Microstructural Characterization of a 1200 MPa Complex-Phase Steel

The demand for new advanced high strength steels (AHSS) has been increasing in the last few decades. A large part of this demand comes from automotive companies. We have produced a new complex-phase (CP) steel with 1200 MPa of mechanical resistance and 8% of elongation, called CP1200. In this paper the dilatometric and microstructural characterization of a newly produced CP1200 steel is presented. The new steel was produced by making changes to the heat treatment of the already industrially available CP1100. The microstructure was quantified using light optical microscopy (LOM) and electron backscatter diffraction (EBSD). The microstructure of both steels was compared to identify the origin of the mechanical properties improvement. A new microstructure distribution, with higher amount of bainite and smaller concentration of ferrite and martensite was identified.


Introduction
Complex-phase (CP) steels belong to a class of advanced high strength steels (AHSS) that has seen some increased use in the automotive industry in the past few years.Steels of this type have been used as substitutes for the more traditional Dual-Phase (DP) steels.The main difference between those classes is that CP steels present a higher yield strength and are less prone to void nucleation than DP steels, which makes the CP steels more workable with a higher stretch-flangeability.Both of these characteristics come from a higher amount of bainite in CP microstructures [1][2][3][4][5][6][7][8][9] .
One important aspect, when working with CP steels, is to control their microstructure.Considerable variation in the mechanical properties can be achieved by manipulating heat treatment, making some industrially available steels even more competitive with small changes to their initial thermal treatment [10][11][12][13][14] .For this purpose, thermomechanical characterization using dilatometry or Gleeble simulations and microstructural characterization are widely applied to achieve improvements in the properties of AHSS.
Once both of those aspects are fulfilled (thermomechanical and microstructural characterization), a new heat treatment can be designed to optimize the mechanical properties.In this way, PUC-Rio and Companhia Siderúrgica Nacional (CSN) have been working in the development of a CP1200 steel that could be produced in an industrial scale to fulfill a commercial demand.Recently, a steel composition used to produce an industrial CP1100 steel had its continuing cooling transformation (CCT) diagram determined using dilatometry, and the microstructure analyzed using LOM and EBSD 14 .In the present work, a new heat treatment for this steel composition was designed using dilatometry and later reproduced in an industrial scale, resulting in a new CP1200 steel.The microstructure characterization of this newly produced steel is reported here.*e-mail: renanmeloclima@gmail.com

Material
The chemical composition (% wt.) of the steel used in the present work contained 0.2Si, less than 0.17C, more than 1.6Mn, 0.015Al and 0.01Nb.The original CP1100 treatment yielded an average of 54 % ferrite, 21 % bainite, 19 % martensite and 2.5 % retained austenite measured by EBSD, LOM and X-ray diffraction 14 .
The production of a CP steel can be largely divided in three steps: hot-rolling, cold-rolling, and heat treatment.The dilatometric results were obtained by treating cold-rolled steel samples extracted from the industrial processing of the CP1100 steel.The supplementary Figure S1 illustrates the sample origin in the industrial process.The cold rolled steel presented a ferritic/perlitic highly banded microstructure, with about 72 % of ferrite and 28 % of pearlite.The quantification was done using LOM and SEM imaging.The supplementary Figure S2, shows the typical microstructure of the steel.
Once the dilatometry results were obtained and the microstructural characterization was performed, a new heat treatment was designed and submitted to an industrial scale process.Samples of this process were used to measure the mechanical properties of the new steel.

Dilatometry
A DIL 805 from TA ® instruments was used to perform controlled dilatometric heat treatments.Samples were machined using wire electrical discharge machining, and were 10 mm long, 4 mm wide and 1.4 mm thick.The derivative of the heating and cooling curves was used to ascertain the transformation temperatures.
Austenitization measurements were done by heating samples to temperatures between Ac 1 and Ac 3 , holding for 5 min and then quenching to room temperature.Three soak temperatures were used in conjunction with the Ac 1 and Ac 3 to produce an austenitization curve by polynomial fitting in the OriginLab software.

Microstructural Characterization
Light optical microscopy (LOM) was conducted using an Olympus BX51M instrument.Samples were ground with silicon paper and polished using diamond suspensions.A color tint etching was performed initially using Nital to etch followed by Na 2 S 2 O 5 + water for tinting.This etching has been shown to reveal ferrite with whitish contrast, brown to dark brown martensite/bainite and black retained austenite in CP steels 15,16 .
For EBSD, those sample were repolished in diamond suspensions up to 0.25 µm, followed by colloidal silica (MasterMet2 ® ) polishing using an Minimet ® and finishing using a 2 s etching in 0,5 % Nital.
A JEOL 7100 scanning electron microscope (SEM) was used to obtain secondary electron (SE) and backscattered electron (BSE) images.Microconstituents were identified by comparison with images from the literature 10,24 .
Images were processed using the open-source software FIJI.The Waikato environment for knowledge analysis (WEKA) 25 plugin was used to train a neural network used for the phase segmentation of LOM and SEM images.
A Nordlys Max 2 detector and the X-Max 80 SDD were used for electron backscattered diffraction (EBSD) and X-ray energy dispersive spectroscopy (EDS), respectively.The EBSD maps were collected in 100x100 µm areas using a step size of 0,08 µm.The sample position required an IPF angles correction of (0°, -90°, 0).Regions for mapping were selected at 1 / 4 of the thickness, avoiding both the edge and the center of the steel sheet.
In the EBSD results, retained austenite was identified by its crystal structure in the phase map.Martensite, due to its high deformation, appeared as zero solution in the phase identification and it was segmented as such.Ferrite and bainite were differentiated by taking in consideration that bainite is a deformed microconstituent, due to its higher dislocation density; therefore, by using the kernel average misorientation (KAM) and grain orientation spread (GOS) maps it was separated from the less deformed ferrite 14,16,17,19,26,27 .
This segmentation process was performed using the AZtecCrystal software 28 .This software applies a machine learning process to identify the microconstituents based on the before mentioned maps.

Mechanical Properties
To measure the mechanical properties of the new industrially processed steel, tensile, bending and hole expansion tests were conducted at CSN production line.An Instron 5585H 25 t instrument with an AVE extensometer and sub dimensioned 50 mm tensile samples following the ASTM E8 29 norm were used for tensile testing.The bending tests were done following the ASTM E290 30 using a vise and a bend supporter with 3.5 times the plate thickness.The bending was first done to 90º for the regular test, and then extended until damage was perceived.The hole expansion test was done following the standard ISO-TS 16630 31 and using a homemade device.
Hardness measurements were done using a Wolpert Wilson Instruments™ universal hardness tester closed loop 930N.The testing was done following the ASTM E92 -17 standard 32 "standard test methods for Vickers hardness and Knoop hardness of metallic materials".

Results
The dilatometric change in length versus temperature curve, resulting from the heat treatment applied in the present work is shown in Figure 1.The heating rate is slowed near the holding temperature to guarantee a more homogeneous heating.This, combined with the austenitization contraction, produces the odd behavior observed during austenite formation.As soon as cooling begins, intercritical ferrite starts to grow and, at the same time, some new ferrite grains will be formed.The fast cooling makes hard to see the change in length caused by the ferrite growth and formation, but in Figure S3 a zoomed segment of the dilatometry curve shows the start and finish of the nucleation of new ferrite grains (the zoomed region is marked in Figure 1).Also in Figure 1, a large non-isothermal bainite transformation can be seen, followed by an isothermal bainite formation.
An industrial scale reproduction of the treatment was conducted to investigate the mechanical properties.The results of the tensile and hardness testing and its comparison to the CP1100 can be seen in Table 1.
The bending test showed satisfactory results, with no cracking in 90º bending.Further testing showed small signs of crack formation above 105º of bending.A λ value of 120 % was obtained in the hole expansion testing.
An austenitization curve was produced by applying a polynomial fitting in the austenite content of the Ac 1 , Ac 3 as well as samples heated and held for 5 min at three temperatures between Ac 1 , Ac 3 followed by quenching using a 50° C/s rate.The ferrite was quantified in those samples and used as a measure of the austenitization; the continuous cooling transformation curve for this composition shows that this cooling rate prevents ferrite formation 14 .Figure 2 shows the resulting austenitization curve.Figure S4 shows the microstructure (Nital etched) from two of the samples used in this process.
Figure 3 presents a LOM image of the treated steel and its segmentation.In Figure 3A whitish regions are ferrite, brown regions are martensite/bainite, and black regions are retained austenite.In Figure 3B the LOM was segmented with red ferrite, green martensite/bainite and magenta retained austenite.
Comparing the microstructure from the material studied in the present work to the prior CP1100 14 , the former presents less ferrite and more bainite/martensite than the CP1100.The LOM images were segmented using WEKA plugin, the result of the segmentation and its comparison to the CP1100 are summarized in Table 2.The table was obtained using 9 LOM images in 500x magnification for a total area of about 0,01 cm 2 .
Figure 4 presents SEM images of the steel subjected to new treatment.The steel preserves a small remnant orientation in the rolling direction, seen by its banded behavior primarily in small magnifications.In Figure 4B, green arrows point to the ferritic regions and black arrows to martensite/austenite islands.It is difficult to properly identify bainite in the SEM image, so EBSD analysis was used for that purpose.Figure 5 shows an example of a segmented EBSD map and the steps that lead to it.Figure 5A shows the Band Contrast (BC). Figure 5B shows the Inverse Pole Figure (IPF).The steel presents a (111) preferential orientation, which was also observed in the CP1100 counterpart.Figure 5C shows the combination of BC, KAM and GOS maps. Figure 5D shows the final segmentation and the insert in the figure highlight the small, retained austenite grains.The segmentation was done using Aztec Crystal software.A total of eleven regions were used for the collection of the EBSD maps in order to have a more representative result; the average microconstituent distribution, and its comparison to the CP1100 14 is presented in Table 3.

Discussion
The steel produced using the new heat treatment results in properties that meet the requirements of a CP1200 classification,   including a 1200 MPa tensile strength, greater than 800 MPa yield strength and at least 8 % of total elongation.Considering that the composition of the steel was the same of the CP1100 14 , it can be stated that the improvement in the mechanical properties came directly from the new microstructural distribution.The LOM results had already presented a substantial decrease in ferrite content and the EBSD quantification further showed an increase of about 15 % in the bainite content, as shown in Table 3.
Prior results from other researchers 7,8,33 have shown that the presence of bainite in the microstructure can reduce or delay the void nucleation.
The increase in the amount of bainite with a corresponding delay in void nucleation can explain the 100 MPa increase in mechanical strength together with the small improvement (1.5 %) to the elongation.A more extensive study of the interface distribution and its influence in the mechanical properties is needed to better understand this aspect.The amount and distribution of interfaces such as ferrite-martensite, bainiteferrite and bainite-martensite can be the key to understand and minimize the void nucleation.
The new treatment also resulted in a higher orientation in the (111) direction.This may be due to the formation of higher amounts of bainite as this microconstituent is known to grow in low energy boundaries 34 .Texture can be an important aspect for automotive steel.Many applications can be benefitted by a texture that resists stamp deformation.Research in the texture evolution from the hot rolling to the final product and its influence in the mechanical properties is being conducted.

Conclusions
A new heat treatment was designed and performed, first in a dilatometer and later at the industrial scale.The steel composition corresponded to an industrial CP1100.The new heat treatment produced a steel with improved mechanical properties that classified the new steel as a CP1200.LOM and EBSD characterization of the new steel showed an increase of about 15 % in the bainite content and a decrease in the ferrite (~10 %) and martensite (~5 %) contents when comparing to the CP1100 microstructure.This new microstructural distribution led to the mechanical property improvement.The improvement was attributed to the higher concentration of bainite that made the steel less prone to void nucleation.Further research in the interface distribution, texture evolution and its influence in the mechanical properties will be conducted.

Figure 1 .
Figure 1.Change in length by temperature result of the new heat treatment.Selected area is shown in higher magnification in Figure S3.

Figure 4 .
Figure 4. SEM image of the CP1200 etched using Nital /Na 2 S 2 O 5 .A -Lower magnification image.B -Higher magnification image, black arrows point to M/A islands, green arrows point to ferrite.

Table 1 .
Tensile and Hardness proprieties of the CP1100 and its comparison to the new treatment.

Table 2 .
Microconstituents quantification comparison from LOM images between the CP1100 and the new treatment.

Table 3 .
EBSD quantification comparison between the CP1100 and the new treatment.