The forging process of plastic mold steel modules of Jiangsu Changli Equipment Manufacturing Co., LTD

Die steel can be roughly classified into three types: cold work die steel, hot work die steel and plastic die steel, which are used for forging, stamping, cutting, die casting, etc.

1. Cold work die steel

High carbon high chromium type

Cr12MoV: High wear resistance for cold stamping dies and drawing dies (corresponding to American standard D3 and Japanese standard SKD1).

Cr12Mo1V1 (D2) : Heavy blanking die, cold extrusion die (corresponding to German standard 1.2379).

DC53: High toughness cold work steel, alternative to Cr12MoV, suitable for precision blanking dies (Hitachi grade).

Impact resistance type

4CrW2Si/5CrW2Si: For shear dies and punches subjected to impact loads.

Carbon tool steel

T7-T12: Simple small molds, low cost but poor wear resistance.

2. Hot work die steel

General-purpose hot work steel

H13 (4Cr5MoSiV1) : Die casting dies, hot forging dies (corresponding to Japanese standard SKD61, German standard 1.2344).

8407: Swedish Imab hot work steel with excellent heat fatigue resistance.

High heat resistance type

3Cr2W8V: High-temperature die-casting mold, but with relatively low toughness.

5CrNiMo: Hammer forging die, suitable for medium modules.

3. Plastic mold steel

Pre-hardened type

P20 (3Cr2Mo) : Universal plastic mold, pre-hardened hardness 30-40HRC (US standard).

1.2738/1.2738H: Large mirror mold, Swedish Imab grade.

Corrosion resistant type

S136 (4Cr13) : Transparent plastic mold, PVC corrosive environment (Swedish Imab).

2316 (German standard) : Corrosion-resistant mirror-finished mold.

Age-hardening type

NAK80: High-precision plastic mold that can be processed directly after pre-hardening (Hitachi grade).

4. International Grade comparison table

China , the United States , Japan , Germany/Sweden uses

Cr12MoV D2  SKD11 1.2379 cold stamping die, wear-resistant parts

4Cr5MoSiV1 H13 SKD61  1.2344 Die casting die, hot forging die

3Cr2Mo P20-1.2311  general-purpose plastic mold

S136 420  1.2083 corrosion-resistant mirror mold

5. Other special die steels

High-speed steel: such as W6Mo5Cr4V2 (M2) for high-wear cutting tools.

Hard alloys: such as YG20 for high-life blanking dies.

         

Vi. Manufacturing process of our company 's plastic mold steel 1.2738 forging die steel

   1.2738 plastic mold steel is an improved grade based on P20. Due to its good hardenability and superior performance, the factory supplies it in a pre-hardened state, and mold manufacturing plants can directly use it for mold processing without heat treatment, which can significantly shorten the mold processing cycle. This steel is mainly used for making large-sized, high-grade plastic molds such as large mirror plastic molds, plastic molds, high-polished mold cavities, hot extrusion molds, etc. Our company has successfully produced 1.2738 with a thickness of 300mm to 600mm and a width of 1100mm to 1,400mm by using the process of "40tEBT eccentric electric furnace smelting +LF furnace refining +VD furnace vacuum treatment + mold casting of 19, 36t ingots +45MN press forging + heat treatment" A large module of plastic mold steel was produced and the control of [O], [H] and non-metallic inclusions in the steel under the mold casting process was studied. The results of the trial production indicated that the metallurgical quality of the large steel ingots produced by our factory reached a relatively high level.

1. Chemical composition requirements and technical difficulties in production

  1. Technical difficulties of 1.2738 plastic mold steel: (1) Foreign successful experience shows that reasonable chemical composition control is the key to ensuring the physical properties of the steel. (2) The steel is a medium carbon plastic mold steel containing 1% nickel, which is sensitive to white spots and cracks. The gas content (T[O], [H]) and non-metallic inclusions in the steel must be strictly controlled during production. (3) Strengthen the control of the smelting process to minimize non-metallic inclusions and meet the requirements of the agreement. According to the standard and the technical requirements of 1.2738 plastic mold steel, a trial production process route and chemical composition control range were developed. The standard, actual production chemical composition and residual, harmful elements and gas contents are shown in Table 1.

2 Key points of control for the trial production process

    The trial production uses high-quality pig iron and scrap steel ingredients. Initial smelting in a 40-ton eccentric electric furnace, with good dephosphorization and end-point carbon control, and pre-deoxidation of molten steel using aluminium blocks and ferrosilicon during the steel tapping process. The LF ladle refining furnace generates high alkalinity reduction slag through electric arc heating, and throughout the refining process, inert gas is blown into the bottom of the ladle to stir the molten steel. Equipped with wire feeding equipment, in the later stage of refining, the aluminum in the molten steel is controlled to keep the oxygen content in the molten steel below 0.002%. Before the VD vacuum refining, to ensure the vacuum treatment effect and improve the purity of the molten steel, on the one hand, reduce the amount of slag to ensure the permeability of the refining slag during the vacuum treatment process, on the other hand, use non-metallic inclusion modified calcium treatment. The vacuum treatment process uses appropriate argon flow control to ensure the ultimate vacuum time and control the hydrogen content in the refined molten steel to be less than 0.0002% to prevent the formation of white spots. A trial production process for casting steel ingots by the drop method, with argon gas under the water mouth protecting the molten steel throughout the pouring process while strictly controlling the pouring temperature and pouring speed of the ingot body and cap mouth. Statistical data show that the metallurgical quality of the large steel ingots produced by our factory has reached a relatively high level.

3. Research on the control of production technical difficulties

3.1 1.2738 Plastic mold steel Chemical Composition Control Theory and trial production situation

   With reference to relevant materials at home and abroad and in combination with the characteristics of the steel and the equipment of our company, internal control requirements for chemical composition were made during the trial production. The main functions of the elements in 1.2738 plastic mold steel are as follows: Carbon ensures strength and heat treatment hardenability; Cr and Mo enhance module hardenability and shift the C curve to the right; Mn and Ni enhance strength and hardenability, and improve module toughness. According to the role of each element, the main elements are required to be controlled above the middle limit of the specification during trial production. To prevent hot brittleness and cold brittleness of the module during heat treatment, the harmful elements phosphorus and sulfur should be controlled to be less than 0.010%. The average results of the trial production statistics are shown in Table 3.

3.2 Gas content and non-metallic inclusions control theory and trial production of the steel during smelting

3.2.1 Theory of hydrogen content control in molten steel and trial results

The solubility of hydrogen in solid and liquid steel follows the square root law. When the hydrogen content in the ingot is greater than 0.0002%, the elongation and reduction of area of the steel will be significantly reduced. It has a greater impact on medium carbon 1.2738 plastic mold steel with Ni1%. When hydrogen content reaches a certain level, white spot defects and hydrogen embrittlement will occur. During the production trial, it is required that the raw and auxiliary materials fed into the furnace be dry, the oxidation period ensure sufficient decarburization amount, the hydrogen in the molten steel is removed by the CO bubbles generated by the intense reaction of carbon and oxygen, the refining time of the LF furnace is controlled within 100 minutes, the gas intake of the molten steel during the refining process is reduced, the VD vacuum equipment is good and combined with a reasonable argon flow rate, It ensures that the hydrogen content in the molten steel is less than 0.00015%, meeting the hydrogen content requirements for 1.2738 plastic mold steel. After vacuum treatment of the steel liquid, the statistics of hydrogen content in the steel liquid were detected on-site using vacuum quartz tubes, indicating that the hydrogen content in the steel liquid was controlled below 0.00016%. The distribution of hydrogen content at the end of VD vacuum treatment was plotted based on the statistics, as shown in Figure 1.

3.2.2 Theory of Oxygen Content Control in Molten Steel and Trial Production Results

During the deP and control process in the electric furnace, a large amount of elements in the molten steel are oxidized, and the free oxygen in the molten steel is very high. The oxidized molten steel needs to be refined and reduced to further remove the oxygen in the molten steel. Deoxidation is an important part of steelmaking, which largely determines the quality of the finished steel. Incomplete deoxidation leads to the formation of Al2O3 and FeS-FeO fusible eutectic precipitates from the grain boundaries when the refined molten steel crystallizes and solidifies with the deoxidizer, reducing the plasticity of the steel and significantly increasing its brittleness. On the other hand, due to the gradual enrichment of oxygen in the molten steel, when its concentration exceeds the oxygen concentration determined by the carbon-oxygen product, bubbles will form in the steel, and during mold casting, they will remain in the ingot body as the molten steel solidifies, causing defects such as porosity, shrinkage cavities, and segregation in the ingot.

 Based on the thermodynamic and kinetic principles of the deoxidation reaction and in combination with the characteristics of the steel, a combined deoxidation method of precipitation deoxidation (adding Al, adding Fe-Si blocks) and diffusion deoxidation (reducing with C, Si, Al powder in the LF ladle refining furnace) is adopted in actual production to control the oxygen content in the LF furnace refining molten steel below 0.0025%, The steel liquid is further reduced by vacuum treatment in the VD furnace. Based on the trial production statistics, a distribution map of oxygen content in 1.2738 plastic mold steel was drawn, as shown in Figure 2.

3.2.3 Actual production of non-metallic inclusions

If the deoxidation products in the molten steel are not removed in time, they will remain in the molten steel as it solidifies, forming non-metallic inclusions, whose presence will reduce the toughness and fatigue limit of the steel and deteriorate its workability. Based on Stokes' principle and the characteristics of our silicate slag system, to ensure that most of the non-metallic inclusions float up, low-density strong deoxidizers aluminum and ferrosilicon were selected during the trial production to deoxidize the molten steel. Under thermodynamic and kinetic conditions met, the inclusions were modified to remove the non-metallic inclusions in the molten steel as much as possible. The trial production statistics are shown in Table 4 below.

During the trial production, gas control in the smelting process was strengthened and inclusions were subjected to metamorphic treatment. Data statistics show that inclusions of Class A and B meet the standard requirements, while inclusions of Class C and D are both grade 0.

4、Post-processing    

During mold casting, argon gas protects the casting throughout the process, and trial casting of 19 t and 36 t ingots. Red delivery of ingots to 45MN press forging to post-forging annealing to module pre-hardening to hardness testing to storage.

5. Trial production module specifications and hardness test results

    After the pre-hardening of the modules, the hardness was tested on-site using a hardness tester. The data of hardness was statistically analyzed based on different ingot types and different module specifications, as shown in Table 5.

According to the results of the statistics, the hardness of modules with thicknesses ranging from 300mm to 600mm fully met the protocol requirements.

6. Conclusion

(1) The plastic mold steel 1.2738 module with a thickness of 300 mm to 600 mm produced by trial production using EBT+LF+VD+ mold casting of 19 tons of octagonal ingot + red send forging can fully meet the user's requirements.

(2) The 19-ton and 36-ton large steel ingots produced by the smelting method of electric furnace + vacuum refining have hydrogen and oxygen content and non-metallic inclusion content that fully meet the standard requirements, the metallurgical quality of the steel ingots can be guaranteed, and all indicators meet the requirements of the agreement.