GB2247246A - Process for producing highly corrosion-resistant low-alloy steel for line pipe - Google Patents

Abstract

A process for producing a highly corrosion-resistant low-alloy steel for line pipes having an improved resistance to corrosion duo to CO2 without detriment to the low-temperature toughness of both the base metal and the part affected by the heat of welding and in site weldability. The steel contains 0.02 to 0.09 wt% of carbon, 0.5 wt% or less of silicon, 0.7 to 1.5 wt% of manganese, 0.03 wt% or less of phosphorus, 0.005 wt% or less of sulphur, 0.02 to 0.06 wt% of niobium, 0.5 to 1.2 wt% or less of chromium, 0.005 to 0.03 wt% of titanium, 0.05 wt% or less of aluminium, 0.002 to 0.005 wt% of nitrogen, and the balance of iron and unavoidable impurities, and satisfies the following relation: 0.35 </= C + (Mn + Cr + V)/5 + (Ni + Cu)/15 </=0.48. The steel is heated in the temperature range of 1100 to 1250 DEG C, is rolled under the condition of a cumulative rolling reduction of 40% or above at a temperature of 950 DEG C or below and a rolling termination temperature of 700 to 850 DEG C, and is subjected to air cooling or accelerated cooling. <IMAGE>

Description

SPECIFICATION MANUFACTURING METHOD OF HIGH CORROSION-RESISTANT LOW-ALLOY STEEL FOR LINE PIPES TECHNICAL FIELD The present invention relates to a manufacturing method of high strength steel plates for line pipes (tensile strength: 50 kg f/mm or more by TS, thickness of 40 mm or less) which are excellent in the corrosion resistance to CO2.

BACKGROUND ART Large-diameter line pipes for transportation of oil or natural gas in cold regions or offshores require not only high strength but also low-temperature toughness and field weldability. Further, the effect of inhibitors is lowered due to injection of CO2 in secondary and tertiary crude oil recovery and increase in the depth of oil wells, and for such reasons, corrosion of line pipes by C 2 gas has become a serious problem lately. Therefore, the corrosion resistance to CO2 has come to be demanded as well.

Although recently it is the knowledge that addition of Cr is effective against C02 corrosion (Journal of Petroleum Technology Association, Vol. 50, No. 2, Figs.

9 and 10), there have not been developed largediameter line pipes having the corrosion resistance to C02 which are perfectly suitable for low-temperature environment yet.

In other words, although a number of steels to which Cr is added in large quantities to improve corrosion resistance have been developed (for example, JP-B-59-19179 and JP-B-59-45750), none of them is excellent in both low-temperature toughness and field weldability as line pipes for low-temperature environment.

Since addition of Cr in large quantities degrades the weldability of the steels, preheating and stress relief heat treatment at high temperature are essential in the view point of preventing weld cracking at the time of welding in fields, thereby the work efficiency is deteriorated to a great extent. Further, addition of Cr in large quantities into the steel deteriorates the toughness of the base material of the steel and weld heat-affected zones (HAZ). Therefore, development of steel for line pipes excellent in the corrosion resistance to CO2 and having favorable low-temperature toughness and favorable field weldability is strongly demanded.

DISCLOSURE OF THE INVENTION Accordingly, the principal object of the present invention is to a provide novel steel for line pipes which is greatly improved in the corrosion resistance to CO2 without deteriorating low-temperature toughness of the base material and the HAZ.

The object is achieved firstly by preparing a steel of which composition is, by weight %, 0.02 to 0.09 carbon, 0.5 or less silicon, 0.7 to 1.5 manganese, 0.03 or less phosphorus, 0.005 or less sulfur, 0.02 to 0.06 niobium, 0.5 to not more than 1.2 chromium, 0.005 to 0.03 titanium, 0.05 or less aluminum, 0.002 to 0.005 nitrogen, and the balance of iron and unavoidable impurities, and which satisfies the equation: 0.35 5 C + (Mn + Cr + V)/5 + (Ni + Cu)/15 < 0.48, and secondly by processing the steel in accordance with the following steps: heating a block of the steel to a temperature in a range of 11000C to 12500C, rolling the block at 9500C or less and at 7000C to 8500C of a finish rolling temperature with a cumulatine rolling reduction of 40% or more, and air-cooling or accelerated cooling the rolled product.Thus, the desired steel product can be obtained.

Further, if necessary, one element or more selected from the group consisting of 0.01 to 0.08 V, 0.05 to 0.5 Ni, 0.05 to 0.5 Cu and 0.001 to 0.005 Ca can be added to the steel of the above mentioned composition.

The present invention will be hereinafter described in detail.

In order to improve the corrosion resistance to CO2 and obtain excellent low-temperature toughness of the base material and the HAZ and excellent field weldability, it is necessary to select a particular chemical composition of the steel. For this reason, the Cr content is set at 0.5 to 1.2% in respect of the corrosion resistance. The Cr content is required to be 0.5% at least to obtain adequate corrosion resistance. However, too much Cr largely deteriorates the low-temperature toughness and the field weldability. Therefore, the upper limit is set at 1.2%.

When a considerable amount of Cr is added to the steel to improve the corrosion resistance, 0.02 to 0.09% of C (carbon) and 0.7 to 1.5% of Mn are necessary for ensuring excellent low-temperature toughness and excellent weldability. The lower limits of C and Mn are minimum amounts for obtaining- the required strength of the base material and welded joints and achieving the effects of precipitation hardening and grain refining of Nb and V when these elements are added to the steel. The upper limits are critical values for obtaining excellent low-temperature toughness and excellent field weldability (most preferable C and Mn contents are 0.03 to 0.06% and 0.8 to 1.2%, respectively).

Yet, it is insufficient to restrict the content of each element only. The following equation has to be satisfied: 0.35% 5 C + (Mn + Cr + V)/5 + (Ni + Cu)/15 5 0.48. This is because low-temperature toughness and weldability are determined by a total amount of chemical components including Cr. The lower limit of 0.35% is a minimum amount for obtaining the required strength of the base material and welded joints, and 0.48% is the upper limit for obtaining excellent low-temperature toughness and excellent weldability.

The steel of the invention contains 0.02 to 0.06% Nb and 0.005 to 0.03% Ti as essential elements.

Nb contributes to refining of grain size and precipitation hardening in controlled rolling, thus toughening the steel. By adding Ti to the steel, fine TiN are formed, and coarsening of y grains is suppressed during slabreheating and welding, effectively improving the base material toughness and the HAZ toughness.

When a large amount of Cr is added to the steel, separations are inhibited on impact fracture surfaces of the control-rolled steel in Charpy test or the like, thereby deteriorating the low-temperature toughness.

Particularly in the steel of the present invention containing small amounts of C and Mn, therefore, addition of Nb and Ti was found to be essential in relation to obtaining the excellent low-temperature toughness.

The lower limits of Nb and Ti contents are minimum amounts for these elements to achieve their effects, and the upper limits are critical values of addition amounts not to deteriorate the HAZ toughness and the field weldability.

Reasons for restricting the amounts of the other elements will be described.

When Si is added excessively to the steel, the weldability and the HAZ toughness are lowered, and consequently, the upper limit is set at 0.5%. Deoxidation of the steel can be sufficiently performed by Ti alone, and it is not always necessary to add Si to the steel.

The reason why contents of impurities of P (phosphorus) and S (sulfur) are set at 0.03% or less and 0.005% or less, respectively, in the steel of the invention is that the low-temperature toughness of the base material and the welded joints can be further improved as a result.

Reduction in the P content prevents inter-granular cracking, and reduction in the S content prevents the roughness from being deteriorated by MnS. Preferable P and S contents are 0.01% or less and 0.003% or less, respectively.

Although al is an element which the steel usually contains for deoxidation, it is not always necessary to add it to the steel because deoxidation can be effected by Ti or Si. When the Al content exceeds 0.05%. Nonmetallic inclusions are increased to degrade cleanliness of the steel. Therefore, the upper limit is set at 0.05%.

Nitrogen (N) serves to form TiN and improve the toughness of the base material and the HAZ through the effect of suppressing the coarsening of y grains. The minimum content for this purpose is 0.002%. However, too much N causes deterioration of the HAZ toughness by solute nitrogen and slab surface defects, so that it is necessary to lower the upper limit to 0.005% or less.

Reasons for adding V, Ni, Cu and Ca to the steel will now be described.

A main object of further adding these elements to the base compositions is to improve the properties such as strength and toughness without spoiling excellent characteristics of the steel of the present invention.

Consequently, addition amounts of them have to be naturally restricted.

Vanadium (V) takes substantially the same effects as Nb such as improvement of the low-temperature toughness and increase in strength due to refinement of the microstructure, increase in the strength owing to precipitation hardening, and so forth. However, excessive addition of V induces deteriorations of the weldability and the HAZ toughness, and therefore, the upper limit is set at 0.08%.

Ni improves both the strength and the toughness without giving unfavorable influences to the wledability and the HAZ toughness, and it is also effective in prevention of hot cracks at the time of addition of Cu to the steel. However, the content exceeding 0.5% is not preferable economically, and accordingly, the upper limit is set at 0.5%.

Although Cu is effective in corrosion resistance and resistance against hydrogen induced cracking, the content exceeding 0.5% causes copper-cracks during hot rolling, resulting in difficulty in the manufacture of the steel. Therefore, the upper limit is set at 0.5%.

Calcium (Ca) controls the shape of a sulfide (MnS) and improves the low-temperature toughness (increase in Charpy absorption energy and the like), and it is also effective remarkably in improvement of the resistance against hydrogen-induced cracking. However, the Ca content not more than 0.001% has no effect in practice, and addition of Ca exceeding 0.005% induces generation of large amounts of CaO and CaS which become to coarse inclusions, not only degrading cleanliness of the steel but also giving unfavorable influences to the toughness and the field weldability.

For this reason, the amount of Ca is limited to 0.001 to 0.005%. In order to improve the resistance against hydrogen-induced cracking, it is especially effective to reduce the sulfur (S) and oxygen (O) contents to 0.001% or less and 0.002% or less, respectively, and to satisfy the following equation: ESSP 2 (Ca) [1 - 124(0)]/1.25(S) 2 1.5. In this case, ESSP stands for Effective Sulfide Shape Controlling Parameter, and indicates a relation in the composition which prevents the sulfide from being extended in the rolling process.

More specifically, when the ESSP is set to 1.5 or more, the amount of MnS is reduced, and the amount of CaS, CaOS which are not easily extended at the time of rolling is formed instead.

As for the above described steel containing Cr, an appropriate manufacturing method must be adopted to improve the low-temperature toughness of the base material, and it is necessary to restrict conditions of reheating, rolling and cooling of the steel (slabs).

First, the reheating temperature is restricted to a range of 1100 to 12500C. The reheating temperature has to be not less than 1100"C to dissolve Nb precipitates into matrix and to obtain a finish rolling temperature.

as high as required. However, when the reheating temperature exceeds 12500C, austenitic (Y) grains become to be considerably coarse, and can not be refined sufficiently even by rolling, so that excellent lowtemperature toughness can not be obtained. Thus, the reheating temperature is set at not more than 12500C (preferably, 1150 to 12000C).

Moreover, the cumulative rolling reduction at 9500C or less must be set at not less than 40%, and the finish rolling temperature has to be set at 700 to 8500C.

This is because y grains which have been refined by recrystallized region rolling are extended by low temperature rolling in the unrecrystallized region, so that resultant ferrite grain size is reduced to a minimum, thus improving the low temperature toughness. When the cumulative rolling reduction is under 40%, extending of the austenitic structure is insufficient, and therefore, fine ferritic grains can not be obtained.

Besides, when the finish rolling temperature is 8500C or more, fine ferrite grains can not be obtained even if the cumulative rolling reduction is not less than 40%. However, when the finish rolling temperature is too low, it results in excessive two phase (y + a) of austenitic and ferritic phases region rolling, thus deteriorating the low temperature toughness. Therefore, the lower limit of the finish rolling temperature is set at 7000C.

Air cooling or accelerated cooling is desirable for cooling after rolling. As a condition of the accelerated cooling, it is preferable to cool the steel to a desired temperature of not more than 6000C at a cooling speed of 10 to 400C/sec immediately after cooling, and to air-cool it thereafter. The advantage of the present invention will not be lost even if the manufactured steel is reheated at a temperature not more than Acl point for the purpose of tempering, dehydrogenation and so forth.

EXAMPLES Steel plates (15 to 32 mm thick) having various steel compositions were manufactured through converter process, continuous casting process and plate rolling process, and examined in respect of strength, toughness, low-temperature toughness and corrosion resistance.

Test items and results are shown in Table 1.

All of the steel plates (steels of the present invention) manufactured according to the invention method have favorable properties. On the other hand, comparative steels which were not manufactured according to the present invention are inferior in strength, low-temperature toughness or corrosion resistance.

As for comparative steels 11 to 19, a steel 11 whose Cr content is low is inferior in the corrosion resistance. A steel 12 whose Cr content is excessive is inferior in the weldability, with Pc (= C + (Mn + Cr + V)/5 + (Ni + Cu)/15) being high, and the HAZ toughness is also inferior. A steel 13 whose carbon content is high is inferior in the low-temperature toughness of both the base material and the HAZ. A steel 14 whose Mn content is high is inferior in the HAZ toughness. A steel 15 contains no Nb so that the strength of the base material is low, and that the toughness is inferior as well. A steel 16 containing no Ti is inferior in the toughness of the base material and the HAZ. As for a steel 17, the reheating temperature is low, and consequently, the base material strength is insufficient.A steel 18 is inferior in the base material toughness because the cumulative rolling reduction at 9500C or less is insufficient. Further, a steel 19 whose finish rolling temperature is too low is inferior in the base material toughness.

Table 1 Chemical Composition (wt%) Class Steel C Si Mn P S Nb Cr Ti Al N 1 0.074 0.22 0.96 0.021 0.003 0.045 0.89 0.012 0.023 0.0023 2 0.074 0.22 0.96 0.021 0.003 0.045 0.89 0.012 0.023 0.0023 Alloy of 3 0.035 0.36 1.31 0.008 0.001 0.028 0.58 0.008 0.012 0.0045 the 4 0.040 0.09 0.90 0.011 0.002 0.021 1.05 0.017 0.003 0.0030 invention 5 0.032 0.33 0.75 0.006 0.001 0.042 1.12 0.014 0.035 0.0038 6 0.082 0.23 0.89 0.008 0.002 0.044 0.92 0.020 0.023 0.0040 7 0.052 0.26 0.88 0.012 0.002 0.046 0.87 0.010 0.018 0.0027 8 0.043 0.45 1.02 0.007 0.003 0.050 0.90 0.012 0.043 0.0032 9 0.068 0.28 0.96 0.012 0.005 0.038 0.79 0.015 0.028 0.0033 10 0.028 0.34 0.88 0.002 0.001 0.024 0.68 0.012 0.018 0.0030 11 0.065 0.23 1.02 0.011 0.002 0.039 0.43 0.012 0.025 0.0028 12 0.076 0.35 0.98 0.008 0.001 0.042 1.33 0.013 0.017 0.0032 13 0.112 0.23 1.12 0.009 0.003 0.045 0.98 0.009 0.025 0.0019 14 0.068 0.22 1.58 0.008 0.001 0.035 0.88 0.013 0.022 0.0040 Compara- 15 0.052 0.19 1.00 0.008 0.003 - 0.90 0.013 0.028 0.0031 tive 16 0.065 0.22 0.98 0.006 0.004 0.046 0.89 - 0.024 0.0032 Alloy 17 0.074 0.22 0.96 0.021 0.003 0.045 0.89 0.012 0.023 0.0023 18 0.074 0.22 0.96 0.021 0.003 0.045 0.89 0.012 0.023 0.0023 19 0.074 0.22 0.96 0.021 0.003 0.045 0.89 0.012 0.023 0.0023 Table 1 (Cont'd) Manufacturing Conditions Reheating Cumulative Finish Accelerated Plate tempera- Bolling Reduc- rolling Cooling Thick Others Pc *1 ture tion at 950 C termperature ness ( C) or less (%) ( C) (mm) 0.44 1200 75 725 No 20 0.44 1200 75 760 Yes 20 0.41 1100 79 740 No 15 0.43 1150 69 780 Yes 25 0.41 1150 75 820 Yes 20 0.44 1250 75 715 No 15 0.05%V 0.46 1150 87 740 No 20 0.13%Ni 0.45 1150 71 800 Yes 32 0.28%Cu 0.35%Ni 0.44 1200 75 780 Yes 20 0.0034%Ca 0.23%Cu 0.36 1150 70 760 Yes 20 0.36 1200 75 760 Yes 20 0.53 1200 75 755 Yes 20 0.53 1200 80 770 Yes 20 0.56 1150 75 740 No 25 0.34%Ni 0.45 1150 75 740 Yes 20 0.0033%Ca 0.44 1200 75 760 No 20 0.44 1050 75 760 Yes 20 0.44 1200 35 760 Yes 20 0.44 1200 75 680 Yes 20 *1: Pc = C+(Mn+Cr+V)/ 5+(Ni+Cu)/15 - To be cont'd - Table 1 (Cont'd) Mechanical Properties of Toughness of Corrosion Resist Base Material*2 weld*3 ance to CO2*4 Y S T S VE-30 VE-30 5 atm CO2, 60 C (kg/mm) (kg/mm) (kg/m) (kg/m) 96hr (mdd) 46.9 57.8 23.8 12.8 260 52.1 62.0 30.6 14.6 248 52.4 59.8 32.5 20.2 290 46.2 58.2 36.7 24.4 239 47.6 57.2 34.9 17.8 221 52.2 61.2 19.4 10.6 267 47.8 59.8 20.6 14.8 258 48.6 60.3 32.1 12.9 265 51.7 63.9 25.2 11.9 273 48.2 57.8 36.7 21.3 282 46.3 56.8 25.2 14.3 412 53.9 66.2 16.7 5.8 210 55.7 67.8 11.5 4.2 252 54.6 63.9 23.9 4.0 268 39.6 53.6 9.8 15.4 266 48.7 60.2 12.1 2.8 271 43.5 54.3 34.5 16.7 258 46.3 60.1 6.7 15.9 265 56.5 64.9 5.8 14.3 261 *2: Perpendicular to a direction of rolling *3: Each-side one-pass submerged arc welding (Notch position HAZ 50%, WM 50%) *4: Test solution: synthetic sea water (ASTM Dll41-75) As will be apparent from the above, according to the present invention, there can be produced line pipes having improved corrosion resistance to CO2 and high strength which are excellent in field weldability. As a result, efficiency of welding work in fields and safety of pipe lines are remarkably improved.

INDUSTRIAL APPLICABILITY The steel manufactured according to the invention method is superior in low temperature toughness and corrosion resistance to CO2, and it is also excellent in field weldability. It is suitable for large-diameter line pipes for transportation of oil or natural gas in cold regions and offshores.

Claims (3)

WHAT IS CLAIMED IS:

1. A manufacturing method of high corrosionresistant low alloy steel for line pipes, comprising the steps of heating the low alloy steel at a temperature in a range of 11000C to 12500C, rolling the steel with the cumulative rolling reduction at 9500C or less being 40% or more, a finish rolling temperature of 700"C to 8500C, and air-cooling or acceleratedly cooling the low alloy steel after rolling, the steel comprising, by weight %, 0.02 to 0.09 carbon, 0.5 or less silicon, 0.7 to 1.5 manganese, 0.03 or less phosphorus, 0.005 or less sulfur, 0.02 to 0.06 niobium, 0.5 to not more than 1.

2 chromium, 0.005 to 0.03 titanium, 0.05 or less aluminum, 0.002 to 0.005 nitrogen and the balance of iron and unavoidable impurities, and the low alloy steel satisfying the following equation: : 0.35 ' C + (Mn + Cr + V)/5 + (Ni + Cu)/15 < 0.48 2. A manufacturing method of high corrosionresistant low-alloy steel for line pipes according to Claim 1, wherein the steel further comprises one element or more selected from the group consisting of 0.01 to 0.08 vanadium, 0.05 to 0.5 nickel, 0.05 to 0.5 copper, and 0.001 to 0.005 calcium, by weight t.

3. A manufacturing method of high corrosionresistant low-alloy steel for line pipes according to Claim 2, wherein the steel satisfies the following equation: ESSP > (Ca) (1 - 124 (oxygen))/l.25 (S).