austenitic stainless steelskonsys-t.tanger.cz/files/proceedings/metal_01/papers/209.pdf ·...

METAL 2001 15. - 17. 5. 2001, Ostrava, Czech Republic

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EFFECT OF RESIDUAL ELEMENTS ON HIGH TEMPERATUREPROPERTIES OF AUSTENITIC STAINLESS STEELS

R.Flesch*, W.Bleck*, P.R.Scheller**

*Institute of Ferrous Metallurgy at RWTH Aachen, D-52056 Aachen, Germany**Krupp Thyssen Nirosta GmbH, D-47794 Krefeld, Germany

ABSTRACT

After partial melting and solidification of cylindrical samples hot tensile tests were performedon austenitic stainless steels containing residual elements such as copper, tin and lead as wellas calcium and magnesium. Using well controlled cooling conditions down to the testingtemperature a radially solidified microstructure in the central part of samples was achieved.The testing material was prepared by remelting of base material from the industrial productionand addition of single elements in the vacuum induction furnace. The maximum force and thereduction of area were determined in the temperature range between liquidus and 1100°C.With regard to the reheating and hot rolling process some samples were thermally treatedunder industrial conditions. The ductility of the material at temperatures down to 950°C wastested and the effect of annealing evaluated.Recommendations for material processing by continuous casting and hot rolling are derivedfrom the tests performed.

1. INTRODUCTION

The continuous casting process is characterised by high yield, constant quality level and thesaving of several processing stages in comparison with the previous ingot casting process. Forthe cost reasons and quality requirements a defect-free cast product has to be charged in thenext stage of processing without, if possible, any previous inspection and machining. To gainknowledge about the way in which faults – inner or surface cracks - occur in continuouscasting and about the material properties at high temperatures associated with this, numeroushot tensile tests have been developed. While the tensile tests at high temperatures up to justbelow 1000oC are known as “warm” tensile tests, tensile tests at temperatures from above1000oC to just below the liquidus temperature are known as “hot” tensile tests. In hot tensiletests, material properties such as strength and ductility at high temperature are investigated,which enable the solidification process and continuous casting process conditions in respect ofproduct quality to be optimised.

The high-temperature properties of metallic materials can be influenced by numerousparameters. The effect of significant influencing factors such as chemical composition, strain


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rate and the temperature-time cycle have been frequently investigated /1-11/. The test resultobtained at various investigative sites, even with the same materials, show up differences dueto the different designs of plant and different test parameters. In the majority of theseinvestigations the specimens were not melted. However, in a previous work it was shown thatthe condition of the microstructure, i.e. the solidification microstructure or deformedmicrostructure, and the alignment of the tensile load in relation to the orientation of themicrostructure has a significant influence on the results of the analysis /12/. The solidificationmorphology and the distribution of δ-ferrite relative to the direction of the deformation, in thecase of stainless steel, for example, have a significant influence on high-temperature ductility/12/.

1.2 Hot crack formation in continuous casting

Hot cracks can occur during casting, welding and hot forming, if the material can no longerdissipate the stresses occurring as a result of stressing and straining. In saying this, two typesof hot cracks have to be distinguished /13/. Type I hot cracks are designated as those whichoccur as inter-crystalline separations, when fluid films wet the grain edges or cells and themicrostructure tears apart under tensile load without plastic deformation. Type II hot cracks,conversely, occur without the involvement of fluid phases in the temperature range of reducedductility, which lies approximately in the range below the re-crystallisation temperature /13/.They are therefore called “ductility-dip cracks”.

Cracks of Type I are called segregation cracks, because the films occur due to micro-segregations during the solidification phase. This type of crack has to be further subdividedinto solidification cracks and melting cracks.

During the solidification process the melt in front of the solidification front enriches inalloying and residual elements so that, by the end of the solidification process, there can be asmall quantity of residual melt, which separates the already solidified microstructures fromeach other. Contraction strain during the solidification and cooling phases, which can beadditionally increased by bulging, can cause surface and internal cracks /14/. Even if thesolidification cracks bond during the subsequent hot forming procedure, the segregated areasremaining in these positions can cause cracking if the material is subjected to greater tensileloads /15/ . If such areas are cut during further processing, they can represent a starting pointfor hardness cracks /16/ or lead to cleavage of the material. Numerous presentations have beendeveloped in the past about the course of solidification /16, 17/. In what follows, the processof solidification crack formation will be explained using a schematic representation infigure 1. As crystallisation progresses, the concatenation of crystals is so far advanced atapprox. 30% remaining melt /18/ that the specimen can take on its first forces. Thistemperature is called the zero-ductility temperature TNF. At this point the specimen,macroscopically, is brittle and breaks completely because the films of melt remaining betweenthe particles are unable to transfer the occurring strains to neighbouring dendrites or grains. Ifthese locally segregated areas begin to solidify as the test temperature drops, the firstmeasurable reductions in area when breaking occur from the so-called zero-ductilitytemperature TNZ. While the strength increases constantly as the temperature falls, reductionsin area when breaking initially increase steeply and then decrease more or less steeply afterreaching a maximum value, which can be almost 100% depending on the type of steel. Thisso-called secondary reduction in ductility can be attributed to the reduced solubility of theaustenite for alloy and residual elements and the associated precipitation of particles /19-23/,


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formation of fluid phases /21/ or pre-eutectoid ferrite /22/ on the edges of the austeniteparticles. This can cause the minimum reductions in area when breaking to fall to very lowvalues.

The temperature range between the zero-strength and the zero-ductility temperaturecharacterises the mechanical properties at the border between solid and liquid phases. The sizeof this temperature difference ∆T0 ≡ ∆TNZF = TNF - TNZ is seen as a measurement of thesusceptibility to internal cracking /24-26/, susceptibility to hot-crack tendency /27/ of the steelin continuous casting process. Industrial investigations confirm that, as the temperatureinterval ∆T0 increases, the observed level of internal cracking increases /26/. Dependent onthe cooling that has taken place in the secondary cooling zone, in the case of carbon steels, theareas of the strand close to the surface, particularly the edges, move in the temperature rangein which the steel only has a limited degree of ductility according to the second reduction oftoughness. Plant operators use the information on the position and formation of the secondreduction of toughness to specify limit temperatures for bending and alignment of the castingor to optimise secondary cooling to such an extent that the frequency of surface cracks isreduced /28/.

The solidification morphology of the austenitic stainless steels is dependent on the balance offerrite and austenite forming elements. The balance is usually described by the quote ofchromium-equivalent to nickel-equivalent, Creq/Nieq. In the present work we calculatedCreq/Nieq after Hammar and Svensson /29/.

Austenitic chromium-nickel steels having a low Creq/Nieq ratio tend towards formation ofinternal cracks. The exit of the strand from the mould is a critical area for crack formation,because the cooling rate is lowered drastically and the temperature gradient in the strand islowered significantly. Consequently, the temperatures at the solidification front increase andcan even reach the liquidus temperature. The consequences of this are melts of segregationzones between the columnar crystals near the solidification front /30/. If the strand reaches thefirst zone of secondary cooling, larger temperature gradients are reached again.

If one wants to simulate the conditions in the solidification of the strand in hot tensile tests, asimilar state of the solidification microstructure has to be set besides the course oftemperature-time and the tensile load /12/. The solidification microstructure of hot tensilespecimens should be similar to an interesting area of the strand in its orientation, itsprecipitation condition and in the changes of phase occurring during the solidification process.Before investigating the susceptibility to hot cracking of cast microstructures, a preliminaryinvestigation of the process to be simulated is required /31/, in order to design test conditionsthat are as close as possible to industrial conditions.

1.3 Influence of chemical composition on the susceptibility to hot cracking

1.3.1 Influence of copper, tin and lead

Schürmann et al. /32/ performed laboratory studies to investigate the influence of copper andtin on the segregation behaviour of chromium and nickel. They discovered that the liquidustemperature drops slightly as the copper content increases. Moreover, they were able to findout that copper is concentrated in the residual melt. After this results the segregation


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behaviour of the principal alloy elements was affected by copper contents higher than approx.4%.Rogberg /33/ investigated the influence of added residual elements on the high-temperatureproperties in re-melted stainless steels alloyed with molybdenum. Contents of up to 4% Cu,0.048% P and 0.09% As were added. He discovered that an addition of Cu, P and As toprimarily austenitic solidifying materials leads to a deterioration of the properties oftoughness, while the addition of approx 0.1% tin had no noticeable influence.

1.3.2 Influence of calcium and magnesium

Because of their great affinity to oxygen and sulphur, the alkaline earth metals calcium andmagnesium are used in the treatment of the widest range of steel grades. They are mainly usedfor the modification of oxide and sulphide inclusions /34,35/. Jolley et al. /36/ investigated thebehaviour at high temperatures of grades of stainless steel containing Nb after the addition ofmagnesium. The susceptibility to cracking of the materials investigated was initially based onthe formation of eutectic areas containing niobium having a low melting point. By addingmagnesium an increase in high-temperature toughness was registered.

1.4 Definition of the problem

Different grades of stainless steels show very significant differences in terms of theirsusceptibility to hot cracking. In these grades of steel the occurrence of hot cracks is closelyassociated with the art of primary phases and the sequence of phases during the solidificationprocess. Primarily austentitic solidifying steel grades are highly susceptible to hot cracking.Besides the high level of contraction on solidification, the reasons for this lie in a reducedsolubility of phosphor and sulphur, a slower rate of diffusion of these elements in the matrixbut also in the high solubility of manganese in the austenite lattice.

There have been very few investigations, on industrially produced material, of the extent towhich susceptibility to hot cracking in the production of stainless, acid and heat-resistantsteels can be reduced by added elements such as calcium and magnesium and increased byresidual elements such as copper, tin and lead brought in from scrap metal. Therefore both theinfluence and the effect of these elements on the high-temperature properties should bestudied.

It is the objective of this work, by varying the content of various residual and added residualelements, to investigate their influence on the ductility and strength of selected steel grades athigh temperatures and therefore on the susceptibility to hot cracking in the temperature rangebetween liquidus and 1100oC. Furthermore, the influence of reheating under industrialconditions on the high-temperature properties in the temperature range of hot ductility is to beworked out.

2. EXPERIMENTAL

2.1 Materials and production of the specimens

Four grades of austenitic chromium-nickel steel 1.4301 (X4CrNi18-10), 1.4311 (X2CrNi18-10), 1.4435 (X2CrNiMo18-14-3) und 1.4439 (X2CrNiMoN17-13-5) were selected for these


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investigations. From industrially produced material, 40 kg melts were produced havingvarious contents of the elements copper, tin, lead, calcium and magnesium and were cast iningots.

The desired contents of additional elements were achieved by addition in an induction furnacein a protected argon atmosphere. An allowed amount was worked with in the addition ofcalcium and magnesium. The melts were quickly cast into moulds after the additives werestirred in. The material was then forged into rods 20 mm in diameter. In order to alloy in lead,previously manufactured electrodes (approx. 90 kg) were re-melted at elevated pressure in anelectroslag re-melting plant (DESU-plant) with the aid of an electromagnetic stirrer havingsimultaneous addition of lead. The following processing of the specimen material was thesame as described above. The chemical composition of the materials is listed in table 1.

The specimens used for tensile testing were 20 mm in diameter and a total of 130 mm inlength. Prior to the tensile test the central section of them 30 mm in length was melted andcooled at 3 K/s such that at least 80% of the cross section had solidified radially. Theperformance of the test has been described in a previous work /12/.

2.2 Investigative measures and test apparatus

The liquidus and solidus temperatures were determined using differential thermoanalysis withthe aid of apparatus Type 404 S, supplied by Netsch AG. The tensile tests were performed ona Trebel high-temperature tensile testing machine (figure 2) following partial melting andcooling of the specimens. The condition of the microstructure of the specimens at the time ofthe tensile test was examined on other specimens of the same material: specimens 5 mm indiameter and 6 to 7 mm in height were then melted in an Al2O3 crucible in a speciallydesigned furnace /24/. After passing through a set temperature-time cycle, the specimenswere quenched in a salt-water bath. The microstructure was then examined metallographicallyand using a microprobe.

The following process was adopted to investigate the effect of added elements on themechanical properties at temperatures, which are in the range within which industrial hotforming processes take place: the middle sections of the specimens prepared for the tensiletest were completely melted in the tensile testing machine and subsequently cooled to atemperature of 700oC at a cooling rate of 3 K/s. These specimens were then put through anindustrial walking beam furnace together with continuously cast slabs. The passage throughthe furnace lasted approx. 4 hours and was done in a slightly oxidising atmosphere atmaximum temperatures of 1290oC-1300oC. Immediately after leaving the reheating furnacethe specimens were quenched in order to freeze in the actual microstructure. The specimenswere then placed into the high-temperature tensile testing machine, heated to the maximumannealing temperature (1290oC-1300oC), maintained there for 360 seconds in order toestablish homogeneous distribution of temperature and then cooled to the testing temperatureat 3 K/s. After a further dwell time of 25 seconds to equalise the temperature over the crosssection of the specimen, the actual tensile test began. The test temperatures were set between1270oC and 950oC, because these are the temperatures reached during the pass through theroughing mill and the finishing roll stands of the hot rolling mill. The maximum (limit of thetesting machine) strain rate of 0.3 s-1, which was considerably higher compared with the testsdescribed above, was chosen which corresponds to an axial speed of 10 mm/s. This was still


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considerably lower than under operating conditions in the rolling mill (approx. 10 s-1), but wassufficient to investigate trends in influence of the additional elements.

3. RESULTS

3.1 The effect of copper, tin and lead on the material properties at high temperatures

For this series of tests two materials, 1.4301 und 1.4439, were selected which solidify in aprimarily ferritic and primarily austenitic mode respectively. Lead was only added to 1.4301.


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Copper

Figure 3a shows the characteristic temperatures for both materials with increasing Cucontent. The addition of copper fundamentally lowers the characteristic temperatures, but hasdifferent effects on both the materials. The effect on the width of the critical temperatureinterval ∆T0, which is defined by the difference between the zero-strength and the zero-ductility temperatures, is shown in figure 3b.It can be clearly seen that, in the case of material1.4439, the critical solidification interval increases from 34oK at 0.07% Cu to 48oK at 2.34%Cu. At the same time the zero-strength temperature and the zero-ductility temperaturesdecrease. In the case of material 1.4301 the liquidus and solidus temperatures, and the zero-strength and zero- ductility temperatures fall by approx. 8K with an increase in the coppercontent from 0.07% to 0.81% and remain at an almost constant level (figure 3a) at coppercontents of up to 2.49%. The critical solidification interval increases by this amountrespectively (figure 3b).

The course of the reduction in area when breaking and the maximum strength withtemperature for the materials alloyed with copper is shown in figure 4. A significant influenceof increasing copper contents on the reduction in area cannot be detected. No reduction in theductility can be detected at decreasing temperatures down to 1100oC in the case of 1.4439,and in the case of 1.4301, figure 5, the reduction in area when breaking even increases.Results of the tensile tests on specimens annealed at temperatures of hot forming processes,are shown in figure 4 and figure 5 as well and are signed by a dashed line for both materials.Specimens with the highest copper content (2.34% for 1.4439 and 2.49% for 1.4301) wereselected for the tensile test respectively. While no change was detected in the values on thereduction in area for 1.4301, they decrease slightly in the case of 1.4439 as the copper contentincreases. In the second case the reduction in ductility is probably attributable to the higherspeed of tension and not to the increased copper content.

Tin

Figure 6a shows the change of the characteristic temperature with increasing Sn-content forboth materials. The addition of Sn also fundamentally lowers the characteristic temperatures,but affects both materials in different ways. The effect on the width of the critical temperatureinterval ∆T0 is shown in figure 6b. In the case of material 1.4439, the critical solidificationinterval is almost doubled from 34 K to 67K by increasing the tin content from 0.006% to0.15% and then it reduces to 58 K at approx. 0.3% Sn. The effect of the tin content onmaterial properties of 1.4301 is different. As the Sn content increases from 0.009% to 0.22%,the critical solidification interval first rises and then remains at a constant level of 16 K up to aSn content of 0.41% Sn (sample Sn 3).

While in the case of 1.4301 both the zero-strength and the zero-ductility temperature(figure 6a), and the level of the reduction in area reduce at T<1250oC as the Sn contentincreases, figure 7, this change is not continuous at 1.4439, figure 8. In this case the lowestvalues are achieved at mean Sn contents of 0.15%. Studies of the microstructure couldattribute this peculiarity to the fact that an increase in the tin content clearly reduces thesegregation of molybdenum. At this Sn content, considerable S and Mn segregations can bederived as having a further influence. Altogether, it can be said of both materials that anincreasing tin content significantly reduces the toughness, particularly in the temperature


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range around 1300oC. The reduction in ductility is greater in the case of 1.4301 than in thecase of 1.4439. After the specimens with the highest Sn contents are annealed under industrial

reheating conditions the toughness reduces at strain rates ε•

of 0.3 s-1 in the case of bothmaterials, but remains at an approximately constant level in the temperature range between1250oC and 950oC. The results of the tests have been drawn in a dashed line in figure 7 andfigure 8. The reduction progresses almost in parallel compared with the non-annealed

condition and lower ε•

-values from which we can conclude that this change is largelyattributable to the influence of the strain rate.

Lead

The influence of lead on the high-temperature properties was studied in steel grade 1.4301(X4CrNi 18-10). A small number of specimens only allowed a limited range of investigationsto be done.

Lead was added in concentrations up to 14 mass ppm. It was discovered from DTAinvestigations that TLiq are reduced by approx. 8 K and TSol by approx. 10 K at Pb contentsbetween 10 and 14 mass ppm. The critical temperature interval between the zero-strength andthe zero-ductility ∆T0 increases from 5 K to approx. 25 K, figure 9.

The slow deformation rate seems to have no effect on the level of toughness. Conversely,tested annealed samples having strain rates of 0.3 s-1 show reduced toughness in thetemperature range between 1100oC and 1200oC.

3.2 Influence of calcium and magnesium on the material properties at hightemperatures.

The influence of the addition of calcium and magnesium on the strength and the ductility ofthe solidification microstructure at high temperatures was investigated in primarily ferriticsolidifying steel 1.4311 and primarily austenitic solidifying steel 1.4435.

Different added amounts of Ca and Mg increase the toughness in the case of 1.4311 over thefull temperature range, figure 10. These additions also have the same positive influence on1.4435; increased Mg contents of 15 to 17 mass ppm shift the zero-ductility temperature andtherefore the whole course of toughness down to temperatures approx. 10 K lower, figure 11.However, the level of toughness is considerably lower (approx. 30%) in comparison with thevalues for 1.4311

4. DISCUSSION OF THE RESULTS

While the critical solidification interval is only increased slightly (approx. 10 K) by theaddition of copper in the case of primarily ferritic solidifying steel 1.4301 accompanying witha simultaneous lowering of the solidus and zero-toughness temperatures, it is considerablyincreased in the case of primarily austentitic solidifying 1.4439. Conversely, the strength andductility at high temperatures is hardly affected by the addition of between 0.7 mass % and 2.5mass % copper. For industrial practice this means that at these Cu-contents the castingtemperature of 1.4301 should be lowered by approx. 10 K to 20 K and that, in the case of


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continuous casting of 1.4439, measures will have to be taken to reduce the mechanical load onthe strand shell.

Tin contents up to 0.4 mass % in 1.4301 increase the width of the critical solidificationinterval by only 10 K approx. Conversely, it is greatly increased in the case of 1.4439 by aslittle as 0.3 mass % Sn. An increasing Sn content significantly reduces the toughness of bothmaterials, especially in the temperature range around 1300oC. Because the decrease intoughness is more pronounced in the case of 1.4301 than in 1.4439, attention must be paid tothe parameters in the hot forming processes in the case of this material.

At lead contents of 14 mass ppm in 1.4301, there is a pronounced minimum toughness atincreased deformation rate in the temperature range between 1200oC and 1100oC. Thereforelead content must be kept below 14 mass ppm in industrial production.

Additions of Ca and Mg to 1.4311 and 1.4439 partially lower the liquidus and the zero-ductility temperatures slightly. They significantly increase toughness in the full temperaturerange from the Tsol down to 1100oC. From this it can be concluded that hot forming of thesematerials is improved with these additives.

5. SUMMARY

The elements copper, tin, lead, calcium and magnesium have different effects on the ductilityand the strength of the solidification microstructure of austenitic steels at high temperatures.In addition, the solidification mode – primarily ferritic or primarily austenitic – is animportant influencing factor.

Copper, tin and lead partially increase the width of the critical temperature interval, whichinfluence the hot-crack susceptibility, and decrease the ductility of the solidifiedmicrostructure under tensile stress. In industrial practice the danger of internal cracks in thecontinuous casting process and of micro-cracks in the hot forming process can be combatedby adjusting the casting parameters and by low deformation rates, respectively, especially inthe case of primarily austentitic solidifying steels.


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