The development of high speed steel rolls by extrusion casting

Introduction

The development of wire rod rolling technology has set higher demands on the properties

of rolls. Previous investigations have indicated that the service lives of traditional rolls such as Ni-

Cr cast iron rapidly chilled rolls, bainitic ductile iron rolls and high Cr cast iron rolls are shorter than

that of hard alloy rolls.15 Hard alloy rolls have excellent wear resistance. However, the cost is

much higher. Technical developments in the field of rolling steel in recent years have been made

to improve the quality of steel products, improve productivity and reduce the manufacturing costs.

With this technical development, the need for the highly reliable roll with a high performance and

long service life has extensively grown. High speed steel (HSS) has the characteristics of high

hardness, good red hardness and fitness for manufacturing of parts working at high temperature

such as rolls, guide rollers and rolling mill guides. An HSS roll is usually manufactured by

centrifugal casting. Owing to different densities of the alloy elements, the centrifugal force enriches

the high density elements (e.g. W and Mo) in the outer layer of the roll, and enriches the low

density elements (e.g. V) in the inner layer, which causes serious segregation in the roll, decreases

the mechanical properties and reduces wear resistance. In order to resolve the segregation

problem in HSS rolls, Ichino et al.6 developed techniques by decreasing the W and Mo contents and

adding a suitable amount of Nb. However, decreased W and Mo contents were found to reduce the

red hardness and high temperature wear resistance of the HSS. Therefore, it is necessary to

develop a new process for manufacturing HSS rolls. The HSS roll manufactured by means of

extrusion casting allowance, no segregation and a simple procedure. It is an effective method of

replacing centrifugal casting for manufacturing HSS rolls.

Composition of the HSS roll

The idea of devising the composition of an HSS roll is as follows. In order to reduce the cost

scrap M2 HSS is added to the HSS roll, replacing tungsten iron and molybdenum iron. Chromium

iron, vanadium iron and graphite are used to complement Cr, V and C. Because V is an essential

element forming high hardness MC type carbide and providing the HSS roll with good wear

resistance, the V content must be over 3%. However, when the V content is over 8%, the melting

point of molten steel rises, the flowing property falls and low hardness M^sub 3^C carbide begins

to appear.7 The V content is limited to between 3.5% and 6.5%.

Experimental

The HSS was melted in a 150 kg medium frequency induction furnace and deoxidised by

aluminum. The REM used in this steel was mainly mixed REM wires in which the mass fraction of

La+ Ce was larger than 98%. The discharge temperature of molten steel was 1600°C.

The equipment used in HSS roll extrusion casting is a 160 t four-post hydraulic press. The

outer diameter, inside diameter and length of the roll are 300, 145 and 100 mm respectively. The

liquid metal, which is poured into the mould and exerted and kept under pressure, begins to

crystallise and solidify. Figure 1 is a sketch of the extrusion castingprocess. The heat treatment

samples with the dimension of 15 × 15 × 25 (mm) were cut from the HSS roll.

Results and discussion

The effects of pressure, pressing time and pressing speed on shrinkage cavity volume

The effects of pressure and pressing time, as well as pressing speed on the shrinkage

cavity volume were investigated. The effects are shown in Figs. 2-4. When the pressure is small,

the shrinkage cavity volume is a constant. When the pressure is over 80 MPa, the shrinkage cavity

volume sharply decreases until it reaches zero, indicating that there is a critical and compacting

pressure in extrusion casting. When the pressure is small, the pressure on the solidified shell is

smaller than the yield limit of the material. For this reason, the pressure will not perform the filling

contract. The curve remains horizontal while the pressure is smaller than the critical pressure. In

the experiment, it was found that a small pressure can enlarge the workpiece shrinkage cavity

volume. The fast thermal transmission in the pressure head quickens the solidification of the

workpiece top and the workpiece forms a molten metal become surrounded by the closed solidified

shell. Neither pressure feeding nor free feeding take place, so the shrinkage cavity volume is larger

for small pressure than for no pressure. When the critical volume of 80 MPa is reached, the

shrinkage cavity volume sharply reduces. When the pressure reaches a compacting pressure of

120 MPa, the shrinkage cavity volume reduces to zero. In the initial stage of extrussion casting, the

shrinkage cavity volume sharply reduces with increasing pressing time and then tendency

moderates. Pressing time and solidification time of the workpiece should coincide. If discharging

pressure in solidification, the volume shrinkage causd in solidification of the roll cannot sustain

feeding, and a shrinkage cavity is formed. If the pressing time is too long, a thermal crack may

arise because the shrinkage of the roll is hindered. The shrinkage cavity volume obviously reduces

as pressing speed increases. When the pressing speed is small, only a partial pressure filling

contraction can be obtained. The shrinkage cavity remains. When the pressing speed is over 14

mm/s, the shrinkage cavity volume is zero. If the pressing speed is too large, the wear of the mould

will increase and the roll may easily crack. The technological parameters of the HSS roll

extrusion casting are as follows: pouring temperature is 1400-1450°C; preheating temperature of

the mould is 180-240°C; the pressure is 150 MPa; the pressing time is 120-150 s; and the pressing

speed is 14-16 mm/s. The extrusion cast rolls have no segregation and the quality of appearance is

superior to rolls produced by common casting methods. The working allowance decreases by 50%

from that of the common casting and the extrusion casting can save metal and working time.

Heat treatment of the HSS roll

The effect of quenching temperature on the HSS hardness is shown in Fig. 5. The hardness

gradually increases with a rise in the quenching temperature, peaking at 1050°C. This is because

the hardness is determined by the alloy structure, the quantity of C and alloy elements in the

martensite, and the nontransformation residual austenite. When the quenching temperature is

lower, the quantity of dissolved C and alloy elements in austenite is smaller, and the quantity of C

and alloy elements in martensite after transformation is also smaller, while the hardness is lower.

When the temperature reaches 1050°C and the temperature continues to increase, excessive

dissolved C and alloy elements in austenite will increase the stability of austenite. The austenite

has no time to transform to martensite during the cooling, which increases the retained austenite

and decreases the hardness. When the temperature is about 1050°C, the peak value of the

hardness can be obtained because the C and elements contained in martensite and the retained

austenite all reach a suitable level.

The tempering treatment of the HSS roll was performed after oil quenching. The effect of

tempering temperature on the hardness of the HSS is shown in Fig. 6. With a rise in the tempering

temperature, the hardness first reduces gradually. It begins to rise at 475°C and reaches a

maximum at 525°C. When the tempering temperature is below 475°C, the C in the martensite is

separated out, forming carbide. Dispersion hardening cannot occur because the carbide content is

small, which decreases both the C content in the martensite and the hardness of the HSS. When

the tempering temperature is about 525°C, the martensite transforms to tempering martensite,

which is separated out into low mass alloy carbides. Residual austenite transforms to martensite

while cooling, which hardens the steel more quickly giving a greater maximum hardness. When the

temperature continues to rise, the alloy carbides begin to gather and grow, which causes the

hardness level to drop. The difference between this and conventional HSS is that the temperature

of the peak hardness reduces. In contrast with conventional HSS (with a quenching temperature

usually above 1150°C, see Ref. 13), the quenching temperature is low. The dissolved C and alloy

elements in the high temperature austenite are smaller and the high temperature austenite is not

stable. The residual austenite content in the quenching structure is smaller and its stability is

lower. The residual austenite is easily transformed to martensite as the tempering temperature is

very low. Therefore, the tempering temperature at which the hardness occurs in the peak value

reduces. In addition, owing to non-stability of residual austenite, the HSS can reach the maximum

hardness after tempering twice. Conversely, the hardness descends after tempering three times.

According to the above results, the heat treatment process of the HSS roll is: 1025-1050°C × 2 h,

oil cooling +520-540°C × 6 h, and air cooling (twice). Figure 7 shows the structure of the heat

treated HSS roll. From Fig. 7, it can be seen that the structure of the HSS roll is very fine and the

carbides are homogeneously distributed. These factors facilitate the increase in the wear

resistance of the HSS.

Effects of extrusion casting on HSS properties

Application of extrusion cast HSS rolls

Service results of extrusion cast HSS rolls

The extrusion cast HSS roll used in the pre-finishing train of a 105 m/s wire rod rolling mill

and rolled metal was plain C steel, and its diameter was 6.5 mm. The results showed that the wear

of the extrusion cast HSS roll was 0.15-0.20 mm when rolling 1000 t of steel. In the use of HSS roll,

the wear of the groove was homogeneous, there was no alligator effect, wear resistance was

better, the surface of rolled metal was smoother, dimensional accuracy was higher and the service

life was five to eight times higher than that of rapidly chilled high Ni-Cr cast iron roll. The

application of extrusion cast HSS roll can elevate the operating rate of rolling mill, reduce the

production cost of rolled metal and generate economic benefit.

Precautions in application of HSS rolls

HSS rolls have properties that are different from other rolls. The cooling of the HSS roll is

stronger than for high chromium cast iron rolls and rapidly chilled high Ni-Cr cast iron rolls. Enough

cooling water should be present, 75% of which should be applied on the side of the hot metal exit.

The surface temperature of the roll should be held below 50°C. Usually the surface temperature of

the roll is measured after the cooling water has stopped for 10-12 min. The quantity of cooling

water should be adjusted according to the surface temperature of the roll. Requirements of the

temperature of the roll surface for different roll materials are shown in Table 3.

The friction coefficient between the HSS roll and the rolled metal is high. Slipping can easily

occur when the rolling load increases. The deformation of each framework must be properly

controlled so as to reduce the rolling load. After grinding, the HSS roll must be examined by eddy

current testing. The roll cannot be reused until the crack and the fatigue layer are completely

eliminated.

Conclusions

This study was carried out to develop extrusion casting techniques for HSS rolls. The

effects of the extrusion casting process on the properties of the HSS roll were investigated. The

results obtained are summarised as follows.

1. By adding vanadium iron and chromium iron to scrap M2 HSS, increasing C by the use of

graphite, and inoculating treatment by titanium and REM, a high C and high V HSS with excellent

properties can be obtained which can be used as the rolling roll.

2. The HSS roll is manufactured by means of extrusion casting. Through monitoring pressure,

pressing time and pressing speed, the obtained roll has no shrinkage cavity, no flaw, dense

structure and small working allowance. Extrusion casting HSS roll also improves strength,

toughness and wear resistance.

3. A rise in the quenching temperature can cause a rise in hardness of the HSS. When the

temperature is over 1050°C, conversely, the hardness can reduce. When the tempering

temperature of the HSS reaches 525°C, the hardness is at its peak value. Maximum hardness can

be obtained through tempering twice. The hardness decreases after tempering three times.

4. By using extrusion cast HSS rolls in the prefinishing train of a high speed wire rod rolling mill, its

service life is five to eight times higher than that of high Ni-Cr rapidly chilled cast iron rolls.