Superior bendability of direct-quenched 960 MPa strip steels

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Procedia Manufacturing 15 (2018) 676–683 Procedia Manufacturing 00 (2017) 000–000

www.elsevier.com/locate/procedia 17th International Conference on Metal Forming, Metal Forming 2018, 16-19 September 2018, Toyohashi, Japan 17th International Conference on Metal Forming, Metal Forming 2018, 16-19 September 2018, Toyohashi, Japan

Superior bendability of direct-quenched 960 MPa strip steels Superior bendability direct-quenched MPa strip steels a, b of International a Manufacturing Engineering Society Conference960 2017, 2017, 28-30 Juneb, Antti Kaijalainen *, Vili Kesti , Lars Troiveb, Anna-Maija ArolaMESIC , Tommi Liimatainen 2017, Vigo (Pontevedra), Spain

b a a Mikko Hemmilä , Jukkab, Kömi , DavidArola Porter b a Antti Kaijalainen *, Vili Kesti , Lars Troive Anna-Maija , Tommi Liimatainenb, b a and Hemmilä Production Engineering, 90014, Finland Mikko , JukkaUniversity Kömiofa,Oulu, David Porter Costing modelsMaterials for capacity optimization inOuluIndustry 4.0: Trade-off a,

a

b SSAB Europe Oy, Rautaruukintie 155, Raahe 92101, Finland Materials and Production Engineering, University of Oulu, Oulu 90014, Finland b SSAB Europe Oy, Rautaruukintie 155, Raahe 92101, Finland

between used capacity and operational efficiency a

Abstract

A. Santanaa, P. Afonsoa,*, A. Zaninb, R. Wernkeb

a University of on Minho, 4800-058 Guimarães, Portugal3-point bending of 6 mm thick ultrahighAbstract The present paper shows the effect of microstructure the press brake and frictionless b 89809-000 Chapecó, SC, Brazil strength steel strips with a yield strength of Unochapecó, 960 MPa. With a traditional press brake machine the minimum bending radii of the The present shows ontimes the press brake andfor frictionless 3-point bending of to 6 mm thick ultrahighstudied steelspaper varied fromthe 1.3effect times of themicrostructure thickness to 3.0 the thickness the bend axis perpendicular the rolling direction strength steel strips yield the strength of 960 Withaxis a traditional brake machine bending radii of the and in the range 2.0 with - 3.5atimes thickness for MPa. the bend parallel topress the rolling direction.the Theminimum frictionless 3-point bendingstudied steels varied from rotatable 1.3 timesdie-rollers the thickness 3.0 times thetothickness for the axis perpendicular to the rolling in direction equipment incorporating hastobeen applied characterize thebend material work hardening behavior a way Abstract and in thetorange 2.0 - 3.5process, times the the bendpunch axis parallel to the rollingdata direction. Thethe frictionless relevant the bending i.e.thickness by usingformeasured force vs. position to derive bending 3-point momentbendingand the equipmentofincorporating rotatable has been applied to characterize the material behavior a way evolution the flow stress and thedie-rollers strip curvature during the bending process. The main aimwork of thehardening present paper is to in establish Under the concept ofprocess, "Industry 4.0", production processes will be pushed to derive be increasingly interconnected, relevant to the bending i.e. by using measured punch force vs. position data to the bending moment and the an understanding of how bendability can be significantly improved and made more isotropic by modifying the subsurface information based on a real time basis and, necessarily, much more efficient. Inand thiswhy context, capacity optimization evolution of the stress the strip curvature during theand bending process. Thelayer main aim of the papermicrostructure is to establish microstructure toflow include a and relatively soft polygonal ferrite granular bainite thepresent subsurface goes beyond the traditional aim of capacity maximization, contributing also more for organization’s profitability value. an understanding of how can be significantly improved and made isotropic by modifying the and subsurface plays such a dominant role.bendability Indeed, lean to management and continuous improvement approaches suggest capacity instead of microstructure include a relatively soft polygonal ferrite and granular bainite layer and why theoptimization subsurface microstructure plays such a dominant role. of capacity optimization and costing models is an important research topic that deserves maximization. The study contributions from both the practical andB.V. theoretical perspectives. This paper presents and discusses a mathematical © 2018 The Authors. Published by Elsevier © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference onA Metal Forming. model for capacity management based on costing models (ABC and TDABC). generic model has been Peer-review under responsibility of the scientificdifferent committee of the 17th International Conference on Metal Forming. © 2018 Theand Authors. by Elsevier developed it wasPublished used to analyze idleB.V. capacity and to design strategies towards the maximization of organization’s Peer-review under responsibility of the scientific committee of theMicrostructure; 17th International Conference on Metal Forming. Keywords: 3-point bending; Curvature; Flow-stress; Frictionless bending; curve; Ultrahigh-stregth steel;that capacity value. The trade-off capacity maximization vs operational efficiency isMoment highlighted and it is shown

optimization might hide operational inefficiency.

Keywords: 3-point bending; Curvature; Flow-stress; Frictionless bending; Microstructure; Moment curve; Ultrahigh-stregth steel;

© 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. Keywords: Cost Models; ABC; TDABC; Capacity Management; Idle Capacity; Operational Efficiency *

Corresponding author. Tel.: +358-29-448-2156; fax: +358-8-344-084.

1.E-mail Introduction address: [email protected]

Corresponding author. Tel.: +358-29-448-2156; fax: +358-8-344-084. The cost of idle capacity is a fundamental for companies and their management of extreme importance E-mail address: [email protected] 2351-9789 © 2018 The Authors. Published by Elsevier information B.V. *

Peer-review under responsibility scientific of unused the 17thcapacity International Conferencepotential on Metal and Forming. in modern production systems.ofInthe general, it iscommittee defined as or production can be measured 2351-9789 2018 The Authors. Published by Elsevier B.V.hours of manufacturing, etc. The management of the idle capacity in several©ways: tons of production, available Peer-review under responsibility thefax: scientific committee * Paulo Afonso. Tel.: +351 253 510of 761; +351 253 604 741 of the 17th International Conference on Metal Forming. E-mail address: [email protected]

2351-9789 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the Manufacturing Engineering Society International Conference 2017. 2351-9789 © 2018 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 17th International Conference on Metal Forming. 10.1016/j.promfg.2018.07.300

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1. Introduction In recent years ultrahigh-strength strip steels produced using thermomechanical rolling and direct quenching have become interesting materials for structural applications because they can exhibit good combinations of mechanical properties and weight saving potential. The microstructures of these steels typically consist of bainite and autotempered lath martensite. In the case of TM-DQ strip steels, cold bending is the most important method of forming in applications such as containers and crane booms. The bendability is generally measured as the minimum ratio of the punch radius (r) to the strip thickness (t) that the steel can tolerate without the appearance of surface defects during bending to an angle of 90º in three-point bending [1]. In the bending process strain increases away from the neutral axis towards the outer surfaces. In some cases strain localization takes place after which the bending process becomes unstable resulting in failures typically due to either the formation of shear bands, or the development of micro-voids [2–4]. According to Dillamore [5], the factors that affect the formation of shear bands are the workhardening exponent, the strain rate sensitivity, the density of mobile dislocations and the Taylor factor. Earlier studies [6, 7] have shown that strain localization is the precursor to failure in the air bending of ultrahigh-strength strip steels and shear bands have been observed to initiate at depths in the range of 1-6 % of the total strip thickness from the surface at angles of ~45° to the surface, i.e. in the maximum shear stress directions. Therefore it is reasonable to suppose that the near-surface properties of the strip govern bendability. The aim of this study is to characterize the material behavior during bending with a newly developed method, which provides improved information about the surface layer properties compared to traditional tensile tests and 3point bending. Nomenclature W Sx Rd Rk t F LN E

die width (c-c measure) horizontal movement of contact point die radius punch radius material thickness bending force horizontal distance between contacts energy

S Sy M α σf 1/R βc r

vertical movement of punch vertical movement of contact point moment bending angle flow-stress curvature contact angle punch radius applied at conventional bending

2. Experimental 2.1. Materials The formability limits of two hot-rolled and direct-quenched ultrahigh-strength strip steels have been investigated. The chemical composition of the steel was 0.09C-0.2Si-1.1Mn-1.2Cr-0.15Mo-0.02Ti (wt.%) and the strip thickness 6 mm. Two strips, coded Novel and Conventional were produced with different subsurface microstructures, as will be described below. More detailed hot rolling schedules are present in Refs. [7, 8]. 2.2. Microstructural characterization General characterization of the transformation microstructures was performed with a laser scanning confocal microscope and a field emission scanning electron microscope (Ultra plus, Zeiss) after etching with nital or picric acid.

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2.3. Mechanical testing Microhardness was measured using a Micro-Hardness Tester (CSM, Switzerland) under a 1 N load and macrohardness using a Duramin-A300 (Struers) under 100 N load. Tensile tests were carried out in accordance with the European standard EN 10002. Three-point bending tests were carried out in an Ursviken Optima 100 bending machine with up to a 90 degree bending angle at room temperature. Plate specimens 6 x 300 x 300 mm3 in size were bent around axes parallel to both the transverse and rolling directions. The die-width (W) employed was 75 mm and the punch radii (r) ranged from 8 mm to 21 mm. After bending, the quality of the bent surface was examined visually, as described in Ref. [1]. The minimum usable bending radius was that resulting in a defect-free bend. Besides the conventional bending experiments, the strip samples were also tested using a novel frictionless 3point bending method to characterize the behavior of the materials in more detail during the bending by measuring the force vs. the position of the punch, see Fig. 1. By the present invented method [9] the force vs. the stroke position was transposed to the moment M determine the real response of the material during bending, according to Eq. (1). 𝑀𝑀𝑀𝑀 =

𝐹𝐹𝐹𝐹 ∙ [𝐿𝐿𝐿𝐿0 − (𝑅𝑅𝑅𝑅𝑘𝑘𝑘𝑘 + 𝑅𝑅𝑅𝑅𝑑𝑑𝑑𝑑 ) sin(0.5𝛼𝛼𝛼𝛼)] 𝐹𝐹𝐹𝐹 ∙ 𝐿𝐿𝐿𝐿𝑚𝑚𝑚𝑚 = , 2 2 cos (0.5𝛼𝛼𝛼𝛼) 2 cos 2(0.5𝛼𝛼𝛼𝛼)

(1)

where Lm is the horizontal distance between the points at which the strip contacts punch and the die assuming a straight flange. However, it can be shown theoretically that the moment M estimated by Eq. (1) relates to the real point of contact for a curved flange, i.e. at LN, see Fig. 2, by following one of current points of contact during the entire punch-stroke S. The total amount of energy can be expressed as: (2)

𝐸𝐸𝐸𝐸 = 2�∫ 𝐹𝐹𝐹𝐹𝑥𝑥𝑥𝑥 ∙ 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑥𝑥𝑥𝑥 + ∫ 𝐹𝐹𝐹𝐹𝑦𝑦𝑦𝑦 ∙ 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑦𝑦𝑦𝑦 �,

where:

𝐹𝐹𝐹𝐹

𝑑𝑑𝑑𝑑𝑥𝑥𝑥𝑥 = 𝑅𝑅𝑅𝑅𝑘𝑘𝑘𝑘 ∙ sin 𝛽𝛽𝛽𝛽𝐶𝐶𝐶𝐶 , 𝑑𝑑𝑑𝑑𝑦𝑦𝑦𝑦 = 𝑑𝑑𝑑𝑑 − 𝑅𝑅𝑅𝑅𝑘𝑘𝑘𝑘 ∙ (1 − cos 𝛽𝛽𝛽𝛽𝐶𝐶𝐶𝐶 ) and 𝐹𝐹𝐹𝐹𝑥𝑥𝑥𝑥 = ∙ tan 𝛽𝛽𝛽𝛽𝐶𝐶𝐶𝐶 2

and

𝐹𝐹𝐹𝐹

𝐹𝐹𝐹𝐹𝑦𝑦𝑦𝑦 = .

(3)

2

By applying Eq. (3) into Eq. (2) it will end with the well-known expressions for the energy in bending; (4)

𝐸𝐸𝐸𝐸 = ∫ 𝑀𝑀𝑀𝑀 ∙ 𝑑𝑑𝑑𝑑𝛼𝛼𝛼𝛼 = ∫ 𝐹𝐹𝐹𝐹 ∙ 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑,

confirming 𝛽𝛽𝛽𝛽𝐶𝐶𝐶𝐶 as the real angle of contact at bending considering curved flanges. However, according to Eq. (5) the bending angle α in Eq. (1), can easily be obtained geometrically as it consider totally straight flanges between the points of contact on the die and the punch. −1

𝛼𝛼𝛼𝛼 = 2 ∙ 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠−1 ��𝐿𝐿𝐿𝐿0 ∙ 𝑄𝑄𝑄𝑄 + �𝐿𝐿𝐿𝐿0 2 + (𝑑𝑑𝑑𝑑 − 𝑄𝑄𝑄𝑄)2 − 𝑄𝑄𝑄𝑄2 ∙ (𝑑𝑑𝑑𝑑 − 𝑄𝑄𝑄𝑄)� ∙ �𝐿𝐿𝐿𝐿0 2 + (𝑑𝑑𝑑𝑑 − 𝑄𝑄𝑄𝑄)2 � � ∙

Where: 𝑄𝑄𝑄𝑄 = 𝑅𝑅𝑅𝑅𝑘𝑘𝑘𝑘 + 𝑅𝑅𝑅𝑅𝑑𝑑𝑑𝑑 + 𝑡𝑡𝑡𝑡 and 𝐿𝐿𝐿𝐿0 = 0.5 ∙ 𝑊𝑊𝑊𝑊.

180 𝜋𝜋𝜋𝜋

[𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑠𝑠𝑠𝑠], (5)

The moment is also used to calculate the evolution of the flow stress and the strip curvature distribution as explained in Ref. [9]. To obtain the real material behavior, all energy loss such as friction has to be reduced. To reduce the friction the die was equipped with rotatable die rollers. In all the tests, one single punch was applied with a radius less than the material thickness. The principal for the setup is very similar to that of the proposed standard VDA238-100 [10], which is mainly applied for thin cold-rolled grades and unable to manage thicker hot-rolled materials. In this case, the tool was a scaled-up design to manage loads up to 150 kN. The diameter of the die-rollers was 80 mm, the punch radius 4 mm and the center to center die roller separation was 105 mm. The width of the specimens, i.e. the bend length, was 70 mm.

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Author / Procedia 00 (2018) 000–000 Antti name Kaijalainen et al.Manufacturing / Procedia Manufacturing 15 (2018) 676–683

Fig. 1. Frictionless bending equipment with rotatable die rollers used in tests measuring force F vs. punch distance S.

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Fig. 2. Parameters involved in model and moment distribution considered.

3. Results and discussion 3.1. Microstructure The microstructures and hardness distributions below the strip surfaces after the two different rolling routes are presented in Fig. 3. It is seen from Fig. 3(a), that in the Novel strip the microstructure down to 70 µm from the surface consists of polygonal ferrite interspersed among other carbon-rich microstructural components while granular bainite is found from 70 to at least 300 µm below the surface (Fig. 3(c)). The subsurface microstructure of the Conventional strip consisted mostly of auto-tempered martensite and upper bainite (Fig. 3(e)). The microstructures at the centerline of both strips consisted of auto-tempered martensite and bainite (in Fig. 3(d) and (f)). In the Novel strip, the macrohardness increases linearly from the surface down to the centerline from 302 to 404 HV, whereas in the Conventional strip the hardness is higher through the thickness (Fig. 3(g)). Below the surface of the Novel strip, granular bainite and ferrite show microhardness values from 275 to 383 HV (Fig. 3(h)). In the Conventional strip, the subsurface martensite and bainite have microhardness values ranging from 382 to 452 HV.

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Fig. 3. LSCM micrograph and microhardness data from the subsurface regions of (a) Novel and (b) Conventional strips after picric acid etching. (c) – (f) FESEM micrographs after nital etching. Novel strip: (c) polygonal ferrite and granular bainite 200 µm from surface and (d) autotempered martensite and bainite at centerline. Conventional strip: (e) upper bainite and martensite 200 µm from surface and (f) auto-tempered martensite and bainite at centerline. (g) Through-thickness macrohardness distributions and (h) mean values of microhardness at various depths below surface.

3.2. Mechanical properties The mechanical properties of the sheets are given in Table 1. The bendability results are grouped such that data for the bend axis parallel to the transverse direction are located in the columns showing longitudinal tensile properties (and vice versa) since both cases lead to principal strains in the same direction. It can be seen that the investigated ultrahigh-strength steels have the yield strength (Rp0.2) of 980 MPa and 1060 MPa (Novel and Conventional, respectively), and the tensile strength (Rm) in longitudinal direction is 1100 MPa and 1160 MPa, respectively. The total elongation to fracture (A5) is almost equal, even though slightly higher at 11.2 % for the longitudinal specimens of the Conventional material. Table 1 also includes the mean macrohardness of the steels as measured on the longitudinal cross-sections taken from Fig. 3(g). The minimum bending radii obtained for the Novel steel were small. With the bend axis along the RD the minimum punch radius for defect-free bending was 12 mm (r/t ratio 2.0) and along the transverse direction 8 mm (r/t ratio 1.33). These values can be considered as good at the strength level concerned. For the Conventional, the minimum usable radius was larger at 21 mm (r/t ratio 3.5) with the bend axis along the RD.

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Table 1. Mechanical properties of investigated materials. Longitudinal straining

Transverse straining

Novel

Conventional

Novel

Conventional

Yield strength (Rp0.2, MPa)

982

1059

987

1052

Tensile strength (Rm, MPa)

1102

1159

1131

1197

Through-thickness mean HV10

356

387

356

387

Total elongation (A5, %)

10.4

11.2

8.7

9.7

Uniform elongation (Ag, %)

2.7

2.6

1.9

2.2

8 (1.33)

18 (3.0)

12 (2.0)

21 (3.5)

Minimum bending radius (r, mm, (r/t))

bend axis perpendicular to RD1

bend axis parallel to RD1

Bending with the bend axis perpendicular to the RD induces principle strains in the longitudinal direction and vice versa. 1

The results obtained from the frictionless bending tests are shown in Figs. 4 to 7. In Fig. 4 the measured forces are shown. As a first observation concerning the loads, both strip versions seem to show quite isotropic behavior as the loads are very similar with both the longitudinal and transverse bending. At the end of the stroke one can also recognize a sudden increase in force that is due to the material that moves forward the punch and starts to slip introducing friction in between. Therefore, this investigation will focus on the part of the stroke before this increase in force. In Fig. 5 the estimated moment is shown. As it can be a small differences between the specimens in width the moment is presented as moment per length unit. When comparing the Conventional and Novel grades, the hardening behavior during bending differs a lot. At the initial state of plastification the Novel steel has a much smoother hardening characteristic and is still hardening at large bending angles where the Conventional grade no longer shows any hardening. Referring to [11] the moment hardening behavior will reflect the curvature of the bend. Whenever a material stops hardening the actual radius or curvature of the strip below the punch decreases rapidly and the material becomes sensitive to strain localizations. It is obvious that for an optimal bend the strains should be even and smoothly distributed and not localized into small areas. Fig. 6 shows estimated flow stress diagrams. The flow stress is estimated by considering the balance between the flow stress and the bending moment assuming the same level of stress distribution in both tension and compression, i.e. with the neutral axis located in the middle of the thickness. This is a good approximation as the moment is calculated from the two points of contact between the punch and the strip material where the neutral axis is still more or less in the middle of the thickness. As can be seen for the Conventional grade the flow stress does decrease immediately after full plastification. One theory is that material deformation goes from pure tension to shearing such that the developed shear bands lower the flow stress. The calculated curvature distribution estimated at a selected bending angle of 50 degrees is shown in Fig. 7. The xaxis represents normalized horizontal distance between the two contact points on the die and the punch at the current state of bending, see Fig. 2. The curvature, 1/R, is linear in the elastically deformed areas and increases plastic deformation occurs. However, it is obvious that the Conventional grade does not show the same smooth distribution of the curvature as the Novel material. The change in curvature for the Conventional strip takes place more or less at a single point close to the punch contact point. Fig. 8 shows the specimens from the frictionless bending, confirming the strong tendency for strain localization in the Conventional strip (Figs. 8(c) and (d)).

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Fig. 4. Bending force vs. punch distance.

Fig. 6. Flow stress vs. bending angle.

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Fig 5. Moment vs. bending angle.

Fig. 7. Distribution of curvature at 50 degrees bend.

(b) Novel (c) Conventional (d) Conventional (a) Novel Longitudinal bending Transverse bending Longitudinal bending Transverse bending Fig. 8. Samples from frictionless bending showing different bending angles due to differences in bendability. The Novel steel, (a) and (b), have a smooth round shape of the outer fibers, whereas Conventional grade, (c) and (d), has a strong tendency to surface flattening due to strain localization.

The frictionless bending results indicate that the Novel strip concept provides smoother and more robust behavior during three-point bending. Unlike traditional tensile tests, which only provide average through-thickness stress – strain data, the frictionless bending results better reveal the properties of the surface layers. In the frictionless bending measurements the properties of the outermost surface layers have a strong impact on the cross-section moment characteristics as the locations of the surface layers are far from neutral axis. As was confirmed by visual inspection, the new method also provides information about the bend shape, i.e. radius of the curvature, and it is evident that the Novel strip forms a better shaped bend while the Conventional strip shows a rapid local change in

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curvature. These findings together with the microstructural characterization support the conclusion that softer surface layers are very beneficial where bendability is concerned. Often very small bend radii are desired. In such cases very large strains are expected near the bend surfaces and therefore the conditions for instability (shear bands etc.) must be avoided even to large strains. According to Dillamore [5], the most critical factors and properties are work-hardening exponent, strain rate sensitivity, density of mobile dislocations and Taylor factor. These factors are affected by the phase structure and hardness of the microstructure, which have been changed in the Novel concept, especially in the critical surface layers. Softer polygonal ferrite/granular bainite subsurface structure enhances the critical properties to delay localization compared to the harder subsurface consisting of tempered martensite/upper bainite. For example, with softer structure the work-hardening capability is increased and dislocation density of polygonal ferrite/granular bainite is known to be smaller than in martensite/upper bainite further delaying the instability condition [12]. This signifies that, concerning bendability, lower hardness of the surface is desirable since it results in the need for larger strains before the condition for strain localization is attained. 4. Summary The aim of this study was to characterize two different steel concepts by means of their bendability properties. Traditional three-point bending tests, microstructural characterization and a newly developed frictionless bending method was applied. The target was to understand and determine the factors governing the bendability of the studied steels. The main observations and conclusions of the work can be summarized as follows: • The novel steel concept provides softer subsurface layers than conventional steel with a similar yield strength. • Relatively soft polygonal ferrite/granular bainite subsurface structure enhances the critical properties, such as work-hardening rate, needed to delay strain localization in bending compared to harder subsurface microstructure. • Delayed strain localization leads to better bendability as crack formation is occurs at larger strains. This means that smaller bending radii and/or larger bending angles can be utilized. • A newly developed frictionless bending method provides better information about the material behavior relevant to bending than does the traditional tensile test because it emphasizes the properties of the surface layers, which are critical during bending. References [1] J. Heikkala, A. Väisänen, Usability testing of ultra high-strength steels, In: Proceedings of the 11th Biennial Conference on Engineering System Desing and Analysis, (2012) 1–13. [2] D. Rèche, T. Sturel, O. Bouaziz, A. Col, A.F. Gourgues-Lorenzon, Damage development in low alloy TRIP-aided steels during air-bending, Materials Science and Engineering: A, 528 (2011) 5241–5250. [3] C. Soyarslan, M.M. Gharbi, A.E. Tekkaya, A combined experimental-numerical investigation of ductile fracture in bending of a class of ferritic-martensitic steel, International Journal of Solids and Structures, 49 (2012) 1608–1626. [4] M. Kaupper, M. Merklein, Bendability of advanced high strength steels—A new evaluation procedure, CIRP Annals - Manufacturing Technology, 62 (2013) 247–250. [5] I.L. Dillamore, J.G. Roberts, A.C. Bush, Occurrence of shear bands in heavily rolled cubic metals, Metal Science, 13 (1979) 73–77. [6] A.M. Arola, A. Kaijalainen, V. Kesti, The effect of surface layer properties on bendability of ultra-high strength steel, AIP Conference Proceedings, 1769 (2016) 200024. [7] A.J. Kaijalainen, P. Suikkanen, L.P. Karjalainen, J.J. Jonas, Effect of austenite pancaking on the microstructure, texture, and bendability of an ultrahigh-strength strip steel, Metallurgical and Materials Transactions A, 45 (2014) 1273–1283. [8] A.J. Kaijalainen, P.P. Suikkanen, L.P. Karjalainen, D.A. Porter, Influence of subsurface microstructure on the bendability of ultrahighstrength strip steel, Materials Science and Engineering: A, 654 (2016) 151–160. [9] L. Troive, New method for evaluation of bendability based on three-point-bending and the evolution of the cross-section moment, Journal of Physics: Conference Series, 896 (2017) 12006. [10] VDA 238-100 test specification draft: Platebending test for metallic materials, (2017). [11] Z. Marciniak, J.L. Duncan, S.J. Hu, Mechanics of Sheet Metal Forming, Butterworth-Heinemann, Oxford, (2002). [12] A. Saastamoinen, A. Kaijalainen, D. Porter, P. Suikkanen, The effect of thermomechanical treatment and tempering on the subsurface microstructure and bendability of direct-quenched low-carbon strip steel, Materials Characterization, 134 (2017) 172–181.