A high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and a low yield ratio without quenching and tempering and a manufacturing method thereof

Abstract

The present invention relates to the technical field of high-strength wear-resistant steel production, specifically a tempering-free, high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and a low yield ratio, and a manufacturing method. The steel plate comprises the following chemical components by weight: C: 0.16% to 0.22%, Si: 0.60% to 0.90%, Mn: 0.80% to 1.00%, P ≤ 0.015%, S ≤ 0.005%, Al: 0.060% to 0.100%, Cr: 0.20% to 0.40%, N ≤ 0.0040%, H ≤ 0.00020%, Ti: 0.045% to 0.060%, with the remainder being Fe and unavoidable impurities. The steel plate has a thickness of ≤ 12 mm, a tensile strength of 1200 MPa or greater, a yield ratio of ≤ 0.75, an elongation of ≥ 12%, a hardness of 400 ± 30 HBW, an impact energy value exceeding 35 J at -40°C, and an unevenness of 5 mm / m. Production costs are greatly reduced, and the product has excellent performance. It not only has high tensile strength, low yield ratio, good plasticity and toughness, but also has high wear resistance, good fracture resistance, formability and low-temperature impact toughness. It can be widely used in high-strength, wear-resistant components in engineering machinery, commercial vehicles and other fields.

Description

Technical Field

[0001] The present invention relates to the technical field of high-strength wear-resistant steel production, in particular to a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and a low yield ratio that does not require quenching and tempering, and a manufacturing method thereof. Background Art

[0002] Against the backdrop of green manufacturing, low-carbon and environmental protection, domestic and foreign industries such as engineering machinery and commercial vehicles are paying more and more attention to product structure upgrades with high strength, weight reduction, greenness and longevity as the direction, and have put forward high requirements for steel materials such as higher strength and greater wear resistance.

[0003] To achieve lightweighting, various industries are using high-strength wear-resistant steel plates to replace conventional, lower-strength, low-alloy steel plates. Currently, the manufacturing technology used for traditional high-strength wear-resistant steel is primarily based on the traditional offline quenching and low-temperature tempering process. While this high-strength wear-resistant steel exhibits excellent strength and wear resistance, it suffers from poor plasticity and formability, making it unsuitable for use in engineering machinery and commercial vehicles. Furthermore, the manufacturing cost is prohibitive. The TMCP online quenching process is being adopted to produce high-strength, low-yield ratio wear-resistant steel, which exhibits high strength, hardness, and toughness, as well as excellent weldability and cold-bending properties. This process also shortens the production process and reduces costs, making it a growing trend.

[0004] Chinese patent publication CN 110079745A discloses "an online-quenched HB400-grade wear-resistant steel plate and its preparation method." While this technology employs online quenching, its offline quenching method simply involves moving the steel to the rolling line and then tempering it offline. This disadvantage is that it still uses the same components as traditional offline-quenched steel, resulting in high costs. Furthermore, its performance still suffers from poor ductility and formability, and offline tempering also increases costs. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the present invention provides a tempering-free, Brinell hardness 400HBW, low yield ratio, high-strength hot-rolled steel plate and a manufacturing method, which not only greatly reduces the production cost, but also has excellent product performance. It has high strength, high toughness, and high wear resistance, while also having a low yield ratio, good plasticity and toughness, and good formability.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A hot-rolled steel plate having a Brinell hardness of 400 HBW, high strength, high toughness, and low yield ratio without quenching and tempering, comprising the following chemical components in percentage by weight:

[0008] C: 0.16% ~ 0.22%, Si: 0.60% ~ 0.90%, Mn: 0.80% ~ 1.00%, P ≤ 0.015%, S ≤ 0.005%, Al: 0.060% ~ 0.100%, Cr: 0.20% ~ 0.40%, N ≤ 0.0040%, H ≤ 0.00020%, Ti: 0.045% ~ 0.060%, the rest are Fe and unavoidable impurities.

[0009] The hot-rolled thin steel plate of the present invention has a thickness of ≤12mm, a tensile strength of more than 1200MPa, a yield ratio of ≤0.75, an elongation of ≥12%, a hardness of 400±30HBW, and an impact energy value of more than 35J at -40°C.

[0010] The reasons for adopting the above ingredients and their weight percentages in the present invention are described in detail below:

[0011] C: 0.16% to 0.22%. Carbon improves the hardenability of the steel plate and has a strong solid solution strengthening effect, significantly increasing the strength and hardness of high-wear-resistant martensitic wear-resistant steel plates. An appropriate amount of carbon combines with titanium to form TiC particles, which significantly improves wear resistance. If the carbon content is too high, the volume fraction of the TiC particles formed will be too high, reducing the elongation and impact energy performance of the high-wear-resistant martensitic wear-resistant steel plate and deteriorating its weldability.

[0012] Si: 0.60% to 0.90%. Si plays a role in solid solution strengthening in steel. Adding Si to metal materials can significantly improve the strength and hardness of the material. In addition, a higher Si content can expand the formation of ferrite phase.

[0013] Mn: 0.80% to 1.00%. Adding Mn to metal materials can expand the austenite region. In addition, this element can significantly improve the hardenability of steel, thereby obtaining more martensite at the same cooling rate. Increased Mn content can increase the high-temperature temper brittleness and heat sensitivity of the steel.

[0014] P ≤ 0.015%, S ≤ 0.005%. Sulfur combines with manganese and other elements in steel to form plastic inclusions called manganese sulfide, which is particularly detrimental to the steel's transverse plasticity and toughness. Therefore, the sulfur content should be kept as low as possible. Phosphorus is also a harmful element in steel, severely impairing the plasticity and toughness of the steel plate. For the purposes of the present invention, sulfur and phosphorus are unavoidable impurity elements and should be kept as low as possible.

[0015] Cr: 0.20% to 0.40%. It can reduce the austenite phase area. Due to the outstanding bonding ability of chromium atoms with carbon atoms, they can form different carbides with the carbon in the steel. At the same time, chromium can sometimes replace some iron atoms in the cementite, thereby forming cementite containing chromium atoms. Cr can also improve the hardenability of steel. This is because the amount of this element in cementite is greater than that in the solid solution. Therefore, when phase transformation occurs, Cr will inevitably diffuse from the cementite to the solid solution. However, due to the small intergranular gaps in austenite, diffusion is more difficult. During the diffusion process, Cr must combine with C atoms, which will slow down the diffusion rate of the C element and ultimately delay the decomposition time of austenite.

[0016] Al: 0.060% to 0.100%. Adding an appropriate amount of Al can accelerate the transformation kinetics of austenite to proeutectoid ferrite during the air cooling stage of the steel plate. On the other hand, the increase in Al content can increase the carbon content in the retained austenite, thereby improving the thermal stability of the retained austenite, and more retained austenite is retained during the cooling process.

[0017] Ti: 0.045% to 0.060%. Ti plays a role in precipitation strengthening and grain refinement. Ti is a strong carbon and nitrogen compound-forming element. Its carbide TiC particles are small and have extremely high hardness. Dispersed in the steel plate matrix, it can effectively improve the hardness and wear resistance of the steel plate.

[0018] In addition, the present invention also provides a method for manufacturing a hot-rolled steel plate with a Brinell hardness of 400 HBW, high strength, high toughness, and low yield ratio without quenching and tempering, the specific steps of which are as follows:

[0019] 1. Smelting and casting

[0020] Smelting and casting into billets according to the above ingredients;

[0021] 2. Slab reheating

[0022] The slab needs to be hot loaded into the heating furnace with a charging temperature of ≥500°C. The heating temperature is 1240-1260°C and the holding time is 180-230 minutes, including a soaking time of 40-60 minutes.

[0023] 3. Controlled rolling

[0024] Rolling is divided into two stages: roughing and finishing. Roughing uses high-reduction rolling. Depending on the slab thickness, roughing can be performed in 4 to 7 passes, with a pass reduction of at least 22%. The intermediate slab to finished product thickness ratio should be ≥5, and the roughing end temperature is 1120-1160°C. An insulation cover is installed between the roughing and finishing stages, and a hot coil box is added before finishing. The finishing inlet temperature is 1070-1100°C. Finishing consists of seven continuous stands, with the first three passes achieving a reduction of no less than 43%, and the final pass achieving a reduction of no more than 12%. The final rolling temperature is controlled between 930-950°C. Finishing is performed at a constant speed, with a rolling speed between 6 and 11 m / s.

[0025] 4. Controlled cooling

[0026] After rolling, the steel plate enters the cooling area and the cooling method is two-stage cooling. The front stage adopts ultra-fast cooling with a cooling rate of 40-70℃ / s and a cooling temperature of 650-700℃, and then air cooling is carried out with an air cooling time of 7-12s. After air cooling, laminar rapid cooling is carried out with a cooling rate ≥20℃ / s. After cooling to 210-290℃, it is coiled and finally a ferrite + martensite structure + a small amount of retained austenite is obtained, of which the ferrite structure is 15-25%, the retained austenite is within 5%, and the rest is martensite.

[0027] 5. Leveling, shearing and straightening

[0028] After rolling, the steel strip is leveled in a leveler with a leveling force controlled between 8,000 and 12,000 kN. The leveler uses positive bending rollers with a bending force ≥ 6% of the leveling force. The leveled steel strip is then cross-cut into steel plates of the desired dimensions and straightened in a leveler to a flatness of 5 mm / m.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] 1. The present invention adopts an economical C-Mn composition design, adds an appropriate amount of Si and Al alloys, supplemented by a small amount of micro-alloying elements such as Cr and Nb, does not require the addition of precious alloying elements such as Ni and Mo, and does not require offline quenching + tempering heat treatment, greatly reducing production costs.

[0031] 2. The slab of the present invention is hot loaded into the heating furnace to prevent the slab from cracking after cooling to room temperature.

[0032] 3. The present invention controls the heating temperature and holding time, controls the rolling mill reduction, sets higher rough rolling end temperature, finishing rolling entrance temperature, and final rolling temperature, and invests in intermediate roller insulation cover and hot coil box to ensure the temperature of the rolled piece, ensure the stability of rolling, and better control the plate shape.

[0033] 4. After rolling, the steel plate enters the cooling zone. Ultra-dense rapid cooling is used in the front section to rapidly cool the rolled strip to the ferrite transformation temperature range, ensuring that some austenite is converted to ferrite. Rapid cooling ensures a fine ferrite grain structure, thereby ensuring good plasticity and formability. Air cooling followed by rapid cooling transforms the remaining austenite into martensite and a small amount of retained austenite, ensuring high strength and hardness, and thus excellent wear resistance.

[0034] 5. This invention significantly improves plate shape through high leveling force and appropriate bending force. After the steel strip is cut into steel plates, it is straightened in a leveler to eliminate residual stress and further improve the plate shape. The finished steel plate has an unevenness of 5mm / m, ensuring good applicability.

[0035] In summary, the wear-resistant steel plate manufactured using the present invention has a thickness of ≤12 mm, a tensile strength of 1200 MPa or greater, a yield ratio of ≤0.75, an elongation of ≥12%, a hardness of 400±30 HBW, an impact energy value exceeding 35 J at -40°C, and a roughness of 5 mm / m for the finished steel plate. This invention significantly reduces production costs and offers excellent product performance, including high tensile strength, a low yield ratio, and good plasticity and toughness, as well as high wear resistance, excellent fracture resistance, formability, and low-temperature impact toughness. The steel plate can be widely used in high-strength, wear-resistant components in fields such as engineering machinery and commercial vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is the metallographic organization diagram of the present invention. DETAILED DESCRIPTION

[0037] The present invention discloses a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and a low yield ratio without tempering and a manufacturing method. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters to achieve the desired effect. It should be noted in particular that all similar replacements and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0038] A hot-rolled steel plate having a Brinell hardness of 400 HBW, high strength, high toughness, and low yield ratio without quenching and tempering, comprising the following chemical components in percentage by weight:

[0039] C: 0.16% ~ 0.22%, Si: 0.60% ~ 0.90%, Mn: 0.80% ~ 1.00%, P ≤ 0.015%, S ≤ 0.005%, Al: 0.060% ~ 0.100%, Cr: 0.20% ~ 0.40%, N ≤ 0.0040%, H ≤ 0.00020%, Ti: 0.045% ~ 0.060%, the rest are Fe and unavoidable impurities.

[0040] The thickness of hot-rolled thin steel plate is ≤12mm, the tensile strength is above 1200MPa, the yield strength ratio is ≤0.75, the elongation is ≥12%, the hardness is 400±30HBW, and the impact energy value at -40℃ exceeds 35J.

[0041] A method for manufacturing a hot-rolled steel plate with a Brinell hardness of 400 HBW, high strength, high toughness, and low yield ratio without quenching and tempering, specifically comprising the following steps:

[0042] 1. Smelting and casting

[0043] According to the above ingredients, it is smelted and cast into billets.

[0044] 2. Slab reheating

[0045] The slab needs to be hot loaded into the heating furnace with a charging temperature of ≥500°C. The heating temperature is 1240-1260°C and the holding time is 180-230 minutes, including a soaking time of 40-60 minutes.

[0046] 3. Controlled rolling

[0047] Rolling is divided into two stages: roughing and finishing. Roughing uses high-reduction rolling. Depending on the slab thickness, roughing can be performed in 4 to 7 passes, with a pass reduction of at least 22%. The intermediate slab to finished product thickness ratio should be ≥5, and the roughing end temperature is 1120-1160°C. An insulation cover is installed between the roughing and finishing stages, and a hot coil box is added before finishing. The finishing inlet temperature is 1070-1100°C. Finishing consists of seven continuous stands, with the first three passes achieving a reduction of no less than 43%, and the final pass achieving a reduction of no more than 12%. The final rolling temperature is controlled between 930-950°C. Finishing is performed at a constant speed, with a rolling speed between 6 and 11 m / s.

[0048] 4. Controlled cooling

[0049] After rolling, the steel plate enters the cooling area and the cooling method is two-stage cooling. The front stage adopts ultra-fast cooling with a cooling rate of 40-70℃ / s and a cooling temperature of 650-700℃, and then air cooling is carried out with an air cooling time of 7-12s. After air cooling, laminar rapid cooling is carried out with a cooling rate ≥20℃ / s. After cooling to 210-290℃, it is coiled and finally a ferrite + martensite structure + a small amount of retained austenite is obtained, of which the ferrite structure is 10-25%, the retained austenite is within 5%, and the rest is martensite.

[0050] 5. Leveling, shearing and straightening

[0051] After rolling, the steel strip is leveled in a leveler with a leveling force controlled between 8,000 and 12,000 kN. The leveler uses positive bending rollers with a bending force ≥ 6% of the leveling force. The leveled steel strip is then cross-cut into steel plates of the desired dimensions and straightened in a leveler to a flatness of 5 mm / m.

[0052] In the method for producing the steel plate of the present invention:

[0053] Slabs must be hot-charged into the furnace at a temperature ≥500°C to prevent cracking after cooling to room temperature. The heating temperature is 1240-1260°C, and the holding time is 180-230 minutes, including a soaking time of 40-60 minutes. This thin steel strip exhibits significant deformation resistance during hot rolling, placing high mill loads. Therefore, the slabs must be evenly and thoroughly fired to ensure rolling stability.

[0054] Rolling is divided into two stages: roughing and finishing. To achieve a fine, original austenite grain size, the roughing stage uses a high reduction, with each pass reducing the steel at a rate of at least 22%. The intermediate billet to finished product thickness ratio is ≥5, ensuring a high reduction ratio and fine grain size, thereby improving the steel structure. Finishing consists of seven stands, with the first three passes reducing the steel at a rate of at least 43%. This ensures high reductions in the upstream stands, reduces the burden on downstream stands, and ensures rolling stability. The final pass of finishing is controlled to a reduction of no more than 12%, ensuring a low reduction in the final stand for better control of plate shape.

[0055] Setting a higher rough rolling end temperature, finishing rolling entrance temperature, final rolling temperature and investing in intermediate roller insulation covers and hot coil boxes are to ensure the temperature of the rolled pieces and facilitate stable rolling.

[0056] The finishing rolling speed of each piece of steel must be kept constant to prevent excessive cooling rate fluctuations during the cooling process, ensuring a constant ferrite-martensite ratio throughout the rolled strip, and minimizing differences in mechanical properties. The rolling speed is controlled within the range of 6-11 m / s to ensure a reasonable rolling speed. If the rolling speed is too slow, the rolling temperature cannot be maintained, while if the rolling speed is too fast, rolling stability and cooling accuracy control will be affected.

[0057] After rolling, the steel plate enters the cooling zone. The front section adopts ultra-dense rapid cooling with a cooling rate of 40-70℃ / s, a cooling temperature of 650-700℃, and an air cooling time of 7-12s. This is to quickly cool the strip steel to the ferrite transformation temperature range after rolling, ensuring that part of the austenite is transformed into ferrite. Rapid cooling is to ensure that the ferrite grain structure is small, its purpose is to ensure that the steel plate has good plasticity and thus has a certain degree of formability. After air cooling, rapid cooling is carried out at a cooling rate of ≥20℃ / s to 210-290℃. The remaining austenite is transformed into martensite and a small amount of retained austenite, ensuring that the steel plate has high strength and hardness, thereby giving the steel plate excellent wear resistance.

[0058] After low-temperature cooling, the steel strip has poor flatness. However, after flattening with high leveling force and appropriate bending roller force, the flatness is significantly improved. After the steel strip is cut into steel plates, they are straightened in a leveler to eliminate residual stress and further improve the flatness. The finished steel plates have an unevenness of 5mm / m, ensuring excellent applicability.

[0059] [Example]

[0060] The chemical composition (Wt%) of the embodiment of the present invention is shown in Table 1; the key process parameters of rolling of the embodiment of the present invention are shown in Table 2; the key process parameters of rolling and leveling of the embodiment of the present invention are shown in Table 3; the technical performance of the present invention is shown in Table 4.

[0061] Table 1 Specific chemical composition of the examples

[0062]

[0063]

[0064] Table 2 Key rolling process parameters

[0065]

[0066] Table 3 Key process parameters of rolling and tempering

[0067]

[0068] Table 4 Technical performance table

[0069]

[0070] like Figure 1 As shown, the structure of the steel plate is ferrite + martensite + a small amount of retained austenite, wherein the ferrite structure is 15-25%, the retained austenite is within 5%, and the rest is martensite.

[0071] The present invention utilizes an economical C-Mn composition design, adding appropriate amounts of Si and Al alloys, supplemented by small amounts of microalloying elements such as Cr and Ti. No precious alloying elements such as Ni and Mo are required, and offline quenching and tempering heat treatment is not required. This significantly reduces production costs and offers excellent product performance, including high tensile strength, low yield ratio, and good plasticity and toughness. This gives the steel plate high wear resistance, excellent fracture resistance, formability, and low-temperature impact toughness. The steel plate can be widely used in high-strength, wear-resistant components in engineering machinery, commercial vehicles, and other fields.

[0072] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A method for manufacturing a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and low yield ratio without quenching and tempering, characterized in that: The steel plate is composed of the following chemical components in percentage by weight: C: 0.16%~0.22%, Si: 0.60%~0.78%, Mn: 0.80%~0.98%, P≤0.015%, S≤0.005%, Al: 0.060%~0.093%, Cr: 0.20%~0.40%, N≤0.0040%, H≤0.00020%, Ti: 0.045%~0.060%, and the remainder is Fe and unavoidable impurities; Steel plate thickness ≤ 12mm, tensile strength above 1200MPa, yield strength ratio ≤ 0.75, elongation ≥ 12%, hardness 400±30HBW, impact energy value at -40℃ exceeding 35J; The steel plate structure is ferrite + martensite + a small amount of retained austenite, of which ferrite is 15-25%, retained austenite is within 5%, and the rest is martensite; The specific steps include: 1) Smelting and casting into billets according to the above composition; 2) The slab needs to be hot loaded into the heating furnace, and the loading temperature should be ≥500℃; Heating temperature: 1240~1260℃, holding time: 180~230min, including soaking time: 40~60min; 3) The roughing reduction is above 22%, the thickness ratio of the intermediate billet to the finished product is ≥5, and the roughing end temperature is 1120~1160℃; Finish rolling is carried out at a constant speed, and the rolling speed is controlled at 6~11m / s; The finishing rolling inlet temperature is 1070~1100℃, and the finishing rolling is carried out with at least 7 stands of continuous rolling. The reduction rate of the first three stands is not less than 43%, and the reduction rate of the last pass of the finishing rolling is controlled not to exceed 12%. The final rolling temperature is controlled at 930~950℃. 4) Two-stage cooling is adopted, with the cooling rate of the first stage being 40~70℃ / s and cooling to a temperature of 650~700℃; Then air-cooling is carried out for 7 to 12 seconds; After air cooling, laminar rapid cooling is carried out with a cooling rate of ≥20℃ / s, and coiling is carried out after cooling to 210~290℃; 5) The rolled steel strip is leveled by a leveler, the leveling force is controlled at 8000~12000KN, and the bending roll force is ≥6% of the leveling force; The flattened steel strip is sheared and straightened with a straightening machine to control the unevenness of the steel plate to below 5mm / m.

2. The method for manufacturing a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and low yield ratio without quenching and tempering according to claim 1, characterized in that: Step 3) The intermediate roller insulation cover between rough rolling and finishing rolling is put into operation.

3. The method for manufacturing a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and low yield ratio without quenching and tempering according to claim 1, characterized in that: Step 3) The hot coil box is put into operation before finishing rolling.

4. The method for manufacturing a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and low yield ratio without quenching and tempering according to claim 1, characterized in that: Step 5) The flattened steel strip is cross-cut.

5. The method for manufacturing a high-strength hot-rolled steel plate with a Brinell hardness of 400 HBW and low yield ratio without quenching and tempering according to claim 1, characterized in that: Step 5) The leveling machine uses positive bending rollers.

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