Forming

Research on steel forming technologies conducted by POSCO which is essential for diverse industries can be classified into the following four categories

Formability evaluation and friction test

In order to check manufacturing formability of new high strength steel a forming test of auto part is performed and then it is possible to confirm the formability by using FLD (Forming Limit Diagram) for Quantitative analysis of strain distribution. FLD is a diagram that expresses the amount of strain in each direction caused by forming, and formability can be determined by using this procedure.

Estimation of springback

Springback of material and shear fracture have recently become important factor in the forming process by adopting AHSS (Advanced High Strength Steel) to auto parts. In order to accurately estimate springback and shear fracture, forming analysis technology with a higher precision than before and advanced material model such as yield function and hardening rule are under research. In addition, we conduct research on production technology with respect to process design including die and part shape to minimize springback and shear fracture. Through prediction of amount of springback and formability of parts which is in development, we are able to propose an optimal solution with consideration on manufacturing conditions of customers.

Parts design and die development technology

We make guidelines on the use of new steel materials for inexperienced customers, providing technical support for parts design and die technology. POSCO has been developing application technologies for new steel materials with OEMs and parts companies through EVI collaboration to promote application of new steel materials.

New forming technology

We develop and supply new forming technologies to increase our market share of steel materials in the automobile market.

Crashworthiness

Crashworthiness is the most important requirement when designing automobiles, because it is directly linked to safety of passengers. Crashworthiness of a car is affected by material, structure, safety device and crash conditions. POSCO conducts researches on dynamic material properties, crashworthiness of automotive parts and crashworthiness of auto-body based on crash simulations.

When an auto-body is deformed by crash accidents, materials of the auto-body are rapidly deformed. Strength of materials generally depends on strain rates. Steel has excellent characteristics for increasement of the strength by strain rates when compared to other materials. POSCO has established the database (DB) for dynamic materials to perform crash simulations and provide dynamic material data to POSCO’s customers.

Crash tests of automotive parts are not carried out often whereas crash tests of a full vehicle are performed in developing a new car. Thus, POSCO has developed its own high speed crash tester to conduct crash tests on high speed compression and bending for automotive parts and simplified specimens. Crashworthiness such as energy absorption and fracture, collapse mode, reaction force, and mean load are evaluated to provide reference data for car design.

The primary purpose of vehicle crash analysis is to propose appropriate steels to POSCO’s customers. POSCO participates in new car development of customers as EVI activities to propose appropriate steels based on the results of crash simulations. Furthermore, new steels developed by POSCO such as AHSS (Advanced High Strength Steel) are applied to automotive parts in order to enhance the lightweight and crashworthiness.

Durability and Fatigue

Light weight of vehicles is receiving great attention as automobile industry started to set targets for reduction of CO₂ emission. With the trend of light weight, chassis parts are increasingly attempting to apply high strength steel as a means to increase strength and stiffness and reduce weight of existing hot rolled PO products. Durability of parts is the most important point to be considered when trying to reduce weight of chassis parts.

POSCO provides physical properties on fatigue test for durability analysis to its clients. We manufacture a variety of test specimens under different steel types, specimen shapes, test conditions, basic materials and welding conditions wanted by customers, providing S/N (stress amplitude-number of cycles) curve and e-N (total strain amplitude-number of reversals) curve as results of fatigue test. We also support stiffness / durability test on parts when requested by clients.

POSCO also conducts EVI activities on suggestion of optimized steel and parts design to be satisfied to customer’s durability performance specifications. Among chassis parts, parts that require durability such as CTBA (Coupled Torsion Beam Axle), wheel, engine cradle, lower control arm and stabilizer bar are interlinked with fatigue data of POSCO steel to provide the results of durability analysis and suggest the most appropriate steel type. Furthermore, we support joint research with clients, parts design, manufacture, and durability / stiffness evaluation. We also analyze causes of fatigue failure through SEM imaging of fatigue fracture surface, measurement of surface roughness, and measurement of strain rate and residual stress.

Welding/Joining

Welding is the most efficient way of joining steels and is an essential process for steel applications in each industry. POSCO is helping to create new demand for automobiles, household appliances and other industries through the development of customer-oriented steel sheet joining technology. In recent years, interest in weldability has been greatly increased due to the increase in use of high-strength steels. We are assuring the quality for new grades through development of cutting-edge technology for welding and joining steel sheet. In addition, we have been accumulating technologies related to resistance welding, arc welding, laser welding, and adhesive bonding of steel sheet, and are now supporting welding education and defect analysis at our customers.

Development of Welding Quality Assurance Technology for Steel Materials Difficult to Weld

We established welding part evaluation technology for actual parts of AHSS, hyper NO steel and high alloy 400 series STS. We also develop customized joining technology for steel materials difficult to weld, and support weld quality assurance technology demanded by clients. As providing welding database that can be reflected on early design of welding part, we support our client in designing an efficient design with more accurate and faster weld analysis and prediction system. In addition, we provide the information of optimal welding materials for steel materials difficult to weld and conduct research on the development of functional and anti-corrosive welding materials.

Development of New Welding and Joining Technology

In addition to further development of existing thin plate joining technology, we have developed advanced welding technologies to enhance weld quality of steel materials difficult to weld. We have an intelligent resistance spot welding system and low heat input welding technology like CMT for spatter reduction. We are continuing our research on TWB, delta spot, HFIW, flash butt pipe and laser hybrid welding technologies, as well as solid state joining technology for high strength steel.

Customized Technical Support on Clients

We conduct regular training that combines general contents of thin plate welding and core technologies for clients. We analyze and support production issues and welding defects to provide customized technical support.

Heat Treatment

The final mechanical properties of heat treated steels are determined by heat treatment. Heat treated steels can obtain high strength, high toughness, durability, wear resistance and fatigue performance through heat treatment.

Heat treatment is usually final process. Forming process is accompanied before heat treatment because it makes forming easily to obtain final shape of each heat treated parts. We can also improve specific properties using heat treatment.

Heat treatment can be classified into two types. The common type of heat treatment is bulk hardening heat treatments using high carbon steels and alloy steels, which are represented by ‘QT’ (Quenching and Tempering) and ‘Austempering’. The heat treatments such as nitriding, carburizing and carbonitriding, applied on only surface of steels, tends to increase in these days. (Especially, nitriding shows smaller distortion and better surface compared to bulk heat treatment in spite of its high cost.) Other heat treatments including annealing, high frequency induction heat treatment, flame hardening (which mostly used in mold steel), plasma heat treatment, and vacuum carburizing have been studied.

POSCO mainly provides technical supports to find and apply a heat treatable steel and heat treatment method satisfied with final customer’s requirements. Especially, PAC have carried out researches to find appropriate heat treatment conditions in order to replace expensive high alloyed high carbon steels into low cost Boron added high carbon steel, resulting in reducing material cost.

Also, PAC have analyzed in various heat treated samples such as automotive parts, tools, etc. to find defects and improve heat treatment method for our customers. We established a steel database on heat treatment characteristics of steels. In addition, we have researched shortening process of our customer’s manufacturing system using heat treatment.

Case: Support for manufacturing IT(Induction heat Treatment) Wire

• Consistent hardness between the center and the surface is imperative during the manufacture of wires (to achieve uniform microstructure)
• Simulation to derive the optimal condition for heat treatment for the customer
• An optimal austenizing temperature recommended during the manufacture of IT wires

Corrosion/Corrosion Control

Stainless steel is an alloy steel with 12 wt. % of chromium or above and corrosion resistance. Since carbon steel has weak adhesion of oxides formed on the surface and many pores on the inside, oxygen can easily react with carbon steel. On the other hand, stainless steel has an oxide film with high adhesion and small defect known as passive film, which protects the base metal from oxygen. When surface of stainless steel is damaged by scratch, it reacts with surrounding oxygen to recreate the passive film by self-healing effect.

Such passive film made of iron and chromium oxides plays the most important role in determining anti-corrosiveness of stainless steel and it has extremely low thickness of about 2nm. Composition and structure of the film is still unknown. POSCO conducts various anti-corrosion analyses on the passive film of stainless steel to examine corrosion mechanism of stainless steel under different corrosive environments and presents guidelines for the applications of each stainless steels type.

For example, stainless steel is a semi-permanent material but it can be occasionally attacked as pitting corrosion, intergranular corrosion and stress corrosion cracking if wrong type of stainless steel is applied under specific corrosive environment. Therefore, it is extremely important to select appropriate steel type with consideration on corrosive environment and economic feasibility. Typical corrosion of stainless steel is pitting corrosion that occurs from local destruction of passive film caused by chlorine ion. When metal action increases inside pitting corrosion, it electrically attracts anion to increase the content of chlorine ion inside. This causes difficulty in formation of passive film on the inside, and pH is also lowered by hydrolysis to create hydrochloric acid (HCl) environment. As a consequence, pitting corrosion is quickly progressed and leads to penetration in severe cases. Therefore, Songdo PAC quantitatively evaluates pitting corrosion resistance of different stainless steel types using electro-chemical methods such as polarization test and suggests optimal steel type to each client.

Motor Performance