Effect of Temperature, Fractional Deformation, and Cooling Rate on ...

Metal Science and Heat Treatment

Vol. 47, Nos. 1 – 2, 2005

UDC 669.14.298:621.77.01

EFFECT OF TEMPERATURE, FRACTIONAL DEFORMATION, AND COOLING RATE ON THE STRUCTURE AND PROPERTIES OF STEEL 09GNB G. E. Kodzhaspirov1 and R. V. Sulyagin1 Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 7 – 10, February, 2005.

The effect of temperature, divisibility of deformation, and cooling rate in high-temperature thermomechanical treatment (HTTMT) on the structure and mechanical properties of low-alloy steel 09GNB is studied. The steel is used as a high-strength material for the production of offshore structures, strips, and other welded articles. The study is performed using the method of experimental design where the parameters are fractional deformation (number of passes in rolling), final temperature of the deformation, and rate of post-deformation cooling. The results of the experiments are used to construct regression equations describing the qualitative and quantitative effect of the parameters of HTTMT on the mechanical properties of the steel. Microstructure and fracture surfaces of the steel are analyzed.

INTRODUCTION

METHODS OF STUDY

High-temperature thermomechanical treatment (HTTMT) is a very effective method for ensuring an optimum combination of strength, ductility, and toughness in low-alloy carbon steels [1 – 5]. The efficiency of the action of such treatment on the structure and properties is ensured by structural and phase transformations that occur directly in the deformation process and during post-deformation cooling of the hot-deformed austenite. Development of the process of production of rolled stock with specific thickness from advanced steels with various mechanical properties requires study of the effect of the parameters of plastic deformation and of the rate of subsequent cooling on the process of structure formation. Such studies are performed under conditions that simulate rolling in commercial mills in order to predict and provide the requisite level of properties [5]. The aim of the present work consisted in studying the effect of HTTMT2 parameters on the structure and mechanical properties of rolled products from low-alloy steel 09GNB.

We melted the steel in an IST-016 induction furnace produced by the Izhorskie Zavody JSC. It had a capacity of 100 kg and a base lining. The chemical composition of the steel was 0.09% C, 0.32% Si, 0.52% Mn, 0.003% S, 0.003% P, 1.38% Ni, 0.03% Nb. Rolling was performed in a DUO “210” mill of the St. Petersburg State Polytechnic University in modes simulating the conditions of deformation in the last passes in a commercial “5000” mill. The initial preform had a cross section of 75 ´ 37 ´ 450 mm. The parameters varied in the HTTMT were the final deformation temperature tf , the number of passes n, and the rate of post-deformation cooling vc . The total degree of deformation remained constant and amounted to about 70%. The microstructure was studied with the help of a Neophot-2 light microscope. The tensile mechanical tests were performed by a standard method on specimens 6 mm in diameter. The tests for impact bending were performed on Charpy specimens. The fracture behavior of specimens of steel 09GNB was studied with the help of a PSEM-500 scanning electron microscope. We analyzed transverse specimens after testing for impact toughness at a temperature of + 20, – 20, and – 40°C. We used a laboratory mill for performing designed experiment of type 23 with variable parameters presented in Table 1.

1 2

St. Petersburg State Polytechnic University, St. Petersburg, Russia; TK OMZ-Izhora Research Center, St. Petersburg, Russia. In the present paper HTTMT is understood as a kind of thermomechanical treatment in which straining occurs in the zone of high-temperature austenite (above A3 ), and the formed transformation products (not only martensite) inherit to this or that degree the features of hot-strained austenite.

43 0026-0673/05/0102-0043 © 2005 Plenum Publishing Corporation

44

G. E. Kodzhaspirov and R. V. Sulyagin

TABLE 1. Parameters of the Designed Experiment and Mechanical Properties of Steel KCV, kJ/m2

Parameters of designed experiment HTTMT modes

1 2 3 4 5 6 7 8

n (X2 )

5 5 5 5 3 3 3 3

tf , vc , °C (X1 ) K/sec (X3 )

1000 1000 800 800 1000 1000 800 800

25 8.6 25.0 8.6 25.0 8.6 25.0 8.6

s0.2 , MPa

sr , MPa

d, %

y, %

longitudinal specimens + 20°C

987 464 820 510 798 520 631 450

1193 669 1020 686 1010 687 987 674

12 25 15 23 17 21 13 23

72 69 73 67 72 68 71 70

1923 (154) 785 (63) 2031 (163) 814 (65) 2315 (185) 1324 (106) 1217 (97) 1040 (83)

transverse specimens + 20°C

– 20°C

HB

– 40°C

1776 (142) 1668 (133) 1246 (100) 471 (38) 294 (20) 128 (10) 1501 (120) 1040 (83) 510 (41) 500 (40) 216 (17) 157 (13) 2267 (181) 893 (71) 206 (17) 893 (71) 294 (24) 157 (13) 1172 (94) 978 (784) 148 (12) 922 (74) 265 (21) 137 (11)

363 192 277 223 266 212 241 185

Notes: 1. In all the treatment modes the rolling was performed after a hold at 1200°C for 1 h 30 min; the total reduction eS is about 70%; partial reductions: about 14% after five passes and about 23% after three passes. 2. The initial thickness of the preform is 37 mm; the thickness of the rolled product is 11 mm. 3. The work of fracture is presented in parentheses in joules. 4. Cooling at a rate vc = 8.6 K/sec was performed in air; cooling at a rate vc = 25 K/sec was performed in water.

Transverse and longitudinal specimens for mechanical tests were fabricated from rolled preforms, five specimens for each test. The mean values of the mechanical properties are presented in Table 1. RESULTS AND DISCUSSION After processing the test data by the methods of regression analysis and analysis of variance we constructed the following regression equations: Y1 = 647.3 + 44.78X1 + 47.78X2 + 161.60X3 ; Y2 = 865.0 + 24.10X1 + 26.16X2 + 186.73X3 ; Y3 = 18.65 + 0.15X1 + 0.10X2 – 4.4X3 ; Y4 = 70.2 + 0.005X1 + 0.005X2 + 1.95X3 ; X1 =

t f -900 ; 100

X2 = X3 =

n -4 ; 1

vc -16.7 , 8.4

where Y1 is the yield strength, Y2 is the ultimate rupture strength, Y3 is the elongation, and Y4 is the contraction. Judging by the values of the regression constants the variable parameters can be arranged in the following order in accordance with their significance: rate of post-deformation cooling, number of passes, final deformation temperature. It follows from the results of mechanical tests that preforms subjected to HTTMT of mode 1 with maximum final rolling temperature of 1000°C, maximum number of passes,

and highest cooling rate (see Table 1) posses the highest strength characteristics (s0.2 = 987 (± 8) MPa). This is connected with the formation of a structure of dispersed lower bainite due to fragmentation of the structure of the initial (prior to the transformation) hot-strained austenite (see Fig. 1a). Dispersion of hot-strained austenite in fractional deformation is described in detail in [6]. In the case of fractional rolling in five passes the decrease in the final deformation temperature from 1000 to 800°C causes the appearance of regions of pearlite and free ferrite in the structure (Fig. 1b ). This is connected with the fact that final deformation temperature falls below Ar3, which also decreases the strength. When the number of passes is reduced from 5 to 3, the strength characteristics decrease somewhat more (by 20 – 25% on the average). This can be associated with the lower density of defects in the crystal structure and larger sizes of fragments of strained austenite as compared to five-pass deformation. The inherited features of this change are manifested in the lower dispersity of the formed bainite, which agrees with the data of [7 – 9]. Cooling in air radically changes the structure and, consequently, the properties of the steel. The diffusion transformation of austenite yields primarily ferrite and pearlite (Fig. 1c). However, the sizes of the pearlite colonies and ferrite grains and their proportion depend on the mode of the HTTMT; at n = 5 the grains have size Nos. 8 – 9, and at n = 3 they have size Nos. 6 – 7. In three-pass deformation the metal treated by mode 5 with maximum final rolling temperature (1000°C) and subsequent cooling in water has the highest strength characteristics (s0.2 = 798 (± 10) MPa, see Table 1). This can be associated with the formation of a bainitic structure comparatively coarser that that formed after HTTMT with n = 5.

Effect of Temperature and Cooling Rate on the Structure and Properties of Steel 09GNB

200 mm

à

45

c

b

Fig. 1. Microstructure of tested steel after deformation in the following modes: a) final rolling temperature tf = 1000°C, number of passes n = 5, rate of post-deformation cooling vc = 25 K/sec; b ) tf = 800°C, n = 5, vc = 25 K/sec; c) tf = 1000°C, n = 5, vc = 8.6 K/sec.

à

s0.2 , ÌPà 1100 1000 900 800 700 600 500

vc

20

5 16

4

, K 12 /s ec 8 3

1200 1100 1000 900 800 700 24

20

vc

5 16

, K 12 /s ec 8 3

4

d

c KCV – 40 , J

d, %

3

n

4

5

13 15 16 17 18 19 20 21 22 23

26 24 22 20 18 16 14

24

20

16

vc

12

,K

8

c /se

156 232 308 384 461 537 613 689 765 841

1200 1000 800 600 400 200 24

vc

20

5 16

,K

2 /s 1 8 3 ec

4

n

24

b

sr , ÌPà

703 747 790 834 877 920 964 1007 1051 1094

n

498 542 585 629 672 716 759 803 846 890

formed in cooling below Ar3 . Such a phenomenon was observed in [5, 6] for medium-carbon steels. The ductility characteristics change in an inverse proportion. Analyzing the dependence of mechanical characteristics (s0.2 , sr , d, KCV – 40 ) of the steel on the parameters of HTTMT, we established that the influence of the cooling rate was the highest (Fig. 2). Growth in vc increases the strength characteristics and decreases the ductility ones, which is naturally connected with the dominant formation of metastable structures of bainitic type.

n

The lowest (s0.2 = 450 (± 7) MPa) strength characteristics are observed in the steel treated by mode 8 (see Table 1). This can be explained by the too low (below Ar3 ) final rolling temperature, at which a part of the strained austenite decomposes. This can be inferred from the large size of pearlitic colonies as compared to the sizes obtained after HTTMT by mode 2. Some increase in s0.2 in more fractional deformation at tf = 800°C can be associated with strain hardening of ferrite

Fig. 2. Dependence of the yield strength (a), ultimate rupture strength (b ), elongation (c), and impact toughness KCV – 40 (d ) of steel 09GNB on the number of passes in rolling n and the cooling rate vc .

46

G. E. Kodzhaspirov and R. V. Sulyagin KCV, kJ/m2 5 2000

1 3

1500

7 8

1000

2

500 0 – 40

6

4 – 10

– 20

0

20

400 mm

à

ttest , °Ñ Fig. 3. Effect of HTTMT mode (the numbers at the curves correspond to the mode numbers in Table 1) on the cold brittleness of steel 09GNB.

We evaluated the cold resistance of the steel in terms of the results of impact tests at + 20°C and – 40°C. It can be seen from the data of Fig. 3 that the metal treated at the highest values of the studied parameters, i.e., the number of passes, the final rolling temperature, and the cooling rate, has the highest cold resistance. The metal with low rate of post-deformation cooling (in air) has the lowest cold resistance. Such a result is explainable by the higher cold resistance of bainitic structures relative to pearlitic ones. Fracture of specimens subjected to HTTMT with postdeformation cooling in water and failure at + 20°C occurs by a ductile mechanism (Fig. 4a ). The width of the zone of crack nucleation is about 300 mm. When the test temperature is decreased to – 20°C, regions of brittle intragrain fracture (up to 15%) emerge by the mechanism of quasi-cleavage, and 85% of the fracture develops by the ductile intragrain mechanism (Fig. 4b ). The width of the zone of crack nucleation decreases to 180 mm. Further decrease in the test temperature to – 40°C increases the area of brittle transcrystalline fracture to 20%. The other part is taken by dimple intercrystalline fracture. Correspondingly, the highest energy intensity of fracture is observed in testing at + 20°C and the lowest one is observed in testing at – 40°C. Fracture of specimens subjected to HTTMT with cooling in air is of another nature. For example, in tests at + 20°C fracture occurs by the mechanism of quasi-cleavage, and areas with this type of fracture occupy about 80% of the total fractured area. The other part is taken by dimple intragrain fracture at a width of the crack nucleation zone of 250 mm (Fig. 4c ). In tests for impact bending at – 20°C we observed 100% fracture by the mechanism of quasi-cleavage at a width of the crack nucleation area of 450 mm. Decrease in the final rolling temperature from 1000 to 800°C in the case of HTTMT with cooling in water virtually does not affect the fracture behavior of the steel. The only difference is that after treatment with tf = 800°C the fracture

b

c

Fig. 4. Fractures in steel 09GNB subjected to the following kinds of HTTMT: a) final rolling temperature tf = 1000°C, number of passes n = 5, rate of post-deformation cooling vc = 25 K/sec, ttest = 20°C; b ) tf = 1000°C, n = 5, vc = 25 K/sec, ttest = – 20°C; c) tf = 1000°C, n = 5, vc = 8.6 K/sec, ttest = 20°C.

surface has zones of quasi-cleavage due to the formation of ferrite-pearlite regions in the structure in addition to bainite. The width of the zone of crack nucleation increases to 600 mm. When the temperature of testing for impact bending is decreased to – 40°C, the area of brittle intragrain fracture increases markedly (to 95%). The remaining part (5%) is taken by ductile intragrain fracture, which corresponds to a substantial decrease in the energy intensity of the fracture process. As we have already mentioned, HTTMT with tf = 800°C, n = 5, and vc = 8.6 K/sec leads to formation of a ferritepearlite structure and thus to a decrease in the energy inten-

Effect of Temperature and Cooling Rate on the Structure and Properties of Steel 09GNB

sity of the fracture process. Fracture of the metal at ttest = + 20°C occurs chiefly by the mechanism of quasicleavage. Decrease in the test temperature to – 40°C causes formation of cleavage zones in addition to quasi-cleavage. The width of the zone of crack nucleation is about 80 mm. CONCLUSIONS 1. Steel 09GNB with a structure of lower bainite obtained at the maximum number of rolling passes (fractional deformation), highest final rolling temperature, and maximum cooling rate has the highest strength characteristics. 2. Steel 09GNB subjected to HTTMT for a structure of lower bainite possesses the highest resistance to brittle fracture. The structure of lower bainite is formed as a result of the transformation of hot-strained dispersed fine-fragmented austenite during cooling. REFERENCES 1. B. Basu, S. M. Tripathi, V. V. Modak, and D. K. Biswass, “Development of TMCP schedule for improving toughness for low carbon HSLA structural steel,” in: Thermomechanical Processing of Steels, London (2000), pp. 754 – 763.

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2. J. K. Patel, B. Wilshire, and P. J. Evans, “Optimization of hot strip mill processing conditions for niobium HSLA steels,” in: Thermomechanical Processing of Steels, London (2000), pp. 265 – 274. 3. G. E. Kodjaspirov and I. Kim, Advanced Strengthening Methods of Metallic Materials, St. Petersburg State Technical University, St. Petersburg (2000). 4. M. L. Bernshtein, V. A. Zaimovskii, and L. M. Kaputkina, Thermomechanical Treatment of Steel [in Russian], Metallurgiya, Moscow (1983). 5. G. E. Kodjaspirov and I. Kim, Thermomechanical Processing of Steels, St. Petersburg Technical University, St. Petersburg (2000). 6. V. V. Rybin, A. S. Rubtsov, and G. E. Kodzhaspirov, “Structural transformations in steel rolled with fractional deformation of various degrees,” Fiz. Met. Metalloved., 58, Issue 4, 774 – 781 (1984). 7. L. M. Utevskii, Electron Diffraction Microscopy in Metals Science [in Russian], Metallurgiya, Moscow (1973). 8. M. L. Bernshtein and M. A. Shtremel’, “Inherited effect of mechanical hardening on the properties of steel,” Fiz. Met. Metalloved., 15, Issue 1, 82 – 90 (1963). 9. V. V. Rybin. G. E. Kodzhaspirov, and A. S. Rubtsov, “Formation of fine structure in steel 38KhS due to deformation close to the temperature of pearlitic transformation,” Fiz. Met. Metalloved., 61, Issue 3, 583 – 591 (1986).