Effects of Heat Treatment on Microstructure and

Materials Science and Technology (MS&T) 2009 October 25-29, 2009, Pittsburgh, Pennsylvania • Copyright © 2009 MS&T’09® Steel Processing, Product and Applications Symposium

Effects of Heat Treatment on Microstructure and Mechanical Properties of Bainitic Single- and Complex-Phase Steel F. Hairer, C. Krempaszky Christian Doppler Laboratory of Material Mechanics of High Performance Alloys, Technische Universität München; Boltzmannstraße 15, 85747 Garching, Germany P. Tsipouridis, E. Werner Chair of Materials Science and Mechanics of Materials, Technische Universität München; Boltzmannstraße 15, 85747 Garching, Germany K. Satzinger, T. Hebesberger, A. Pichler voestalpine Stahl Linz, B3E Research and Development; VOEST-ALPINE Straße 3, Postfach 3, Linz A-4031, Austria Keywords: advanced high strength steels, complex-phase steels, microstructure, heat treatment

Abstract Different heat treatments were applied to process low-alloyed bainitic single- and complex-phase steel grades with strength levels above 800 MPa. Specimens were heat treated in a multi-purpose annealing simulator (MULTIPAS) in order to identify microstructures with optimum mechanical properties. Light optical microscopy (LOM) as well as scanning electron microscopy (SEM) were used to characterize the microstructure of the specimens. The fraction of retained austenite was measured by a magnetic-volumetric method. To characterize the mechanical properties of the investigated materials, tensile and hole expansion tests were conducted. The effects of selected heat treatments on the resulting microstructure and on the mechanical properties are discussed.

Introduction Increased ecological awareness and enhanced demands on passenger safety led to the development of high strength sheet steels. The importance of automotive lightweight construction increases in order to minimize the fuel consumption and CO2 emission as passenger safety and crash worthiness are to be maximized. Therefore advanced high strength steels (AHSS) are developed continuously. For specific applications the microstructure can be modified in order to enhance total elongation for stretch forming or local elongation for sheared edge stretching limits. Besides transformation induced plasticity (TRIP) and ferritic-martensitic dual-phase (DP) steels, bainitic single- and multi-phase (complex-phase) steels are gaining interest due to their combination of high strength and a high hole expansion ratio, which is an important property for flanging and stretching operations. In this work the influence of annealing, quenching and overaging temperature is studied on the laboratory scale to optimize the strength and stretch flangeability of a low alloyed complex-phase steel, whose microstructure is mainly bainitic.

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Experimental Procedure - Heat Treatment and Microstructure Development Steel plates with dimensions of 170 x 450 mm2 were machined from industrially produced lowalloyed cold-rolled sheet steel with a thickness of 1.2 mm (voestalpine Stahl Linz, Austria). The chemical composition of the steel grade is given in Table 1. Table 1 Chemical composition of the steel [wt%]

C 0.16

Si 0.035

Mn+Cr+Mo 2.2

Al 0.05

Ti+Nb+B 0.005

The heat treatments were conducted using a Multi-Purpose Annealing Simulator (MULTIPAS) built by vatron GmbH, Linz, Austria. This setup enables the investigation of the impact of heat treatment on the microstructure and on the mechanical properties. By applying electrical resistance heating and gas-jet cooling, heating rates of up to 50 K/s and cooling rates from the annealing to the overaging temperature of -100 K/s can be obtained. To study the influence of the annealing conditions on the microstructure evolution and on the mechanical properties, a wide variety of heating cycles were performed. These cycles are conducted within the limits of an industrial hot dip galvanizing line, as schematically shown in Fig. 1. The specimens were heated up to the annealing temperature, which was varied from 780°C to 840°C in steps of 20°C; the holding time at each temperature was set to 45 s (allowing for complete recrystallization to take place). The austenitization temperature for the analyzed alloy was calculated with TermoCalc and measured in a dilatometer. The calculations did not yield reliable results because of the traces of boron in the alloy which could not be taken into account in the calculations due to a lack of thermodynamic data regarding this element. The dilatometric study revealed an austenitization temperature of 813°C which is about 10°C higher than the calculated one. At annealing temperatures above 813°C the microstructure is fully austenitic while the microstructure annealed at temperatures below is ferritic-austenitic. After annealing the specimens were cooled to the quenching temperature (selected in a range between 650°C and 750°C) and then quenched to the overaging temperature. The high cooling rate from quenching temperature prevents the formation of ferrite and perlite. The small amounts of soluble boron retard ferrite formation, which normally shields the bainitic transformation. The overaging takes place at temperatures between 400°C and 500°C (in steps of 25°C). During isothermal holding at the overaging temperature a bainitic transformation takes place. The upper bainitic transformation (generally between 550°C to 400°C) can be divided into two stages. Firstly bainitic ferrite forms, characterized by a low carbon solubility (< 0.03 wt.%) . The formation of ferrite enriches the remaining austenite with carbon. Cementite precipitates from the austenite between the ferrite sub-units. The amount of cementite depends on the carbon concentration and on additional alloying elements like Si. Silicon retards the precipitation of cementite, and as a result the microstructure consists rather of bainitic ferrite than of bainite at higher overaging temperatures [1-3]. After overaging, the specimens were heated up or cooled down to 460°C (temperature of hot dip galvanizing) and air-cooled down to room temperature with 15K/s. The heating or cooling to the galvanizing temperature does not cause significant changes in the microstructure. During the final quenching to room temperature, only the unstable austenite grains transform into martensite, while the stable one are maintained in the microstructure in form of retained austenite.

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TAN

Temperature [°C]

TQ CR=-3 K/s TOA CR=-50 K/s

Time [s]

Figure 1 Schematic representation of applied annealing cycles (T AN=annealing temperature, T Q=quenching temperature, TOA=overaging temperature, CR=cooling rate)

Microstructural Characterization The specimens for the microstructural characterization were wire cut (parallel to the rolling direction) from the annealed plates. For the investigations via light optical microscopy (LOM), specimens were embedded in epoxy resin, ground and polished to a grid of 1 μm and subsequently etched with LePera etchant. In microstructures etched with LePera ferrite (F) grains appear blue, bainite (B) brown, pearlite (P) black and martensite (M) white or brown. Martensite with a high amount of carbon appears white, while martensite with low carbon content appears brown and is partly structured. Line intercept method was used for a quantitative description of the martensite volume fraction, employing a line distance of 5 m and an analyzed area of 50.000 m2 (10 images) for statistical significance [4]. The scanning electron microscopy (SEM) analysis was conducted on Nital etched specimens to distinguish bainitic-ferrite from bainite by means of the quantity of cementite in the microstructure. In SEM micrographs ferrite appears dark, cementite in bainite bright and martensite either bright or dark. The content of retained austenite (RA) was determined using the magnetic-volumetric method [5]. Determination of Mechanical Properties Specimens for mechanical testing were wire cut from the annealed plates to avoid predamage of the microstructure during machining. Tensile tests were performed according to the standard EN 10002 parallel to the rolling direction. The hole expansion specimens were pieces of 100 x 100 mm2 with a hole (diameter 10 mm) introduced in the center of the specimens. The impact of the machining method on the quality of the hole edge and the resulting hole expansion ratio has been intensively studied [6]. Based on this knowledge, wire cutting was used for hole preparation. Hole expansion testing was performed according to the standard ISO/TS 1393

16630:2003. The holes were expanded by a conical punch of 60° top angle until the first crack at the hole edge was observed. The tests were conducted on an instrumented hole expansion device with a load-drop control [7]. The hole expansion ratio (HE) is calculated from the equation HE[%]

df  d0 ˜100% , (1) d0

where d 0 is the initial hole diameter and d f is the average hole diameter after testing [7]. Hardness testing (HV1) was realized according to the standard DIN EN ISO 6507-1:2005 using an indentation load of 9.807 N.

Results and Discussion Annealing temperature In order to study the influence of T AN in the range of 780°C-840°C, both the quenching and the overaging temperature were held constant at 750°C and 425°C, respectively. The resulting microstructures are shown in Figs. 2-5, the impact of annealing temperature on the hardness is plotted in Fig. 6. The initial state of the microstructure after annealing at TAN=840°C is fully austenitic. The specimen is cooled moderately to the quenching temperature TQ=750°C and then is rapidly quenched to the overaging temperature TOA=425°C. During overaging, the bainitic transformation takes place. The remaining austenite partly transforms to 6.4% martensite (2% “white” martensite and 4.4% “brown” martensite) during the final quenching to room temperature while a small fraction remains untransformed as retained austenite (0.6%). Annealing at TAN=820°C in the austenitic range results in a bainitic-martensitic microstructure with a retained austenite fraction of 1.4% and a martensite fraction of 2% (1% “white” and 1% “brown” martensite). The lower annealing temperature impedes austenite grain growth, resulting in smaller martensite and bainite grains. The hardness decrease from 314 HV1 to 300 HV1 (Fig. 6) can be explained by the slightly lower martensite fraction produced when annealing at TAN=820°C in comparison to 840°C. Annealing at 800°C is intercritical with an austenitization degree of 92%. The remaining 8% of ferrite causes a softening of the microstructure (reflected in the hardness drop shown in Fig. 6). Decreasing the annealing temperature to 780°C decreases the austenite fraction to 83%. Ferrite can be detected in form of dark, flat areas in Fig. 5b. Besides ferrite the microstructure consists of 2.5% martensite (1% “white” and 1.5% “brown” martensite) and contains 3.3% retained austenite. M

B

B

M

a) b) Figure 2 TAN=840°C, TOA=425°C, TQ=750°C, a) LOM (LePera etched), b) SEM (Nital etched)

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M

B

M

B

a) b) Figure 3 TAN=820°C, TOA=425°C, TQ=750°C, a) LOM (LePera etched), b) SEM (Nital etched)

M B M B a) b) Figure 4 TAN=800°C, TOA=425°C, TQ=750°C, a) LOM (LePera etched), b) SEM (Nital etched)

B B

F M M

a) b) Figure 5 TAN=780°C, TOA=425°C, TQ=750°C, a) LOM (LePera etched), b) SEM (Nital etched)

Figure 6 Hardness as a function of annealing temperature

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The results of the tensile and hole expansion tests are summarized in Figs. 7-8. Tensile strength and yield strength both decrease with decreasing annealing temperature while the elongation values increase. The maximum values of 896 MPa tensile strength and 757 MPa yield strength are achieved at 840°C together with a total elongation of 5.8%. The strength values decrease to 740 MPa tensile strength and 536 MPa yield strength at a total elongation of 13.1% at 780°C. Even though the elongation values of the investigated materials are rather low, the hole expansion ratios exceed 100%. This is plausible because failure in tensile testing is induced by necking whereas in hole expansion testing this plastic instability is shifted to higher strains by the stabilizing effect of the lesser strained material in the vicinity of the hole edge [8]. Therefore, failure in hole expansion specimens is more affected by local damage resulting from a high contrast of phase properties than by a plastic instability.

a)

b)

Figure 7 Influence of annealing temperature on tensile properties a) strength, b) elongation

Figure 8 Hole expansion ratio as a function of the temperature

Overaging temperature To investigate the effects of overaging temperature, the annealing temperature was fixed at TAN=840°C, because specimens annealed at this temperature showed the highest tensile strength and stretch flangeability. The quenching temperature again was set to TQ=750°C. The resulting microstructures after overaging at temperatures from 400°C to 500°C are shown in Fig. 2 and in Figs. 9-12. The hardness is summarized in Fig. 13.

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The microstructure after overaging at 500°C is ferritic-bainitic-martensitic with 20.9% martensite (11,5% “white” and 9,4% “brown” martensite) and 4.5% retained austenite. Ferrite is of type bainitic-ferrite with small cementite precipitates. The high overall hardness (305 HV1) can be explained by the rather high martensite fraction. Lowering TOA to 475°C decreases the martensite fraction to 13% (5.2% of “white” and 7.8% of “brown” martensite) and the fraction of retained austenite to 3.4%, but increases the amount of precipitated cementite in the bainitic ferrite. Due to the lower martensite fraction the hardness decreases to 270 HV1 (Fig. 13). Decreasing the overaging temperature to 450°C increases the hardness to 285 HV1 because of the harder bainitic matrix (due to significant cementite precipitations) with embedded martensite and retained austenite islands (2.6% “white”, 6.7% “brown” martensite and 2.3% of retained austenite). The microstructure obtained after overaging at 425°C consists of bainite, 6.4% martensite and 0.6% retained austenite, see Fig. 2. The hardness increases to 314 HV1 due to the precipitated cementite in bainite. A fully bainitic microstructure was produced by overaging at 400°C (Fig. 12). This microstructure exhibits the highest hardness (322 HV1) because all carbon is precipitated as cementite.

M

M

BF

BF a) b) Figure 9 TOA=500°C, TQ=750°C, TAN=840°C, a) LOM (LePera etched), b) SEM (Nital etched)

BF BF M

M

a) b) Figure 10 TOA=475°C, TQ=750°C, TAN=840°C, a) LOM (LePera etched), b) SEM (Nital etched)

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B

M B M

a) b) Figure 11 TOA=450°C, TQ=750°C, TAN=840°C, a) LOM (LePera etched), b) SEM (Nital etched)

B

B

a) b) Figure 12 TOA=400°C, TQ=750°C, TAN=840°C, a) LOM (LePera etched), b) SEM (Nital etched)

Figure 13 Hardness as a function of annealing temperature

The specimen overaged at 500°C shows a typical microstructure and mechanical properties of a dual-phase steel. The low yield strength to tensile strength ratio and the low hole expansion ratio are indications of this dual-phase steel character. It has been shown that a high contrast in phase specific properties (high strength differences between the phases) leads to improved tensile uniform elongation and deep-drawability, while a mechanically homogeneous microstructure enhances stretch flangeability [9-13]. By decreasing the overaging temperature, an increase in the tensile strength from 846 MPa to 966 MPa was observed, while the elongation properties decrease (Fig. 14). By lowering the overaging temperature from 500°C to 400°C the microstructure becomes more homogeneous until a single-phase microstructure is reached at 400°C (yield strength to tensile strength ratio increases). This improves the hole expansion ratio

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from 55% to 124%. The fully bainitic microstructure exhibits the highest strength (Rm=966 MPa) and the highest stretch flangeability (HE=124%)

a) b) Figure 14 Influence of overaging temperature on tensile test properties a) strength, b) elongation

Figure 15 Hole expansion ratio as a function of the overaging temperature

Quenching temperature Varying the quenching temperature impacts the microstructure and the mechanical properties only very little. Annealing and overaging temperature were set constant to the experimentally determined optimum temperatures of TAN=840°C and TOA=400°C while the quenching temperature was varied between 650°C and 750°C in steps of 25°C. The microstructure of all specimens was fully bainitic, as shown in Fig. 12, with small amounts of martensite (between 1% to 3%) and retained austenite (between 0.2% and 0.5%). These contents of retained austenite are in the error range of the magnetic volumetric method. The quenching temperature has only a minor impact, also on the mechanical properties. The tensile strength lies between 957 MPa at TQ=650°C and 966 MPa at TQ=750°C. The hole expansion ratio ranges between 119% and 124%. The hardness varies between 314 HV1 and 322 HV1. Summary Different heat treatment cycles were applied on industrially produced low-alloyed sheet steel to produce complex-phase steels with strength levels above 800 MPa and hole expansion ratios higher than 100%. The microstructural evolution was studied by light optical microscopy

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and scanning electron microscopy. To characterize the mechanical properties, tensile, hole expansion and hardness tests were performed. The microstructure varies from ferritic-bainiticmartensitic complex-phase to fully bainitic. Annealing at 780°C forms a complex-phase microstructure with ferrite, bainite and martensite with relatively high elongation values (13.1%) but poor tensile strength (740 MPa) while the microstructure annealed at TAN=840°C is bainiticmartensitic with a tensile strength of 896 MPa with a total elongation of only 5.8%. A high overaging temperature (500°C) produces a complex-phase steel with bainitic ferrite and martensite, while a lower overaging temperature (400°C) generates a fully bainitic microstructure. Decreasing the overaging temperature deteriorates the total elongation but increases the strength level. The hole expansion ratio was generally higher for specimens with high tensile strength and low elongation properties, which can be explained by the contrast in the mechanical properties of the phases. A mechanically homogeneous microstructure with a high yield to tensile strength ratio such as fully bainitic microstructures, exhibits both high strength and excellent formability. Variations in the heating cycles showed that full austenitization at an annealing temperature of 840°C, cooling to 750°C and quenching to an overaging temperature of 400°C produces a microstructure with an optimized combination of strength and stretch flangeability. The obtained fully bainitic microstructure exhibits a tensile strength of 966 MPa and a hole expansion ratio of 124%. A variation of the quenching temperature (in the range 650°C-750°C) in this annealing cycle has no significant impact on the microstructure and the resulting properties. Acknowledgments The authors gratefully acknowledge the financial support of the Christian Doppler Research Association (CDG).

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