Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-08-30
2002-03-19
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C148S334000, C148S335000, C420S106000, C420S110000, C420S111000
Reexamination Certificate
active
06358336
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a heat-resistant Cr—Mo alloy steel which has excellent high-temperature strength and toughness and which is suitable for use in steel tubes for heat exchangers and piping, heat-resistant valves, and joints employed in the field of boiler, chemical and atomic industries. The invention also relates to a process for producing the steel.
2. Description of the Related Art
Heat-resistant steels which are used at temperatures as high as 400° C. or more are broadly classified into four types: (1) austenitic stainless steel; (2) high-Cr ferritic steel containing 9-12% Cr; (3) Cr—Mo alloy steel containing a few % Cr; and (4) carbon steel.
Steels of these types are appropriately selected in consideration of economical advantage and service conditions, such as temperature and pressure, under which the steel is to be used.
Among these steels, Cr—Mo alloy steel is a heat-resistant steel which typically contains a few % of Cr, and Mo and W as the optional alloying elements and has a tempered martensite or tempered bainite structure.
Cr—Mo alloy steel, due to the element Cr contained, is characterized by its superiority to carbon steel in terms of excellent oxidation resistance, high-temperature corrosion resistance, and high-temperature strength. Cr—Mo alloy steel is inexpensive, has a small thermal expansion coefficient, and has excellent toughness, weldability, and thermal conductivity.
High-temperature strength is a very important property in designing pressure member (i.e., material to be used in under high pressure), and steels for producing pressure member should preferably have high strength regardless of the temperature at which the steel is to be used. Particularly, the wall thickness of heat- and pressure-resistant steel tubes employed in the boiler, chemical and atomic industries is determined in accordance with the high-temperature strength of the steel.
High-temperature strength of Cr—Mo alloy steel is improved by solution strengthening and precipitation strengthening. Typically, solution strengthening is attained by adding appropriate amounts of C, Cr, Mo, and W into steel, to thereby improve high-temperature strength. However, when the thus-strengthened steel is used at high temperature for a long period of time, carbide particles are coarsened and intermetallic compounds precipitate, thereby lowering creep strength under high-temperature conditions and after passage of a prolonged period of time. In order to enhance high-temperature strength, an increase in amounts of solute elements is a possible means for potentiating solution strengthening. However, addition of solute elements beyond their solubility limit causes precipitation of these elements, thereby lowering ductility, workability, and weldability.
Precipitation strengthening is attained by adding precipitation-strengthening elements such as V, Nb, and Ti Such Cr—Mo steels are disclosed in, for example, Japanese Patent Application Laid-Open (kokai) Nos. 57-131349, 57-131350, 59-226152, and 8-158022 and some of them have already been put into practical use. In addition, as precipitation-strengthened Cr—Mo alloy steels, 1Cr—1Mo-0.25V steel serving as turbine material and 2.25Cr—1Mo—Nb steel serving as material used for a fast-breeder reactor are well known.
Japan Kohyo Patent Publication No. 11-502259 discloses heat-resistant 0.5-1.5% Cr-0.1-1.15% Mo ferritic steel to which the following elements have been added: V and Nb serving as precipitation-strengthening elements; B serving as a control element of a matrix structure; and optionally W and Ti.
However, in case of precipitation strengthening, the control of microstructure is difficult, and the following problems arise:
(a) Although strengthened steel as produced or strengthened steel which is used at high temperature for only a short period of time exhibits high strength, the strengthening effect deteriorates when these steels are exposed to high temperature for 10,000 hours or more, and thus high-temperature strength deteriorates. Carbides and nitrides precipitated in as-produced steel or short-time-served steel are effective for precipitation strengthening. However, these precipitates are coarsened by an aging which occurs during a long term use at high-temperature, and strengthening effect deteriorates; and
(b) Since precipitation-strengthened steels strengthen inside grains, strength of grain boundaries becomes relatively weak, thereby lowering toughness, ductility and corrosion resistance.
If high-temperature strength of Cr—Mo alloy steel can be further enhanced, the following advantages are obtained:
1) Conventionally, austenitic stainless steel or high-Cr ferritic steel has been employed so as to ensure high-temperature strength even under conditions of use which do not require strict high-temperature corrosion resistance. If Cr—Mo alloy steel of improved high-temperature strength is employed in place of these steels, there can be obtained beneficial properties inherent to Cr—Mo steel, such as excellent weldability, thermal conductivity, fatigue resistance, and low cost;
2) The thickness of conventionally used steel product can be reduced, thereby elevating thermal conductivity and improving thermal efficiency of plants. In addition, thermal stress caused by startup and shutdown of plants can be mitigated; and
3) the decrease in weight of steel products due to reduction of thickness results in size-reduction of plants and reduction of production costs.
SUMMARY OF THE INVENTION
In view of the foregoing, an object of the present invention is to provide a Cr—Mo alloy steel which exhibits high creep strength at temperatures as high as approximately 400-600° C.; which maintains strength even when the steel is used for long periods within such a temperature range; which further exhibits suppressed temper embrittlement; and which has excellent toughness. Another aspect of this invention is to provide a process for producing the steel. The summary of the invention will be described next. Accordingly, the present invention provides the following [1] to [3].
[1] a Cr—Mo alloy steel which comprises, on a mass % basis,
C: 0.01-0.25%,
Si: 0.01-0.7%
Mn: 0.01-1%,
P: 0.03% or less,
S: 0.015% or less,
Cr: 0.1-3%,
Nb: 0.005-0.2%,
Mo: 0.01-2.5%,
Ca: 0.0001-0.01%,
N: 0.0005-0.01%,
B: 0.0001-0.01%, and
which satisfies the following expression:
0.1≦Nb+Mo+V
wherein each element symbol denotes content thereof (mass %), wherein MX-type complex precipitates formed inside grains of the steel contain 30 mass % or more of Mo and 7 mass % or more of Nb.
[2] a process for producing Cr—Mo alloy steel product which has excellent high-temperature strength and toughness, which process comprises: casting a Cr—Mo alloy steel having a chemical composition as described in [1] into a product; optionally forging and hot-working the product; normalizing the as cast, forged or hot-worked product at 950° C. or higher; cooling the product to room temperature; and tempering the product, wherein cooling in the temperature range of 850° C. to 650° C. is carried out at an average cooling rate equal to or faster than both a cooling rate A represented by the following equation (1) and a cooling rate B represented by the following equation (2), and tempering is carried out in a temperature range defined by the following equations (3) and (4):
A=0.6×log(Nb)+1.24 (1);
B=0.1×log(C+N)+0.3 (2);
C=780−125×Mo/(Mo+Nb) (3); and
D=780+100×Nb/(Mo+Nb) (4).
[3] a process for producing Cr—Mo alloy steel product which has excellent high-temperature strength and toughness, which process comprises: hot-rolling a Cr—Mo alloy steel having a chemical composition as described in [1] into a product; finishing the product in a temperature range of 1100° C. to 900° C.; cooling the product to 200° C. or lower; and tempering the produ
Clark & Brody
Sumitomo Metal Industries Ltd.
Yee Deborah
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