of vehicle structural parts.
In response to the development of ultra-high strength steel and the current market changes, it is desired that ultra-high strength steel can have both low cost and high performances. At present, 780DP (dual-phase) steel is still the mainstream steel in applications, accounting for about 60% of the total amount of dual-phase steel, widely used in various types of structural parts and safety parts.
Canadian Patent Application CA2526488 (published on December 2, 2004 ) discloses a cold-rolled steel sheet having a chemical composition consisting of: C: 0.05-0.09%; Si: 0.4-1.3%; Mn: 2.5-3.2%; optionally Mo: 0.05-0.5% or Ni: 0.05-2%; P: 0.001-0.05%; S≤0.08*Ti-3.43*N+0.004; N≤0.006%; Al: 0.005-0.10%; Ti: 0.001-0.045%; optionally Nb≤0.04% or B: 0.0002-0.0015%; optionally Ca for treatment; and a balance of Fe and unavoidable impurities. The steel sheet has a bainite content of greater than 7%, and Pcm≤0.3. It is obtained by hot rolling at a temperature not less than Ar3, coiling at a temperature of 700°C or lower, cold rolling, annealing in the range of 700 - 900°C, and rapid cooling which starts from 550-700°C, so that a high-strength steel with a strength of at least 780MPa is obtained ultimately. The steel has the characteristics of high local deformability and low hardness in the welding zone. However, the use of a high content of Mn in the design of the steel will cause severe banded structure and thus cause non-uniform mechanical performances of the steel. In addition, the addition of a relatively large amount of Si is not conducive to the surface quality and welding performance of the steel while a high content of Mn is added.
U.S. Patent Application US20050167007 (published on August 4, 2005 ) discloses a method for manufacturing a high-strength steel sheet having a chemical composition consisting of: C: 0.05-0.13%, Si: 0.5-2.5%, Mn: 0.5-3.5%, Cr: 0.05-1%, Mo: 0.05-0.6%, Al≤0.1%, S≤0.005%, N≤0.01%, P≤0.03%, and optionally at least one of Ti: 0.005-0.05%, Nb: 0.005-0.05%, or V: 0.005-0.2%. It is obtained by hot rolling at a temperature not less than Ar3, coiling at 450-700°C, annealing, quenching by cooling from 700-600°C at a cooling rate of 100°C/s, and then tempering between 180-450°C, so that a high-strength steel with a tensile strength of 780MPa and a hole expansion ratio of more than 50% is obtained ultimately. The main problem with this steel is that the total amount of alloying elements is too high and the Si content is high, which is not conducive to the weldability and phosphatability of the steel.
Chinese Patent Application CN101363099A (published on February 11, 2009 ) discloses an ultra-high strength dual-phase steel comprising C: 0.14-0.21%, Si: 0.4-0.9%, Mn: 1.5-2.1%, P≤0.02%, S≤0.01%, Nb: 0.001-0.05%, V: 0.001-0.02%, and a balance of Fe and unavoidable impurities. The high-strength dual-phase steel is obtained by holding the steel at 760-820°C after hot rolling and cold rolling, cooling at a rate of 40-50°C/s, and aging at 240-320°C for 180-300s. However, the carbon equivalent in this steel is designed to be high, and the steel does not have balanced performances.
Chinese Patent Application CN103060703A discloses a 780MPa-grade cold-rolled dual-phase strip steel having a microstructure of a fine equiaxed ferrite matrix and martensite islands evenly distributed on the ferrite matrix, and comprising chemical elements by mass percentage of: C: 0.06-0.1%; Si≤0.28%; Mn: 1.8-2.3%; Cr: 0.1-0.4%; no Mo in the case of Cr≥0.3%; Mo=0.3-Cr in the case of Cr <0.3%,; Al: 0.015-0.05%; at least one of Nb and Ti elements with Nb+Ti being in the range of 0.02-0.05%; and a balance of Fe and other unavoidable impurities. This 780MPa-grade cold-rolled dual-phase strip steel has high strength, good elongation, good phosphatability and small anisotropy of mechanical performances. However, the alloy designed in this disclosure includes a large amount of alloying elements such as Cr and Mo, which is not conducive to reducing costs.
As it can be seen, in the prior art, although some dual-phase steels have good formability, they have a high C content or a high Si content, or contain a relatively large amount of Cr, Ni, Mo and other alloying elements, not conducive to the weldability, surface quality and phosphatability of the steels, and the cost is relatively high. Some steels with a high Si content have a high hole expansion ratio and good bendability, but they have a high yield ratio and reduced stamping performance. So far, 780MPa dual-phase steel with both high mechanical performances and low cost has not been obtained.

Summary

In order to solve the above problems, the present disclosure provides a dual-phase steel by reasonably designing the element composition, and the dual-phase steel has both low cost and excellent mechanical performances.
In a first aspect of the present disclosure, there is provided a dual-phase steel comprising, in addition to at least 90% of Fe and unavoidable impurities, the following components in mass percentage: C: 0.09% - 0.11%, Si: 0.1% - 0.3%, Mn: 1.4% - 1.6%, Al: 0.01% - 0.03%, Nb: 0.01% - 0.03%, Ti: 0.01% - 0.03%, and B: 0.0020% - 0.0030%.
In a second aspect of the present disclosure, there is provided a dual-phase steel comprising the following components in mass percentage: C: 0.09% - 0.11%, Si: 0.1% - 0.3%, Mn: 1.4% - 1.6%, Al: 0.01% - 0.03%, Nb: 0.01% - 0.03%, Ti: 0.01% - 0.03%, and B: 0.0020% - 0.0030%, and a balance of Fe and unavoidable impurities.
Preferably, the dual-phase steel of the present disclosure is free of Mo and Cr.
The composition design of the dual-phase steel of the present disclosure is mainly based on C-Si-Mn, and a trace amount of high hardenability element B is added to further reduce the Mn content. In addition, the addition of trace amounts of Nb and Ti can inhibit the growth of austenite grains and refine the grains effectively. Owing to the above appropriate composition design, the present disclosure allows for the acquisition of a dual-phase steel with a strength of 80 kg grade, having both low cost and excellent mechanical performances, without addition of precious alloying elements such as Mo and Cr.
In the present disclosure, the chemical elements are designed according to the following principles:

C: The addition of the C element can improve the strength of the steel and increase the hardness of martensite in the steel. If the C content in the steel is lower than 0.09%, 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 0.11%, 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 the present disclosure, the C content is controlled in the range of 0.09% - 0.11%.
Si: The addition of the Si element to the steel can improve hardenability. In addition, 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 if the Si content in the steel is too high, it will be detrimental to the control of surface quality. Therefore, in the present disclosure, the Si content is controlled in the range of 0.1% - 0.3%.
Mn: The addition of the Mn element is beneficial to improving the hardenability of the steel, and can improve the strength of the steel sheet effectively. If the Mn content in the steel is lower than 1.4%, the strength of the steel sheet will be insufficient; if the Mn content in the steel is higher than 1.6%, the strength of the steel sheet will be too high, such that its formability will be degraded. Therefore, in the present disclosure, the Mn content is controlled in the range of 1.4% - 1.6%.
B: The addition of the B element is beneficial to improving the hardenability of the steel, and can improve the strength of the steel sheet effectively. If the B content 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 too high, such that its formability will be degraded. Therefore, in the present disclosure, the B content is controlled in the range of 0.0020% - 0.0030%.
AI: The Al element may be added to the steel to play a role in deoxidation and grain refinement. On the other hand, the lower the Al content is, the more favorable it is to the castability during smelting. In the present disclosure, the Al content is controlled in the range of 0.01% - 0.03%.
Nb: Nb is a strong carbide-forming element that refines grains. After adding a small amount of Nb to microalloyed steel, during a controlled rolling process, it can lead to a strain-induced precipitation phase to significantly reduce the recrystallization temperature of deformed austenite through particle pinning and subgrain boundaries, provide nucleation particles, and have a significant effect on grain refinement. In the process of continuous annealing and austenitization, the soaked, undissolved carbide and nitride particles will prevent coarsening of the soaked austenite grains by the mechanism of grain boundary pinning by particles, thereby refining the grains effectively. However, the Nb content shall not be too high; otherwise, the production cost will be increased. Therefore, in the present disclosure, the Nb content is controlled in the range of 0.01% - 0.03%, preferably in the range of 0.015% - 0.025%.
Ti: Similar to the function of Nb, the strong carbide-forming element Ti added to the steel also shows a strong effect of inhibiting the growth of austenite grains at high temperatures, which helps to refine the grains. At the same time, if Ti is added in a large amount, the production cost will also be increased. Therefore, in the present disclosure, the Ti content is controlled in the range of 0.01% - 0.03%, preferably in the range of 0.015% - 0.025%.

Owing to the above 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 economical efficiency. It should be ensured that the alloying elements C, Mn and B are added to the dual-phase steel of the present disclosure in sufficient amounts to provide the dual-phase steel with sufficient hardenability, and ensure that the dual-phase steel acquires a high strength of 80 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.
Preferably, in the dual-phase steel of the present disclosure, the Nb content is 0.015% - 0.025%, and/or the Ti content is 0.015% - 0.025%. If the Nb and Ti contents are too low, the corresponding grain refinement effect will not be significant. If the Nb and Ti contents are too high, the cost will be increased.
Preferably, the hardenability factor Y_Q of the dual-phase steel of the present disclosure satisfies: 1.9≤Y_Q≤2.1, which is calculated according to the following equation: Y_Q=Mn+200×B, wherein Mn and B each represent the numerical value before the percentage sign of the mass percentage content of the corresponding element. The hardenability factor Y_Q reflects the combined effect of B and Mn 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. If the Y_Q value is lower than 1.9, the steel strength obtained cannot reach the 80 kg grade; if the Y_Q value is higher than 2.1, the corresponding elongation of the steel cannot meet the requirement. 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.
Preferably, in the dual-phase steel of the present disclosure, the contents of the impurity elements in mass percentage satisfy: P≤0.015%, S≤0.003%, and N≤0.005%.
P, N and S are all unavoidable impurity elements in the steel. The lower the contents of P, N and S in the steel, the better the performances of the steel. In particular, MnS formed from S affects the formability seriously, and N causes cracks or bubbles on the slab surface readily. Therefore, in the dual-phase steel of the present disclosure, it's controlled that P≤0.015%, S≤0.003%, and N≤0.005%.
In the present disclosure, the microstructure of the dual-phase steel includes martensite and ferrite. Preferably, the microstructure of the dual-phase steel of the present disclosure consists of martensite and ferrite, wherein the volume percentage content of martensite is 55% or higher and 85% or lower, preferably 58-80%. The content of martensite in the steel structure of the present disclosure is directly related to the strength of the dual-phase steel. At the same time, the presence of some ferrite in the steel is necessary, so that the soft and hard phases can cooperate and coordinate when the steel is deformed, thereby improving the overall performances of the steel.
Preferably, the average grain sizes of both martensite and ferrite in the dual-phase steel of the present disclosure are 5 µm or less, and more preferably, the grain sizes of both martensite and ferrite in the dual-phase steel are 5 µm or less, which helps to improve the strength and processing performance of the steel.
Preferably, the dual-phase steel of the present disclosure is a 80 kg-grade dual-phase steel having a yield strength of ≥420 MPa, preferably ≥430 MPa, a tensile strength of ≥800 MPa, preferably ≥820 MPa, and an A₅₀-gauge-length elongation at break of ≥18%, preferably ≥19%. In some embodiments, the dual-phase steel of the present disclosure has a yield strength of ≥450 MPa, a tensile strength of ≥820 MPa, and an A₅₀-gauge-length elongation at break of ≥20%.
The "80 kg grade" referred to in the present disclosure means that the steel of the present disclosure is able to withstand a force of at least 80 kg per square centimeter. In some embodiments, the steel of the present disclosure is able to withstand a force of 80 - 90 kilograms per square centimeter, preferably 83 - 90 kilograms per square centimeter.
Another aspect of the present disclosure provides a method for manufacturing the above dual-phase steel, comprising the following steps: smelting and continuously casting molten steel to obtain a continuously cast product; hot rolling the continuously cast product; cold rolling; annealing; tempering; and temper rolling.
Preferably, in the annealing step, the annealing soaking temperature is controlled to be 825-855°C, and the annealing time is 40-200s. Then, the temperature is reduced to the rapid cooling start temperature of 735-760°C at a rate of 3-5°C/s, followed by rapid cooling at a rate of 40-100°C/s (e.g., 40-80°C/s), with the rapid cooling end temperature being 220-260°C.
Preferably, the annealing soaking temperature is 830-840°C. When the annealing soaking temperature is within the above numerical range, the grain size of the dual-phase steel obtained can be finer, and the mechanical performances and formability can be better.
Preferably, in the hot rolling step, the continuously cast product is first heated to 1160-1190°C, held for 150 minutes or longer (e.g., 150-210 minutes), 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; coiled with the coiling temperature being controlled to be 500-540°C, and then air-cooled after the coiling. For the 80 kg-grade dual-phase steel of the present disclosure, the production requirements can be met without slow cooling of or other treatments on the hot-rolled coil.
Preferably, in the cold rolling step, the cold rolling reduction ratio is 50 - 70%.
Preferably, in the tempering step, the tempering temperature is 220-260°C, and the tempering time is 100-400 s. A long tempering time is beneficial to reducing the hardness difference between the ferrite and martensite phases of the dual-phase steel. Nonetheless, the tempering time should not be too long; otherwise, the steel with a strength lower than 80 kg grade will be obtained.
Preferably, in the temper rolling step, the temper rolling reduction ratio is ≤ 0.3%.
Owing to the reasonable chemical composition design in cooperation with the optimized manufacturing process, the present disclosure 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 Mo and Cr. The dual-phase steel is an 80 kg-grade dual-phase steel, and it has a microstructure comprising martensite + ferrite, a yield strength of ≥420MPa, a tensile strength of ≥800MPa, and an A₅₀-gauge-length elongation at break of ≥18%.

Description of the Drawing

FIG. 1 is a photograph showing the microstructure of the dual-phase steel in Example 1 according to the present disclosure.

Detailed Description

The dual-phase steel and the manufacturing method thereof according to the present disclosure will be further explained and illustrated with reference to the specific Examples below. However, the following description is an illustrative description for explaining the present disclosure, with no intention to limit the technical scope of the present disclosure exclusively to the scope of the description.

Examples 1-5 and Comparative Examples 1-14

The dual-phase steels of Examples 1-5 according to the present disclosure were each prepared by the following steps:

(1) According to the formulas shown in Table 1 below, molten steel was smelted and continuously cast to obtain a continuously cast product;
(2) The continuously cast product was subjected to hot rolling, wherein the continuously cast product was first heated to 1160-1190°C, held for at least 150 minutes, 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;
(3) Cold rolling: The cold rolling reduction ratio was 50-70%;
(4) Annealing: The annealing soaking temperature was 825-855°C, the annealing time was 40-200 seconds; then, the temperature was reduced to the rapid cooling start temperature of 735-760°C at a rate of 3-5°C/s, followed by rapid cooling at a rate of 40-100°C/s, with the rapid cooling end temperature being 220-260°C;
(5) Tempering: The tempering temperature was 220-260°C, and the tempering time was 100-400 seconds;
(6) Temper rolling: The temper rolling reduction ratio was ≤ 0.3%.

The steels of Comparative Examples 1-14 were also prepared according to the formulas shown in Table 1 below using substantially the same process as the above Examples of the present disclosure, except that at least one of the chemical element contents or the manufacturing process parameters of Comparative Examples 1-14 did not meet the requirements of the present disclosure.
The 80 kg-grade dual-phase steel of the present disclosure means that the microstructure of the steel of the present disclosure includes two phases, and the steel of the present disclosure is able to withstand a pressure of 80 kg per square centimeter.

Table 1 lists the chemical compositions of the dual-phase steels of Examples 1-5 and the steels of Comparative Examples 1-14, and the values of the corresponding hardenability factor Y_Q.

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

	Steel type	Chemical elements										Y_Q
	Steel type	C	Si	Mn	B	Al	P	S	N	Nb	Ti	Y_Q
Ex. 1	A	0.093	0.15	1.40	0.0025	0.025	0.013	0.0025	0.0045	0.016	0.021	1.90
Ex. 2	B	0.11	0.30	1.60	0.0022	0.010	0.012	0.0016	0.0033	0.017	0.010	2.04
Ex. 3	C	0.108	0.16	1.55	0.0020	0.014	0.009	0.0018	0.0042	0.010	0.022	1.95
Ex. 4	D	0.090	0.17	1.48	0.0024	0.012	0.011	0.002	0.0022	0.030	0.030	1.96
Ex. 5	E	0.098	0.10	1.49	0.0030	0.030	0.012	0.0012	0.0028	0.023	0.023	2.09
Comp. Ex. 1	F	0.088	0.26	1.56	0.0029	0.019	0.011	0.0024	0.0045	0.021	0.016	2.14
Comp. Ex. 2	G	0.117	0.17	1.43	0.0021	0.021	0.014	0.0022	0.0037	0.015	0.024	1.85
Comp. Ex. 3	H	0.098	0.14	1.38	0.0023	0.013	0.011	0.0022	0.0027	0.016	0.018	1.84
Comp. Ex. 4	I	0.108	0.25	1.69	0.0026	0.016	0.009	0.0023	0.0034	0.02	0.023	2.21
Comp. Ex. 5	J	0.094	0.22	1.47	0.0013	0.023	0.012	0.0015	0.0028	0.021	0.021	1.73
Comp. Ex. 6	K	0.106	0.23	1.51	0.0035	0.015	0.014	0.0016	0.0035	0.016	0.019	2.21
Comp. Exs. 7-14	L	0.107	0.28	1.49	0.0024	0.017	0.013	0.0017	0.0024	0.017	0.024	1.97

Table 2-1 and Table 2-2 list the specific process parameters for the dual-phase steels of Examples 1-5 and the steels of Comparative Examples 1-14.

Table 2-1

No.	Steel type	Hot rolling					Cold rolling
No.	Steel type	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	A	1180	175	865	32	505	52
Ex. 2	B	1175	165	874	62	525	57
Ex. 3	C	1165	202	890	68	535	58
Ex. 4	D	1187	185	866	76	515	60
Ex. 5	E	1169	188	864	46	520	55
Comp. Ex. 1	F	1185	192	885	74	538	65
Comp. Ex. 2	G	1190	174	858	30	540	68
Comp. Ex. 3	H	1187	163	875	50	534	50
Comp. Ex. 4	I	1175	158	850	62	516	56
Comp. Ex. 5	J	1168	182	890	48	535	55
Comp. Ex. 6	K	1169	174	880	76	505	62
Comp. Ex. 7	L	1153	169	875	46	516	57
Comp. Ex. 8	L	1207	207	870	58	533	54
Comp. Ex. 9	L	1163	184	866	55	488	64
Comp. Ex. 10	L	1177	175	865	50	555	50
Comp. Ex. 11	L	1185	180	884	70	522	55
Comp. Ex. 12	L	1176	190	875	75	506	68
Comp. Ex. 13	L	1169	165	858	69	538	56
Comp. Ex. 14	L	1163	160	864	55	540	60

Table 2-2

No.	Annealing						Tempering		Temper rolling
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	825	50	5	745	45	220	220	120	0.3
Ex. 2	845	190	3	737	54	250	250	300	0.1
Ex. 3	835	115	5	750	65	255	255	240	0.3
Ex. 4	834	180	3	740	75	240	240	250	0.2
Ex. 5	828	90	5	755	60	235	235	375	0.3
Comp. Ex. 1	842	150	3	745	48	246	246	340	0.1
Comp. Ex. 2	839	70	4	739	66	248	248	200	0.3
Comp. Ex. 3	840	125	4	742	78	252	252	180	0.2
Comp. Ex. 4	828	180	5	748	82	225	225	210	0.3
Comp. Ex. 5	833	65	4	752	58	237	237	250	0.2
Comp. Ex. 6	847	75	5	740	90	252	252	140	0.1
Comp. Ex. 7	836	130	4	743	70	227	227	320	0.1
Comp. Ex. 8	844	145	3	739	85	256	256	125	0.3
Comp. Ex. 9	827	95	3	751	45	240	240	175	0.3
Comp. Ex. 10	836	160	5	752	80	225	225	285	0.1
Comp. Ex. 11	812	125	5	746	55	235	235	380	0.2
Comp. Ex. 12	860	55	4	735	68	242	242	280	0.3
Comp. Ex. 13	833	75	3	750	47	212	212	300	0.2
Comp. Ex. 14	835	120	4	744	72	272	272	280	0.3

It should be noted that in Table 2-2, 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.
The resulting dual-phase steels of Examples 1-5 and the resulting steels of Comparative Examples 1-14 were sampled respectively to obtain the corresponding samples. The performances of the steel samples obtained were subjected to tensile tests and Charpy impact tests to obtain the performance data of the steels of the Examples and Comparative Examples. The test results of the steels of the Examples and Comparative Examples are listed in Table 3.

Table 3 lists the performance test results of the dual-phase steels of Examples 1-5 and the 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	434	822	22.5	83.9
Ex. 2	485	839	21.4	85.6
Ex. 3	474	866	19.6	88.4
Ex. 4	429	804	23.5	82.0
Ex. 5	458	825	20.8	84.2
Comp. Ex. 1	396	772	24.2	78.8
Comp. Ex. 2	528	939	14.6	95.8
Comp. Ex. 3	363	767	24.8	78.3
Comp. Ex. 4	519	940	15.2	95.9
Comp. Ex. 5	404	777	19.8	79.3
Comp. Ex. 6	538	928	16.6	94.7
Comp. Ex. 7	389	766	24.8	78.2
Comp. Ex. 8	526	936	16.8	95.5
Comp. Ex. 9	548	955	15.5	97.4
Comp. Ex. 10	394	771	24.8	78.7
Comp. Ex. 11	399	762	24.5	77.8
Comp. Ex. 12	544	949	16.5	96.8
Comp. Ex. 13	534	947	15.8	96.6
Comp. Ex. 14	395	774	24.4	79.0

Note: Kgf, 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, the overall performances of Examples 1-5 of the present disclosure were excellent, including a yield strength of ≥420 MPa, a tensile strength of ≥800 MPa, and an A₅₀-gauge-length elongation at break of ≥18%. The dual-phase steel of each Example acquired a tensile strength of greater than 800 MPa and a good elongation with no addition of precious alloying elements such as Mo and Cr. The overall performances of the dual-phase steels of Examples 1-5 of the present disclosure were significantly better than those of Comparative Examples 1-14.
The methods for testing the various performances listed in Table 3 were conducted according to GB/T228-2010 : Metallic materials - Tensile test - Method of test at room temperature. The A₅₀-gauge-elongation at break means that the parallel length × width of the tensile sample is 50 mm × 25 mm.
As it can be seen from Tables 1 to 3, in contrast to the steels of Comparative Examples 1-14, the chemical compositions of the steels of Examples 1-5 of the present disclosure fall within the claimed scope, and cooperate with the optimized process parameters to provide dual-phase steels with both low cost and high performances.
For each of Examples 1-5 and Comparative Examples 1-14, the dual-phase steel or the steel was corroded with 4% (by volume) Nital and then observed using an optical microscope for its microstructure. The volume fractions and sizes of martensite and ferrite in the steel were determined using image analysis software. The microstructure of Example 1 is shown in Fig. 1. As it can be seen from Fig. 1, the microstructure of the dual-phase steel of Example 1 of the present disclosure included martensite and ferrite. The percentage of martensite in the figure was 63%. As the dual-phase steel of the present disclosure has a uniform structure, the percentage of martensite shown in any cross section of the steel may be regarded as the volume percentage content of martensite in the steel. Hence, the volume fraction in the dual-phase steel of Example 1 was 63%. In addition, neither the average particle size of martensite nor the average particle size of ferrite exceeded 5 µm.

Table 4 shows the microstructures of the steels of Examples 1 - 5 and Comparative Examples 1 - 14, the volume fractions of martensite in the steels, and the average grain sizes of martensite and ferrite in the steels.

Table 4.

No.	Microstructure	Volume fraction of martensite/%	Average grain size of martensite/um	Average grain size of ferrite/um
Ex. 1	Martensite+ferrite	63	4.3	3.8
Ex. 2	Martensite+ferrite	69	4.2	4.1
Ex. 3	Martensite+ferrite	78	4.5	3.7
Ex. 4	Martensite+ferrite	58	3.8	4.0
Ex. 5	Martensite+ferrite	66	3.9	4.2
Comp. Ex. 1	Martensite+ferrite	52	5.5	5.8
Comp. Ex. 2	Martensite+ferrite	92	3.5	3.8
Comp. Ex. 3	Martensite+ferrite	49	5.6	5.7
Comp. Ex. 4	Martensite+ferrite	90	4.2	4.4
Comp. Ex. 5	Martensite+ferrite	53	5.8	5.2
Comp. Ex. 6	Martensite+ferrite	90	4,1	4.3
Comp. Ex. 7	Martensite+ferrite	48	6.2	6.0
Comp. Ex. 8	Martensite+ferrite	94	4.4	3.9
Comp. Ex. 9	Martensite+ferrite	95	3.4	3.7
Comp. Ex. 10	Martensite+ferrite	50	5.8	6.0
Comp. Ex. 11	Martensite+ferrite	47	6.3	6.5
Comp. Ex. 12	Martensite+ferrite	95	3.8	4.0
Comp. Ex. 13	Martensite+ferrite	92	3.5	4.4
Comp. Ex. 14	Martensite+ferrite	51	6.2	5.8

As it can be seen from Table 4, the dual-phase steels of Examples 1-5 had a structure of martensite and ferrite. In addition, the volume fraction of martensite in all of the steels was in the range of 55% - 85% as defined in the present disclosure, and the average grain sizes of both martensite and ferrite were 5 µm or less. The steels of Comparative Examples 1-14 also had a structure of martensite and ferrite. However, since their chemical compositions or manufacturing processes didn't meet the conditions defined by the present disclosure, the microstructure as desired by the present disclosure couldn't be obtained, and the volume fraction of martensite was outside of the range defined by the present disclosure.
In summary, the present disclosure provides a dual-phase steel with both low cost and excellent performances by designing a reasonable chemical composition in coordination with an optimized process.
It should be noted that all technical features recorded in this application can be combined freely or associated in any way unless a contradiction occurs. It will be apparent to those skilled in the art that various modifications and changes can be made to the present disclosure without departing from the scope of the present disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a further embodiment. Thus, it is intended that the present disclosure covers such modifications and changes falling in the scope of the appended claims and their equivalents.

Claims

A dual-phase steel, comprising, in addition to at least 90% of Fe and unavoidable impurities, the following components in mass percentage: C: 0.09% - 0.11%, Si: 0.1% - 0.3%, Mn: 1.4% - 1.6%, Al: 0.01% - 0.03%, Nb: 0.01% - 0.03%, Ti: 0.01% - 0.03%, and B: 0.0020% - 0.0030%.
The dual-phase steel of claim 1, comprising the following components in mass percentage: C: 0.09% - 0.11%, Si: 0.1% - 0.3%, Mn: 1.4% - 1.6%, Al: 0.01% - 0.03%, Nb: 0.01% - 0.03%, Ti: 0.01% - 0.03%, and B: 0.0020% - 0.0030%, and a balance of Fe and unavoidable impurities.
The dual-phase steel of claim 1 or 2, wherein the dual-phase steel is free of Mo and Cr.
The dual-phase steel of claim 1 or 2, wherein the dual-phase steel has a hardenability factor Y_Q that satisfies: 1.9≤Y_Q≤2.1, wherein Y_Q=Mn+200×B, wherein Mn and B each represent a numerical value before a percentage sign of a mass percentage content of a corresponding element.
The dual-phase steel of claim 1 or 2, wherein the contents of impurity elements in mass percentage satisfy: P≤0.015%, S≤0.003%, and N≤0.005%.
The dual-phase steel of claim 1 or 2, wherein the dual-phase steel has a microstructure comprising martensite and ferrite, preferably consisting of martensite and ferrite; more preferably, martensite has a volume percentage content of 55% or higher and 85% or lower, preferably 58-80%.
The dual-phase steel of claim 6, wherein martensite and ferrite each have an average grain size of 5 µm or less; preferably, martensite and ferrite each have a grain size of 5 µm or less.
The dual-phase steel of claim 1 or 2, wherein the dual-phase steel is a 80 kg-grade dual-phase steel having the following performances: a yield strength of ≥420 MPa; a tensile strength of ≥800 MPa; and an A₅₀-gauge-length elongation at break of ≥18%.
The dual-phase steel of claim 1 or 2, wherein the dual-phase steel has a yield strength of ≥450 MPa, a tensile strength of ≥820 MPa, and an A₅₀-gauge-length elongation at break of ≥20%, and the dual-phase steel is able to withstand a force of 83 - 90 kilograms per square centimeter.
A method for manufacturing the dual-phase steel of any one of claims 1-9, wherein the method includes the following steps:
1) Smelting and continuously casting molten steel to obtain a continuously cast product;

2) Hot rolling the continuously cast product;

3) Cold rolling;

4) Annealing;

5) Tempering; and

6) Temper rolling to obtain the dual-phase steel.
The method of claim 10, wherein in the step of annealing, an annealing soaking temperature is 825-855°C; an annealing time is 40-200s; then, the temperature is reduced to a rapid cooling start temperature of 735-760°C at a rate of 3-5°C/s, followed by rapid cooling at a rate of 40-100°C/s, with a rapid cooling end temperature being 220-260°C; preferably, the annealing soaking temperature is 830-840°C.
The method of claim 10 or 11, wherein in the step of hot rolling, the continuously cast product is first heated to 1160-1190°C, held for 150 minutes or longer, then hot rolled with a 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.
The method of claim 10 or 11, wherein in the step of cold rolling, a cold rolling reduction ratio is 50 - 70%.
The method of claim 10 or 11, wherein in the step of tempering, a tempering temperature is 220-260°C, and a tempering time is 100-400 s.
The method of claim 10 or 11, wherein in the step of temper rolling, a temper rolling reduction ratio is ≤ 0.3%.