Steel for pipeline

Abstract

The present invention provides a steel for pipeline which has excellent strength and toughness at room temperature and high temperature without the necessity of adding expensive alloy elements. The steel for pipeline of present invention comprises 0.04 to 0.16% of C, not greater than 0.5% of Si, 0.8 to 1.7% of Mn, not greater than 0.015% of P, not greater than 0.003% of S, not greater than 0.04% of Ti, 0.001 to 0.07% of Al, not greater than 0.01% of N, simultaneously satisfying expression:Al(N−Ti/3.4)≦0.00015, and is constituted of structure comprising martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof. The steel for pipe of the present invention is suitably for the use as the flow line in severe environment such as seabed oil fields or the like, and can be produced at low cost.

Description

TECHNICAL FIELD

The present invention relates to steel for pipeline which is highly tensile steel having excellent strength, toughness and, particularly, high-temperature strength.

TECHNICAL BACKGROUND

Steel used for pipeline is required to have high strength and excellent toughness as well. Particularly, since the exploitation of oil field and natural gas field has been transferring to more severe environment, the steel is even required to have excellent strength under high temperatures around 200° C.

In consideration of the anticipated exhaustion of the resources of crude oil and is natural gas in the near future, the exploitation of oil fields and natural gas fields have been transferring to more severe environment such as deep-seated oil field, and the product fluids are under the state of high temperature and high pressure.

On the other hand, although the exploitation of seabed oil field is becoming popular, the comparatively small oil fields do not hold their own product fluid treating equipments from the viewpoint of reducing equipments investment and maintenance expense. Therefore, a currently main treating mode is to transport product fluids through seabed pipelines so-called “flow line” to an oil field nearby and carry out convergence treatments. In this case, since high-pressure and high-temperature (until about 200° C.) fluids flow in the flow lines, the pipelines used therefor are required to have excellent strength under high temperature until about 200° C. As a measure to solve this problem, Japanese Patent Laid-Open No. S56-166324 has published a method wherein a seamless steel pipe, which comprises either or both of Mo and V added to a special composition containing C, Si, Mn and Al, is made by hot rolling, and then immediately subject to direct quenching, and tempering thereafter. However, according to this method, the addition of expensive alloy elements Mo and V is indispensable. Further, austenite crystal grains would become large due to the direct quenching and thus impairing the toughness of base metal. Therefore, the pipes made by this method are not suitable to be used in cold districts.

In addition, Japanese Patent Laid-Open No. H02-50917 has published a method of using Mannesman pipe-making process to form a seamless steel pipe from a round ingot having a specific composition with Cr and V as its necessary added elements, and subjecting the steel pipe to heating at 850 to 1000° C., quenching, and then tempering at temperature from 500 to 700° C. The steel pipe made according to this method is still expensive since the addition of expensive alloy elements Cr and V is indispensable.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide steel for pipeline which has excellent strength and toughness at both high-temperature and room temperature without the addition of expensive alloy elements.

In order to solve the problems described above, the inventor made the present invention on the basis of the following findings concluded from repeated researches:

(1) In order to obtain steel used for pipeline with excellent strength and toughness at room temperature and high temperature without addition of expensive alloy elements, the structure of the steel can be made to comprise martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

(2) Although, generally, the structure having such large austenite average grain sizes would impair the toughness of the steel, the excellent toughness can still be obtained without making fine-grain structure but by controlling the addition amounts of Al, Ti and N.

The present invention provides the steel for pipeline as follows.

(1) Steel for pipeline comprising, by mass percent, 0.04 to 0.16% of C, not greater than 0.5% of Si, 0.8 to 1.7% of Mn, not greater than 0.015% of P, not greater than 0.003% of S, not greater than 0.04% of Ti, 0.001 to 0.07% of Al, not greater than 0.01% of N, and simultaneously satisfying expression (1):
Al(N−Ti/3.4)≦0.00015  (1)
and said steel for pipeline being constituting of structure comprising martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

(2) The steel for pipeline according to (1) further comprising, in addition to the components described above, one or more of the components, by mass percent, not greater than 0.6% of Cr, not greater than 0.3% of Mo, not greater than 0.4% of Cu, not greater than 0.4% of Ni, not greater than 0.01% of Nb, not greater than 0.08% of V and not greater than 0.001% of B.

(3) The steel for pipeline according to (1) or (2) further comprising, by mass percent, not greater than 0.005% of Ca.

BEST MODES OF THE INVENTION

The best modes of the highly tensile steel for pipeline according to the present invention are described in details as follows.

First, the reasons for limiting the chemical composition of the steel in the present invention are described. In this specification, “%” generally represents “mass %” unless there is other specific explanation.

C is an indispensably added element for improving the strength of steel. In order to ensure the required strength of steel at room temperature and high temperature, the addition of C should be 0.04% or more. However, if the C content exceeds 0.16%, the toughness of base metal and heat affected zones at welded joints would be impaired. Therefore, the C content is defined within a range of 0.04 to 0.16%, and preferably 0.06 to 0.13%.

Si is added for the purpose of deoxidization, and it also contributes to the improvement of strength at room temperature and high temperature. However, if the Si content exceeds 0.5%, the toughness of base metal and heat affected zones at welded joints would be impaired. Therefore, the Si content is defined not greater than 0.5%, and preferably not greater than 0.35%.

Mn is an effective element for ensuring the strength and toughness. If the Mn content is lower than 0.8%, it is unable to realize desirable effects. On the other hand, if the Mn content exceeds 1.7%, the toughness of base metal would decrease. Therefore, the Mn content is defined within a range of 0.8 to 1.7%.

P exists in steel as an impurity that impairs the toughness of base metal. Therefore, the P content is desirably controlled as low as possible, not greater than 0.015%, and preferably not greater than 0.012%.

S, similar to P, exists in steel as an impurity that impairs the toughness of base metal. Therefore, the S content is desirably controlled as low as possible, not greater than 0.003%, and preferably not greater than 0.001%.

Ti, in general, combines with N and C contained in the steel and thus existing in forms of TiN and TiC. Either TiN or TiC would impair the toughness when the structure of base metal is coarse. So, from this aspect of view, Ti is preferably not added. But, on the other hand, those N not fixed in the form of TiN would combine with Al to perform AlN which would impair the toughness of base metal. Further, the excessive existence of TiN would cause the refinement of austenite grains due to the pinning effect, and thereby decreases the hardness of steel, thus failing to obtain sufficient strength. Therefore, first, the addition of Ti should be controlled to satisfy expression (1). Secondly, even though the Ti content is controlled to satisfy expression (1), the toughness would still be decreased when it exceeds 0.04%, so that the upper limit of Ti content is defined to 0.04%.

Al is added for the purpose of deoxidization. The Al content lower than 0.001% would cause insufficient deoxidization, and thus bringing about steel deterioration. However, when the Al content exceeds 0.07%, the deoxidization effect does not change any longer, while the content of impurities is increased, and thus decreasing the toughness of steel. Therefore, the Al content is defined within a range of 0.001 to 0.07%, and preferably 0.01 to 0.05%. However, even if the Al content falls within this range, the pinning effect caused by excess of AlN formed by the combination of Al and N in steel would induce the refinement of austenite grains, and thus decreasing the hardness of steel. Thereby, sufficient strength of steel cannot be obtained. Further, excessive AlN in such coarse grain structure would degrade the toughness of base metal. Therefore, the Al content should be controlled to satisfy expression (1).

N exists in steel as an impurity that combines with Ti to form TiN, and those not fixed in the form of TiN combines with Al to form AlN. The existence of AlN and TiN would cause the refinement of austenite grains due to the pinning effect, and thereby decreases the hardness of steel, thus failing to obtain sufficient strength. Meanwhile, TiN and AlN in the coarse structure would degrade the toughness of base metal. Therefore, the N content is desirably as low as possible, not greater than 0.01% and simultaneously satisfying expression (1).

Next, the reasons for limiting the contents of Al, N and Ti according to expression (1) are described.

As described above, Al and Ti combine with N contained in steel to form AlN and TiN. Either of AlN and TiN would cause the refinement of austenite grains due to the pinning effect, and thereby decrease the hardness of steel, thus failing to obtain sufficient strength. Meanwhile, TiN and AlN in the coarse structure would degrade the toughness of base metal. Therefore, in addition to the individual limitation of the contents of these elements, the value obtained from expression (1) should be controlled at not greater than 0.00015, so as to ensure the high strength and high toughness of the steel.

Next, the reasons for adding Cr, Mo, Cu, Ni, Nb, V, B and Ca as desirably added elements are described as follows. Any one of these elements is added for improving the strength of the steel. The strength and toughness of steel are expected to be further improved by addition of one or more of these elements to the basic essential components of the steel.

Cr is an element contributing to high strength by improving hardness. The addition of Cr contributes to high strength at room temperature and high temperature. However, when the Cr content exceeds 0.6%, it would impair the toughness of heated affected zones at welded joints. Therefore, the Cr content is controlled at not greater than 0.6%.

Mo, similar to Cr, is an element contributing to high strength by improving hardness. The addition of Mo can contribute to high strength at room temperature and high temperature. However, when the Mo content exceeds 0.3%, it would impair the toughness of heated affected zones at welded joints. Therefore, the Mo content is controlled at not greater than 0.3%.

Cu can improve corrosion resistance and high strength. However, its excessive addition would impair the field weldability and increase material cost. Therefore, the Cu content is controlled at not greater than 0.4%.

Ni contributes to the improvement of strength without impairing the toughness. However, its excessive addition would impair the field weldability and increase material cost. Therefore, the Ni content is controlled at not greater than 0.4%.

Nb can improve strength by promoting the precipitation. However, when its addition exceeds 0.01%, it would impair the toughness. Therefore, the Nb content is controlled at not greater than 0.01%.

V can improve strength at room temperature and high temperature by promoting the precipitation. However, when its addition exceeds 0.08%, it would impair the toughness. Therefore, the V content is controlled at not greater than 0.08%.

Slight addition of B can ameliorate strength by improving hardness. However, when its addition exceeds 0.001%, it would impair the toughness of base metal and heat affected zones at welded joints. Therefore, the B content is controlled at not greater than 0.001%.

Ca reacting with S contained in steel to form sulfide but it maintains the spherical shape after rolling without extending along the rolling direction. Therefore, the generation of hydrogen-inducing cracking or the like that starts from tips of extended impurities such as MnS can be suppressed. However, the excessive addition of Ca would deteriorate affect the cleanness of steel and impair the toughness of base metal. Therefore, the Ca content is controlled at not greater than 0.005%.

Next, the reasons for limiting the structure of the steel are described.

By making the structure steel for pipeline comprise martensite or bainite with an austenite average grain size of 30 μm or larger, or their tempered structure, excellent strength at room temperature and high temperature can be obtained without adding alloy elements such as Cr, Mo and V. However, when austenite average grain size is greater than 200 μm, the toughness would become lower even if the chemical composition is defined. Therefore, the structure of steel of the present invention is defined to be the structure comprising martensite or bainite having an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

There is no any special limitation on the manufacturing method of the steel for pipeline according to the present invention provided that the steel has the composition and the structure prescribed in the present invention.

The effects of the present invention are further described using the examples shown as follows, but these examples would not introduce any limitation to the present invention.

EXAMPLES

Each of the steel having the chemical compositions shown in Table 1 was melt in a vacuum-melt furnace, and the molten steel is solidified to produce a round ingot with a weight of 50 Kg. The ingot thus obtained was subjected to hot rolling and heat treatment to produce steel plate under the condition shown in Table 2.

Then, JIS Z2202 V-notch Charpy impact test piece and JIS Z2201 round stick tensile test piece were taken from each of said steel plates along the plumb direction of rolling, and then subjected to the Charpy impact test and Tensile test at room temperature and high temperature. The tensile test at high temperature was carried out according to the standard of JIS G0567 with round stick tensile test pieces having a parallel portion space of 6 mm and a punctuation space of 30 mm.

The test results are shown in Table 2. Examples marked with 1 to 17 in Table 2 showed tensile strength greater than 500 MPa at room temperature and high temperatures till 200° C., and fracture transition temperature of −70° C. and lower.

Comparative examples marked with 18 to 29 in table 2, which had components exceeding the content ranges of the present invention, showed lower strength and toughness.

Comparative examples marked with 30 and 31 in table 2, which had the components within the content ranges of the present invention while the austenite average grain size exceeding the range of the present invention, also showed lower strength and toughness.

Comparative examples marked with 32 in table 2, which had the components within the content ranges of the present invention while being subjected normalizing as the heating treatment after rolling, also showed lower strength since the structure comprising martensite or bainite was not obtained.

EFFECTS OF THE INVENTION

The steel for pipeline according to the present invention has excellent strength and toughness at room temperature and high temperature without the necessity of adding expensive alloy elements, and therefore realizing high production efficiency and low production cost.

	TABLE 1
	Components (mass %)
	Steel									Expression
Sort	Mark	C	Si	Mn	P	S	Ti	Al	N	(1)
Present	A	0.07	0.27	1.38	0.011	0.001	0.006	0.036	0.0039	0.00008
Invention	B	0.16	0.25	1.24	0.013	0.001	0.007	0.038	0.0037	0.00006
	C	0.08	0.46	1.35	0.015	0.001	0.008	0.041	0.0040	0.00007
	D	0.11	0.30	0.83	0.010	0.001	0.009	0.033	0.0038	0.00004
	E	0.07	0.28	1.64	0.010	0.001	0.008	0.033	0.0038	0.00005
	F	0.07	0.30	1.43	0.010	0.003	0.008	0.033	0.0038	0.00005
	G	0.07	0.27	1.37	0.011	0.002	0.034	0.062	0.0081	−0.00012
	H	0.06	0.25	1.25	0.012	0.001	0.007	0.033	0.0036	0.00005
	I	0.05	0.25	1.31	0.012	0.001	0.010	0.033	0.0036	0.00002
	J	0.06	0.26	1.24	0.013	0.001	0.006	0.036	0.0035	0.00006
	K	0.07	0.26	1.25	0.015	0.001	0.008	0.038	0.0036	0.00005
	L	0.06	0.27	1.26	0.011	0.002	0.008	0.035	0.0033	0.00003
	M	0.12	0.26	1.32	0.014	0.001	0.008	0.034	0.0040	0.00006
	N	0.06	0.27	1.36	0.015	0.001	0.006	0.039	0.0052	0.00013
Comparative	O	0.02	0.25	1.30	0.010	0.001	0.006	0.033	0.0038	0.00007
Example	P	0.23	0.27	1.32	0.012	0.001	0.007	0.037	0.0046	0.00009
	Q	0.07	0.67	1.29	0.013	0.001	0.006	0.035	0.0036	0.00006
	R	0.07	0.25	0.71	0.011	0.001	0.006	0.036	0.0031	0.00005
	S	0.06	0.27	1.85	0.011	0.001	0.007	0.037	0.0037	0.00006
	T	0.06	0.21	1.35	0.021	0.001	0.005	0.040	0.0028	0.00005
	U	0.08	0.23	1.31	0.013	0.006	0.006	0.038	0.0037	0.00007
	V	0.05	0.24	1.32	0.011	0.001	0.051	0.041	0.0039	−0.00046
	W	0.06	0.22	1.28	0.010	0.002	0.007	0.0004	0.0042	0.00000
	X	0.07	0.25	1.33	0.013	0.001	0.008	0.082	0.0054	0.00025
	Y	0.06	0.24	1.30	0.012	0.001	0.007	0.037	0.0118	0.00036
	Z	0.05	0.22	1.26	0.012	0.001	0.005	0.053	0.0063	0.00026
		Components (mass %)
	Steel	Others
	Sort	Mark	Cr	Mo	Cu	Ni	Nb	V	B	Ca
	Present	A	—	—	—	—	—	—	—	—
	Invention	B	—	—	—	—	—	—	—	—
		C	—	—	—	—	—	—	—	—
		D	—	—	—	—	—	—	—	—
		E	—	—	—	—	—	—	—	—
		F	—	—	—	—	—	—	—	—
		G	—	—	—	—	—	—	—	—
		H	0.52	—	—	—	—	—	—	—
		I	—	0.25	—	—	—	—	—	—
		J	0.16	—	—	—	—	0.06	—	—
		K	—	—	0.29	0.25	—	—	—	—
		L	—	—	—	—	0.007	—	—	—
		M	—	—	—	—	—	—	0.0007
		N	—	—	—	—	—	—	—	0.0035
	Comparative	O	—	—	—	—	—	—	—	—
	Example	P	—	—	—	—	—	—	—	—
		Q	—	—	—	—	—	—	—	—
		R	—	—	—	—	—	—	—	—
		S	—	—	—	—	—	—	—	—
		T	—	—	—	—	—	—	—	—
		U	—	—	—	—	—	—	—	—
		V	—	—	—	—	—	—	—	—
		W	—	—	—	—	—	—	—	—
		X	—	—	—	—	—	—	—	—
		Y	—	—	—	—	—	—	—	—
		Z	—	—	—	—	—	—	—	—

	TABLE 2
		Wall
		thickness
	Finish	Of		Average	Strength at	Strength at
	Temp	Steel slab	Heat	Size of	room temp.	high temp.	Fracture
	Test		Heat	after	after	treatment	Tempering	Austentite	Yield	Tensile	Yield	Tensile	Transition
	Piece	Steel	Temp	rolling	Rolling	after	Temp.	Grains	Strength	Strength	Strength	Strength	Temp.
Sort	Mark	Mark	° C.	° C.	° C.	rolling	° C.	μm	MPa	MPa	MPa	MPa	° C.
Present	1	A	1250	1050	25.4	Quench in water	—	66	591	643	573	612	−72
Invention	2	A	1250	1050	25.4	Quench in water	650	70	473	559	451	534	−86
	3	A	1250	1150	25.4	Quench in water	650	92	520	604	497	577	−78
	4	A	1250	950	25.4	Quench in water	650	54	449	537	429	513	−91
	5	B	1250	1050	25.4	Quench in water	650	72	519	629	485	591	−70
	6	C	1250	1050	25.4	Quench in water	650	66	475	558	449	529	−84
	7	D	1250	1050	25.4	Quench in water	650	62	407	528	391	504	−90
	8	E	1250	1050	25.4	Quench in water	650	67	529	594	508	567	−74
	9	F	1250	1050	25.4	Quench in water	650	68	476	547	456	522	−82
	10	G	1250	1050	25.4	Quench in water	650	65	511	574	491	549	−80
	11	H	1250	1050	25.4	Quench in water	650	69	513	570	491	544	−70
	12	I	1250	1050	25.4	Quench in water	650	71	494	564	465	531	−76
	13	J	1250	1050	25.4	Quench in water	650	61	543	609	515	584	−75
	14	K	1250	1050	25.4	Quench in water	650	66	500	582	478	553	−83
	15	L	1250	1050	25.4	Quench in water	650	47	490	567	466	536	−73
	16	M	1250	1050	25.4	Quench in water	650	68	503	603	475	573	−71
	17	N	1250	1050	25.4	Quench in water	650	39	444	526	422	505	−101
Comparative	18	O	1250	1050	25.4	Quench in water	650	62	391	447	366	418	−112
Example	19	P	1250	1050	25.4	Quench in water	650	59	628	701	587	659	−41
	20	Q	1250	1050	25.4	Quench in water	650	66	468	550	437	513	−59
	21	R	1250	1050	25.4	Quench in water	650	65	346	455	321	423	−98
	22	S	1250	1050	25.4	Quench in water	650	67	573	637	539	600	−49
	23	T	1250	1050	25.4	Quench in water	650	67	471	551	438	513	−52
	24	U	1250	1050	25.4	Quench in water	650	63	464	555	431	518	−51
	25	V	1250	1050	25.4	Quench in water	650	35	719	757	674	709	−39
	26	W	1250	1050	25.4	Quench in water	650	77	456	546	426	511	−45
	27	X	1250	800	25.4	Quench in water	650	21	327	413	314	396	−35
	28	Y	1250	1200	25.4	Quench in water	650	17	249	364	239	350	−48
	29	Z	1250	1050	25.4	Quench in water	650	20	305	397	293	381	−44
	30	A	1250	800	25.4	Quench in water	650	12	345	419	330	398	−90
	31	A	1250	1200	25.4	Quench in water	650	263	756	839	709	790	5
	32	A	1250	1050	25.4	normal	650	67	260	336	239	305	−94

Claims

1. Steel for pipeline comprising, by mass percent, 0.04 to 0.16% of C, not greater than 0.5% of Si, 0.8 to 1.7% of Mn, not greater than 0.015% of P, not greater than 0.003% of S, not greater than 0.04% of Ti, 0.001 to 0.07% of Al, not greater than 0.01% of N, and simultaneously satisfying expression (1): Al(N−Ti/3.4)≦0.00015  (1) and said steel for pipeline being constituted of structure comprising martensite or bainite with an austenite average grain size of 30 to 200 μm, or the tempered structure thereof.

2. The steel for pipeline according to claim 1, further comprising one or more, by mass percent, not greater than 0.6% of Cr, not greater than 0.3% of Mo, not greater than 0.4% of Cu, not greater than 0.4% of Ni, not greater than 0.01% of Nb, not greater than 0.08% of V and not greater than 0.001% of B.

3. The steel for pipeline according to claim 1, further comprising, by mass percent, not greater than 0.005% of Ca.

4. The steel for pipeline according to claim 2, further comprising, by mass percent, not greater than 0.005% of Ca.