Surface Hot Shortness of Copper Containing Steel

in a Compact Strip Production Process*

Akihiro Takemura1, Yusuke Ugawa2, Kazutoshi Kunishige3, Yasuhiro Tanaka3,Shunichi Hashimoto4 and Kazuya Ootsuka5

1Department of Mechanical Engineering, Faculty of Engineering, Tokyo University of Science,Yamaguchi, Sanyo Onoda 756-0884, Japan2Sumitomo Metal Industries, Ltd., Amagasaki 660-0856, Japan3Department of Advance Materials Science, Faculty of Engineering, Kagawa University,Takamatsu 760-8521, Japan4Kobelco Research Institute Inc., Kobe 651-2271, Japan5Nippon Steel Corporation, Futtsu 293-8511, Japan

The compact strip production (CSP) process has received much attention because it is environment friendly and can be used for recyclingresources. However, steel scrap, the main material used in the CSP process, causes surface cracks in the hot strip being manufactured as a resultof surface hot shortness by Cu and Sn in the steel scrap. This study investigates the influence of heat pattern on the brittleness of Cu-containingsteels prior to treatment in a reheating tunnel furnace at 1100�C. The prior austenite grain size of a sample that was reheated from roomtemperature before being transferred to a tunnel furnace was finer than that obtained by the direct transfer process, and the crack depth wasinhibited by 50%. In contrast, the prior austenite grain size of a sample obtained by a process in which austenite is reheated from a cooling stoptemperature of 650�C–850�C was almost the same as that obtained by direct transfer to the tunnel furnace. However, the crack depth in the caseof reheating from the cooling stop temperature of 650�C–850�C was greater than in the case of direct transfer. This deep cracking was caused bya noninternal oxidation area at the scale/steel interface. [doi:10.2320/matertrans.H-M2011818]

(Received April 27, 2011; Accepted July 15, 2011; Published September 25, 2011)

Keywords: copper-containing steel, surface hot shortness, compact strip production process, copper-enriched alloy, oxidized scale structure,

iron-selective oxidation

1. Introduction

The compact strip production (CSP) process is used tomake hot-rolled coils. In this process, a continuous cast slabmade from refined scrap in an electric furnace is sent directlyto a tunnel furnace. This process has received much attentionbecause it is environment friendly.1) However, trampelements such as Cu and Sn, which are present in steelscrap, cause surface cracks during hot rolling.2) It isextremely difficult to remove Cu from steel scrap duringthe steel-refining process because Fe is more oxidizable thanCu.3–5) Fe is selectively oxidized at the slab surface duringheating, and the Cu-enriched layer formed at the scale/steelinterface causes surface hot shortness. This Cu-enrichedlayer melts and penetrates into austenite grain boundariesbecause the melting point of this phase is relatively low.For these reasons, surface cracks appear as a result of liquidembrittlement during hot rolling.6–9)

Many studies have shown that Ni can suppress surfacehot shortness.6–8,10–12) One study found that conducting theoxidation process at a temperature higher than 1200�Calleviated surface hot shortness.7–11,13) The addition of Cr iseffective in suppressing surface hot shortness.14) However,these solutions have their own problems. The addition of Niincreases production costs, whereas the use of temperaturesgreater 1200�C in the heating process increases fuel costs.We investigated the effect of shot peening on the suppressionof surface hot shortness.15) In this paper, we focus on the

influence of austenite grain size on surface hot shortness.In addition, the influence of the cooling stop temperatureon austenite grain size and the suppression of surface hotshortness is discussed.

2. Experimental Procedure

Table 1 shows the chemical composition of steel used inthis study. This steel was produced in a vacuum meltingfurnace and contains 0.3%Cu–0.04%Sn as tramp elements.The amount of Ni was 0.02%, which is inevitably includedin normal steels. The melt was divided into two batches.The amount of Nb in one batch was adjusted to 0.03%.We produced 100-mm-thick ingots from two types of steel.The ingots were then heated at 1200�C and rolled into 40-mm-thick plates by hot rolling at 900�C–1100�C. In thispaper, surface hot shortness was investigated using variousspecimen shapes made of hot-rolled Cu-bearing low carbonsteels.

The specimens were compressed by Thermecmaster (Fuji-Denpa, Japan), a thermomechanical simulator, to investigatesurface hot shortness. The transformation temperature wasmeasured by Formaster (Fuji-Denpa, Japan), a thermalexpansion testing machine. Figures 1(a) and (b) show theThermecmaster and Formaster specimens, respectively.

Table 1 Chemical composition of steel (mass%).

C Si Mn P S Al Cu Sn Ni N

0.05 0.10 0.90 0.02 0.005 0.04 0.29 0.04 0.02 0.008*This Paper was Originally Published in Japanese in Netsu Shori 50 (2010)

138–143.

Materials Transactions, Vol. 52, No. 10 (2011) pp. 1905 to 1911#2011 The Japan Society for Heat Treatment

2.1 Simulation of CSP processFigure 2 illustrates the experimental procedure in which

the CSP process is simulated using Thermecmaster. First, thesample was heated to 1300�C at a rate of 2.5�C/s in avacuum. Subsequently, the sample was either heated directlyin a tunnel furnace or cooled to a temperature between 950�Cand room temperature, to investigate the influence of thecooling stop temperature on surface hot shortness duringthe CSP process. Hereafter, the heating process from 1300�Cto the tunnel furnace condition is called the direct transferprocess, the heating process once the sample cools is calledhot-charged substance (HC), and the heating process once thesample cools to room temperature is called RT.

The oxidation period in the tunnel furnace was 30 min at1100�C in air. A compression test using Thermecmaster wasconducted at 1100�C to investigate cracking caused bysurface hot shortness. The strain rate was 0.01/s and theamount of deformation was 50% for the compression test.Mica glass foils, which are usually laid on the top and bottomof a sample for lubrication between the sample and pistonheads of the compression test machine, did not use toenhance bulging. The compression test revealed that periph-eral length of specimens showed a maximum expansion ofapproximately 40%–45% at the center of the height. Surfacecrack depth was measured with an optical microscope. Inaddition, oxidized specimens that had the same shape andwere quenched before the compression test were obtained toinvestigate the causes of brittleness in Cu-bearing steelduring the CSP process. The microstructure at the scale/steel

interface of these specimens after oxidation was closelyobserved using an optical microscope.

2.2 Measurement of prior austenite grain size andtransformation temperature

The Formaster specimens shown in Fig. 1(b) were ob-tained using a wire electric discharge machine to investigatethe grain size of prior austenite. They were quenched in Hegas in a vacuum before the compression test by using thesame heat pattern as in the Thermecmaster process. Theywere galvanically corroded at approximately 8 V for 15 s inan electrolyte of 200 mL distillated water, 53.5 g chromiumacid, and 80 g phosphoric acid, to expose the prior austenitegrain boundaries. Then, the microstructures were observedusing an optical microscope.

The transformation temperature was measured using aFormaster test machine. The heat pattern is shown in Fig. 3.The heating and cooling speeds and the 15-min holdingperiod at 1300�C used in heat treatment were the same asthose for the simulation experiment during the CSP process.The transformation temperature during cooling was meas-ured using these test specimens.

3. Results

3.1 Results of maximum crack depthFigure 4 shows the influence of the cooling stop temper-

ature on the maximum crack depth at the surface of thespecimens. This figure also shows the grain size of prioraustenite before the compression test and the transformationtemperature measured with the Formaster test machine. Themaximum crack depth was 111 micrometers in the directtransfer process. These cracks increased with decrease in thecooling stop temperature to 750�C. The maximum crackdepths in the 950�C, 850�C, and 750�C HC processes were128, 210, and 319 micrometers, respectively. However, themaximum crack depth decreased to 245 micrometers in the650�C HC process and 119 micrometers in the 500�CHC process. The crack depth in the RT process was 62micrometers. This maximum crack depth in the RT processwas 20% of that of the 750�C HC process and 50% that ofthe 500�C HC process. Thus, the maximum crack depth isunderstood to change greatly depending on the cooling stop

(a) (b)

Fig. 1 Dimensions of specimens: (a) for compression test and (b) for

measuring grain size of prior austenite and transformation temperatures.

Fig. 2 CSP process laboratory simulation.

Fig. 3 Experiment to measure the transformation temperatures.

1906 A. Takemura et al.

temperature prior to reheating for 30 min oxidation at1100�C, which imitates conditions in a tunnel furnace.

3.2 Results of measurement of prior austenite grain sizeand transformation temperature

The metallographic structure after 30 min oxidation in airat 1100�C was observed in order to measure the prioraustenite grain size. Figure 5 shows optical micrographs ofthe Formaster specimens that were quenched in He gas beforethe compression test. The prior austenite grain boundaries ofthe specimens were visualized by galvanic corrosion. Theprior austenite grain size was 90 micrometers in the directtransfer process and 114–126 micrometers in the 950�C–750�C HC process. However, no influence of the cooling stoptemperature was observed on the prior austenite grain size inthe 950�C–750�C HC process. In contrast, the boundary ofthe prior austenite grain size was refined under the 750�C HCprocess. The grain size was refined to 92 micrometers in the650�C HC process. The prior austenite grain size in the650�C HC process was the same as that in the direct transferprocess and the refined grain sizes were 51 and 54 micro-meters in the 500�C HC and RT processes, respectively.The grain sizes of prior austenite in the 500�C HC and RTprocesses were 50% that of the direct transfer process.

The transformation temperature of the specimens wasmeasured using the Formaster test machine. Figure 6 showsthe thermal dilatation curve during cooling from 1300�C. Thetransformation temperature was determined by measuringthermal dilatation of the specimen. Dilatation started at743�C, which shows the start of transformation fromaustenite to ferrite. It was finished at 570�C, which showsthe completion of transformation. Pf can be regarded as theAr1 point because the cooling rate is as low as 2.5�C/s. Thisresult suggests that the specimens had an Ar3 point at 743�Cand an Ar1 point at 570�C. In the temperature range from743�C to 570�C, the metallographic structure was estimatedto be a two-phase structure of ferrite and austenite. When thecooling stop temperature was 650�C or less, which is lowerthan the transformation temperature at 743�C, the prioraustenite grain size was refined by the A3 transformation. Theprior austenite grain size was refined at the start of trans-formation and was refined significantly at a temperature of500�C or less.

Figure 6 shows the relevance of the transformationtemperature to the prior austenite grain size. Comparison ofthe direct transfer process and the RT process reveals thatsurface hot shortness in the RT process was suppressed andwas influenced by the grain refinement of prior austenite.However, the reason for increase in the maximum crackdepth at 750�C–850�C, where the grain sizes were almost thesame, cannot be explained from the prior austenite grain size.

4. Discussion

4.1 Refinement of prior austenite grain by cooling stepbefore transfer to tunnel furnace and suppression ofsurface hot shortness

The maximum crack depth was suppressed in the 500�CHC and RT processes. Figure 7 shows the mechanism of therefinement of the prior austenite grain size during the coolingand reheating steps. Transformation from austenite to ferriteduring cooling and the reverse transformation from ferriteto austenite during reheating create fine grains. The matrixwas austenite at 1300�C, which simulated slab solidification.The metallographic structure seems to be transformed fromaustenite to ferrite by cooling before heating at 1100�C in asimulated tunnel furnace. For the specimens used in thisstudy, the transformation start temperature was 743�C, thetransformation finish temperature was 570�C, and the heatingand the cooling rate was 2.5�C/s. Thus, the thermal treatmentperiod of the specimens in the HC and RT processes werelonger than those in the direct transfer process, because inthe HC and RT processes, the samples were cooled beforetransfer to a tunnel furnace and were then reheated. Thespecimens in the 950�C–750�C HC process were not cooledto a transformation temperature. For this reason, effect ofgrain refining by cooling and reheating before transfer to thetunnel furnace was not observed in these specimens. There-fore, it seems that the specimens in the 950�C–750�C HCprocess were coarsened by the long thermal treatment time.

The refinement of grain size started in the 650�C HCprocess because of the following reasons: The specimensused here had an Ar3 point at 743�C and an Ar1 point at570�C. It seems that the cooling to 650�C before transfer tothe tunnel furnace transformed the austenite specimen to atwo-phase structure of ferrite and austenite. A slight refine-ment of the prior austenite grain size was achieved by reversetransformation from partial ferrite.

Specimens cooled below 500�C transformed austeniteinto ferrite. At this time, the ferrite phase was refined bytransformation sources, i.e., the grain boundary of austeniteand the triple point of the grain boundary. Then, a reversetransformation from ferrite to austenite occurred by reheatingat 1100�C in a simulated tunnel furnace. The refinementof grain size caused a reverse transformation from the grainboundary of the ferrite phase and the triple point of the grainboundary. The grain size was more refined as a result ofthe transformation from austenite to ferrite and the reversetransformation from ferrite to austenite.

The refinement of grain size is believed to improve thesuppression of surface hot shortness.7,8) Figure 8 showsschematic figures of the influence of grain size refinementon the suppression of surface hot shortness. The area of the

Fig. 4 Relationship between maximum crack depth, grain size of prior

austenite and phase region where cooling stop temperature is located.

Surface Hot Shortness of Copper Containing Steel in a Compact Strip Production Process 1907

grain boundary is increased by the refinement of the grainsize, which was achieved by cooling before transfer andreheating in a tunnel furnace. If the Cu segregation at thescale/steel interface is independent of the grain size, the areaof the grain boundary increases with decrease in grain size.Hence, Cu-enriched alloy at the scale/steel interface isdispersed and the depth of Cu penetration decreases.Therefore, surface hot shortness is suppressed by therefinement of grain size. Figure 9 shows the edge of hot-

rolled 40-mm-thick steel that was finished at temperatures inexcess of 900�C after being heated to 1200�C. Figure 9(a)shows interstitial free steel bearing 0.3%Cu–0.04%Sn,which is called Nb free steel. Figure 9(b) shows interstitialfree steel bearing 0.3%Cu–0.04%Sn–0.03%Nb, which iscalled 0.03%Nb bearing steel. The prior austenite grain sizesin the Nb free steel and 0.03%Nb bearing steel were 154 and102 micrometers, respectively. In case of 0.03%Nb bearingsteel used in this study, Nb was completely melted at

Fig. 5 Optical microstructures of the steels were subjected to each heat treatment changing the following cooling stop temperatures:

(a) direct, (b) 950�C HC, (c) 850�C HC, (d) 750�C HC, (e) 650�C HC, (f) 500�C HC and (g) RT.

1908 A. Takemura et al.

1073�C, explaining equilibrium solubility product.16) Theprior austenite grain size is refined by the solute drag effectof the dissolved Nb.17)

4.2 Change in austenite grain size and surface crackdepth at 650�C–850�C

Figure 5 shows the prior austenite grain size in the 650�C–850�C HC process, which shows no change compared withthe direct transfer process. However, the specimens in the650�C–850�C HC process have large cracks with more than200 micrometers depth. This suggests that there may beanother cause that enhances surface hot shortness, in additionto the coarsening of the austenite grain.

The microstructure of the scale/steel interface of theoxidized specimens was investigated in detail. A character-istic phase area that had no internal oxidation was found insamples at the cooling stop temperature of 850�C or less.Large oxidized particles surrounding this area were alsofound. This phase area increased with decrease in the coolingstop temperature. An example of this phase area is shown inFig. 10. Hereafter, this phase area is called the noninternaloxidation area.

The chemical composition of this area was analyzed usingEDX. Fe and O characteristic X-ray peaks were detected.However, no enriched Cu or Sn phase was detected. Normalinterstitial free steel was used in this study, which containssmall amounts of Si and Mn. Si and Mn are more easilyoxidized than Fe. A solute of Si and Mn as the solid solutionwas oxidized by O2� ion, diffused from air. This results in theinternal oxidation of Si and Mn, which are more oxidizablethan Fe at the initial stage of oxidation. In addition, theaustenite grain size was refined by cooling before transfer toa tunnel furnace and reheating. In this manner, the grainboundary diffusion of O2� ion was accelerated duringoxidation at 1100�C. Furthermore, decarburization occurredduring oxidation at the same temperature.18) The internaloxidation of Si and Mn near the decarburized area and theoxidized grain boundary massed together. It seems thatthis decarburized area became a noninternal oxidation area.Figure 11 shows optical micrographs of a specimen from the650�C HC process at the scale/steel interface. Cu-enrichedalloy is observed in large amounts near the noninternaloxidation area. It seems that formation of the noninternaloxidation area, which increased internal oxidation arounditself, accelerated surface hot shortness.

Figure 12 shows the influence of the cooling stop temper-ature, the prior austenite grain size, and the noninternaloxidation area on the embrittlement of Cu-bearing steelduring the CSP process.

Fig. 6 Thermal contraction curve on cooling.

Fig. 7 Schematic illustration of the structural refinement through trans-

formation.

Fig. 8 Schematic illustrations of an enriched copper phase at the scale/

steel interface: (a) coarse grain and (b) fine grain.

Fig. 9 Appearance of surface hot shortness in 0.30% Cu-0.04% Sn steels:

(a) Nb free and (b) 0.03% Nb bearing.

Surface Hot Shortness of Copper Containing Steel in a Compact Strip Production Process 1909

Fig. 10 Optical microstructures at the scale/steel interface.

Fig. 11 Optical microstructure at the scale/steel interface of the 650�C HC

specimen.

Fig. 12 Schematic diagram for the severity of cracking against cooling

stop temperatures, being influenced by two factors of grain size of prior

austenite and non internal oxidation area.

1910 A. Takemura et al.

5. Conclusions

Interstitial free steel bearing 0.30%Cu–0.04%Sn, theconstituents of which were designed to imitate a standardelectric furnace steel, was melted in an experimentalprocedure. Processing heat treatment that imitated theCSP process was applied using a thermomechanicalsimulator. Before transfer to a tunnel furnace, the coolingstop temperature that affects surface hot shortness wasinvestigated. The parameter set for the tunnel furnace wasa holding period of 30 min in air at 1100�C. The amountof deformation during a compression test at 1100�C was50%. The surface crack depth at the center part of theheight, which occurs by bulging, was examined andmeasured for surface hot shortness. The results obtainedare as follows:

(1) The surface crack depth in the RT process was thesmallest. This crack depth was 50% smaller that in the directtransfer process. However, it was large in the 650�C–850�CHC process.

(2) The results of measuring the transformation temper-ature showed that the Ar3 and the Ar1 point were 743�Cand 570�C, respectively. The prior austenite grain size wascoarse in the 650�C–950�C HC process. When the specimenswere cooled to 500�C or less, the prior austenite grain sizewas refined by 50 micrometers, suppressing surface hotshortness.

(3) The results of the examination of the scale/steelinterface of the He-quenched oxidized specimens revealed apeculiar phase area that had no internal oxidation at thecooling stop temperature of 850�C or less. Large oxidizedparticles were found around this area.

(4) It seems that the formation of a noninternal oxidationarea, which increased internal oxidation around itself,accelerated surface hot shortness in the 650�C–850�C HCprocess.

Acknowledgement

The authors are greatly indebted to Dr. N. Yoshinaga andD. Maeda, Nippon Steel Corporation, for their discussionsand valuable advice.

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Surface Hot Shortness of Copper Containing Steel in a Compact Strip Production Process 1911