Microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels

The microstructures and mechanical properties of C-Mn-Cr-Nb and C-Mn-Si-Nb ultra-high strength dual-phase steels were studied by scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, and tensile test. The results show that Si can promote the transformation of austenite （γ） to ferrite （α）, enlarge the （α＋γ） region, and increase the aging stability of martensite by inhibiting carbide precipitation. Adding Cr leads to the formation of retained austenite and martensite/austenite （M/A） constituents, as well as the decomposi- tion of martensite during the overaging stage. Both of the steels show higher initial strain-hardening rates and two-stage strain-hardening characteristics. The C-Mn-Si-Nb steel shows the higher strain-hardening rate than the C-Mn-Cr-Nb steel in the first stage; however, there is no significant difference in the second stage. Although the tensile strength and elongation of the two steels both exceed 1000 MPa and 15%, respectively, the comprehensive mechanical properties of the C-Mn-Si-Nb steel are superior.

Numerical simulation on the microstress and microstrain of low Si-Mn-Nb dual-phase steel

According to the stress-strain curves of single-phase martensite and single-phase ferrite steels,whose compositions are similar to those of martensite and ferrite in low Si-Mn-Nb dual-phase steel,the stress-strain curve of the low Si-Mn-Nb dual-phase steel was simulated using the finite element method（FEM）.The simulated result was compared with the measured one and they fit closely with each other, which proves that the FE model is correct.Based on the FE model,the microstress and microstrain of the dual-phase steel were analyzed. Meanwhile,the effective factors such as the volume fraction of martensite and the yield stress ratio between martensite and ferrite phases on the stress-strain curves of the dual-phase steel were simulated,too.The simulated results indicate that for the low Si-Mn-Nb dual-phase steel, the maximum stress occurs in the martensite region,while the maximum strain occurs in the ferrite one.The effect of the volume fraction of martensite（fm） and the yield stress ratio on the stress-strain curve of the dual-phase steel is small in the elastic part,while it is obvious in the plastic part.In the plastic part of this curve,the strain decreases with the increase of f_M,while it decreases with the decrease of the yield stress ratio.

Effect of Holding Time on the Microstructure and Mechanical Properties of Dual-Phase Steel during Intercritical Annealing

Continuous annealing simulation tests were conducted by using a continuous annealing thermomechanical simulator. Holding times of 5, 60, 180, and 480 seconds for an intercritical annealing temperature of 820℃ were adopted to investigate the evolution of the mierostructure and mechanical properties of ferrite-bainite dual-phase steel. The ferrite-bainite dual-phase steel was characterized by high strength and low yield ratio due to the presence of the constituents （polygonal ferrite, bainite, martensite and retained austenite） of the steel microstructure. Specimen 3 exhibits the highest value of A50 （7.67%） and a product of Rm × A50 （10453MPa%） after a 180s holding. This is likely attributed to the presence of a C-enriched retained anstenite in the microstructure. And the effect of martensite islands and carbide precipitate is thought to be able to contribute in strengthening the present steel. It is expected that equilibrium of anstenite fraction would be reached for reasonable intercritical holding period, regardless of the heating temperature. The results suggest that long increasing holding times may not be needed because the major phase of the microstructure does not change very significantly. It is favorable for industrial production of DP steels to shorten holding times. Key words： ferrite-bainite dual-phase steel; holding time; martensite islands; mechanical properties

Effects of pre-strain and baking parameters on the microstructure and bake-hardening behavior of dual-phase steel

In a typical process, C-Mn steel was annealed at 800℃ for 180 s, and then cooled rapidly to obtain the ferrite-martensite microstructure. After pre-straining, the specimens were baked and the corresponding bake-hardening （BH） values were determined as a function of pre-strain, baking temperature, and baking time. The influences ofpre-strain, baking temperature and baking time on the microstructure evolution and bake-hardening behavior of the dual-phase steel were investigated systematically. It was found that the BH value apparently increased with an increase in pre-strain in the range from 0 to 1%; however, increasing pre-strain from 1% to 8% led to a decrease in the BH value. Furthermore, an increase in baking temperature favored a gradual improvement in the BH value because of the formation of Cottrell atmosphere and the precipitation of carbides in both the ferrite and martensite phases. The BH value reached a maximum of 110 MPa at a baking temperature of 300℃. Moreover, the BH value enhanced significantly with increasing baking time from 10 to 100 min.

Corrosion behavior of tempered dual-phase steel embedded in concrete

Dual-phase （DP） steels with different martensite contents were obtained by appropriate heat treatment of an SAE1010 structural carbon steel, which was cheap and widely used in the construction industry. The corrosion behavior of DP steels in concrete was investigated under various tempering conditions. Intercritical annealing heat treatment was applied to the reinforcing steel to obtain DP steels with different contents of martensite. These DP steels were tempered at 200, 300, and 400℃ for 45 min and then cooled to room temperature. Corrosion experiments were conducted in two stages. In the first stage, the corrosion potential of DP steels embedded in concrete was measured every day for a period of 30 d based on the ASTM C 876 standard. In the second stage, the anodic and cathodic polarization values of these steels were obtained and subsequently the corrosion currents were determined with the aid of cathodic polarization curves. It was observed that the amount of second phase had a definite effect on the corrosion behavior of the DP steel embedded in concrete. As a result of this study, it is found that the corrosion rate of the DP steel increases with an increase in the amount of martensite.

Effect of intercritical annealing on microstructure,mechanical properties,and work-hardening behavior of ultrahigh-strength dual-phase steels with different silicon contents

In this study,three kinds of dual-phase(DP) steels were used to investigate the influence of silicon content and intercritical annealing temperature on their microstructures,mechanical properties,and work-hardening behaviors. By adding silicon and matching the critical annealing temperature,a new DP steel(1.0Si and intercritically annealed at 790 ℃) that exhibits an excellent combination of ultrahigh strength and adequate ductility was obtained. Variations in the strength,elongation,and fracture mechanism of the specimens with respect to different intercritical annealing temperatures were correlated to microstructural features. With an increase in the silicon content,there is no significant change in the martensitic band structure or ferrite morphology. At the same annealing temperature,the yield strength and yield strength ratio of the specimens decreased,but at different annealing temperatures,the tensile strength was reduced. The Hollomon analysis results indicate that the workhardening behavior obeys a two-stage work-hardening mechanism. With an increasing intercritical annealing temperature,the "transition strain"shifts to the left,and with an increasing silicon content,the "transition strain"shifts to the right. The surface exhibits ductile fractures characterized by a high density of microvoid dimples. With an increase in the silicon content,the average dimple size on the fracture surface decreases and the plasticity of the material increases.

Effects of overaging temperature on the microstructure and properties of 600MPa cold-rolled dual-phase steel

C–Mn steels prepared by annealing at 800°C for 120 s and overaging at 250–400°C were subjected to pre-straining（2%） and baking treatments（170°C for 20 min） to measure their bake-hardening（BH_2） values. The effects of overaging temperature on the microstructure, mechanical properties, and BH_2 behavior of 600 MPa cold-rolled dual-phase（DP） steel were investigated by optical microscopy, scanning electron microscopy, and tensile tests. The results indicated that the martensite morphology exhibited less variation when the DP steel was overaged at 250–350°C. However, when the DP steel was overaged at 400°C, numerous non-martensite and carbide particles formed and yield-point elongation was observed in the tensile curve. When the overaging temperature was increased from 250 to 400°C, the yield strength increased from 272 to 317 MPa, the tensile strength decreased from 643 to 574 MPa, and the elongation increased from 27.8% to 30.6%. Furthermore, with an increase in overaging temperature from 250 to 400°C, the BH_2 value initially increases and then decreases. The maximum BH_2 value of 83 MPa was observed for the specimen overaged at 350°C.

Plastic Deformation Mechanism of Dual-phase Steel at Different Strain Rates

The mechanical properties of dual-phase steel (DP1000) over the strain rate range of 10^-3-10^3 s^-1 were studied using an electronic universal testing machine and a high-speed tensile testing machine.The plastic deformation mechanism was investigated from the perspectives of the strain rate sensitivity index,activation volume and dynamic factors.The results show that the tensile strength and yield strength of DP1000 increase as the strain rate increases.The elongation increases without any change after fracture,and then decreased rapidly when the strain rate reaches 103 s^-1.The true strain curves of DP1000 show three stages:the point of instability decreases in the strain range of 10^-3-10^-1 s^-1;the instability point increases between 100-5×10^2 s^-1;above 5×10^2 s^-1,and the instability strain becomes smaller again.The plastic deformation mechanism of the DP was determined by the competitive contributions of work hardening (strain hardening,strain rate hardening) and softening effects due to the adiabatic temperature rise.

CHARACTERISTICS OF MARTENSITIC TRANSFORMATION IN DUAL-PHASE STEELS

A theorectical expression for the driving force and M_(?) point of martensitic transformation has been proposed.The M_(?) values using this expression are in good agreement with that obtained experimentally.It was found that the values of M_(?) and M_(?) are not only related to the carbon content in martensite,but also to the volume fraction of ferrite.

MICROFRACTOGRAPHY OF NEAR-THRESHOLD FATIGUE CRACK PROPAGATION IN DUAL-PHASE STEELS

SEM microfractography of near-threshold fatigue crack propagation were carried out in the dual-phase steels of 3 martensite morphologies and 6 volume fractions of martensite (V_m). All of them are featured by cyclic cleavage characteristics in near-threshold region,i.e.,main- ly controlled by mode Ⅱ stress.In the higher ΔK regions,the fracture surfaces are character- ized by mixed modes including cyclic cleavage facets,two types of secondary cracks and striations,etc..The roughness-induced crack closure of fracture surface is attributed primarily to extreme high fatigue crack growth threshold values.

The Study of Dual-phase Steel after Cyclic Deformation at Various Strain Amplitudes

Low cycle fatigue tests under plastic strain control were carried out with a dual-phase steel containing 23 Vol.-％ martensite. Specimens hardened rapidly at first few cycles followed by a slight softening to saturation stages when cycled at higher strain amplitudes, whereas at lower strain amplitudes the specimens presented continually hardening for a long time until saturation. TEM examination of the saturation dislocation structures show that clusters, parallel walls and cells were found at low, medium and high strain amplitude, respectively. It also has been found that the martensite/ferrite interfaces did not affect the dislocation structures signi- ficantly when a specimen was fatigued at lower strain amplitude. However, the dislocation struc- ture adjacent to the two-phase boundary is dif- ferent to some extent from that in the remote regions in the ferrite when a higher strain amplitude is applied.

EFFECT OF CRACK CLOSURE ON SLOW PROPAGATION IN DUAL-PHASE STEEL

The effect of ferrite content in ferrite-martensite dual-phase steel on the initiation and prop- agation of fatigue crack and the plastic deformation at crack tip has been studied.In the range of ferrite content from 24.2 to 41.5％,the optimum seems to be 33.8％,of which the crack ini- tiation will be prolonged,the threshold value increased,the propagation rate decreased and the closure stress intensity factor increased.As the propagation force is described by effective stress intensity factor,three steels with various ferrite contents will show the same propagation behaviour on da/dN vs △ K_(eff)curve.It is shown that the closure effect increases with the decrease in △K at the fatigue crack tip.When △K equals to △K_(th),the closure effect reaches a maximum value of0.7 in a dual-phase steel with 33.8％ferrite.

Effect of hot-dip galvanizing processes on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel

A C–Mn dual-phase steel was soaked at 800°C for 90 s and then either rapidly cooled to 450°C and held for 30 s(process A) or rapidly cooled to 350°C and then reheated to 450°C(process B) to simulate the hot-dip galvanizing process. The influence of the hot-dip galvanizing process on the microstructure and mechanical properties of 600-MPa hot-dip galvanized dual-phase steel(DP600) was investigated using optical microscopy, scanning electron microscopy(SEM), transmission electron microscopy(TEM), and tensile tests. The results showed that, in the case of process A, the microstructure of DP600 was composed of ferrite, martensite, and a small amount of bainite. The granular bainite was formed in the hot-dip galvanizing stage, and martensite islands were formed in the final cooling stage after hot-dip galvanizing. By contrast, in the case of process B, the microstructure of the DP600 was composed of ferrite, martensite, bainite, and cementite. In addition, compared with the yield strength(YS) of the DP600 annealed by process A, that for the DP600 annealed by process B increased by approximately 50 MPa because of the tempering of the martensite formed during rapid cooling. The work-hardening coefficient(n value) of the DP600 steel annealed by process B clearly decreased because the increase of the YS affected the computation result for the n value. However, the ultimate tensile strength(UTS) and elongation(A80) of the DP600 annealed by process B exhibited less variation compared with those of the DP600 annealed by process A. Therefore, DP600 with excellent comprehensive mechanical properties(YS = 362 MPa, UTS = 638 MPa, A_(80) = 24.3%, n = 0.17) was obtained via process A.

Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase （DP） steels with almost constant martensite contents of 20vo1% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature （1043 K, FH1） with fast-heating （300 K/s） show banded ferrite/martensite structure, whereas those soaked at high temperature （1103 K, FH2） with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated （5 K/s） reference material （SH0）. The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard-Jaoul technique demonstrate two stages of work hardening for all samples.

Microstructure evolution and mechanical properties of 1000 MPa cold rolled dual-phase steel

The microstructure evolution of 1 000 MPa cold rolled dual-phase (DP) steel at the initial heating stages of the continuous annealing process was analyzed. The effects of different overaging temperatures on the microstructures and mechanical properties of 1 000 MPa cold rolled DP steel were investigated using a Gleeble-3500 thermal/mechanical simulator. The experimental results show that ferrite recovery and recrystallization, pearlite dissolution and austenite nucleation and growth take place in the annealing process of ultra-high strength cold rolled DP steel. When being annealed at 800 ℃ for 80 s, the tensile strength and total elongation of DP steel can reach 1 150 MPa and 13%, respectively. The microstructure of DP steel mainly consists of a mixture of ferrite and martensite. The steel exhibits low yield strength and continuous yielding which is commonly attributed to mobile dislocations introduced during cooling process from the intercritical annealing temperature.

Fatigue Crack Propagation in Plain Carbon Dual-phase Steel

The effect of volume fraction V_M and carbon content(%C)_M of martensite was studied on the fa- tigue crack propagation behaviour of plain carbon martensite plus ferrite M+F dual-phase steel.The experimental results show that the △K_(th)decreases and da/dn increases with increasing V_M and (%C)_M at a load ratio R=0.05.High△K_(th)values are obtained by obvious crack closing ability caused by high roughness of the fractured surfaces.

Mechanism and control of nonuniform phase transformation of microalloyed dual-phase steel during cooling process after hot rolling

After cooling in the hot rolling process,the metallographic structure of microalloyed dual-phase steel is nonuniform along the rolling direction,while the thickness fluctuation of microalloyed dual-phase steel with a nonuniform metallographic structure will occur during cold rolling.The mechanism of nonuniform phase transformation of microalloyed dual-phase steels was studied during the cooling process after hot rolling,and the nonuniform phase transformation of microalloyed dual-phase steel was regulated during the cooling process after hot rolling through process optimization.First,the empirical equation of phase transformation temperature was measured by a dilatometer considering thermal expansion.Then,the phase field and temperature field of laminar cooling process were calculated to provide initial boundary conditions for the finite element model.After that,the coupling finite element model of the temperature phase transformation of the strip steel in coiling transportation process was established.The simulation results show that the different thermal contact conditions of the microalloyed dual-phase steel during coil transportation lead to uneven cooling of the coil,which leads to nonuniform transformation of the coil along the rolling direction.In addition,by prolonging the time interval from coiling to unloading,the phenomenon of nonuniform phase transformation of microalloyed dual-phase steel can be effectively controlled.The simulation results are applied to industrial production.The application results show that prolonging the time interval from coiling to unloading can effectively improve the nonuniform phase transformation of microalloyed dual-phase steel in the cooling process after hot rolling.

Precipitation Behavior and Textural Evolution of Cold-Rolled High Strength Deep Drawing Dual-Phase Steels

Low-carbon Cr-Mo micro-alloyed deep drawing dual-phase steels were designed in laboratory. As the mi- crostructure and texture evolution in hot-rolled strips and annealed sheets were investigated using SEM, TEM and XRD technologies, the attribution of solute Mo and MoC particles to DP sheets＇ drawing capacity was investigated. The precipitation thermodynamics were also calculated by Thermo-calc software. Results show that the precipitates in hot-rolled strips mainly are MoC, AIN and MnS, and with the increase of Mo addition, finer and denser MoC par- ticles precipitated in matrix and along grain boundaries of ferrite more easily. Weak textures are shown in the hot- rolled strips, and {112}~110~ and {223}%110~ components tend to be stable in subsequent cold rolling process. During annealing, on one hand, the development of ~lll~//ND texture is suppressed because finer MoC particles prevent the grain boundary migration. On the other hand, unfavorable texture {001 } %110：〉 significantly reduces with Mo increasing, which is attributed to that part of solution C in matrix has been fixed during recrystallization. In addition, the addition of Mo can enhance hardenability strongly and MoC easily re-dissolve at high temperature, which is favor to form martensite in dual-phase steel.

Effect of Heat-Treatment Schedule on the Microstructure and Mechanical Properties of Cold-Rolled Dual-Phase Steels

Low-carbon （0.08 wt% C） steel has been subjected to three different heat treatments to obtain dual-phase steels with different microstructures. An understanding of structure-property was established through tensile tests, in conjunction with scanning electron microscope and transmission electron microscope. The results show that the steel after intermediate quenching （IQ） consisting of fine and fibrous martensite exhibited the intermediate strength, highest elongation and the best comprehensive performance of mechanical properties, whereas the steel subjected to intercritical annealing （IA） produced a network martensite along ferrite grain boundaries, having the lowest strength and intermediate elongation. Besides, step quenching （SQ） resulted in a coarse and blocky ferrite-martensite microstructure showing the worst mechanical properties of the three different heat-treatment conditions. The strain-hardening behavior was studied through the modified Crussard- Jaoul model, indicating two stages of strain-hardening behavior for all three samples. The highest magnitude of strain- hardening ability was obtained by IQ annealing routes. The analysis of the fractured surface revealed that ferrite/martensite interfaces are the most susceptible for microvoid nucleation. However, martensite microcracks were also observed in SQ sample, and the microvoids are nucleated within the ferrite grain in IA sample as well. The variations in strength, elongation, strain-hardening behavior and fracture mechanism of the steel with different heat-treatment schedules were further discussed in relation to the microstructural features.