The use of high strength steel sheets is increasing in Japan, due to the need for autobody weight reduction and strengthening as well as the prevention of surface damage during production. A current trend in Japan is that towards the use of HS steel sheets not only for the bumpers, door impact bars and some structural members but also for the outer and inner body panels. This trend has increased due to demand from autobody engineers, and the corresponding development of new types of steel sheet is a result of their cooperative efforts towards the same goal.
Trials of the HS steel sheets application started in 1972 and have been continued for about 10 years, with improving material properties and forming techniques, making possible the large scale application of HS steel sheets and prediction of its future status.

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In the near future, we expect the further application of high-strength sheet steel in vehicles. Through the repetition of actual parts tests and through the optimization of vehicle performance and manufacturing requirements, we will increase the usage of this material.
We believe we can assemble cars more efficiently through the use of such a steel. We expect 35-45 kgf/mm² tensile strength steel for outer panels due to the need for panel stiffness and dent resistance, 45-60kgf/mm² tensile strength for the inner parts, and 60-80kgf/mm² tensile strength for bumpers and door impact beams.
From a car engineer's point of view, we would like the steel industry to produce a high performance, high-strength sheet steel with improved formability and weldability. We look forward to using the precoated high-strength sheet steel, as well as the recently developed, bake-hardenable steel in the near future.

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In order to decrease fuel consumption, reducing the weight of autobody is greatly expected. To solve this problem, the developments of high strength steel sheets and its application to autobody have been made. Therefore, we have tried to discuss with these materials about press formability, surface deflection, dent resistance through several experimental studies in actual press shops. And we have obtained some results to the required mechanical properties of high strength steel sheet for autobody parts.

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A new type of cold rolled dual phase steel sheet with good formability and dent-resistance has been developed for automobile outer body panels. The steel, basically a low carbon and manganese aluminum killed steel, is manufactured by means of hot rolling with high temperature coiling, cold rolling, and continuous annealing with water-quenching and tempering. The mechanical properties after temper rolling are characterized by yield strength lower than 24kgf/mm², total elongation larger than 40% and r-value higher than 1.5. Moreover the steel sheet has not only high bake hardenability but also high anti-aging property.

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The planar anisotropy of the r value
(Δr=|r₀+r₉₀/2-r₄₅|)
in Nb-treated high strength hot-rolled steel sheet is usually high. Methods of decreasing Δr have been studied in laboratory-melted materials and with an experimental rolling mill.
The mechanism which gives rise to an increase in Δr is probably that rolling conditions such as rolling sequence or lubrication affect the development of austenite texture in the temperature range where recrystallization does not take place. This texture is then inherited by the ferrite during the γ-α transformation.
Unidirectional rolling makes Δr higher than does reverse rolling, and water-lubrication or no lubrication makes Δr higher than does oil-lubrication.
Ti, V and Mo do not cause such a large increase in the lowest critical temperature for γ recrystallization (T_c) as does Nb and so these elements are suitable as additions to decrease Δr in high strength hot-rolled steel sheet for deep drawing.
(r₀, r₄₅, r₉₀: r values at 0°, 45°, and 90° to the rolling direction)

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The metallurgical factors affecting the press formability of cold rolled high strength steels are discussed and the properties of the following 40 kgf/mm² class tensile strength steels are examined: dual phase steel consisting of a ferrite-martensite structure, produced by rapid cooling in a continuous annealing line, which has a very low yield to tensile strength ratio and high n-value; rephosphorized steel, which is a batch-annealed aluminum-killed steel hardened with phosphorus and manganese, having a high r-value and a relatively low yield strength; and a recently developed steel of extra deep drawing quality and high strength which is characterized by its extremely high r-value, over 2.0, and low yield strength. This can be produced by continuous annealing of niobium-stabilized, extra low carbon, phosphorus-bearing steel. The formability and brittleness of the steels are also examined.

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The bake-hardenable cold-rolled steel sheets for the outer panels of auto-bodies exhibiting an excellent resistance to the denting have been developed in the box-annealing process, by examining the effects of chemical compositions and box-annealing conditions on the bake-hardenability and on Snoek peak height of Al-killed steels. It has been found that lowering the total carbon content to less than 0.02% or raising the annealing temperature to air intercritical range bears a bake-hardenable steel sheet in the box-annealing process. Additions of silicon and phosphorus and reduction of manganese are also found to increase the bake-hardenability. The bake-hardening is due to the strain aging caused by the solute carbon of approximately 10ppm. Furthermore, it is shown that the bake-hardenable steel sheets can be successfully manufactured in steel mills and exhibits non-aging quality as well as the high r-value.

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Dual-phase structure formation in the intercritical annealing method is strongly affected by the cooling stage. Role of the cooling stage consists of
1) carbon enrichment into the untransformed austenite,
2) avoidance of pearlite formation, and
3) martensite and retained austenite formation.
Significance of a two-stage cooling can be explained on the above basis.
Employment of a rapid heating and a short-time intercritical holding produces a dual-phase steel having both a high bake-hardening and a high work-hardening characteristics. This phenomenon is supposedly related to a transient state of solute carbon partitioning in the intercritical temperature range.

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Several microalloyed steels were intercritically annealed and then aircooled to room temperature or water-quenched to obtain different microstructures. The effects of the amount and the nature of low-temperature transformation products on the deformation behavior, stretch-flangeability and fracture-toughness after cup deep-drawing of multi-phase sheet steels were examined. Bainite has a slight deleterious effect on strength-uniform elongation relationship but has a favorable effect on stretch-flangeability and on fracture toughness after cup deep-drawing as compared with martensite. It is also suggested that ferritic-bainitic (-martensitic) steel may be preferred to “dual phase” steel in some complex pressings.

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