Carbon Diffusion in Hot Strips of Low Carbon Steel Produced by CSP Line under Different Thermal Histories

Two experiments were carried out on the same compact strip production (CSP) line, which differs in that one of them experienced γ→α→γ thermal history. The differences in microstructure, precipitation, misorientation etc between two experiments were investigated by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back-scattered diffraction (EBSD) and positron annihilation technique (PAT). The carbon concentration in matrix is more inhomogeneous in the experiment than that with γ→α→γ7 thermal history. The specific precipitation characteristic in the experiment without γ→α→γ thermal history is discussed on the basis of different carbon diffusion behavior and interaction between dislocation and excess carbon.

Precipitation characteristic of high strength steels microalloyed with titanium produced by compact strip production

Transmission electron microscopy （TEM） and physics-chemical phase analysis were employed to investigate the precipitates in high strength steels microalloyed with Ti produced by compact strip production （CSP）. It was seen that precipitates in Ti microalloyed steels mainly included TiN, Ti4C2S2, and TiC. The size of TiN particles varied from 50 to 500 nm, and they could precipitate during or before soaking. The Ti4C2S2 with the size of 40-100 nm might precipitate before rolling, and the TiC particles with the size of 5-50 nm precipitated heterogeneously. High Ti content would lead to the presence of bigger TiC particles that precipitated in austenite, and by contrast, TiC particles that precipitated in ferrite and the transformation of austenite to ferrite was smaller. They were less than 30 nm and mainly responsible for precipitate strengthening. It should be noted that the TiC particles in higher Ti content were generally smaller than those in the steel with a lower Ti content.

Thermodynamic Research on Precipitates in Low Carbon Nb-Microalloyed Steels Produced by Compact Strip Production

Microalloying element Nb in low carbon steels produced by compact strip production （CSP） process plays an important role in inhibiting recrystallization, decreasing the transformation temperature and grain refinement.With decreasing the rolling temperature, dislocations can be pinned by carbonitrides and the strength is increased. Based on the two sublattice model, with metal atom sublattice and interstitial atom sublattice,a thermodynamic model for carbonitride was established to calculate the equilibrium between matrix and carbonitride. In the steel produced by CSP, the calculation results showed that the starting temperature of precipitation of Ti and Nb are 1340℃ and 1040℃, respectively. In the range of 890-950℃, Nb rapidly precipitated. And the maximum of the atomic fraction of Nb in carbonitride was about 0.68. The morphologies and energy spectrum of the precipitates showed that （NbTi） （CN） precipitated near the dislocations. The experiment results show that Nb rapidly precipitated when the temperature was lower than 970℃, and the atomic fraction of Nb in carbonitride was about 60%-80%. The calculation results are in agreement with the experiment data. Therefore the thermodynamic model can be a useful assistant tool in the research on the precipitates in the low carbon steels produced by CSP.

Effects of Ti addition on low carbon hot strips produced by CSP process

Large quantity of fine Ti（C,N） particles, 15-30 nm in size, were observed in low carbon hot strips added to a small amount of Ti and produced by CSP process. The results showed that the precipitation of Ti（C,N） mostly took place during soaking and hot rolling, which is significantly different from that in the conventional production. These fine Ti carbonitride particles could be very effective on the austenite grain refinement by hindering grain growth of recrystallized austenite. Their precipitation behavior was discussed and compared with that of the steels produced in the conventional production.

Effect of boron on hot strips of low carbon steel produced by compact strip production

The effect of boron on hot strips of low carbon steel produced by compact strip production （CSP） to reduce the strength to a certain degree was investigated, which is quite different from that of high-strength low alloy steel. The mechanical properties and microstructural evolution of the hot strip were studied using optical microscopy and tensile tests. By means of an electrolytic dissolution technique and Thermo-Cal calculation, the precipitates containing boron were analyzed and detected. From the electron backscattered diffraction analysis, it can be deciphered whether the microstructure has recrystallized or not. Furthermore, the effect of boron segregation on the recrystallization or non-recrystallization conditions can be distinguished. The segregation behavior of boron was investigated in boron-containing steel. The nonequilibrium segregation of boron during processing was discussed on the basis of the forming complexes with vacancies that migrate to the boundaries prior to annihilation, which was confirmed by the subsequent cold rolling with annealing experiments.

Numerical Simulation of Coupled Molten Steel Flow and Temperature Fields in Compact Strip Production Casting

Based on the casting manufacture practice of steel slabs by CSP technology, the flow and the temperature fields of the funnel mould and the secondary cooling segment were simulated using the commercial code, CFX4. Compared with other physical investigations, the correlative data of the present simulation results are in good agreement with them. Therefore, a more comprehensive survey for metallurgy characteristic of the flow and the temperature fields in CSP continuous casting process can be achieved.

Correlation of morphological anisotropy and Lankford value of deep drawing sheets hot-rolled by compact strip production

Cold-rolled steel sheets in automotive applications require an excellent deep draw ability, which is characterized by the Lankford value （r-value）. In this study, a correlation was identified between r-value and pancake-shaped grain flatness which is indicated as the ratio of grain diameter in the rolling direction （RD） and normal direction （ND） of sheets （dr/dn）. A mathematical model （ r = e^0.345（dn^1/2-dr^1/2） ） was developed to calculate r-value by the microstructure of steel sheets hot-rolled by compact strip production （CSP）. It is shown that the r-value is higher, if the microstructure of steel sheet is of pancake-shaped grains elongated in the rolling direction. The calculated r-value is confirmed to fit exactly to the measured one from the large-scale production.

Morphology and Precipitation Kinetics of MnS in Low-Carbon Steel During Thin Slab Continuous Casting Process

The morphology of manganese sulfide formed during thin slab continuous casting process in low-carbon steel produced by compact strip production （CSP） technique was investigated. Using transmission electron microscopy analysis, it was seen that a majority of manganese sulfides precipitated at austenite grain boundaries, the morphologies of which were spherical or close to the spherical shape and the size of MnS precipitates ranged from 30 nm to 100 nm. A mathematical model of the manganese sulfide precipitation in this process was developed based on classical nucleation theory. Under the given conditions, the starting and finishing precipitation temperatures of MnS in the continuous casting thin slab of the studied low-carbon steel are 1 189 ℃ and 1 171 ℃, respectively, and the average diameter of MnS precipitates is about 48 nm within this precipitation temperature range. The influences of chemical components and thermo-mechanical processing conditions on the precipitation behavior of MnS in the same process were also discussed.

Comparison and analysis of dislocation density, morphology and evolution in microstructure of low-carbon steel produced using different technologies

Three kinds of specimens were produced from hot strips of similar composition and same thickness (nominal gauge 4.0mm) but produced using different technologies, and the dislocation density of these strips was quantitatively measured by positron annihilation technique test. The dislocation morphology and evolution in microstructure of each pass for producing the 1.9mm hot strip using CSP (compact strip production) technology were observed under an H-800 transmission electron microscope; its density was also quantitatively measured using the positron annihilation technique test, and the factors influencing the dislocation density during the production process were analyzed. The experimental results show that the dislocation density in the microstructure produced using CSP technology is higher than that in the microstructure produced using conventional technology. This result was discussed and confirmed on the basis of the finite element simulation and the theory relevant to dislocations.

HMPT-BOF-RH-CSP process for SPHE substrate of cold rolled deep drawing steel

Based on hot metal pretreatment （HMPT）-basic oxygen furnace （BOF）-Rheinstahl Heraeus （RH）-compact strip production （CSP） process, parameters controlling on cold rolling deep drawing substrate SPHE were investigated during smelting and rolling process by composition design and technology control. The influence of parameters on chemical compositions, mechanical properties and microstructure was revealed by scanning electron microscope （SEM）. The results show that, 1） main chemical components in SPHE are w（C）_〈40×10^-6, w（Si）_〈 0.01%, w（S）_〈0.009%, w（N）〈20×10^-6, w（O）〈_ 25×10^-6; 2） main mechanical properties of the SPHE are Crs=274 MPa, 00=334 MPa, A=48.9%; 3） main performances of deep drawing quality （DDQ） grade steel produced by SPHE are as follows, transversely crs=167 MPa, 00=298 MPa, n=0.219, r=2.46; vertically σs=166 MPa, 00=298 MPa, n=0.226, r=2.39; in 45° direction σ=171 MPa, 00=308 MPa, n=0.214, t=2.26; 4） microstrueture of DDQ is ferrite, average grain size is Grade 7.5, and inclusion size is 3-10μm.

Comparison and analysis of dislocation density,morphology and evolution in mi-crostructure of low-carbon steel produced using different technologies

Three kinds of specimens were produced from hot strips of similar composition and same thickness （nominal gauge 4.0 mm） but produced using different technologies, and the dislocation density of these strips was quantitatively measured by positron annihilation technique test. The dislocation morphology and evolution in microstructure of each pass for producing the 1.9 mm hot strip using CSP （compact strip production） technology were observed under an H-800 transmission electron microscope; its density was also quantitatively measured using the positron annihilation technique test, and the factors influencing the dislocation density during the production process were analyzed. The experimental results show that the dislocation density in the microstructure produced using CSP technology is higher than that in the microstructure produced using conventional technology. This result was discussed and confirmed on the basis of the finite element simulation and the theory relevant to dislocations.

Distribution behavior of inclusions in compact-strip-produced martensitic steel and its influence on mechanical properties

The distribution behavior of inclusions in martensitic steel produced by compact strip production process(CSP-MS)and its influence on mechanical properties were systematically investigated.The inclusions in the CSP-MS specimen are mainly composed of spherical Al_(2)O_(3)-CaO-CaS,MnS with high aspect ratio and small-sized TiN,whereas many coarse cuboidal TiN inclusions do exist in conventional martensitic steel(Con-MS).The high inclusion density of the CSP-MS specimen resulted in lower total elongation and impact toughness,and the MnS inclusions with high aspect ratio led to significantly stronger mechanical anisotropy than that for Con-MS specimen.The in-situ tensile results indicated that when the fracture direction is parallel to MnS direction,the microcracks induced by MnS inclusions tend to propagate into the matrix,leading to the formation of valley-like features,which significantly deteriorate the properties such as the total elongation and impact toughness.The microcracks caused by TiN inclusions are sharper than those caused by spherical Al_(2)O_(3)-CaO-CaS inclusions,and are easy to propagate into the matrix.This work is expected to guide the optimization of the mechanical properties of martensitic steels produced by CSP process and provide a theoretical basis for the CSP process design.

Improving flangeability of multiphase steel by increasing microstructural homogeneity

Multiphase microstructure significantly increases the strength,usually at the expense of flangeability because of lacking microstructure homogeneity.To further improve the strength-flangeability of multiphase steel,the microstructural homogeneity was advanced by adjusting the hard martensite/austenite(M/A)islands.The strength-flangeability was measured via uniaxial tensile tests and hole expansion tests.Their microstructures were characterized using a scanning electron microscope equipped with an electron backscatter diffraction detector and a transmission electron microscope.Nanoindentation tests were supplementally used to quantitatively reveal the microstructural homogeneity of the steels.Results show that the adjusted multiphase steel achieves an excellent ultimate tensile strength(~800 MPa)and flangeability(~135%hole expansion ratio).A promising homogeneous multiphase microstructure was obtained by controlling undercooled austenite transformed at about 600℃.This microstructure consists of soft polygonal ferrite,blocky bainitic ferrite,and hard M/A islands.The volume fraction of M/A islands is around 5%,and the average size is less than l pm.Detailed nanoindentation analysis indicated that the participation of M/A islands impressively influenced the microstructural homogeneity.Weakened strain partition and better mechanical compatibility were present in the adjusted multiphase steel since the plasticity initiation started late,which resulted in a positive flangeability.Moreover,avoiding M/A islands distributed in the chain along the rolling direction on the matrix hindered the possibility of voids coalescing into cracks and stabilized the flanging performance.

Mechanism of Black Strips Generated on Surface of CSP Hot-Rolled Silicon Steel

Because of the effect of silicon on the formation of oxide scale, black strip defect is common on the surface of compact strip production （CSP） hot-rolled silicon steel, which leads to difficulty in pickling process compared with plain carbon steels. Although high-temperature oxidation of Fe-Si alloy has been discussed a lot, few studies have explained the mechanism of black strip defect generated on the surface of hot-rolled silicon steel and proposed means to prevent it effectively. Thermogravimetric analyzer （TGA） is used to simulate oxidation of Fe-2.2Si alloy for 30 min under air condition, and temperature range is from 1000 to 1150 ℃. Effect of rolling deformation on scale morphology is also discussed. Electron probe microanalysis （EPMA） is used to analyze cross-sectional morphology and elemental distribution of the oxide scale. Schematic diagram of formation of black strip defect is obtained, which helps to provide theoretic basis for proposing of solutions for the Problem. It is proposed that lowering furnace tem-perature and shortening time to stay at high teniperature condition after rolling will be helpful tO relieve the black strip problem.

Analysis of Microstructure Characteristics and Precipitation Behavior of Automobile Beam Steels Produced by Compact Strip Production

The microstructure characteristics and precipitation behavior of automobile beam steels produced by compact strip production （CSP） were investigated by use of scanning electron microscopy （SEM） transmission electron microscopy （TEM） and X-ray energy dispersive spectroscopy. The result shows that the final microstructure is mainly composed of polygonal ferrite and small amount of pearlite, and the average ferrite grain size is about 3-6μm. Small amount addition Ti to aluminium-killed steel can help to refine the microstructure and improve the mechanical properties. A large number of fine precipitates have been observed in automobile beam steels. The mean particle size is about 10-30 nm. Remarkable strengthening and grain refinement can be obtained by these nano-particles.

Comprehensive Shape Control Technology for CSP Hot Strip Mills

With the increasing demand on higher strip quality, the profile and flatness of hot rolling strips have become subjects of concern, particularly for compact strip product(CSP) hot strip mills. Based on the roll contour, control model, and rolling process, a comprehensive shape control technology is proposed and applied to CSP hot strip mill of Lianyuan steel, which includes optimization and design of the work roll contour and varying contact back-up roll(VCR) plus backup roll contour, analysis of the flatness feedback control model, as well as improvement of the rolling process control system. The application of the technology has significantly improved the shape control performance. The roll wear is improved and the general roll consumption of the finishing mill is reduced by 29.86%.The percentages that satisfy the control target ranges of the average strip flatness and crown are increased by approximately 15.40%and 14.82%, respectively. The rejection rate of grade Q235 due to shape quality problem is reduced monthly by 39.69%, which creates significant economic benefits for the plant.

Analysis on the Microstructure and Performance Evolvement of Alloyed Tool Steel Hot Rolling Strip Produced by CSP Technology

This thesis determinates the microstructure and inclusions of the alloyed tool steel by CSP processing with the help of SEM.The results show that the slab microstructures are the fine branched crystalline grains and the branch width has little disparity from the surface to the central region.For the product,the grain microstructures are fine pearlites and few ferrites with the pearlite space 0.4μm to 0.2μm.

In-situ observation and analysis of solidified structure evolution of S50C steel in soaking furnace

The evolution of solidified structure of S50C steel during heat treatment in compact strip production process was studied through an ultra-high-temperature confocal scanning laser microscope.It was found that the solidified structure consisted of dendritic crystals with secondary dendrite arm spacing ranging in 32-120μm,where carbon segregation was evident,and the dendrite arms wereα-Fe.The insignificant change was observed at a soaking temperature of 1180℃,whereas at 1300℃,the finer structure firstly disappeared,and then,the coarsening decreased,indicating that carbon tended to be homogenized.Therefore,the microsegregation was improved at 1300℃for 15 min.The phase transformation ofα-Fe→γ-Fe enhanced the carbon diffusion,and the evolution of the equivalent radius req was controlled by carbon diffusion.The diffusion coefficient of carbon(D=15μm2/s)was determined by using the inverse problem method.

Precipitates in Steels With Ti Additive Produced by CSP Process

Hot strips of low carbon steels with Ti additive [-contain C 0.04 % -0. 07 % , Si≤0.6%, Mn≤0.6%, Ti 0. 060/00- 0.14% （mass percent）] prodvced by EAF-CSP （Electric Arc Furnaces-Compact Strip Production） process were examined by TEM, HREM and XRD. Carbonitrides with different N/C ratio were found in the sam- ples. The varying composition of the Ti-carbonitrides resulted from the supersaturation of Ti and temperature at which the compound was formed. In the tested steel, total mass fraction of the precipitates including cementite, carbonitride and a small quantity of Fe3O4, AO2O3 , Ti2 CS and A1N was about 0. 305 %. XRD results showed that about a quarter of the powder extracted by electrolysis was titanium nitrides, carbonitrides and carbides. Particle arrays formed by interphase precipitation could be observed either in slabs or in hot strips. The dominant reaction mecha- nisms were discussed. Compared with the conventional cold charge process, small amount of Ti addition would be more effective for orecipitation of fine orecioitates in the steels oroduced by CSP process.

Influence of inclusions on hydrogen-induced delayed cracking in hot stamping steels

The hydrogen-induced delayed cracking(HIDC)behaviors of two types of 1500 MPa grade hot stamping steels(HSSs)have been investigated by the method of slow strain rate tensile test and hydrogen permeation,where one is manufactured by compact strip production(CSP)process which is a revolution to the traditional HSS and the other by the traditional cold rolling process.The results show that the performance of HSS produced by CSP is superior to that of the traditional HSS,due to lower hydrogen embrittlement index,lower hydrogen diffusion coefficient and lower hydrogen content.It has been found that HIDC behavior is closely associated with inclusions.The inclusions of HSS produced by CSP are mainly spherical Al-Ca-O and CaS,while the inclusions in the traditional HSS are TiN+AI2O3+MnS with sharp edges and corners.Based on these results,the influence of composition,shape and distribution of inclusions in HSS on HIDC and the mechanism of HIDC from the perspective of inclusions were analyzed and discussed.