IMPACT STRENGTH OF STEEL FOR

ROLLING MILLS

Yu. A. Bashnin, I. V. Paisov, and V. N. Tsurkov

ROLLS IN HOT

UDC 621.771.073:620.178.4.72

The heat t reatment of forgings for rolls used in hot rolling consists of normalizat ion and prolonged soaking at 550-680~ with slow cooling at the rate of 15-40 deg/h. The t rea tment increases the mechanical propert ies of the steel due to improvement of the s t ructure and reduction of the hydrogen content. Roils for hot rolling are usually made of low-alloy steels 60KhN, 60KhG, and 55Kh. After the heat t rea tment the s t ructure consists of sorbite and pearli te of different sizes.

The mechanical proper t ies of the rolls, par t icular ly the impact strength, do not always meet the spee- ificationso

The steels used to manufacture the rolls are susceptible to temper bri t t leness [1], and therefore the present heat t reatment often resul ts in inconsistent and low values of the impact strength. Also, the impact strength can be affected by the austenite grain size and the phosphorus and hydrogen content of the steel. The present work concerns the effect of these factors on the impact strength of the steels.

The investigation was made with forged pieces and under laboratory conditions. Longitudinal Mesna- ger samples were prepared from the necks of forgings 600-700 mm in diameter; samples were taken from the surface, at a depth one-third the radius, and from the center. The heat t reatment of the samples under laboratory conditions was selected on the basis of previous work [2]. Samples 12 x 12 x 60 mm were heated to normalizat ion tempera ture and cooled in the furnace at the rate of 25 deg/h, and were also held at 550- 680~ and cooled at the rate of 15 deg/h, which corresponds to the normal cooling rate of forgings under shop conditions. Figure 1 shows the variat ion of the impact strength with the test ing tempera ture at dif- ferent austenite grain sizes for steel 60KhN with the following chemical composition: 0.68% C, 0.80% Cr, 1.15% Ni, 0.72% Mn, 0.020% S, 0.022% P, 0.25% St.

a n , kg-m/cm 2

-20 0 20---"--~ ~ Testing temperature 0~OlO g~02O 0,030 ~

Fig. I Fig. 2

Fig. 1. Impact strength of steel 60KhN in relation to grain size and cooling rate at different testing tem- peratures.- ) Grain size grade 7-8; .... ) grade 3. 1) Cooled in water; 2) cooled in furnace.

Fig. 2. Variation of impact strength with phosphorus content of 60KhN forgings.

a n . kg-m/cm z

Moscow Institute of Steel and Alloys. Translated from Metailovedenie i Termicheskaya Obrabotka Metallov, No.5, pp. 49-51, May, 1969.

384

a n, kg-m/cm 2 a n, kg-m/cm z

2

I I 7 - - L -60 -20 l? 20 gO ~ -GO-bO-20 l? 20 50 60

Testing temperature Testing temperature

Fig. 3 Fig. 4

q

80 ~

Fig. 3. Variation of impact strength with testing tempera ture for steel 60KhN from different sections of forging. 1) Surface, 0.0016% P; 2) at a depth of one-third the radius, 0.0016% P; 3) center, 0.0022% P.

Fig. 4. Variation of impact strength with test ing tempera ture for steel 60KhG from different sections of the f o r g i n g . ~ ) Surface, 0.0019% P; . . . . ) at a depth of one-thid the radius, 0.0021% P; . . . . . ) center, 0.0021% P.

Grain refining from grade 3 to grade 7-8 leads to a substantial difference in the values of the impact strength af ter rapid and slow cooling, which is due to the g rea te r length of the grain boundaries. The ab- solute value of the impact strength in the f ine-grained steel is higher than in the coa r se -g ra ined steel - by 140% in the rapidly cooled steel and 125% in the steel cooled in the furnace. An increase of the cooling rate ra i ses the impact strength, par t icu lar ly in the f ine-grained steel.

Figure 2 shows the variat ion of the impact strength with the phosphorus content of steel 60KhN from forged rools 550-700 mm in diameter . The curve was plotted af ter s tat is t ical t rea tment of the exper iment- al data by l inear correlat ion. The scat ter ing of the experimental points is due to the difference in austenite grain size, the degree of forging reduction, and the variation of the chemical composition within the nominal Iimits. Reduction of the phosphorus content f rom 0.030% to 0.015% has a negligible effect on the impact strength at room temperature , while reduction of the phosphorus content f rom 0.015% to 0.012% sharply in- c reases the impact strength, which conforms with the findings in [3].

The same variat ion of impact strength with phosphorus content was found in steels 60KhG and 55Kho

Figures 3, 4, and 5 show the variat ion of the impact strength with the test ing tempera ture for samples of 60KhN, 60KhG, and 55Kh from three places in forged roils 550-700 mm in d iameter - the surface, at a depth of one-third the radius, and from the center. The scat ter ing of the resul ts after mathematical t r e a t - merit is shown for steel 60KhN. Even a slight increase of the phosphorus content (a few thousandths of one per cent) with increas ing distance f rom the surface of the forging inc reases the susceptibil i ty of the steel to t emper bri t t leness. Thus, in the forging of steel 60KhN the phosphorus eontent in the surface is 0.006% less than in the eenter. This ra i ses the cold br i t t leness threshold 30~ in the center, andthe impact strength at 20~ decreases 1.4 t imes by compar ison with the surface.

With equal phosphorus concentrations the values of the impact strength are almost identical r egard less of which section of the forging the samples are taken from. For example, the impact strength in the center and at a depth of one-third the radius in the forging of steel 60KhG is the same at all testing tempera tures for equal phosphorus concentrat ions (0.021%).

In the forgings investigated the austenite grain size did not vary through the section and was grade 6-7. The hydrogen content varied from 1 to 2.6 cmZ/100 g of metal through the section of the forgings.* Therefore changes in the austenite grain size and hydrogen content within the limits given cannot be the reason for reduction of the impact strength with inereasing distance from the surface of the forging. The impact strength in the center of the forging is lower than in the surface because of phosphorus segregat ion (even a few thousandths of one per cent).

* Our data and those from [4] indicate that the presence of less than 4 cm 3 of hydrogen per 100 g of metal has no effect on the impact strength.

385

a n, kg-m/cm 2

- - ~ . J l

-50 -40 -20 g 20 40 ~

Testing temper arurr

Fig. 5

a n, kg-m/cm 2

11 . . . . . . [ I 2 48 y~ h

Tempering time

Fig. 6

Fig. 5. Variation of impact strength with test ing tempera ture for steel 55Kh from different sections of forging. - - ) Sur- face, 0.018% P; . . . . ) at a depth of one-third the radius, 0.0020% P; . . . . . . -) center, 0.0025% P.

Fig. 6. Impact strength of steel 60KhN in relation to tempering tempera ture and time and cooling rate after t e m p e r i n g . ~ ) Cooled in water; - - - -) cooled in furnace. 1) Tempered at 650~ 2) at 600~ 3) at 550~

Large forgings are usually held at 600 :~ 50~ for 100 h or more. Figure 6 shows the variation of the impact strength in a forging of steel 60KhN with the tempering temperature , the holding time, and the cool- ing rate when cooled in water and in the furnace at the rate of 15 deg/h, which corresponds to the cooling rate of forgings under shop conditions.

An increase of the holding time to 96 h ra i ses the impact strength 1.15 t imes after tempering at 050~ and reduces it 1.3 and 1.4 t imes after tempering at 600 and 550~ respect ively. At 600 and 550~ the sec - tions near the surface are impoverished in carbon and carb ide- forming elements, and phosphorus migra tes into these sections, leading to revers ib le temper bri t t leness [3]. Independently of the cooling conditions, an increase of the holding t ime at these t empera tures reduces the impact strength, although cooling in wa- te r after tempering resul ts in a higher impact strength in all cases (by 15-30%), since with rapid cooling the embri t t iement process ceases abruptly. Complete suppression of the susceptibili ty to revers ib le temper bri t t leness on prolonged holding is not attained even with cooling in water, since the alloying elements and phosphorus are part ial ly redistr ibuted in the surface a reas during holding.

The increased impact strength of the steel with increasing holding t ime at 650~C may be due to spheroi+ dization of the carbide phase and an increase in the solubility of alloying elements and phosphorus in a - phase, which reduces the phosphorus concentration in the boundaries. The lower values of impact strength resul t ing from cooling in the furnace in this case is due to the prolonged holding of the steel at temper br i t - t leness t empera tures during cooling.

1.

2 ,

3. 4.

L I T E R A T U R E C I T E D

G. V. Kurdyumov and R. I. l~ntin, Temper Brit t leness of Structural Steels [in Russian], MetMturg- izdat, Moscow (1945). I. V. Paisov, Yu. A. Bashnin, V. N. Tsurkov, et al., Vestnik Mashinostroeniya, No. 1 (1967). L. M. Utevskii, Temper Brit t leness of Steel [in Russian], Metallurgizdat, Moscow (1961). P. V. Sklyuev, Hydrogen and Flakes inLarge Forgings [in Russian], Mashgiz, Moscow (1963).

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