The major scientific and technological advances and breakthroughs of advanced high strength steels (AHSS) were achieved due to the strong demands of automotive industry. The development of AHSS began in the early 1980s with the aim of improving passenger safety and weight-saving. The present paper presents the driving forces and logic of development of various AHSS for automotive applications since 1980s. The importance of crash performance, weight-saving, formability, and rigidity are critically reviewed for the development of new steel grades for automotive applications. The logical sequences of the development of dual phase (DP) steel, transformation induced plasticity (TRIP) steels, tempered DP steels, complex phases (CP) steels, Ferrite-Bainite steels, hot-stamping technology, twinning induced plasticity (TWIP) steels, Quench and Partitioning (Q&P) steels, Medium Mn steels, and steels–polymer composites are presented.

Driving Force and Logic of Development of Advanced High Strength Steels for Automotive Applications

In order to clarify the effects of the martensite distribution on the mechanical properties of low-carbon dual-phase steel, four types of dual-phase steel with different ferrite grain sizes and martensite distributions were prepared using a thermomechanical treatment. The tensile properties of these steels were investigated; in particular, the strain hardening and the ductile fracture behaviors were discussed in terms of the strain partitioning between the ferrite and martensite and the formation and growth of micro-voids, respectively. When the martensite grains surround the ferrite grains and form a chain-like networked structure, the strain hardenability is greatly improved without a significant loss of elongation, while the necking deformability is considerably reduced. A digital-image correlation analysis revealed that the tensile strain in the martensite region in the chain-like networked dual-phase structure is markedly increased during tensile deformation, which leads to an improvement in the strain hardenability. On the other hand, the joint part of the martensite grains in the structure acts as a preferential formation site for micro-voids. The number density of the micro-voids rapidly increases with increasing tensile strain, which would cause the lower necking deformability.

Effect of the martensite distribution on the strain hardening and ductile fracture behaviors in dual-phase steel

In this paper, we examine the key factors influencing ductile failure of various grades of dual phase (DP) steels using the microstructure-based modeling approach. Various microstructure-based finite element models are generated based on the actual microstructures of DP steels with different martensite volume fractions. These models are, then, used to investigate the influence of ductility of the constituent ferrite phase and also the influence of voids introduced in the ferrite phase on the overall ductility of DP steels. It is found that with volume fraction of martensite in the microstructure less than 15%, the overall ductility of the DP steels strongly depends on the ductility of the ferrite matrix, hence pre-existing micro-voids in the microstructure significantly reduce the overall ductility of the steel. When the volume fraction of martensite is above 15%, the pre-existing voids in the ferrite matrix does not significantly reduce the overall ductility of the DP steels, and the overall ductility is more influenced by the mechanical property disparity between the two phases. The applicability of the phase inhomogeneity driven ductile failure of DP steels is then discussed based on the obtained computational results for various grades of DP steels, and the experimentally obtained scanning electron microscopy (SEM) pictures of the corresponding grades of DP steels near fracture surface are used as evidence for result validations.

On key factors influencing ductile fractures of dual phase (DP) steels

Nanoindentation measurements were obtained on eight commercially-produced DP980 dual-phase steels to quantify the hardness of the individual constituents, ferrite and martensite, in each steel. Each microstructure was also evaluated to determine grain size, martensite volume fraction (MVF), and retained austenite content. Nanoindentation hardnesses and quantitative microstructural measurements were correlated with tensile properties and performance in hole expansion tests to assess the importance of the individual constituent properties. Hole expansion samples were prepared with both sheared edges produced by mechanical punching, and non-deformed edges produced by electric discharge machining (EDM). Average material hardness based on nanoindentation data correlated directly to Vickers hardness measurements, verifying the capability of the nanoindentation technique to produce data consistent with traditional hardness measurements. Yield strength (YS) correlated directly to ferrite hardness indicating that, for a similar MVF and microstructural morphology, the YS is controlled by the strength of the softer matrix phase (ferrite). Hole expansion ratios (HER) on EDM samples decreased with an increase in both martensite and ferrite hardness, indicating that EDM HER values can be enhanced by softening both constituents. Punched-hole HER values decreased with increasing martensite hardness and martensite-to-ferrite hardness ratio, but were independent of ferrite hardness, indicating that softening the martensite while increasing the ferrite hardness could produce a higher HER.

https://inside.mines.edu/~cpackard/papers/Correlations%20between%20nanoindentation%20hardness%20and%20macroscopic.pdf

Correlations between nanoindentation hardness and macroscopic mechanical properties in DP980 steels

In the present paper, the effects of process parameters on phase transformations during non-isothermal deformations are described and discussed. Non-isothermal high temperature compressive deformations were conducted on 22MnB5 boron steel by using deformation dilatometry. Cylindrical samples were uniaxially deformed at different strain rates ranging from 0.05 to 1.0 s −1 to a maximum compressive strain of 50%. Qualitative and quantitative investigations were carried out using surface hardness mapping data as well as dilatation curves. It was observed that a higher initial deformation temperatures resulted in a higher martensite fraction of the microstructure, while a variation in the martensite start temperature was negligible. Another conclusion was that by applying larger amounts of strain as well as higher force levels, not only the martensite start temperature, but also the amount of martensite was reduced. Moreover, it was concluded that using surface hardness mapping technique and dilatometry experiments were very reliable methods to quantify and qualify the coexisting phases.

The effects of non-isothermal deformation on martensitic transformation in 22MnB5 steel

Stretch-flange-formability is an important property for ultra high strength steel (UHSS) sheets for press-forming. In this study, microscopic deformation behaviors during punching and following stretch-flange-forming were investigated using three types of 980 MPa grade UHSS sheets with either two ferrite/martensite dual-phase structures or a martensite single-phase structure in order to clarify how the microstructure affects the stretch-flange-formability of UHSS sheets. The results of this investigation revealed following conclusions. Microscopic plastic-flow or micro-void density generated by punching is not the dominant factor of the stretch-flange-formability of UHSS sheets. During hole-expanding, cracks were mainly initiated at the fractured surface part and the cracks became longer and deeper from the punched surface with the increase of hole-expanding ratio. Deep cracking resistance in this process is important to improve the stretch-flange-formability. The existence of strain gradient induced by hole punching is considered to be one of the reason for the highest hole-expanding ratio of the martensite single phase steel. During hole-expanding, the micro-cracks propagate mostly along the phase interfaces in the dual-phase steel sheets in the case of poor stretch-flange-formability, while the micro-cracks are tend to propagate through ferrite and martensite phases in the case of high stretch-flahge-formability. The analysis of the hardness of ferrite and martensite suggests that the difference in hardness is the dominant factor of the stretch-flange-formability of the dual-phase steel. In addition, the volume fractions of phases also influence the formability.

Effects of Microstructure on Stretch-flange-formability of 980 MPa Grade Cold-rolled Ultra High Strength Steel Sheets

PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet reduced in dispersion in mechanical properties and its manufacturing method. SOLUTION: The high strength cold rolled steel sheet has a composition which consists of 0.06 to 0.15% C, 0.5 to 1.5% Si, 1.5 to 3.0% Mn, ≤0.05% P, ≤0.01% S, 0.5 to 1.5% Al and the balance Fe with inevitable impurities and in which the value of A, defined by A=Si+9×Al, satisfies 6.0≤A≤20.0. Moreover, the steel sheet has a structure which is composed of a dual-phase structure of ferrite and martensite and in which the area ratio of the ferrite is 40 to 90%. When manufacturing this steel sheet, recrystallization annealing/tempering treatment is carried out by carrying out: holding at a temperature ranging from the Ac 1 point and the Ac 3 point for ≥10 sec; cooling down to 500 to 750°C at ≤20°C/sec cooling rate; rapid cooling down to ≤100°C at ≥100°C/sec cooling rate; and tempering at 300 to 500°C. COPYRIGHT: (C)2007,JPO&INPIT

High strength cold rolled steel sheet reduced in dispersion in mechanical characteristic, and its manufacturing method

Disclosed is a super-high strength cold-rolled steel sheet having excellent bending properties and lagging destruction resistance and having a small thickness. Specifically disclosed is a super-high strength cold-rolled steel sheet having excellent bending properties, characterized by containing a soft steel sheet surface layer part that comprises 0.15-0.30% of C, 0.01-1.8% of Si, 1.5-3.0% of Mn, 0.05% or less of P, 0.005% or less of S, 0.005-0.05% of Al, 0.005% or less of N, and the remainder made up by Fe and unavoidable impurities, and meeting the formulae shown below, wherein the soft steel sheet surface layer part contains tempered martensite at a ratio of 90% or more by volume, the structure of the center part of the steel sheet comprises tempered martensite, and the tensile strength of the steel sheet is 1270 MPa or more. Hv(S)/Hv(C) <= 0.8... (1) (wherein Hv(S) represents the hardness of the soft steel sheet surface layer part; and Hv(C) represents the hardness of the steel sheet center part.) 0.10 = t(S)/t = 0.30 ... (2) (wherein t(S) represents the thickness of the soft steel sheet surface layer part; and t represents the thickness of the sheet.

Super-high strength cold-rolled steel sheet having excellent bending properties

Provided are a high-strength steel sheet and a method for producing the same. A high-strength steel sheet has a composition containing 0.10% to 0.18% C, more than 0.5% to 1.5% Si, 0.5% to 1.5% Mn, 0.05% or less P, 0.005% or less S, and 0.05% or less Al on a mass basis, the remainder being Fe and inevitable impurities and also has a microstructure containing ferrite and pearlite. The volume fraction of the ferrite is 70% to 97%. The volume fraction of the pearlite is 3% or more. The volume fraction of cementite present at grain boundaries of the ferrite is 2% or less. The sum of the volume fractions of phases other than the ferrite, the pearlite, and the cementite is less than 3%. The average grain size of the ferrite is 7 μm or less.

High-strength steel sheet and method for producing the same

PROBLEM TO BE SOLVED: To provide a dual phase type high tensile strength steel sheet having excellent elongation and stretch flanging properties, and exhibiting the value of TS (tensile strength)×{El (elongation)×λ} 1/2 satisfying ≥30,000, and to provide a production method therefor. SOLUTION: The steel sheet has a composition comprising, by mass, 0.03 to 0.20% C, ≤2% Si, 0.5 to 3% Mn, ≤0.10% P, ≤0.01% S, 0.01 to 0.1% Sol.Al, ≤0.005% N, ≤1% Cr and ≤1% Mo, and the balance iron with inevitable impurities, and a structure in which the average crystal grain size of martensite is ≤2 μm, the average crystal grain size of ferrite is ≤2 μm, and the volume ratio of martensite is 20 to <60%. COPYRIGHT: (C)2004,JPO&NCIPI

Kenji Kawamura

Papers

Effects of Microstructure on Stretch-flange-formability of 980 MPa Grade Cold-rolled Ultra High Strength Steel Sheets

High strength cold rolled steel sheet reduced in dispersion in mechanical characteristic, and its manufacturing method

Super-high strength cold-rolled steel sheet having excellent bending properties

High-strength steel sheet and method for producing the same

Dual phase type high tensile strength steel sheet having excellent elongation and stretch flanging property, and production method therefor