Research on Fabrication of TRIP Steel with High Durability Phuoc

Many weapons and mechanical parts such as bullet shells, jet engine covers, bulletproof armor shells operated in harsh conditions require durability when used and good flexibility when forming. Many countries have used steel with phase transition effects due to plastic deformation - TRIP effect (Transformation Induced Plasticity) to replace low carbon steel, high-strength low-alloy steel fabricating them. The paper presents research on the technology of metallurgy of a CMnSiAl steel grade from sponge iron materials and application of mechanical-thermal technology to create small particles and organize three phases: Ferrite, Bainite, residual Austenite to stabilize and increase plasticity. The results show that the impurity content is low, S% is less than 0.025%; magnitude of small ferrite particles below 20μm, ratio of residual austenitic fγ = 6.39%. Mechanical properties: durable limit σb on 590 MPa, yield limit σc over 390 MPa, ratio σb/σc> 1.5 and relative elongation δ50 over 28%. Research results for the prospect of applying this steel grade in the manufacture of some defense products, instead imported materials.


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Abstract-Many weapons and mechanical parts such as bullet shells, jet engine covers, bulletproof armor shells operated in harsh conditions require durability when used and good flexibility when forming.Many countries have used steel with phase transition effects due to plastic deformation -TRIP effect (Transformation Induced Plasticity) to replace low carbon steel, high-strength low-alloy steel fabricating them.The paper presents research on the technology of metallurgy of a CMnSiAl steel grade from sponge iron materials and application of mechanical-thermal technology to create small particles and organize three phases: Ferrite, Bainite, residual Austenite to stabilize and increase plasticity.The results show that the impurity content is low, S% is less than 0.025%; magnitude of small ferrite particles below 20μm, ratio of residual austenitic fγ = 6.39%.Mechanical properties: durable limit σb on 590 MPa, yield limit σc over 390 MPa, ratio σb/σc> 1.5 and relative elongation δ50 over 28%.Research results for the prospect of applying this steel grade in the manufacture of some defense products, instead imported materials.

I. INTRODUCTION
Materials in general and alloy steel, in particular, are a key development direction of each country.Since the late 60s of the 20th century, new metallurgical technology was born -the technology of producing cast iron by direct reconstitution of iron ore -the production technology of sponge iron was born, gradually replacing the furnace technology [1]- [4].The technology of sponge iron production has many advantages: without using cokes, it reduces a part of the environmental pollution, but more importantly, provides the metallurgical industry with an advanced material, sponge iron, with low carbon content, low content of P, S% impurities, containing trace elements, so that they are more durable and ductile than steel grades with corresponding components made from cast iron and scrap steel [5]- [9].
From the new material sources, the countries have created many advanced steel families: steel has very high plasticity used in deep stamping -IF steel, advanced steel grades of high strength AHSS -DP duplex steel, phase shift steel due to TRIP plastic deformation, and other special steel grades.Structural steel grades are classified according to durable limits combined with elongation (Fig. 1) [10].
Unlike the Conventional High Strength Steels (Conventional HSS), it is strengthened by solid solution, dispersion phase, and granulation, advanced high strength steel is strengthened thanks to the transformation of phase and in the nest.The function usually has ferrite, bainite, residual austenite and may have martensite (Fig. 2) [11], [12].These steels have high durability, some have good plasticity (suitable for forming), so they are applied strongly and increasingly to make structural details in the automobile and ship industries Marine, defense industry to meet the technical criteria (high durability, lightweight) and safety  In advanced high-strength steel families, steel with phase change due to plastic deformation (TRIP (Transformation Induced Plasticity) Steel -hereinafter referred to as TRIP steel) is a new steel line, with durability, high plasticity, ability to Good fatigue resistance, very suitable for making bearing details needed through shaping [13]- [15].The

Research on Fabrication of TRIP Steel with High Durability
Phuoc Quy Phong Nguyen plastic deformation transformation effect was introduced by Zackay in 1967 when the study of transforming austenite into martensite appeared during deformation of some stainless steels (Table 1).Particularly, residual austenite in the structure of TRIP steel is very rich in carbon and sensitive to plastic deformation.When affected by plastic deformation, residual austenite will transform into martensite and become a hard phase in the substrate [16]- [18].This phenomenon is similar to transforming austenite into martensite in high manganese austenitic steel when subjected to impact loads.As a result, this steel has high plasticity due to the initial residual austenite in the structure, both high strength after plastic deformation due to austenite transformed into martensite.

II. MANUFACTURING METHODS
Content 1: Cooking and making ingot cast properly with chemical components, casting into billet:  Samples after heat treatment were reheated before rolling and rolling on the test rolling machine; Then continue to heat treatment on the school's heat treatment equipment.According to the thermo-mechanical treatment method, TRIP steel also has two main lines: TRIP steel for hot-rolled heat treatment (referred to as hot-rolled TRIP steel) and TRIP steel for processing cold-rolled thermally (referred to as rolling steel TRIP) cool) (Fig. 5).The process of thermo-mechanical engineering to create steel billets of this type is quite complicated.The original billet (slab cast or slab rolled) is hot rolled at a temperature of T1 (the temperature at which the austenitic steel is held), to a certain thickness, then, the workpiece can be passed through hot rolling or used to cold rolled.For hot-rolled TRIP steel, the workpiece is rolled to end at T2 temperature (the temperature at which the steel consists of both ferrite and austenitic phases), then the steel billet is rapidly cooled to T3 (banit turning temperature)., keep the heat at T3 for a certain period of time so that austenite does not completely transform into banit but retain a certain amount, then cool quickly to room temperature to obtain a mixture of residual ferrite, banit and austenite.For cold-rolled TRIP steel, the billet is cold-rolled with a certain amount of deformation, then heated to T2 temperature and heat treated similarly to hot-rolled TRIP steel.Hot rolling method is often used to create TRIP billets of thickness greater than 3mm, while the cold rolling method is often used to create TRIP billets of smaller thickness.In general, the technology to create steel billets of this type is complex and requires high technology.The process of making billets must go through two stages: the stage of casting and refining of cast ingot and the process of processing heat to produce billet with organized requirements.More modern technology can use continuous casting -rolling, alternating heat treatment.
In the world, most of country smelting steel from the ore then refined and mixed with alloy elements.After creating the cast of swex castings, it is treated with the thermomechanical process according to hot-rolled or cold-rolled process.However, this process makes the post-cast model vulnerable to defects.

III. RESULTS AND DISCUSSION
A. Compare with mechanical properties of some highstrength steels The addition of alloy elements Mn, Al, and Si in steel has the effect of extending, stabilizing austenitic and slowing the reaction of forming the bits, especially the chromite.Therefore steel TRIP family CMnAlSi is a basic system and has been widely researched in the automotive industry because of its high synthesis properties (Fig. 6) in comparison to properties of TRIP steel, duplex steel (DP) and composite steel High strength low needle (HSLA)), while the price is not expensive.Analyzing the microorganism image of the steel sample after implementing the hot rolling, cold rolling, and heat treatment process, it was found that after hot rolling, the post-casting natural area was removed; Particle size more uniform, more edge.Continuing to deform the sample with the deformation of up to 70%, the deformed particles in the direction of rolling with the dimension perpendicular to the rolling direction are smaller and finer (average width of the grain only reaches about 10μm).It is this, when carrying out the next heat treatment, the particle has a small size and increases the properties of the material.After performing the heat treatment according to the above procedure, it is shown that the fine particle size is obtained with light and dark color phases (Fig. 4).However, by conventional optical photography, it is difficult to detect phase composition in an organization.This is further explained by color-imaged images.
The impregnated image analysis of the sample color after deformation is found to include: ferrite and austenite.Sample impregnated color samples after heat treatment showed ferrite phases; Bainite and residual austenite.Analysis of phase rates showed that the ratio of ferrit phase was about 52%.The amount of austenite is about 16% and the rest is Bainit.

IV. CONCLUSION
Smelted steel from sponge iron with low content of nonmetallic impurities P%, S% is a basic condition for training steel grades that require a high purity of non-metallic impurities, while no need for refining.Using iron-smelted iron is a scientific problem that needs to be decoded.Using mechanical-thermal technology, including: hot forging (forging ratio y = 4) and quick cooling to create small particles (average ferrite particle size ~ 20 m); Heat treatment in the two-phase ferrite + Austenite phase at 820 o C / 10 minutes and carburate isothermal in the Bainite transformation area at 400 o C during 10 minutes has created three ferrites, Bainit and Austenite residual phases (6.39%) to Chemical stability and increased plasticity.Calculated results: σb = 800 MPa, σc = 471 MPa.
Published on February 22, 2019.Phuoc Quy Phong Nguyen is with Ho Chi Minh city University of Transport, Ho Chi Minh city, Vietnam.

Fig. 1 .
Fig. 1.Classified steel according to plasticity and durability IF-steel has free intermediate phase transformation, IF-HS -high strength IF steel, BH -baked hardened steel, HSLA -high strength low alloy steel, TRIP -steel with phase change due plastic deformation, DP -duplex steel, CP -steel with complex phase structure, MARTENSITE -martensitic steel

Fig. 2 .
Fig. 2. Organization formation of advanced high strength steel Martensite -martensitic steel, Dual phase -duplex steel, TRIP -steel with phase change due to plastic deformation, Complex phase -mixed steel

Fig. 3 .
Fig. 3. Cooking diagram for create casting ingot With available materials in the country; The authors tested the cooking and refining process with a medium frequency furnace.Content 2: Processing of billet deformation by combining deformation and heat treatment, creating an organized billet of TRIP steel.Survey the organization and mechanics of TRIP steel after each step of technology.

Fig. 5 .
Fig. 5. Thermo-mechanical diagram for making TRIP billet a-through hot rolling, b-through cold rolling.

Fig. 6 .
Comparison of properties of steel TRIP, DP, HSLA a-stress-strain relationship, b-the relationship between the coefficient of hard-deformation.B. Microstructure Analysis Sample after forging Sample after deformation Sample after heat treatment Sample after deforming (color-imbued) Sample after heat treatment (color impregnation) Fig. 7. Microstructure before and after deforming and heat treatment

TABLE I :
MECHANICAL PROPERTIES OF STEEL TRIP590, TRIP690,