Effect of continuous annealing parameters on the mechanical properties and microstructures of a cold rolled dual phase steel

A cold rolled dual phase （DP） steel with the C-Si-Mn alloy system was trial-produced in the laboratory, utilizing a Gleeble-3800 thermal simulator. The effects of continuous annealing parameters on the mechanical properties and microstructures of the DP steel were investigated by mechanical testing and microstructure observation. The results show that soaking between 760 and 820℃ for more than 80 s, rapid cooling at the rate of more than 30℃/s from the quenching temperature between 620 and 680℃, and overaging lower than 300℃ are beneficial for the mechanical properties of DP steels. An appropriate proportion of the two phases is one of the key factors for the favorable properties of DP steels. If the volume fraction of martensite and, thereby, free dislocations are deficient, the tensile strength and n value of DP steels will decrease, whereas, the yield strength will increase. But if the volume fraction of martensite is excessive to make it become a dominant phase, the yield and tensile strength will increase, whereas, the elongation will decrease obviously. When rapid cooling rate is not fast enough, pearlite or cementite will appear, which will degrade the mechanical properties. Even though martensite is sufficient, if it is decomposed in high temperature tempering, the properties will he unsatisfied.

Stress-strain partitioning analysis of constituent phases in dual phase steel based on the modified law of mixture

A more accurate estimation of stress-strain relationships for martensite and ferrite was developed, and the modified law of mixture was used to investigate the stress-strain partitioning of constituent phases in dual phase （DP） steels with two different martensite volume fractions. The results show that there exist great differences in the stress-strain contribution of martensite and ferrite to DP steel. The stress-strain partitioning coefficient is not constant in the whole strain range, but decreases with increasing the true strain of DP steel. The softening effect caused by the dilution of carbon concentration in martensite with the increase of martensite volume fraction has great influence on the strain contribution of martensite. The strain ratio of ferrite to martensite almost linearly increases with increasing the true strain of DP steel when the martensite volume fraction is 22%, because martensite always keeps elastic. But the strain ratio of ferrite to martensite varies indistinctively with the further increase in true strain of DP steel above 0.034 when the martensite volume fraction is 50%, because plastic deformation happens in martensite. The stress ratio ofmartensite to ferrite decreases monotonously with increasing the true strain of DP steel whether the martensite volume fraction is 22% or 50%.

Dry Sliding Oxidative Wear in Plain Carbon Dual Phase Steel

To investigate the tribological potential of the dual phase （DP） steel as a wear resistant material, the wear and the friction characteristics of this steel, which consists of hard martensite islands embedded in a ductile ferrite matrix, have been investigated and compared with those observed in plain carbon hardened （H） steel that has the same carbon content of 0.2%. Dry sliding wear tests have been carried out using a pin-on-disk wear testing machine at different normal loads of 21.3 N, 28. 5 N, 35.7 N, and 42.6 N and at a constant sliding velocity of 1.20 m/s. The analysis of surface and wear debris of samples showed that the wear mechanism was mainly mild oxidative. The friction and the wear rate of the H steel and the DP Steel have been explained with respect to the microstructure and the wear mechanism.

A Study on the Wear Behaviour of Dual Phase Steels

Dual phase （DP） steels containing four different amounts of martensite ranging from 43 vol. pct to 81 vol. pct have been developed from 0.2 wt pct carbon steel by intercritical heat treatment at a fixed temperature of 780℃ with varying holding times followed by water quenching. Dry sliding wear tests have been conducted on DP steels using a pin- on-disk machine under different normal loads of 61.3, 68.5, 75.7 and 82.6 N and at a constant sliding speed of 1.20 m/s. At these loads, the mechanism of wear is primarily delamination, which has been confirmed by SEM micrographs of subsurface and wear debris of samples. Wear properties have been found to improve with the increase in martensite volume fraction in dual phase steels.

Austenite formation during intercritical annealing in C-Mn cold-rolled dual phase steel

Two different kinds of experimental techniques were used to in-situ study the austenite formation during intercritical annealing in C-Mn dual phase steel. The microstructure evolution was observed by confocal laser scanning microscope, and the austenite isothermal and non-isothermal transformation kinetics were studied by dilatometry. The results indicate that banded structure is produced for the reason of composition segregation and the competition between recrystallization and phase transformation. Austenite prefers to nucleate not only at ferrite/ferrite grain boundaries, but also inside the grains of ferrite.Furthermore, the austenitizing process is accomplished mainly via migration of the existing austenite/ferrite interface rather than nucleation of new grains. The incubation process can be divided into two stages which are controlled by carbon and manganese diffusion, respectively. During the incubation process, the nucleation rate of austenite decreases, and austenite growth changes from two-dimensional to one-dimensional. The partitioning coefficient, defined as the ratio of manganese content in the austenite to that in the adjacent ferrite, increases with increasing soaking time.

Development of Tailored Structure and Tensile Properties of Thermomechanical Treated Micro Alloyed Low Carbon Dual Phase Steel

Direct hot rolled dual phase steel production represents a challenging route, compared with cold rolled and intercritical annealing process, due to complex and sophisticated control of the hot strip mill processing parameters. Instead, high technology compact slab production plant offers economic advantages, adequate control and prompt use of the advanced thermomechanical controlled rolling. The current work aims to obtain different structures and tensile properties by physical simulation of direct hot rolled niobium micro alloyed dual phase low carbon steel by varying the metallurgical temperatures of hot strip mill plant. This starts with adaptation of the chemical analysis of a low carbon content to fall far from the undesired peritectic region to avoid slab cracking during casting. Thermodynamic and kinetics calculations by Thermo-Calc 2020 and JMat pro software are used to define the transformation’s temperatures Ae1 and Ae3 as well as processing temperatures;namely of reheating, finishing rolling, step cooling and coiling temperatures. The results show that the increase of finishing rolling temperature from 780°C to 840°C or decreasing either of step cooling duration at ferrite bay from 7 to 4 seconds, enhances yield and tensile strengths, all due to more martensite volume fraction formation. The yield and tensile strengths also increase with decreasing coiling temperature from 330°C to 180°C, which is explained due to the increase of dislocation densities resulted from the sudden shape change during martensite formation at the lower coiling temperature in additional to the self-tempering of martensite formed at higher coiling temperatures which soften the dual phase steel.

Effect of controlled rolling on the martensitic hardenability of dual phase steel

The C-Mn and C-Mn-Nb steels were thermo-mechanically processed to develop dual phase steel and to study the effect of controlled rolling on the martensitic hardenability of austenite. The steel specimens were intercritically annealed at 790℃, rolled at that temperature to the reductions of 10%, 23%, and 47% and immediately cooled at different rates. Quantitative metallography was used to construct the microstructure map, which illustrated that increasing deformation progressively reduced the proportion of new ferrite formed at all cooling rates and increased the amount of martensite at fast and intermediate rates. The martensitic hardenability of austenite remaining after all the rolling reductions was plotted as a function of cooling rates. It was observed that for the austenite-martensite conversion efficiencies greater than about 25%, controlled rolling increased the martensitic hardenability of austenite.

EFFECT OF TRACE B AND Ga ON TRANSFORMATION CONTROL OF Mn-Si-Cr AS-ROLLED DUAL PHASE STEELS

Trace of B and Ga can substitute for Mo in Mn-Si-Cr dual phase steels.The technological process and CCT curves of steels treated with B and Ga are similar to those for Mn-Si-Cr-Mo steel.

RELATIONSHIP BETWEEN TENSILE DEFORMATION BEHAVIOUR AND MICROSTRUCTURAL PARAMETERS OF A DUAL PHASE STEEL

Bused on the deformation model of dual phase steels, an expression for the stress of martensite in dual phase steels is derived, it predictes that the onset of plastic deformation of martensite (transition strain) depends on the strain hardening of ferrite and on the strength of martensile. The relationship between the flow stress and microstructural parameters of a 0.12C-0.9Mn dual phase steel was investigated using the expression for the flow stress of dual phase steel[1] . By calculating the stress ratio and the stress-strain partition coefficient, the loud transition and the stress-strain partition between two phases are studied. It shows that the deformation of dual phase steel lies between the isostress and isostrain states and the stress-strain pratition changes continuously during the deloramtion.

Influence of silicon on the microstructures, mechanical properties and stretch-flangeability of dual phase steels

Uniaxial tension tests and hole-expansion tests were carried out to determine the influence of silicon on the microstructures, mechanical properties, and stretch-flangeability of conventional dual-phase steels. Compared to 0.03wt% silicon, the addition of 1.08wt% silicon induced the formation of finer ferrite grains （6.8μm ） and a higher carbon content of martensite （Cm≈ 0.32wt%）. AS the silicon level increased, the initial strain-hardening rate （n value） and the uniform elongation increased, whereas the yield strength, yield ratio, and stretch-flangeability decreased. The microstructures were observed after hole-expansion tests. The results showed that low carbon content martensite （Cm ≈ 0.19wt%） can easily deform in coordination with ferrite. The relationship between the mechanical properties and stretch-flangeability indicated that the steel with large post-uniform elongation has good stretch-flangeability due to a closer plastic incom- patibility of the ferrite and martensite phases, which can effectively delay the production and decohesion of microvoids.

Microstructure-Properties Correlation of Dual Phase Steels Produced by Controlled Rolling Process

The purpose of this research is to quantify the effects of compositional and processing parameters on the microstruc-ture and properties of dual phase steel produced directly by hot rolling and rapid cooling. Steels with the base composition of 0.1%C, 1.4%Si, and 1.0%Mn with additions of 0.5%Cr to influence hardenability, 0.04%Nb to retard recrystallization in the latter stages of rolling, or 0.02%Ti to inhibit grain growth during and after reheating were investigated. Investigation was made to predict microstructure evolution and to correlate microstructure with processing parameters. The effects of the important microstructure parameters such as ferrite grain size, martensite volume fraction (VM) and morphology (polygonal or fibrous) on the tensile and impact properties are discussed. Multiple linear regression analysis of the ultimate tensile strength has shown that, increasing VM and martensite microhardness and grain refinement of ferrite are the major contributions to increase the strength of the steel. It was found that the dual-phase steel produced by controlled rolling process, with a microstructure which consisted of fine grained ferrite (4 um) and 35%~40% fibrous martensite, presented optimum tensile and impact properties because of enhanced resistance to crack propagation.

Corrosion Behavior of Fe-Al-Zn Interphased Dual Phase Steel in Marine Environment

Intercritical heat treatment operation has been in use for the development of Dual Phase Steel (DPS) and has been found to improve the mechanical properties of the steel. In spite of the enhancement a limitation was however observed as to its corrosion susceptibility. In a bid to further enhance the corrosion resistance of the DPS while maintaining its mechanical properties, galvanealing operation was adopted which involving the immersion of the DPS into Al-Zn melt and subsequently subjecting it to annealing operation at 550°C. Weight loss and linear polarization technique were used to measure or evaluate its resistance in 3.5% NaCl (a simulated marine environment). A minimum of 3 samples was used per immersion time. From the result, it was observed that there is a general sharp decrease in the corrosion rate of the GAS as compared to the control sample. The sample immersed and allowed to dwell in the melt for 20 seconds and further annealed with a soaking time of 20 minutes exhibited the highest corrosion resistance. The polarization curve also shows that the substrate was generally passivated, and this is as a result of the Al-Zn/Fe adhesiveness.

STUDY ON STRUCTURE AND PROPERTIES OF 18Si MnCu DUAL PHASE STEEL

After intercritical treatment,the structure and properties of 18SiMnCu cold rolled sheet steel is studied. When the sheet steel held at 780℃ and followed by 25℃/ s colling rate, a composite structure of (F+M+A) and optimum mechanical properties were obtained:σ_s=300MPa,σ_b = 750MPa,δ=27%, n= 0.27. The transformation process of retained austenite was examined by x-ray and TEM.

Effect of post rolling stress on phase transformation behavior of microalloyed dual phase steel

The slow phase transformation of microalloyed dual phase steel makes the nonuniform stress and temperature fields during the post rolling cooling process have a significant impact on the phase transformation process.Given the relatively slow phase transformation of DP780 steel within the microalloyed dual phase steel series,the influence of stress on the phase transformation behavior of DP780 steel was investigated.To quantify the nonuniform thermal and stress conditions in the steel coil,a thermo-mechanical coupled finite element model of the hot-rolled strip cooling process was established.Based on the simulation data,DP780 steel was chosen as the research material,and Gleeble 3500 thermal simulation equipment was used for experimental validation.The thermal expansion curves were analyzed through regression to establish the dynamic model of DP780 steel phase transformation under stress.Subsequently,metallographic analysis was conducted to determine phase transformation type and grain size of DP780 steel.The results confirmed that the stress promotes the occurrence of semi-diffusion-type bainite transformation.Furthermore,an appropriate level of stress facilitates the growth of bainitic grains,while the increased stress inhibits the growth of ferritic grains.

Effects of Silicon Content and Intercritical Annealing on Manganese Partitioning in Dual Phase Steels

Steels of constant manganese and carbon contents with silicon content of 0.34 %- 2.26% were cast. The as-cast steels were then hot rolled at 1 100 ℃ in five passes to reduce the cast ingot thickness from 80 to 4 mm, air cooled to room temperature and cold rolled to 2 mm in thickness. Dual phase microstructures with different volume fraction of martensite were obtained through the intercritical annealing of the steels at different temperatures for 15 min followed by water quenching. In addition to intercritical annealing temperature, silicon content also altered the volume fraction of martensite in dual phase steels. The partitioning of manganese in dual phase silicon steels was investigated using energy-dispersive spectrometer （EDS）. The partitioning coefficient, defined as the ratio of the amounts of alloying element in the austenite to that in the adjacent ferrite, for manganese increased with increasing intercritieal annealing temperature and silicon content of steels. It was also found that the solubility of manganese in ferrite and austenite decreased with increasing intereritical temperature. The results were discussed by the diffusivity and the solubility of manganese in ferrite and austenite existed in dual phase silicon steels.

Heat Treatment of Cold-Rolled Low-Carbon Si-Mn Dual Phase Steels

The effects of overageing （OA） and vanadium addition on microstructure and properties of cold-rolled lowcarbon Si-Mn dual phase （DP） steel sheets are studied. The results show that the microstructure and mechanical properties of DP steels are greatly affected by OA temperature. When OA is done at low temperatures （473 -- 573 K）, the elongation rate increases sharply while both the yield strength and tensile strength decrease significantly, When OA is done at high temperatures （〉573 K）, the elongation does not further increase, but the yield strength increases and tensile strength decreases, which deteriorates the mechanical properties of the steel. The results also show that both the strength and the tempering stability can be significantly improved by vanadium microalloying. Finally, the optimum OA temperature for the traditional steel appears to be around 523 K, while it is around 573 K for the V-containing steel, resulting in the best overall mechanical properties.

Effect of Heat Treatment on Microstructure,Mechanical Properties and Fracture Behaviour of Ship and Dual Phase Steels

Grade A （GA） and high strength steel DH36 ship steels possessing different chemical compositions were used, and strength properties of GA steel and DH36 steel were compared. Additionally, 4 types of dual phase （DP） steels with different martensite volume fractions （MVFs） were produced from GA steel by means of heat treatment and they were compared with other steels through conducting mierostructure, microhardness, tensile and impact tests. The fracture surfaces of specimens （DH36, GA and DP steels） exposed to tensile and Charpy impact tests were investigated by scanning electron microscope. Furthermore, it was found that the specimens quenched from 800 and 900℃ had better strength than DH36 steel. The tensile test results indicated that the tensile strength of DP steel water quenched from 900℃ was 3 times that of GA steel and twice that of DH36 steel.

Effects of TMCP Parameters on Microstructure and Mechanical Properties of Hot Rolled Economical Dual Phase Steel in CSP

Effects of chemical compositions, finish rolling temperature, isothermal temperature on runout table and coiling temperature on microstructure and mechanical properties of economical dual phase steel produced on CSP line were investigated. Experimental results showed that martensite volume fraction could be enhanced and banding mi- crostructure could be reduced by controlling Mn, Si contents and applying proper finish rolling temperature. Opti- mized processing-parameters were obtained for DP580 production on CSP line of Wuhan Iron and Steel （group） Co （WISCO） in China. Optimal mierostructure and mechanical properties could be achieved when the strip was finished rolling at the range of 790 to 830 ~C, isothermally holding at 680 to 740 ~C and coiling below 250 ~C.

Effect of Strengthening Phase on Deformation Behaviour during Uniaxial Tension of Hot-rolled Dual Phase Steel

Two kinds of C-Si-Mn-Cr series tested steels were designed to obtain dual phase microstructures of ferrite （F） ＋martcnsite （M） or ferrite （F）-bainite （B） with different mechanical properties. Effects of strengthening phase on yielding and fracture behaviours during uniaxial tension of dual phase steel were discussed. Compared with hot-rolled martensite dual phase steel, ferrite-bainite dual phase steel has high ratio of yield strength to tensile strength （YS/TS） and low elongation. During necking process of uniaxial tension, microvoids of ferrite-martensite steel are generated by fracture of ferrite/martensite boundary or martensite islands with irregular shape. But ferrite matrix elongated remarkably along deformation direction, and strengthening phase also coordinated with ferrite matrix. Compatible de formation between ferrite and bainite is distinct. Ferrite-bainite dual phase steel has fine and less microvoid, and phase boundary of ferrite and bainite is beneficial for restraining generation and extending of microvoid.

Influence of Mo on Dynamic Continuous Cooling Transformation of 1000 MPa Cold Rolled Dual Phase Steel

The expanding curves of two kinds of 1000 MPa ultra-high strength cold rolled dual phase steels,which were C-Si-Mn-Cr and C-Si-Mn-Cr-Mo steel respectively,were detected on Gleeble-1500 thermal-mechanical simulator at different cooling rates.Combined with metallographic and hardness methods,the continuous cooling transformation curves (CCT) of the two steels were obtained.The results showed that Mo could raise the A r3 temperature,and strongly restrain the pearlite and bainite transformation.The reason for this was the interaction that the addition of Mo could increase the chemical driving force of ferrite transformation and the activation energy for the diffusion of carbon in austenite,which could decelerate the ferrite transformation.The hardness of the two steels was similar in the cooling rates range of this experiment and got higher with the increase of the cooling rates.When the cooling rates were above 7 ℃/s,the hardness almost kept constant because the most part of the microstructure was martensite.