Weldable low-chromium ferritic cast steel, having excellent high-temperature strength

Abstract

 This invention relates to low-Cr ferritic cast steels having excellent high-temperature strength, weldability, oxidation resistance and high-temperature corrosion resistance. These low-Cr ferritic cast steels consist essentially of, on a weight percentage basis, 0.03 to 0.12% C, 0.03 to 0.7% Si, 0.02 to 1% Mn, up to 0.3% Co, up to 0.025% P, up to 0.015% S, 0.8 to 3% Cr, 0.01 to 1% Ni, 0.01 to 0.5% V, 0.1 to 3% W, 0.01 to 0.2% Nb, 0.001 to 0.05% Al, 0.0001 to 0.02% B, 0.001 to 0.05% N, up to 0.03% O, 0.0005 to 0.05% Mg, and the balance being iron and incidental impurities, provided that the Mg content satisfies the following inequality as expressed on a weight percentage basis:

As a result, they exhibit excellent weldability and markedly improved high-temperature strength.

Description

BACKGROUND OF THE INVENTION

Field of the invention

[0001] This invention relates to low-Cr ferritic cast steels which have excellent high-temperature strength, weldability, oxidation resistance and high-temperature corrosion resistance and are suitable for use as cast materials, especially for use in a high-temperature environment at or above 450°C, in the fields of boilers, nuclear power industry, chemical industry and the like.

Description of the related art

[0002] Materials for use as heat-resistant and pressure-tight members in various types of equipment in the fields of boilers, nuclear power industry, chemical industry and the like include austenitic steels, high-Cr ferritic steels having a Cr content of 9 to 12%, low-Cr ferritic steels having a Cr content of 3.5% or less (e.g., 2.1/4Cr-1Mo steel), and carbon steel. These materials are suitably selected according to the service temperature, pressure and atmosphere for the particular member and with consideration for economic efficiency. Among others, high-Cr ferritic steels having a Cr content of 9 to 12% and low-Cr ferritic steels having a Cr content of 3.5% or less have been extensively investigated with respect to alloy systems containing various trace elements. As a result, ferritic steels having high-temperature strength equal to or greater than that of austenitic steels have been developed. However, most of them are intended to be used after being worked by forging, rolling or the like, and there are very few materials (such as cast steels) which are used without requiring forging and rolling. The reason for this seems to be that it has been difficult to develop a material having excellent overall performance from the viewpoint of high-temperature strength, weldability, impact resistance, economic efficiency and the like.

[0003] As compared with forged steels, cast steels have the advantage that they can be easily formed into articles of complicated shapes without requiring a forging step and hence involve a less working cost. With the recent progress of casting techniques, the reliability of cast steels which was apprehended in the past has made a marked improvement. Accordingly, there is a need for an inexpensive cast steel having excellent high-temperature strength and weldability.

[0004] As described above, the existing Cr-containing ferritic cast steels have the following problems: (1) Low-Cr ferritic cast steels tend to develop a material deterioration due to the production of porosity and high-temperature cracking, especially in thick-walled members. (2) Their high-temperature creep strength at 450°C or above is low. (3) They have poor impact resistance. (4) They require preheating prior to welding.

[0005] Accordingly, an object of the present invention is to provide low-Cr ferritic cast steels which develops no casting defect even in thick-walled members, show a marked improvement in high-temperature strength (particularly high-temperature creep strength) at 450°C or above as compared with conventional cast steels, have performance equal to or higher than that of the existing forged steels with respect to toughness and weldability, and achieve high economic efficiency.

SUMMARY OF THE INVENTION

[0006] The present inventors have made an effort to solve the above-described problems on the basis of the fundamental conceptions that (1) internal defects should be minimized even in thick-walled cast steels, (2) creep strength at 450°C or above should be improved as a result of precipitation hardening by V and Nb and solid solution strengthening by W, Mo and Cu, and (3) weldability should be improved by controlling the contents of C, Mn and B. As a result, the following facts have been found.

[0007] Low-Cr ferritic cast steels most probably tend to suffer from the macrosegregation of S, and this tendency becomes more pronounced in large ingots and weakly deoxidized materials. Even if sufficient deoxidation is effected, porosity tends to be concentrated in the parts where the macrosegregation of S occurs. Consequently, the macrosegregation of S also needs to be suppressed for the purpose of minimizing material deterioration due to porosity. Moreover, the macrosegregation of S causes the following problems: (1) the promotion of high-temperature cracking, for example, during welding, (2) a reduction in oxidation resistance and high-temperature corrosion resistance due to the destabilization of Cr₂O₃ film, and (3) a reduction in grain boundary strength.

[0008] Accordingly, the present inventors have investigated various methods for suppressing the segregation of S in low-Cr ferritic cast steels, and have discovered the following solution.

[0009] S can be stabilized by effecting sufficient deoxidation with Al and, at the same time, adding Mg having a strong affinity for S. Thus, the macrosegregation and microsegregation of S can be markedly suppressed. As a result, internal defects and high-temperature cracking during welding which are caused by the segregation of S can be minimized.

[0010] Besides Mg, Ca and rare earth elements are also effective for the stabilization of S. However, in the low-Cr ferritic cast steels of the present invention which are used at high temperatures, it is also important to secure the stability of scale at high temperatures. Since Mg also has the effect of stabilizing scale of, e.g, Cr₂O₃, it is desirable to add Mg for the purpose of stabilizing S. When Mg is added, its effect is governed by the balance between the Mg content and the S, O and Al contents. Accordingly, the Mg content must satisfy the following inequality:

That is, Mg has not only the effect of stabilizing S in the form of MgS, but also the effect of stabilizing scales in itself.

[0011] As described above, the present inventors have completed the present invention on the basis of the synergistic effect of a measure for suppressing the segregation of S and an optimization of the contents of other alloying elements.

[0012] That is, the present invention provides low-Cr ferritic cast steels having the compositions defined in paragraphs (1) to (4) below.

(1) A low-Cr ferritic cast steel having excellent weldability and markedly improved high-temperature strength which consists essentially of, on a weight percentage basis, 0.03 to 0.12% C, 0.03 to 0.7% Si, 0.02 to 1% Mn, up to 0.3% Co, up to 0.025% P, up to 0.015% S, 0.8 to 3% Cr, 0.01 to 1% Ni, 0.01 to 0.5% V, 0.1 to 3% W, 0.01 to 0.2% Nb, 0.001 to 0.05% Al, 0.0001 to 0.02% B, 0.001 to 0.05% N, up to 0.03% O, 0.0005 to 0.05% Mg, and the balance being iron and incidental impurities, provided that the Mg content satisfies the following inequality as expressed on a weight percentage basis:

(2) A low-Cr ferritic cast steel having excellent weldability and markedly improved high-temperature strength which further contains 0.01 to 0.2% by weight of one or more elements selected from the group consisting of Ca, Ti, Zr, Y, La, Ce and Ta, in addition to the components described in the above paragraph (1).

(3) A low-Cr ferritic cast steel having excellent weldability and markedly improved high-temperature strength which further contains 0.01 to 3% by weight of Mo in addition to the components described in the above paragraph (1) or (2).

(4) A low-Cr ferritic cast steel having excellent weldability and markedly improved high-temperature strength which further contains 0.1 to 2.5% by weight of Cu in addition to the components described in any of the above paragraphs (1), (2) and (3).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0013] The action of various components in the low-Cr ferritic cast steels of the present invention, and the reasons for the selection of their content ranges are described below. In the following description, all percentages are by weight.

[0014] C combines with Cr, Fe, W, V and Nb and with optionally added Mo and Cu to form carbides and thereby contributes to the improvement of high-temperature strength. At the same time, C itself acts as an austenite-stabilizing element to stabilize the structure. If its content is less than 0.03%, the precipitation of carbides will be insufficient to achieve adequate high-temperature strength. If its content is greater than 0.12%, excessive amounts of carbides will precipitate, resulting in marked hardening of the steel. Accordingly, the proper content of C is in the range of 0.03 to 0.12%. In this range, lower C contents provide better weldability. Consequently, the content of C should preferably be in the range of 0.05 to 0.08%.

[0015] Si is an element which acts as a deoxidizer and improves steam oxidation resistance. If its content is greater than 0.7%, Si will cause a marked reduction in toughness and will be detrimental to creep strength. If its content is less than 0.03%, the melt flowability during casting will become poor. Accordingly, the content of Si should be in the range of 0.03 to 0.7% by weight. Where greater importance is attached to creep strength than to melt flowability, the content of Si should preferably be in the range of 0.03 to 0.30% by weight.

[0016] Mn has desulfurizing and deoxidizing effects, and is effective in stabilizing the structure. If its content is less than 0.02%, no sufficient effect will be produced. If its content is greater than 1%, Mn will harden the steel and enhance sensitivity to temper embrittlement. When the content of S is particularly low, the content of Mn may be reduced. Accordingly, the content of Mn should be in the range of 0.02 to 1%. When the content of S is particularly low, the content of Mn may be in the range of 0.02 to 0.30%.

[0017] Depending on the history of melting, Co may be present as a steel impurity in an amount of up to 0.3%. However, Co will exert no appreciable harmful effect at a content of up to 0.3%. Accordingly, the content of Co as an inevitable impurity should be up to 0.3%. Thus, Co need not be positively added during compositional adjustment.

[0018] Both P and S are elements which are detrimental to toughness. Since even a very slight amount of S destabilizes grain boundaries and Cr₂O₃ scale film and thereby causes a reduction in high-temperature strength and toughness, its content should preferably be as low as possible within the aforesaid limit. Accordingly, the contents of P and S as inevitable impurities should be up to 0.025% and up to 0.015%, respectively.

[0019] Cr is an element which is indispensable from the viewpoint of the oxidation resistance and high-temperature corrosion resistance of low-alloy steels. If its content is less than 0.8%, Cr will fail to produce sufficient oxidation resistance and high-temperature corrosion resistance. On the other hand, Cr added in an amount of greater than 3% will detract from strength and toughness. Accordingly, the content of Cr should be in the range of 0.8 to 3% by weight.

[0020] Ni is an austenite-stabilizing element and contributes to the improvement of toughness. However, if its content is less than 0.01%, no sufficient effect will be produced. If its content is greater than 1%, Ni will detract from high-temperature creep strength. Moreover, the addition of large amounts of Ni is also disadvantageous from an economic point of view. Accordingly, the content of Ni should be in the range of 0.01 to 1% by weight.

[0021] V combines with C and N to form a fine precipitate comprising V(C,N) and the like. This precipitate contributes greatly to the improvement of long-time creep strength at high temperatures. However, if its content is less than 0.01%, no sufficient effect will be produced. If its content is greater than 0.5%, the precipitation of V(C,N) will become excessive and, on the contrary, detract from creep strength and toughness. Accordingly, the proper content of V is in the range of 0.01 to 0.5%.

[0022] W acts as a solid solution strengthening and fine carbide precipitation strengthening element and is effective for the improvement of creep strength. Although Mo has a similar effect, W has a lower diffusion rate in Fe and is hence more excellent in the high-temperature stability of its fine carbide which contributes to the improvement of creep strength. When added in combination with Mo, W brings about a greater improvement in strength, particularly in high-temperature creep strength, than when added alone. If its content is less than 0.1%, no effect will be produced, and if its content is greater than 3%, W will harden the steel and detract from its toughness. Accordingly, the content of W should be in the range of 0.1 to 3%. In this range, the content of W should preferably be in the range of 1.0 to 2.0%.

[0023] Nb, like V, combines with C and N to form Nb(C,N) and thereby contributes to the improvement of creep strength. In particular, Nb shows a marked strength-improving effect at relatively low temperatures of 600°C or below. If its content is less than 0.01%, the above-described effect will not be produced. If its content is greater than 0.2%, Nb will harden the steel significantly and detract from its toughness and weldability. Accordingly, the content of Nb should suitably be in the range of 0.01 to 0.2%. In order to achieve a satisfactory combination of weldability and creep strength, the content of Nb should desirably be in the range of 0.03 to 0.15%.

[0024] Al is an indispensable deoxidizing element and forms a carbonitride. Moreover, Al also has the effect of making the structure finer. If its content is less than 0.001%, no effect will be produced, and if its content is greater than 0.05% by weight, Al will detract from creep strength and workability. Accordingly, the content of Al should be in the range of 0.001 to 0.05% by weight.

[0025] The addition of a very slight amount of B has the effect of dispersing and stabilizing carbides and thereby contributes to the improvement of long-time creep strength. If its content is less than 0.0001%, no sufficient effect will be produced, and if its content is greater than 0.02%, B will detract from workability. Accordingly, B should be added so as to give a B content in the range of 0.0001 to 0.02%. In this range, the addition of B is also effective for the improvement of hardenability. Consequently, it is necessary from the viewpoint of structure control to regulate the amount of B added as required.

[0026] N is necessary for the formation of carbonitrides by combination with V and Nb. If its content is less than 0.001%, no effect will be produced. However, as its content becomes higher, N in solid solution will increase and the nitrides will become coarse, resulting in a reduction in creep strength. Moreover, if its content is greater than 0.05%, N may be responsible for the formation of blow-holes during casting. Accordingly, the content of N should be in the range of 0.001 to 0.05%.

[0027] O increases casting defects such as pipe flaws and blow-holes, and also exerts an adverse influence on toughness and hot workability. Accordingly, the content of O should be up to 0.03% and preferably up to 0.02%.

[0028] Mg is an element which stabilizes S and is effective for the suppression of porosity resulting from the segregation of S during casting, the suppression of weld defects, and the strengthening of grain boundaries. Moreover, Mg is also an important element which stabilizes Cr₂O₃ film and, in the case of Cu addition as will be described later, Cu-O film. However, if its content is less than 0.0005% or does not satisfy the following inequality as expressed on a weight percentage basis:

the desired effect will not be produced. On the other hand, even if Mg is added in an amount greater than 0.05%, its effect will become saturated. Accordingly, the content of Mg should be in the range of 0.0005 to 0.05% and, at the same time, should satisfy the following inequality:

The above inequality means that it is necessary to secure a certain amount of Mg which is not fixed by S or O but exists in solid solution as a free metal. This inequality has been formulated by considering the atomic weights of Mg, S, O and Al to be 24, 32, 16 and 27, respectively.

[0029] Ca, Ti, Zr, Y, La, Ce and Ta combine with P, O and S that are impurities. In order to control the morphology of the resulting precipitates (or inclusions), one of more of these elements are added in very small amounts. The addition of 0.01% of each element makes it possible to free the steel of such impurities as P, O and S, and thereby improve its strength and toughness. This is particularly effective for the improvement of creep strength. However, if the content of each element is greater than 0.2%, the resulting inclusion will increase and, on the contrary, detract from toughness. Accordingly, the content of each of these elements should be in the range of 0.01 to 0.2%.

[0030] Mo, like W, is effective for the improvement of creep strength. However, Mo need not necessarily be added to the steels of the present invention which contain a large amount of W. Nevertheless, Mo produces a strength-improving effect when added in combination with W, and is also effective for the improvement of toughness when added in small amounts. If the content of Mo is less than 0.01%, the above-described effects will not be produced. If its content is greater than 3%, intermetallic compounds will precipitate at high temperatures, resulting in not only a reduction in toughness but also the loss of its effect on strength. Accordingly, when Mo is added, its content should be in the range of 0.01 to 3%.

[0031] Cu not only improves the strength of the steel owing to solid solution strengthening and precipitation hardening, but also contributes to the improvement of oxidation resistance. Moreover, Cu changes the structure into martensite or bainite and is hence effective for the improvement of toughness. However, the addition of excessive amounts of Cu will harden the steel to an undue extent. When Cu is added to the steels of the present invention which need not be worked by forging, rolling or the like, the content of Cu should be up to 2.5% and its lower limit is 0.1%.

Example

[0032] 30 kg each of steels having the respective chemical compositions shown in Tables 1 were melted in a vacuum melting furnace, cast into the form of Y-type test pieces, and then cooled slowly. Steels A and B are typical conventional cast steel materials which have chemical compositions corresponding to SCPH 21 and SCPH 32, respectively, of JIS (Japanese Industrial Standards). Steels C and D have chemical compositions corresponding to those of heat-resisting steels for small-diameter pipes which are used in boilers and the like. Steels E to M are comparative steels in which the contents of some alloy components are modified so as to be outside the scope of the present invention. Steels 1 to 24 are steels in accordance with the present invention.

[0033] As a conventional heat treatment, steels A to D were normalized by heating at 950°C for 2 hours and air cooling, and then tempered by heating at 730°C for 2 hours and air cooling. Steels E to M and the inventive steels 1 to 24 were normalized by heating at 1,050°C for 2 hours and air cooling, and then tempered by heating at 770°C for 1.5 hours and air cooling.

[0034] With respect to each steel, the presence or absence of internal defects was examined by performing a dye-check test on sections corresponding to 1/4 and 1/2 of the thickness of the ingot. In comparative steel N having a Mg content outside the scope of the present invention, defects were observed in both sections corresponding to 1/4 and 1/2 of the thickness of the ingot. Moreover, its creep resistance and weldability were also insufficient. On the other hand, no internal defect was observed in the cast steels of the present invention.

[0035] In order to compare mechanical properties, room temperature tension tests, Charpy impact tests and creep rupture tests were performed on the comparative steels and inventive steels. Moreover, y-type weld cracking tests were performed in order to evaluate weldability. For use in the room temperature tension tests and the creep rupture tests, test pieces having a diameter of 6 mm and a gage length of 30 mm were cut out from the bottom of the Y-type test pieces in a direction perpendicular to the direction of solidification. The tension tests were performed at room temperature. In the creep rupture tests, long-time rupture tests were performed at 500°C, 550°C, 600°C and 650°C for a period of time up to about 10,000 hours, and the 600°C x 10,000 hour creep rupture strength was determined. The Charpy impact tests were performed according to JIS Z2202. That is, using No. 4 test pieces, the impact value at 0°C was measured three times, and the average of the three impact values was obtained. The y-type weld cracking tests were performed according to JIS Z3158 by using a plate thickness of 20 mm and without preheating (i.e., at 20°C). The weldability was evaluated in terms of longitudinal section cracking rate.

[0036] The test results thus obtained are shown in Table 2. In the tension tests, the inventive steels exhibit a tensile strength in the range of 600 to 700 MPa and an elongation of 20% or greater. With respect to 600°C x 10,000 hour creep rupture strength which indicates high-temperature strength, the comparative steels including conventional steels have a value of at most 84 MPa. In contrast, the inventive steels have a value of 130 MPa or greater, indicating a marked improvement in high-temperature strength by a factor of more than 1.5 times. Among them, steels 4 and 5 containing Mo have a higher creep rupture strength than steels 1-3, and steel 11 additionally containing Cu shows a further increase in creep rupture strength. Steels 16-24, which contain one or more of Ca, Ti, Zr, Y, La, Ce, Ta and Mg, shows no reduction in creep rupture strength and hence have excellent high-temperature strength, even in the presence of relatively large amounts of impurities such as P and S.

[0037] Of the comparative steels, even those having the most excellent impact resistance exhibit an impact value of 126 J/cm² or less. In contrast, the inventive steels exhibit an impact value of 176 J/cm² or greater, indicating that they have excellent toughness at low temperatures.

[0038] The y-type weld cracking tests have revealed that the occurrence of full cracking or partial cracking was observed in all of the comparative steels, but the inventive steels undergo no cracking even at 20°C. Thus, it can be seen that the inventive steels have very excellent weldability and their preheating during welding may be omitted.

Table 2

Test results
	Designation	Room temperature tension test			600°C x 10,000 h creep rupture strength (MPa)	Impact value (0°C) in Charpy impact test (J/cm2)	y-type weld cracking test	Casting defects
		Tensile strength	0.2% veld strength	Elongation
		(MPa)	(MPa)	(%)
Comparative cast steels	A	479	321	38	35	29	Δ	xx
	B	559	372	33	70	34	Δ	xx
	C	545	368	26	75	48	Δ	xx
	D	568	380	24	73	56	Δ	xx
	E	658	510	32	80	53	x	xx
	F	652	478	29	78	35	x	xx
	G	661	465	25	70	68	x	xx
	H	721	498	19	80	80	x	xx
	I	694	503	20	71	119	x	xx
	J	668	480	21	77	126	Δ	xx
	K	671	474	24	81	27	Δ	xx
	L	764	567	17	84	97	x	xx
	M	758	543	18	84	31	x	xx
	N	632	503	32	101	48	Δ	xx

Test results
	Designation	Room temperature tension test			600°C x 10,000 h creep rupture strength	Impact value (0°C) in Charpy impact test	y-type weld cracking test	Casting defects
		Tensile strength	0.2% yield strength	Elongation
		(MPa)	(MPa)	(%)	(MPa)	(J/cm2)
Inventive cast steels	1	648	518	28	130	198	O	OO
	2	657	526	29	131	191	O	OO
	3	641	513	28	135	222	O	OO
	4	633	506	26	159	231	O	OO
	5	642	514	30	168	205	O	OO
	6	672	538	28	145	237	O	OO
	7	623	498	31	144	241	O	OO
	8	602	482	28	151	208	O	OO
	9	604	483	26	149	197	O	OO
	10	612	490	29	159	223	O	OO
	11	666	533	27	178	242	O	OO
	12	633	506	28	160	220	O	OO
	13	641	513	26	161	189	O	OO
	14	652	522	25	151	189	O	OO
	15	644	515	24	167	180	O	OO
	16	655	524	26	164	207	O	OO
	17	639	511	23	166	210	O	OO
	18	672	538	21	156	176	O	OO
	19	654	523	23	162	178	O	OO
	20	634	507	25	147	257	O	OO
	21	655	524	24	156	231	O	OO
	22	639	511	27	167	289	O	OO
	23	653	522	26	169	246	O	OO
	24	647	518	26	157	264	O	OO
x: full cracking Δ: partial cracking O: no cracking xx: casting defects were observed OO: no casting defect was observed

[0039] The low-Cr ferritic steels of the present invention are materials which show a marked improvement in high-temperature strength over conventional low-Cr ferritic steels and also have excellent impact resistance and weldability. Consequently, the steels of the present invention having such excellent properties may be substituted for forged steels in parts which have conventionally required the use of forged steels, resulting in a reduction in cost and an increase in reliability. The steels of the present invention can be widely used for cast steel articles of various shapes which are used as heat-resistant and pressure-tight members in the industrial fields of boilers, chemical industry, nuclear power industry and the like.

Claims

1. A low-Cr ferritic cast steel consisting essentially of, on a weight percentage basis, 0.03 to 0.12% C, 0.03 to 0.7% Si, 0.02 to 1% Mn, up to 0.3% Co, up to 0.025% P, up to 0.015% S, 0.8 to 3% Cr, 0.01 to 1% Ni, 0.01 to 0.5% V, 0.1 to 3% W, 0.01 to 0.2% Nb, 0.001 to 0.05% Al, 0.0001 to 0.02% B, 0.001 to 0.05% N, up to 0.03% 0, 0.0005 to 0.05% Mg, and the balance being iron and incidental impurities, provided that the Mg content satisfies the following inequality as expressed on a weight percentage basis:

2. A low-Cr ferritic cast steel further containing 0.01 to 0.2% by weight of one or more elements selected from the group consisting of Ca, Ti, Zr, Y, La, Ce and Ta, in addition to the components described in claim 1.

3. A low-Cr ferritic cast steel further containing 0.01 to 3% by weight of Mo in addition to the components described in claim 1 or 2.

4. A low-Cr ferritic cast steel further containing 0.1 to 2.5% by weight of Cu in addition to the components described in any of claims 1, 2 and 3.