introduce
65NB mold steel is a niobium containing matrix steel. The content design of alloying elements Cr, W, Mo, and V in steel is taken from the matrix composition of quenched W6Mo5Cr4V3 high-speed steel. The role of alloying elements in the mold is similar to that in high-speed steel.
A small amount of niobium, a strengthening carbide forming element, is added to the steel grade to form highly stable NbC with the carbon in the steel grade, preventing the growth of austenite grains during quenching and heating. Compared with mold steel without Nb, the grain refinement temperature of Aishiti is increased by 40-50 ℃, and Nb is partially dissolved in carbides of Cr, W, Mo, and V. The stability of the enhancer reduces the collective carbon content after quenching, significantly improves the strength and toughness of the steel, and improves the processability of the mold steel. The chemical composition of 65Nb mold steel is shown in the table below.
2 Mechanical properties
65Nb mold steel has high strength and toughness, which is significantly improved compared to the parent material W6Mo5Cr4V2 and high carbon and high chromium mold steel. When applied to cold extrusion and cold heading molds with pressures below 2450MPa, its service life is doubled compared to high-speed mold steel and high carbon and high chromium mold steel. However, when applied to molds with pressures exceeding 2450MPa and requiring high wear resistance, the compressive yield strength and wear resistance of 65Nb mold steel appear insufficient.
3 Process performance
Forging process
65Nb mold steel belongs to the Lai style body mold steel and needs to be forged. The forging performance of 65Nb mold steel is good, but it should be heated slowly to ensure complete burning. Forging with slow cooling.
In order to break, refine, and evenly distribute the original banded carbides and network carbides, for upsetting billets, especially large-sized billets, they should be repeatedly upsetting and pulled after modification forging; For molds with cutting edges, such as edge cutting molds, after repeated upsetting and pulling, the phenomenon of edge peeling is basically overcome, and the service life is increased by 4-5 times compared to molds that only undergo pulling.
Annealing process
There are two annealing processes for 65Nb mold steel: conventional spheroidization annealing and isothermal spheroidization annealing. The conventional annealing process involves heating at 800 ℃ for 3-4 hours and slowly cooling to 500 ℃ before exiting the furnace; The isothermal spheroidization annealing process involves heating at 860 ℃ for 3-4 hours, cooling to 740 ℃ for 5-6 hours, and then cooling to 500 ℃ before exiting the furnace. After annealing, the hardness is 217HBS. If the isothermal time is extended from 6h to 9h, the hardness will further decrease to 187HBS, and cold extrusion forming can be used, which is a major advantage of 65Nb die steel.
Quenching and tempering process
The quenching temperature of 65Nb mold steel is 1080~1180 ℃, and the quenching heating time should ensure that the carbides are fully dissolved and homogenized, while not causing grain growth. The heating coefficient in a salt bath furnace should be 15~20s/mm. According to the shape of the mold and the requirements for deformation, cooling methods can include oil cooling, oil quenching air cooling, and graded quenching. 65Nb mold steel is generally subjected to secondary tempering, with a tempering temperature range of 520~560 ℃.
The hardness values obtained through quenching at different temperatures and secondary tempering at different temperatures will all exhibit secondary hardening during the tempering process. The peak hardness value appears between 520~540 ℃ and increases with the increase of quenching temperature.
4 Application scope
65Nb mold steel is a high-strength and toughness dual-purpose cold and hot mold steel, widely used in the production of various cold work molds, especially suitable for cold extrusion molds for large or difficult to deform metals and cold forging molds with high impact loads. It is sometimes also used for hot work molds, but mainly for cold work molds. 65Nb mold steel can also be used for warm molds of steel materials. Due to insufficient compressive strength and wear resistance of 65Nb mold steel, it cannot be used for extruding steel materials with a pressure of 2500MPa and molds that require high wear resistance.
65Cr4W3Mo2VNb (65Nb) steel
Standard: GB/T 9943-1988
chemical composition 
Carbon C: 0.50-0.60 (allowable deviation: ± 0.01)
Sulfur S: ≤ 0.030
Phosphorus P: ≤ 0.030
Chromium Cr: 3.80-4.40 (allowable deviation: ± 0.05)
Nickel Ni: allowable residual content ≤ 0.30
Copper Cu: allowable residual content ≤ 0.25
Vanadium V: 1.00-1.20 (allowable deviation: ± 0.05)
Molybdenum Mo: 2.00-2.20 (allowable deviation: size ≤ 6, ± 0.05; size>6, ± 0.10)
Tungsten W: 2.95-3.25 (allowable deviation: size ≤ 10, ± 0.10; size>10, ± 0.20)
High speed base steel 1. 65Cr4W3Mo2VNb (65Nb) steel
(1) The annealed billet is annealed according to the process shown in Figure 9-4, with a hardness of around 217HBS. By extending the isothermal time at 740 ℃ from 6h to 9h, the hardness decreases to around 187HBS; If the time is shortened to 3 hours, the hardness will be 217-229HSS.