OA11422A - Ultra-high strength steels with excellent cryogenic temperature toughness. - Google Patents

Abstract

An ultra-high strength, weldable, low alloy steel, containing less than 9 wt.% nickel, with excellent cryogenic temperature toughness in the base plate and in the heat affected zone (HAZ) when welded, having a tensile strength greater than 830 MPa (120 ksi) and a microstructure comprising predominantly fine-grained lath martensite and/or fine-grained lower bainite, is prepared by heating a steel slab comprising iron and some or all of the additives carbon, manganese, nickel, nitrogen, copper, chrominum, molybdenum, silicon, niobium, vanadium, titanium, aluminum, and boron; reducing the slab to form plate in one or more passes in a temperature range in which austenite recrystallizes; finish rolling the plate in one or more passes in a temperature range below the austenite recrystallization temperature and above the Ar3 transformation temperature; quenching the finish rolled plate (10''') to at a suitable Quench Stop temperature; stopping the quenching; and tempering the plate (10''') at a suitable temperature for a period of time sufficient to cause precipitation of hardening particles.

Description

011422

ULTRA-HIGH STRENGTH STEELS WITH EXCELLENT

CRYOGENIC TEMPERATURE TOUGHNESS

5 FIELD OF THE INVENTION

This invention relates to ultra-high strength, weldable, low alloy Steel plates . with excellent cryogénie température toughness in both the base plate and in the heataffected zone (HAZ) when welded. Furthermore, this invention relates to a methodfor producing such Steel plates. 10

BACKGROUND OF THE INVENTION

Various tenns are defined in the following spécification. For convenience, a

Glossary of ternis is provided herein, immediately preceding the daims.

Frequently, there is a need to store and transport pressurized, volatile fluids at 15 cryogénie températures, i.e., at températures lower than about -40°C (~40°F). Forexample, there is a need for containers for storing and transporting pressurizedliquefied natural gas (PLNG) at a pressure in the broad range of about 1035 kPa (150psia) to about 7590 kPa (1100 psia) and at a température in the range of about -123°C(-190°F) to about -62°C (-80°F). There is also a need for containers for safely and 20 economically storing and transporting other volatile fluids with high vapor pressure,such as methane, ethane, and propane, at cryogénie températures. For such containersto be constructed of a welded Steel, the Steel must hâve adéquate strength to withstandthe fluid pressure and adéquate toughness to prevent initiation of a fracture, i.e., afailure event, at the operating conditions, in both the base Steel and in the HAZ. 25 The Ductile to Brittle Transition Température (DBTT) delineates the two fracture régimes in structural steels. At températures below the DBTT, failure in theSteel tends to occur by low energy cleavage (brittle) fracture, while at températuresabove the DBTT, failure in the Steel tends to occur by high energy ductile fracture.Welded steels used in the construction of storage and transportation containers for the 30 aforementioned cryogénie température applications and for other load-bearing, cryogénie température service must hâve DBTTs well below the service températurein both the base Steel and the HAZ to avoid failure by low energy cleavage fracture. 2 011422

Nickel-containing steels conventionally used for cryogénie températurestructural applications, e.g., steels with nickel contents of greater than about 3 wt%,hâve low DBTTs, but also hâve relatively low tensile strengths. Typically,commercially available 3.5 wt% Ni, 5.5 wt% Ni, and 9 wt% Ni steels hâve DBTTs of 5 about -100°C (-150°F), -155°C (-250°F), and -175°C (-280°F), respectively, andtensile strengths of up to about 485 MPa (70 ksi), 620 MPa (90 ksi), and 830 MPa(120 ksi), respectively. In order to achieve these combinations of strength andtoughness, these steels generally undergo costly processing, e.g., double annealingtreatment. In the case of cryogénie température applications, industry currently uses 10 these commercial nickel-containing steels because of their good toughness at lowtempératures, but must design around their relatively low tensile strengths. Thedesigns generally require excessive Steel thicknesses for load-bearing, cryogénietempérature applications. Thus, use of these nickel-containing steels in load-bearing,cryogénie température applications tends to be expensive due to the high cost of the 15 Steel combined with the Steel thicknesses required.

On the other hand, several commercially available, state-of-the-art, low and medium carbon high strength, low alloy (HSLA) steels, for example AISI 4320 or4330 steels, hâve the potential to offer superior tensile strengths (e.g., greater thanabout 830 MPa (120 ksi)) and low cost, but suffer from relatively high DBTTs in 20 general and especially in the weld heat affected zone (HAZ). Generally, with thesesteels there is a tendency for weldability and low température toughness to decreaseas tensile strength increases. It is for this reason that currently commerciallyavailable, state-of-the-art HSLA steels are not generally considered for cryogénietempérature applications. The high DBTT of the HAZ in these steels is generally due 25 to the formation of undesirable microstructures arising from the weld thermal cyclesin the coarse grained and intercritically reheated HAZs, i.e., HAZs heated to atempérature of from about the Aci transformation température to about the AC3transformation température. (See Glossary for définitions of Aci and AC3transformation températures.) DBTT increases significantly with increasing grain 30 size and embrittling microstructural constituents, such as martensite-austenite (MA)islands, in the HAZ. For example, the DBTT for the HAZ in a state-of-the-art HSLAsteel, XI00 linepipe for oil and gas transmission, is higher than about -50°G (-60°F). 011422

There are significant incentives in the energy storage and transportation sectors for thedevelopment of new steels that combine the low température toughness properties ofthe above-mentioned commercial nickel-containing steels with the high strength andlow cost attributes of the HSLA steels, while also providing excellent weldability and 5 the desired thick section capability, i.e., substantially uniform microstructure andproperties (e.g., strength and toughness) in thicknesses greater than about 2.5 cm (1inch).

In non-cryogenic applications, most commercially available, state-of-the-art,low and medium carbon HSLA steels, due to their relatively low toughness at high 10 strengths, are either designed at a fraction of their strengths or, altematively,processed to lower strengths for attaining acceptable toughness, In engineeringapplications, these approaches lead to increased section thickness and therefore,higher component weights and ultimately higher costs than if the high strengthpotential of the HSLA steels could be fully utilized. In some critical applications, 15 such as high performance gears, steels containing greater than about 3 wt% Ni (suchas AISI48XX, SAE 93XX, etc.) are used to maintain sufïïcient toughness. Thisapproach leads to substantial cost penalties to acccss the superior strength of theHSLA steels. An additional problem encountered with use of standard commercialHSLA steels is hydrogen cracking in the HAZ, particularly when low heat input 20 welding is used.

There are significant économie incentives and a definite engineering need forlow cost enhancement of toughness at high and ultra-high strengths in low alloysteels. Particularly, there is a need for a reasonably priced Steel that has ultra-highstrength, e.g., tensile strength greater than 830 MPa (120 ksi), and excellent cryogénie 25 température toughness, e.g. DBTT lower than about -73°C (-100°F), both in the baseplate and in the HAZ, for use in commercial cryogénie température applications.

Consequently, the primary objects of the présent invention are to improve thestate-of-the-art high strength, low alloy Steel technology for applicability at cryogénietempératures in three key areas: (i) lowering of the DBTT to less than about -73°C 30 (-100°F) in the base Steel and in the weld HAZ, (ii) achieving tensile strength greater than 830 MPa (120 ksi), and (iii) providing superior weldability. Other objects of theprésent invention are to achieve the aforementioned HSLA steels with substantially 4 011422 uniform through-thickness microstructures and properties in thicknesses greater thanabout 2.5 cm (1 inch) and to do so using current commercially available processingtechniques so that use of these steels in commercial cryogénie température processesis economically feasible.

SUMMARY OF THE INVENTION

Consistent with the above-stated objects of the présent invention, a processingmethodology is provided wherein a low alloy Steel slab of the desired chemistry isreheated to an appropriate température then hot rolled to form Steel plate and rapidlycooled, at the end of hot rolling, by quenching with a suitable fluid, such as water, to asuitable Quench Stop Température (QST), to transform the microstructure of the Steel topreferably predominantly fine-grained lath martensite, fine-grained lower bainite, ormixtures thereof, and then by tempering within a suitable température range to produce amicrostructure in the tempered Steel preferably coinprising predominantly temperedfine-grained lath martensite, tempered fine-grained lower bainite, or mixtures thereof,or, more preferably comprising substantially 100% tempered fine-grained lathmartensite. As used in describing the présent invention, quenching refers to acceleratedcooling by any means whereby a fluid selected for its tendency to increase the coolingrate of the Steel is utilized, as opposed to air cooling the Steel to ambient température. Inone embodiment of this invention, the Steel plate is air cooled to ambient températureafter quenching is stopped and prior to tempering.

Also, consistent with the above-stated objects of the présent invention, steelsprocessed according to the présent invention are especially suitable for manycryogénie température applications in that the steels hâve the followingcharacteristics, preferably for Steel plate thicknesses of about 2.5 cm (1 inch) andgreater: (i) DBTT lower than about -73 °C (-100°F) in the base Steel and in the weldHAZ, (ii) tensile strength greater than 830 MPa (120 ksi), preferably greater thanabout 860 MPa (125 ksi), and more preferably greater than about 900 MPa (130 ksi),(iii) superior weldability, (iv) substantially uniform through-thickness microstructureand properties, and (v) improved toughness over standard, commercially available,HSLA steels. These steels can hâve a tensile strength of greater than about 930 MPa s 011422 (135 ksi), or greater than about 965 MPa (140 ksi), or greater than about 1000 MPa(145 ksi).

DESCRIPTION OF THE DRAWINGS 5 The advantages of the présent invention will be better understood by refemng to the following detailed description and the attached drawings in which: FIG. 1A is a schematic illustration of austenite grain size in a Steel slab afterreheating according to the présent invention; FIG. IB is a schematic illustration of prior austenite grain size (see Glossary) in a 10 Steel slab after hot rolling in the température range in which austenite recrystallizes, butprior to hot rolling in the température range in which austenite does not recrystallize,according to the présent invention; and FIG. IC is a schematic illustration of the elongated, pancake grain structure inaustenite, with very fine effective grain size in the through-thickness direction, of a Steel 15 plate upon completion of TMCP according to the présent invention.

While the présent invention will be described in connection with its preferred embodiments, it will be understood that the invention is not limited thereto. On thecontrary, the invention is intended to cover ail alternatives, modifications, andéquivalents which may be included within the spirit and scope of the invention, as 20 defined by the appended daims.

DETAILED DESCRIPTION OF THE INVENTION

The présent invention relates to the development of new HSLA steels meeting the above-described challenges. The invention is based on a novel combination of 25 Steel chemistry and processing for providing both intrinsic and microstructural toughening to lower DBTT as well as to enhance toughness at high tensile strengths.Intrinsic toughening is achieved by the judicious balance of critical alloying élémentsin the Steel as described in detail in this spécification. Microstructural tougheningresults from achieving a very fine effective grain size as well as producing 30 fine-grained martensitic and/or lower bainitic laths occurring in fine packets with amean dimension much finer than the prior austenite grain. Additionally, in theprésent invention, dispersion strengthening from fine copper précipitâtes and mixed 6 011422 carbides and/or carbonitrides is utilized to optimize strength and toughness during thetempering of the martensiticÆiainitic structure.

In accordance with the foregoing, a method is provided for preparing a Steelplate having a microstructure comprising predominantly tempered fine-grained lath 5 martensite, tempered fine-grained lower bainite, or mixtures thereof, wherein themethod comprises the steps of (a) heating a Steel slab to a reheating températuresufficiently high to (i) substantially homogenize the Steel slab, (ii) dissolvesubstantially ail carbides and carbonitrides of niobium and vanadium in the Steel slab,and (iii) establish fine initial austenite grains in the Steel slab; (b) reducing the Steel 10 slab to form Steel plate in one or more hot rolling passes in a first température range inwhich austenite recrystallizes; (c) further reducing the Steel plate in one or more hotrolling passes in a second température range below about the T^- température and above about the A13 transformation température; (d) quenching the Steel plate at acooling rate of about 10°C per second to about 40°C per second (18°F/sec - 72°F/sec) 15 to a Quench Stop Température below about the Ms transformation température plus200°C (360°F); (e) stopping the quenching; and (f) tempering the Steel plate at atempering température from about 400°C (752°F) up to about the Acj transformationtempérature, preferably up to, but not including, the Acj transformation température,for a period of tune sufficient to cause précipitation of hardening particles, i.e., one or 20 more of ε-copper, M02C, or the carbides and carbonitrides of niobium and vanadium.The period of time sufficient to cause précipitation of hardening particles dépendsprimarily on the thickness of the Steel plate, the chemistry of the steel plate, and thetempering température, and can be determined by one skilled in the art. (See Glossaryfor définitions of preddminantly, of hardening particles, of T^ température, of Ar3, 25 Ms, and Act transformation températures, and of Mo2C.)

To ensure ambient and cryogénie température toughness, steels according tothis invention preferably hâve a microstructure comprised of predominantly temperedfine-grained lower bainite, tempered fine-grained lath martensite, or mixtures thereof.It is préférable to substantially minimize the formation of embrittling constituents 30 such as upper bainite, twinned martensite and MA. As used in describing the présent 011422 invention, and in the daims, “predominantly” means at least about 50 volume percent.More preferably, the microstructure comprises at least about 60 volume percent to about80 volume percent tempered fine-grained lower bainite, tempered fine-grained lathmartensite, or mixtures thereof. Even more preferably, the microstructure comprises at 5 least about 90 volume percent tempered fine-grained lower bainite, tempered fine-grained lath martensite, or mixtures thereof. Most preferably, the microstructurecomprises substantially 100% tempered fine-grained lath martensite. A Steel slab processed according to this invention is manufactured in acustomary fashion and, in one embodiment, comprises iron and the following alloying 10 éléments, preferably in the weight ranges indicated in the following Table I:

Table I 15 20 25

Alloying Elément carbon (C)manganèse (Mn)nickel (Ni)copper (Cu)molybdenum (Mo)niobium (Nb)titanium (Ti)aluminum (Al)nitrogen (N)

Range (wt%) 0.04 - 0.12, more preferably 0.04 - 0.070.5 - 2.5, more preferably 1.0-1.81.0 - 3.0, more preferably 1.5 - 2.50.1 - 1.5, more preferably 0.5 -1.00.1 - 0.8, more preferably 0.2 - 0.50.02 -0.1, more preferably 0.03 - 0.050.008 - 0.03, more preferably 0.01 - 0.020.001 - 0.05, more preferably 0.005 - 0.030.002 - 0.005, more preferably 0.002 - 0.003

Vanadium (V) is sometimes added to the Steel, preferably up to about 0.10wt%, and more preferably about 0.02 wt% to about 0.05 wt%.

Chromium (Cr) is sometimes added to the Steel, preferably up to about 1.0wt%, and more preferably about 0.2 wt% to about 0.6 wt%. 30 Silicon (Si) is sometimes added to the Steel, preferably up to about 0.5 wt%, more preferably about 0.01 wt% to about 0.5 wt%, and even more preferably about0.05 wt% to about 0.1 wt%. δ 011422

Boron (B) is sometimes added to the Steel, preferably up to about 0.0020 wt%,and more preferably about 0.0006 wt% to about 0.0010 wt%.

The Steel preferably contains at least about 1 wt% nickel. Nickel content ofthe Steel can be increased above about 3 wt% if desired to enhance performance after 5 welding. Each 1 wt% addition of nickel is expected to lower the DBTT of the Steel byabout 10°C (18°F). Nickel content is preferably less than 9 wt%, more preferably lessthan about 6 wt%. Nickel content is preferably minimized in order to minimize costof the Steel. If nickel content is increased above about 3 wt%, manganèse content canbe decreased below about 0.5 wt% down to 0.0 wt%. 10 Additionally, residuals are preferably substantially minimized in the Steel.

Phosphorous (P) content is preferably less than about 0.01 wt%. Sulfur (S) content ispreferably less than about 0.004 wt%. Oxygen (O) content is preferably less thanabout 0.002 wt%. 15

Processing of the Steel Slab

(1) Lowering of DBTT 20 Achieving a low DBTT, e.g., lower than about -73°C (-100°F), is a key challenge in the development of new HSLA steèls for cryogénie températureapplications. The technical challenge is to maintain/increase the strength in theprésent HSLA technology while lowering the DBTT, especially in the HAZ. Theprésent invention utilizes a combination of alloying and processing to alter both the 25 intrinsic as well as microstructural contributions to fracture résistance in a way toproduce a low alloy Steel with excellent cryogénie température properties in the baseplate and in the HAZ, as hereinafter described.

In this invention, microstructural toughening is exploited for lowering the baseSteel DBTT. A key component of this microstructural toughening consiste of refming 30 prior austenite grain size and modifying the grain morphology, aimed at enhancingthe interfacial area of the high angle boundaries per unit volume in the Steel plate. Asis familiar to those skilled in the art, "grain" as used herein means an individual 9 011422 crystal in a polycrystalline material, and "grain boundary" as used herein means anarrow zone in a métal corresponding to the transition from one crystallographicorientation to another, thus separating one grain from another. As used herein, a"high angle grain boundary" is a grain boundary that séparâtes two adjacent grains 5 whose crystallographic orientations differ by more than about 8°. Also, as used herein, a "high angle boundary" is a boundary that effectively behaves as a high anglegrain boundary, i.e., a boundary that tends to deflect a propagating crack or fractureand, thus, induces tortuosity in a fracture path.

The contribution from thermo-mechanical controlled rolling processing 10 (TMCP) to the total interfacial area of the high angle boundaries per unit volume, Sv,is defined by the following équation:

Sv = 1^1 + R ++ 0.63(r - 30) where: 15 J is the average austenite grain size in a hot-rolled Steel plate prior to rolling in the température range in which austenite doesnot recrystallize (prior austenite grain size); R is the réduction ratio (original Steel slab thickness/final Steel 20 plate thickness); and r is the percent réduction in thickness of the Steel due to hotrolling in the température range in which austenite does notrecrystallize. 25

It is well known in the art that as the Sv of a Steel increases, the DBTTdecreases, due to crack deflection and the attendant tortuosity in the fracture path atthe high angle boundaries. In commercial TMCP practice, the value of R is fixed fora given plate thickness and the upper limit for the value of r is typically 75. Given 30 fixed values for R and r, Sv can only be substantially increased by decreasing d , asévident from the above équation. To decrease d in steels according to the présentinvention, Ti-Nb microalloying is used in combination with optimized TMCPpractice. For the same total amount of réduction during hot rolling/defonnation, aSteel with an initially finer average austenite grain size will resuit in a finer finished 10 011422 average austenite grain size. Therefore, in this invention the amount of Ti-Nbadditions are optimized for low reheating practice while producing the desiredaustenite grain growth inhibition during TMCP. Referring to FIG. 1 A, a relativelylow reheating température, preferably between about 955°C and about 1065°C(1750°F - 1950°F), is used to obtain initially an average austenite grain size D1 of lessthan about 120 microns in reheated Steel slab 10' before hot deformation. Processingaccording to this invention avoids the excessive austenite grain growth that resultsfrom the use of higher reheating températures, i.e., greater than about 1095°C(2000°F), in conventional TMCP. To promote dynamic recrystallization inducedgrain refining, heavy per pass réductions greater than about 10% are employed duringhot rolling in the température range in which austenite recrystallizes. Referring nowto FIG. IB, processing according to this invention provides an average prior austenitegrain size D" (i.e., d ) of less than about 30 microns, preferably less than about 20microns, and even more preferably less than about 10 microns, in Steel slab 10" afteThot rolling (deformation) in the température range in which austenite recrystallizes,but prior to hot rolling in the température range in which austenite does notrecrystallize. Additionally, to produce an effective grain size réduction in thethrough-thickness direction, heavy réductions, preferably exceeding about 70%cumulative, are carried out in the température range below about the T^ température but above about the Ar3 transformation température. Referring now to FIG. 1C,TMCP according to this invention leads to the formation of an elongated, pancakegrain structure in austenite in a finish rolled Steel plate 10"' with very fine effectivegrain size D’" in the through-thickness direction, e.g., effective grain size D"’ less thanabout 10 microns, preferably less than about 8 microns, and even more preferably lessthan about 5 microns, thus enhancing the interfacial area of high angle boundaries,e.g., 11, per unit volume in Steel plate 10"', as will be understood by those skilled inthe art.

In somewhat greater detail, a Steel according to this invention is prepared byforming a slab of the desired composition as described herein; heating the slab to atempérature of from about 955°C to about 1065°C (1750°F - 1950°F); hot rolling theslab to form Steel plate in one or more passes providing about 30 percent to about 70 11 • 011422 percent réduction in a first température range in which austenite recrystallizes, i.e.,above about the Tm· température, and further hot rolling the Steel plate in one or morepasses providing about 40 percent to about 80 percent réduction in a secondtempérature range below about the température and above about the Ar3 5 transformation température. The hot rolled Steel plate is then quenched at a coolingrate of about 10°C per second to about 40°C per second (18°F/sec - 72°F/sec) to asuitable QST below about the Ms transformation température plus 200°C (360°F), at which time the quenching is terminated. In one embodiment of this invention, theSteel plate is then air cooled to ambient température. This processing is used to 10 produce a microstructure preferably comprising predominantly fine-grained lathmartensite, fine-grained lower bainite, or mixtures thereof, or, more preferablycomprising substantially 100% fine-grained lath martensite.

The thus direct quenched martensite in steels according to this invention hashigh strength but its toughness can be improved by tempering at a suitable 15 température from above about 400°C (752°F) up to about the Aci transformation température. Tempering of Steel within this température range also leads to réductionof the quenching stresses which in tum leads to enhanced toughness. Whiletempering can enhance the toughness of the Steel, it normally leads to substantial lossof strength. In the présent invention, the usual strength loss from tempering is offset 20 by inducing precipitate dispersion hardening. Dispersion hardening from fine copperprécipitâtes and mixed carbides and/or carbonitrides are utilized to optimize strengthand toughness during the tempering of the martensitic structure. The uniquechemistry of the steels of this invention allows for tempering within the broad rangeof about 400°C to about 650°C (750°F - 1200°F) without any significant loss of the 25 as-quenched strength. The Steel plate is preferably tempered at a temperingtempérature from above about 400°C (752°F) to below the Aci transformationtempérature for a period of time sufficient to cause précipitation of hardening particles(as defined herein). This processing facilitâtes transformation of the microstructure ofthe Steel plate to predominantly tempered fine-grained lath martensite, tempered 30 fine-grained lower bainite, or mixtures thereof. Again, the period of time sufficient tocause précipitation of hardening particles dépends primarily on the thickness of the 01 1422 12

Steel plate, the chemistry of the Steel plate, and the tempering température, and can bedetermined by one skilled in the art.

As is understood by those skilled in the art, as used herein percent réduction inthickness refers to percent réduction in the thickness of the Steel slab or plate prior to the 5 réduction referenced. For purposes of explanation only, without thereby limiting thisinvention, a Steel slab of about 25.4 cm (10 inches) thickness may be reduced about 50%(a 50 percent réduction), in a first température range, to a thickness of about 12.7 cm (5inches) then reduced about 80% (an 80 percent réduction), in a second températurerange, to a thickness of about 2.5 cm (1 inch). As used herein, “slab” means a piece of 10 Steel having any dimensions.

The Steel slab is preferably heated by a suitable means for raising the températureof substantially the entire slab, preferably the entire slab, to the desired reheatingtempérature, e.g., by placing the slab in a fumace for a period of time. The spécifiereheating température that should be used for any Steel composition within'the range of 15 the présent invention may be readily determined by a person skilled in the art, either byexperiment or by calculation using suitable models. Additionally, the fumacetempérature and reheating time necessaiy to raise the température of substantially theentire slab, preferably the entire slab, to the desired reheating température may be readilydetermined by a person skilled in the art by reference to standard industry publications. 20 Except for the reheating température, which applies to substantially the entire slab, subséquent températures referenced in describing the processing method of thisinvention are températures measured at the surface of the Steel. The surfacetempérature of Steel can be measured by use of an optical pyrometer, for example, orby any other device suitable for measuring the surface température of Steel. The 25 cooling rates referred to herein are those at the center, or substantially at the center, ofthe plate thickness; and the Quench Stop Température (QST) is the highest, orsubstantially the highest, température reached at the surface of the plate, afterquenching is stopped, because of heat transmitted from the mid-thickness of the plate.For example, during processing of experimental heats of a Steel composition 30 according to this invention, a thermocouple is placed at the center, or substantially atthe center, of the Steel plate thickness for center température measurement, while thesurface température.is measured by use of an optical pyrometer. A corrélation 13 011422 between center température and surface température is developed for use duringsubséquent processing of the same, or substantially the same, Steel composition, suchthat center température maÿ be determined via direct measurement of surfacetempérature. Also, the required température and flow rate of the quenching fluid to 5 accomplish the desired accelerated cooling rate may be determined by one skilled inthe art by reference to standard industry publications.

For any Steel composition within the range of the présent invention, thetempérature that defines the boundary between the recrystalüzation range andnon-recrystallization range, the Tm- température, dépends on the chemistry of the steel, 10 particularly the carbon concentration and the niobium concentration, on the reheatingtempérature before rolling, and on the amount of réduction given in the rolling passes.Persons skilled in the art may détermine this température for a particular Steel accordingto this invention either by experiment or by model calculation. Similarly. the Aci, Ar3,and Ms transformation températures referenced herein may be determined by persons 15 skilled in the art for any Steel according to this invention either by expenment or bymodel calculation.

Although the microstructural approaches described above arc uscful forlowering DBTT in the base Steel plate, they are not fully effective for maintainingsufficiently low DBTT in the coarse grained régions of the weld HAZ. Thus, the 20 présent invention provides a method for maintaining sufficiently low DBTT in thecoarse grained régions of the weld HAZ by utilizing intrinsic efïects of alloyingéléments, as described in the following.

Leading feiritic cryogénie température steels are generally based onbody-centered cubic (BCC) crystal lattice. While this crystal system offers the 25 potential for providing high strengths at low cost, it suffers ffom a steep transitionfrom ductile to brittle fracture behavior as the température is lowered. This can befundamentally attributed to the strong sensitivity of the critical resolved shear stress(CRSS) (defined herein) to température in BCC Systems, wherein CRSS rises steep lywith a decrease in température thereby making the shear processes and consequently 30 ductile fracture more difficult. On the other hand, the critical stress for brittle fractureprocesses such as cleavage is less sensitive to température. Therefore, as the 011422 14 température is lowered, cleavage becomes the favored fracture mode, leading to tbeonset of low energy brittle fracture. The CRSS is an intrinsic property of the Steel andis sensitive to the ease with which dislocations can cross slip upon deformation; thatis, a steel in which cross slip is easier will also hâve a low CRSS and hence a low 5 DBTT. Some face-centered cubic (FCC) stabilizers such as Ni are known to promotecross slip, whereas BCC stabilizing alloying éléments such as Si, Al, Mo, Nb and Vdiscourage cross slip. In the présent invention, content of FCC stabilizing alloyingéléments, such as Ni and Cu, is preferably optimized, taking into account costconsidérations and the bénéficiai effect for lowering DBTT, with Ni alloying of 10 preferably at least about 1.0 wt% and more preferably at least about 1.5 wt%; and thecontent of BCC stabilizing alloying éléments in the steel is substantially minimized.

As a resuit of the intrinsic and microstructural toughening that results from theunique combination of chemistry and processing for steels according to this invention,the steels hâve excellent cryogénie température toughness in both the base plate and 15 the HAZ after welding. DBTTs in both the base plate and the HAZ after welding ofthese steels are lower than about -73°C (-100°F) and can be lower than about -107°C(-160°F). (2) Tensile Strength greater than 830 MPa (120 ksi) and Through-Thickness 20 Uniformity of Microstructure and Properties

Generally, upon tempering, plain carbon and low alloy martensitic steels withno strong Carbide formers soften or lose their as-quenched strength, the degree of thisstrength loss being a function of the spécifie chemistry of the steel and of the 25 tempering température and duration. In the steels of the présent invention, the loss instrength during tempering is substantially ameliorated by fine précipitation ofhardening particles. The unique chemistry of the steels of this invention allows fortempering within the broad range of about 400°C to about 650°C (75O°F - 1200°F)without any significant loss of the as-quenched strength. Within this broad tempering 30 range, strengthening results from hardening particle précipitation occurring or peakingat various température régimes; i.e., within this broad range, sufficient précipitation ofhardening particles occurs to provide cumulative strength adéquate to compensate for 15 011422 the loss of strength normally associated with tempering. The processing flexibilityprovided.by the ability to temper within this broad range is advantageous.

In the présent invention, the desired strength is obtained at a relatively lowcarbon content with the attendant advantages in weldability and excellent toughness 5 in both the base steel and in the HAZ. A minimum of about 0.04 wt% C is preferredin the overall alloy for attaining tensile strength greater than 830 MPa (120 ksi).

While alloying éléments, other than C, in steels according to this invention aresubstantially inconsequential as regards the maximum attainable strength in the steel,these éléments are désirable to provide the required through-thickness uniformity of 10 microstructure and strength for plate thickness greater than about 2.5 cm (1 inch) andfor a range of cooling rates desired for processing flexibility. This is important as theactual cooling rate at the mid section of a thick plate is lower than that at the surface.The microstructure of the surface and center can thus be quite different unless thesteel is designed to eliminate its sensitivity to the différence in cooling rate between 15 the surface and the center of the plate. In this regard, Mn and Mo alloying additions, and especially the combined additions of Mo and B, are particularly effective. In theprésent invention, these additions are optimized for hardenability, weldability, lowDBTT and cost considérations. As stated previously in this spécification, from thepoint of view of lowering DBTT, it is essential that the total BCC alloying additions 20 be kept to a minimum. The preferred chemistry targets and ranges are set to meetthese and the other requirements of this invention. (3) Superior Weldability For Low Heat Input Welding 25 The steels of this invention are designed for superior weldability. The most important concem, especially with low heat input welding, is cold cracking orhydrogen cracking in the coarse grained HAZ. It has been found that for steels of theprésent invention, cold cracking susceptibility is critically affected by the carboncontent and the type of HAZ microstructure, not by the hardness and carbon 30 équivalent, which hâve been considered to be the critical parameters in the art. Inorder to avoid cold cracking when the steel is to be welded under no or low preheat(lower than about 100°C (212°F)) welding conditions, the preferred upper limit ίοτ 011422 16 carbon addition is about 0.1 wt%. As used herein, without limiting this invention inany aspect, “low heat input welding” means welding with arc energies of up to about 2.5 kilojoules per millimeter (kJ/irun) (7.6 kJ/inch).

Lower bainite or auto-tempered lath martensite microstructures offer superiorrésistance to cold cracking. Other alloying éléments in the steels of this invention arecarefiilly balanced, commensurate with the hardenability and strength requirements,to ensure the formation of these désirable microstructures in the coarse grained HAZ. Rôle of Alloying Eléments in the Steel Slab

The rôle of the various alloying éléments and the preferred limits on theirconcentrations for the présent invention are given below:

Carbon (C) is one of the most effective strengthening éléments in Steel. It alsocombines with the strong Carbide formers in the Steel such as Ti, Nb, V and Mo toprovide grain growth inhibition and précipitation strengthening during terapering.Carbon also enhances hardenability, i.e., the ability to form harder and strongermicrostructures in the Steel during cooling. If the carbon content is less than about0.04 wt%, it is not sufficient to induce the desired strengthening, viz., greater than 830MPa (120 ksi) tensile strength, in the Steel. If the carbon content is greater than about0.12 wt%, the Steel will be susceptible to cold cracking during welding and thetoughness is reduced in the Steel plate and iis HAZ on welding. Carbon content in therange of about 0.04 wt% to about 0.12 wt% is preferred to produce the desiredstrength and HAZ microstructures, viz., auto-tempered lath martensite and lowerbainite. Even more preferably, the upper limit for carbon content is about 0.07 wt%.

Manganèse (Mn) is a matrix strengthener in steels and also contributesstrongly to the hardenability. A minimum amount of 0.5 wt% Mn is preferred forachieving the desired high strength in plate thickness exceeding about 2.5 cm (1 inch),and a minimum of at least about 1.0 wt% Mn is even more preferred. However, toomuch Mn can be harmful to toughness, so an upper limit of about 2.5 wt% Mn ispreferred in the présent invention. This upper limit is also preferred to substantiallyminimize centerline ségrégation that tends to occur in high Mn and continuously caststeels and the attendant through-thickness non-uniformity in microstructure and 17 011422 properties. More preferably, the upper limit for Mu content is about 1.8 wt%. Ifnickel content is increased above about 3 wt%, the desired high strength can beachieved without the addition of manganèse. Therefore, in a broad sense, up to about 2.5 wt% manganèse is preferred.

Silicon (Si) may be added to Steel for deoxidation proposes and a minimum ofabout 0.01 wt% is preferred for this propose. However, Si is a strong BCC stabilizerand thus raises DBTT and also has an adverse effect on the toughness. For thesereasons, when Si is added, an upper limit of about 0.5 wt% Si is preferred. Morepreferably, when Si is added, the upper limit for Si content is about 0.1 wt%. Siliconis not always necessary for deoxidation since aluminum or titanium can perform thesame function.

Niobium (Nb) is added to promote grain refînement of the rolledmicrostructure of the Steel, which improves both the strength and toughness. Niobiumcarbide and carbonitride précipitation during hot rolling serves to retard recrystallization and to inhibit grain growth, thereby providing a means of austenitegrain refînement. Also, précipitation of carbides and carbonitrides of niobium duringtempering provides the desired secondary hardening to offset the strength lossnormally observed in Steel when it is tempered above about 500°C (930°F). For thesereasons, at least about 0.02 wt% Nb is preferred, and at least about 0.03 wt% Nb iseven more preferred. However, Nb is a strong BCC stabilizer and thus raises DBTT.Too much Nb can be harmful to the weldability and HAZ toughness, so a maximumof about 0.1 wt% is preferred. More preferably, the upper limit for Nb content isabout 0.05 wt%.

Vanadium (V) is sometimes added to give précipitation strengthening byforming fine particles of the carbides and carbonitrides of vanadium in the Steel ontempering and in its HAZ on cooling after welding. When dissolved in austenite, Vhas a strong bénéficiai effect on hardenability. When V is added to the steels of theprésent invention, at least about 0.02 wt% V is preferred. However, excessive V willhelp cause cold cracking on welding, and also deteriorate toughness of the base Steeland its HAZ. The V addition, therefore, is preferably limited to a maximum of about0.1 wt%, and even more preferably is limited to a maximum of about 0.05 wt%. 18 011422

Titanium (Ti), when added in a small amount, is effective in forming finetitanium nitride (TiN) particles which refine the grain size in both the rolled structureand the HAZ of the Steel. Thus, the toughness of the Steel is improved. Ti is added insuch an amount that the weight ratio of Ti/N is preferably about 3.4. Ti is a strong 5 BCC stabilizer and thus raises DBTT. Excessive Ti tends to deteriorate the toughnessof the Steel by forming coarser TiN or titanium Carbide (TiC) particles. A Ti contentbelow about 0.008 wt% generally can not provide sufficiently fine grain size or tie upthe N in the Steel as TiN while more than about 0.03 wt% can cause détérioration intoughness. More preferably, the Steel contains at least about 0.01 wt% Ti and no 10 more than about 0.02 wt% Ti.

Aluminum f Al) is added to the steels of this invention for the purpose ofdeoxidation. At least about 0.001 wt% Al is preferred for this purpose, and at leastabout 0.005 wt% Al is even more preferred. Al also ries up nitrogen dissolved in theHAZ. However, Al is a strong BCC stabilizer and thus raises DBTT. If the Al 15 content is too high, i.e., above about 0.05 wt%, there is a tendency to form aluminumoxide (AI2O3) type inclusions, which tend to be harmful to the toughness of the Steeland its HAZ. Even more preferably, the upper limit for Al content is about 0.03 wt%.

Molvbdenum (Mo) increases the hardenability of Steel on direct quenching,especially in combination with boron and niobium. Mo is also désirable for 20 promoting secondary hardening during tempering of the Steel by providing fine M02Ccarbides. At least about 0.1 wt% Mo is preferred, and at least about 0.2 wt% Mo iseven more preferred. However, Mo is a strong BCC stabilizer and thus raises DBTT.Excessive Mo helps to cause cold cracking on welding, and also tends to deterioratethe toughness of the Steel and HAZ, so a maximum of about 0.8 wt% is preferred, and 25 a maximum of about 0.5. wt% is even more preferred.

Chromium (Cr) tends to increase the hardenability of Steel on direct quenching. It also improves corrosion résistance and hydrogen induced cracking(HIC) résistance. Similar to Mo, excessive Cr tends to cause cold cracking inweldments, and also tends to deteriorate the toughness of the Steel and its HAZ, so 30 when Cr is added, a maximum of about 1.0 wt% Cr is preferred. More preferably,when Cr is added the Cr content is about 0.2 wt% to about 0.6 wt%. 19 011422

Nickel (Ni) is an important alloying addition to the steels of the présentinvention to obtain the desired DBTT, especially in the HAZ. It is one of thestrongest FCC stabilizers in Steel. Ni addition to the Steel enhances the cross slip andthereby lowers DBTT. Although not to the same degree as Mn and Mo additions, Ni 5 addition to the Steel also promûtes hardenability and therefore through-thicknessuniformity in microstructure and properties in thick sections (i.e., thicker than about 2.5 cm (1 inch)). For achieving the desired DBTT in the weld HAZ, the minimum Nicontent is preferably about 1.0 wt%, more preferably about 1.5 wt%. Since Ni is anexpensive alloying element, the Ni content of the Steel is preferably less than about 10 3.0 wt%, more preferably less than about 2.5 wt%, more preferably less than about 2.0 wt%, and even more preferably less than about 1.8 wt%, to substantially minimizecost of the Steel.

Copper (Cu) is a useful alloying addition to provide hardening duringtempering -via ε-copper précipitation. Preferably at least about 0.1 wt%, more 15 preferably at least about 0.5 wt%, of Cu is added for this purpose. Cu is also an FCCstabilizer in Steel and can contribute to lowering of DBTT in small amounts. Cu isalso bénéficiai for corrosion and HIC résistance. At higher amounts, Cu inducesexcessive précipitation hardening and can lower the toughness and raise the DBTTboth in the base plate and HAZ. Higher Cu can also cause embrittlement during slab 20 casting and hot rolling, requiring co-additions of Ni for mitigation. For the abovereasons, an upper limit of about 1.5 wt% Cu is preferred, and an upper limit of about1.0 wt% is even more preferred.

Boron ÎB) in small quantifies can greatly increase the hardenability of Steeland promote the formation of Steel microstructures of lath martensite, lower bainite, 25 and ferrite by suppressing the formation of upper bainite both in the base plate and thecoarse grained HAZ. Generally, at least about 0.0004 wt% B is needed for thispuipose. When boron is added to steels of this invention, from about 0.0006 wt% toabout 0.0020 wt% is preferred, and an upper limit of about 0.0010 wt% is even morepreferred. However, boron may not be a required addition if other alloying in the 30 steel provides adéquate hardenability and the desired microstructure.

This step-out combination of properties in the steels of the présent invention provides a low cost enabling technology for certain cryogénie température operations, 20 011422 for example, storage and transport of natural gas at low températures. These newsteels can provide significant material cost savings for cryogénie températureapplications over the current state-of-the-art commercial steels, which generallyrequire far higher nickel contents (up to about 9 wt%) and are of much lower 5 strengths (less than about 830 MPa (120 ksi)). Chemistry and microstructure designare used to lower DBTT and provide uniform mechanical properties in thethrough-thickness for section thicknesses exceeding about 2.5 cm. (1 inch). Thesenew steels preferably hâve nickel contents lower than about 3 wt%, tensile strengthgreater than 830 MPa (120 ksi), preferably greater than about 860 MPa (125 ksi), and 10 more preferably greater than about 900 MPa (130 ksi), ductile to brittle transitiontempératures (DBTTs) below about -73°C (-100°F), and offer excellent toughness atDBTT. These new steels can hâve a tensile strength of greater than about 930 MPa(135 ksi), or greater than about 965 MPa (140 ksi), or greater than about 1000 MPa(145 ksi). Nickel content of these Steel can be increased above about 3 wt% if 15 desired to enhance performance after welding. Each 1 wt% addition of nickel isexpected to lower the DBTT of the Steel by about 10°C (18°F). Nickel content ispreferably less than 9 wt%, more preferably less than about 6 wt%. Nickel content ispreferably minimized in order to minimize cost of the Steel. 20 While the foregoing invention has been described in tenus of one or more preferred embodiments, it should be understood that other modifications may be madewithout departing from the scope of the invention, which is set forth in the followingdaims. 21 011422

Glossarv of terms:

Acj transformation température: 5

Ac3 transformation température: the température at which austenite begins to formduring heating; the température at which transformation of ferriteto austenite is cornpleted during heating; A12O3: 10

Ar3 transformation température: aluminum oxide; the température at which austenite begins totransform to femte during cooling; BCC: 15 cooling rate: body-centered cubic; cooling rate at the center, or substantially at thecenter, of the plate thickness; CRSS (critical resolved shear stress): an intrinsic property of a Steel, sensitive to the20 ease with which dislocations can cross slip upon deformation, that is, a Steel in which cross slip iseasier will also hâve a low CRSS and hence alow DBTT; 25 cryogénie température: any température lower than about -40°C (-40°F); 011422 22 DBTT (Ductile to Brittle T ransition T emperature) : delineates the two fracture régimes in structural steels; at températures below the DBTT, failure tends to occur by low energy cleavage (brittle) 5 fracture, while at températures above the DBTT, failure tends to occur by high energy ductile fracture; FCC: face-centered cubic; 10 grain: an individual crystal in a polycrystalline material; grain boundary: a narrow zone in a métal corresponding to the 15 transition from one crystallographic orientation to another, thus separating one grain from another; hardening particles one or more of ε-copper, M02C, or the carbides 20 and carbonitrides of niobium and vanadium; HAZ: heat affected zone; HIC: hydrogen induced cracking; 25 high angle boundary: a boundary that effectively behaves as a high angle grain boundary, i.e., a boundary that tends to deflect a propagating crack or fracture and, thus, induces tortuosity in a fracture path; 30 011422 23 high angle grain boundary: a grain boundary that séparâtes two adjacent grains whose crystallographic orientations differby more than about 8°; 5 HSLA: high strength, low alloy; intercritically reheated: heated (or reheated) to a température of from about the Aci transformation température toabout the AC3 transformation température; 10 low alloy Steel: a Steel containing iron and less than about 10 wt% total alloy additives; low heat input welding: welding with arc energies of up to about 2.5 15 kJ/mm (7.6 kJ/inch); MA: martensite-austenite; M02C: a form of molybdenum Carbide; 20

Ms transformation température: the température at which transformation of austenite to martensite starts during cooling; predominantly: as used in describing the présent invention, means 25 at least about 50 volume percent; prior austenite grain size: average austenite grain size in a hot-rolled Steel plate prior to rolling in the température range inwhich austenite does not recrystallize; 24 011422 quenching: as used in describing the présent invention,accelerated cooling by any means whereby a fluidselected for its tendency to increase the coolingrate of the Steel is utilized, as opposed to air 5 cooling; Quench Stop Température (QST): the highest, or substantially the highest,température reached at the surface of the plate,after quenching is stopped, because of heat 10 transmitted from the mid-thickness of the plate; slab: a piece of Steel having any dimensions; Sv : total interfacial area of the high angle 15 boundaries per unit volume in Steel plate; tensile strength: in tensile testing, the ratio of maximum load to original cross-sectional area; 20 TiC: titanium Carbide; TiN: titanium nitride; Tut température: the température below winch austenite does not 25 recrystallize; and TMCP: thermo-mechanical controlled rolling Processing.

Claims (10)

1. 011422 25 We Claim: A method for preparing a Steel plate having a DBTT of lower than about-73°C (-100°F) in both said Steel plate and its HAZ, a tensile strength greaterthan 830 MPa (120 ksi), and a microstructure comprising predominantlytempered fine-grained lath martensite, tempered fine-grained lower bainite, ormixtures thereof, said method comprising the steps of: 10 (a) heating a Steel slab to a reheating température (i) sufficiently high tosubstantially homogenize said Steel slab and dissolve substantially ailcarbides and carbonitrides of niobium and vanadium in said Steel slab,and (ii) low enough to establish initial austenite grains having a grainsize of less than about 120 microns in said Steel slab; 15 (b) reducing said Steel slab to form Steel plate in one or more hot rollingpasses in a first température range in which austenite recrystallizes; 20 (c) further reducing said steel plate in one or more hot rolling passes in asecond température range below about the T^· température and aboveabout the Ar3 transformation température; 25 (d) quenching said Steel plate at a cooling rate of about 10°C per second toabout 40°C per second (18°F/sec - 72°F/sec) to a Quench StopTempérature below about the Ms transformation température plus200°C (360°F); (e) stopping said quenching; and (f) tempering said steel plate at a tempering température from about400°C (752°F) to about the Aci transformation température for aperiod of time sufficient to cause précipitation of hardening particles, 30 26 011422 5 2. 3. 10 4. 5. 15 6. 20 25 so as to facilitate transformation of said microstructure of said Steel plateto predominantly tempered fîne-grained lath martensite, temperedfine-grained lower bainite, or mixtures thereof. The method of claim 1 wherein said reheating température of step (a) isbetween about 955°C and about 1065°C (1750°F - 1950°F). The method of claim 1 wherein a réduction in thickness of said Steel slab ofabout 30% to about 70% occurs in step (b). The method of claim 1 wherein a réduction in thickness of said Steel plate ofabout 40% to about 80% occurs in step (c). The method of claim 1 further comprising the step of allowing said Steel plateto air cool to ambient température from said Quench Stop Température priorto tempering said Steel plate in step (f). The method of claim 1 wherein said Steel slab of step (a) comprises iron andthe following alloying éléments in the weight percents indicated: about 0.04% to about 0.12% C, at least about 1% Ni to less than about 9% Ni, about 0.1% to about 1.5% Cu, about 0.1% to about 0.8% Mo,about 0.02% to about 0.1% Nb,about 0.008% to about 0.03% Ti,about 0.001% to about 0.05% Al, andabout 0.002% to about 0.005% N. The method of claim 6 wherein said Steel slab comprises less than about6 wt%Ni. 30 27 011422
2. 8. The method of claim 6 wherein said Steel slab comprises less than about 3 wt% Ni and additionally comprises about 0.5 wt% to about 2.5 wt% Mn.
3. 9. The method of claim 6 wherein said Steel slab further comprises at least one 5 additive selected from the group consisting of (i) up to about 1.0 wt% Cr, (ii) up to about 0.5 wt% Si, (iii) up to about 0.1 wt% V, and (iv) up to about 2.5 wt%Mn.
4. 10. The method of claim 6 wherein said Steel slab further comprises about 0.0004 10 wt% to about 0.0020 wt% B.
5. 11. The method of claim 1 wherein said Steel plate comprises substantially 100%tempered fine-grained lath martensite after the tempering of step (f). 15 12. A steel plate having a DBTT of lower than about -73°C (-100°F) in both said Steel plate and its HAZ, a tensile strength greater than 830 MPa (120 ksi), and amicrostructure comprising predominantly tempered fine-grained lathmartensite, tempered fine-grained lower bainite, or mixtures thereof, andwherein said Steel plate is produced from a reheated Steel slab comprising iron 20 and the following alloying éléments in the weight percents indicated: about 0.04% to about 0.12% G,at least about 1% Ni to less than about 9% Ni,about 0.1% to about 1.5% Cu,about 0.1% to about 0.8% Mo, 25 about 0.02% to about 0.1% Nb, about 0.008% to about 0.03% Ti,about 0.001 % to about 0.05% Al, andabout 0.002% to about 0.005% N.
6. 13. The steel plate of claim 12 wherein said steel slab comprises less than about 6 wt%Ni. 28 011422
7. 14. The Steel plate of claim 12 wherein said Steel slab comprises less than about 3wt% Ni and additionally comprises about 0.5 wt% to about 2.5 wt% Mn.
8. 15. The Steel plate of claim 12 further comprising at least one additive selected 5 from the group consisting of (i) up to about 1.0 wt% Cr, (ii) up to about 0.5 wt% Si, (iii) up to about 0.1 wt% V, and up to about 2.5 wt% Mn.
9. 16. The Steel plate of claim 12 further comprising about 0.0004 wt% to about0.0020 wt% B. 0
10. 17. A method for obtaining a DBTT of lower than about -73°C (-100°F) in theHAZ of Steel plate by adding at least about 1.0 wt% Ni and at least about 0.1wt% Cu, and by substantially minimizing addition of BCC stabilizingéléments.