Cold work die steel and preparation method thereof

Abstract

The invention discloses cold work die steel and a preparation method thereof, and belongs to the technical field of metallurgy. In order to improve the flaw detection qualification rate of products, the invention provides a preparation method of cold work die steel, which comprises the following steps of: slowly casting the front section of molten steel and quickly casting the rear section; when the molten steel is poured to half height of a riser, adding an exothermic agent and carbonized rice husks; the steel ingot is heated to 830-860 DEG C at the speed of 80-100 DEG C / h, heat preservation is conducted for 2-3 h, then the steel ingot is heated to 1180-1200 DEG C at the speed of 180-200 DEG C / h, heat preservation is conducted for 4-8 h, and turning over is conducted once every 1.5- The rolling pass is 15-25, the reduction rate of the first five passes is 10-15%, the reduction rate of the middle pass is 20-30%, and the reduction rate of the last five passes is 5-10%. The casting, heating and pressing procedures are optimized, so that the defects such as looseness and shrinkage cavities in the cast ingot are effectively controlled, and the flaw detection qualification rate is remarkably increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of metallurgy, and particularly relates to a cold work die steel and a preparation method thereof. Background Art

[0002] Cold work die steels are mainly used for the production of cold stamping dies, cold extrusion dies, cold heading dies, cold drawing dies, etc. These dies generally require high hardness, high wear resistance, sufficient toughness, good fatigue resistance, and anti-seizure bonding ability during use. Therefore, the basic composition design of general cold work die steels mostly adopts high carbon and high chromium. More typical steel grades such as Cr12, Cr12MoV, Cr12Mo1V1, D2, SKD11, etc. have a carbon content as high as over 1.40% and a chromium content of 11.00% - 13.00%. Due to the relatively high carbon and chromium contents in cold work die steels, a well-developed ledeburite structure is easily formed, resulting in uneven structure and eutectic segregation areas with lower melting points in the as-cast state. During the processing, it shows low plasticity, high deformation resistance, and is very sensitive to cracks, making the production of such steel grades very difficult. This is particularly prominent when rolling square steel. Currently, when enterprises use the rolling method to produce Cr12 series square steel, it usually shows that the cross-section of the square steel cracks in a crescent shape. When inspected, the defect type reflected is F > B, and the inspection pass rate is about 70%.

[0003] CN115747671B discloses a preparation method of a cold work die steel, which includes arc furnace melting, LF refining, VD refining, ingot casting, forging, rolling, and isothermal spheroidizing annealing carried out in sequence. During the preparation process of this cold work die steel, the parameters in the melting and forging processes are adjusted, ultimately reducing the non-metallic inclusions and minimizing the macrostructure defects of the prepared cold work die steel. However, the ingot casting + rolling method adopted by this method has many processes, a relatively cumbersome process, and a relatively high manufacturing cost.

[0004] CN118910490A discloses a preparation method of Cr12MoV steel, including EBT electric arc furnace melting, LF furnace refining and VD furnace refining carried out in sequence, casting to obtain a steel billet, and the steel billet is successively subjected to upsetting and drawing forging, annealing, flaw detection, sampling and finishing treatments to obtain Cr12MoV steel; during the electric arc furnace melting process: the content of C is 1.45%-1.70%, the content of Mo is 0.40%-0.60%, and the content of V is 0.75-1.00%; the casting temperature is 1435°C-1445°C, the ingot body casting time is 290s-370s, and the riser time is 150s-210s. This method uses high-temperature homogenization treatment to basically eliminate the small particle eutectic carbides in the ingot, partially dissolve the large particle eutectic carbides, significantly improve the composition segregation, and further make the carbides in the steel fine and uniform. However, this method requires gradient heating in the forging process and heat preservation treatment above 1160°C and repeated upsetting and drawing forging, resulting in a more cumbersome process and a relatively high rolling cost. Summary of the Invention

[0005] In order to improve the quality of rolled products of Cr12 series square steel, increase the flaw detection qualification rate of products, simplify the production process, and reduce the production cost, the present invention optimizes the casting process, heating process and rolling process, and develops a preparation method of cold work die steel, which is mainly used to improve the quality of square steel products with a cross-sectional size of (150-200 mm)*(150-200 mm) in the rolling production of Cr12 series steel grades.

[0006] The present invention provides a preparation method of cold work die steel, which includes the following steps:

[0007] A. The molten steel after being smelted by an electric furnace→LF→VD is cast. During the casting process, the front section of 40%-60% of the molten steel amount is slowly cast, and the casting speed is controlled at 40-55 kg / s. The remaining 60%-40% of the molten steel amount in the rear section is quickly cast, and the casting speed is controlled at 55-80 kg / s, and the casting speed in the rear section is 10-30 kg / s faster than that in the front section; when the molten steel is poured to half the height of the riser, a heating agent and carbonized rice husk are added, and after casting is completed, an ingot is obtained;

[0008] B. The ingot obtained by casting in step A is sent to a heating furnace while it is red-hot. First, it is heated at a heating rate of 80-100°C / h to 830-860°C and kept warm for 2-3 h, and then heated at a heating rate of 180-200°C / h to 1180-1200°C and kept warm for 4-8 h; the ingot is turned over once every 1.5-2 h in the heating furnace.

[0009] C. Rolling: The number of rolling passes is distributed between 15 and 25 passes; during rolling, the reduction ratio in the first 5 passes is controlled at 10 - 15%, large reduction ratios are adopted in the middle passes, and the reduction ratio is controlled at 20 - 30%, and the reduction ratio in the last 5 passes is controlled at 5 - 10%; after the rolling is completed, it is cooled to obtain the cold work die steel.

[0010] Among them, in the preparation method of the above-mentioned cold work die steel, the cold work die steel is a steel grade of Cr12 series cold work die steel, and its carbon content is not less than 1.40 wt%, and the chromium content is 11.00 wt% - 13.00 wt%.

[0011] Preferably, in the preparation method of the above-mentioned cold work die steel, the cold work die steel is Cr12, Cr12MoV, Cr12Mo1V1, D2 or SKD11.

[0012] Preferably, in the preparation method of the above-mentioned cold work die steel, in step A, the first half of the molten steel with 49% - 51% of the total amount is slowly poured, and the second half of the molten steel with 51% - 49% of the total amount is quickly poured, and the pouring speed of the second half is 15 - 25 kg / s faster than that of the first half.

[0013] Among them, in the preparation method of the above-mentioned cold work die steel, in step A, the addition amount of the heating agent is 2.0 - 2.5 kg / t of steel ingot, and the addition amount of the carbonized rice husk is 0.6 - 1.0 kg / t of steel ingot.

[0014] Among them, in the preparation method of the above-mentioned cold work die steel, in step A, the molten steel temperature is ensured to be 1460 - 1490 °C during the pouring process.

[0015] Among them, in the preparation method of the above-mentioned cold work die steel, in step A, the obtained steel ingot is a rectangular ingot with a weight of 3.0 - 3.75 t.

[0016] Among them, in the preparation method of the above-mentioned cold work die steel, in step B, when the steel ingot is placed in the heating furnace, the riser end is placed upward.

[0017] Among them, in the preparation method of the above-mentioned cold work die steel, in step C, the final rolling temperature is controlled ≥ 960 °C.

[0018] Among them, in the preparation method of the above-mentioned cold work die steel, in step C, the nozzle end is forward and the riser end is backward during rolling.

[0019] Among them, in the preparation method of the above-mentioned cold work die steel, in step C, the rolling is carried out on a reversing mill of the 1350 rolling mill.

[0020] Among them, in the preparation method of the above-mentioned cold work die steel, in step C, the obtained cold work die steel is a square steel with a cross-sectional size of (150 - 200 mm) * (150 - 200 mm).

[0021] The present invention also provides a cold work die steel, which is prepared by the above preparation method. Through the optimization of the casting process, heating process, rolling process, etc. by the present invention, the product quality is improved, and the flaw detection qualification rate of the product is increased. Therefore, when performing flaw detection, there are no F>B type defects, and the flaw detection equivalent The flaw detection qualification rate is more than 92%.

[0022] Advantages of the present invention:

[0023] By controlling the pouring temperature, pouring speed and the timing of adding auxiliary materials, strictly controlling the heating rate, and adopting segmented heat preservation, small reduction in the early stage, large reduction in the middle passes, and small reduction in the last 5 passes for control, the present invention finally realizes the effective control of the internal porosity and shrinkage cavity defects in the ingot, significantly improves the quality of the rolled square steel products of the Cr12 series cold work die steel, and there are no F>B type defects when flaw detecting the square steel products with a cross-sectional size of (150-200mm)*(150-200mm) in the rolling production, and the flaw detection qualification rate is significantly increased to more than 92%. Description of the drawings

[0024] Figure 1 It is an internal defect diagram of a square steel with a cross-sectional size of 180*180mm of Cr12MoV steel.

[0025] Figure 2 It is a macrostructure diagram of the cross-section of the square steel in Example 3. Detailed implementation manners

[0026] Specifically, a preparation method of a cold work die steel includes the following steps:

[0027] A. Pour the molten steel after being smelted by an electric furnace→LF→VD. During the pouring process, the front section with 40%-60% of the molten steel amount is slowly poured, and the pouring speed is controlled at 40-55 kg / s. The rear section with 60%-40% of the molten steel amount is quickly poured, and the pouring speed is controlled at 55-80 kg / s, and the pouring speed of the rear section is 10-30 kg / s faster than that of the front section. When the molten steel is poured to half of the height of the riser, add a heating agent and carbonized rice husk. After pouring is completed, an ingot is obtained;

[0028] B. Send the ingot obtained by pouring in step A to a heating furnace while it is still hot. First, heat it at a heating rate of 80-100 °C / h to 830-860 °C and keep it warm for 2-3 h, then heat it at a heating rate of 180-200 °C / h to 1180-1200 °C and keep it warm for 4-8 h. The ingot is turned over once every 1.5-2 h in the heating furnace;

[0029] C. Rolling: The number of rolling passes is distributed from 15 to 25 passes; during rolling, the reduction ratio of the first 5 passes is controlled at 10 - 15%, large reduction is adopted for the middle passes, the reduction ratio is controlled at 20 - 30%, and the reduction ratio of the last 5 passes is controlled at 5 - 10%; after rolling is completed, it is cooled to obtain the cold work die steel.

[0030] In the present invention, the cold work die steel is a steel grade of Cr12 series cold work die steel, and its common feature is that the carbon content is not less than 1.40 wt%, and the chromium content is 11.00 wt% - 13.00 wt%; due to the relatively high carbon and chromium contents, it is easy to form a developed ledeburite structure, resulting in non-uniform structure and eutectic segregation area with relatively low melting point in the as-cast state. During the processing, it shows low plasticity, high deformation resistance, and is very sensitive to cracks, making the production of such steel grades very difficult. More specifically, in the present invention, the cold work die steel is Cr12, Cr12MoV, Cr12Mo1V1, D2 or SKD11.

[0031] In the present invention, by controlling the pouring temperature, pouring speed and the timing of adding auxiliary materials, the feeding of the ingot riser end and the central part is achieved, ensuring the soundness of the internal solidification quality of the ingot without large-area porosity shrinkage defects. Preferably, in step A, the first half of 49% - 51% of the molten steel amount is slowly poured, and the second half of the remaining 51% - 49% of the molten steel amount is quickly poured, and the pouring speed of the second half is 15 - 25 kg / s faster than that of the first half.

[0032] In step A of the present invention, the addition amount of the heat - generating agent is 2.0 - 2.5 kg / t of ingot, and the addition amount of the carbonized rice husk is 0.6 - 1.0 kg / t of ingot.

[0033] In step A of the present invention, the molten steel temperature is ensured to be 1460 - 1490 °C during the pouring process.

[0034] In step A of the present invention, the obtained ingot is a 3.0 - 3.75 t rectangular ingot.

[0035] In step B of the present invention, by strictly controlling the heating rate, it is ensured that no thermal stress is generated during the heating process of the ingot to cause it to crack; on the other hand, by turning the ingot over within a fixed time, the temperature uniformity of the ingot can be ensured. At the same time, placing the riser end upwards can increase the uniformity of the temperature drop of the air - contacting section after the ingot is taken out of the furnace, preventing the stress difference in the rolling process caused by different temperature drops at the riser and nozzle ends, and reducing the risk of longitudinal cracking caused by large temperature difference between the head and tail during the rolling deformation process. In step B of the present invention, when the ingot is placed in the heating furnace, the riser end is placed upwards.

[0036] After the heating and heat preservation in step B of the present invention are completed, the ingot can be transferred to the rolling mill by the transfer roller table, that is, rolling begins, and the entire transfer process takes 60 - 80 s.

[0037] In step C of the present invention, through small reduction in the early stage, it can ensure that deformation occurs in the equiaxed crystal region of the ingot, playing a role in deformation but not causing cracking of the relatively fragile parts of the ingot; while large reduction is adopted in the intermediate passes, which can make the rolling force fully penetrate into the core of the ingot, playing a role in fully welding the porosity and shrinkage cavity in the core; the reduction rate of the last 5 passes is controlled at 5-10%. Considering that the rolling force can fully penetrate at this time, small deformation can not only meet the need of deformation control but also will not cause the generation of internal defects.

[0038] In step C of the present invention, the final rolling temperature is controlled ≥960°C.

[0039] In step C of the present invention, during rolling, the nozzle end faces forward and the riser end faces backward.

[0040] In step C of the present invention, rolling is carried out on a reversing mill of the 1350 mill.

[0041] In step C of the present invention, after pressing is completed, the steel ingot is placed on the cooling bed and allowed to cool naturally in the air to room temperature, then the final product, cold work die steel, can be obtained. In step C of the present invention, the obtained cold work die steel is square steel with a cross-sectional size of (150-200 mm)×(150-200 mm).

[0042] The present invention also provides a cold work die steel prepared by the above preparation method. Through the optimization of the casting process, heating process, rolling process, etc. of the present invention, the product quality is improved and the flaw detection qualification rate of the product is increased. Therefore, when detecting flaws, there are no F>B type defects, and the flaw detection equivalent The flaw detection qualification rate is more than 92%.

[0043] The present invention will be further described in detail below through embodiments, but the protection scope of the present invention is not limited to the scope of the described embodiments.

[0044] The terms "first", "second", etc. in the specification, claims and drawings of the present invention are used to distinguish similar objects and do not necessarily need to describe a specific order or sequence. It should be understood that such used data can be interchanged under appropriate circumstances, so that the embodiments of the present invention described here, for example, can be implemented in an order other than those illustrated or described here.

[0045] The terms "comprising" and "having" in the present invention and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device comprising a series of steps or sub-modules does not necessarily have to be limited to those clearly listed steps or sub-modules, but may include other steps or sub-modules not clearly listed or inherent to these processes, methods, products or devices.

[0046] Example 1

[0047] A preparation method of cold work die steel, including ingot casting, ingot heating and rolling. The specific implementation steps are as follows:

[0048] 1. Ingot casting

[0049] Ladle the molten steel after being smelted by electric furnace → LF → VD to the automatic casting area. During the automatic casting process, ensure that the temperature of the molten steel is between 1460 and 1490 °C, and the casting speed is controlled between 40 and 80 kg / s. The first half (the first 50% of the molten steel volume) is slowly cast (the casting speed is controlled between 40 and 55 kg / s), and the second half (the remaining 50% of the molten steel volume) is quickly cast (the casting speed is controlled between 55 and 80 kg / s); when the molten steel is poured to half the height of the riser, start adding 7.5 kg of heating agent and 3 kg of carbonized rice husk, and finally cast into a 3.5 t rectangular ingot.

[0050] 2. Ingot heating

[0051] 2.1. Red-send the ingot obtained from the above casting into the heating furnace, heat it to between 830 and 860 °C at a heating rate of ≤100 °C / h, hold for 2 to 3 h, and then heat it to between 1180 and 1200 °C at a heating rate of ≤200 °C / h, and hold for 4 to 8 h.

[0052] 2.2. The ingot needs to be turned over once every 1.5 h in the heating furnace.

[0053] 2.3. When placing the ingot in the heating furnace, ensure that the riser end is placed upward.

[0054] 3. Rolling

[0055] 3.1. The rolling deformation is carried out on a reversing mill of the 1350 unit. According to the different finished product specifications, the number of rolling passes is distributed between 15 and 25 passes.

[0056] 3.2. During rolling, the nozzle end is forward and the riser end is backward; control the final rolling temperature ≥960 °C.

[0057] 3.3. During rolling, the reduction rate of the first 5 passes is controlled at 10 - 15%, the middle passes use large reduction, the reduction rate is controlled at 20 - 30%, and the reduction rate of the last 5 passes is controlled at 5 - 10%.

[0058] Differences in the preparation method of the comparative example:

[0059] Compared with the method of the present invention, the comparative example adopts the conventional method for production. The main differences are as follows: 1. When casting the ingot, the conventional method usually adopts overall slow casting (it was theoretically considered in the past that slow casting is beneficial to reducing or avoiding the generation of porosity and shrinkage cavities), which is different from the segmented control of pouring speed casting adopted by the present invention; 2. When casting the ingot, the conventional method adds the exothermic agent and protective slag after the riser casting is completed, while in the present invention, the exothermic agent and protective slag can be added when casting reaches half the height of the riser; 3. The ingot is not turned over during heating in the conventional method; 4. When rolling in the conventional method, it uses 1-hole flat rolling with large reduction until the cross-sectional size reaches 280*550mm and then transfers to 3-hole rolling. The reduction rate of the first 5 passes reaches more than 15% correspondingly. This method is extremely likely to cause further cracking and expansion of the porosity and shrinkage cavity defects inside the ingot itself, while in the present invention, small deformation rate rolling is adopted for the first 5 passes.

[0060] Table 1 Steel grades and chemical compositions (mass percentage / wt%) of each example and comparative example

[0061]

[0062]

[0063] Table 2 Pouring and heating parameters of each example and comparative example

[0064]

[0065] Table 3 Rolling parameters of each example and comparative example

[0066]

[0067] Table 4 Flaw detection equivalent and flaw detection qualification rate of each example and comparative example

[0068]

[0069] Figure 1 It is an internal defect diagram of a Cr12MoV steel produced by the conventional method, with a cross-sectional size of 180*180mm square steel. This defect is caused by the cracking and expansion starting from the defect position under the action of rolling force during the rolling deformation of the typical porosity and shrinkage cavity inside the ingot. Figure 2 It is a macrostructure diagram of the cross-section of the square steel in Example 3.

[0070] It can be seen from the examples and comparative examples that the present invention controls the pouring temperature, pouring speed and the timing of adding auxiliary materials to achieve feeding at the riser end and the central part of the ingot, ensuring good solidification quality inside the ingot without large-area porosity shrinkage defects; by strictly controlling the heating rate and adopting segmented heat preservation, it is ensured that the ingot will not crack due to thermal stress during the heating process, and by turning the ingot over within a fixed time, the temperature uniformity of the ingot can be ensured. At the same time, placing the riser end upwards can increase the uniformity of the temperature drop in the section where the ingot contacts the air after being taken out of the furnace, preventing the increase in the stress difference during the rolling process caused by different temperature drops at the riser and nozzle ends, and reducing the risk of longitudinal cracking caused by a large temperature difference between the head and tail during the rolling deformation process; by applying small reduction in the early stage, it can ensure that the deformation occurs in the equiaxed crystal zone of the ingot, playing a role in deformation but not causing tensile cracking of the relatively fragile parts of the ingot. A large reduction is adopted in the middle passes to make the rolling force fully penetrate into the core of the ingot, playing a role in fully welding the porosity shrinkage in the core. The reduction rate in the last 5 passes is controlled at 5-10%. The small deformation can not only meet the need of deformation control but also prevent the generation of internal defects; finally, effective control of the internal porosity shrinkage defects in the ingot is achieved. When detecting the square steel products with a cross-sectional size of (150-200mm)*(150-200mm) produced by rolling, there are no F>B type defects, and the detection qualification rate is significantly increased to over 92%.

Claims

1. A preparation method of cold work die steel, characterized in that: It includes the following steps: A. Pour the molten steel after being smelted in an electric furnace → LF → VD. During the pouring process, slow-pour the first 40% - 60% of the molten steel volume at the beginning, control the pouring speed at 40 - 55 kg / s, and fast-pour the remaining 60% - 40% of the molten steel volume at the end, control the pouring speed at 55 - 80 kg / s, and the pouring speed at the end is 10 - 30 kg / s faster than that at the beginning. When the molten steel is poured to half the height of the riser, add a heating agent and carbonized rice husk. After pouring is completed, an ingot is obtained. B. Send the ingot obtained by pouring in step A to the heating furnace while it is still hot. First, heat it at a heating rate of 80 - 100 °C / h to 830 - 860 °C and keep it warm for 2 - 3 h, then heat it at a heating rate of 180 - 200 °C / h to 1180 - 1200 °C and keep it warm for 4 - 8 h. The ingot is turned over once every 1.5 - 2 h in the heating furnace. C. Rolling: The number of rolling passes is distributed in 15 - 25 passes. During rolling, the reduction ratio in the first 5 passes is controlled at 10 - 15%, large reduction is adopted in the middle passes, the reduction ratio is controlled at 20 - 30%, and the reduction ratio in the last 5 passes is controlled at 5 - 10%. After pressing is completed, it is cooled to obtain cold work die steel.

2. The preparation method of the cold work die steel according to claim 1, wherein: The cold work die steel is a steel grade of Cr12 series cold work die steel, its carbon content is not less than 1.40 wt%, and its chromium content is 11.00 wt% - 13.00 wt%. Preferably, the cold work die steel is Cr12, Cr12MoV, Cr12Mo1V1, D2 or SKD11.

3. The preparation method of the cold work die steel according to claim 1, wherein: In step A, slow-pour the first half of 49% - 51% of the molten steel volume, and fast-pour the second half of the remaining 51% - 49% of the molten steel volume. The pouring speed in the second half is 15 - 25 kg / s faster than that in the first half.

4. The preparation method of the cold work die steel according to claim 1, characterized in that: In step A, the addition amount of the heating agent is 2.0 - 2.5 kg / t of ingot, and the addition amount of the carbonized rice husk is 0.6 - 1.0 kg / t of ingot.

5. The preparation method of the cold work die steel according to claim 1, characterized in that: In step A, ensure that the molten steel temperature is 1460 - 1490 °C during the pouring process.

6. The preparation method of the cold work die steel according to claim 1, characterized in that: In step A, the obtained ingot is a 3.0 - 3.75 t rectangular ingot.

7. The preparation method of the cold work die steel according to claim 1, characterized in that: In step B, when placing the ingot in the heating furnace, place the riser end upward.

8. The preparation method of the cold work die steel according to claim 1, characterized in that: In step C, at least one of the following is satisfied: Control the final rolling temperature ≥ 960 °C; During rolling, the nozzle end faces forward and the riser end faces backward; Rolling is carried out on a reversing mill of a 1350 mill.

9. The preparation method of the cold work die steel according to any one of claims 1 to 8, characterized in that: In step C, the obtained cold work die steel is square steel with a cross-sectional size of (150 - 200 mm) * (150 - 200 mm) specification.

10. The cold work die steel prepared by the preparation method of the cold work die steel according to any one of claims 1 to 9, characterized in that: During its flaw detection, there are no F > B type defects, the flaw detection equivalent is φ1.2 - 2.0, and the flaw detection qualification rate is over 92%.

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