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Bainitic steels with boron

a technology of boron and steel, which is applied in the field of seamless pipes, can solve the problems of poor toughness of steel and overshadow the improvement of property, and achieve the effect of excellent toughness and good weldability

Inactive Publication Date: 2010-12-23
TENARIS CONNECTIONS
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

[0007]In an embodiment, a method of making a boron-titanium steel with yield strength of at least 100 ksi (690 MPa), excellent toughness, and good weldability is provided. The method comprises providing a composition comprising carbon, titanium, and boron. The method may additionally comprise providing one or more of manganese, silicon, nickel, chromium, molybdenum, vanadium, and niobium to the composition. The method may also comprise cooling the composition from casting at a cooling rate sufficiently high to inhibit coarsening of titanium nitride (TiN) precipitates within the composition and to limit the size of the TiN precipitates to less than about 50 nm. The method may further comprise hot rolling the composition so as to refine the microstructure and achieve grain sizes of about 20 to 50 μm, prior to transformation. The method may further include cooling the composition in air after hot rolling and subjecting the composition to austenization and quenching; cooling the composition in air after hot rolling and subjecting the composition to austenization, quenching and tempering; or forced cooling the composition immediately after hot rolling at rates between about 5 to 50° C. / sec without any subsequent heat treatment. In certain embodiments, the steel composition may be formed into a steel pipe, for example, a seamless pipe.

Problems solved by technology

These particles, which may further grow during reheating prior to hot rolling, can lead to poor toughness in the steel and overshadow the property improvements yielded by the boron addition.

Method used

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  • Bainitic steels with boron
  • Bainitic steels with boron
  • Bainitic steels with boron

Examples

Experimental program
Comparison scheme
Effect test

example 1

Continuous Cooling Transformation (CCT) Behavior and Microstructural Evaluation of Composition 1 for Cooling Rates Between about 0.5 and 50° C. / s.

[0113]The CCT diagram derived from dilatometric measurements of composition 1 is shown in FIG. 2. Illustrated in FIG. 2 are traces of temperature as a function of cooling rate for transformations of about 5%, 20%, 50%, 80%, and 95%. Due to the reheating condition, about 920° C. over about 10 min, the austenitic grain size prior to transformation was estimated to be about 10-20 μm, based upon the sample cooled at about 50° C. / sec.

[0114]In the CCT diagram of FIG. 2, two transformation regions may be observed, corresponding to cooling rates lower and higher than about 5° C. / sec. For cooling rates less than about 5° / sec, the phase transformation is observed to start at about 550-600° C. The microstructure resulting under these conditions was mainly bainitic, with some retained austenite, as illustrated in the micrographs corresponding to cooli...

example 2

Toughness Evaluation of Composition 1—Accelerated Cooling Condition

[0117]In order to study the impact properties of composition 1 under accelerated cooling conditions, several Charpy tests were conducted using thermal cycles discussed above with respect to the CCT diagram. Samples prepared using cooling rates of about 5° C. / sec, 10° C. / sec, 30° C. / sec, and 45° C. / sec were examined. Charpy tests were performed at temperatures of about 25° C., 0° C., and −20° C. The results of these impact tests are illustrated in Table 3 and FIGS. 4A and 4B, which complementarily plot impact energy (Charpy V-Notch, CVN) as a function of cooling rate and test temperature, respectively.

TABLE 3Impact Energy and Hardness of composition 1 after acceleratedcooling simulations performed at GleebleCoolingImpact EnergyCom-RateHardnessTCVNShearpositionCondition(° C. / sec)(Hv)(° C.)(J)Area (%)1Accelerated 528025284100Cooling 028791−20286891Accelerated1030525335100Cooling027889−20257781Accelerated3033125251100Coo...

example 3

Mechanical Evaluation of Composition 1—As-Quenched

[0119]The tensile and impact properties of composition 1 in the as-quenched condition are illustrated in Tables 4 and 5.

TABLE 4Tensile properties of composition 1 after quenchingYSUTSCompositionCondition(ksi)(ksi)YS / UTSEl (%)1As-1211560.7816Quenched

TABLE 5Impact Energy and Hardness of composition 1 after quenchingCharpy (10 × 10 mm)ShearHardnessCVNAreaCompositionCondition(Hv 1 kg)T (° C.)(J)(%)1As-Quenched363251507908243−204224

[0120]In general, the as-quenched composition exhibited improvements in strength and impact energy over that of as-rolled samples (YS˜69 ksi, UTS˜99 ksi, CVN˜6-8 J at 25° C. to −20° C.). This improvement may be ascribed to a general refinement of the microstructure and the substantial disappearance of large, blocky austenitic regions.

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Abstract

Steel compositions contain micro-alloying additions of boron and titanium, with yield strength of at least 100 ksi (690 MPa), excellent toughness and good weldability. Boron additions are used to increase hardenability. Strong nitride formers, such as titanium, may be added to the steel composition in order to prevent boron nitrides from forming. These compositions may be cooled from hot rolling in air or using accelerated cooling. After air cooling, the composition may be quenched or quenched and tempered. The compositions are suitable for high strength line pipes (for example, X100 in API 5L standard) and other applications.

Description

BACKGROUND OF THE INVENTION[0001]1. Field[0002]Embodiments of the present disclosure pertain to seamless pipes formed from steels containing micro-alloying additions of boron and titanium, with yield strengths of at least 100 ksi (690 MPa), excellent toughness, and good weldability. Such pipes are suitable for use as high strength line pipes, for example X100 in API 5L standard, and other possible applications.[0003]2. Description of the Related Art[0004]Micro-alloying additions of boron to steel are desirable, as such additions may improve the mechanical properties of the steel. For example, boron additions may increase hardenability, the ability of steel to be hardened by heat treatment. By migrating to grain boundaries, where they inhibit austenite to ferrite phase transformation, boron additions may improve the ease with which martensite may be formed. Furthermore, boron is effective at very low concentrations, providing significant improvements in hardenability at relatively lo...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): C21D11/00C21D9/08C22C38/00
CPCC21D1/18C21D1/25C21D8/0226C21D8/10C21D2211/008C21D9/08C21D9/085C21D2211/002C21D8/105
Inventor PEREZ, TERESA ESTELAGOMEZ, GONZALO ROBERTO
Owner TENARIS CONNECTIONS
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