Ultra-high Strength Structural Steels Research Articles

Reliability analysis of thermal cycle method on the prediction of residual stresses in arc-welded ultra-high strength steels

The large amount of calculation time is one of the major obstacles to the industrial application of numerical welding simulation. The thermal cycle method is a newly developed approach, which can significantly reduce the computational time. The present work aims to systematically analyze the reliability of the thermal cycle method in the prediction of residual stresses in the arc-welded ultra-high strength steels (UHSS). Both numerical simulation and experiments have been performed for estimating and validating its feasibility in this study. The increasingly used ultra-high strength structural steel S960 in industrial sectors was selected for investigation here. The results indicate that the calculated longitudinal residual stress applying the moving heat source and thermal cycle method is almost the same in and near the weld, while that is quite different mainly in the base metal directly under the weld area for UHSS. The predicted transverse residual stress using the thermal cycle method and moving heat source is significantly different, especially for the transforming UHSS. Although the thermal cycle method can obviously reduce the computing time, the moving heat source gives much better agreement with experimental results than the thermal cycle method in the assessment of welding residual stresses (WRS) overall in UHSS thick plates. Thus, the thermal cycle method is not reliable to predict WRS in thick structures made of UHSS. Furthermore, it is not recommended to use the thermal cycle method in numerical welding simulation for UHSS thin-walled structures.

Evolution of inclusions with cerium addition and effects of C-containing rare earth inclusions on the toughness of ultra-high-strength structural steel

ABSTRACT Ultra-high strength structural steels with Ce (cerium) content of 0, 0.0367, 0.0559 and 0.0792 wt.% respectively are prepared to study the effect of Ce addition on inclusions and the toughness of ultra-high-strength steel. The results show that the 0.0367 wt.% Ce can refine the size of oxides or sulfides to about 1 μm and improve the inclusion morphology to spherical, resulting in improved toughness of L-Ce steel. The number of inclusions in the H-Ce steel increases sharply, mainly due to the precipitation of C-containing rare earth inclusions. The rare earth carbides are hard and brittle phases, with the size of about 1 μm and a spherical shape. In addition to the increase in the number and size of inclusions, the reason for the deterioration of the toughness of H-Ce steel is that the C-containing rare earth inclusions are easily broken during hot rolling act as stress concentrators under impact loading.

Design of tough, ductile direct quenched and partitioned advanced high-strength steel with tailored silicon content

The novel processing concept of direct quenching and partitioning (DQ&P) has been explored with a medium-carbon (0.4 wt.% C) steel to evaluate and optimize the processing route for excellent property combinations. New compositional design approach was based on physical simulation studies aiming to understand the influence of varying silicon contents (1.5, 0.75 and 0.25 wt.%) and Q&P processing parameters on microstructural development including carbide formation and retained austenite stabilization. Optimized Q&P parameters were selected to design a DQ&P processing route for laboratory hot-rolling trials based on the analyses of physical simulation data. The overall aim of the study was to produce ultrahigh-strength structural steels with yield strength ≥1100 MPa combined with high uniform and total elongation and impact toughness, achieved through designing a low-temperature quenching and partitioning route with effective carbon partitioning. The DQ&P steels gained an excellent combination of mechanical properties comprising of high yield strength of ∼1000–1200 MPa and tensile strength ∼2100–2300 MPa, good elongation (∼11–13%), and moderate impact toughness transition temperature T28J ∼ (−5 to +12 °C). Straining of austenite prior to DQ&P led to an extensive refinement of the final martensitic-austenitic nanostructure. Formation of nanoscale lath-martensite and fine film-like retained austenite structures enabled the observed improvement of mechanical properties. Besides the formation of nano-twinned martensite, inter-lath austenite and transitional carbides were comprehensively characterized. No adverse effect of prolonged partitioning during slow cooling, simulating coiling in actual industrial practice, has been noticed suggesting new possibilities for developing tough, ductile structural steels both for strip/plate products.

Effect of the Welding Thermal Cycles Based on Simulated Heat Affected Zone of S1300 Ultrahigh Strength Steel

In the automotive industry there is an increasing demand for the wider application of high strength steels due to their favourable mechanical properties. The steel producers continuously developing new generations of high strength steels to insure higher strength and toughness properties. Since in most cases these steels are joined in welded structures, great attention must be taken to their weldability. The weldability of high strength steels has still challenges which are as follows: cold cracking sensitivity; reduction of strength and toughness of heat affected zone (HAZ); filler metal selection. Because the mechanical properties of ultrahigh strength steels are provided by using various alloying elements, micro alloys, and by different metallurgical methods, the steels may lose their outstanding properties during welding. In real welded joints the critical parts of the HAZ have small extent so their properties can be limitedly analysed by conventional material testing methods. With the help of physical simulators, the different parts of the heat affected zone can be produced in an adequate size for subsequent tests. In our research work the weldability, especially the HAZ properties of an ultrahigh strength structural steel (Rp0.2 = 1300 MPa) were investigated on thermal simulated samples with the help of Gleeble 3500 physical simulator. Three relevant technological variants for gas metal arc welding (GMAW), t8/5 = 5 s, 15 s and 30 s were applied during the HAZ simulations in the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones. Both the microstructure was studied by optical microscope and the mechanical properties were analysed by Vickers hardness tests and Charpy V-notch impact tests at -40 °C. According to the results the investigated ultrahigh strength steel was softened on account of the welding heat cycles, besides that the strength of the investigated ultrahigh strength steel can be better with the application of shorter t8/5 cooling time.

Hitsiliitoksen geometrian vaikutus väsymismitoituksessa

Tässä tutkimuksessa tarkasteltiin kahdella eri lämmöntuonnilla MAG-hitsattujen päittäisliitosten poikkileikkausten geometriaa sekä hitsiliitoksen geometrian vaikutusta väsymiskestävyyteen lovenvaikutuslukujen avulla S700-lujuusluokan rakenneteräksellä. Hitsiliitosten poikkileikkausten geometriat tuotiin elementtimenetelmäohjelmistoon, jossa määritettiin malli lovenvaikutuslukujen laskemiseksi. Laskettuja lovenvaikutuslukuja verrattiin kirjallisuuteen, sekä lovenvaikutuslukujen avulla määritettiin S‑N ‑käyrät, joita verrattiin mitoitusohjeiden S-N -käyriin. Lisäksi tarkasteltiin elementtikoon vaikutusta lovenvaikutuslukuun sekä alkusärön koon vaikutusta kestoikään murtumismekaniikan avulla. Tutkimuksessa havaittiin, että geometria ei yksin selitä hitsiliitoksen huonompaa väsymiskestävyyttä ehjään perusaineeseen verrattuna, vaan lähes puolet erosta johtuu muusta kuin geometriasta, esimerkiksi hitsauksen aiheuttamista mikrorakenteen muutoksista tai jäännösjännityksistä. Ainoastaan lovenvaikutusluku huomioiden saadaan hitsiliitoksille ylioptimistisia väsymiskestävyyden suuruuksia sekä kirjallisuuteen että mitoitusohjeisiin verrattuna. Elementtimenetelmää mitoituksessa hyödynnettäessä on käytettävä riittävän tiheää elementtiverkkoa, jotta rakenteen ja hitsiliitoksen geometria kuvataan oikein ja laskentajännitykset ovat todellisuutta vastaavia.

Development of high ductile ultra high strength structural steel through stabilization of retained austenite and stacking fault

In the current study, a bainitic grade steel of 1.5 GPa and ~35% elongation and a martensitic grade steel with 2 GPa and ~20% elongation have been developed. The uniqueness of the work lies in achieving high strength with extended ductility in the bainitic and martensitic grade steels through solid solution strengthening and a possible twin plasticity mechanism. Both the microstructures consist of mechanically stable retained austenite with different morphology and characteristics. The synergic effect of carbon in lowering stacking fault energy and the stability of RA is proposed based on microstructural and mechanical characterisation of the developed steels. TEM investigations reveal the presence of stacking fault in the microstructure. This is one of the major indications of lowering of SFE in the retained austenite and subsequently augment the twin formation during tensile straining and thus contribute towards ductility.

Microstructural Evolution and Toughness of the Various HAZs in 1300-MPa-Grade Ultrahigh-Strength Structural Steel

The effect of heat input on the microstructure and toughness of simulated subregions was investigated by welding thermal simulations and Charpy impact tests. The results indicate that the microstructures of the simulated coarse and fine grain heat affected zone (CGHAZ and FGHAZ) gradually changed from lath martensite to a mixture of lath martensite/lath bainite and finally to granular bainite with the increase in heat input. The microstructure of the simulated intercritical heat affected zone (ICHAZ) was mainly composed of granular bainite and blocky martensite regardless of heat input. When the heat input increased, the toughness of the simulated CGHAZ and ICHAZ continuously decreased. Nevertheless, the simulated FGHAZ still displayed good toughness (53.16 J) due to its fine structure. The occurrence of martensite-austenite (M-A) constituents was the main reason for the decrease in crack initiation energy of the simulated CGHAZ and ICHAZ at high values of heat input, and the toughness deteriorated as the size of M-A constituents increased. It should be noted that high-misorientation packet and block boundaries can effectively deviate or arrest the propagation of microcracks. When the heat input was in the approximate range of 8.63-14.95 kJ cm−1, all of the simulated subregions exhibited good toughness.

Digital image correlation and optical strain measuring in bendability assessment of ultra-high strength structural steels

Abstract Air-bending is a widely used forming process for ultra-high strength steel because it is fast, cost-effective and flexible way to form material. The bendability of a material can be expressed by minimum bending radius R/tmin, which is the relation of the smallest inner radius to the sheet thickness the material can be bent without damage. Damage usually occurs on the outer surface of the bend in the form of intense strain localization that further progresses to cracking. The minimum bending radius contains no other information that affects the bendability such as the lower tool width or the desired bending angle. Hence, developing more detailed test procedure is critical to better describe the behavior of ultra-high strength steel sheet in bending. In this paper, a method for more detailed assessment of bendability for ultra-high strength structural steel is presented. Using optical strain measuring techniques and digital image correlation coupled with bending tests in a universal tensile test machine one can measure the strain evolution at the outer surface of the bend and determine the critical strains that limit the bendability of these materials

Experimental cyclic behavior and constitutive modeling of high strength structural steels

High strength structural steels have recently achieved increasing application in civil and structural engineering worldwide. For instance, the sony center tower located in Berlin were constructed through steel truss made of S460 and S690 high strength steel whose nominal yield stress is 460 MPa and 690 MPa respectively. The utilization of high strength steel considerably reduced the structure deadweight and produced larger architecture space. Besides, steel connections of one military steel bridge in Mittådalenthe, Sweden were fabricated of 960 MPa ultra-high strength steel. This application remarkably increases the construction convenience since high strength steel presents excellent ability to reduce member size as well as improve structure performance. Nevertheless, limited test data have been available on the monotonic properties and cyclic behavior of these structural steels. In this study, uniaxial coupons were tested for four types of Chinese high strength structural steels, including grades Q460D, Q550D, Q690D as well as ultra-high strength structural steel Q890D, to investigate their stress-strain response and energy-dissipation behavior under monotonic and various cyclic strain demands. The monotonic test results of the tested steels were then compared with the requirements of current seismic design codes GB 50011-2010 and Eurocode8. Besides, cyclic strength, deformability and energy dissipation capacity were evaluated through the tested strain-stress loops. The experimental observations in cyclic tests show that cyclic softening, which significantly affects the shape of cyclic loops and energy dissipation capacity, is an important hysteretic characteristic for all tested high strength structural steels. Moreover, cyclic backbones of these four steels were calibrated using Ramberg-Osgood model. To fundamentally define the cyclic stress-strain relationship, essential parameters of three commonly used constitutive models, including Chaboche model, Dong-Shen model and Giuffre-Menegotto-Pinto model were calibrated based on the experimental data under cyclic loading. Those calibrated models were then employed in simulations of aforementioned cyclic loading tests and their validity were compared. Good agreements between experimental response and simulated performance by Dong-Shen model were achieved, demonstrating that the calibrated Dong-Shen model can elaborately describe the cyclic behavior of these four types of high strength structural steels.

An Appraisal of Direct Quenching for the Development and Processing of Novel Super-High Strength Steels

Recent interests in developing novel super-high strength steels have led to extensive research efforts in direct quenching with or without tempering (DQ, DQT) or combined with partitioning (DQP). Both strip and plate products have been targeted for different applications. For boron-microalloyed DQ/DQT steels, the ASTM A255 approach for predicting the hardenability was considered inapplicable. Fresh attempts were made to develop new hardenability models through non-linear regression analysis by dynamically varying both the boron factor and multiplying factors of most elements in the alloy factor. Based on the recent concept of quenching and partitioning (Q&P), a novel processing route comprising thermomechanical rolling followed by direct quenching and partitioning (TMR-DQP) has been established for the development of ultra-high strength structural steels with yield strengths ≈1100 MPa combined with good uniform and total elongations and impact toughness. Examples of recent advances made in DQ processing and associated challenges, such as those related to the bendability of low carbon martensitic-bainitic steels and influence of boron on the toughness of Nb-bearing martensitic steels are presented.

Effect of Cooling Procedure on Tensile and Charpy Impact Properties of Cr-Mo Ultra-High Strength Steel

The effect of cooling procedure on the transformation behavior of low-carbon Cr-Mo microalloyed steel was investigated by using microstructural observations, mechanical properties and impact fractographs. Three steel plates were adopted under three different cooling rates, and their microstructure, tensile and impact properties were evaluated. The results indicated that the strength of experimental steels was increased and the impact toughness was decreased with decreasing the coiling temperature. Steel A consisted of granular bainite, coarse bainitic ferrite lath and M/A constituent subjected to a coiling temperature of 560 oC. The yield strength, tensile strength and impact energy of 1/2-size Charpy impact at-20 oC were 740MPa, 1020MPa, and 33.5J, respectively, which were imperfect in strength. The effects of coiling temperature were potent on the refinement of microstructure and the size of M/A constituents. Steel B consisted of a small amount of lath bainite, fine M/A constituents and bainitic ferrite lath subjected to a lower coiling temperature of 520 oC. The yield strength, tensile strength and impact energy of 1/2-size Charpy impact at-20°C were 840MPa, 1030MPa, and 30.7J, respectively. However, steel C was composed of lath bainite and lath martensite subjected to the lowest coiling temperature of 380 oC (slightly above Ms point). The yield strength, tensile strength and impact energy of 1/2-size Charpy impact at-20 oC were 985MPa, 1200MPa and 22.5J, respectively, which could meet the demand of ultra high strength structural steel applications.

Microstructure, Properties and Design of Direct Quenched Structural Steels

Direct quenching (DQ) is one of the latest process routes in production of ultra-high strength, high performance steels and Ruukki one of the pioneering companies in the utilization of direct quenching. Ruukki has applied direct quenching for the production of ultra-high-strength structural steels in the form of hot-rolled strip and plate. The paper briefly summarizes the physical metallurgy fundamental of direct steels and shows some selected examples of the microstructures and properties of steels produced by direct quenching. In addition, a brief review on the usability properties and design rules of ultra-high strength structural steels is made.

Innovation and Processing of Novel Tough Ductile Ultra-High Strength Steels through TMR-DQP Processing Route

Based on the recent concept of quenching and partitioning (Q&P), a novel TMR-DQP (thermomechanical rolling followed by direct quenching and partitioning) processing route has been established for the development of ultra-high strength structural steels with yield strengths ≈1100 MPa combined with good uniform and total elongations and impact toughness. Suitable compositions were designed based on high silicon and/or aluminium contents with or without small additions of Nb, Mo or Ni. The DQP parameters were established with the aid of physical simulation on a Gleeble simulator. Finally, the TMR-DQP processing route was designed for trials on a laboratory rolling mill. Metallographic studies showed that the desired martensite-austenite microstructures were achieved thus providing the targeted mechanical properties. The advantage of strained austenite in refining the martensite packets/blocks was clearly evident. No adverse effect of prolonged partitioning simulating the coiling stage has been noticed suggesting new possibilities for strip and plate products. Promising results in respect of microstructures and mechanical properties indicate that there are possibilities for developing tough ductile structural steels through the TMR-DQP route.

Designing a Novel DQ&P Process through Physical Simulation Studies

This study presents the use of physical and laboratory rolling simulations for the development of a novel direct quenching and partitioning (DQ&P) process for the development of tough ductile ultra-high strength structural steels with yield strengths ~1100 MPa and reasonable ductility and toughness. Suitable compositions were designed based on high silicon and/or aluminium content. The DQ&P parameters were established with the aid of physical simulation on a Gleeble simulator. Two types of dilatation tests were carried out: with or without prior straining in the no-recrystallization regime to establish the influence of controlled deformation on subsequent transformation structures and properties. Based on dilatation results, simulated rolling trials were conducted on a laboratory rolling mill and the rolled samples were direct quenched in water to the desired quench stop temperatures followed by partitioning in a furnace held at this temperature. Detailed microstructural examination confirmed that the desired martensite-austenite microstructures were achieved. Besides high strengths, the ductility (including uniform elongation) and impact toughness were quite improved in comparison to that of a direct quenched carbon steel in the same strength class.

Tests and numerical study of ultra-high strength steel columns with end restraints

High strength steels with the nominal yield strength more than 460 MPa have begun to be applied in the construction of many steel structures, but there are short of sound researches on the major axis buckling behavior of such steel welded I-section columns, especially for the ultra-high strength steels having the nominal yield strength more than 690 MPa. In this paper, the experimental research is described on the overall buckling behavior about the major axis of ultra-high strength steel compression I-section columns with end restraints. In this research 8 columns made from 2 kinds of ultra-high strength structural steels S690 and S960, with nominal yield strengths of 690 MPa and 960 MPa, respectively, were tested. Based on the test results, the finite element analysis (FEA) model was validated to analyze this behavior of ultra-high strength steel columns, and the buckling strength of pin-ended columns fabricated from such steels were calculated by the verified FEA model, which were compared with the design buckling strengths according to the Eurocode 3, the American specification for structural steel buildings ANSI/AISC 360–05, and the Chinese codes for steel structures design GB50017-2003 respectively. It shows that the major axis nondimensional buckling strengths of the ultra-high strength steel compression columns, whose buckling curve is type b according to Eurocode 3 and GB50017-2003, are much higher than that calculated according to the column curve b, even higher than the curve a 0 in Eurocode 3 and the curve a in GB50017-2003 on average, and they are also higher than the design values according to ANSI/AISC 360–05. It is therefore indicated that the buckling strength about the major axis of the ultra-high strength steel I-section columns is improved a lot compared with the ordinary strength steel columns on a non-dimensional basis, and the column curve a 0 and curve a can be adopted to design this behavior in Eurocode 3 and GB50017-2003, respectively. Besides, there is no obvious difference between the major axis nondimensional buckling strengths of the pin-ended I-section columns fabricated from these two kinds of ultra-high strength steels: S690 and S960. These research works will provide the test basis to complete the buckling design method and theory of the ultra-high strength steel columns, and also be helpful for the application of ultra-high strength steel structures.

A study on deformation characteristic of ultra high strength steel and its fracture

A new shape on the load and elongation curve of ultra high strength structural steel in tension is measured by means of the advanced physical methods. A method of calculating true stresstrue strain is found in accordance with the whole load-elongation curve. The calculation results show that during the period of deformation, whetner prior to or after the maximum load, all are staged power hardening. The method of calculating hardening in dex is finally studied for discussion.

Welding of High-Strength Steel

In the last few years, the use of low alloy high strength structural steels and also ultra high-strength structural steels has slowly increased due to the fact that several earth moving machinery manufacturing concerns have taken to these steels in view of the strength-to-weight advantage they get from these very sophisticated varieties of products.