UDC 669.14.018.255

CHOICE OF STEEL FOR ROLLING MILL ROLLS

I. A. Borisov1 and S. S. L’vova1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 6, pp. 8 – 14, June, 2009.

The most important properties of test Cr – Ni – Mo – V-steels, answering the main requirements specified for

roll steels are studied: quenchability, hardenability, tempering stability, mechanical properties and impact

strength, contact fatigue resistance. Dependences on roll cooling rate are determined for these properties. The

Cr – Ni – Mo – V-steels with a carbon concentration of 0.50 – 0.85% developed on the basis of these studies

are proposed for industrial approval.

Key words: roll steels, mechanical properties, quenchability, hardenability, contact fatigue resis-

tance, cooling schedules.

INTRODUCTION

Among the problems facing contemporary rolling pro-

duction an important place is occupied by development of

materials for a new generation of rolls. Here it is necessary to

consider not only the level of contact stresses, reaching

1100 MPa or more, but also the size of rolls. For example,

the supporting rolls may have a barrel diameter up to

2200 mm. In addition, attention should be drawn to the

whole metallurgical cycle of roll production, including final

heat treatment technology. Alongside heating with high and

industrial frequency current, use is being started of cheaper

volumetric and surface heating of rolls for hardening, and

also controlled water-air cooling in special devices [1 – 4].

In view of this for the new generation of roll materials, in

addition to traditional high surface hardness, requirements

are laid down for improved hardenability that should provide

a sufficient thickness of active layer and a smooth change in

hardness from the surface of the hardened layer to the core of

the roll, and also a high set of strength and ductility proper-

ties for the barrel and neck. This does not remove a require-

ment for providing traditional specifications for wear resis-

tance, cracking and fatigue resistance. This set of properties

may be provided, as our studies have shown, solely for steels

developed on a base of a Cr – Ni – Mo – V-composite

[5 – 7].

The aim of this work is to study the effect of carbon con-

centration within the limits 0.45 – 0.85% on the physical and

mechanical properties of test roll Cr – Ni – Mo – V-steels and

the choice of this base for rational alloying limits.

METHODS OF STUDY

In choosing a steel composition for study attention was

drawn to the requirement for obtaining properties that satisfy

the specifications for roll material. Previously it has been

shown that [5] in order to achieve high hardenability in steels

they should contain chromium and nickel simultaneously. In

view of this the steel composition contained 1 – 4% Cr and

1.5% Ni. In order to obtain high steel thermal stability up to

1% Mo was added alongside chromium. It is well known that

in order to obtain high reliability during operation, and also

expansion of the operating temperature range, elements are

required both for austenite grain refinement and increasing

the temperature for the start of rapid grain growth. As a result

of this vanadium, niobium (0.06%) and aluminum were

added to the steel. The high strength and wear resistance

should be provided by carbon (0.45 – 0.85%), and also car-

bide-forming elements, i.e. chromium and vanadium [6]. In

addition, a nickel-free steel was proven with a high silicon

and manganese content (up to 1% of each).

Steel smelting was carried out in a 150-kg induction fur-

nace. Pouring was carried out into moulds with a capacity of

50 kg. after pouring and cooling to 300°C the ingots were ex-

tracted from the mould and heat treated in a chamber furnace

at 700°C, 10 h. Hot deformation of ingots was performed by

compaction and forging. The billets were obtained with a di-

ameter of 30 – 50 mm for subsequent specimen manufacture.

With the aim of crack prevention for these billets after defor-

mation and cooling to 300°C they were placed in a furnace

where they were annealed at 600°C for 10 h.

After hot deformation billets were heat treated, i.e. iso-

thermal annealing by a regime: heating to 850°C, holding for

2 h, furnace cooling to 700°C, holding for 20 h, further cool-

Metal Science and Heat Treatment Vol. 51, Nos. 5 – 6, 2009

272

0026-0673/09/0506-0272 © 2009 Springer Science + Business Media, Inc.

1Central Research Institute for Heavy Mechanical Engineering

(NPO TsNIITMASh JSC), Moscow, Russia.

ing to the workshop temperature. The chemical composition

is given in Table 1 for the steels and critical temperatures A1,

determined on heating specimens in a dilatometer at a rate of

200 K�h. Quenchability for the test steels was determined

from the results of measuring the hardness of specimens after

water quenching from 800 – 1050°C, soaking time 1 h. Har-

denability was evaluated by the standard procedure of end

quenching from three temperatures: 850, 900 and 950°C.

Resistance to loss of strength during tempering was eval-

uated from the reduction in hardness after quenching from

900°C (0.5 h) and tempering at 20 – 650°C, 2 h. Mechanical

properties of the test steels were determined with tensile test-

ing for specimens 6 mm in diameter (GOST 1497–84) and

impact of type 1 specimens (GOST 9454–78). Heat treat-

ment of billet specimens for mechanical tests was; quenching

from 900°C (0.5 h). Here with the aim of approaching quen-

ching conditions for actual objects specimens were cooled in

oil, in air and in a device for simulating cooling rates with

vcool

= 500, 130 and 70 K�h.

For example: in the case of quenching a roll with a barrel

diameter of 1400 mm in a sprayer unit after volumetric heat-

ing [average heat transfer coefficient during cooling is

3000 W�(m2� K)] cooling rate in the temperature range

700 – 300°C is: in the center 85 K�h, at the center of the ra-

dius 90 K�h, at one third of the radius 125 K�h, and at the

surface 40,500 K�h.

Testing for contact fatigue resistance of the test steels

was carried out in an MKV-K machine. Special cylindrical

specimens with a projecting conical surface with a track

width of 0.45 mm were tested. Contact of specimens with

circular diameter of 200 mm, made of steel ShKh15, was

carried out by means of a special clamping spring. The con-

tact strength was estimated from the number of specimen ro-

tations to development of pitting failure, and this was re-

corded by a special signal device. Heat treatment of speci-

mens was: oil quenching from 900°C, tempering at 200°C,

2 h. It was not possible to determine the contact strength

limit by the experimental conditions. In view of this an esti-

mate was made of its qualitative property, i.e. the number of

cycles to failure with a constant nominal load. The structure

and nature of specimen failure was studied metallogra-

phically and by a fractographic method.

RESULTS AND DISCUSSION

Steel Quenchability

It is well known that steel quenchability depends mainly

on the concentration of carbon and alloying elements, and

mainly carbide-forming elements [8]. Results of measuring

hardness after water quenching for the test steels are pro-

vided in Table 2. It can be seen that for all of the steels there

are three temperature ranges for a change in hardness:

800 – 850°C for an increase in HRC; 900 – 950°C for the

maximum HRC value; 1000 – 1050°C for a reduction in

HRC. An increase in hardness as the quenching temperature

increases is connected with phase transformation and subse-

quent dissolution of carbide phase in austenite, that increases

the alloying capacity for austenite and the hardness of

martensite formed within it. Insignificant oscillations in the

hardness value for steel after quenching from 900 – 950°C is

connected with stabilization of the austenite composition,

and also the opposite effect on hardness of two processes:

1 ) an increase in austenite alloying capacity; 2 ) an increase

in the amount of residual austenite ion the quenched struc-

ture. A reduction in hardness in the temperature range

1000 – 1050°C is caused by grain growth and a further in-

crease in austenite alloying capacity due to vanadium car-

bide, that leads to a reduction in martensitic transformation

temperature, and as a consequence of this to an increase in

residual austenite. It should be noted that in spite of the simi-

lar nature of the change in hardness for the test steels it dif-

Choice of Steel for Rolling Mill Rolls 273

TABLE 1. Chemical Composition and Test Steel A1 Critical Temperature

Steel

Alloying element content, %

A1

, °C

C Si Mn S P Cr Ni Mo V Al

50Kh2NMF 0.49 0.30 0.43 0.013 0.015 1.80 0.65 0.65 0.10 0.006 780 – 815

55Kh3NMF 0.56 0.38 0.40 0.011 0.010 2.70 0.70 0.60 0.13 0.011 780 – 830

55Kh4NMF 0.55 0.33 0.40 0.015 0.016 3.89 0.60 0.58 0.11 0.009 785 – 840

80Kh2GSMF 0.81 0.88 0.90 0.018 0.013 2.20 – 0.40 0.09 0.005 760 – 810

80KhNMF 0.78 0.35 0.45 0.017 0.015 1.30 1.20 0.60 0.14 – 770 – 810

70Kh3NMF 0.72 0.35 0.42 0.016 0.015 2.85 0.60 0.35 0.12 0.015 780 – 800

TABLE 2. Steel Hardness after Water Quenching from Different

Temperatures

Steel

Hardness HRC after quenching from temperature, °C

800 850 900 950 1000 1050

50Kh2NMF – 57 58 58 57 56

55Kh3NMF – 53 58 61 59 58

55Kh4NMF – 53 58 59 58 57

80Kh2GSMF – 61 63 64 59 57

80KhNMF – 63 64 64 57 53

70Kh3NMF 53 60 65 65 62 61

fers in value. The hardness of steel 70Kh3NMF after quench-

ing from the temperature of the first range exceeds the hard-

ness of other steels, and this is explained by the narrow phase

transformation range for it (780 – 800°C). The latter indi-

cates that steel composition is balanced and close to the

eutectoidal. Also noted is the high hardness of the steel over

the whole test heating temperature range for hardening.

Metallographic analysis showed that rapid austenite grain

growth for this steel occurs after heating above 950°C due to

presence within it of niobium carbonitride. This points to

presence of a broad temperature range of heating for harden-

ing for this steel and its advantage with respect to this index

over other steels. It should be noted that with respect to hard-

ness steel 80Kh2GSMF is only slightly better than steel

70Kh3NMF. Among other composites it possible to separate

steel 55Kh3NMF, that has a stable hardness of 58 – 61 HRC

in the hardening temperature range 900 – 1000°C.

Test Steel Hardenability

Steel hardenability was studied for two groups broken

down with respect to carbon content: 1 ) 0.45 – 0.60% C;

2 ) 0.65 – 0.85% C. Hardenability curves for the test steels

are presented in Fig. 1. It can be seen that for all cases an in-

crease in heating temperature fro hardening from 85 to

900 – 950°C leads to a marked increase in hardenability, i.e.

its depth increases from 20 – 30 to 100 mm or more. This is

connected with the fact that with tq

= 850°C austenite is suf-

ficiently alloyed, and with distance from the surface there is

conversion of austenite into bainite and even into pearlite.

An increase in hardenability with tq

> 900°C points to pre-

sence of a reserve with respect to an increase in the depth of

hardenability with an increase in the temperature fro harden-

ing. However, in steels of the first group with a carbon con-

tent of 0.45 – 0.60% (Fig. 1, a – c) only in the composition

55Kh3NMF is it possible to achieve a hardness above

60 HRC. For all of the steels of the second group with 0.65 –

0.85% C the hardness is above 65 HRC (see Fig. 1, d – f ).

Steel 70Kh3NMF has the best indices for this property among

all of the test compositions: the hardness varies within the

limits 65 – 62 HRC and the average scatter is less than

1 HRC. For comparison: similar properties for steel 80KhNMF

are 64 – 60 HRC and 2 HRC, for steel 80Kh2GSMF they are

64 HRC and 3 HRC, respectively. Analysis of the data ob-

tained that in the first group of test alloy composition steels

55Kh3NMF is preferred, and in the second group steel

70Kh3NMF is preferred.

274 I. A. Borisov and S. S. L’vova

HRCe

HRCe

HRCe

HRCe

HRCe

HRCe

60

55

50

45

65

60

55

50

55

50

45

55

50

45

55

50

45

55

50

45

40

3

3

3

3

3

3

2

2

2

2

2 2

1

1

1

1

1 1

à

b

e

d

c

f

0 20 40 60 80 0 20 40 60 80h, mm h, mm

Fig. 1. Hardenabilty curves for steels

50Kh2NMF (a), 55Kh3NMF (b ),

55Kh3NMF (c), 80Kh2GSMF (d ),

80KhNMF (e) and 70Kh3NMF ( f )

(h is distance from the surface):

1, 2, 3 ) heating temperatures for har-

dening 850, 900, 950°C, respectively.

Stability and Strength Loss During Tempering

This is an important property of roll steels, since it af-

fects the contact fatigue resistance of the roll surface, in par-

ticular it governs the sensitivity of the metal towards surface

tears and chips. In addition, the supporting rolls are subject

to tempering at 500°C, and the cold working rolls are subject

to tempering at 600°C in order to obtain a high set of proper-

ties for the core of the roll and the neck. It should be noted

that loss of strength for the test steels with an increase in

tempering temperature occurs with approximately the same

intensity (Table 3). Some lower tendency is noted towards

loss of strength of steels 80Kh2GSMF and 70Kh3NMF, that

after tempering at 200°C have a hardness of 60 and 61 HRC,

respectively. The higher temper stability of steel 80Kh2GSMF

may be explained by the high silicon content. At the same

time steels 50Kh2NMF and 80KhNMF have the lowest tem-

per stability. After tempering at 650°C the hardness of all of

the test steels levels out and it is within the limits of

30 – 32 HRC.

Steel Mechanical Properties

The properties of the three most promising test steels

55Kh3NMF, 80KhNMF and 70Kh3NMF are provided in

Fig. 2, and values of the dimensionless parameter �

0.2��

rare

given in Table 4. it is noted that all of the steels have high

Choice of Steel for Rolling Mill Rolls 275

� �

r 0.2, ; ÌPà

� �

r 0.2, ; ÌPà

� �

r 0.2, ; ÌPà

1200

1000

800

600

1200

1000

800

600

1800

1600

1400

1200

1000

800

600

� �, ; %

� �, ; %

� �, ; %

KCU,

ÌJ m� 2

KCU,

ÌJ m� 2

KCU,

ÌJ m� 2

40

30

20

40

30

20

30

20

18,000 2000 500 130 70

18,000 2000 500 130 70

18,000 2000 500 130 70

vcool

, K h�

vcool

, K h�

vcool

, K h�

�

r

�

r

�

r

�

0.2

�

0.2

�

0.2

�

�

�

�

�

�

KCU

KCU

KCUà b

c

0.2

0.1

0.5

0.4

0.3

0.6

0.5

0.4

0.3

0.2

Fig. 2. Dependence of mechanical properties

for steels 55Kh3NMF (a), 80Kh2GSMF (b ),

70Kh3NMF (c) on cooling rate during quench-

ing from 900°C (0.5 h): solid lines are temper-

ing at 500°C; broken lines are at 600°C.

TABLE 3. Steel Hardness after Tempering for 10 min at Different

Temperatures

Steel

Hardness HRC after tempering at temperature, °C

20 200 300 400 500 550 600

50Kh2NMF 58 55 52 49 46 44 40

55Kh3NMF 58 57 55 52 48 45 42

55Kh4NMF 58 56 53 52 48 44 40

80Kh2GSMF 63 60 56 53 50 47 44

80KhNMF 64 56 51 48 46 43 40

70Kh3NMF 65 61 56 53 48 45 42

strength properties with high cooling rates (18,000 –

20,000 K�h). As the cooling rate is reduced to 500, 130, and

70 K�h the strength properties are reduced, and as a rule the

ductility properties are increased significantly. Here an unex-

pected effect was detected, i.e. a reduction in impact

strength. Steels 55Kh3NMF and 70Kh3NMF have a favor-

able combination of strength and ductility properties. After

with the minimum and maximum cooling rates (18,000 and

70 K�h) and tempering at 600°C, steel 70Kh3NMF has the

following properties: �

r= 1250 and 700 MPa, �

0.2= 1100

and 500 MPa, � = 11 and 17%, � = 43 and 48%, KCU = 0.45

and 0.25 MJ�m2. Steel 55Kh3NMF after quenching with

similar cooling rates and tempering at 500°C, has the follow-

ing properties: �

r= 1350 and 850 MPa, �

0.2= 1200 and

550 MPa, � = 18 and 22%, � = 56 and 52%, KCU = 0.67 and

0.47 MJ�m2. As follows from analyzing the values of dimen-

sionless parameter and the microstructure of steels, with

vcool

= 18,000 and 2000 °Ch there is austenite decomposition

by a martensitic mechanism, and with vcool

= 500 K�h it is

bainitic decomposition, and with vcool

= 130 and 70 K�h it is

mixed in the pearlitic and bainitic regions. The level of me-

chanical properties for these two steels makes it possible to

recommend them for manufacturing rolls with a barrel dia-

meter up to 1400 mm. In order to use these steels for large

diameter rolls it is necessary to increase their heating tempe-

rature for hardening to 930 – 950°C. Here it is desirable to

introduce into their composition strong carbide-forming ele-

ments such as zirconium and tantalum.

Steel 80Kh2GSMF in nature of the change in mechanical

properties is markedly different from the steels considered

above. With the two extreme cooling rates vcool

= 18,000 and

70 K�h and two tempering temperatures 500 and 600 °C, the

steel has the following level of properties: �

r= 1840 and

1040; 1380 and 1000 MPa, �

0.2= 1640 and 550; 1250 and

500 MPa, � = 6 and 14; 12 and 17%, � = 15 and 33; 25 and

40%, KCU = 0.29 and 9.27; 0.37 and 0.28 MJ�m2.

On the basis of analyzing values of dimensionless pa-

rameter (Table 4) and steel microstructure with vcool

= 18,000

and 2000 K�h there is austenite decomposition in the

martensitic region and with vcool

= 500 K�h it is in the pear-

lite-bainite region, and with vcool

= 130 and 70 K�h it is in

the pearlitic region. The small increase noted in impact

strength after tempering at 500 °C is apparently connected

with an increase in metal stress level. The data obtained indi-

cate that steel 80Kh2GSMF compared with steels 55Kh3NMF

and 70Kh3NMF exhibits higher strength and lower ductility

and considerably lower impact strength, and also reduced

hardenability. At the same time, data for the high strength

level, especially after cooling with decomposition in the

pearlitic region indicates that work for improvement of the

composition of steel 80Kh2GSMF is very promising.

Attention should also be drawn to the change in

dimensionless parameter for the test steels. The value of the

parameter decreases not only with a reduction in cooling

rate, that is connected with decomposition of austenite from

the martensitic region into the pearlitic region, but also with

an increase in tempering temperature from 500 to 600°C, that

may be explained by carbide transformation and coalescence

of the carbide phase. In addition, apart from the dimen-

sionless parameter, the impact strength of roll steels appeared

to be typical, more sensitive to structure and stressed state of

the metal, than relative elongation and reduction of area.

The contact fatigue resistance plays an important role in

the operating life of roll steels (Table 5).

Testing for contact fatigue resistance revealed a leader

with respect to this property, i.e. steel 70Kh3NMF, that has

an advantage at all specific loads. Steel 55Kh3NMF also has

stable results, although with respect tot eh number of cycles

to failure with high specific loads it is surpassed by the rest.

It is important to stay with steel 80Kh2GSMF, that by exhi-

biting high strength properties loses out to steel 70Kh3NMF

due to formation of coarse chips during contact fatigue tests.

Fractographic studies of specimens showed that the reason

276 I. A. Borisov and S. S. L’vova

TABLE 4. Dimensionless Parameter �0.2 ��r after Quenching with

a Different Cooling Rate vcool and Tempering at 500 and 600°C

Steel ttemp

, °C

�

0.2��

rwith v

cool, °C

18,000 2000 500 130 70

55Kh3NMF 500 0.89 0.87 0.73 0.64 0.64

600 0.86 0.77 0.68 0.63 0.63

80Kh2GSMF 500 0.90 0.90 0.60 0.55 0.53

600 0.89 0.87 0.60 0.50 0.50

70Kh3NMF 500 0.78 0.78 0.77 0.59 0.57

600 0.74 0.71 0.76 0.64 0.56

TABLE 5 Steel Contact Fatigue Properties

Steel psp

, N�mm2 Nf� 10 – 6 Form of surface

behavior

55Kh3NMF 4900 0.6 Crumbling

4600 0.8

4200 8.0

3900 13.5

80Kh2GSMF 4900 1.3 Deep crumbling

4600 2.0

4200 9.0 Crumbling

3900 17.0

80KhNMF 4900 1.4 Crumbling

4600 2.0

4200 5.3

3900 9.8

70Kh3NMF 4900 2.4 Crumbling

4600 5.7

4200 8.9

3900 16.0

Notations: psp

is specific load, Nf

is number of cycles to failure.

for this chipping is a s a rule coarse particles of non-metallic

inclusions or carbides. High internal stresses in the metal

structure also promote coarse chipping.

These studies have confirmed the desirability of increas-

ing the chromium content to 2.5 and 3.0% and nickel to 0.8%

with a simultaneous reduction in carbon concentration to

0.70 and 0.55% in steels for the working and support rolls of

cold rolling. This relationship between chromium, nickel and

carbon has a favorable effect on quenchability, hardenability,

temper stability, contact fatigue resistance of steel, and en-

tirely overcomes the danger of the occurrence of a cementite

network and it increases brittle failure resistance.

Additional alloying with molybdenum up to 0.70%, va-

nadium up to 0.15% and niobium up to 0.06%, combined

with aluminum also increases the brittle strength of steels as

a result of an increase in hardenability and preparation of a

fine-grained structure.

An increase in manganese and silicon content to 1.0% of

each markedly increases the strength properties of steels with

martensitic, pearlitic and bainitic structures, prepared during

tempering. However, simultaneously there is a sharp reduc-

tion in ductility and impact strength for the steel, that makes

it impossible to recommend it for industrial approval.

For final clarification of the limits of the content of al-

loying elements in the recommended steels it is necessary to

perform additional studies on the metal of an industrial melt

with specific heat treatment parameters and service charac-

teristics.

CONCLUSIONS

1. On the basis of Cr – Ni – Mo – V-composites steel

70Kh3NMF for the working rolls of cold rolling with a bar-

rel diameter up to 1400 mm and steel 55Kh3NMF for sup-

port rolls have been developed. The steels exhibit good

hardenability, they answer the specifications with respect to

the main properties laid down for roll materials, and they are

recommended for approval under industrial conditions.

2. It is shown that tempering at 600°C during preliminary

heat treatment in a regime for improving steel 70Kh3NMF

provides a high level of mechanical properties (including im-

pact strength) in the necks and roll barrel cross section, and

also in the zone of its hardened surface.

SUMMARY

In choosing steels for cold rolling rolls of contemporary

mills it is necessary to consider not only the operating condi-

tions for rolls and their dimensions, but also technological

features of their metallurgical production, in particular the fi-

nal heat treatment technology. A requirement arises for de-

velopment of compositions for steels within which there is

consideration of both the operating requirements and the size

of rolls, and also hardening features (HFC, IFC, graded and

volumetric heating for hardening, cooling in water, oil, wa-

ter-air medium, water spray). In this connection development

of deeply hardened Cr – Ni – Mo – V-steels for rolls with a

diameter of 1400 mm or more, considering experience of

producing large forgings of turbine rotors and generators, is

promising. Here we should not forget that the recommenda-

tions made in this article are not final and they require ap-

proval under industrial conditions.

REFERENCES

1. V. P. Polukhin, V. A. Nikolaev, M. A. Tylkin, et al., Reliability

and Endurance of Cold Rolling Rolls [in Russian], Metallurgiya,

Moscow (1976).

2. E. I. Treiger and V. P. Prikhodko, Improvement in the Quality and

Operational Life of Sheet Mill Rolls [in Russian], Metallurgiya,

Moscow (1989).

3. A. S. Trekalo and I. A. Borisov, “Improvement in manufacturing

technology and heat treatment of large objects,” in: Technology,

Organization and Mechanization of metal Heat Treatment and

Chemical Heat Treatment [in Russian], NIIinformtyazhmash,

Moscow (1977).

4. E. T. Dolbenko, A. I. Surovtsev, and A. I. Borisov, “Future deve-

lopment of technology and equipment for heat treatment of large

forgings,” in: Technology, Organization of Production and Con-

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5. I. A. Borisov, “Effect of carbon, chromium and nickel on carbide

transformation in Cr – Ni – Mo – V-steels during tempering,”

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ments on rotor steel properties,” Metalloved. Term. Obrab. Met.,

No. 10, 33 – 35 (1994).

7. I. A. Borisov, “Some assumptions of steel alloying theory for tur-

bine rotor and generator steels,” Metalloved. Term. Obrab. Met.,

No. 3, 22 – 25 (2001).

8. I. A Borisov, “Effect of overheating on the mechanical properties

of roll and forging steels, Metalloved. Term. Obrab. Met., No. 8,

20 – 26 (2001).

Choice of Steel for Rolling Mill Rolls 277