The production flow sheet for test steel is 2 t medium frequency induction furnace-200 kg ESR (Φ180 mm) -annealing-upset forging to Φ325 mm-spheroidal annealing-quenching at 1050 ℃-tempering at 590 ℃，furnace cooling. The effect of 0 - 0.001 0% Mg on structure and mechanical properties of hot-working-die steel H13 (/% : 0.40-0.41C, 0.98-1.01Si, 0.29Mn, 4.95-5.08Cr, 1.21-1.22Mo, 0.91-0.93V, 0.023-0.027P, 0.006-0.007S) has been tested and studied. Results show that with increasing Mg content in steel from 0 to 0. 0010% , the average tensile strength and HRC value of test die steel H13 increases respectively from 1617.6 MPa and 45.7 to 1702.9 MPa and 47.8 ； Mg is available to increase the stability of tempered martensite in steel, in high temperature tempered die steel H13 containing Mg, a lot of tempering troostite occurs in structure and partial globular carbides with size less than 10 μm precipitate around Mg-Al inclusions, it is favourable to improvement of performance of steel.

The steelmaking process for test hot working die steel H13 (/% : 0.36-0.38C, 0.36-0.38Mn, 0.89-0.91Si, 0.007-0.010P, 0.006-0.011S, 5.10-5.15Cr, 1.12-1.18Mo, 0.84-0.89V) is 40 t eccentric bottom tapping arc furnace-ladle furnace refining-VD treatment-4. 65 t ingot casting process. With analysis on total oxygen and nitro gen content in liquid- T [O] and [ N] , and analysis on morphology and ingredient of inclusions in steel, the cleanliness of steel H13 produced by this process flow sheet has been studied. Results show that after refining by LF-VD process the average total oxygen content in steel H13- T[O] is 22 x 10 ^-6 and the average nitrogen- [N] is 95 x 10 ^-6 ； the main inclusions in Φ200 mm forging are deoxidation products- aluminium oxide with sulHdes including manganese sulfide and nitride formed by segregation ； the sizes of inclusions are mainly about 5 μm, the sizes of some inclusions can come up to about 10 μm. The steel H13 steelmaking by this flow sheet could meet the requirement for cleanliness of that steel grade.

The production flowsheet of test cold-working die flat steel Crl2MolVI (D2) (/% ： 1. 50C, 0. 35Si, 0. 40Mn, 12. 00Cr, 0. 95Mo, 0. 80V) is 30 t EAF-LF-VD-3 t ingot casting-rolling-annealing at 890 °C process. The effect of cross-section size (/mm: 30 x 400, 70 x 360 and 90 X 605) of cold-working die flat steel D2 on the size, distribution and uniformity of eutectic carbide, the structure and the impact toughness of steel has been studied. The results show that with decreasing the cross-section size, the size of particles, morphology and distribution uniformity of eutectic carbide in flat are improved, the rating of carbide in flat decreases from 4 ~6 rating of 90 mm x605 mm flat to 1 ^4 rating of 30 mm x 400 mm flat, and the impact toughness of steel increases ； the distribution of eutectic carbide at core of flat is worst ( rating 4 ~ 6) and that at edge is best (rating 1 ~ 4) , and the impact toughness of steel at core is obviously lower than that at edge.

With using the process measures including controlling mold inlet water temperature 40℃ , outlet water temperature 68 ℃ ； controlling model of computer auto back feeding for shrinkage ； at end auto back feeding, changing consumable electrode to graphite electrode withing 90 s and power supply with 53 V, 3 000 A melting for 3 ~5 min, then again changing to consumable electrode and power supply with 50 V, 4 000 A for 6 min, the depth of top shrinkage cavity defect of electroslag remelting ( ESR) ingot decreases to 30 ram from original 100 mm. Production practice shows that the process operation of back feeding for shrinkage is simple and easier to do. The forged billets of remelting 210 piece tool steel 9Cr2Mo Φ 280 mm x 1 000 mm, 480 kg ESR ingot and 60 piece hot die working steel H13 420 mm x 1 400 mm , 1 500 kg ESR ingot by optimized process have not been rejected due to ingot top shrinkage cavity defect, the ingot yield increases obviously.

By using combination teachnology of 20 t EAF-LF-VD, 20 t argon protection ESR unit remelting, 1 250 ~1 280 ℃ high temperature homogenization treatment, 60 MN oil pressure quick forging machine multiple forging and 1 000 〜1 050 ℃ high temperature solution treatment process, the inclusion(≤0.5 rating) , structure and lateral impact energy (KV2 25 〜31 J) of produced 640 mm x 1 100 mm 4Cr5Mo2V heavy die steel are higher than the requirement in standard NADCA#207-2018 for advanced steel leave,and the isotropy of steel reaches 0.95.

The 2311 die steel 16 ~ 80 mm plate is produced by KR desulfphrization-130 t BOF-LF-RH-300 mm cast slab-rolling-580 -610 °C tempering process. The results show that for 2311 steel (/% : C 0. 38 ~0.42, Si 0.25 ~0. 35 , Mn 1.30 ~ 1.50, P≤0.020, S≤0.005, Cr 1.80 ~2.00, Mo 0.17 ~0.25, Als 0.015 ~0.045) , through alloys rational division in converter-LF-RH process, the composition hit rate of the molten steel in tundish can be improved; using weak cooling and low speed casting (0.70 ~0.75 m/min) , pit cooling and slow heating (10-12 min/cm) , the slab cracking can be avoided； with thermo-mechanical rolling control process (TMCP) +580 -610 °C tempering process, the thickness 16 - 80 mm steel plate with hardness value 28 ~33HRC, cross-sectional hardness deviation value W3HRC, metallographic organization of tempered bainite, and eliminating internal stress of microstructure can be achieved. Finally, the performance and microstructures of the steel plate can meet the requirements of 2311 pre-hardened die steel.

The effect of C and V content on hardness of non-quenched-tempered pre-hardening plastic die steel (/% : 0.36-0.45C, 0.42-0.46Si, 1.41-1.49Mn, 0.11-0.15 V) has been tested and researched by a 50 kg medium frequency induction furnace in laboratory, and according to regression analysis the vanadium non-quenched-tempered prehardening plastic die steel 48MnV (/% : 0.45-0.50C, 0.30-0.60Si, 1.20-1.40Mn, ≤0.020P, ≤0.020S, 0.15-0.20V) is developed to replace the quenched-tempered plastic die steel P20 (/% : 0.35-0.40C, 0.40-0.60Si, 0.80 -1.00Mn, ≤0.015P, ≤0.015S, 1.50-1.80Cr, 0.30-0.50Mo ). The production results of 110 heats commercial production by 120t BOF-360 mm x 450 mm casting-≤85 mm flat rolling-inline pre-hardening treated with controled cooling rate 0.6-1.0 °C/s flowsheet show that the HRC hardness value of non-quenched-tempered flat of steel 48MnV is 29.5-33.5 to meet the requirement of relative specificalion.

In order to recover and apply the cutting scrap of die steel H13 and increase the yield of Cr, Mo, V etc alloy elements, the die steel H13 is melted by return cutting scrap method in 50 t medium frequency induction furnace- LF- VD flowsheet. The effect of curing pressed block and low vacuum melting process on Cr yield and the effect of BaO content in CaO-BaO compound slag series on dephosphorizing results are analyzed. Results show that with curing pressed block treatment and melting on ≤1 333 Pa low vacuum condition the yield of Cr, Mo and V in cutting scrap increases to more than 96% , as compared with non-using curing pressed block treatment, the melting time decreases by 25 min/heat. During melting period, dephosphorization process is carried out by high basicity CaO-20% BaO compound slag series, the P content in products shall be controlled to 0. 01%.

The designed tested steels (steel 0.45C-12. 5Cr-0. 41 Mo-0. 22V and steel 0. 85C-10. 5Cr-0. 91 Mo-0. 25V) are melted by a 50 kg vacuum induction furnace and forged to tested specimen. The effect of quenching temperature (950 ~ 1 150 ℃)，tempering temperature (once tempering at 200 ~ 400 ℃. and double tempering at 500 ~ 600 ℃)on structure, strength, elongation, hardness and impact energy of steels is tested and studied. Results show that the properties of steel 0.45C-12.5Cr-0.41 Mo-0. 22V quenched at 1 050 ℃+ double tempered at 500 ℃ are best： tensile strength 1712.3 MPa, yield strength 1 476.5 MPa, elongation 7. 8% , HRC hardness value 69. 3 and impact energy 7. 3 J. The hardness value of die steel increases by secondary hardening, but during tempering process the carbides in steel grow up with uneven distribution easily led to impact toughness of steel decreasing. The strength index of tested new stainless die steels is high than that of normal commercial die steel 42Gr3 Mo2MnVl.

With using the process measures including modifying the low basicity (2. 6) CaO-Al ₂O ₃ -SiO ₂ slag series to high basicity ( ≥5. 0) CaO-Al ₂0 ₃-SiO ₂-CaF ₂ slag series, increasing the [Als] in steel before LF refining from 0. 02% ~ 0. 03% to M 0. 06% , improving the atmosphere casting to box type Ar shielding casting and improving the 3t ingot body casting time from 6 ~8 min to 4 ~5 min, the [O] and [S] in cold-working die steel Cr12MoV are respectively [O]≤ 18 X 10 ^-6 and [S]≤ 0. 005% , and the comprehensive products yield increases from 74. 35% to 80. 12%.

The production flowsheet of steel P80R flat products (/% ： 0. 15C, 0.25Si, 1. 70Mn, 0. 012P, 0.002S, 3. 01 Ni, 0. 34Mo, 1. 07Cu, 1. 10AI) is 110 t LD-LF-RH-casting 4. 2 t ingot-breakdown 150 mmx615 mm slab-rolling to 60 mm x610 mm flats. It is obtained by examination and analysis that the Ni and Cu is seriously enriched near the cracks, the Ni content and Cu content in area near cracks are respectively up to 32. 55% ~ 33. 53% and 7. 97% 〜8. 86% , and the enrichment of Cu leads to surface grain boundary oxidation to form cracks, therefore the Cu content in steel, slab heating temperature and time should be controlled. By the process measures including decreasing Cu content in steel from 1.07% to 0. 92% , Al content in steel decreasing from 1. 10% to 0. 88% , increasing Ni/Cu from 2. 7 to 3. 3, reducing heating time at 1100 ~ 1140 ℃. from 30 ~50 min to 20 ~30 min and slab peeling treating, the occurrence rate of surface cracks of mirror plastic die steel P80R flats decreases from 70% to ≤0.5%.

The effect of solid solution treatment at 830~910°C and aging at 480~540°C on structure and hardness of Φ20mm plastic die steel NAK80 (/%: 0.15C, 0.30Si, 1.50Mn, 3.00Ni, 1.00Cu, 1.00Al, 0.30Mo, 0.30Cr) is tested and its aging kinetics behavior has been studied. Results show that after solid solution at 890℃，the structure of steel NAK80 is lath martensite and granular bainite, the HRC hardness value is up to 42.4, and with aging at 500℃ for 4h, the structure of steel is tempered troostite and precipitated phase, its HRC hardness value is up to 45.5. With calculat ing by using JMA equation it is obtained that the Avrami indexes n of steel aged at 480°C and 500°C are respectively 2.20 and 1.63 , and the apparent activation energy is 96.787J/mol.

The process flowsheet of commercial-produced steel 4Cr5MoSiVl (/% : 0.38~0.40C, 0.88~0.92Si, 0.42~0.44Mn, 0.001〜0.003S, 0.007~0.014P, 5.00~5.55Cr, 1.37~1.40Mo, 0.98~1.00V, 0.015~0.025 Alt) is 65% hot metal +35% scrap-100 t EAF-LF-VD-Φ500mm bloom casting. With the process measures including controlling EAF end [C]0.06%~0.10% and end [P]0.006% , pre-deoxidizing by adding 1kg/t aluminium block during tapping, controlling LF refining slag basicity 2.5~3.0, soft argon stirring for ≥10 min after feeding calcium wire, VD ≤67 Pa, argon stirring with 100 x2 L/min for ≥15 min, supper heat extent of liquid in tundish 20~30℃ , casting mold electromagnetic stirring with 310 A and 1.5 Hz in casting, shielding casting and casting speed 0.34~0.35 m/min, the [O], [N] and [ N ] in 10 heats of steel 4Cr5MoSiVl is respectively 10 x 10 ^-6 ~ 12 x 10 ^-6, 72 x 10 ^-6 ~ 80 x 10 ^-6 and 1. 2 x 10 ^-6〜1.4 x 10 ^-6 , and each index of steel all meet the requirement of specification.

The die steel 4Cr5MoSiV1 was produced with the path of hot metal + scrap→100 t EAF→double LF→ double VD→ round billet continuous casting (section Φ 500 mm)→ step reheating furnace→ 950 rolling mill,in the special steel division new district of Shandong Steel. There are defects in the detection of individual batches die steel 4Cr5MoSiV1. By using the ultrasonic testing to locate the defect location, and analyzing with the way of Metallographic analysis and scanning electron microscopy (SEM) , finally confirmed the center defect and the rolling compression ratio small are the main reasons of the testing substandard batch of steel 4Cr5MoSiV1, and the corresponding improved measures are put forward.

The microstructure, hardness and corrosion resistance of mold steel U338ESR for plastics treated at various solution and aging temperatures, were investigated by means of optical microscope (OM) , scanning electron microscope (SEM) , transmission electron microscope (TEM) , hardness tester and electrochemical analyzer. The results showed that with the increase of solution temperatures, which were at the range of 1 000 ~ 1 090 °C , alloying elements and NbC dissolved more into matrix, the uniformity of microstructures and the amount of martensite increased after water cooling, leading to the increase of hardness and the enhancement of corrosion resistance. When the aging temperatures were at the range of 300 -450 °C ，with the increase of temperatures, small Cu-rich phases nucleated and formed coherent bcc structure with the matrix and the coherent strain energy increased gradually, resulting in the increase of hardness and the decrease of corrosion resistance ； when the aging temperatures were at the range of 450 〜600°C , with the increase of temperatures, the Cu-rich phases transferred from coherent to incoherent state gradually, the coherent strain energy decreased, resulting in the decrease of hardness and the enhancement of the corrosion resistance. Reversed austenite formed at 600 °C, enhanced corrosion resistance further.

Hot compression tests of DIN 1. 2738 die steel for plastic (/% ：0. 35 ~0. 46C,0, 20 ~ 0.40Si, 1.30 ~ 1.60Mn,L 80 ~2.20Cr,0.90 ~ 1.20Ni,0.15 ~0.23Mo) are carried out by a Gleeble-3800 thermal-mechanical simulator to get stress-strain curves of steel at 850 ~ 1250℃  with strain rate 0. 01 ~30 s ^-1. Based on the hot deformation data,the peak stresses of steel, the strain-compensated constitutive equation and hot processing maps are established. Suitable hot deformation parameters are also identified by analyzing the processing maps. The results show that the hot deformation activation energy of DIN 1.2738 steel is 354. 21 kj/mol. Adopting the established strain-compensated constitutive equation, the flow curve can be predicted accurately. The suitable hot deformation parameters for DIN 1.2738 steel are： (1) temperature between 950 ℃ and 1150 ℃，strain rate between 0.01 s ^-1 and 0.7 s ^-1 ； (2) temperature between 1170℃. and 1200 ℃, strain rate between 0. 01 s ^-1 and 1 s ^-1.

The microhardness, impact toughness, wear resistance, phases composition and microstructure of die steel for auto engine crankshaft forging of boroning process at 850 ~950 ℃ 3 ~6 h are studied. The research results show that the steel boronized at 925 ℃ for 4 h has better performance, that is the maximum HV microhardness value of tlie boronizing lay¬er 1712,the thickness of the boronizing layer 58.6 pjn,and the impact toughness value at 400 ℃ and 24 ℃ respectively 39 J • cm ^-2 and 27 J • cm ^-2 ,and the wear amount after 90 min friction test 6.7 mg of die steel. The phase composition shows that the main phase of the boronizing layer is Fe ₂B, while the boriding layer,transition layer and matrix microstructure are obviously observed in microstructure.

Change rule the microstructure evolution and mechanical properties of 55NiCrMoV7 die steel under different quenching temperature (790~910°C) , and tempering temperature (100 ~650°C ) are studied by means of scanning electron microscopy (SEM) , hardness test, V-impact test, uniaxial tensile test combined with finite element modeling and simulation. The results show that, with the increase of quenching temperature, spherical caibide gradually dissolves into martensite matrix, martensite structure grows and coarsens, and residual austenite increases gradually； after quenching, the HRC hardness value is basically stable at 42 ~ 46, and the yield and tensile strength first increase and then decrease, reaching a maximum of 1380 MPa and 1485 MPa after quenching at 870 °C ,and the maximum value of impact toughness is 26 J after quenching at 850 °C. In different temperature tempering process, the content of martensite structure basically stable, with the increase o£ tempering temperature, the retained austenite gradually disappears, fine and dispersed carbides begin to precipitate at the boundary of martensite. Quenching at 870°C 4 h and tempering at 560°C 6 h can make 55NiCrMoV7 steel to have good comprehensive mechanical properties.

The microstructure and hardness of precipitated hardening die steel 0Cr3Ni3MoCuAl with solution-treated at 860~1010 ℃ for 35 min and aging treated at 450~560 ℃ for 3 h are tested and studied.The results show that when the solution treatment temperature is 950 ℃,the holding time is 35 min and air cooling to room temperature,the HRC hardness number is up to 39.5,the structure is granular bainite and low carbon lath martensite and a small amount of residual austenite.When the aging-treated at 500 ℃ for the holding time 3 h and air cooling to room temperature,the HRC hardness number is up to 44.1 and the microstructure is tempered bainite.

The research and application achievements of die casting steel materials in China are introduced.By studying the influence of alloying elements on material properties of die material, a new die casting die steel with low Si, V and Ni, CO and W has been developed. By reducing the content of S in steel, using RE to change the shape of inclusion and optimizing forging process, the isotropic properties of the die-steel can reach more than 90%；To make a comparison between conventional electric slag, gas shielded electric slag and vacuum consumable  process remelted three die-casting material,  nitrogen in steel is respectively  150×10 ^-6,90 ×10^-6 and 40×10 ^-6, oxygen content in steel is respectively 24×10 ^-6, 15×10 ^-6 and 6×10 ^-6; By ASPEX Explorer metal quality analyzer inclusions index statistics and analysis of three kinds of technology, using vacuum consumable smelting exponential decline in non-metallic inclusions, non-metallic inclusions and no larger than 5 um, material impact performance boost, 7 mm × 10 mm gap impact energy can reach 450 J；To develop high-end die-casting mould material microstructure, the non-notched impact evaluation index standard and specification are established with the development of integrated die casting technology, the future mold material shall develop toward large-scale, super pure, high precision and long life direction.