The Development of High Hard and 
Tough Matrix Type High Speed Tool Steels 
By SHUN-SUKE NAKAHAMA*1, YUKINORI MATSUDA*2, KUNIO NAMIKI*3, KOZO OZAKI*4

3. The concept of the development(continued)

3.4 The development of the new steels

The fundamental concept to develop new steel is to decrease primary carbides and the specific measures are summarized as follows. 

1. Alloy designing to optimize the carbide type and the amount of carbides 
2. Production technologies to refine primary carbides To control carbides solution and precipitation in hot forging and rolling processes 
3. Three steels to meet wide variety application from cold to hot forging 

First, by using the phase calculation software, Thermocalc, we have tried to attain the chemistry with which there exist austenite single region in wide temperature range. The result is shown in Fig. 10 (below). In spite that there is no austenite single phase region with AISI M2, we can enlarge this region with new alloy designing.

Next, Fig. 11 (below) shows the effect of solution time, carbide types and initial carbide sizes on carbide solution behavior obtained by calcu1ation.8~ It is clear that it requires long time to solute large sized carbides. And this solution time can be shortened by controlling the original primary carbides size.

a) Effect of carbide types.   b) Effect of carbide sizes. 

For this purpose Electro Slag Remelting is applied for the production of the new steels to reduce original primary carbide size. Figure 12 (below) shows the schematic structure and the features of ESR. ESR is progressing type melting in water cooled Cu mold with using specially blended slag and by resisting heat. Therefore, molten steel in the poo1 can be refined and solidified rapidly, which results in clean and uniform steel with finely distributed carbides from the surface to the center and from the top to the bottom as well. The examples of the microstructures of high speed steel M2 melted by conventional furnace and ESR are shown in Fig. 13 (below). The steel produced by ESR shows finer carbides and less segregation than conventionally melted steel. ESR also can reduce sulfur content to 0.00 1% level resulting clean steel with extremely small amount of MnS.

Thirdly, finely solidified primary carbides are remained to be fine by controlling the heating temperature prior to forging and re-heating temperature for final forging or rolling. 

By applying these technologies we have newly developed two steels: DRM1 and 2. Furthermore, by adding high hardness type high carbon steel, DRM3 which contains a little coarse primary carbides, we have completed DRM series : Daido's Revolutionary Matrix type High Speed Tool Steels and launched to the market. Principal chemistries of these steels are shown in Table2.

<PREV  |  TABLE OF CONTENTS  |  NEXT>