A method for producing weathering steel using scrap steel and weathering steel

Abstract

The invention belongs to the technical field of steel production, and discloses a method for producing weathering steel using scrap steel and weathering steel. The composition of the molten steel of the present invention includes: Mn: 0.4-1.0%; Si: 0.2-0.4%; Cr: 0.3-0.5%; P: 0.07-0.18%; S: ≤0.003%; O: 0.004-0.008%; H≤0.0005%. The composition of the molten steel of the present invention also includes: Cu: 0.25-0.4%; Sn: 0-0.12%; Sb: 0-0.15%. The content of the C element in the molten steel of the present invention is controlled according to 0.03-0.009*Si-0.01*Si*Mn-0.0015*Mn-0.013*Cu. The method of the present invention makes full use of the weathering elements Cu, Sn, and Sb that cannot be removed by scrap steel smelting, and the scrap steel ratio is 60-100%, which greatly promotes the recycling of scrap steel. The method of the invention reduces the crack detection rate of the twin-roller thin strip casting weathering steel to below 1.0% by optimizing the casting process.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production, and particularly relates to a method for producing weathering steel by utilizing scrap steel and the weathering steel. Background Art

[0002] Compared with the blast furnace-converter steelmaking process, the scrap-based electric arc furnace steelmaking process is more environmentally friendly. Data from the World Steel Association proves that the carbon emissions of the scrap-based electric arc furnace steelmaking process are only 13.6% of the blast furnace-converter steelmaking process.

[0003] The proportion of hot-rolled weathering steel products accounts for more than 90% of the entire weathering steel variety. After the molten steel is obtained, the technical equipment routes for producing weathering steel include conventional hot rolling, thin slab continuous casting and rolling, and twin-roller thin strip casting and rolling. Among them, twin-roller thin strip casting and rolling integrates continuous casting, rolling, and even heat treatment into one, so that the thin strip produced can be formed into a thin steel strip at one time after one online hot rolling, which greatly simplifies the production process, shortens the weathering steel production cycle and production process, and significantly reduces the cost, carbon emissions and energy consumption of the product.

[0004] The scrap-based electric arc furnace steelmaking + twin-roller thin strip casting and rolling technology equipment route is the best choice for producing weathering steel. However, no matter which process route is used, cracking is one of the key issues in container plate production.

[0005] Patent document 201811587434.0 discloses the "Production Method of Weathering Steel Q355NHD", which belongs to the category of conventional hot rolling and suppresses the occurrence of cracks in the corners of weathering steel ingots.

[0006] The document "Analysis and Improvement of Causes of Center Cracks in SPA-H Steel Plate" also belongs to the category of conventional hot rolling. It believes that the central eccentricity, looseness and MnS inclusions enriched in the center of the ingot are the root causes of the cracks. The cracks are prevented by reducing the S content to below 80ppm, controlling the superheat at around 25°C, optimizing the electromagnetic stirring process, optimizing the dynamic light reduction process and other measures.

[0007] The document "Research on the Quality of Hot-rolled Container Plate Continuous Casting Billets" belongs to the category of thin slab continuous casting and rolling. It believes that C is in the range of peritectic reaction and the excessive heat flux density of the crystallizer is the cause of cracks. Cracks can be prevented by reducing the C content to below 0.06% and reducing the continuous casting speed.

[0008] The document "Study on the Heating Process in a Roller-Heat Heating Furnace for SPA-H Container Plate Continuous Casting" also belongs to the category of thin slab continuous casting and rolling. It believes that copper brittleness is the cause of cracks, and that cracks can be prevented by adding a certain amount of Ni or Si.

[0009] Patent document 200910048141.X disclosed in "A weathering steel and its manufacturing method" and patent document 201410591781.6 disclosed in "A weathering steel hot-rolled thin strip with no P segregation and its manufacturing method" belong to the category of double-roll thin strip casting and rolling. The carbon content of this method falls within the range of sub-peritectic steel, and the steel strip is prone to cracks.

[0010] Patent document 201710888181.X disclosed in "A weathering steel and its production method" and patent document 201910130658.7 disclosed in "A low-cost weathering steel and its production method" also belong to the category of double-roller thin strip casting and rolling, but the problem of weathering steel cracks has not been completely solved.

[0011] In addition, Sn, Cu, and Sb in steel are difficult to remove during the steelmaking process. With the recycling of scrap steel, the content of Sn, Cu, and Sb in scrap steel gradually increases. Sn, Cu, and Sb are easily enriched at grain boundaries, causing defects such as cracks. The crack problem of weathering steel produced using scrap steel is aggravated.

[0012] Sn, Cu, and Sb can improve the weather resistance of weathering steel. If the cracks caused by their enrichment can be suppressed, it will "turn harm into benefit", promote the recycling of scrap steel, reduce production costs, and achieve green and sustainable development of weathering steel. Summary of the invention

[0013] The purpose of the present invention is to promote the recycling of scrap steel, make full use of the weathering elements Cu, Sn and Sb in the scrap steel, greatly reduce the probability of cracks in twin-roll cast weathering steel, develop a weathering steel, and realize the economical, industrialized and green large-scale production of twin-roll cast weathering steel.

[0014] To achieve the purpose of the present invention, the present invention adopts the following technical solution:

[0015] According to a first aspect of the present invention, a method for producing weathering steel is disclosed, the method comprising the following steps:

[0016] (1) Using EAF+VD+LF steelmaking process to obtain molten steel,

[0017] The molten steel comprises the following components:

[0018] Si: 0.20-0.40%;

[0019] Mn: 0.40-1.0%;

[0020] Cr: 0.3-0.5%;

[0021] P: 0.07-0.18%;

[0022] S: ≤0.003%;

[0023] The balance is Fe and unavoidable impurities;

[0024] (2) subjecting the molten steel obtained in step (1) to thin strip continuous casting using a twin-roll casting device under the protection of an inert gas to obtain a cast strip;

[0025] (3) placing the cast strip obtained in step (2) into a hot box, wherein the hot box includes a hot box No. 1, a hot box No. 2, and a hot box No. 3;

[0026] (4) hot rolling the cast strip obtained in step (3) into a thin strip through one pass;

[0027] (5) cooling the thin strip obtained in step (4) by mist cooling, winding and air cooling to room temperature;

[0028] Wherein, the molten steel further comprises the following components:

[0029] Cu: 0.25-0.40%;

[0030] Sn: 0-0.12%;

[0031] Sb: 0-0.15%.

[0032] According to the method for producing weathering steel of the present invention, preferably, the C element of the molten steel is controlled by the Si, Mn and Cu contents according to the following relationship:

[0033] C: 0.03-0.009*Si-0.01*Si*Mn-0.0015*Mn-0.013*Cu.

[0034] According to the method for producing weathering steel of the present invention, preferably, the proportion of scrap steel in EAF is 60-100%.

[0035] According to the method for producing weathering steel of the present invention, preferably, the tundish is set to O, and the O content is 0.004-0.008%; the tundish is set to H, and the H content is ≤0.0005%.

[0036] According to the method for producing weathering steel of the present invention, preferably, in the step (2), the thickness of the cast strip is 1.85-2.5 mm.

[0037] According to the method for producing weathering steel of the present invention, preferably, in the step (2), the roller brush torque is ≥250 lb·in, the roller brush speed is 400±20 rpm, and the hot box looper amount is controlled at 20-40%.

[0038] According to the method for producing weathering steel of the present invention, preferably, in step (3), the inert gas flow rate in the No. 1 hot box is ≥ 600m 3 / h.

[0039] According to the method for producing weathering steel of the present invention, preferably, in the step (4), the hot rolling reduction ratio is ≥ 22%.

[0040] According to the method for producing weathering steel of the present invention, preferably, in the step (5), the cooling temperature of the mist cooling is adjusted according to the LF endpoint C, comprising:

[0041] (a) When the refining endpoint C>0.016%, the mist cooling temperature is controlled to be 680±20℃;

[0042] (b) When the refining endpoint C is 0.01-0.016%, the mist cooling temperature is controlled to be 650±20°C;

[0043] (c) When the refining endpoint C is less than 0.01%, the mist cooling temperature is controlled to be 620±20℃.

[0044] According to a second aspect of the present invention, a weathering steel is disclosed, which is produced according to the aforementioned method for producing weathering steel.

[0045] According to the weathering steel of the present invention, preferably, the metallographic structure of the weathering steel comprises polygonal ferrite precipitated along austenite grain boundaries surrounding acicular ferrite.

[0046] According to the weathering steel of the present invention, preferably, the yield strength of the weathering steel is ≥355 MPa, the tensile strength is ≥490 MPa, and the elongation after fracture is ≥25%.

[0047] According to the weathering steel of the present invention, preferably, the crack detection rate of the weathering steel is reduced to below 1.0%.

[0048] Technical concept

[0049] The conception of the above technical solution of the present invention is as follows:

[0050] The present invention believes that the leftward shift of the peritectic point leads to an early solid phase transformation from delta ferrite to gamma austenite, which causes insufficient strength of the solidified shell and makes it difficult to resist phase transformation shrinkage, resulting in cracks. The present invention effectively avoids this phenomenon by optimizing the composition.

[0051] Si, Mn, Cu and S cause the peritectic point in the phase diagram to shift to the left. While meeting weathering steel standards or customer requirements, the Si, Mn and Cu contents are controlled, and the C content is further limited to offset the impact of the left shift of the peritectic point.

[0052] In the present invention, Si, Mn, Cu and C are controlled as follows:

[0053] Si: 0.20-0.40%;

[0054] Mn: 0.40-1.0%;

[0055] Cu: 0.25-0.40%

[0056] C: 0.03-0.009*Si-0.01*Si*Mn-0.0015*Mn-0.013*Cu;

[0057] S: ≤0.003%.

[0058] The present invention believes that when the density of P combined with the solidified metal surface is higher than a certain level, it hinders the solidification of the metal and is easy to form a closed liquid film. When the tensile stress on the casting strip is too large, the grains on both sides of the closed liquid film are pulled apart to form crack sources. The strength of the billet shell is too low, and the crack sources expand to form cracks.

[0059] Since P is an important weather-resistant element, it is impossible to eliminate it. The negative effects of P need to be eliminated by other means:

[0060] (1) The present invention limits the thickness of the casting strip to 1.85-2.5 mm, ensuring that a certain amount of liquid core is retained when the casting strip passes through the hot box, reducing the probability of the occurrence of a closed liquid film and the probability of the formation of a crack source.

[0061] (2) The present invention limits the thickness of the casting strip to 1.85-2.5 mm, increases the thickness of the casting strip, reduces the casting strip temperature, increases the strength of the billet shell, and inhibits the expansion of the crack source.

[0062] (3) The present invention limits the inert gas flow rate of the No. 1 hot box to ≥ 600m 3 / h, reducing the temperature of the casting strip when passing through the hot box looper, increasing the strength of the billet shell and inhibiting the expansion of the crack source.

[0063] (4) The present invention limits the amount of hot box loopers to 20-40%, thereby reducing the tensile stress on the casting belt when passing through the hot box and inhibiting the formation of crack sources.

[0064] The present invention believes that low-frequency vibration during casting and rolling can cause cracks:

[0065] The present invention limits the O content of the tundish to 0.004-0.008%, which can effectively suppress the occurrence of low-frequency vibration.

[0066] H gathers between the casting rolls and the casting belt, seriously affecting the interface heat transfer, causing uneven solidification and forming cracks: the present invention limits the H content of the tundish to ≤0.0005%.

[0067] The thickness of the oxide film on the casting roll in the twin-roll casting process significantly affects the interfacial heat transfer between the molten steel and the casting roll. If the oxide film is too thin, too thick or uneven, it will inhibit the interfacial heat transfer and cause cracks: the present invention limits the roller brush torque to ≥250lb·in and the roller brush speed to 400±20rpm, which can ensure a uniform and appropriate oxide film and inhibit the occurrence of cracks.

[0068] Sufficient hot rolling reduction can weld some cracks: the present invention limits the hot rolling reduction to ≥ 22%.

[0069] In traditional processes, Sn and Sb are easily segregated elements. A small amount of Sn and Sb will be enriched at the grain boundary, causing cracks. The double-roll thin strip technology belongs to the category of sub-rapid solidification, which greatly reduces the tendency of element segregation and can greatly increase the solid solution content of elements. Under the above process guarantee, the upper limit of Sn in the present invention is controlled to 0.12%, and the upper limit of Sb is controlled to 0.15%. After multiple trial productions, no cracks have occurred.

[0070] Beneficial technical effects

[0071] Compared with the prior art, the technical concept and corresponding technical solution of the present invention can at least achieve the following beneficial technical effects:

[0072] (1) The weathering steel of the present invention makes full use of Cu, Sn and Sb in scrap steel that cannot be removed by smelting. The Cu content is 0.25-0.4%, the Sn content is 0-0.12%, the Sb content is 0-0.15%, and the scrap steel ratio is 60-100%, which greatly promotes the recycling of scrap steel.

[0073] (2) The present invention reduces the crack detection rate of twin-roll thin strip cast weathering steel to below 0.8%. BRIEF DESCRIPTION OF THE DRAWINGS

[0074] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings of the embodiments are briefly introduced below. Obviously, the drawings in the following description only relate to some embodiments of the present invention, but are not intended to limit the present invention.

[0075] Figure 1 The present invention is a schematic diagram of the production process of producing thin-gauge weathering steel according to the method of the present invention.

[0076] Figure 2 This is the metallographic structure diagram of the weathering steel produced according to Example 1 of the present invention.

[0077] Figure 3 This is the metallographic structure diagram of the weathering steel produced according to Example 2 of the present invention.

[0078] Figure 4 This is the metallographic structure diagram of the weathering steel produced according to Example 3 of the present invention. DETAILED DESCRIPTION

[0079] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0080] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one of ordinary skill in the art to which the present invention belongs.

[0081] The following are embodiments of the present invention. The described embodiments are only a part of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments that can be obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0082] Example 1

[0083] (1) Molten steel smelting: Electric furnace steelmaking, VD vacuum furnace degassing, LF furnace refining are adopted to obtain molten steel with qualified composition, which is as follows by weight: C: 0.018%; Si: 0.31%; Mn: 0.56%; Cu: 0.35%; Cr: 0.42%; P: 0.081%; S: 0.0021%; Sn: 0.062%; Sb: 0.08%; the remainder is iron and unavoidable impurity elements.

[0084] (2) The content of O and H in the tundish is 0.0051% for O and 0.00032% for H.

[0085] (3) Strip casting:

[0086] The thickness of the casting strip is 1.95 mm, and the inert gas flow rate of the No. 1 hot box is 980 m 3 / h, the hot box looper amount is controlled at 30%, the roller brush torque is 270-290lb·in, and the roller brush speed is 410-420rpm.

[0087] (4) Hot rolling: The cast strip was hot rolled in one pass at a reduction of 24.6% to form a hot rolled thin strip with a thickness of 1.47 mm. The mist cooling temperature was 675°C.

[0088] Under this process parameter, the probability of steel strip cracks is 0%, the yield strength of weathering steel is 363MPa, the tensile strength is 502MPa, the elongation is 28%, and the metallographic structure of weathering steel is as follows: Figure 2 shown.

[0089] Example 2

[0090] (1) Molten steel smelting: Electric furnace steelmaking, VD vacuum furnace degassing, LF furnace refining are adopted to obtain molten steel with qualified composition, which is as follows by weight: C: 0.016%; Si: 0.26%; Mn: 0.88%; Cu: 0.32%; Cr: 0.38%; P: 0.075%; S: 0.0021%; Sn: 0.11%; Sb: 0.11%; the remainder is iron and unavoidable impurity elements.

[0091] (2) The content of O and H in the tundish is 0.0065% and 0.00045%.

[0092] (3) Thin strip continuous casting:

[0093] The thickness of the casting strip is 2.1 mm, and the inert gas flow rate of the No. 1 hot box is 1030 m 3 / h, the hot box looper amount is controlled at 35%, the roller brush torque is 250-270lb·in, and the roller brush speed is 400-420rpm.

[0094] (4) Hot rolling: The cast strip was hot rolled in one pass with a reduction of 30% to form a hot rolled thin strip with a thickness of 1.47 mm. The mist cooling temperature was 630°C.

[0095] Under this process parameter, the probability of steel strip cracks is 0%, the yield strength of weathering steel is 373MPa, the tensile strength is 515MPa, the elongation is 25%, and the metallographic structure of weathering steel is as follows: Figure 3 shown.

[0096] Example 3

[0097] (1) Molten steel smelting: Electric furnace steelmaking, VD vacuum furnace degassing, LF furnace refining are adopted to obtain molten steel with qualified composition, which is as follows by weight: C: 0.019%; Si: 0.38%; Mn: 0.90%; Cu: 0.35%; Cr: 0.35%; P: 0.11%; S: 0.0024%; Sn: 0.071%; Sb: 0.04%; the remainder is iron and unavoidable impurity elements.

[0098] (2) The content of O and H in the tundish is 0.0060% and 0.00048% respectively.

[0099] (3) Thin strip continuous casting:

[0100] The thickness of the casting strip is 2.1 mm, and the inert gas flow rate of the No. 1 hot box is 780 m 3 / h, the hot box looper amount is controlled at 38%, the roller brush torque is 310-320lb·in, and the roller brush speed is 400-420rpm.

[0101] (4) Hot rolling: The cast strip was hot rolled in one pass with a reduction of 30% to form a hot rolled thin strip with a thickness of 1.47 mm. The mist cooling temperature was 650°C.

[0102] Under this process parameter, the probability of steel strip cracks is 0%, the yield strength of weathering steel is 407MPa, the tensile strength is 522MPa, the elongation is 25%, and the metallographic structure of weathering steel is as follows: Figure 4 shown.

[0103] The above description is only a specific implementation of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for producing weathering steel using scrap steel, characterized in that: The method comprises the following steps: (1) Using EAF+VD+LF steelmaking process to obtain molten steel, The molten steel is as follows by weight percentage: Si: 0.20-0.40%; Mn: 0.40-1.0%; Cr：0.3-0.5％； P：0.07-0.18％； S：≤0.003％； Cu: 0.25-0.40%; Sn: 0-0.12%; Sb: 0-0.15%, The C element is controlled by the Si, Mn, and Cu contents according to the following relationship: C: 0.03-0.009*Si-0.01*Si*Mn-0.0015*Mn-0.013*Cu, The balance is Fe and unavoidable impurities; (2) subjecting the molten steel obtained in step (1) to thin strip continuous casting using a twin-roll casting device under the protection of an inert gas to obtain a cast strip; (3) placing the cast strip obtained in step (2) into a hot box, wherein the hot box includes a hot box No. 1, a hot box No. 2, and a hot box No. 3; (4) hot rolling the cast strip obtained in step (3) into a thin strip through one pass; (5) cooling the thin strip obtained in step (4) by mist cooling, winding and air cooling to room temperature; Among them, the tundish is O, and the O content is 0.004-0.008%; the tundish is H, and the H content is ≤0.0005%. Wherein, in the step (5), the cooling temperature of the mist cooling is adjusted according to the LF refining end point C, comprising: (a) When the LF refining endpoint C>0.016%, the mist cooling temperature is controlled to be 680±20℃; (b) When the LF refining endpoint C is 0.01-0.016%, the mist cooling temperature is controlled to be 650±20℃; (c) When the LF refining endpoint C is less than 0.01%, the mist cooling temperature is controlled to be 620±20℃.

2. The method for producing weathering steel using scrap steel according to claim 1, characterized in that: The proportion of scrap steel in EAF is 60-100%.

3. The method for producing weathering steel using scrap steel according to claim 1, characterized in that: In the step (2), the thickness of the cast strip is 1.85-2.5 mm.

4. The method for producing weathering steel using scrap steel according to claim 3, characterized in that: In the step (2), the thickness of the cast strip is 1.95 mm.

5. The method for producing weathering steel using scrap steel according to claim 1, characterized in that: In step (3), the inert gas flow rate in the No. 1 hot box is ≥ 600m 3 / h.

6. The method for producing weathering steel using scrap steel according to claim 1, characterized in that: In the step (4), the hot rolling reduction ratio is ≥ 22%.

7. A weathering steel, characterized in that: The weathering steel is produced according to the method for producing weathering steel using scrap steel according to any one of claims 1 to 6.

8. The weathering steel according to claim 7, characterized in that: The metallographic structure of the weathering steel includes polygonal ferrite precipitated along austenite grain boundaries and surrounding acicular ferrite.

9. The weathering steel according to claim 7 or 8, characterized in that: The yield strength of the weathering steel is ≥355MPa, the tensile strength is ≥490MPa, and the elongation after fracture is ≥25%.

10. The weathering steel according to claim 7 or 8, characterized in that: The crack detection rate of the weathering steel is reduced to below 1.0%.

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