With environment and safety guidelines pushing for fuel efficiency and safety, automakers are more than ever exploring lighte and strong car bodies. Lightweight construction reduces the weight of an electric vehicle and substantially increases its fuel economy, and body stiffness contributes to crash safety performance.
One of the recent automotive trends is "die-quenched steel plate" of high strength for more parts of the car body. Parts relying on die-quenching solutions include, for example, reinforcement parts inside doors and bumpers, and frame parts such as pillars, those required to be strong enough to protect the occupants in case of a collision.
In hot forming die-quenching, steel sheets are, as its name suggests, heated at 900 to 1000 °C and then transferred to a die and pressed in a mold, where they are rapidly cooled and quenched at the same time. The parts resultantly have enormously improved hardness and tensile strength. Die-quenching offers the flexibility of mold shape selection, allowing a geometrical variety of parts. Ultra-high-tensile steel sheet, another name for die-quenched steel sheet, comes from its tensile strength* that can reach as high as 980 to 1500 MPa, nearly twice the 500 to 800 MPa common high-tensile steel sheets have.
Aisin Takaoka is home to metal parts professionals specializing in casting, machining, and stamping of automobile parts. High-strength body parts including die-quenched parts are produced here.
Automotive die-quenched parts, before being delivered to customers, need to be tested to ensure they conform to the designed strength. Destructive testing is commonly used to determine the hardness of materials. In these hardness tests, a small piece cut out from a sampled part is mounted in resin. The test piece is machine-polished, and the test surface is indented with a diamond indenter. The hardness of the sample is determined by the average length of diagonals of the indentations. The idea is that materials that are hard enough should have sufficient strength.
However, destructive testing is done with samples, since we cannot test products we ship. Destructive testing methods require detailed procedures and inevitably produce waste. Another drawback is that they are time-consuming. A substantial number of nonconforming products could be churned out by the time a test report of the nonconformity comes out. And, the more the products and models, the more laboratory operators are needed.
There a more efficient, less time-consuming way to test all of the parts produced? That was the question that fueled the spirit to develop a new non-destructive test method.
* Tensile strength: The capacity of a material or structure to withstand loads that can cause it to elongate, i.e., strength to resist tension.
Nondestructive testing (NDT) is a wide group of analysis techniques that include frequently used radiographic and ultrasonic testing. It was the magnetic property that drew the attention of laboratory engineers at Aisin Takaoka. A research presentation by Iwate University at a technology tradeshow caught their eyes.
Die-quenching forms a steel crystalline structure called “martensite” that locks in the carbon atoms. This is the secret that delivers the highest level of hardness among pressed parts. Martensite shows increasing coercivity on a magnetic hysteresis loop, which is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. The hardness and coercivity of a die-quenched steel sheet are correlated, meaning that the hardness can be known by measuring the magnetic properties.
Correlation between Coercivity and Hardness
The coercivity is a horizontal intercept of the hysteresis loop.
The greater the coercivity, the longer the horizontal intercept.
An engineer of Aisin Takaoka who was interested in the presentation proposed giving it a try. Takeshi Seo, who had joined Aisin Takaoka four years before, was selected to lead the project. His major at university was semiconductors, so magnetism was a new domain to him, but the natural born engineer's gut feeling told him the technique would work.
Without experience or specialist to help him, Seo had to start the project virtually alone under his boss. It was back in 2013 when he took his first move to visit to Iwate University for advice from Prof. Yasuhiro Kamada, a material engineering and NDT specialist.
Seo recalls, "You may call it a sort of industry-academia collaboration, but it was more like a hidden project I was doing along with other research assignments. I felt like I was back in school as a graduate student working in the professor's lab.”