A thin gauge yield strength 800mpa grade high strength steel and a method of producing the same

Abstract

The present application belongs to the technical field of steel production, and particularly relates to a low-cost thin-gauge high-strength steel with a yield strength of 800 MPa and a production method thereof, and the thickness of the steel coil is 0.8-1.6 mm. The present application is based on a thin-strip casting and rolling production method, molten steel is obtained by thin-strip continuous casting, the cast strip is obtained by single-pass hot rolling and air mist cooling, and the hot-rolled coil is obtained by coiling. The chemical composition of the molten steel includes, by mass percentage, C: 0.20-0.25%; Si: 0.30-0.60%; Mn: 1.20-1.70%; Al: ≤0.003%; P: ≤0.020%; S: ≤0.004%, and the balance is Fe and inevitable impurity elements. The present application solves the problem that the traditional hot-rolling process is difficult to produce thin-gauge high-strength products, and solves the problem that the cost of the cast-rolling process for producing high-strength steel is high. The method of the present application can produce thin-gauge steel strips with a yield strength of ≥800 MPa and a breaking elongation of ≥10% by using low-cost molten steel ingredients, in combination with subsequent process flows and specific parameters.

Description

Technical Field

[0001] This invention belongs to the field of steel production technology, specifically relating to a thin-gauge high-strength steel with a yield strength of 800MPa and its production method. Background Technology

[0002] With the increasing prominence of global energy and environmental protection issues, industries such as automobiles, agricultural machinery, and equipment manufacturing are placing great emphasis on lightweighting. Reducing the thickness of steel plates by increasing their strength is a crucial means of achieving lightweighting. The application of high-strength steel plates with a yield strength of 800 MPa is becoming increasingly widespread. However, thin-gauge high-strength steels rely more heavily on cold rolling following hot rolling to ensure their thinness, and annealing following cold rolling to guarantee their performance. Producing ultra-high-strength steel plates using traditional hot rolling or thin slab continuous casting and rolling processes, such as CSP, is highly challenging, especially for plates with a thickness ≤1.5 mm.

[0003] Patent document 202010486978.9 discloses "TMCP process for ultra-high strength steel with yield strength > 800MPa and its production method." This method involves adding a significant amount of Nb, Ti, and Mo elements, resulting in high alloy costs. The cooling process of this method is a three-stage cooling process: water cooling-air cooling-water cooling. After the first stage of water cooling to 640–680℃, air cooling is applied for a period, followed by continued water cooling to the coiling temperature of 600–630℃. When producing thin steel sheets, it is difficult to accurately control the coiling temperature; the product thickness of this method is 3–8mm.

[0004] Patent document 202010263681.6 discloses "Hot-rolled steel sheet with yield strength of 800MPa and its manufacturing method". This method adds a large amount of Nb, Ti and Mo elements, resulting in high alloy cost. After hot rolling and coiling, this method requires leveling and tempering processes to obtain hot-rolled steel sheet with yield strength of 800MPa, which results in high process cost.

[0005] Patent document 201710398334.2 discloses "Hot-rolled thin plate with yield strength ≥800MPa directly rolled from thin slab and production method", which adds more Nb, Ti and Mo elements, resulting in high alloy cost.

[0006] It is evident that the methods described above in the prior art have at least the following drawbacks:

[0007] (a) The addition of more alloying elements such as Nb, Ti, Mo, and Cr results in higher alloy costs;

[0008] (b) The product thickness cannot cover the range of thin specifications below 1.5mm.

[0009] The purpose of this invention is to overcome the above-mentioned defects of the prior art—to provide a method for producing thin-gauge high-strength steel with a yield strength of 800 MPa in a low-cost manner. Summary of the Invention

[0010] To achieve the objectives of this invention, the technical solution described below is adopted.

[0011] According to a first aspect of the present invention, a method for producing thin-gauge high-strength steel with a yield strength of 800 MPa is provided, the method comprising the following steps:

[0012] (1) Steel is obtained by smelting, wherein the steel is formulated according to the following chemical composition and mass fraction:

[0013] C: 0.20–0.25%;

[0014] Si: 0.20–0.60%;

[0015] Mn: 1.20–1.70%;

[0016] P: ≤0.020%;

[0017] Al: ≤0.003%;

[0018] S: ≤0.004%;

[0019] The balance consists of Fe and unavoidable impurities;

[0020] (2) The molten steel obtained in step (1) is subjected to a twin-roll casting process under inert gas protection to obtain a thin strip. The casting speed of the thin strip is 40-70 m / min, and the thickness of the obtained strip is 1.5-1.8 mm.

[0021] (3) The cast strip obtained in step (2) is hot rolled into a thin strip in one pass. The hot rolling reduction rate is 10-50%, the hot rolling exit temperature is ≥900℃, and the thickness of the thin strip after hot rolling is 0.8-1.6mm.

[0022] (4) The steel strip obtained in step (3) is cooled to 540-600°C by an air mist cooling system, and then air-cooled to room temperature after winding. The winding temperature is 440-500°C.

[0023] According to the method for producing thin-gauge high-strength steel with a yield strength of 800 MPa according to the present invention, preferably, in step (2), the initial pouring temperature of the molten steel is 1500-1600℃.

[0024] According to the method for producing thin-gauge high-strength steel with a yield strength of 800 MPa according to the present invention, preferably, in step (2), the casting speed of the thin strip continuous casting is 42 to 70 m / min.

[0025] According to the method for producing thin-gauge high-strength steel with a yield strength of 800 MPa according to the present invention, preferably, in step (3), the hot rolling reduction rate is 10-40%.

[0026] According to the method for producing thin-gauge high-strength steel with a yield strength of 800 MPa according to the present invention, preferably, in step (3), the hot rolling exit temperature is ≥910℃.

[0027] According to the method for producing thin-gauge high-strength steel with a yield strength of 800 MPa according to the present invention, preferably, in step (4), the steel strip is cooled to 540-571°C by an air mist cooling system.

[0028] In the method for producing thin-gauge high-strength steel with a yield strength of 800 MPa according to the present invention, preferably, in step (4), the winding temperature is 440-475°C.

[0029] According to a second aspect of the present invention, a thin-gauge high-strength steel with a yield strength of 800 MPa is provided, said thin-gauge high-strength steel with a yield strength of 800 MPa is produced using the aforementioned method.

[0030] The thin-gauge high-strength steel of the present invention has a yield strength of 800 MPa and preferably has an elongation after fracture of >10%.

[0031] The thin-gauge high-strength steel of the present invention with a yield strength of 800 MPa is preferably characterized by a microstructure comprising lath bainite and granular bainite.

[0032] Beneficial technical effects

[0033] Compared with the prior art, the technical advantages and beneficial effects of the present invention are at least as follows:

[0034] (1) Low alloy cost: This invention only adds Si: 0.2-0.6% and Mn: 1.2-1.7%, which is significantly lower than the existing technology. It does not add high-cost alloying elements such as Nb, Ti, Mo and Cr, which makes the alloy cost of this invention low. That is, with low-cost molten steel materials, combined with the subsequent process flow and specific parameters of this invention, it is still possible to produce thin steel strips with a yield strength ≥800MPa and an elongation after fracture ≥10%.

[0035] (2) Thin casting strip thickness: The casting strip thickness of the present invention is only 1.5mm to 1.8mm, which is much lower than the thickness of the casting strip or billet in the prior art. With the complete process route of the present invention, it is very easy to obtain a thin strip with a thickness of 0.8mm to 1.6mm.

[0036] (3) Hot rolling process: This invention only undergoes one 10% to 50% hot rolling, with a hot rolling exit temperature of ≥900℃. Combined with other process parameters of this invention, it can produce thin strips with excellent mechanical properties—yield strength ≥800MPa and elongation after fracture ≥10%.

[0037] (4) Cooling and coiling process: After the steel strip passes through the hot rolling mill, it undergoes a period of air mist cooling. The cooling temperature is 540℃~600℃ and the coiling temperature is 440℃~500℃. It is easy to hit the process target, the production control is simple, and the requirements for the coiling machine are low. It is not easy to have defects such as coil collapse during coiling. Attached Figure Description

[0038] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below. Obviously, the drawings described below only relate to some embodiments of the present invention and are not intended to limit the scope of the invention.

[0039] Figure 1 The metallographic structure of the high-strength steel produced according to Embodiment 1 of the present invention.

[0040] Figure 2 The metallographic structure of the high-strength steel produced according to Embodiment 2 of the present invention.

[0041] Figure 3 The metallographic structure of the high-strength steel produced according to Embodiment 3 of the present invention. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0043] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0044] Example 1

[0045] (1) Steelmaking: Electric furnace steelmaking is adopted, VD vacuum furnace degassing and LF furnace refining are used to obtain qualified molten steel with the following weight percentages: C: 0.235%; Si: 0.35%; Mn: 1.48%; Al: 0.0022%; P: 0.013%; S: 0.0018%; the balance is iron and unavoidable impurity elements.

[0046] (2) Thin strip continuous casting: qualified molten steel is continuously cast using twin-roll casting process. The initial pouring temperature of the molten steel is 1560℃, the casting speed is 42m / min, and the molten steel is continuously cast in thin strip under inert gas protection. The thickness of the cast strip is 1.8mm.

[0047] (3) Hot rolling: The cast strip is hot rolled into a 1.50 mm thick strip in one pass with a reduction rate of 17%. The hot rolling exit temperature is 912℃, the strip temperature after air mist cooling is 542℃, and the coiling temperature is 468℃.

[0048] The high-strength steel produced according to the above composition and process has the following mechanical properties: yield strength 804MPa, tensile strength 1013MPa, and elongation 12%.

[0049] According to the above embodiments, the metallographic structure of the thin-gauge high-strength steel with a yield strength of 800 MPa is as follows: Figure 1 As shown.

[0050] Example 2

[0051] (1) Steelmaking: Electric furnace steelmaking is adopted, VD vacuum furnace degassing and LF furnace refining are used to obtain qualified molten steel with the following weight percentages: C: 0.211%; Si: 0.37%; Mn: 1.62%; Al: 0.0028%; P: 0.012%; S: 0.0015%; the balance is iron and unavoidable impurity elements.

[0052] (2) Thin strip continuous casting: qualified molten steel is continuously cast using twin-roll casting process. The initial pouring temperature of the molten steel is 1530℃, the casting speed is 57m / min, and the molten steel is continuously cast in thin strip under inert gas protection. The thickness of the cast strip is 1.6mm.

[0053] (3) Hot rolling: The cast strip is hot rolled into a 1.20 mm thick strip in one pass with a reduction rate of 25%. The hot rolling exit temperature is 922℃, the strip temperature after air mist cooling is 551℃, and the coiling temperature is 460℃.

[0054] The high-strength steel produced according to the above composition and process has the following mechanical properties: yield strength 834MPa, tensile strength 1044MPa, and elongation 11%.

[0055] According to the above embodiments, the metallographic structure of the thin-gauge high-strength steel with a yield strength of 800 MPa is as follows: Figure 2 As shown.

[0056] Example 3

[0057] (1) Steelmaking: Electric furnace steelmaking is adopted, VD vacuum furnace degassing and LF furnace refining are used to obtain qualified molten steel with the following weight percentages: C: 0.225%; Si: 0.55%; Mn: 1.57%; Al: 0.0021%; P: 0.0110%; S: 0.0021%; balance is iron and unavoidable impurity elements.

[0058] (2) Thin strip continuous casting: qualified molten steel is continuously cast using twin-roll casting process. The initial pouring temperature of the molten steel is 1570℃, the casting speed is 55m / min, and the molten steel is continuously cast in thin strip under inert gas protection. The thickness of the cast strip is 1.5mm.

[0059] (3) Hot rolling: The cast strip is hot rolled into a 0.90 mm thick strip in one pass with a reduction rate of 40%. The hot rolling exit temperature is 910℃, the strip temperature after air mist cooling is 571℃, and the coiling temperature is 475℃.

[0060] The high-strength steel produced according to the above composition and process has the following mechanical properties: yield strength 860MPa, tensile strength 1056MPa, and elongation 13%.

[0061] According to the above embodiments, the metallographic structure of the thin-gauge high-strength steel with a yield strength of 800 MPa is as follows: Figure 3 As shown.

[0062] The above description is only a specific embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for producing thin-gauge high-strength steel with a yield strength of 800 MPa, characterized in that, The method includes the following steps: (1) Steel is obtained by smelting, wherein the steel is formulated according to the following chemical composition and mass fraction: C：0.20~0.25%； Si: 0.35~0.55%; Mn: 1.48~1.57%; P：≤0.020%； Al：≤0.003%； S：≤0.004%； The balance consists of Fe and unavoidable impurities; (2) The molten steel obtained in step (1) is subjected to a twin-roll casting process under inert gas protection to obtain a thin strip. The casting speed of the thin strip is 42~70m / min, and the thickness of the obtained strip is 1.5~1.8mm. (3) The cast strip obtained in step (2) is hot rolled into a thin strip in one pass. The hot rolling reduction rate is 10~50%, the hot rolling exit temperature is ≥910℃, and the thickness of the thin strip after hot rolling is 0.8~1.6mm. (4) The steel strip obtained in step (3) is cooled to 540~600℃ by an air mist cooling system, and then air-cooled to room temperature after winding. The winding temperature is 440~475℃.

2. The method according to claim 1, characterized in that: In step (2), the initial pouring temperature of the molten steel is 1500~1600℃.

3. The method according to claim 1, characterized in that: In step (2), the casting speed of the thin strip continuous casting is 42~57m / min.

4. The method according to claim 1, characterized in that: In step (3), the hot rolling reduction rate is 10-40%.

5. The method according to claim 1, characterized in that: In step (3), the hot rolling exit temperature is 910~922℃.

6. The method according to any one of claims 1-5, characterized in that: In step (4), the steel strip is cooled to 540~571℃ by an air mist cooling system.

7. The method according to any one of claims 1-5, characterized in that: In step (4), the winding temperature is 460~475℃.

8. A thin-gauge high-strength steel with a yield strength of 800 MPa, characterized in that: The high-strength steel is produced using the method described in any one of claims 1-7.

9. The thin-gauge high-strength steel with a yield strength of 800 MPa according to claim 8, characterized in that: The high-strength steel has a yield strength of 800 MPa and an elongation after fracture of >10%.

10. The thin-gauge high-strength steel with a yield strength of 800 MPa according to claim 8 or 9, characterized in that: The microstructure of the high-strength steel includes lath bainite and granular bainite.

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