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Journal of Failure Analysis and Prevention

, Volume 8, Issue 6, pp 492–497 | Cite as

Investigation into Surface Defects Arising in Hot-Rolled SUP 11A Grade Spring Billets

  • Santosh Kumar
  • Vinod Kumar
  • R. K. Nandi
  • T. S. Suresh
  • Ramen Datta
Case History---Peer-Reviewed

Abstract

Silicomanganese grade billets are the most commonly used steels for manufacture of automobile leaf springs. However, Cr-Mn-B grade steel known by trade name of SUP 11A grade is replacing the conventional silicomanganese grades such as 60Si7 or 65Si7 steels because it has become a competitive alternative in the market. Three heats of SUP11A grade spring steel were made through BOF-VAD-CC route and continuously cast into 125 × 125 mm billets. Some of the billets contained blowholes and piping. Rolling of selected billets into 85 × 15 mm flats revealed occasional slivers, seams, and a few shallow hairline surface cracks. A detailed metallurgical investigation was carried out to understand the genesis of these defects. A pearlite-free ferritic microstructure near the cracks combined with the presence of dispersed inclusions resulting from internal oxidation in the vicinity of cracks and the presence of scales within the shallow discontinuous short-length longitudinal cracks indicated that these defects resulted from pre-existing subsurface blowholes lying within 1 mm of billet surface. Reduction of the gas content of liquid steel in the mold, optimization of electromagnetic stirring (EMS) current, and control of superheat are some of the broad measures identified to improve the cast quality of SUP 11A spring steel billets and minimize the rejection of rolled flats.

Keywords

Billets Continuous cast steel Failure analysis Spring steel 

Introduction

Spring steels find application in automobiles, railways wagons, machine tools, and many load-bearing elastic members. A mechanical spring may be defined as an elastic body having a primary function of deflecting under load and returning to the original shape on removal of applied load. However, often the spring is expected to perform other functions, as for example the rear leaf spring in an automobile acts structurally to maintain rear-axle alignment. For the manufacture of automobile leaf springs, silicomanganese grade billets are the most commonly used steels [1], but Cr-Mn-B grade steel known by the trade name of SUP 11A is replacing 60Si7, 65Si7, and other conventional silicomanganese grades.

Because of the emerging market for SUP 11A grade spring steels, a development program for SUP 11A grade spring steel billets was initiated at Durgapur Steel Plant of Steel Authority of India Limited. The chemical composition for SUP 11A grade is 0.55 to 0.65% C, 0.7 to 1.0% Mn, 0.035% max for both S and P, 0.15 to 0.35% Si, 0.7 to 1.0% Cr, and 0.0005% min B. The limit for Cu and Sn is specified by Cu + 8Sn < 0.4%. Specified inclusion rating for spring steel billet calls for 2.0 max A, B, C, and D both thin and thick series [2]. The macrostructure of the billet should conform to C2 of ASTM E 381 specification [3], and cast billets should be free from flaking, piping, surface patches, and porosity.

Three heats (314 tons) of SUP11A were made for this study, and billets (125 × 125 mm) were supplied to a major leaf spring manufacturer for processing into leaf spring. Some of the billets were found to contain blowholes, piping, and other defects. Rolling of selected billets into 85 × 15 mm flats revealed that the surface quality was inadequate because of slivers, seams, shallow hairline surface cracks. A detailed metallurgical investigation was carried out to understand the genesis of these defects. This article collates and analyzes the investigation results and presents a future course of actions to minimize such defects.

Experimental

Samples were taken from billets of three heats of SUP 11A. The samples included ground finished transverse section of the billets that were macroetched using 50–50 HCl-H2O solution heated to 65 to 75 °C. Transverse sections of the rolled flats (15 mm thick) were also examined for macrostructure. Samples were cut from the rolled flats to reveal short surface cracks, and the metallurgical investigation included chemical analysis, assessment of macro- and microstructural features, electron probe microanalysis near the crack area, and analysis of gas content of the samples.

Results

Steelmaking and Casting

Steelmaking and casting parameters and chemical analysis of the three heats are shown in Tables 1 and 2, respectively. The oxygen content was 15.2, 19.8, and 18.5 ppm for the three heats, respectively, and met the 15 to 20 ppm specification. The Mn-to-Si ratio was controlled to greater than 3 in each heat, and the casting speed was regulated within 1.8 to 2.5 m/min. The electromagnetic stirring (EMS) current was 200 A for the first heat and was raised to 250 A for the second and third heats. With the exception of P (0.040%) in first heat, which is marginally high, the chemical composition of the three heats was found to conform to the specification.
Table 1

Steelmaking and casting parameters

Parameter

First heat, No. 05303449

Second heat, No. 05103656

Third heat, No. 05203683

Temperature at LTS, °C

1589

1578

1579

O2 in LTS, ppm

15.2

19.8

18.5

Mn/Si

>3

>3

>3

Average casting speed, m/min

1.9–2.2

1.9–2.5

1.8–2.4

Casting superheat, °C

60–75

40–50

50–60

Tundish temperature, °C

1539, 1542, 1553

1516, 1518, 1526

1533, 1538, 1544

EMS current, A

200

250

250

EMS frequency, Hz

8

8

8

Table 2

Chemical composition of SUP 11A grade steel billets

 

Composition, wt.%

C

Mn

S

P

Si

Cr

B

Cu + 8Sn

Specified

0.55–0.65

0.7–1.0

0.035 max

0.035 max

0.15–0.35

0.7–1.0

0.0005 min

0.40 max

First heat, No. 05303449

0.65

0.85

0.030

0.040

0.28

0.78

0.004

···

Second heat, No. 05103656

0.63

0.90

0.028

0.034

0.27

0.87

0.002

···

Third heat, No. 05203683

0.60

0.88

0.025

0.028

0.25

0.85

0.004

···

Visual Observations

Figure 1(a) shows fractured surface of a billet containing typical subsurface blowholes. These blowholes (Fig. 1b) have dimension of 1 to 2 mm in diameter and 15 to 20 mm in length and are located as near as 0.5 to 1 mm from the billet surface. Figure 2 shows the pickled surface of the hot-rolled flats containing surface defects.
Fig. 1

(a) Typical subsurface blowholes observed on the fractured surface of billet. (b) Macroetched billet samples containing blowholes

Fig. 2

Hairline cracks parallel to rolling direction near edge of the flats

Macrostructure Analysis

Figures 3(a) to (c) show the transverse section of billets from the three heats. In case of the first heat (Heat No. 05303449), Fig. 3(a) reveals presence of central piping and some general “central looseness,” while the other two heats show a relatively higher area of looseness (Fig. 3b and c). Off-corner cracks at a distance of about 5 mm from the billet surface are also observed, although they do not appear to affect the surface quality of the rolled flats processed from these billets. Figure 4 shows macroetched transverse section of the rolled flats, delineating the extent of healing of the central piping. However, the presence of defects below the surface was observed in the rolled flat specimen, and the central looseness observed in the rolled flat indicates inadequate healing of the central piping.
Fig. 3

Macrostructure of the cast 125 × 125 mm billets of SUP 11A Grade. (a) Heat No. 05303449. (b) Heat No. 05103656. (c) Heat No. 05203683

Fig. 4

A typical macrostructure of rolled flat specimen

Microstructure Analysis

Microstructure is largely pearlitic (Fig. 5) and, as Fig. 6 shows, a decarburized zone is observed near both the billet and the crack surfaces. The extent of decarburization was observed to be 0.12 mm, barely within the permissible limit of 0.15 mm. The inclusion rating was evaluated and is shown in Table 3. Against the specified limit of 2.0 max A, B, C, and D for spring steel billet, the rating was found to be 1.5 and lower. Longitudinal section of the rolled flat showed presence of silicate inclusions. Table 4 shows results of oxygen and nitrogen analysis for billets of the three heats. High values of oxygen (>40 ppm) were observed in all cases. The billet sample pertaining to the Fig. 1 showed very high value of oxygen (156 ppm), which is unacceptable.
Fig. 5

A typical microstructure (800×) revealing pearlitic structure in the as-rolled condition

Fig. 6

Microstructure (100×) reveals presence of decarburized zone near the crack

Table 3

Typical inclusion rating of SUP 11A grade spring steel

 

Inclusion rating

A

B

C

D

Observed

1.5

0.5

1.0

1.5

Specified

2.0

2.0

2.0

2.0

Table 4

Oxygen and nitrogen analysis of hot-rolled flats of SUP 11A grade spring steels

Heat No.

O, ppm

N, ppm

05303449

52

105

05103656

38

102

05203683

40

99

Electron Probe Microanalysis

The material found within the cracks was analyzed using EPMA and showed the presence of iron oxide (Fig. 7). X-ray analysis for Si, Ca, Mn, Mg, S, and Cr suggested their absence. However, the dispersion of globular inclusions around the crack, Fig. 8, showed that these inclusions contained Cr, Si, Mn, and O.
Fig. 7

Electron probe analysis of the short longitudinal crack. (a) Secondary electron image. (b) X-ray image of Fe. (c) X-ray image of O

Fig. 8

Electron probe microanalysis of the small inclusions near crack tip. (a) Secondary electron image. (b) X-ray image of Fe. (c) X-ray image of Cr. (d) X-ray image of Si. (e) X-ray image of Mn. (f) X-ray image of O

Discussion

The incidence of blowholes (Fig. 1) in some samples of cast billets from first heat is consistent with the measured values (>38 ppm) of oxygen in rolled flats (Table 4). However, the incidence of blowholes was unexpected in view of reported oxygen control (Table 1). A possible reason for the aforementioned discrepancy may be related to the fact that the oxygen potential was measured in VAD, and there could be subsequent oxygen pickup because the billet caster is an open-stream caster. The role of EMS [4, 5, 6, 7] in suppressing the columnar zone is well known; however, the inadequacy of 200 A of EMS current (Table 1) is evident from the observation of wide columnar zones (Fig. 3a) and the presence of piping. In the case of billets from the initial stage of casting, blowhole formation is a common observation for medium-carbon steel billets, and the EMS current, which suppresses its formation, was kept low deliberately to avoid breakout. Very high superheat of >60 °C (Table 1) has also led to extensive growth of columnar zone in the case of first heat leading to central piping. Positive influence of increasing EMS current is evident in macrostructure of billets of the subsequent two heats.

The presence of subsurface defects observed in the macrostructure of the rolled flats (Fig. 4) and blowhole defects (Fig. 1b)/interdendritic cracks (Fig. 3a-c) at matching locations indicate that the defects existed in the billet. Relation between concentration of an element in liquid and solid phase during solidification is given by [8]:
$$ C_{ 1} = C_{\text{O}} \left( { 1 - f_{\text{S}} } \right)^{{k_{\rm e} - 1}} $$
(1)
where C 1 is the concentration of elements in liquid phase (wt.%), C O is the concentration of solute in liquid before start of solidification (wt.%), f S is the fraction of solidification, and k e is the equilibrium partition ratio of solute.

As solidification proceeds, oxygen potential rises in the interdendritic liquid, which favors the formation of blowhole at these locations. Effective EMS minimizes the buildup of localized gas and helps in the suppression of blowhole formation. A scale loss of 1.5% suggests that for billet of 125 × 125 mm size, approximately 0.9 mm metal will undergo oxidation during reheating. This implies that during reheating of billets prior to hot rolling, the subsurface blowholes may be exposed if the distance of these defects from surface is less than 1 mm. An exposed blowhole would lead to a streak on the hot-rolled surface.

The size of a pre-existing defect should change during the hot rolling of 125 mm thick billet to a 15 mm hot-rolled flat. This change should be in proportion to the change in dimensions of the component. Calculations show that an original defect in the form of blowholes of 2 mm diameter will yield a defect size of roughly 20 mm in length, whereas a blowhole of 15 mm length will compress into a defect having a depth of about 1.8 mm. Sizes of 20 mm length of short longitudinal surface defect observed in Fig. 2 and matching depth of crack observed in Fig. 6 are in agreement with the above calculations.

The off-corner cracks can also give rise to surface defects such as streaks if they are located close enough to the billet surface (<1.0 mm). The rhombodity apparent in billets is undesirable [9] as the obtuse angled corners of rhomboid billet experience tensile stress. For minimizing the off-corner cracks, machine alignment and secondary cooling condition must be controlled.

In open stream casting, aluminum killing is never practiced because nozzle blockage occurs because of accretion of alumina through reaction of Al with atmospheric oxygen. Under these circumstances, the dominant inclusion becomes silicates as observed in the rolled flats. In the inclusion rating, B-type inclusions (alumina) are accordingly the minimum at 0.5 (Table 3).

Microanalysis of material in the cracks confirmed presence of oxide scales (Fig. 7). No inclusions were detected within the crack. Further, the presence of ferrite around the crack (Fig. 4) indicates that decarburization took place. These observations imply that the defect that pre-existed in cast billet was exposed to the oxidizing furnace atmosphere before the rolling operation and that adequate time was available for the decarburization. Evidence of internal oxidation (Fig. 8) establishes that oxygen from the crack surface diffused into the steel and reacted with Cr, Si, and Mn, which have a higher affinity for oxygen compared to Fe. Oxidation in the solid state led to dispersed oxide inclusions around the crack (Figs. 7, 8) combined with pearlite-free ferrite matrix near the crack (Fig. 6). Surface defect in the form of streaks/shallow short-length longitudinal cracks in the hot-rolled flats of spring steels owe their genesis to the pre-existing blowholes. This study showed that suitable control of gases in steel, optimization of EMS current, control of superheat during casting, and measures to check rhombodity of cast billets should correct the observed problem. These controls were instituted and have resulted in an all-round improvement in the quality of spring steel (Fig. 9).
Fig. 9

A typical macro of SUP 11A billets after introduction of corrective measures

Conclusions

  • Macroanalysis of SUP11A billets revealed the presence of blowholes, off-corner cracks, and central piping.

  • Hot rolling of billets into 85 × 15 mm flats resulted in occasional occurrence of surface defects such as slivers, seams, and shallow hairline cracks. The intensity of these defects was relatively greater in the first heat compared to the second and third heats.

  • Optical microscopy studies revealed the presence of ferrite around the crack, indicating the possibility of oxidation of a pre-existing defect during reheating of billets. Electron probe microanalysis showed the presence of iron oxide within the crack, which further corroborates the possibility of a defect pre-existing in the billets that manifests in the form of a crack after the rolling process.

  • The genesis of hairline cracks and seams in the rolled flats is related to the subsurface blowholes lying within 1 mm of the billet surface. The removal of the surface scale during the reheating process opens up these blowholes to oxidation. Subsequent rolling leads to elongation of these blowholes into shallow short-length longitudinal surface cracks. The size of these cracks corresponds very well with the reduction ratio and size of the blowholes.

  • Control of oxygen level in liquid steel and the introduction of other technological measures such as higher EMS current and control of superheat within 40 °C has led to a significant reduction in the extent of blowholes and incidence of defects in the rolled flats.

Notes

Acknowledgments

The authors are grateful to the management of RDCIS and DSP for encouragement and support. Invaluable help rendered by Mr. K. Patwari, Mr. B.B. Reddy, and Mr. S.R. Sarkar during heat making and casting is acknowledged. Authors are thankful to Mr. C.B. Sinha and Dr. S.S. Dutta for coordination with plant and active support during different stages of planning, production, and dispatch. Fruitful technical discussions with Dr. Niladri Sen and Dr. C.D. Singh is acknowledged with pleasure. Keen interest and support provided by Dr. S.K. Chaudhuri, GM(P), RDCIS, and Sri A. Bhattacharyya, retired GM(RCL), DSP are gratefully acknowledged. We are indebted to application engineers, Sri S. Bhattacharyya, Sri N.K. Mehta, and Sri G.K. Sodani for their painstaking technical help during performance trials. Help received from Sri R.R. Storte, Sri Abhishekh Sinha, and Smt. Mausumi Bhattacharyya in coordinating the trials at customers’ end is acknowledged with pleasure.

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Copyright information

© ASM International 2008

Authors and Affiliations

  • Santosh Kumar
    • 1
  • Vinod Kumar
    • 1
  • R. K. Nandi
    • 2
  • T. S. Suresh
    • 3
  • Ramen Datta
    • 1
  1. 1.RDCIS, SAILRanchi India
  2. 2.RDCIS Plant Centre, DSP DurgapurIndia
  3. 3.Durgapur Steel PlantDurpaur India

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