TWI897360B - High-strength and multi-phase steel and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a high-strength and multi-phase steel and manufacturing method thereof. Slab with a specific composition is subjected to a cooling treatment process with specific conditions in the present manufacturing method for changing the metallographic structure of the obtained steel. The obtained steel has specific tensile strength and bake hardness.

Description

High-strength composite steel and its manufacturing method

The present invention relates to a high-strength composite steel and a method for manufacturing the same, and in particular to a method for manufacturing the high-strength composite steel comprising a multi-stage cooling step.

It is known that grain refinement can be achieved by adding niobium (Nb). Specifically, niobium combines with carbon and nitrogen atoms to form nano-precipitates. These nano-precipitates inhibit the movement of the grain boundaries of the austenite, preventing it from re-crystallizing into equiaxed crystals during rolling, resulting in flattened rolled grains. During the subsequent cooling process, when the austenite transforms into ferrous iron (FFI), the FFI has less room for growth, resulting in grain refinement. This, in turn, improves the steel's strength through grain boundary strengthening.

It is known that nioin nanoprecipitates precipitate within austenite during cooling. As the temperature decreases, the supersaturation increases, and the precipitation rate gradually accelerates. However, this decrease in temperature also reduces the atomic diffusion rate, slowing the precipitation rate. Consequently, an optimal precipitation temperature, known as the nose of the curve, is reached. However, the rolling temperature of steel billets is generally between 800°C and 1100°C, which is precisely the temperature range for nioin nanoprecipitates. Nioin nanoprecipitates can cause a sharp increase in steel strength, leading to rolling instability and complicating thin sheet production.

On the other hand, steel strip production is typically based on single-stage cooling, which involves water cooling to the target temperature followed by air cooling. However, this cooling method generates random distribution of precipitates, resulting in significant differences in the density of precipitates in different areas. Furthermore, precipitates formed at different temperatures also vary in size, easily leading to inhomogeneous steel and insufficient strength.

Therefore, there is an urgent need to provide a high-strength composite steel and a manufacturing method thereof to solve the above problems.

The steel blank of this invention does not contain niobarium, so the aforementioned rolling instability issue does not occur. Instead, titanium, vanadium, and chromium are added to the blank, and titanium carbide (TiC) and vanadium carbide (VC) are generated during the manufacturing process to strengthen the resulting steel.

The manufacturing method of the high-strength composite steel of the present invention includes a multi-stage cooling process with two cooling stages, and an air cooling step between the two cooling stages. This allows the steel to form neatly arranged precipitates during the air cooling stage, which contribute significantly to precipitation strengthening, thereby increasing the overall strength of the steel and generating equiaxed, ferrous iron grains, thereby improving the steel's workability.

At least one embodiment of the present invention provides a high-strength composite steel. Based on the total weight of the high-strength composite steel being 100 weight percent, the high-strength composite steel comprises 0.03 to 0.12 weight percent carbon, 1 to 2 weight percent manganese, 0.05 to 0.3 weight percent titanium, 0.02 to 0.15 weight percent vanadium, and 0.2 weight percent tantalum. The high-strength composite steel contains up to 0.8 weight percent of chromium, 0.02 weight percent to 0.08 weight percent of aluminum, not more than 0.03 weight percent of phosphorus, not more than 0.01 weight percent of sulfur, and the remainder of iron and inevitable impurities. The high-strength composite steel does not contain nioin. The tensile strength of the high-strength composite steel is not less than 760 MPa, and the bake hardening value of the high-strength composite steel is not less than 30 MPa.

In at least one embodiment of the present invention, the high-strength composite steel comprises granular iron, tantalum, matan loose iron, and chrysotile carbon iron.

In at least one embodiment of the present invention, the yield strength of the high-strength composite steel is not less than 660 MPa.

In at least one embodiment of the present invention, the porosity of the high-strength composite steel is not less than 45%.

In at least one embodiment of the present invention, the elongation of the high-strength composite steel is not less than 13%.

At least one embodiment of the present invention provides a method for manufacturing a high-strength composite steel, comprising the following steps: First, providing a steel ingot, wherein, based on the total weight of the steel ingot as 100 weight percent, the steel ingot contains 0.03 to 0.12 weight percent carbon, 1 to 2 weight percent manganese, 0.05 to 0.3 weight percent titanium, 0.02 to 0.15 weight percent vanadium, 0.2 to 0.8 weight percent chromium, 0.02 to 0.08 weight percent aluminum, no more than 0.03 weight percent phosphorus, no more than 0.01 weight percent sulfur, and the remainder iron and unavoidable impurities, wherein the steel ingot does not contain niobia. Then, the steel ingot is heated. Then, the steel blank is hot rolled to obtain a hot rolled steel plate. Then, the hot rolled steel plate is cooled to obtain a steel plate, wherein the cooling step comprises (1) cooling the hot rolled steel plate to not less than 600°C at a cooling rate of 20°C/second to 200°C/second; (2) air cooling the hot rolled steel plate, wherein the air cooling time is 2 seconds to 10 seconds; and (3) cooling the hot rolled steel plate to a coiling temperature at a cooling rate of 50°C/second to 200°C/second, wherein the coiling temperature is 400°C to 650°C. The cooled steel plate is then coiled to produce high-strength composite steel.

In at least one embodiment of the present invention, in the cooling step (1), the cooling temperature is 600°C to 800°C.

In at least one embodiment of the present invention, the hot-rolled steel plate has a finishing temperature of 800°C to 1000°C.

In at least one embodiment of the present invention, the high-strength composite steel produced using the above-mentioned manufacturing method has a tensile strength of not less than 760 MPa and a bake hardening value of not less than 30 MPa.

In at least one embodiment of the present invention, a high-strength composite steel is produced by the above-mentioned method for producing high-strength composite steel, wherein the high-strength composite steel comprises granular iron, tantalum, matan loose iron, and chrysotile carbon iron.

100: Manufacturing method

110, 120, 130, 140, 142, 144, 146, 150, 160: Steps

To make the above and other objects, features, advantages and embodiments of the present invention more clearly understood, the attached drawings are described in detail below.

Figure 1 is a schematic flow diagram of a method for manufacturing high-strength composite steel according to some embodiments of the present invention.

The following details the making and using of embodiments of the present invention. However, it should be understood that the embodiments provide many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are for illustrative purposes only and are not intended to limit the scope of the present invention.

In this document, a range expressed as "from a value to another value" is a summary method that avoids listing all the values within the range in the specification. Therefore, the description of a specific range of values covers any value within the range and any smaller range of values defined by any value within the range, just as if the value and the smaller range were explicitly stated in the specification.

Some embodiments of the present invention provide a high-strength composite steel. Based on 100 weight percent of the total weight of the high-strength composite steel, the high-strength composite steel comprises 0.03 to 0.12 weight percent of carbon, 1 to 2 weight percent of manganese, 0.05 to 0.3 weight percent of titanium, 0.02 to 0.15 weight percent of vanadium, 0.2 to 0.8 weight percent of chromium, 0.02 to 0.08 weight percent of aluminum, no more than 0.03 weight percent of phosphorus, no more than 0.01 weight percent of sulfur, and the remainder being iron and unavoidable impurities.

The high-strength composite steel has a yield strength of not less than 660 MPa, a tensile strength of not less than 760 MPa, an elongation of not less than 13%, a porosity of not less than 45%, and a bake hardening value of not less than 30 MPa. In some embodiments, the yield strength of the high-strength composite steel is 700 MPa to 800 MPa, for example, 750 MPa to 800 MPa. In some embodiments, the tensile strength of the high-strength composite steel is 760 MPa to 900 MPa, for example, 800 MPa to 900 MPa. In some embodiments, the bake hardening value of the high-strength composite steel is 40 MPa to 50 MPa.

The metallographic structure of the high-strength composite steel of the present invention is primarily composed of martensite and tantalum, with small amounts of martensite and austenite. It should be noted that the term "martensite" here refers generally to the martensite-austenite constituent (M-A).

It is noteworthy that the high-strength composite steel of the present invention does not contain niobium. Therefore, there will be no niobium nano-precipitates causing rolling instability and making thin plate production difficult.

The addition of carbon causes carbon atoms to accumulate in high-density dislocation sites, hindering their movement and achieving a strengthening effect. Carbon also reacts with titanium and vanadium to form TiC and VC, strengthening the resulting steel. When the carbon content in the aforementioned steel is less than 0.03 weight percent or greater than 0.12 weight percent, the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values cannot be formed.

The addition of manganese can increase the volume fraction of varnish in steel and improve the strength of varnish, thereby enhancing the hardenability of varnish. However, manganese addition can also promote the formation of manganese sulfide intermediaries or cause segregation, both of which are detrimental to the steel's porosity. Therefore, when the manganese content in the aforementioned steel is less than 1 weight percent or greater than 2 weight percent, it is impossible to form the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values described in this application.

Titanium reacts with carbon to form TiC nanoprecipitates. TiC effectively inhibits dislocation and grain boundary movement, thereby strengthening the steel. When the titanium content in the aforementioned steel is less than 0.05 weight percent or greater than 0.3 weight percent, the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values cannot be formed.

Vanadium reacts with carbon to form VC nanoprecipitates, which also strengthen steel. However, vanadium is more expensive than titanium, so titanium is still used as the primary alloying addition. However, when titanium-vanadium composite additions are used, the resulting composite titanium-vanadium nanoprecipitates are less susceptible to grain coarsening at high temperatures than conventional titanium nanoprecipitates, thus more effectively suppressing dislocation movement and thus optimizing the precipitation strengthening effect. When the vanadium content in the aforementioned steel is less than 0.02 weight percent or greater than 0.15 weight percent, the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values cannot be formed.

The addition of chromium increases the volume fraction of varnish in steel and improves the strength of varnish, thereby increasing its hardenability. However, varnish accumulates a large number of dislocations, which in turn affects the steel's elongation. Therefore, when the chromium content in the aforementioned steel is less than 0.2 weight percent or greater than 0.8 weight percent, it is impossible to form the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values described in this application.

Aluminum acts as a deoxidizer in steel, reducing the oxygen content in molten steel and improving its toughness and workability. When the aluminum content exceeds 0.02 weight percent, the deoxidation effect is significant. However, when the aluminum content exceeds 0.08 weight percent, the steelmaking process becomes significantly more difficult, making it impossible to produce the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values.

While phosphorus addition has a solid solution strengthening effect on ferrous iron, it tends to segregate at the grain boundaries of the pre-existing austenitic iron, weakening the grain boundary strength and, in turn, deteriorating the workability of the resulting steel. When the phosphorus content in the aforementioned steel exceeds 0.03 weight percent, it is impossible to form the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values described in this case.

The presence of sulfur tends to segregate at the grain boundaries of the previously formed austenitic iron, weakening the grain boundary strength and, consequently, deteriorating the workability of the resulting steel. Furthermore, sulfur promotes the formation of manganese sulfide, which is detrimental to the steel's porosity. When the sulfur content in the aforementioned steel exceeds 0.01 weight percent, it is impossible to form the high-strength composite steel with the specified yield strength, tensile strength, elongation, porosity, and bake hardening values described herein.

Please refer to Figure 1, which is a schematic flow diagram of a method 100 for manufacturing a high-strength composite steel according to some embodiments of the present invention. A steel ingot is provided, as shown in step 110. Based on the total weight of the steel ingot as 100 weight percent, the steel ingot comprises 0.03 to 0.12 weight percent carbon, 1 to 2 weight percent manganese, 0.05 to 0.3 weight percent titanium, 0.02 to 0.15 weight percent vanadium, 0.2 to 0.8 weight percent chromium, 0.02 to 0.08 weight percent aluminum, no more than 0.03 weight percent phosphorus, no more than 0.01 weight percent sulfur, and the remainder iron and unavoidable impurities.

Thereafter, the steel blank is heated, as shown in step 120. In some embodiments, the heating temperature of the steel blank is 1150°C to 1300°C.

The steel blank is then hot rolled to obtain a hot-rolled steel plate, as shown in step 130. The hot-rolled steel plate has a finishing temperature of 800°C to 1000°C. When the finishing temperature is between 800°C and 1000°C, the steel produced subsequently has a specific yield strength, tensile strength, elongation, porosity, and bake hardening value.

Next, the hot-rolled steel plate is subjected to a cooling step to obtain a cooled steel plate, as shown in step 140. The cooling step includes (1) first cooling the hot-rolled steel plate to not less than 600°C at a cooling rate of 20°C/second to 200°C/second, as shown in step 142; (2) then air-cooling the hot-rolled steel plate for an air cooling time of 2 seconds to 10 seconds, as shown in step 144; and (3) then cooling the hot-rolled steel plate to the coiling temperature at a cooling rate of 50°C/second to 200°C/second, as shown in step 146.

In the aforementioned stage (1), the hot-rolled steel plate may be cooled (e.g., by water cooling) to a temperature range of the ferrous iron phase. In some embodiments, the hot-rolled steel plate may be water-cooled to a temperature of 600°C to 800°C. In some embodiments, the cooling rate in stage (1) is preferably 20°C/second to 200°C/second, and more preferably 40°C/second to 185°C/second. If the cooling rate in stage (1) is less than 20°C/second, the ferrous iron grains may coarsen, thereby affecting the toughness of the subsequently obtained steel. When the cooling rate in stage (1) is greater than 200℃/s, the grains of the granulated iron will become irregular, thus affecting the toughness of the subsequent steel.

The above-mentioned stage (2) can, for example, be to air-cool the hot-rolled steel plate to 600°C to 800°C. When the air-cooling temperature is between 600°C and 800°C, a certain proportion of TiC and VC can be generated, and granular iron grains of a specific size can be generated. In addition, when the air-cooling temperature of the above-mentioned stage (2) is between 600°C and 800°C, fine and neat TiC and VC will be generated. The more precipitates, the higher the strength, but too many precipitates will make the strength of the resulting steel too high. Therefore, when the air-cooling temperature is preferably controlled between 600°C and 800°C, the resulting steel has a specific yield strength, tensile strength, elongation, porosity and bake hardening value.

When the air cooling time in the above-mentioned stage (2) is less than 2 seconds, there is insufficient time to generate granular iron, which will result in insufficient elongation. When the air cooling time in the above-mentioned stage (2) is greater than 10 seconds, the austenitic iron will generate too much granular iron, and too much TiC and VC will be generated, causing the solid solution carbon in the steel to be excessively consumed, resulting in insufficient bake hardening value.

During the above-mentioned stage (3), a faster cooling rate helps to form tantalum and TiC precipitates in the steel plate. In some specific examples, stage (3) can be a water cooling stage. In some embodiments, the cooling rate of stage (3) is preferably 50°C/s to 200°C/s, and more preferably 80°C/s to 200°C/s. Since the formation mechanism of tantalum is a non-diffusion phase transformation, a high cooling rate (e.g., 50°C/s to 200°C/s) is required to promote its phase transformation. When the cooling rate is less than 50°C/s or greater than 200°C/s, it is impossible to obtain a steel with a specific yield strength, tensile strength, elongation, porosity, and bake hardening value.

After the cooling step 140, the cooled steel plate is subjected to a coiling step, as shown in step 150. In some embodiments, the coiling temperature in the above-mentioned stage (3) is 400°C to 650°C. When the coiling temperature is lower than 400°C, too little TiC will be produced. When the coiling temperature is higher than 650°C, the formation of tantalum will be suppressed. Specifically, when the temperature on the steel coil is high enough, it is sufficient to provide rapid atomic diffusion, which can cause the precipitation of precipitates to continue, and at the same time, sufficient tantalum will be generated, so that the steel obtained later has a specific yield strength and tensile strength.

Thereafter, a high-strength composite steel is obtained, as shown in step 160. The high-strength composite steel produced using the manufacturing method 100 has a yield strength of not less than 660 MPa, a tensile strength of not less than 760 MPa, an elongation of not less than 13%, a porosity of not less than 45%, and a bake hardening value of not less than 30 MPa.

The following experimental examples illustrate the application of the present invention, but are not intended to limit the present invention. Anyone skilled in the art will be able to make various modifications and improvements without departing from the spirit and scope of the present invention.

Experimental Example

First, a steel slab with a specific composition is heated. The slab is then hot-rolled to produce a hot-rolled steel plate, with the final rolling temperature being 940°C. Next, the hot-rolled steel plate is subjected to a cooling step to obtain a cooled steel plate, wherein the cooling step comprises (1) first cooling the hot-rolled steel plate to not less than 600°C at a cooling rate of 20°C/second to 200°C/second; (2) then air-cooling the hot-rolled steel plate, wherein the air-cooling time is 2 seconds to 10 seconds and the air-cooling temperature is 700°C; and (3) then cooling the hot-rolled steel plate to a coiling temperature at a cooling rate of 50°C/second to 200°C/second. Thereafter, the cooled steel plate is subjected to a coiling step to obtain a steel material, wherein the coiling temperature is 550°C.

The steel was evaluated using the following evaluation methods to measure yield stress (YS), tensile strength (TS), elongation (EL), hole expansion (HE), and bake hardness (BH). The results are shown in Table 1.

Comparative Example 1 to Comparative Example 3

Comparative Examples 1 through 3 were conducted in a similar manner to the experimental examples. The differences were that Comparative Example 1 employed single-stage cooling, with a cooling rate of 82°C/second and a cooling temperature of 553°C. The air cooling temperature in Comparative Example 2 was 807°C. The coiling temperature in Comparative Example 3 was 369°C. In addition to the process parameters specifically disclosed above, the other process parameters in Comparative Examples 1 through 3 are within the specific parameters of the process of the present invention. The specific parameter conditions and evaluation results for Comparative Examples 1 through 3 are shown in Table 1.

Table 1

Evaluation method

The yield strength, tensile strength, elongation, porosity, and bake hardening value of the steel were measured using known instruments and methods. The evaluation results are shown in Table 1.

1. Yield Strength (YS)

The yield strength referred to herein in this invention is measured in accordance with the standard method JIS Z2241, and is expressed in MPa, for the steel materials in the Experimental Example and Comparative Examples 1 to 3. Table 1 shows that the yield strengths of the Experimental Example are 790 MPa and 798 MPa, respectively, while the yield strengths of Comparative Examples 1 to 3 are all less than or equal to 760 MPa.

2. Tensile Strength (TS)

The tensile strength referred to herein is the tensile strength of the steel used in the Experimental Examples and Comparative Examples 1 through 3, measured in MPa using the standard test method JIS Z2241. Table 1 shows that the tensile strengths of the Experimental Examples are 832 MPa and 847 MPa, respectively, while the tensile strengths of Comparative Examples 1 through 3 are all less than or equal to 800 MPa.

3. Elongation (EL)

The elongation referred to herein is determined by testing the steel materials in the experimental examples and comparative examples 1 to 3 in accordance with the standard method JIS Z2241. Table 1 shows that the elongation of the experimental examples is 16.1% and 17.1%, respectively, while the elongation of comparative examples 1 to 3 ranges from 16.2% to 20.2%.

4. Porosity (HE)

The porosity referred to herein is determined by testing the steel materials in the Experimental Examples and Comparative Examples 1 to 3 in accordance with the standard method JFS T 1001. Table 1 shows that the porosity of the Experimental Examples is 66% and 74%, respectively, while the porosity of Comparative Examples 1 to 3 ranges from 28% to 71%.

5. Bake Hardening Value (BH)

The bake hardening values referred to herein in this invention were measured in accordance with the standard method JIS Z2241, and are expressed in MPa for the steel materials in the Experimental Example and Comparative Examples 1 to 3. Table 1 shows that the bake hardening values for the Experimental Example are 51 MPa and 46 MPa, respectively, while the bake hardening values for Comparative Examples 1 to 3 range from 28.4 MPa to 85.5 MPa.

Because Comparative Example 1 employed single-stage cooling, uneven precipitate density easily led to uneven strength. Because the air cooling temperature in Comparative Example 2 was outside the 600°C to 800°C range, insufficient TiC was produced, resulting in insufficient steel strength. Because the coiling temperature in Comparative Example 3 was outside the 400°C to 650°C range, insufficient TiC was produced, resulting in insufficient steel strength.

The high-strength composite steel of the present invention contains specific composition ratios and does not contain niobium, thus avoiding rolling instability. The manufacturing method of the high-strength composite steel of the present invention utilizes a two-stage cooling step with specific process parameters, with an air cooling step between the two stages. This allows for the formation of neatly arranged TiC and VC precipitates during the air cooling stage, enhancing the overall strength of the steel and producing equiaxed, rich iron grains, which improves the steel's workability.

It should be understood that although this invention uses a specific manufacturing method and a specific evaluation method as examples to illustrate the high-strength composite steel and its manufacturing method, anyone with ordinary skill in the art will recognize that the invention is not limited thereto and that other manufacturing methods or other evaluation methods may be used without departing from the spirit and scope of the invention.

Although the present invention has been disclosed above in terms of embodiments, this is not intended to limit the present invention. Anyone with ordinary skill in the art to which the present invention pertains may make various modifications and improvements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope of the patent application attached hereto.

100: Manufacturing method

110, 120, 130, 140, 142, 144, 146, 150, 160: Steps

Claims (9)

A high-strength composite steel comprising, based on 100 weight percent of the total weight of the high-strength composite steel, the following: 0.03 weight percent to 0.12 weight percent carbon; 1 weight percent to 2 weight percent manganese; 0.05 weight percent to 0.3 weight percent titanium; 0.02 weight percent to 0.15 weight percent vanadium; 0.2 weight percent to 0.8 weight percent chromium; 0.02 weight percent to 0.08 weight percent aluminum; not more than 0.03 weight percent phosphorus; not more than 0.01 weight percent sulfur; and The remainder is iron and inevitable impurities, wherein the high-strength composite steel does not contain nioin, the high-strength composite steel has a tensile strength of not less than 760 MPa, and a bake hardening value of not less than 30 MPa, wherein the high-strength composite steel has a porosity of not less than 45%, and the porosity is tested in accordance with the standard method JFS T 1001. The high-strength composite steel as claimed in claim 1, wherein the high-strength composite steel comprises granular iron, tantalum, matan loose iron, and chrysoprase carbon iron. The high-strength composite steel as described in claim 1, wherein a yield strength of the high-strength composite steel is not less than 660 MPa. The high-strength composite steel as described in claim 1, wherein the high-strength composite steel has an elongation of not less than 13%. A method for manufacturing a high-strength composite steel comprises: providing a steel billet, wherein, based on 100 weight percent of the total weight of the steel billet, the steel billet comprises: 0.03 weight percent to 0.12 weight percent carbon; 1 weight percent to 2 weight percent manganese; 0.05 weight percent to 0.3 weight percent titanium; 0.02 weight percent to 0.15 weight percent vanadium; 0.2 weight percent to 0.8 weight percent chromium; 0.02 weight percent to 0.08 weight percent aluminum; not more than 0.03 weight percent phosphorus; not more than 0.01 weight percent sulfur; and the remainder being iron and unavoidable impurities, wherein the steel billet does not contain niobia; and subjecting the steel billet to a heating step; After the heating step, the steel blank is subjected to a hot rolling step to obtain a hot rolled steel plate; the hot rolled steel plate is subjected to a cooling step to obtain a cooled steel plate, wherein the cooling step comprises: cooling the hot rolled steel plate to not less than 600°C at a cooling rate of 20 ^° C/second to 200 ^° C/second; after cooling the hot rolled steel plate to not less than 600 ^° C, air cooling the hot rolled steel plate, wherein an air cooling time is 2 seconds to 10 seconds; and after air cooling the hot rolled steel plate, cooling the hot rolled steel plate to not less than 600 ^° C at a cooling rate of 50 ^° C/second to 200 ^°C/ second. C/s to cool the hot-rolled steel plate to a coiling temperature of 400 ^° C to 650 ^° C; and coiling the cooled steel plate to obtain the high-strength composite steel, wherein the high-strength composite steel has a porosity of not less than 45%, the porosity being tested in accordance with standard method JFS T 1001. The method for manufacturing a high-strength composite steel as described in claim 5, wherein in the step of cooling the hot-rolled steel plate to not less than 600 ^° C at the cooling rate of 20 ^° C/s to 200 ^° C/s, the hot-rolled steel plate is cooled to 600 ^° C to 800 ^° C. The method for manufacturing a high-strength composite steel as described in claim 5, wherein a finishing temperature of the hot-rolled steel plate is 800 ^° C to 1000 ^° C. The method for manufacturing a high-strength composite steel as described in claim 5, wherein the high-strength composite steel has a tensile strength of not less than 760 MPa and a bake hardening value of not less than 30 MPa. A high-strength composite steel is produced by the method for producing a high-strength composite steel as described in any one of claims 5 to 8, wherein the high-strength composite steel comprises ferrous granular iron, tantalum, maitainite, and tantalum carbon.