Bainite plastic die steel based on laterite nickel ore and manufacturing method thereof

Abstract

The application discloses a bainite plastic die steel based on laterite nickel ore and a manufacturing method thereof, and ingredients of the bainite plastic die steel are as follows in percentage by mass: C 0.01-0.15%, Si 0.25-1.0%, Mn 0.5-2.0%, P≤0.045%, S≤0.015%, Cr 2.0-5.0%, Ni 1.01-2.0%, B 0.0010-0.0050%, N 0.004-0.008%, and the rest contains Fe and other inevitable impurities, and the following conditions are simultaneously met: hardness index: 28≤13.8+132.8C+2.34Mn+1.84Cr-1.12Ni≤38, bainite index: C 0.5 ×Mn 1.4 ×Cr 4 ×Ni×N ‑1.05 / 20≤2000. The die steel has a tensile strength of greater than or equal to 800 MPa, a hardness of 28-38 HRC, a cross-section hardness difference of a steel plate with a thickness of 15-300 mm of less than or equal to 2 HRC, flatness of less than or equal to 2 mm / m, and flaw detection quality meeting the E / e level requirements in the SP1921-84 standard. The die steel can be applied to the manufacturing of plastic parts in the fields of automobiles, household appliances, medical treatment and electronics.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of die steel, in particular to a bainite plastic die steel based on laterite nickel ore and a manufacturing method thereof. BACKGROUND

[0002] The key challenge currently faced by the die manufacturing industry is the dual pressure of rising raw material costs and the demand for technological upgrading. Traditional plastic die steels represented by P20 (1.2311), 1.2738, etc. have high material costs due to the presence of about 0.30%-0.50% of the noble metal molybdenum (Mo) in their composition. The traditional molybdenum-containing plastic die steel is shown in Table 1.

[0003]

[0004] The market demand for pre-hardened plastic die steel is continuously strong. According to industry statistics, the monthly demand for various thick specifications of plastic die steel in China has reached 50,000 tons, mainly applied in the fields of automobile parts, household appliance shells, electronic equipment, and daily products, etc. With the acceleration of the lightweight and precision trend of plastic products, especially the surge in demand for high-precision molds for new energy vehicle parts.

[0005] Chinese patent CN201510338198.9 discloses "a bainite large cross-section plastic die steel and its manufacturing method", the composition design (wt%) is C 0.05-0.19%, Si 0.10-0.60%, Mn 1.0-1.65%, V 0.04-0.20%, Cr 1.0-1.70%, Mo 0.15-0.50%, ≤0.02% P, S ≤0.01%, Ni 0-0.50%, N ≤0.01%, Ti 0.005-0.025%, Ca 0-0.0050%, Al 0.01-0.04%, the rest is Fe and inevitable impurities. After hot rolling and air cooling, tempering at not higher than 600℃ is applied to obtain granular bainite structure, although the process flow is shortened compared with the traditional die steel offline quenching + tempering process.

[0006] Chinese patent CN202510609306 discloses "a bainite non-quenched and tempered steel and its preparation method", the composition design (wt%) is: C: 0.25%-0.27%, Si: 0.32%-0.36%, Mn: 2.00%-2.04%, Cr: 0.50%-0.54%, Al: 0.005%-0.015%, V: 0.11%-0.13%, P ≤0.0205%, S: 0.038%-0.045%, the balance is Fe and inevitable impurities. Its technical route is to sequentially perform hot metal pretreatment, converter smelting, ladle refining, vacuum refining, vacuum degassing, continuous casting, and rolling on the blast furnace hot metal.

[0007] Chinese patent CN202411725741 discloses "a low Mo plastic mold steel plate and its production method", the component design (wt%) is: C: 0.32~0.42%, Si: 0.2~0.4%, Mn: 1.1~1.6%, Al: 0.01~0.05%, Cr: 1.5~2.0%, Mo: 0.10~0.30%, Ni: 0.9~1.1%, P: ≤0.02%, S: ≤0.01%, the rest is Fe and inevitable impurity elements. Through steelmaking, continuous casting, hot rolling, stack cooling, normalizing, tempering and other processes to prepare plastic mold steel plate. Although this patent reduces the Mo content compared with traditional plastic mold steel, it still contains about 0.2% Mo element, the cost is still high, and the production process is more complicated than the mainstream TMCP process, the production cost is high, and it does not have market competitiveness.

[0008] Chinese patent CN201510527554 discloses "a plastic mold steel and its manufacturing method", the component design (wt%) is: C 0.30~0.50%; Si 0.30~0.60%; Mn 1.40~1.80%; P≤0.030%; S≤0.015%; Cr 1.00~2.00%; V 0.10~0.20%; Cu≤0.25%; Cu+Ni≤0.55%; Al 0.02~0.05%; Ti 0.01~0.03%; B 0.001~0.0025%, the rest is inevitable impurities and Fe. This patent uses converter + LF + RH + continuous casting + controlled rolling and controlled cooling + high temperature tempering to produce plastic mold steel plate, and the steel plate is martensite + bainite structure. Although Cu and Ni elements are used instead of Mo element, due to the high C content, the hardness of the martensite structure formed after hot rolling and controlled cooling is high, and offline high temperature tempering is still needed, the production process is complex, and the economy is low.

[0009] Chinese patent CN202110451551 discloses "a preparation method of a special thick Mn-Cr mold steel", the component design (wt%) is: C: 0.38~0.42%, Si: 0.20~0.40%, Mn: 1.4~1.6%, Cr: 1.8~2.0%, S: ≤0.005%, P≤0.020%, Al: 0.010~0.025%, the balance is Fe and inevitable impurities. The "hot rolling + stacking + tempering heat treatment" process can obtain a plastic mold steel with tempered martensite structure. The stacking cooling process needs more than 48h, and then tempering heat treatment is carried out.

[0010] Chinese patent CN201910398089 discloses "a hot work die steel with three-dimensional isotropic performance and a preparation method thereof", the component design (wt%) is: C: 0.28%~0.31%, Si: 0.55%~0.75%, Mn: 1.00%~1.20%, P≤0.015%, S≤0.015%, V: 1.20%~2.00%, Cr: 4.00%~4.50%, Ni: 0.20%~1.00%, Als: 0.01%~0.03%, W: 0.20%~0.30%, Cu: 0.05%~0.10%, Ca≤0.05%, [O]≤20ppm, [H]≤3ppm, [N]≤70ppm, the rest is Fe and inevitable impurities. Through smelting-large slab continuous casting-heating-rolling-high temperature pretreatment-quenching and tempering treatment, a hot work die steel with martensite structure is obtained. The patent adds a large amount of V and W elements in alloy elements, which increases the difficulty of hot rolling manufacturing, and needs a complex heat treatment process, the manufacturing cost is high, and it is not suitable for application in the field of plastic die steel.

[0011] In summary, the newly developed bainite die steel of each patent still contains a high Mo element, the alloy cost is high, and the process is hot rolling controlled cooling + offline tempering process. Some low Mo or Mo-free plastic die steels still have a structure mainly of martensite, and the process relies on controlled rolling and controlled cooling + offline tempering. There is still a deficiency of high alloy cost or process cost, and with the progress of the steel industry technology, the long process heat treatment technology has no economic advantage. It is urgent to develop a plastic die steel product with low alloy, low process cost and excellent performance. SUMMARY

[0012] The purpose of the present application is to provide a bainite plastic die steel based on laterite nickel ore and a manufacturing method thereof, the die steel has uniform structure and excellent mechanical properties, the die steel has a tensile strength of ≥800MPa, a hardness of 28~38HRC, a hardness difference of ≤2HRC in a cross section of a 15~300mm thick steel plate, a flatness of ≤2mm / m, and a flaw detection quality meeting the E / e level requirements in the SP1921-84 standard; in addition, the present application does not need a complex heat treatment process, and has high production efficiency; the plastic die steel can be applied to the manufacturing of plastic parts in the fields of automobiles, household appliances, medical treatment and electronics.

[0013] To achieve the above purpose, the technical scheme of the present application is:

[0014] A bainite plastic die steel based on laterite nickel ore, the chemical component mass percentage is:

[0015] C: 0.01~0.15%,

[0016] Si: 0.25~1.0%,

[0017] Mn: 0.5-2.0%,

[0018] P≤0.045%,

[0019] S≤0.015%,

[0020] Cr: 2.0-5.0%,

[0021] Ni: 1.01-2.0%,

[0022] B: 0.0010-0.0050%,

[0023] N: 0.004-0.008%,

[0024] the balance being Fe and other unavoidable impurities, and, simultaneously satisfying the following relationship:

[0025] hardness index: 28≤13.8+132.8C+2.34Mn+1.84Cr-1.12Ni≤38;

[0026] bainite index: C 0.5 x Mn 1.4 x Cr 4 x Ni x N -1.05 / 20≤2000.

[0027] Further, the chemical composition of the plastic mold steel further comprises one or more of: V≤0.2%, Ca: 0.0005-0.0050%, Cu≤0.35%, in mass percent.

[0028] The plastic mold steel according to the present application is a bainite structure, the tensile strength is≥800MPa, the hardness is: 28-38HRC, the hardness difference of the 15-300mm thick steel plate cross section is≤2HRC, the flatness is≤2mm / m, and the flaw detection quality meets the E / e level requirements in the SP1921-84 standard.

[0029] In the component design of the bainite plastic mold steel according to the present application:

[0030] C, carbon is a key element that determines the hardness and strength of steel, which improves the matrix strength by forming carbides and solid solution strengthening. Low carbon design avoids the toughness decline caused by high carbon, and ensures the toughness and impact resistance of bainite steel. Low carbon content promotes the formation of carbide-free bainite, reduces brittle phase, and improves plasticity and toughness.

[0031] Si, steelmaking deoxidizer, improves elastic limit and resistance to temper softening. Inhibits cementite precipitation, promotes the formation of carbide-free bainite, optimizes the strength and toughness combination, the deoxidizing ability of silicon reduces the oxide inclusions in the steel, and improves the surface finish after mold polishing.

[0032] Mn, Mn has the effect of improving the hardenability, strength and toughness, and neutralizing the hot brittleness hazard of sulfur. The laterite nickel ore contains a high amount of manganese, and 0.5-2.0% of manganese can fully utilize the resources and reduce the smelting cost, in addition, manganese improves the hardenability, and ensures that the uniform bainite structure is obtained in the center of the large-size plastic mold.

[0033] Cr, improves the corrosion resistance, wear resistance and strength of the plastic mold steel, forms a chromium oxide passivation film on the steel, slows down the rust corrosion, the chromium carbide (Cr7C3) is combined with the bainite matrix to resist the wear of the plastic filler, the medium chromium design avoids the high chromium cost, and the natural content of chromium in the laterite nickel ore is matched.

[0034] Ni, improves the toughness and weather resistance of the steel, and stabilizes the austenite structure. Nickel reduces the brittle transition temperature, enhances the crack resistance of the mold under impact load, forms a "chromium-nickel corrosion resistant system" with chromium, resists acidic plastic decomposition products (such as fluorine-containing polymers), directly uses the nickel element in the laterite nickel ore, and reduces the raw material cost.

[0035] N, can significantly promote the mold steel of the application to form bainite under high cooling rate, and greatly improve the production efficiency, but excessive nitrogen also increases the content of residual austenite.

[0036] B, a small amount of boron is adsorbed on the grain boundary to inhibit the nucleation of ferrite, significantly improve the hardenability of bainite transformation, 0.002% of boron can replace 1-2% of nickel or molybdenum, and reduce the alloy cost.

[0037] In addition to the above-mentioned necessary elements and the purpose characteristics obtained, one or more of V≤0.20%, Ca: 0.0005-0.0050%, Cu≤0.5% can be selectively added according to the desired characteristics, in mass percent.

[0038] V is a ferrite forming element, and excessive V content is not conducive to the stability of the austenite structure, so V must be controlled within an upper limit. A small amount of V element can form fine VN precipitates, which can strengthen the material and improve the strength of the material. Therefore, the content of V is controlled to be ≤0.20%.

[0039] A small amount of Ca element mainly modifies the inclusions in stainless steel, so that the poor plastic inclusions are modified to good ductility inclusions. Therefore, the content of Ca is controlled to be 0.0005-0.0050%.

[0040] Cu element cooperates with Ni and Cr, and Cu can make up for the insufficient corrosion resistance of low-chromium steel, significantly prolongs the service life of the mold in a humid or acidic plastic environment (such as HCl decomposed by PVC), therefore, the content of Cu is controlled to be ≤0.35%.

[0041] The component design innovation feature of the present application based on laterite nickel ore bainite plastic mold steel is that:

[0042] 1. The Cr and Ni rich in laterite nickel ore are innovatively used to replace Mo. At present, the mainstream plastic mold contains about 0.3% Mo element to improve the hardenability of the steel plate. Although some manufacturers have launched low-Mo or Mo-free plastic mold steel products, it is also easy to lead to insufficient hardenability and uneven section structure, the core hardness is difficult to reach the surface hardness, and the market promotion is difficult. The present application innovatively designs the composition to replace Mo element with Cr and Ni through the laterite nickel ore nickel iron water smelted by blast furnace, changes the structure from martensite to bainite with more uniform performance, and can obtain uniform bainite structure in a larger cooling rate range. The higher Cr content improves the corrosion resistance of the plastic mold steel, and it can be applied to the forming field of corrosive plastics.

[0043] 2. The hardness index: 28≤13.8+132.8C+2.34Mn+1.84Cr-1.12Ni≤38 is innovatively proposed. Due to the existence of a certain fluctuation range of the alloy composition of the laterite nickel ore nickel iron water smelted by blast furnace, the change of the alloy elements will lead to the change of the product structure and hardness. In order to improve the influence of the composition fluctuation of the laterite nickel ore nickel iron water smelted by blast furnace on the product hardness, based on the hardness index formula, the C element content and the mixing ratio of the scrap iron water smelted by electric furnace are adjusted to adjust the fluctuation of the product hardness caused by the change of the alloy elements and control the product hardness.

[0044] After research, the C element is the most obvious for hardness improvement. Increasing the C content can promote the precipitation of carbide in the bainite ferrite or between the laths, and strongly increase the lattice distortion, directly enhance the hardness and wear resistance. The Mn and Cr elements strongly delay the pearlite transformation, expand the bainite transformation interval, and improve the hardenability, but the enhancement effect of Cr on hardness is weaker than that of Mn element. Both Cr and Mn can still form bainite under a slower cooling rate, and can refine the ferrite lath and increase the dislocation density, strengthen the matrix and thus improve the overall hardness. In addition, Cr can also form fine chromium carbide (such as M 23 C6), which is uniformly distributed in the bainite ferrite, and significantly improves the hardness and wear resistance. Unlike the above elements, the Ni element increases the residual austenite content to reduce the overall hardness, so the coefficient of Ni on the hardness index is negative.

[0045] 3. The bainite index: C 0.5 ×Mn 1.4 ×Cr 4 ×Ni×N -1.05 / 20≤2000, traditional die steel is mostly martensitic structure, which needs to be cooled and then high-temperature tempered to reduce the hardness to about 35HRC, and at the same time, the martensitic residual stress is eliminated. While the traditional bainite steel needs to be cooled at a slow rate or isothermally for a long time to obtain complete bainite structure, which is low in production efficiency. The present application, through the research on the bainite transformation of elements and the composition design, innovatively finds that the addition of a small amount of N element can greatly improve the bainite transformation cooling rate, and full bainite can be obtained under the condition of faster cooling after hot rolling, thereby improving the hot rolling production efficiency. The CCT curves of different N contents are shown in Figure 1 、 Figure 2 When the bainite index is greater than 2000, the on-line tempering is more likely to form martensitic structure on the surface and the edge, resulting in poor uniformity of the cross-section hardness.

[0046] After research, in the bainite transformation process, the C element only inhibits the diffusion of C atoms from austenite to ferrite when the C content is high, thereby reducing the bainite transformation rate, but the C content of the present application is low, so the influence is small. The Cr atom has strong affinity with C, which greatly inhibits the diffusion of C atoms, thereby greatly delaying the formation of bainite. Mn and Ni both stabilize austenite, reduce the free energy difference of phase change, and reduce the bainite nucleation rate, but the influence is limited. These elements all prolong the incubation and transformation time of bainite to different extents, and only the N element can greatly improve the transformation rate of bainite. The reason is that N can be enriched near the grain boundary and dislocation line after being dissolved in austenite, which reduces the interface energy barrier of bainite ferrite nucleation, and also reduces the diffusion activation energy of C, accelerates the redistribution of C in austenite, forms a carbon-poor zone (which is beneficial to the nucleation of bainite ferrite), thereby shortens the incubation period of phase change. Therefore, the present application can control the bainite isothermal transformation time within 10 minutes by adjusting the bainite index, thereby improving the production efficiency of continuous hot rolling-cooling.

[0047] In summary, the present application innovatively uses the nickel-iron melt smelted from laterite nickel ore as raw material, supplemented with electric furnace smelting waste iron and other low-cost elements (C, N), that is, the hardness index can be adjusted according to the actual hardness requirement to control the hardness range of the product, the bainite transformation rate can be controlled by adjusting the bainite index, the production on-line tempering efficiency is guaranteed, and full bainite structure is guaranteed. The appropriate and uniform hardness distribution characteristics are obtained, which ensures good material temperature, machining property, polishing uniformity and long service life during the use of the mold.

[0048] The manufacturing method of the bainite plastic mold steel based on laterite nickel ore provided by the present application comprises the following steps:

[0049] 1) Smelting and casting

[0050] The electric furnace melting scrap iron is added into the nickel-iron melt obtained by smelting laterite nickel ore in a blast furnace, and the steel liquid with the above components is obtained after AOD furnace and LF furnace smelting, and then the slab is continuously cast;

[0051] 2) Hot rolling

[0052] The slab is heated, phosphorus is removed, and hot rolling is performed; wherein the slab heating temperature is 1150-1250℃, the heating time is 180-240min, and the finish rolling temperature is 850-1000℃.

[0053] 3) Cooling

[0054] After the hot rolling is completed, the slab is cooled by a segmented water cooling method, the temperature is reduced by 100-150℃ in each segment, the cooling speed is ≥10℃ / s, and then air cooling is performed for 0.5-1.5min, so that the slab is cooled by two or three segments, and the temperature of the slab is reduced to 550-700℃.

[0055] 4) Heat treatment

[0056] After the cooling, the slab is straightened by a hot straightening machine, and then the slab is air cooled to the furnace entry temperature of 400-450℃ on a roller way, and finally the slab is slowly cooled in an online tempering furnace, the furnace temperature is 200-450℃, the cooling time in the furnace is 8-30min, and the slab is cooled after being discharged from the furnace.

[0057] Preferably, in step 1), the content of the electric furnace melting scrap iron in the steelmaking mother liquid is not more than 30%, and the composition satisfies the hardness index: 28≤13.8+132.8C+2.34Mn+1.84Cr-1.12Ni≤38, the bainite index: C 0.5 ×Mn 1.4 ×Cr 4 ×Ni×N -1.05 / 20≤2000.

[0058] Preferably, in step 2), the finish rolling temperature is ≥A c3 +80, A c3 =896-237C-28Mn-8Cr-26Ni, unit: ℃.

[0059] Preferably, in step 3), the temperature is reduced by 100-150℃ in the first segment, the cooling speed is ≥10℃ / s, and then air cooling is performed for 0.5-1.5min, so that the heat is transferred from the core to the surface of the slab; then the slab is cooled in the second segment, the temperature is reduced by 100-150℃, the cooling speed is ≥10℃ / s, and then air cooling is performed for 0.5-1.5min; if the temperature of the slab is reduced to 550-700℃, the third segment is not performed, if the temperature of the slab is higher than 700℃, the third segment is performed to reduce the temperature to 550-700℃, and the temperature reduction is controlled to be 100-150℃, and the cooling speed is ≥10℃ / s.

[0060] Preferably, in step 4), after hot straightening, the steel plate is cooled to 400-450℃ in an air cooling manner at a cooling speed of 0.5-1℃ / s; then the steel plate is cooled in an online tempering furnace, the temperature in the furnace is 200-450℃, the cooling speed of the steel plate is 0-0.5℃ / s, and the cooling time t in the tempering furnace is (0.1-0.4)×H, t is in min, and H is the thickness of the steel plate, in mm.

[0061] In the manufacturing method described in the application, the following steps are included:

[0062] In step 1), during the steelmaking process, the nickel-iron melt smelted by the nickel laterite ore by the blast furnace has a Ni content of 0.8-2.6% and a Cr content of 3.0-5.5%, which fluctuates greatly, and the alloy composition has a great influence on the structure and hardness of the steel plate. In order to make the product meet the performance requirements of the structure and hardness, and at the same time have the optimal economy, in the application, the proportion of the scrap iron melt added in the electric furnace smelting is adjusted according to the actual alloy content of the nickel-iron melt, and the content of the scrap iron melt in the mother liquor is not more than 30wt%. If the added amount of the scrap iron melt is higher than 30wt%, the alloy cost will increase. The mother liquor is further adjusted in the AOD furnace and the LF furnace to adjust the contents of C and N elements and remove inclusions, so that the hardness index and the bainite index meet the control requirements.

[0063] In step 2), the finish rolling temperature is 850-1000℃, and the finish rolling temperature needs to be controlled to be greater than or equal to A c3 +80℃, A c3 =896-237C-28Mn-8Cr-26Ni, in ℃. C, Mn and Ni are austenite forming elements, which can greatly expand the austenite phase region and significantly reduce the A c3 temperature. When the content of Cr is less than 7%, Cr can expand the austenite region, but the effect is not as good as that of Mn and Ni. Through phase transition dynamics and thermodynamics calculation, the correlation between the A c3 temperature of the application and the main alloy elements C, Mn, Cr and Ni is fitted as the above calculation formula. Controlling the finish rolling temperature to be higher than A c3 +80℃ is beneficial to the dynamic recrystallization of the grains after rolling, which can homogenize the structure, and at the same time, it can prevent the finish rolling temperature from being too low, so that the deformation banded structure can not affect the uniformity of the cross-section structure and hardness.

[0064] In Step 3), since the current plastic mold steel adopts quenching (online or offline) + high-temperature tempering, the plate shape problem is not excessively worried about during quenching (water cooling), and the steel plate can still be straightened after high-temperature tempering. However, all the processes of the present application are completed online, and there is no subsequent tempering heat treatment, so it is necessary to ensure that the plate shape is good during hot rolling and cooling. Therefore, the following water cooling process is formulated: After hot rolling, in order to prevent a large amount of single-stage water cooling from causing excessive residual stress of the steel plate and causing the plate shape to warp, the steel plate is gradually cooled to 550-700°C by using a segmented water cooling method. The temperature drop of the first stage of water cooling is controlled to be 100-150°C, the cooling rate is ≥10°C / s, and then air cooling is performed for 0.5-1.5 min to allow heat transfer from the core to the surface of the steel plate. Then, the second stage of water cooling is entered, the temperature drop is 100-150°C, the cooling rate is ≥10°C / s, and then air cooling is performed for 0.5-1.5 min. If the temperature of the steel plate is reduced to 550-700°C, the third stage of water cooling is not performed. If the temperature of the steel plate is higher than 700°C, the third stage of water cooling is performed to reduce the temperature to 550-700°C, the temperature drop is controlled to be 100-150°C, the cooling rate is ≥10°C / s, and then a hot straightening machine is entered for straightening. During water cooling, the surface temperature of the thick steel plate rapidly decreases, but the core of the thickness still maintains a high temperature. If the temperature drop is too large in a short time, the temperature difference between the core and the surface will cause the steel plate to deform and bend in the width direction. This bending cannot be straightened by a straightening machine. Therefore, the present application strictly controls the temperature drop of single-stage water cooling to prevent the temperature difference between the surface and the core from being too large. In addition, after single-stage water cooling, air cooling is performed to allow the high temperature of the core to be transferred to the surface, so as to homogenize the temperature in the thickness direction and also facilitate the release of stress caused by water cooling. Therefore, the temperature drop of single-stage water cooling is controlled to be within 100-150°C, and air cooling is performed for 0.5-1.5 min. This cooling process is repeated until the temperature of the steel plate is uniformly reduced to 550-700°C, so as to reduce the residual stress.

[0065] Since the bainite mold steel has high strength, cold straightening cannot straighten the thick steel plate. In addition to being controlled through the hot rolling and cooling process, the steel plate must also be further optimized through hot straightening at high temperature. Therefore, the following hot straightening process is formulated: After segmented water cooling to 550-700°C, online hot straightening is immediately performed. If the hot straightening temperature exceeds 700°C, the iron oxide scale will adhere to the straightening roll during hot straightening, causing the iron oxide scale to be pressed into the surface of the steel plate during subsequent hot straightening, thereby affecting the surface quality of the steel plate. If the hot straightening temperature is lower than 550°C, the strength of the plastic mold steel will be greatly increased, making it difficult to straighten the plate shape. Therefore, straightening at 550-700°C is beneficial to preventing the scale from adhering to the roll and damaging the surface quality of the steel plate, and is also easy to control the plate shape.

[0066] Since the present application no longer adopts offline quenching or offline tempering process, the cooling process after hot straightening needs to be controlled to prevent deterioration of the plate shape caused by improper cooling. Therefore, after hot straightening, uniform cooling is carried out by air cooling, and the following air cooling process is formulated: the steel plate is slowly oscillated on the conveying roller way between the hot straightening machine and the tempering furnace at a cooling rate of 0.5-1℃ / s to 400-450℃. The air cooling process should be uniformly cooled on the whole plate, and local contact with spray water and other rapid cooling media should not be allowed. Too fast local cooling rate will cause martensite to appear at the rapid cooling position, resulting in increased local stress of the steel plate and deterioration of the plate shape. At the same time, the cooling rate of the whole plate should not exceed 1.0℃ / s, which will produce a small amount of martensite, affecting the uniformity of the plate shape and hardness. The cooling rate lower than 0.5℃ / s will lead to pearlite precipitation, reducing the local hardness and affecting the hardness uniformity and machining performance of the steel plate.

[0067] Based on the component design of the present application, the bainite transformation temperature of the present application is 400-450℃, so isothermal heat treatment needs to be carried out at 400-450℃ to form full-thickness bainite structure, which can obtain uniform hardness and machining performance. Therefore, furnace cooling process is adopted, which includes: the steel plate is cooled to 400-450℃ by air cooling, and immediately enters the online tempering furnace. This temperature range is the bainite transformation temperature of the present application. If the furnace temperature is lower than 400℃, it will lead to premature entry into the martensite transformation point, resulting in the formation of hard and brittle martensite. If the furnace temperature is higher than 450℃, it will lead to entering the residual austenite interval, reducing the hardness. In order to control the cooling rate and achieve the purpose of eliminating the residual stress of thick steel plate, the temperature in the tempering furnace is specially set to 200-450℃, which can control the cooling rate to 0-0.5℃ / s. If the furnace temperature is lower than 200℃, the cooling speed will increase, which will lead to the formation of a small amount of martensite and deterioration of the plate shape. If the furnace temperature is higher than 350℃, the cooling rate will be too low, which will lead to the formation of residual austenite and decrease of the hardness uniformity. Further, in order to reduce production cost and carbon emission, the heating device of the online tempering furnace can not be opened, and the residual heat of other hot-rolled products in the production line is used to heat the tempering furnace to 200-450℃, further improving the production efficiency and saving energy consumption. In addition, the cooling time in the tempering furnace t=(0.1-0.4)×H, unit min, H is the thickness of the steel plate, unit mm. Too long cooling time in the furnace will affect the production efficiency, and too short cooling time in the furnace will be difficult to eliminate the residual stress, affecting the stability of die manufacturing.

[0068] In summary, the application adopts the whole-process control of the innovative combined process of hot rolling homogenization and recrystallization + segmented water cooling + online hot straightening + online air cooling + online tempering furnace cooling, on the one hand, good plate shape, full bainite structure and uniform hardness are obtained, on the other hand, the production efficiency of the plastic mold steel is greatly improved, and the production cost and carbon emission are reduced. Compared with the maximum hardness and minimum hardness difference of 2~3HRC in the thickness section of the bainite structure, the hardness of the full bainite structure obtained by the above component design and process design is uniformly improved by more than 50%, and the maximum hardness and minimum hardness difference in the thickness section is controlled to be within 2HRC, so that the mold steel is more uniform after polishing.

[0069] The manufacturing method process innovation feature of the application is that:

[0070] 1. Overturn the traditional mold steel smelting method: the traditional mold steel adopts blast furnace molten iron to add the required alloy for smelting, and a certain amount of Mo element is added to improve the hardenability of the steel plate. The application cancels the expensive Mo element, and innovatively uses red soil nickel ore smelting nickel molten iron as the main raw material for steelmaking, directly uses the rich Fe, Cr, Ni and Mn elements in the red soil nickel ore, adjusts the organization by a small amount of electric furnace molten waste iron and other low-cost elements (C, N), and obtains bainite structure with appropriate hardness.

[0071] 2. Innovative mold steel heat treatment system: the traditional mold steel usually adopts hot rolling + quenching + high-temperature tempering or controlled rolling and controlled cooling + high-temperature tempering heat treatment process, and the application adopts the process of online segmented water cooling + online hot straightening + online air cooling + online furnace cooling (see Figure 3 ), fully utilizes the self-heat of the steel plate and the heat treatment furnace waste heat, not only shortens the process flow, but also has excellent plate shape and stable performance, and basically realizes carbon-free heat treatment.

[0072] The beneficial effects of the application are:

[0073] The application uses blast furnace smelting red soil nickel ore nickel molten iron as raw material, uses Cr, Ni and other elements contained in the red soil nickel ore to replace Mo element, and adds a certain proportion of electric furnace smelting waste iron, which greatly reduces the alloy cost compared with the smelting method of adding alloy elements in the traditional blast furnace molten iron.

[0074] The application can obtain uniform bainite structure and hardness distribution by adjusting the hardness index and bainite index.

[0075] The application innovatively proposes the process of online segmented water cooling + online hot straightening + online air cooling + online tempering at residual temperature, which greatly shortens the process flow, and the cooling time after hot rolling is only 30~60min, reduces the carbon emission of heat treatment, and obtains good plate shape and low residual stress. Attached Figure Description

[0076] Figure 1 The CCT curve is shown when the N content of the plastic mold steel described in this invention is 0.002%.

[0077] Figure 2 The CCT curve is shown when the N content of the plastic mold steel described in this invention is 0.006%.

[0078] Figure 3 This is a process flow diagram of the hot rolling and heat treatment process of the plastic mold steel described in this invention;

[0079] Figure 4 The metallographic structure of the plastic mold steel of Example 9 of the present invention;

[0080] Figure 5 The thickness section hardness distribution of the plastic mold steel of Example 9 of the present invention. Detailed Implementation

[0081] The present invention will be further described below with reference to the embodiments and accompanying drawings.

[0082] The chemical composition of the embodiments and comparative examples of this invention is shown in Table 2, with the balance including Fe and other unavoidable impurity elements. The manufacturing process parameters of the embodiments and comparative examples are shown in Table 3. The performance results of the embodiments and comparative examples are shown in Table 4.

[0083] Example 1

[0084] A method for manufacturing bainitic plastic mold steel based on laterite nickel ore includes the following steps:

[0085] 1) Smelting and casting

[0086] The molten nickel iron produced by blast furnace smelting laterite nickel ore was mixed with no more than 30 wt% of electric furnace molten scrap iron as the mother liquor for steelmaking. After smelting in AOD furnace and LF furnace, the molten steel was continuously cast into slabs. The composition of the molten steel is shown in Table 2.

[0087] 2) Hot-rolled

[0088] The slab undergoes heating and heat preservation, descaling, and hot rolling; the slab heating temperature is 1158℃, the heating time is 232min, and the final rolling temperature is 980℃.

[0089] 3) Cooling

[0090] After hot rolling, the steel plate is cooled by a two-stage water cooling method. The first stage of water cooling drops the temperature by 149℃ with a cooling rate of ≥10℃ / s, followed by air cooling for 0.7min. The second stage of water cooling drops the temperature by 119℃, followed by air cooling for 1.2min, which reduces the temperature of the steel plate to 681℃.

[0091] 4) Heat treatment

[0092] After cooling, the steel plate is straightened by a hot straightening machine, and after straightening, the steel plate is air-cooled to the furnace inlet temperature of 448℃ on a roller way at the air-cooling rate of 0.97℃ / s, and finally slowly cooled in an online tempering furnace at the furnace temperature of 443℃ and the furnace cooling rate of 0.01℃ / s, and the cooling time in the tempering furnace is 8min, and after being discharged from the furnace, the steel plate is stack-cooled.

[0093] Examples 1-9 are produced according to the composition and process of the present application, and the performance meets the requirements, wherein the thickness of Example 9 reaches 300mm, the metallographic structure of the core of the thickness is full bainite as shown in Figure 4 , and the hardness distribution of the thickness section is as shown in Figure 5 , and the hardness distribution is uniform.

[0094] Compared with Examples 1-9, the hardness index of Comparative Example 1 is 25.0, which is lower than the range value of 28-38, resulting in the measured hardness value of the comparative example being only 26.1HRC, which does not reach the design value.

[0095] Compared with Examples 1-9, the hardness index of Comparative Example 2 is 41.1, which is higher than the range value of 28-38, resulting in the measured hardness value of the comparative example being 41.3HRC, which exceeds the design value.

[0096] Compared with Examples 1-9, the bainite index of Comparative Example 3 is 2502, which is higher than the range value of ≤2000, resulting in the formation of part of martensite during the cooling process, resulting in the local hardness value increasing, the hardness difference of the section being 3.5HRC, and the hardness uniformity being poor, which affects the polishing and milling performance.

[0097] Compared with Example 1, the finish rolling temperature of Comparative Example 4 is 822℃, which is lower than the limited value of ≥A c3 +80=870℃, resulting in insufficient dynamic recrystallization of the steel plate, a large amount of deformed band structure, uneven hardness distribution on the section thickness, and the hardness difference reaching 2.6HRC.

[0098] Compared with Example 2, the water cooling temperature drop of Comparative Example 5 is too large, the hot straightening temperature is too low, the residual stress is too large, the steel plate is difficult to straighten, and the flatness reaches 14.0mm / m.

[0099] Compared with Example 3, the furnace inlet temperature of Comparative Example 6 is too high, resulting in the precipitation of pearlite at the center of the thickness, the decrease of hardness and strength, and the deterioration of hardness uniformity, and the hardness difference reaching 2.6HRC.

[0100] Compared with Example 4, the furnace inlet temperature of Comparative Example 7 is too low, resulting in the steel plate entering the martensite transformation point in advance, the appearance of martensite structure, the increase of hardness, and the deterioration of hardness uniformity, and the hardness difference reaching 3.4HRC.

[0101] Comparative Example 8 is a traditional plastic mold steel (grade 1.2738) with a Mo content of about 0.4%, and its process is quenching + high-temperature tempering, and the thickness cross-section hardness uniformity is poor, and the thickness cross-section hardness difference is 2.9HRC, which is higher than that of the bainite plastic mold steel of the application.

[0102] It is illustrated by the above examples and comparative examples that the qualified plastic mold steel product can be obtained by the component design and process parameters according to the application.

[0103] The plastic mold steel of the application has excellent mechanical properties: the plastic mold steel of the examples has a bainite structure obtained under the component design and process parameters, the tensile strength is ≥800MPa, the hardness is 28-38HRC, the thickness steel plate cross-section hardness difference of the plastic mold steel of 15-300mm is ≤2HRC, the flatness is ≤2mm / m, and the flaw detection quality meets the E / e level requirements in the SP1921-84 standard.

[0104]

[0105]

[0106]

Claims

1. A bainite plastic mold steel based on laterite nickel ore, characterized in that, The component mass percentage is: C：0.01~0.15%， Si: 0.25~1.0%, Mn: 0.5~2.0%, P≤0.045%， S≤0.015%， Cr：2.0~5.0%， Ni: 1.01~2.0%, B：0.0010~0.0050%， N：0.004~0.008%， The balance is Fe and other inevitable impurities, and the following relationship must be met simultaneously: Hardness index: 28≤13.8+132.8C+2.34Mn+1.84Cr-1.12Ni≤38; Bainite index: C 0.5 x Mn 1.4 x Cr 4 x Ni x N -1.05 / 20 < 2000.

2. The lateritic nickel ore-based bainite plastic mold steel according to claim 1, characterized in that, The component of the die steel also includes one or more of the following: V≤0.2%, Ca: 0.0005~0.0050%, Cu≤0.35%, in mass percentage.

3. The lateritic nickel ore-based bainite plastic mold steel according to claim 1 or 2, characterized in that, The tensile strength of the die steel is ≥800MPa, the hardness is 28~38HRC, the cross-section hardness difference of the 15~300mm thick steel plate is ≤2HRC, and the flatness is ≤2mm / m.

4. The method of manufacturing a bainite plastic mold steel based on laterite nickel ore according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: 1) Smelting and casting The laterite nickel ore is smelted into nickel molten iron by a blast furnace, and no more than 30wt% of scrap iron is added as a steelmaking mother liquor, the molten iron is smelted into a molten steel liquid with the component of the die steel by an AOD furnace and an LF furnace, and then the molten steel liquid is continuously cast into a slab; 2) Hot rolling The slab is heated, dephosphorized and hot-rolled, wherein the slab heating temperature is 1150~1250℃, the heating time is 180~240min, and the final rolling temperature is 850~1000℃; 3) Cooling After hot rolling, the steel plate is cooled by a segmented water cooling mode, the temperature drop of each segment is 100~150℃, the cooling speed is ≥10℃ / s, and then air cooling is performed for 0.5~1.5min, so that the steel plate is cooled by two or three segments of water cooling until the temperature of the steel plate is reduced to 550~700℃; 4) Heat treatment After cooling, the steel plate is straightened by a hot straightening machine, and then air cooling is performed on the roller way until the temperature of the steel plate is 400~450℃, and finally the steel plate is slowly cooled in an online tempering furnace, the furnace temperature is 200~450℃, the cooling time in the tempering furnace is 8~30min, and the steel plate is cooled after being discharged.

5. The method of manufacturing a bainite plastic mold steel based on laterite nickel ore according to claim 4, characterized in that, In step 1), the content of scrap iron melted by the electric furnace in the steelmaking mother liquor is not more than 30%, and the composition needs to satisfy the hardness index: 28≤13.8+132.8C+2.34Mn+1.84Cr-1.12Ni≤38, the bainite index: C 0.5 ×Mn 1.4 ×Cr 4 ×Ni×N -1.05 / 20≤2000.

6. The method of manufacturing ferritic bainitic plastic mold steel based on laterite nickel ore according to claim 4, characterized in that, In step 2), the finishing temperature ≥ A c3 + 80, A c3 = 896 - 237C - 28Mn - 8Cr - 26Ni, in °C.

7. The method of manufacturing ferritic bainitic plastic mold steel based on laterite nickel ore according to claim 4, characterized in that, In step 3), the temperature drop of the first segment of water cooling is 100~150℃, the cooling speed is ≥10℃ / s, and then air cooling is performed for 0.5~1.5min, so that heat is transferred from the core of the steel plate to the surface; If the temperature of the steel plate is reduced to 550~700℃, the third segment of water cooling is not performed, and if the temperature of the steel plate is higher than 700℃, the third segment of water cooling is performed to reduce the temperature of the steel plate to 550~700℃, and the temperature drop of water cooling is controlled to be 100~150℃ and the cooling speed is ≥10℃ / s.

8. The method of manufacturing a bainite plastic mold steel based on laterite nickel ore according to claim 4, characterized in that, In step 4), after the steel plate is straightened, the steel plate is cooled by air cooling until the temperature of the steel plate is 400~450℃, the cooling speed is 0.5~1℃ / s, and then the steel plate is slowly cooled in an online tempering furnace, the furnace temperature is 200~450℃, the cooling speed of the steel plate is 0~0.5℃ / s, the cooling time in the tempering furnace is t=(0.1~0.4)×H, t is in min, and H is the thickness of the steel plate, in mm.

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