Hot rolling of AISI 304 tailored strips produced by ...

e.Proofing

Seite 1 von 19

Hot rolling of AISI 304 tailored strips produced by twin roll strip casting M. Vidoni 1,* Email [email protected] M. Daamen 1 Email [email protected] J. Gastreich 1 Email [email protected] N. Offermanns 1 Email [email protected] G. Hirt 1 Email [email protected] URL http://www.ibf.rwth-aachen.de 1 Institute of Metal Forming (IBF), RWTH Aachen University, Intzestraße 10, 52056 Aachen, Germany

Abstract A process for the production of steel strips with an optimized cross section by means of twin-roll strip casting has recently been developed at the Institute of Metal Forming and offers considerable technical and commercial potentials. This production concept is based on the use of profiled casting rolls which continuously transfer the profile geometry to the solidified strip. In order to reduce the internal porosity and improve the surface finish the cast strip was hot rolled using profiled rolls. Offline and inline rolling experiments were performed to prove the effectiveness of this combined forming

http://eproofing.springer.com/journals/printpage.php?token=g6l9YzkZaPDOxb1WXc... 26.06.2014

e.Proofing

Seite 2 von 19

process. This paper describes the design of a proper rolling tool and the effect of this forming step on the final profile geometry, surface quality, internal porosity and mechanical properties of the strip. The experiments showed that a rolling step with 15 % thickness reduction already allows the global internal porosity to be reduced by 48 % and the strip surface roughness to reach a minimum value of Ra = 2.3 µm. The tensile strength is improved after the hot rolling operation and the obtained values are within the 500–700 MPa range specified by the norm EN 10088-3 for the stainless steel AISI 304. The prescribed ultimate elongation of 45 % is also reached.

Keywords Profile casting Twin-roll strip casting Tailored strip Hot rolling AISI 304

1. Introduction Lightweight construction methods increasingly involve the use of tailored products. Tailored blanks and tailored strips are frequently used in the automotive industry. Two application examples are car body side panels [ 1 ], and exhaust systems [ 2 ]. The production of transversally profiled strips is already possible with strip tailor welding and profile rolling processes [ 3 – 5 ]. In the welding method, a separate uncoiling and welding unit needs to be included in the production line for each required thickness, increasing the complexity and cost of the line itself. The joining process requires the initial leveling of the material, provided by manufacturers as coil [ 6 ]. The flatness of the base strips is relevant not only for the quality of the end product, but also because it impacts on the reliability of the welding process itself [ 7 ]. After uncoiling and straightening, the edges of the strips are milled or cut with precision shears [ 7 ]. The typical welding technique is laser welding due to the relatively small extension of the heat affected zone [ 8 ].


e.Proofing

Seite 3 von 19

Strip profile rolling is a forming process able to produce a localized thickness reduction in the cross section of an initially flat strip. This cold rolling process adopts a narrow deformation zone inducing a lateral material flow. During investigations performed at the Institute of Metal Forming (IBF), a bulging phenomenon around the deformation area could be observed during the profiling experiments [ 3 ]. The adoption of multiple rolling passes and of an optimized roll geometry lead to a reduction of bulging. Strip profile rolling, however, requires a relatively large number of rolling steps to produce a profile in the transversal direction. In the profile rolling experiments a profile width of 64 mm and a depth of 1 mm required 29 rolling passes and a final flattening pass [ 3 ]. The target of this work is to investigate the potential of the twin-roll profile casting process with an integrated hot rolling step for the production of transversally profiled steel strips by investigating the properties of the profile strip produced with specifically developed casting and rolling tools. An illustration of the profile strip casting principle with an inline rolling step is provided in Fig. 1 . In comparison to the welding method and to strip profile rolling, this method allows more than two different thicknesses and curved transitions are in principle possible. In this work hot rolling experiments were carried out. The surface quality, internal porosity and mechanical properties of the strip after hot rolling were investigated and compared with the corresponding values before hot rolling. Fig. 1 Illustration of the twin-roll profile casting principle with inline rolling


e.Proofing

Seite 4 von 19

In twin-roll strip casting, liquid steel can be cast and directly hot rolled to hot strip in thicknesses ranging from 1 to 5 mm. This process is industrially used for the production of a variety of steel grades. Castrip LLC (USA) produces mainly carbon steels [ 9 , 10 ], while ThyssenKrupp Nirosta (now Outokumpu, Germany) and POSCO (Korea) focused the research on stainless steels [ 11 ]. Also Japan and China invested in this production technology. The advantages of this process lay on the compact process route, reduced energy consumption, but also in the possibility to produce steel grades in thin gauges which until now could be produced at great expense such as high-manganese steels [ 12 ]. Currently, strip casting is used industrially only for the production of strip with a constant thickness distribution. Using specifically profiled casting rolls it is possible to produce strip with an optimized crosssection. Numerical and experimental studies demonstrated the feasibility and the limitations of this concept by producing strip with a thickness difference up to 1 mm in a stable process [ 13 ]. It was found that the difference in the solidification conditions introduced by the thickness variation has an impact on the microstructure and on the internal porosity of the strips, leading to an increased porosity in the zone with higher thickness.

2. Methods


e.Proofing

Seite 5 von 19

In order to reduce the internal porosity and improve the strip surface finish, a hot rolling step is investigated in this work. Both offline and inline hot rolling were tested. The material chosen for the experiments is the austenitic stainless steel AISI 304 and its chemical composition is reported in Table 1 . Table 1 Target composition of the material AISI 304 used for the experiments Element

C

Si

Mn

Cr

Ni

Mass%

0.08

0.4

1.2

17.5

8.3

2.1. Experimental setup The vertical twin-roll strip caster used for the experiments enables the production of a strip with a thickness ranging from 0.8 to 3.0 mm and a width ranging from 130 to 250 mm according to the mounted casting rolls. In this work, the casting rolls used were 148 mm wide. A schematic representation of the used equipment which is operated in cooperation with ThyssenKrupp Steel Europe AG is shown in Fig. 2 . After solidification the strip can be thermo-mechanically treated. A rolling stand situated at the base level of the casting facility and two optional water cooling zones allow the investigation of a wide range of strip inline treatments. The integrated rolling mill has a maximum rolling force of 700 kN, work roll diameter of 166 and 200 mm barrel width. The profiled strips used in the present work were produced with a casting speed in the range from 0.4 to 0.7 m/s, target melt delivery temperature of 1,460 °C. Fig. 2 Schematic representation of the experimental facility of the IBF


e.Proofing

Seite 6 von 19

2.2. Geometrical definition of the used profiles Two different profile geometries were used for the casting rolls and two corresponding profiles were applied to the work rolls to be used for the inline and for the offline hot rolling tests. A schematic description of the profile geometries used for the experiments is provided in Fig. 3 . Fig. 3 Schematic representation of the profiles used for the experiments

The profile width (38 mm) is the same in all profiles used for the experiments. The profile depth of the work-rolls is defined depending on the casting-roll geometry in order to achieve a constant height


e.Proofing

Seite 7 von 19

reduction across the strip width. A detailed definition of casting and rolling geometries is reported in Table 2 . The investigated profiles are not symmetrical (αL ≠ αR) in order to study different transition geometries with a single roll preparation. The principle adopted for the preparation of the work rolls is to achieve the same reduction in the thin and thick area in order to avoid the occurrence of flatness problems. This implies that the rolls need to be designed for a defined thickness and a defined reduction. Table 2 Geometrical parameters used for the experiments Exp. type (Unit)

Offline

Process step –

Thin zone thickness hThin (mm)

Δh = hThick−hThin (mm)

Profile transition steepness α (Degrees)

Casting

1.10–1.50 –2.00

0.50

L 5.7 R 2.8

Rolling 15 % red

0.94–1.28 –1.70 0.36

L 3.7 R 1.4

Rolling 25 % red

0.83–1.13 –1.50

Casting

From 1.40 to 2.20

0.70

L 7.7 R 45.0

Rolling 15 % red

From 1.19 to 1.87

0.52

L 5.9 R 34.4

Inline

Preliminary calculations showed that with the casting width and thickness range of the casting facility, with an initial rolling temperature of 1,000 °C and AISI 304 as cast material it would not be possible to obtain a constant reduction higher than 15 % with the available maximum rolling force in the inline rolling experiments. Therefore, for the offline experiments the strip width W was reduced to 80 mm and the profile was designed for 25 % reduction. The profiles used for the inline rolling were designed for a 15 % uniform reduction with an initial thickness of 1.5 mm in the thinner region of the profile. Flatness problems are expected when rolling with


e.Proofing

Seite 8 von 19

a reduction different from the design value as well as in the transition time while closing the rolling gap to the target value.

2.3. Offline and inline rolling experiments For the offline rolling experiments strip samples were reheated in a furnace under argon atmosphere up to 1,100 °C. This temperature is a representative value for the rolling entry temperature measured at the mill entry during the combined casting and inline rolling process. The rolling speed for the offline rolling experiments was also selected in a range typical for the casting operation with the used casting equipment: 0.35 m/s. During the offline experiments the strips were manually extracted from the reheating furnace and introduced in the rolling gap. Three different strip thicknesses were hot rolled (initial strip thickness before rolling, in the thin zone h0–Thin: 1.1, 1.5 and 2.0 mm) with two reductions: 15 and 25 % using the same profiled rolls. Flatness defects were expected when rolling with 15 % height reduction. The mill entry and exit temperatures were monitored by means of pyrometers. In the inline rolling experiments the casting process starts with an opened rolling gap. When the casting process is stabilized, the rolling gap is closed to its target operating point. In contrast to the offline experiments, the inline experiments allow a continuous variation of the cast and rolled thickness by introducing ramps and different set-points. Four casting experiments with integrated rolling were performed with thicknesses ranging from 1.4 to 2.2 mm in the thinner region of the profile and a 15 % uniform reduction. Figure 4 illustrates one of the profile inline hot rolled coils. Fig. 4 Cast and inline hot rolled coil, material AISI 304 with a profile thickness difference of 0.7 mm, and a thickness in the thin region ranging from 1.5 to 2.2 mm


e.Proofing

Seite 9 von 19

2.4. Strip analysis The tailored strips produced by twin-roll strip casting were analyzed before and after the rolling operation in order to provide a statement on the effect of rolling on the strip quality. The surface of samples taken from the as-cast, the offline hot rolled and the inline hot rolled strips were optically analyzed. In all samples a thin adherent scale layer was present and was removed by pickling. Roughness measures were optically performed on the pickled samples using a confocal microscope (Nanofocus µSurf 6.x). The profile transition regions of the pickled samples have been observed with a 3D microscope to compare their morphology before and after the rolling operation. The internal porosity of the strip represents an important quality parameter as it may reduce the reliability of the final products by causing premature component failure. The produced strip was then sectioned and embedded in resin to investigate optically the internal microstructure and porosity. For each strip condition ten complete transversal sections were analyzed. In the present study a quantitative analysis method to compare the porosity of the as-cast and hot rolled strip is proposed. The pores´ sizes were measured and classified in three categories (small, medium, large) according to their maximum size as reported in Table 3 . Table 3 Pore classification according to their maximum size Pore max dimension (µm)

Category name

30, 60

L—large

The majority of pores were concentrated in the strip central globulitic zone. Because of this the number of pores per centimeter width was chosen as comparison criteria. An example of the obtained micrographic image and of the measured pores is reported in Fig. 5 . Fig. 5 Example micrographic image with pore measurement and corresponding classification

In this work the number of pores divided by the width of the analyzed strip section is proposed as comparison quantity and expressed in pores/cm width. Tensile tests were performed on strip samples to compare the local mechanical properties in the thin and thick regions of the profile before and after the hot rolling operation. Due to the limited width of the cast strip and the presence of the longitudinal profile, all the probes for the tensile tests were oriented along the casting direction as depicted in Fig. 6 . To ensure reliable results, for each strip condition six samples were cut in the lateral thin zones and six in the central thick zone of the profile. Fig. 6


e.Proofing

Seite 11 von 19

Top view of the tensile test samples orientation in relation to the cast strip

3. Results and discussion The profile rolling experiments showed that the rolling step is useful to improve the profile geometrical precision and the surface finish of the final strip. The offline experiments did not show any flatness defects even applying different reductions from the designed one, probably due to the reduced strip width.

3.1. Surface characterization The hot rolling step reduces sensibly the surface roughness and this effect is already visible without using optical instruments as depicted in Fig. 7 . Fig. 7 Surface detail of the as-cast (a), offline hot rolled (b) and inline hot rolled (c) strips after pickling


e.Proofing

Seite 12 von 19

The results of the roughness measurements are reported in Fig. 8 . The roughness Ra is sensibly reduced through the rolling process in both the thick and in the thin region of the profile. The surface of the work-rolls was prepared by grinding and had an initial roughness Ra of 0.37 µm. The Ra values for the inline hot rolled samples are higher and more dispersed if compared to the ones of the offline experiments. Fig. 8 Roughness comparison before and after the rolling operation

This is due to the longer duration of the rolling operation in the inline case which made the effect of roll wear noticeable.

3.2. Geometrical accuracy Figure 9 compares the geometrical accuracy of the profile transition zone before and after the rolling operation. Fig. 9 Measured strip profiles in a smooth transition zone before and after offline rolling


e.Proofing

Seite 13 von 19

The local deviations from the target geometry are reduced due to the rolling step. The profiled work rolls used for the rolling operation are very smooth and redefine the profile transition.

3.3. Microstructure and porosity The microstructure in proximity of the surface results refined due to the hot rolling. Figure 10 illustrates a comparison of the microstructures in the as-cast and in the hot rolled condition. Fig. 10 Strip microstructure before (a) and after the inline rolling operation (b)


e.Proofing

Seite 14 von 19

The porosity in the thick areas of the as-cast strips used for the inline experiments is sensibly higher than the porosity in the same profile region of the strips used for the offline rolling experiments. This is attributed to the different geometry of the cast strip, which influences the solidification homogeneity. The initial thickness difference was Δh = 0.5 mm for the offline and Δh = 0.7 mm for the inline rolling experiments. The initial difference in pores/cm between the strips used for the offline and inline rolling experiments is 53 %. The hot rolling operation reduces this difference and after a 15 % rolling pass the porosity is reduced to