Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/003—Cementite

Definitions

• the present invention relates to a Ni-containing steel product usable at low temperatures and a method for producing the same. More specifically, the present invention relates to a Ni-containing steel suitable for structural material for low temperature storage tanks such as for LNG (Liquefied Natural Gas), and a method for producing the same.
• LNG Liquefied Natural Gas
• Excellent fracture toughness is required for a steel which is usable at low temperature storage tanks for low temperature substances such as LNG, in view of safety.
• a representative example of the steel to meet such demand is a 9% Ni steel.
• Ni steels have experienced various improvements, including reduction in impurities such as P and S, reduction in C, and the use of a three-step heat treatment process, namely, "quenching (Q), lamellartizing (L) and tempering (T).” Also an attempt has been made with the addition of Mo as an effective alloying element in order to improve the strength and toughness of the Ni-containing steel.
• Q quenching
• L lamellartizing
• T tempering
• Patent Document 1 discloses a 9% Ni steel, which contains 0.04 to 0.5% Mo and has a thickness of 40 mm or more, being produced by a three-step heat treatment process (QLT) or a direct quenching-lamellartizing (DQ-LT).
• QLT three-step heat treatment process
• DQ-LT direct quenching-lamellartizing
• Patent Document 2 discloses a method for producing a 9% Ni steel having a thickness of 40 mm or more, by using a quenching-tempering treatment process (Q-T) or a direct quenching-tempering treatment process (DQ-T).
• Q-T quenching-tempering treatment process
• DQ-T direct quenching-tempering treatment process
• the prices of steel products have been rapidly increasing for many reasons including the rising prices of alloying elements.
• the prices of the 9% Ni steels may particularly rise since they need a large amount of expensive alloying elements such as Ni.
• In order to curtail the steel cost there is a need for development of a cost-reducing, low-Ni steel that has an equivalent or superior property, for example excellent toughness, to that of the 9% Ni steel.
• the state of the art in the low-Ni type steel usable at a low temperature includes the following.
• Patent Document 3 discloses a steel usable at a low temperature containing 4.0 to 7.5% Ni and having an Ms of 370°C or lower.
• the above Patent Document 2 discloses a steel containing 7.5 to 10% Ni and a method for producing the same by using the DQ-LT process.
• Patent Document 4 discloses a steel containing 5.5 to 10% Ni and a method for producing the same by using a continuous casting process.
• Patent Documents 5 and 6 steels containing 1.5 to 9.5% Ni and 0.02 to 0.08% Mo are disclosed.
• Patent Document 1 Japanese Laid-open Patent Publication No. 04-371520 .
• Patent Document 2 Japanese Laid-open Patent Publication No. 06-184630 .
• Patent Document 3 Japanese Laid-open Patent Publication No. 06-136483 .
• Patent Document 4 Japanese Laid-open Patent Publication No. 07-90504 .
• Patent Document 5 Japanese Laid-open Patent Publication No. 09-302445 .
• Patent Document 6 Japanese Laid-open Patent Publication No. 2002-129280 .
• Patent Document 1 gives no detailed conditions for rolling and provides no steel having equivalent properties to those of the steel of the present invention, described later, when the Ni content is more than 6% to less than 8%.
• Patent Document 2 describes a steel containing 7.52 % Ni as a comparative example. Because of an unsuitable chemical composition and producing method, the amount of retained austenite is 1.5%, which is not enough to realize the equivalent properties to those of the 9% Ni steel, thus being referred to as a comparative example.
• Patent Document 3 discloses a method in order to improve toughness in the weld heat affected zone (HAZ). However, it fails to disclose a chemical composition design and a producing method for obtaining base material properties comparable to those of the 9% Ni steel. Moreover, the base material properties themselves are nowhere disclosed in the said document.
• Patent Document 2 describes rolling reductions of 20 to 90% at 700 to 900°C, which is, however, not a rolling reduction per pass.
• the toughness of the steel thus produced falls short of 250 J at -196°C.
• Patent Document 4 describes components capable of continuous casting. However the said patent document fails to disclose a method for producing a base material and its properties. Further, the minimum content of Ni disclosed concretely in the said document is 9.08%, and thus no means are disclosed in order to obtain base material properties equivalent to those of the 9% Ni steel with a low Ni content.
• Patent Documents 5 and 6 disclose the DQ-LT process that discontinues water cooling at 400°C or lower. However, no conditions for the heating temperature and rolling are disclosed. Further, both of the documents disclose no properties for a Ni content of around 7%. Instead, inventive examples of the documents show that as a base material property, the 9% Ni steel has a vTs of lower than -196°C, whereas the vTs of 5.0% Ni steel is -160°C and that of 1.5% Ni steel is -125°C. Thus, the decrease in Ni content has a direct adverse influence on toughness.
• the objective of the present invention is to provide a steel product having equivalent properties to those of the 9% Ni steel with a Ni content lower than the 9% Ni steel, and also to provide a method for producing the same.
• the present inventors in an attempt to accomplish the above objective, conducted an extensive study on the above-described prior arts. As a result, the inventors have found that the prior arts are insufficient in the refinement of the microstructure and also insufficient in securing the amount of retained austenite. That is to say, there is a need for the means to make the base material microstructure itself fine while at the same time stabilizing austenite with a Ni content lower than the 9% Ni steel.
• the first means is to introduce a lattice defect in the untransformed austenite in order to lower the Mf, in which the martensite transformation finishes.
• the transformation from austenite to martensite is a shearing type transformation, which involves dislocation migration, and the lattice defect in the austenite serves as an obstruction to dislocation migration. This procedure impedes the finishing of the shearing type transformation from austenite to martensite, thereby lowering the Mf. Lowering the Mf increases the amount of the retained austenite at room temperature.
• the second means is related to refinement of the untransformed austenite phase.
• the minimum unit (lath) of instantaneously formed martensite is approximately 0.5 to 1 ⁇ m in the thickness direction.
• This transformation in reality involves an increase in volume. This leads to the finding that if the size of the untransformed austenite phase is equal to or smaller than the minimum unit of the instantaneously formed martensite, the volume-increasing martensite transformation is significantly inhibited and consequently the said untransformed austenite phase exists more stable than would be expected from the actual amount of the chemical composition.
• the retained austenite in a steel of low Ni content thus obtained is not only comparable in the amount to that was obtained in the conventional quenched-tempered material obtained from the 9% Ni steel, but also is characterized in the following respect.
• the austenite in the low Ni steel is an extremely fine granular structure in a two-dimensional view, even though the overall amount of the austenite is substantially the same as that of the austenite in the 9% Ni steel. For this reason, the retained austenite can be secured stably even with a low Ni content.
• the conditions for heating, rolling and cooling are important.
• a high rolling reduction at a low temperature is known to introduce a large amount of lattice defects (dislocations) and to make the resulting microstructure fine.
• the addition of Nb as a trace element is particularly effective. This is based on the fact that the finely precipitated Nb(C,N) impedes the dislocation migration and consequently the lattice defect (dislocation) density in the austenite increases.
• the present invention has been made on the basis of the above findings.
• the gists of the present invention are steel products and methods for producing the said steel products described in the following.
• C is an effective element for lowering the Mf and stabilizing the retained austenite.
• C hardens the martensite matrix itself and thus causes a deterioration of toughness, overwhelming its improvement realized by an increase in the amount of the austenite. Therefore, C is contained to an amount necessary to secure strength or somewhat more than that amount, but it is vital to avoid an excessive C content that might cause a deterioration of toughness. If the content of C is less than 0.01%, the strength is insufficient. On the other hand, if the content of C exceeds 0.1%, the toughness is deteriorated. Accordingly, the content of C is set to 0.01 to 0.1%. A more preferable C content range is 0.03 to 0.07%.
• Si is effective as a deoxidizing element. Also Si is effective as an element to inhibit the precipitation of cementite and improve the stability of the austenite in tempering. However, an excessive content of Si causes a deterioration of toughness. Therefore, the content of Si is set to 0.005 to 0.6%. A more preferable Si content range is 0.03 to 0.5% and further more preferable content range of Si is 0.1 to 0.3%.
• Mn is effective for lowering the Mf and stabilizing the austenite, and the more the Mn content is, the more austenite can be obtained.
• the content of Mn is set to 0.3 to 2%.
• a more preferable lower limit of Mn content is 0.5% and further an even more preferable lower limit of Mn content is 0.7%.
• a more preferable upper limit of Mn content is 1.5% and further an even more preferable upper limit of Mn content is 1.0%.
• Ni more than 6% to less than 8%
• Ni is the most important element in order to enhance the strength of the steel and to contribute to the stability of the austenite.
• a more Ni content level is preferable, since the more the Ni content is, the higher strength can be obtained and moreover the lower Mf, which increases the amount of the retained austenite, can be gained.
• a large amount of Ni causes an increase in cost, and therefore the content of Ni is set to less than 8%.
• a more preferable upper limit of Ni content is 7.5%.
• a more preferable lower limit of Ni content is 6.5%.
• sol.Al 0.005 to 0.05%
• Al is effective as a deoxidizing element and as an element in order to inhibit precipitation of cementite and improve the stability of the austenite in tempering. Further, Al forms AlN with N, and the said AlN has an effect on refinement of the austenite grains during heating. Therefore, the content of 0.005% or more of Al as sol.Al is needed. However, an excessive content of Al causes a deterioration of toughness. Accordingly, the content of Al as sol.Al is set to 0.005 to 0.05%. A more preferable content range of sol.Al is 0.02 to 0.04%.
• N is an element to contribute to the stability of the austenite and therefore it is preferably contained. Further, N forms AlN with Al, and the said AlN has an effect on refinement of the austenite grains during heating. In order to obtain the said effects, the content of 0.005% or more of N is needed. On the other hand, the content of N must be set to 0.005% or less, since an excessive content of N causes a deterioration of the martensite matrix. A more preferable content range of N is 0.002 to 0.004%.
• One of the steel products of the present invention is a steel product, which contains the above-described components and the balance of Fe and impurities.
• Another steel product of the present invention is a steel product containing, in addition to the above-described components, one or more selected from among Mo, Cu, Cr, V, Nb, Ti, B, Ca and Mg. These components will be described below.
• Mo is an austenite stabilizing element and effective for increasing the amount of the austenite.
• the content of Mo must be set to 0.1% or less, since if the content of Mo exceeds 0.1% it causes a deterioration of toughness of the martensite matrix.
• a more preferable lower limit of Mo content is 0.02%.
• a more preferable upper limit of Mo content is 0.06% and further more preferable upper limit of Mo content is 0.05%.
• the dissolved Cu in the matrix is effective to stabilize the austenite. Therefore, in order to obtain the said effect, it is preferable to contain 0.05% or more of Cu. Although Cu is effective for enhancing strength, it deteriorates toughness, because the dissolved Cu precipitates in the form of ⁇ -Cu by tempering treatment. Accordingly, the upper limit of Cu content is set to 2.0%.
• Cr is an element effective for enhancing strength. In order to obtain this effect, it is preferable to contain 0.05% or more of Cr. However, if the content of Cr exceeds 0.8%, toughness is deteriorated. Therefore, the upper limit of Cr content is set to 0.8%.
• V 0.08% or less
• V is an element effective for enhancing steel strength, that is, it forms precipitates on tempering treatment and strengthens the steel.
• the content of V is preferably set to 0.005% or more.
• the content of V exceeds 0.08%, the said precipitates become excessive and they deteriorate toughness. Therefore, the content of V is set to 0.08% or less.
• Nb enlarges the non-recrystallization temperature region in rolling and thus is effective for the refinement of microstructures after rolling and enhancement of toughness. In order to obtain these effects, it is preferable to contain 0.005% or more of Nb. However, if the content of Nb exceeds 0.08%, toughness is deteriorated. Accordingly, the upper limit of Nb content is set to 0.08%.
• Ti is an element effective for preventing cracks of slab. In order to obtain this effect, it is preferable to contain 0.005% or more of Ti. However, if the Ti content exceeds 0.03%, toughness is deteriorated. Therefore, the upper limit of Ti content is set to 0.03%.
• B is an element effective for enhancing strength. In order to obtain this effect, it is preferable to contain 0.0002% or more of B. However, if the B content exceeds 0.0030%, toughness is deteriorated. Accordingly, the upper limit of B content is set to 0.0030%.
• Ca is an element effective for improving toughness. In order to obtain this effect, it is preferable to contain 0.0002% or more of Ca. However, if the Ca content exceeds 0.0050%, toughness is deteriorated. Therefore, the upper limit of Ca content is set to 0.0050%.
• Mg is an element effective for improving toughness.
• the content of Mg is preferably set to 0.0005% or more.
• the upper limit of Mg content is set to 0.0050%.
• a certain amount of the austenite in the steel product serves as important means to improve toughness with a low Ni content.
• it In order to obtain a low Ni steel with equivalent toughness to that of the 9% Ni steel, it must contain austenite of 1.7% or more in area ratio.
• a more preferable lower limit of the amount of the austenite is 2.0% and further more preferable its lower limit is 3.0%.
• the upper limit of the amount of the austenite is not specified, since the more the austenite there is, the more effectively the toughness is improved. However, an amount that exceeds 40% causes a lack of strength. Therefore, it is preferable to set the upper limit of the amount of the austenite at 40%.
• the austenite phase is fine.
• a preferable aspect ratio is 2.5 or less.
• the above circle-equivalent grain diameter refers to the diameter of a circle of an equivalent area to the projected area of the austenite.
• a microstructure which is observed as a result of cutting off the steel product along a plane parallel to the rolling direction (vertical direction to the thickness) is measured.
• the projected area of the austenite can be measured with an image analyzing apparatus.
• Cementite precipitates from the martensite matrix and is also formed by a decomposition of the untransformed austenite.
• the above precipitation of cementite decreases strength and deteriorates toughness. Therefore, it is necessary to keep the size of cementite at 0.6 ⁇ m or less in the average circle-equivalent diameter.
• the average circle-equivalent diameter of cementite is the same as described above. That is, regarding the circle-equivalent diameter of cementite, the measurement is made for cementite instead of austenite grain.
• the heating temperature of the steel slab prior to rolling is set at 850 to 1050°C. Heating at lower than 850°C causes a lack of strength, on the other hand, heating at higher than 1050°C deteriorates toughness. It is preferable to set the said heating temperature at 900 to 1000°C.
• the rolling which has a rolling reduction of 5% or more per pass and a cumulative rolling reduction of 25% or more, in a temperature region of 700 to 830°C is necessary for introducing a lattice defect (dislocation) in austenite in the non-recrystallization temperature region thereof and thereby inhibiting the untransformed austenite from transforming to martensite.
• the said rolling must be finished within a temperature region of 700 to 800°C. If the finishing temperature is lower than 700°C, the anisotropy of the steel product becomes noticeable. If the finishing temperature exceeds 800°C, toughness is deteriorated.
• a cooling rate of 10°C/s or higher from the starting temperature of the said accelerated cooling to at least 600°C is required. The purpose of this is to maximize the amount of the lattice defects (dislocations) which are introduced in finish rolling. Also in order to obtain a martensite phase, a cooling rate of 5°C/s or higher from the said starting temperature of the accelerating cooling to 200°C is required. If the said accelerated cooling is finished at a temperature higher than 200°C, martensite cannot be sufficiently obtained which results in deterioration of strength.
• the time from the above-mentioned roll finishing to the starting of the above accelerated cooling is as short as possible. A preferable period of time from the above roll finishing to the starting of the above accelerated cooling is 30 seconds or less.
• the resulting product After the said accelerating cooling, the resulting product must be tempered at a temperature of 650°C or lower.
• the martensite, which was formed by the cooling treatment, that is, quenching, can be tempered by this treatment.
• the said tempering treatment it is possible to adjust strength and at the same time to improve toughness. If the tempering treatment is carried out at a temperature higher than 650°C, strength is deteriorated.
• the lamellartizing is necessary to heat the resulting product in a temperature region of 600 to 800°C and then to cool the same to a temperature region of 200°C or lower, at a cooling rate of 5°C/s or higher.
• a more preferable heating temperature region of the said lamellartizing is 680 to 750°C.
• the "inventive examples” shown in Table 3 are those having the chemical composition specified in the present invention and produced by the method according to the present invention.
• the inventive examples also satisfy the above-described formulas (1) and (2), and the inventive conditions for the area ratio and configuration of austenite and moreover the configuration of cementite.
• Each of the inventive examples has a YS of 585 MPa or more, a TS of 690 to 825 MPa, and a Charpy impact energy of 250 J or more at -196°C.
• Toughness is particularly improved to have an absorbed energy of 290 J or more in the cases (testing numbers T2, T4, T6, T7, T8, T10, T13 and T15) where the examples, which contain austenite of 1.7% or more in area ratio, satisfy both of the two microstructural requirements: (1) the austenite has an aspect ratio of 3.5 or less in average and an average circle-equivalent grain diameter of 1.0 ⁇ m or less, and (2) cementite has an aspect ratio of 5.0 or less in average and an average circle-equivalent diameter of 0.6 ⁇ m or less.
• the comparative examples where any one of the conditions such as chemical composition is outside the inventive ranges, have low impact energy, resulting in insufficient low temperature toughness.
• the present invention provides a steel product that has equivalent or superior mechanical properties to those of the steel containing 9% Ni even though it has a low Ni content of more than 6% to less than 8%.
• the steel product has excellent low temperature toughness as well as low cost, and therefore it is suitable for structural material for storage tanks for low temperature substances such as LNG.

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