Kiyoyuki Fukui

Prediction of Post Weld Hardness of Advanced High Strength Steels for Automotive Application using a Dedicated Carbon Equivalent Number

Weldability of advanced high strength steels in automotive manufacturing is a key issue. There are two important aspects to weldability: producing the welds and the quality of the welds. Producing the welds concerns the process to be used, possible addition of filler materials, the electrodes to be used, et cetera. Weld quality concerns the performance of the welds in a construction (e.g. strength and crash). With advanced high strength steels issues arise with increasing strength levels concerning the weld-quality. Traditionally carbon equivalent numbers are used to predict weldability. These traditional carbon equivalent numbers are not sufficient to predict post weld hardness of advanced high strength steels. Sumitomo Metal Industries and Corus cooperate to research weldability of advanced high strength steels. This paper concentrates on the influence of the chemical composition on weldability, as a first step to assess weldability of advanced high strength steels. This is done in two steps. First the traditional use of carbon equivalent numbers to predict weldability is explored. Literature is reviewed and possible issues with welding of advanced high strength steels are identified. Next the application of carbon equivalent numbers to predict post weld hardness for various welding processes (e.g. laser beam welding and resistance spot welding) is discussed. A wide range of steels was evaluated experimentally to determine the relationship between chemical composition and post weld hardness. The influence of welding processes expressed in terms of the cooling rates. The results are combined into simple models to predict post weld hardness of advanced high strength steel joints.

Tailored Blanks of High Strength Steels — Comparison of Welding Processes

The purposes of this work are: 1) To determine optimum welding parameters for high strength steels, and to define an upper steel strength limit that can be welded. 2) To obtain fundamental data on the formability and the fatigue properties of high strength steels. The strength of steel sheets used in tailored blanks for automotive bodies has been rising. In order to obtain guidelines for the choice of appropriate welding methods to high strength steels, formability and fatigue performance were investigated. Laser, mash seam and plasma welding were employed to 590 MPa and 780 MPa high strength steels. The results suggested that laser welding was the best welding process for both formability and fatigue performance. The mash seam and plasma welding thermal cycles deteriorate the mechanical properties of the weld metal and HAZ. As a result, the mash seam and arc welded 780 MPa steel sheet broke at the weld or in the HAZ during forming. Laser and arc welded high strength steel sheets exhibited a high fatigue strength. However, the fatigue strength of the mash seam welds was low due to stress concentration.

The Effect of Ageing on the Spot Weld Strength of AHSS and the Consequences for Testing Procedures

Market trends within the automotive industry are leading to an ever-increasing use of high-strength and advanced high-strength steels (AHSS). The attraction of these materials is the advantage of excellent formability, combined with increasingly high tensile strength. It is a well-known fact that weld performance can be an issue with AHSS, where susceptible weld microstructures can lead to low strengths and undesirable failure modes. Much research has been conducted and published on this subject. A less well-documented effect in the weld performance of AHSS is ‘ageing’, whereby a weld exhibits poor mechanical performance immediately after welding, but after a certain period of time, the weld properties improve significantly. In the Corus — SMI research cooperation, this ‘ageing’ effect was first observed in weld samples in 2004, since this time ageing has been a major topic of combined research. This presentation is a summary of the Corus — SMI weld ageing study, highlighting the major issues and characteristics of the effect: the influence of process parameters, the susceptible alloying systems and the possible mechanisms that can cause ageing of the weld. The ‘ageing’ effect has serious implications for standardized testing procedures, where the timescale between welding and testing is not specified, ageing can have a huge influence on the welding results obtained in ageing susceptible materials. The final aspect of this report is to consider the consequences of ageing for weld testing procedures.

Fatigue Strength Prediction of Spot-Welded Joints Using Small Specimen Testing

It is well known that fatigue strength of spot weld of high strength steel sheet is not improved, compared with that of mild steel sheet. In this study, the governing factors and the effects of steel grade on fatigue strength of spot weld is investigated. Firstly, small specimens with total length of less than 3mm are taken from the spot weld of mild steel sheet (270MPa-grade) and high strength steel sheet (590MPa-grade). And then, tensile and high cycle fatigue properties are individually evaluated by newly-developed testing technique. Secondly, finite element analyses of tensile-shear specimen of spot-welded joints under cyclic loading are carried out and fatigue limit of the joints are predicted, using the above-mentioned local material strength properties and considering welding residual stresses around spot weld. Predicted results are nearly equal in both steels, which coincides with experimental results. It is found that fatigue strength of HAZ, which is the crack initiation site in joint, of 590MPa-grade steel is higher than that of 270MPa-grade steel. However, residual stress in 590MPa-grade steel is also higher and this may be one of the reasons why 590MPa-grade steel exhibits little improvement in fatigue strength of the joint over 270MPa-grade steel.

TAILORED BLANKS OF HIGH STRENGTH STEELS - COMPARISON OF WELDING PROCESSES

The strength of steel sheets used in tailored blanks for automotive bodies has been rising. In order to obtain guidelines for the choice of appropriate welding processes and materials, the formability of welded steel sheets were investigated. The purposes of this work are 1) to determine an upper steel strength limit that can be welded and 2) to obtain the requirements of materials from a viewpoint of formability. Laser, mash seam, and plasma arc welding were employed up to 980MPa high strength steels. The results suggested that laser welding was the best welding process because of its small heat input. It could be applied to 980MPa steels. 590MPa was the maximum steel strength grade to which mash seam and plasma arc welding could be applied, because the mash seam and the plasma arc welding thermal cycles softened the HAZ of high strength steel. The requirement of materials for the formability of laser welded steels is high elongation of base metals. On the other hand, the requirement for the formability of plasma arc welded steels is low weld hardness caused by low carbon content. (A) For the covering abstract see ITRD E121867.

Tailored blank technology of high strength steel sheet

Optimization of component structures, reduction in the number of components and improved yield of steel sheet are sought for tailored blank (hereafter referred to as TWB) technology, in which a welded blank material is subjected to press forming by the optimum arrangement of multiple steel sheet and one-piece forming. This technology has been employed by various automobile manufacturing companies. The demand for both safety improvements as well as for light weight automobiles has been increasing year after year and there is a developing requirement in the application of effective high tensile strength steel sheet year by year. Also, the move to high tensile strength TWB components has been taken into consideration; for example, high tensile strength steel of more than 500 MPa is employed for structural components, such as pillars and rails. TWB technology consists of the welding process and the subsequent press process of blank materials, as indicated in figure 1. Needless to say, a quality such that no cracks occur during press forming is required for welded blank materials. However, the strain direction and value near the weld line are varied since a combination of components of dissimilar strength and dissimilar sheet thickness are welded and press formed. Accordingly, when considering press forming cracking of actual components, it is necessary to consider not only the maintenance of quality of the welded blank material and the characteristics of the base metal and weld zone but also the blank configuration/component/ metal mould design etc., for which the arrangement of the weld line is taken into account. The purpose of this paper is to deepen the understanding of TWB welding technology of high tensile strength steel sheet. The laser, mash seam and plasma welding processes, which are employed for TWB were reviewed and the characteristics of the welding processes and case studies of the weld zone formability of high tensile strength steel using these welding processes are introduced in this paper.