阅读说明：本技术 450MPa级热镀锌双相钢及其生产方法 (450 MPa-grade hot-galvanized dual-phase steel and production method thereof ) 是由 余灿生 郑之旺 周伟 王敏莉 郑昊青 苏冠侨 于 2021-08-05 设计创作，主要内容包括：本发明公开了一种450MPa级热镀锌双相钢及其生产方法,属于冷轧板带生产技术领域。所述双相钢化学成分按质量百分比为：C 0.03％～0.10％,Si 0.20～0.60％,Mn 1.20％～2.00％,P≤0.020％,S≤0.010％,Als 0.015％～0.070％,N≤0.0060％,其余元素是Fe及不可避免的杂质；其微观组织按体积百分比为：铁素体90～96％、马氏体4～10％。制备方法包括冶炼、热轧、酸轧和热镀锌步骤。本发明双相钢的生产方法采用稀疏冷却方式有利于组织性能的稳定控制；明确了除鳞、机架间冷却水的开闭方式,有利于获得良好的表面质量。配合以冶炼、热镀锌工艺获得了450MPa级热镀锌双相钢板的良好力学性能及表面质量,具有良好的市场竞争力。(The invention discloses 450 MPa-grade hot-galvanized dual-phase steel and a production method thereof, belonging to the technical field of cold-rolled strip production. The dual-phase steel comprises the following chemical components in percentage by mass: 0.03-0.10% of C, 0.20-0.60% of Si, 1.20-2.00% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 0.015-0.070% of Als, less than or equal to 0.0060% of N, and the balance of Fe and inevitable impurities; the microstructure comprises the following components in percentage by volume: 90-96% of ferrite and 4-10% of martensite. The preparation method comprises the steps of smelting, hot rolling, acid rolling and hot galvanizing. The production method of the dual-phase steel adopts a sparse cooling mode, which is beneficial to the stable control of the structure performance; the open-close mode of the cooling water between the descaling machine frame and the machine frame is determined, and the good surface quality is obtained. The high-strength zinc-plated dual-phase steel plate with 450MPa grade is obtained by matching with smelting and hot-dip galvanizing processes, has good mechanical property and surface quality, and has good market competitiveness.)

1.450 MPa-grade hot-galvanized dual-phase steel is characterized by comprising the following chemical components in percentage by mass: 0.03-0.10% of C, 0.20-0.60% of Si, 1.20-1.80% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 0.015-0.070% of Als, less than or equal to 0.0060% of N, and the balance of Fe and inevitable impurities; the microstructure comprises the following components in percentage by volume: 90-96% of ferrite and 4-10% of martensite.

2. The 450MPa grade hot dip galvanized dual phase steel according to claim 1, characterized in that: the dual-phase steel comprises the following chemical components in percentage by mass: 0.04 to 0.07 percent of C, 0.40 to 0.55 percent of Si, 1.60 to 1.75 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.010 percent of S, 0.03 to 0.050 percent of Als, less than or equal to 0.0060 percent of N, and the balance of Fe and inevitable impurities.

3. The 450MPa grade hot dip galvanized dual phase steel according to claim 1 or 2, characterized in that: the ferrite has an average grain size of 7 to 9 μm.

4. The method for producing 450MPa grade hot dip galvanized dual phase steel according to any one of claims 1 to 3, characterized by comprising the following steps:

a. smelting: smelting according to the chemical components of 450 MPa-grade hot-galvanized dual-phase steel, and casting into a plate blank;

b. hot rolling: b, heating the plate blank obtained in the step a, removing phosphorus, initially rolling, finish rolling and laminar cooling to obtain a hot rolled coil; laminar cooling adopts a sparse cooling mode, the cooling rate of the upper surface and the lower surface is 50% and 75%, the intermediate temperature is 700-750 ℃, and the coiling temperature is 560-620 ℃;

c. acid rolling: c, washing the hot rolled coil obtained in the step b with acid, and cold-rolling the hot rolled coil into cold-rolled thin strip steel;

d. hot galvanizing: heating the cold-rolled thin strip steel obtained in the step c to 740-780 ℃; soaking and preserving heat for 50-120 s, slowly cooling to 680-740 ℃ at the speed of 1-5 ℃/s, rapidly cooling to 450-470 ℃ at the speed of 10-25 ℃/s, entering a zinc pool for galvanizing for 3-10 s, discharging from the zinc pool, and cooling to room temperature at the speed of more than or equal to 5 ℃/s.

5. The method for producing 450MPa grade hot-dip galvanized dual-phase steel according to claim 4, characterized in that: in the step b, the heating temperature is 1200-1250 ℃, the finish rolling start temperature is 1030-1150 ℃, and the finish rolling finishing temperature is 850-950 ℃; the finish rolling reduction rate is more than or equal to 85 percent.

6. The method for producing 450MPa grade hot-dip galvanized dual-phase steel according to claim 5, characterized in that: in the step b, carrying out primary rolling dephosphorization after heating, wherein the descaling pressure is more than or equal to 15 Mpa; and (4) maintaining the dephosphorization pressure of more than or equal to 15Mpa at the finish rolling inlet for dephosphorization.

7. The method for producing 450MPa grade hot-dip galvanized dual-phase steel according to claim 4, characterized in that: in the step c, the cold rolling reduction is 55-77%.

8. The method for producing 450MPa grade hot-dip galvanized dual-phase steel according to claim 4, characterized in that: in the step d, the heating of the cold-rolled thin strip adopts a sectional heating mode, firstly the cold-rolled thin strip is heated to 300 ℃ at a heating rate of 15-20 ℃/s, then the cold-rolled thin strip is heated to 700 ℃ at a heating rate of 8-12 ℃/s, and finally the cold-rolled thin strip is heated to 740-780 ℃ at a heating rate of 3-6 ℃/s.

Technical Field

The invention belongs to the technical field of cold-rolled strip production, and particularly relates to 450 MPa-grade hot-galvanized dual-phase steel and a production method thereof.

Background

With the development of light weight and the improvement of safety requirements of passengers, the proportion and the amount of high-strength steel in the body-in-white of the automobile are rapidly increased in recent years, and dual-phase steel is more and more favored by the automobile industry due to high strength, low yield ratio and good formability, and the application proportion of the dual-phase steel in the automobile is continuously improved. Compared with cold-rolled dual-phase steel, the galvanized dual-phase steel also has excellent corrosion resistance, can ensure that an automobile has good corrosion resistance and perforation resistance, and is mainly applied to structural parts and reinforcing parts of medium-high-grade cars at present. With the continuous improvement of the equipment capability of the hot galvanizing unit, most of the hot galvanizing unit has the pre-oxidation-reduction function, and the capabilities of slow cooling at a high temperature section and strong air cooling after being discharged from a zinc pot provide possibility for further reducing the cost and improving the performance.

CN108754343A discloses a zinc-iron alloy coating dual-phase steel plate for 450 MPa-level automobile outer plates and a manufacturing method thereof, wherein the steel plate comprises the following chemical components in percentage by weight: c: 0.05-0.08%, Si is less than or equal to 0.005%, Mn: 0.70-1.00%, P is less than or equal to 0.005%, S is less than or equal to 0.003%, Als: 0.010-0.050%, Cr: 0.20-0.40% and Mo: 0.05 to 0.20% by weight, and the balance Fe and unavoidable impurities. The rolling process adopts finish rolling at 900-930 ℃, coiling at 620-660 ℃, and cold rolling reduction rate of 62-75%; the galvanizing is carried out at 760-800 ℃ by soaking, wherein the atmosphere dew point in an annealing furnace is controlled below-40 ℃, the oxygen content is less than or equal to 2ppm, the temperature of a zinc pot is 460-480 ℃, the Al content in zinc liquid components is 0.100-0.105 wt%, the alloying temperature is controlled to be 480-500 ℃, the heat preservation time is 10-25 s, the iron content of a coating is 9-11 wt%, the finishing rate is 0.4-0.6%, the surface roughness of an alloyed steel plate is controlled to be 0.5-1.2 mu m, and the surface PC value is 100-150/cm. The precious alloy elements such as Cr and Mo are added, so that the alloy cost is increased, the recrystallization temperature is increased, the temperature of a two-phase region is increased during galvanizing, and the process cost is increased. In addition, the patent only describes the rolling temperature, the finishing temperature and the coiling temperature, and does not specify the cooling path, the rolling reduction of the hot rolling finish rolling, and the like, which is not favorable for stable control of the structure property and the surface quality.

CN105369135A discloses a 450MPa class galvanized dual-phase steel for cars and a production method thereof, wherein the steel comprises the following chemical components in percentage by weight: c: 0.04-0.09%, Si is less than or equal to 0.01%, Mn: 1.0-2.0%, P is less than or equal to 0.015%, S is less than or equal to 0.010%, Als: 0.01 to 0.08%, Mo: 0.01-0.30%, Cr: 0.01 to 1.0%, Nb: 0.001-0.03%, N is less than or equal to 0.005%, and the balance is Fe and unavoidable impurities. The rolling process comprises the following steps: final rolling at 890-920 ℃, finish rolling at 580-620 ℃, and cold rolling reduction rate of 45-75%; the hot galvanizing process comprises the following steps: annealing temperature: 770-810 ℃, the dew point in the annealing furnace is controlled below minus 40 ℃, and the oxygen content is less than or equal to 3 ppm; the temperature of the zinc liquid is controlled to be 455-465 ℃, the aluminum content in the zinc liquid is controlled to be 0.18-0.23%, the Fe content is less than or equal to 0.009%, and the finishing elongation is controlled to be 0.7-1.1%. The addition of noble alloy elements such as Cr, Mo and Nb not only increases the alloy cost, but also increases the recrystallization temperature, so that the temperature of a two-phase region is increased during galvanizing, and the process cost is increased. In addition, the patent only describes the rolling temperature, the finishing temperature and the coiling temperature, and does not specify the cooling path, the rolling reduction of the hot rolling finish rolling, and the like, which is not favorable for stable control of the structure property and the surface quality.

CN106011644B discloses a high-elongation cold-rolled high-strength steel plate and a preparation method thereof, wherein the high-elongation cold-rolled high-strength steel plate comprises the following chemical components in percentage by weight: c: 0.03-0.05%, Si: 0.40 to 0.50%, Mn: 1.35-1.50%, Al: 0.030-0.050%, P is less than or equal to 0.015%, S is less than or equal to 0.010%, N is less than or equal to 0.005%, and the balance is Fe and inevitable impurities. The initial rolling temperature of hot rolling is 1000-1100 ℃, the final rolling temperature is 850-950 ℃, and the coiling temperature is 610-690 ℃. And (3) cold rolling by adopting a cold rolling reduction ratio of 50-70%, soaking at 780-820 ℃, then slowly cooling to 640-680 ℃ in sequence, wherein the slow cooling rate (CR1) is 1-7 ℃/s, immediately rapidly cooling to the overaging temperature of 240-345 ℃, the rapid cooling rate (CR2) is 10-30 ℃/s, and finally cooling to room temperature, thereby obtaining the high-elongation cold-rolled high-strength steel plate. The patent is used for producing the normal-cooling dual-phase steel instead of the hot-dip galvanized dual-phase steel by continuous annealing, and does not consider the heat preservation process of the medium-temperature transformation zone entering a zinc pot.

CN104233068A discloses a hot-dip galvanized steel sheet with tensile strength of 440MPa and a production method thereof, wherein the hot-dip galvanized steel sheet comprises the following chemical components in percentage by weight: c: 0.0036-0.0049%, Si is less than or equal to 0.020%, Mn: 1.55-1.75%, Al: 0.015 to 0.030%, Nb: 0.015-0.035%, P: 0.078-0.095%, S is less than or equal to 0.00: 3%, Ti: 0.025-0.035%, B: 0.0010-0.0014%, N is less than or equal to 0.003%, and the balance of Fe and inevitable impurities. The hot rolling adopts final rolling at 920-940 ℃ and reeling at 580-600 ℃, and the hot galvanizing adopts soaking at 810-830 ℃. The method adopts an ultra-low carbon (0.0036-0.0049% C) route, increases the process cost by a vacuum RH furnace during smelting, adds a large amount of Mn, Nb, Ti and B elements, increases the alloy cost, obviously improves the recrystallization temperature (the soaking temperature is 810-830 ℃) and has high process cost. Most importantly, the steel is strengthened by phosphorus (P) (P: 0.078-0.095%), grain boundary segregation is easy to generate, and the forming effect is poor although the elongation rate is possibly good.

Disclosure of Invention

The invention aims to provide 450 MPa-grade hot-dip galvanized dual-phase steel with good mechanical property and surface quality.

In order to realize the purpose, the invention firstly provides 450 MPa-grade hot-dip galvanized dual-phase steel which comprises the following chemical components in percentage by mass: 0.03-0.10% of C, 0.20-0.60% of Si, 1.20-1.80% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 0.015-0.070% of Als, less than or equal to 0.0060% of N, and the balance of Fe and inevitable impurities; the microstructure comprises the following components in percentage by volume: 90-96% of ferrite and 4-10% of martensite.

The 450 MPa-grade hot-dip galvanized dual-phase steel comprises the following chemical components in percentage by mass: 0.04 to 0.07 percent of C, 0.40 to 0.55 percent of Si, 1.60 to 1.75 percent of Mn, less than or equal to 0.020 percent of P, less than or equal to 0.010 percent of S, 0.03 to 0.050 percent of Als, less than or equal to 0.0060 percent of N, and the balance of Fe and inevitable impurities.

Wherein the average grain size of the ferrite of the 450 MPa-grade hot-dip galvanized dual-phase steel is 7-9 mu m.

The invention also provides a production method of the 450 MPa-grade hot-dip galvanized dual-phase steel, which comprises the following steps:

a. smelting: smelting according to the chemical components of 450 MPa-grade hot-galvanized dual-phase steel, and casting into a plate blank;

b. hot rolling: b, heating the plate blank obtained in the step a, removing phosphorus, initially rolling, finish rolling and laminar cooling to obtain a hot rolled coil; laminar cooling adopts a sparse cooling mode, the cooling rate of the upper surface and the lower surface is 50% and 75%, the intermediate temperature is 700-750 ℃, and the coiling temperature is 560-620 ℃;

c. acid rolling: c, washing the hot rolled coil obtained in the step b with acid, and cold-rolling the hot rolled coil into cold-rolled thin strip steel;

d. hot galvanizing: heating the cold-rolled thin strip steel obtained in the step c to 740-780 ℃; soaking and preserving heat for 50-120 s, slowly cooling to 680-740 ℃ at the speed of 1-5 ℃/s, rapidly cooling to 450-470 ℃ at the speed of 10-25 ℃/s, entering a zinc pool for galvanizing for 3-10 s, discharging from the zinc pool, and cooling to room temperature at the speed of more than or equal to 5 ℃/s.

In the production method of the 450 MPa-grade hot-galvanized dual-phase steel, in the step b, the heating temperature is 1200-1250 ℃, the rolling start temperature of finish rolling is 1030-1150 ℃, and the rolling finish temperature of finish rolling is 850-950 ℃; the finish rolling reduction rate is more than or equal to 85 percent.

In the production method of the 450 MPa-grade hot-galvanized dual-phase steel, in the step b, the steel is heated and then is subjected to primary rolling dephosphorization, and the descaling pressure is more than or equal to 15 MPa; and (4) maintaining the dephosphorization pressure of more than or equal to 15Mpa at the finish rolling inlet for dephosphorization.

In the production method of the 450MPa grade hot galvanizing dual-phase steel, in the step c, the cold rolling reduction rate is 55-77%.

In the production method of the 450MPa grade hot-dip galvanized dual-phase steel, in the step d, the cold-rolled thin strip steel is heated in a sectional heating mode, firstly, the cold-rolled thin strip steel is heated to 300 ℃ at a heating rate of 15-20 ℃/s, then, the cold-rolled thin strip steel is heated to 700 ℃ at a heating rate of 8-12 ℃/s, and finally, the cold-rolled thin strip steel is heated to 740-780 ℃ at a heating rate of 3-6 ℃/s.

Compared with the prior art, the invention has the beneficial effects that:

the 450Mpa hot galvanizing dual-phase steel produced by the method properly improves the content of Si, on one hand, the solid solution strengthening effect is utilized to reduce the addition of precious elements, and on the other hand, the characteristic that Si inhibits cementite is utilized to enrich C into austenite in the hot galvanizing process so as to improve the hardenability. The hot rolling adopts a low-temperature coiling mode to refine crystal grains, prevents the occurrence of coiling defects (caused by phase change), and obtains more obvious fine grain strengthening effect. Meanwhile, the sparse cooling mode is more favorable for the uniformity of the structure performance, so that more stable mechanical properties are obtained; the open-close mode of descaling and cooling water between the racks is determined, the cleaning effect of the hot-rolled iron scale is ensured, and good surface quality is obtained. Precious alloy elements such as Cr, Mo, Nb and Ti are not added, so that the production cost is reduced, the recrystallization temperature is reduced, the deformation energy storage is increased by large cold rolling reduction (55-77%), the good mechanical property can be obtained by relatively low soaking temperature (740-780 ℃), the production process cost is further reduced, the good mechanical property and surface quality of 450 MPa-grade hot-dip galvanized dual-phase steel plates are obtained, and the market competitiveness is good.

Drawings

FIG. 1 is a metallographic structure diagram of the structure of example 1 of the present invention;

FIG. 2 is a SEM scanning electron micrograph of example 1 of the present invention.

FIG. 3 is a schematic view of the surface of a steel strip of example 1 of the present invention.

FIG. 4 is a schematic diagram showing the surface quality of the plating layer in example 1 of the present invention.

Detailed Description

Specifically, the 450 MPa-grade hot-galvanized dual-phase steel comprises the following chemical components in percentage by mass: 0.03-0.10% of C, 0.20-0.60% of Si, 1.20-2.00% of Mn, less than or equal to 0.020% of P, less than or equal to 0.010% of S, 0.015-0.070% of Als, less than or equal to 0.0060% of N, and the balance of Fe and inevitable impurities; the microstructure comprises the following components in percentage by volume: 90-96% of ferrite and 4-10% of martensite.

C, one of the most important components of the dual phase steel, determines the strength, plasticity and formability of the steel sheet. C is the most obvious element for the solid solution strengthening effect in the steel material, the solid solution C content in the steel is increased by 0.1 percent, and the strength can be improved by about 450 MPa. When the content of C is too low, the stability of austenite and the martensite hardenability are reduced, so that the strength is low, and the content of C in the dual-phase steel is generally not lower than 0.02%; when the content of C is too high, the plasticity and welding performance of the dual-phase steel are reduced, and the content of C in the dual-phase steel is generally not higher than 0.15%. Therefore, the content of C in the invention is 0.03-0.10%, preferably 0.04-0.07%.

Si plays a remarkable role in solid solution strengthening in steel, effectively inhibits precipitation of carbide, delays pearlite transformation and the like in the phase transformation process, but the deformation resistance in thin gauge rolling can be remarkably increased due to the excessively high content of Si, so that the thin gauge rolling is not facilitated, the activity of carbon element can be improved by the Si, and segregation of carbon in a manganese-rich area is promoted. When the two-phase region is subjected to heat preservation, the diffusion of carbon to austenite is accelerated, the ferrite is obviously purified, the purity of the ferrite in the dual-phase steel is improved, the formation of the ferrite is promoted, and the process window of the ferrite formation is enlarged, so that the lower yield ratio is obtained. On the other hand, too high a silicon content increases the brittleness of martensite to deteriorate the toughness, and a high-melting oxide is formed on the surface of the steel sheet to affect the surface quality of the steel sheet, and it is necessary to reduce the silicon content in the steel as much as possible. Therefore, the Si content of the present invention is 0.20 to 0.60%, preferably 0.40 to 0.55%.

Mn is a good deoxidizer and desulfurizer, and is also a common solid solution strengthening element in steel, and the content of Mn in dual-phase steel is generally not less than 1.20%. Mn can be combined with C to form various carbides to play a role in precipitation strengthening, and can also be dissolved in a matrix to enhance the solid solution strengthening effect. Mn is easily combined with S to form a high melting point compound MnS, thereby eliminating or weakening hot embrittlement caused by FeS and improving hot workability of the steel. Mn can improve the stability of austenite and shift the C curve to the right, thereby obviously reducing the critical cooling rate of martensite. However, when the Mn content is too high, the surface is easily enriched in the annealing process to form a large amount of manganese compounds, thereby causing the reduction of the surface galvanizing quality. Therefore, the Mn content in the present invention is 1.20% to 2.00%, preferably 1.60% to 1.75%.

Al is a common deoxidizer in steel, and can form an AlN pinning grain boundary so as to play a role in refining grains; in addition, Al acts similarly to Si, and suppresses carbide precipitation, thereby making austenite sufficiently rich in carbon. Therefore, the Al content in the invention is 0.015-0.070%, preferably 0.03-0.05%

The invention also provides a production method of the 450 MPa-grade hot-dip galvanized dual-phase steel, which comprises the following steps:

a. smelting: smelting according to the chemical components of 450 MPa-grade hot-galvanized dual-phase steel, and casting into a plate blank;

b. a hot rolling procedure: heating the plate blank to 1200-1250 ℃ for more than or equal to 250 minutes in the furnace, and removing scale from the heating furnace by primary rolling, wherein the descaling pressure is more than or equal to 15MPa, and all descaling nozzles are guaranteed to be free of blockage; the descaling pressure of a finish rolling inlet is still kept to be more than or equal to 15MPa for descaling, the water of the machine frame of each pass of the finish rolling unit is fully opened, the angle is kept consistent, and the blockage phenomenon does not occur, and the finish rolling temperature is 850-950 ℃; the finish rolling reduction rate is more than or equal to 85 percent; laminar cooling adopts a sparse cooling mode, the cooling rates of the upper surface and the lower surface are 50% and 75%, the intermediate temperature is 700-750 ℃, and the coiling temperature is 560-620 ℃.

c. Acid rolling process: after acid washing, the hot rolled coil is cold rolled into cold rolled thin strip steel, and the cold rolling reduction rate is 45 to 67 percent

d. Hot galvanizing procedure: the cold-rolled thin strip steel is firstly heated to 300 ℃, 700 ℃ and 740-780 ℃ in stages at the heating rates of 15-20 ℃/s, 8-12 ℃/s and 3-6 ℃/s respectively; after soaking and heat preservation for 50-120 s, slowly cooling to 680-740 ℃ and rapidly cooling to 450-470 ℃ in sequence at the speed of 1-5 ℃/s and 10-25 ℃/s respectively, then entering a zinc pool for galvanizing treatment for 3-10 s, and cooling to room temperature at the speed of more than or equal to 5 ℃/s after being taken out of the zinc pool.

The following examples are provided to further illustrate the embodiments of the present invention and are not intended to limit the scope of the present invention.

Example 1: the present example provides two sets of low-cost 450MPa grade hot-dip galvanized dual-phase steels, whose chemical compositions are shown in table 1:

table 1 example 1 cold rolled dual phase steel chemical composition (wt.%)

Numbering	C	Si	Mn	P	S	N	Als
								1	0.055	0.42	1.70	0.010	0.002	0.0024	0.045
2	0.060	0.44	1.75	0.012	0.001	0.0036	0.042

The preparation method of the low-cost 450 MPa-grade hot-galvanized dual-phase steel comprises the following specific processes:

A. smelting: preparing a dual-phase steel plate blank with chemical components shown in the table 1 through a smelting process;

B. a hot rolling procedure: heating, dephosphorizing, hot rolling and laminar cooling the plate blank to obtain a hot rolled coil, wherein the specific hot rolling process parameters are shown in table 2:

table 2 example 1 hot rolling of cold rolled dual phase steel the main process parameters

C. Acid rolling process: pickling the hot rolled coil, and cold rolling the hot rolled coil into thin strip steel, wherein the cold rolling reduction rate of the thin strip steel with the number 1 is 62.5%; the cold rolling reduction of number 2 was 61.0%;

D. hot galvanizing procedure: the cold-rolled thin strip steel is firstly heated to 300 ℃, 700 ℃ and 750-770 ℃ in stages at heating rates of 15-20 ℃/s, 8-12 ℃/s and 3-6 ℃/s respectively; after soaking and heat preservation for 50-120 s, slowly cooling to 680-740 ℃ and rapidly cooling to 450-470 ℃ in sequence at the speed of 1-5 ℃/s and 10-25 ℃/s respectively, then entering a zinc pool for galvanizing treatment for 3-10 s, and cooling to room temperature at the speed of more than or equal to 5 ℃/s after being taken out of the zinc pool. The specific hot galvanizing process parameters are shown in table 3:

table 3 example 1 main process parameters for continuous annealing

Numbering	Annealing temperature/. degree.C	Slow cooling rate/. degree.C/s	Fast cooling onset temperature/. degree.C	Fast cooling rate ℃/s	temperature/DEG C of zinc bath
						1	761	1.2	697	18	458
2	768	1.3	710	22	451

FIG. 1 is a metallographic structure drawing of example 1 of the present invention, and FIG. 2 is an SEM (scanning Electron microscope) image of example 1 of the present invention, from which it can be seen that the microstructure of the dual phase steel of the present invention is composed of about 95% of ferrite (average grain size of 8.0 μm) and about 5% of martensite distributed in island form.

The product obtained by the process according to publication CN108754343A, CN105369135A, CN106011644B, CN104233068A was tested as four comparative examples according to GB/T228-2010 "method for testing metallic materials at room temperature, and its mechanical properties are shown in table 4 below:

table 4 mechanical properties of the steels of the examples and comparative examples

Numbering	Yield strength/MPa	Tensile strength/MPa	Elongation A80％	Yield ratio/%
					1	328	509	33.5	64.4
2	303	512	35.0	59.2
					CN 108754343 A	265	477	33.0	55.6
CN 105369135 A	275	490	30.5	56.1
					CN 106011644 B	318	496	36.8	64.1
CN 104233068A	320	455	35.0	70.3

As shown in Table 4, the dual phase steel of the present invention has a yield strength of 310 to 355MPa, a tensile strength of 480 to 530MPa, and an elongation A₈₀31.0 to 36.0%.