Investigation of separation phenomenon in high-toughness X70 thick-walled linepipe steel

Separation is a commonly observed phenomenon during drop weight tear testing in high-toughness linepipe steels. Severe separation is harmful because it may cause fracturing or bursting of pipes. In this study, a quantitative measurement of separation was proposed, and using this new method, a relationship between the separation and microstructure was determined and discussed. The microstructures observed using optical and scanning electron microscopy revealed that the separation was related to the mixed ferfitic and bainitic microstructures, or rather,it was related to the carbon enrichment in bainite.

Comparative Study of Tensile and Charpy Impact Properties of X70 and X80 Linepipe Steels After Ultra Fast Cooling Processing

Ultra fast cooling（UFC） processing after hot deformation was conducted on X70 and X80 linepipe steels. Tensile and charpy impact properties of both steels have been investigated in this work. The results have shown that the mechanical properties satisfy all the standard requirements of the X70 and X80 steels. UFC results in a presence of microstructure containing quasi polygonal（QF）, acicular ferrite（AF） and granular bainite（GB）. The alloying elements and UFC enhance the strengthening contribution caused by solid solution, dispersion, dislocation and precipitation strengthening. The size and distribution of precipitates in the linepipe steels are fine and dispersed. MA is also homogeneously dispersed due to UFC. Average grain size in the X80 steel is finer than that in the X70 steel. The volume fractions of secondary phases in the X80 steel are greater than those in the X70 steel. The X80 steel remains finer and more dispersed precipitates compared to the X70 steel. As a result, the tensile properties of X80 steel are higher than those of X70 steel. The Charpy absorbed energies of X70 and X80 steels at-10 ℃ reached 436 and 460 J, respectively. They reached 433 and 461 J at-15 ℃, respectively. This is mainly attributed to the presence of larger amounts of AFs in the X80 steel. A microstructure of AF for the X80 steel results in combining high strength and high toughness.

Effects of Processing Parameters on Microstructure and Properties of Ultra High Strength Linepipe Steel

To examine the effect of processing parameters on microstructural evolution and to obtain the excellent combination of strength and toughness, simulation of thermo-mechanical processing was conducted using the Gleeble machine. Trial production was then conducted under the conditions obtained by Gleeble tests. Based on the results of microstructure analysis and mechanical property evaluation, the relationship between microstructural features and mechanical properties was elucidated. The result shows that the volume fraction of constituted phases can be controlled through adjusting the cooling rate and finish cooling temperature in order to get different strength levels. As cooling rate increases, the volume fraction of upper bainite increases, which leads to the increase of strength. The upper shelf energy （USE） increases with increasing volume fraction of acicular ferrite in bainite base because of the small effective acicular ferrite grain size. Ductile-brittle transition temperature （DBTT） decreases with increasing acicular ferrite volume fraction. High reduction in the rough stage has great influence on grain refinement.

Prediction of spatial distribution of the composition of inclusions on the entire cross section of a linepipe steel continuous casting slab

In the current study, the transformation in the composition of non-metallic inclusions from the molten steel to the solidified steel was studied and the composition distribution of inclusions on the cross section of a linepine continuous casting slab was predicted. During cooling and solidification of the continuous casting strand, Al_(2)O_(3)-CaO inclusions reacted with the bulk steel and transformed to CaS-Al_(2)O_(3)-MgO-(CaO) ones in the continuous casting slab. The composition of inclusions on the cross section of the slab varied with locations due to the varied cooling rate. A model was established to predict the distribution of the composition of inclusions on the cross section of the continuous casting slab, coupling solidification and heat transfer of the continuous casting slab, the kinetic mass transfer of the dissolved elements in the solid steel, and thermodynamic calculation of inclusion transformation at different temperatures. The composition transformation of inclusions mainly occurred at the temperature between the liquidus and solidus of the linepipe steel. Inclusions were mainly CaS-Al_(2)O_(3)-MgO-(CaO) in slab center and were MgO-Al_(2)O_(3)-CaO-CaS within the subsurface of the slab. In the slab, the transformation fraction of inclusions was less than 10 % at corners while it reached 70 % at 50 mm below the surface of the slab.

Stress Oriented Hydrogen Induced Cracking of Linepipe Steel

The stress oriented hydrogen induced cracking (SOHIC) is a typical hydrogen embrittlement phenomenon occurring in the linepipe steels exposed to sour environment containing H 2 S gas.However,even recently,the cracking mechanism of SOHIC has not been clarified because of lacking in the empirical data on the actual failure mode of SOHIC cracking.The factors affecting SOHIC are discussed in terms of metallurgy of high strength linepipe steel and hydrogen electrochemistry.The cracking mechanisms of SOHIC are examined by comparing them with the empirical failure mode of SOHIC which is developed by observation of the actual fracture sites of the hydrogen induced blister cracking (HIBC) and secondary cracks.Finally,the correlation between SOHIC and HIC is discussed.

Effect of Nb on Austenite Formation and Decomposition in an X80 Linepipe Steel

Advanced high strength steels for pipeline applications,e.g.X80 grades,have complex microstructures and are frequently microalloyed with Nb.In the hot rolled product it is sought to have Nb precipitated as Nb(CN).However,when processing these steels Nb may be in solution and critically affects the microstructure evolution,e.g.austenite decomposition on the run-out table of a hot mill.Further,microstructure changes in the heat affected zone (HAZ) during girth welding of these linepipe steels may occur with Nb precipitated or in solution.In the HAZ,depending on welding procedures,the material undergoes a number of austenite formation and decomposition cycles and the amount of Nb in solution varies along these stages.In selected positions of the HAZ,thermal cycles peak at the intercritical region and the partial formation of austenite and subsequent decomposition constitutes additional complexity.Developing reliable process models for run-out table cooling and the HAZ hinges on an accurate tracking of microstructure evolution,which is strongly influenced by the amount of Nb in solution.The present study provides more insight into the effect of Nb on austenite formation and decomposition.Firstly,a novel experimental methodology is presented to measure quantitatively the effect of Nb on transformation temperatures pertinent to austenite decomposition,notably ferrite.A model for ferrite formation that accounts for solute drag of Nb is proposed to describe the experimental observations.Secondly,an experimental study will be presented to quantify the effect of Nb in and out of solution on austenite formation in the intercritical region.It is found that the morphology of intercritical austenite,as well as the kinetics of its formation is strongly affected by the starting microstructure and the state of Nb.

Evolution of Microalloyed Linepipe Steels With Particular Emphasis on the “Near Stoichiometry” Low Carbon,0.10 Percent Niobium “HTP” Concept

Microalloyed linepipe steels were first introduced in 1959 at strength levels around 52-60 ksi [1] [2].The steels were generally strengthened by niobium or vanadium used singly.As strengths levels increased to X-70 in the early 1970’s niobium and vanadium were used in combination and controlled rolling was introduced on a broad basis.Higher strengths decrease tolerance for impurities and non-metallic inclusions which required improved steelmaking practices.Sulfur,carbon,and nitrogen were reduced and continuous casting was increasingly applied which required improved ladle refining and deoxidation practices.Niobium-molybdenum steels were introduced by IPSCO in 1972 [3] and by Italsider in 1974 [4].Steels with higher niobium contents were also introduced in the early 1970’s but available steelmaking technologies limited the use of optimum niobium to carbon ratios,i.e.those approaching stoichiometry.Nevertheless the steels benefited from the effect of niobium in retarding austenite recrystallization at relatively high rolling temperatures and they developed attractive combinations of strength and toughness in old manufacturing facilities using simple (relaxed) rolling schedules.Fluctuations in the price of molybdenum and vanadium in the past decade and expanded application of API Grade X-80 linepipe,have led to more widespread use of niobium contents up to 0.11 percent,in combination with 0.02-0.03 percent carbon (near stoichiometry) thus maximizing the effects of solute niobium during rolling,in lowering transformation temperature,and thereby reducing reliance on other alloying,often expensive,elements.The history of these developments will be presented and applications of the concept will be summarized.

Development and Production of X120 Strip With Excellent Toughness in Shougang Steel

In this paper,the product designs,mechanical properties and microstructure of ultra-high strength linepipe steel in grade X120 strip with a thickness of 16mm have been shown and analyzed systematically.The tensile test results with different directions,including transverse,longitudinal and 30° to the rolling direction,showed that the yield strength and ultimate tensile strength reach and exceed 900MPa and 1000MPa respectively,which are far higher than requirements of X120,such as 827MPa and 931MPa.On the other hand,as shown from the test results of Charpy impact tests at different temperatures from 20 ℃ to-60 ℃,the absorbed energies and fracture shear area at-60℃ are about 200J and 100% respectively,which is very exciting and interesting.In order to clarify the strengthening mechanism and effect of X120 strip further,microstructure has been observed through metallography.Analysis of the metallography revealedthat the microstructure was composed of lower bainitewith a size of 1um and fine M/A components.The X120 strip was formed into 1420 mm diameter spiral welded pipe in Huabei pipe mill.Tested results showed that ultra-high strength and high toughness linepipe could be achieved through reasonable alloy composition design and optimized rolling processes.