MXPA99001850A - Ultrafine-grain steel pipe and process for manufacturing the same - Google Patents
Ultrafine-grain steel pipe and process for manufacturing the sameInfo
- Publication number
- MXPA99001850A MXPA99001850A MXPA/A/1999/001850A MX9901850A MXPA99001850A MX PA99001850 A MXPA99001850 A MX PA99001850A MX 9901850 A MX9901850 A MX 9901850A MX PA99001850 A MXPA99001850 A MX PA99001850A
- Authority
- MX
- Mexico
- Prior art keywords
- steel tube
- less
- ferrite
- reduction
- temperature
- Prior art date
Links
- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 274
- 239000010959 steel Substances 0.000 title claims abstract description 274
- 238000004519 manufacturing process Methods 0.000 title claims abstract description 45
- 238000000034 method Methods 0.000 title abstract description 11
- 229910000529 magnetic ferrite Inorganic materials 0.000 claims abstract description 114
- 229910000859 α-Fe Inorganic materials 0.000 claims abstract description 114
- 238000010438 heat treatment Methods 0.000 claims abstract description 58
- 230000001131 transforming Effects 0.000 claims abstract description 18
- 229910052799 carbon Inorganic materials 0.000 claims abstract description 16
- 229910052710 silicon Inorganic materials 0.000 claims abstract description 11
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- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 8
- 238000005496 tempering Methods 0.000 claims description 33
- 239000000203 mixture Substances 0.000 claims description 29
- 239000011521 glass Substances 0.000 claims description 27
- 239000012535 impurity Substances 0.000 claims description 18
- 238000000265 homogenisation Methods 0.000 claims description 10
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- 230000001186 cumulative Effects 0.000 claims description 7
- 229910052804 chromium Inorganic materials 0.000 abstract description 5
- 229910052750 molybdenum Inorganic materials 0.000 abstract description 5
- 229910052759 nickel Inorganic materials 0.000 abstract description 5
- 229910052720 vanadium Inorganic materials 0.000 abstract description 5
- 229910052758 niobium Inorganic materials 0.000 abstract description 4
- 238000005096 rolling process Methods 0.000 abstract description 4
- 229910052719 titanium Inorganic materials 0.000 abstract description 4
- 229910052802 copper Inorganic materials 0.000 abstract description 3
- 239000000047 product Substances 0.000 description 61
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- 230000001939 inductive effect Effects 0.000 description 24
- 238000005260 corrosion Methods 0.000 description 22
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- 229910001563 bainite Inorganic materials 0.000 description 14
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- 230000004059 degradation Effects 0.000 description 9
- 238000006731 degradation reaction Methods 0.000 description 9
- 229910000734 martensite Inorganic materials 0.000 description 9
- 229910001566 austenite Inorganic materials 0.000 description 8
- 239000011575 calcium Substances 0.000 description 8
- 238000005275 alloying Methods 0.000 description 7
- 230000001965 increased Effects 0.000 description 7
- 229910001562 pearlite Inorganic materials 0.000 description 7
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- 235000010446 mineral oil Nutrition 0.000 description 4
- QTBSBXVTEAMEQO-UHFFFAOYSA-N acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 description 3
- 229910045601 alloy Inorganic materials 0.000 description 3
- 239000000956 alloy Substances 0.000 description 3
- REDXJYDRNCIFBQ-UHFFFAOYSA-N aluminium(3+) Chemical class [Al+3] REDXJYDRNCIFBQ-UHFFFAOYSA-N 0.000 description 3
- OKTJSMMVPCPJKN-UHFFFAOYSA-N carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 3
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- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 2
- GRYLNZFGIOXLOG-UHFFFAOYSA-N nitric acid Chemical compound O[N+]([O-])=O GRYLNZFGIOXLOG-UHFFFAOYSA-N 0.000 description 2
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- 241001517310 Eria Species 0.000 description 1
- 210000001503 Joints Anatomy 0.000 description 1
- 210000002445 Nipples Anatomy 0.000 description 1
- 208000001285 Stress Fractures Diseases 0.000 description 1
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminum Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 description 1
- 238000009835 boiling Methods 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- 239000012267 brine Substances 0.000 description 1
- OYPRJOBELJOOCE-UHFFFAOYSA-N calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 description 1
- 229910052791 calcium Inorganic materials 0.000 description 1
- 238000005255 carburizing Methods 0.000 description 1
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- 230000000670 limiting Effects 0.000 description 1
- 239000000314 lubricant Substances 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000006011 modification reaction Methods 0.000 description 1
- TWXTWZIUMCFMSG-UHFFFAOYSA-N nitride(3-) Chemical compound [N-3] TWXTWZIUMCFMSG-UHFFFAOYSA-N 0.000 description 1
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- 229920003245 polyoctenamer Polymers 0.000 description 1
- 238000004881 precipitation hardening Methods 0.000 description 1
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- 238000007493 shaping process Methods 0.000 description 1
- 238000004904 shortening Methods 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N silicon Chemical group [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 230000005476 size effect Effects 0.000 description 1
- 238000005549 size reduction Methods 0.000 description 1
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- QAOWNCQODCNURD-UHFFFAOYSA-L sulfate Chemical compound [O-]S([O-])(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-L 0.000 description 1
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- 229910052717 sulfur Inorganic materials 0.000 description 1
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- UCKMPCXJQFINFW-UHFFFAOYSA-N sulphide Chemical compound [S-2] UCKMPCXJQFINFW-UHFFFAOYSA-N 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
Abstract
A steel pipe which comprises ultrafine ferrite grains and possesses excellent toughness, ductility, ductility-strength balance, and impact resistance;and a process for manufacturing the same, comprising heating an untreated steel pipe, with the average grain diameter of ferrite being di(&mgr;m), comprising C, Si, Mn, and Al each limited in optimal content range and optionally added Cu, Ni, Cr and/or Mo, or Nb, Ti, V and/or B, to a temperature of Ac3 transformation point or below and then subjecting the heated pipe to stretch reduction with a total reduction of Tred (%) at an average rolling temperature of&thgr;m (°C) in the transformation temperature range of from 400 to Ac3 in such a manner that di,&thgr;m, and Tred satisfy a predetermined relational expression.
Description
STEEL PIPE DB GRAIN STJPBR PINE AND METHOD TO PRODUCE IT
TECHNICAL FIELD DB THE INVENTION
The present invention relates to a steel tube containing superfine glass grains, which has excellent strength, toughness and ductility, and superior resistance to collision impact, and to a method for producing the same.
ANTECEDENTS OF THE TECHNIQUE
The strength of steel materials has been increased so far by adding alloy elements such as Mn and Si, and by using, for example, controlled boring, controlled cooling, heat treatments such as rapid and tempering cooling, or by adding elements of Precipitation hardening such as Nb and V. However, in the case of steel material, not only strengths but also high ductility and toughness are required. Therefore, a steel material with balanced strength and ductility as well as tenacity is demanded. The reduction in crystal size is important insofar as it is one of the few means to increase not only the strength, but also the ductility and tenacity at the same time. Glass beads sufficiently reduced in size can be produced, for example, by a method which comprises avoiding the formation of coarse austenite grains and obtaining fine ferritic glass grains from fine austenite grains when using the austenite transformation. ferrite; a method which comprises obtaining fine ferrite grains from fine austenite grains made by worked; or a method which comprises using martensite or lower bainite which results from rapid and tempered cooling. In particular, controlled boring comprises intense work in the austenitic region and reducing the size of ferrite grains by using the subsequent transformation of austenite-ferrite which is widely used for the production of steel materials. In addition, a method for further reducing the size of ferrite grains by adding a trace amount of Nb and thus suppressing the recrystallization of austenite grains is also known in the art. By working at a temperature in the non-recrystallizing temperature region, the austenite grains grow so as to form a transgranular deformation band and ferrite grains are generated from the deformation band so that the size of the grains is further reduced. Ferrite grains. In addition, controlled cooling which comprises cooling during or after work has also been used.
However, the fine grains available with the above methods have lower limits in the grain size of about 4 to 5 μm. In addition, the methods are too complicated to be applied to the production of steel tubes. In light of such circumstances, a method comprising simple process steps and yet capable of further reducing the grain size of ferrite crystals to improve the toughness and ductility of steel tubes has been required. In addition, the concern of the increasing recent demand for steel tubes that have higher impact strengths by collision to obtain the improved safety objective of automobiles, limits cutting costs and has long been found with the methods listed above. , which are used, because they require considerable modification in the process steps including the replacement of equipment and the like. In addition, the improvement in tensile corrosion resistance with disulfide fractures of steel tubes for use in line pipes, to date, the hardness control is done to decrease the concentration of impurities and control the concentration of alloying elements. Conventionally, the resistance to fatigue has been improved by using thermal treatments such as hardening with rapid cooling and tempering, induction hardening and carburizing, or when adding expensive alloying elements such as Ni, Cr, Mo, etc., in large quantities. However, these methods have problems of damaging the susceptibility to welding and also of increasing the cost. A high-strength steel tube having a tensile strength exceeding 600 MPa is produced by using a material rich in carbon containing carbon (C) at a concentration of 0.30% or greater, or by a material containing C a a high concentration and other alloying elements added in large quantities. In the case of high strength steel tubes, this resistance increased by the above methods, however, tend to be damaged to the elongation properties. Therefore, in general the application of intense work is avoided, * in case intensive work is necessary, an intermediate annealing is carried out during the work and additional thermal treatments are applied such as normalization, rapid cooling and tempering, etc. However, the application of additional heat treatment such as intermediate annealing returns to the complicated process. "In light of the above circumstances, a method is demanded which allows the intense work of a high strength steel tube without applying intermediate annealing, and also, a further reduction in the glass grains is desired for the improvement in the working capacity of the high strength steel tubes.An objective of the present invention is to advantageously solve the above problems and provide an improved steel tube in ductility and impact resistance by collision without incorporating a considerable change in the production process Another object of the present invention is to provide a method for producing the same steel.Another object of the present invention is to provide a steel tube and a method to produce the same, the steel tube contains superfine grains that have excellent tenaci ductility and ferrite grains of 3 μm or less in size, preferably with a size of 2 μm, and more preferably 1 μm or less. A further object of the present invention is to provide a high strength steel tube containing superfine glass grains, which is improved in its working capacity and has a tensile strength of 600 MPa or greater, and with a method for produce the same.
DESCRIPTION DB LA TUVENCjg
The present inventors conducted extensive and intensive studies with respect to a method for producing high strength steel tubes having excellent ductility and still a high production rate. As a result, it has been found that high strength and high ductility steel tubes having well balanced properties of strength and ductility can be produced by applying reduce to a steel tube having a specified composition in a recovery temperature range of ferrite or recrystallization. First, the experimental results from which the present invention is derived are described below. A steel tube welded by stitching (f 42.7 mm D x 2.9 mmt) having a composition of 0.09% by weight of C - 0.40% by weight Si - 0.80% by weight of Mn - 0.04% Al is heated to each one of the temperatures in the range from 750 to 10 550 ° C, and the reduction is made by using a reduction mill to obtain product tubes that differ in outer diameter in a range of f 33.2 to 15.0 mm, while adjusting the output speed of stretching at 200 m / min. After boring, the tensile strength (TS) and the elongation (El) are measured for each of the product tubes, and the relationship between elongation and strength is graphically shown, as shown in Figure 1 ( plotted by black circles in the figure). In the figure, the white circles show the relationship between the elongation and the strength of the steel tubes 20 welded by stitching of different size which are obtained by welding but without applying boring. For the elongation values (El), a reduced value obtained by the following equation:
The * E10 x (V (aO / a) ° -4 (where, E10 represents the observed elongation, aO is a value equivalent to 292 mm2, already represents the cross-sectional area of the sample (mm2)). Referring to Figure 1, it can be seen that a higher elongation can be obtained if the base steel tube is subjected to reduction in the temperature range from 750 to 550 ° C compared to the elongation of a steel tube welded by stitching as it is welded to the same strength, that is, the present inventors have found that a high strength steel tube having a good balance in ductility and strength can be obtained by heating a base steel tube having a specified composition up to a temperature range of 750 to 400 ° C and apply reduction.In addition, it has been found that the steel tube produced by the above production method contains fine ferrite grains of 3 μm or smaller in size. In addition to impact strengths due to collision, the present inventors also obtained the relationship between tensile strength
(TS) and ferrite grain size while greatly changing the degree of deformation of 2,000 s "1. As a result, it has been found that the tensile strength increases considerably when the diameter of ferrite grain decreases. 3 μm or less, and that the increase in TS is particularly large to the impact deformation by collision in the case where the degree of deformation is high.Therefore, it has been * found additionally that the steel tube having Fine ferrite grains show not only a superior balance in ductility and strength, but also considerably improved properties of collision impact resistance The present invention, which enables a superfine granular steel tube further reduced in grain size to 1 μm or less, it provides a method for producing steel comprising heating or homogenizing a steel tube having an ODi (mm) outer diameter and having grain s of ferrite with an average crystal diameter of di (μm) in the cross section perpendicular to the longitudinal direction of the steel tube, and then apply tempering at an average bore temperature of? m (° C) and a ratio of total reduction Tred (%) to obtain a product tube having a diameter of ODf (mm), wherein the tempering comprises performing it in a temperature range of 400 ° C or more, but not greater than the heating or homogenization temperature, and in such a way that the average glass diameter of di (μm), the average bore temperature of? m (° C) ) and the ratio of total reduction Tred (%) are in a relation that satisfies equation (1) as follows: di =. (2 .65 - .003 x? M) x lo * 10 -008 + ßm / soooo) x xrßd}
- - - ( 1 )
where, di represents the average glass diameter of the steel tube base (μm), *? m represents the average bore temperature (° C) (= (? i +? f) / 2; where? i is the temperature Initial bore (° C) and? f is the finished bore temperature (° C)), * and Tred represents the ratio of total reduction (%) (= ODi - ODf x 100 / ODi; where, ODi is the outer diameter of the steel tube (mm) and ODf is the outer diameter of the product tube (mm)). In the present invention, the reduction is preferably carried out in the temperature range from 400 to 750 ° C. It is also preferred that the heating or homogenization of the base steel tube is carried out at a temperature not higher than Ac3 at the transformation temperature. It is further preferred that the heating or homogenization of the base steel tube is carried out at a temperature in a range defined by (Acx + 50 ° C) when considering the Acx transformation temperature as the reference temperature. In addition, quenching is preferably carried out under lubrication. Preferably, the reduction process is adjusted such that it comprises at least one step having a reduction ratio per step of 6% and wherein the cumulative reduction ratio is 60% or greater.
In addition, the method for producing a superfine granular steel tube containing superfine grains having an average grain size of 1 μm or less according to the present invention preferably uses a steel tube containing 0.70% by weight or less of C as the base steel tube, and preferably a steel tube containing, by weight, 0.005 to 0.30% of C, 0.01 to 3.0% of Si, 0.01 to 2.0% of Mn, 0.001 to 0.10% of Al and the rest of Faith with unavoidable impurities. In the present invention, in addition, the above composition may additionally contain at least one type that is selected from one or more groups that are selected from groups A to C shown below: Group A: 1% or less Cu, 2 % or less of Ni, 2% or less of Cr, and 1% or less of Mo; Group B: 0.1% or less of Nb, 0.5% or less of V, 0.2% or less of Ti, and 0.005% or less of B; and Group C: 0.02% or less of REM and 0.01% or less of Ca. Additionally, the present inventors have found that by restricting the composition of the base steel tube in the appropriate range, a steel tube can be produced which it has high strength and toughness and even superior resistance against stress corrosion fractures when using the above method to produce steel tubes, and that such steel tubes can be advantageously used as steel tubes for line or pipe lines.
In order to improve the strength properties of stress corrosion fracture, conventionally, steel tubes for use in in-line tubes have been subjected to hardness control which comprises reducing the content of impurities such as S or control of the elements of alloy. However, such methods have limits in improving resistance and present problems of increasing the cost. By further restricting the composition of the base steel tube to an appropriate range, and by applying reduction of the base steel tube in the ferritic recrystallization region, fine ferrite grains and fine carbides can be dispersed so that a steel tube with high strength and high tenacity. At the same time, the alloying elements can be controlled in a manner that decreases the hardening by welding, while suppressing the generation and development of fractures so that the resistance to fracture by stress corrosion is improved. present invention provides a steel tube having excellent ductility and impact resistance by collision, and still improvement in resistance to stress corrosion fractures when applying tempering under conditions satisfying equation (1) to a base steel tube that contains, by weight, 0.005 to 0.10% of C, 0.01 to 0.5% of Si, 0.01 to 1.8% of Mn, 0.001 to 0.10% of Al and which also contain at least one or more types that are selected from the type that consists of 0.5% or less of Cu, 0.5% or less of Ni, 0.5% or less of Cr and 0.5% or less of Mo; or in addition, one or more selected from the group consisting of 0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti and 0.004% or less of B; or additionally, one or more selected from the group consisting of 0.02% or less of REM and 0.01% or less of Ca; and the rest of Faith with inevitable impurities. In addition, the present inventors have found that, by restricting the composition of the base steel tube in a suitable additional range, a steel tube having high strength and toughness and still having superior fatigue resistance properties can be produced. using the above method to produce steel tubes, and that such steel tubes can advantageously be used as steel tubes with high fatigue strength. By restricting the composition of the base steel tube to an appropriate range, and by applying tempering to the base steel tube in the ferritic recovery region and recrystallization, fine ferrite grains and fine precipitates can be dispersed so that an Steel tube with high strength and high tenacity. At the same time, the elements of the alloy can be controlled in such a way that the hardening by welding decreases, while the generation and development of fatigue fractures is suppressed so that the fatigue resistance properties are improved.
That is, the present invention provides a steel tube having excellent ductility and impact resistance by collision, and which is still improved in fatigue resistance properties by applying tempering under conditions satisfying equation (1) to a tube of base steel containing, by weight, 0.06 to 0.30% C, 0.01 to 1.5% Si, 0.01 to 2.0% Mn, 0.001 to 0.10% Al, and the rest of Fe with unavoidable impurities. Additionally, it is possible to obtain a high strength steel tube having excellent working capacity, characterized in that it has a composition containing, by weight, more than 0.30% at 0.70% C, 0.01 at 2.0% Si, 0.01 to 2.0 % of Mn, 0.001 to 0.10% of Al, and the rest of Fe with unavoidable impurities, and a texture consisting of ferrite and a second phase different from the ferrite that constitutes more than 30% in proportion of area, with the cross section perpendicular to the longitudinal direction of the steel tube containing superfine ferrite grains having an average crystal grain size of 2 μm or less; otherwise, with a cross section perpendicular to the longitudinal direction of the steel tube containing superfine ferrite grains having an average crystal grain size of 1 μm or less.
BRIEF DESCRIPTION OF IOS DIBOJOS
Figure 1 is a graph showing the relationship between the elongation and the tensile strength of the steel tube, * Figure 2 is a graph showing the influence of the tensile strain rate on the relationship between the resistance to the tensile and grain size of ferrite crystals of the steel tube; Figure 3 is an electron micrograph showing the metallic texture of the steel tube obtained as an example in accordance with the present invention; Figure 4 is a schematically drawn diagram of an example of equipment line according to a preferred embodiment of the present invention, * Figure 5 is a schematically drawn diagram of an example of a production equipment for pressure-welded steel pipes. solid state and a production line for continuous production according to a preferred embodiment of the present invention; Figure 6 is a graph showing the relationship between the total reduction ratio and the average glass grain size of the steel tube, which are the parameters that affect the reduction of the size of the glass beads of the product tube.; and Figure 7 is an explanatory diagram, schematically drawn, showing the shape of the test sample for use in the sulfide stress corrosion fracture resistance test. 5 (Explanation of Symbols)
1 Flat strip 2 Preheating furnace 10 3 Forming and working apparatus 4 Induction heating device for preheating the edges 5 Induction heating device for heating the edges 15 6 Pressing roller 7 Open tube 8 Base steel tube 14 Unwinder 15 Jointing device 20 16 Product tube 17 Oiler 18 Cutter 19 Tube straightening device 20 Thermometer 25 21 Reducing mill 22 Homogenizing oven (seam cooling and tube heating apparatus) 23 Rust removing apparatus 24 rapid cooling 25 Reheating device 26 Cooling device
MK.TOR MODE FOR CARRYING OUT THE INVENTION
In the present invention, a steel tube is used as the starting material. There is no particular limitation regarding the method to produce the base steel tube. Thus, a steel pipe welded by electrical resistance (steel pipe welded by stitching) can be favorably used using electric resistance welding, a solid-pressure steel tube welded by heating both edge portions of a pipe open to a region of pressure welding temperature in the solid state and realization of pressure welding, a steel tube welded by forging, or a seamless steel pipe obtained by using a Mannesmann punch. The chemical composition of the base steel tube or steel tube of product is limited according to the following reasons: C: 0.07% or less:
Carbon is an element that increases the strength of steel by forming a solid solution with the matrix or by precipitation as a carbide in the matrix. It also precipitates as a second hard phase in the form of fine cementite, martensite or bainite, and contributes to increasing ductility (uniform elongation). To obtain a desired strength and to obtain the improved ductility effect by using cementite and similar precipitate as the second phase, C must be present in a concentration of 0.005% or greater, and preferably 0.04% or higher. Preferably, the concentration of C is in the range of not greater than 0.30% and more preferably of 0.10% or less. In view of these requirements, the concentration of C is preferably limited in a range from 0.005 to 0.30%, and more preferably, in a range from 0.04 to 0.30%. To improve the stress corrosion fracture resnce of the steel tube to make it suitable for use in in-line pipes, the concentration of C is preferably controlled in a range of 0.10% or less. If the concentration exceeds 0.10%, the resnce to fracture by stress corrosion decreases due to the hardening of the welded portion. To improve the fatigue strength properties of the steel tube to make it suitable for use as a steel tube with high fatigue resnce, the C concentration is preferably controlled at a range from 0.06 to 0.30%. If the concentration is less than 0.06%, the fatigue resnce properties decrease due to insufficiently low strength. To obtain a desired strength of 600 MPa or greater, the concentration of C must exceed 0.30%. However, if C is added at a concentration exceeding 0.70%, the ductility is adversely affected. Therefore, the concentration of C must be in a range that exceeds 0.30% but not greater than 0.70%. Yes: 0.01 to 3.0%: Silicon functions as a deoxidizing element, which increases the strength of the steel by forming a solid solution with the matrix. This effect is observed in the case where Si is added at a concentration of 0.01% or higher, preferably 0.1% or higher, but an addition exceeding 3.0% damages the ductility. In case of a high strength steel tube, the upper limit in concentration is set at 2.0% when considering the ductility problem under consideration. Therefore, the concentration of Si is established in a range from 0.01 to 3.0%, or from 0.01 to 2.0%. However, preferably, the range is between 0.1 and 1.5%. To improve the resnce to stress corrosion fracture of the steel tube to make it suitable for use in in-line or pipe tubes, the Si concentration is preferably controlled at 0.5% or less. If the concentration exceeds 0.5%, the fracture resnce due to stress corrosion decreases due to the hardening of the welded portion. To improve the fatigue strength properties of the steel tube to make it suitable for use as a high strength steel tube, the Si concentration is preferably controlled at 1.5% or less. If the concentration exceeds 1.5%, the fatigue resnce properties decrease due to the formation of inclusions. Mn: 0.01 to 2.0%: Manganese increases the strength of the steel, and accelerates the precipitation of a second phase in the form of fine cementite, or martensite or bainite. If the concentration is less than 0.01%, not only is it impossible to obtain the desired strength, but also the fine precipitation of cementite or the precipitation of martensite and bainite is damaged. If the addition should exceed 2.0%, the strength of the steel increases excessively and adversely damages the ductility. Therefore, the concentration of Mn is limited in a range from 0.01 to 2.0%. From the point of view of making a balance between resnce and elongation, the Mn concentration is preferably in the range of 0.2 to 1.3% and more preferably in a range of 0.6 to 1.3%. To improve the tensile strength fracture resistance of the steel tube to make it suitable for use in in-line pipes, the concentration of Mn is preferably controlled at 1.8% or less. If the concentration exceeds 1.8%, the fracture resistance due to stress corrosion decreases due to the hardening of the welded portion. Al: 0.001 to 0.10%: Aluminum provides fine crystal grains. To obtain such fine crystal grains, Al must be added at a concentration of at least 0.001%. However, an addition in excess of 0.10% increases the inclusions that contain oxygen which damages clarity. Therefore, the concentration of Al is set in a range from 0.001 to 0.10%, and preferably in a range from 0.015 to 0.06%. In addition to the above basic steel composition, at least one type of an alloying element can be added which is selected from one or more groups from A to C below. Group A: Cu: 1% or less, Ni: 2% less, Cr: 2% or less, and Mo: 1% or less: Any element selected from the group of Cu, Ni, Cr and Mo improves the property of rapid cooling of the steel and increases the resistance. Therefore, one or two or more elements can be added based on the requirements. These elements decrease the transformation point and effectively generate fine ferrite or second phase grains. However, the upper limit for the concentration of Cu is adjusted by 1% due to the fact that Cu incorporates large quantities and damages the hot working capacity. The Ni increases not only the resistance but also the tenacity. However, the effect of Ni saturates an addition that exceeds 2%, and also increases the costs excessively. Therefore, the upper concentration limit is set at 2%. The addition of Cr or Mo in large quantities not only damages the susceptibility to welding, but also increases the total expense. Therefore, its upper limits are set at 2% and 1%, respectively. Preferably, the concentration range for the elements in group A is from 0.1 to 0.6% for Cu, from 0.1 to 1.0% for Ni, and from 0.1 to 1.5% for Cr, and from 0.05 to 0.5% for Mo. To elaborate Useful steel pipes for line pipe to improve the resistance against stress corrosion fractures, the concentration of Cu, Ni, Cr and Mo are each restricted to be 0.5% or less. If any of them is added in large quantities so that they exceed the concentration of
0. 5%, hardening of the welded portion occurs in a manner that degrades the fracture resistance by stress corrosion. Group B: Nb: 0.1% or less, V: 0.5% or less, Ti: 0.2% or less, and B: 0.005% or less: Any element of the group consisting of Nb, V, Ti and B precipitates as a carbide , neither a nitride nor a carbonitride, and contributes to the production of fine crystal grains and higher strength. In particular, for steel tubes which have joints and which are heated to high temperatures, these elements effectively function to produce fine crystal grains during the heating for bonding, or as precipitation cores for ferrite during cooling. Therefore they are effective in preventing hardening in the joint portions. Therefore, one or two or more elements can be added, based on the requirements. However, since the addition in large quantities leads to degradation in the susceptibility to welding and toughness, the upper limits for the concentration of the elements are established as follows: 0.1% for Nb; 0.5%, preferably 0.3% for V; 0.2% for Ti, and 0.005%, preferably 0.004% for B. More preferably, the concentration range for the elements in group B is from 0.005 to 0.05% for Nb, 0.05 to 0.1% for V, from 0.005 to 0.10% for Ti, and from 0.0005 to 0.002% for B. To make useful steel pipes for line pipes by improving resistance against stress corrosion fractures, the concentration of Nb, V and Ti is each restricted to be less than 0.1% or lower. If either of them is added in larger amounts so that they exceed the 0.1% concentration, hardening of the welded portion occurs so that it degrades the fracture resistance by stress corrosion. Group C: REM: 0.02% or less and Ca: 0.01% or less: REM and calcium Ca control the form of inclusions and improve the working capacity. Any element in this group precipitates as a sulphide, an oxide or a sulphate and prevents hardening from occurring in the joint portions of steel tubes. Therefore, one or more elements can be added based on the requirements. However, if the addition should not exceed the limits of 0.02% for REM and 0.01% for Ca, too many inclusions are formed in a way that decreases clarity, and a degradation in ductility results as a result. It should be noted that an addition of less than 0.004% for REM, or an addition of less than 0.001% of Ca shows a small effect. Therefore, it is preferred that REM be added as such to provide a concentration of 0.004% or greater and that Ca be added at 0.001% or higher. The base steel tubes and the steel product tubes contain, in addition to the components described above, the rest of Fe with unavoidable impurities. The permissible as unavoidable impurities are 0.010% or less of N, 0.006% or less of O, 0.025% or less of P, and 0.020% or less of S. N: 0.010% or less: Ni is allowed at a concentration of 0.010 %; a quantity necessary for it to be combined with Al to produce fine crystal grains. However, an incorporation of it in excess of this limit damages the ductility. Therefore, it is preferred that the concentration of N is decreased to 0.010% or less, and more preferably, the concentration thereof is controlled to be in the range from 0.002 to 0.006%.
O: 0.006% or less: O damages clarity by forming oxides. Its incorporation is not desirable, and its tolerable limit is 0.006%. P: 0.025% or less: P is preferably not incorporated, because it damages tenacity by segregation in grain boundaries. The permissible limit of the same is 0.025%. S: 0.020% or less: S is preferably not incorporated, because it increases the sulfides, and leads to degradation of clarity. The permissible limit of the same is 0.020%. A description of the structure of the product tubes is given below. 1) The steel tube according to the present invention has excellent ductility and impact resistance properties by collision, and comprises a texture based on ferrite grains having an average crystal diameter of 3 μm or less. If the size of the ferrite grains exceeds 3 μm, no apparent improvement in ductility can be obtained as well as in the collision impact resistance properties, that is, the properties of resistance against impact weight. Preferably, the average crystal size of the ferrite grains is 1 μm or less.
The average crystal diameter of the ferrite grains in the present invention is obtained by observation under an optical microscope or an electron microscope. More specifically, a cross section obtained by cutting the steel tube perpendicular to the longitudinal direction thereof, and the observation is made on the attacked surface using a reagent for Nital attack. Therefore, the diameter of the equivalent circle is obtained for 200 or more grains, and the average thereof is used as the representative value. The structure based on ferrite grains as referred to in the present invention includes a structure containing only ferrite and having no precipitation from a second phase, and a structure containing ferrite and a second phase other than ferrite. It is mentioned as the second phase different to martensite ferrite, bainite and cementite, which can precipitate alone or as a compound of two or more of them. The area ratio of the second phase should constitute 30% or less. The second phase precipitated in this way contributes to the increase in uniform elongation in case of deformation. Therefore, it improves the ductility and impact resistance properties due to collision. However, this effect becomes less apparent if the area ratio of the second phase exceeds 30%.
2) The high-strength steel tube according to the present invention comprises a ferrite-based structure and a second phase that constitutes more than 30% in area ratio, and contains grains having an average crystal diameter of 2 μm or less as they are observed in a cross section cut perpendicular to the longitudinal direction of the steel tube. As the second phase different from ferrite, martensite, bainite and cementite are mentioned, which can precipitate alone or as a compound of two or more of them. The area ratio of the second phase should constitute more than 30%. The second phase precipitated in this way contributes to the increase in strength and a uniform elongation in a way that improves strength and ductility. However, such an effect is small if the area ratio of the second phase is 30% or less. Therefore, the area ratio of the second phase other than the ferrite is preferred to be greater than 30% but not greater than 60%. If the proportion of area "exceeds 60%, the ductility is damaged due to the formation of coarse cementite grains.If the average glass diameter exceeds 2 μm, a distinctive improvement in ductility is no longer observed, and therefore both there is no apparent improvement in working capacity Preferably, the average grain diameter of ferrite is 1 μm or less.
The average crystal grain diameter according to the present invention is obtained by observation under an optical microscope or an electron microscope. More specifically, a cross section obtained by cutting the zero tube perpendicular to the longitudinal direction thereof, and the observation is made on the attacked surface using a reagent for Nital attack. Therefore, the diameter of the equivalent circle is obtained for two or more grains, and the average of them is used as the representative value. The grain diameter of the second phase is obtained by taking the boundary of the perlite colony as the grain limit case in the case where pearlite is the second phase and, when taking the package limit as the grain limit in the case of bainite or martensite as the second phase. In Figure 3 an example of a steel tube according to the present invention is provided. The "method for producing the steel tube according to the present invention is described below." The base steel tube of the composition described above is heated in a temperature range of Ac3 at 400 ° C, preferably to a range from (ACj. + 50 ° C) to 400 ° C, and more preferably, in a range of 750 to 400 ° C. If the heating temperature exceeds the Ac3 transformation point, not only degradation of the surface properties occurs, but also formation of coarse crystal grains occurs. Accordingly, the heating temperature for the base steel tube is preferably set at a temperature no higher than the Ac3 transformation point, preferably not greater than (Acj, + 50 ° C), and more preferably, not higher of 750 ° C. On the other hand, if the heating temperature is less than 400 ° C, a favorable bore temperature can not be carried out. Therefore, the heating temperature is preferably not less than 400 ° C. Then, the heated base steel tube is subjected to tempering. Although not limiting, quenching is preferably carried out using a three-roll type reducing mill. The reducing mill preferably comprises a plurality of stations, so that the boring is carried out continuously. The number of stations can be determined based on the size of the base steel tube and the product steel tube. The boring temperature for reduction is in a range corresponding to the recovery of ferrite and the recrystallization temperature range, ie from Ac3 to 400 ° C, but preferably in a range of (Acx + 50 ° C) to 400 ° C, and more preferably, in a range from 750 to 400 ° C. If the boring temperature exceeds the Ac3 transformation point, superfine glass grains are no longer available, and the ductility does not increase as expected at the expense of a decreased resistance. Therefore, the boiling temperature is set to a temperature no greater than the transformation point Ac3, preferably at a temperature not higher than (Acx + 50 ° C), and more preferably not higher than 750 ° C. If the bore temperature should be less than 400 ° C, on the other hand, the material becomes brittle due to a blue shortening (brittle condition) and may experience breakage. In addition, at lower boring temperatures of 400 ° C, not only does the material's deformation resistance increase in such a way that boring becomes difficult, but also the working voltage tends to remain due to insufficient recovery and recrystallization of the material . Therefore, the tempering is carried out in a temperature range limited from Ac3 to 400 ° C, preferably in a range of (Acx + 50 ° C) to 400 ° C, and more preferably in a range from 750 to 400 ° C. More preferably, the temperature range is from 600 to 700 ° C. The cumulative reduction ratio in diameter during tempering is adjusted to 20% or greater. If the proportion of cumulative reduction in diameter, which is equivalent to. { [(external diameter of the base steel tube) - (outer diameter of the product tube)] /
(outer diameter of the base steel tube) x 100.}. , it must be less than 20%, the crystal grains subjected to recovery and recrystallization tend to be insufficiently reduced in size. Such a steel tube can not show superior ductility. In addition, the production efficiency becomes low due to the low speed of tube production. Accordingly, in the present invention, the cumulative reduction ratio in the diameter is adjusted to 20% or higher. However, at a cumulative reduction rate of 60% or more, not only does increased strength due to work hardening occur, but also fine structures become prominent. Therefore, even in a steel tube having a component system containing the alloying elements at a lower concentration compared to the composition ranges mentioned above, a well-balanced strength and ductility can be imparted thereto. It can be understood from the foregoing that, more preferably, the cumulative reduction ratio in diameter is set at 60% or higher. When performing the tempering, it is preferred that the bore comprises at least one step having a diameter reduction ratio per pitch of 6% or greater. If the ratio of diameter reduction per pass during tempering is set lower than 6%, the fine crystal grains which result from the recovery and recrystallization processes tend to be insufficiently reduced in size. On the other hand, with a diameter reduction ratio per step of 6% or more, a rise in temperature is produced by the working heat, which prevents a decrease in temperature from occurring. Therefore, the reduction ratio of diameter per passage is preferably set to 8% or greater, so that a high effect is obtained by producing finer crystal grains. The tempering process of the steel tube according to the present invention produces a biaxial tensioned mandrel, which is particularly effective for obtaining fine crystal grains. In contrast to this, the boring of the steel sheet is under uniaxial tension because the free end is present in the width direction of the sheet (ie, in the direction perpendicular to the boring direction). Therefore, the reduction in grain size becomes limited. In the present invention, it is preferred that the tempering is carried out under lubrication conditions, when the tempering is performed under lubrication, the deformation distribution in the thickness direction becomes uniform so that the distribution of "crystal" size distribution is also it becomes uniform in the direction of thickness If a non-lubricated bore is to be made, the deformation is concentrated only on the surface layer portion of the material so that it alters the uniformity of the glass grains in the thickness direction. lubricant can be carried out by using a boring oil well known in the art, for example, a mineral oil or a mineral oil mixed with a synthetic ester can be used without limitation.After reducing, the steel material is cooled At room temperature, cooling can be done by using air cooling, but from a point of view of suppression of grain growth as much as possible, in any of the cooling methods known in the art, for example, cooling with water, cooling with haze or cooling with forced air, are applicable. The cooling rate is 1 ° C / second or higher, and preferably
° C / second or greater. In addition, the stepped cooling so as to retain the middle part of cooling, can be used based on the requirements of the properties of the product. In the method according to the present invention, the tempering as described below can be applied to the base steel tube by stably maintaining the glass grain diameter of the product tube at 1 μm or less, or at 2 μm or less in the case of a steel tube of high resistance Assume that the average crystal grain diameter of the ferrite grains or, even of the second phase in the case of the high strength steel tube, is di (μm), as seen in the cross section cut perpendicular to the direction Longitudinal of the steel tube in an ODi outside diameter (mm). The base steel tube is then heated or homogenized, and is subjected to tempering at an average bore temperature of? M (° C) and a ratio of total reduction in diameter of Tred (%) so that a tube is obtained of finished product having an outer diameter of ODf (mm). The reduction is preferably applied by using a plurality of step rollers called reducers. An example of a line of equipment suitable for carrying out the present invention is shown in Figure 4. Figure 4 shows a boring apparatus 21 comprising a plurality of stations having a pitch. The number of stations of the laminator is determined appropriately based on the combination of the diameter of the base steel tube and the product tube. For the step rollers, any selected type of rollers well known in the art, for example, two rollers, three rollers or four rollers, can be applied favorably. There is no particular limitation regarding the heating or homogenization method, however, it is preferred that it be heated using a heating or induction heating oven. In particular, the method of induction heating is preferred from the viewpoint of a high rate of heating and high productivity, or from the viewpoint of its ability to suppress the growth of crystal grains. (Figure 4 shows an overheating apparatus 25 of a type of induction heating). The heating or homogenization is carried out at a temperature not higher than the transformation point Ac3 which corresponds to a temperature range in which there is no formation of glass grain thickness, or at a temperature not higher than (Acx + 50 ° C) ), by taking the Acx transformation point of the base steel tube as the standard, or more preferably, in the temperature range from 600 to 700 ° C. In the present invention, as a matter of course, the product tube results with fine glass grains even if the heating or homogenization of the base steel tube is to be carried out at a temperature which deviates from the above temperature range. In the case where the second phase in the texture of the base steel tube is pearlite, the stratified cementite incorporated in the perlite undergoes size reduction by separation when performing the boring in the above temperature range. Therefore, the working capacity of the product tube is improved because better elongation properties are acquired. Similarly, in the case where the second phase in the structure of the base steel tube is bainite, the bainite undergoes recrystallization after working so as to form a fine bainite ferrite structure. Therefore the working capacity of the product tube is improved due to the improved elongation properties. The reduction is carried out at a temperature range of 400 ° C or higher but not greater than the heating or homogenization temperature. Preferably, the temperature is not higher than 750 ° C. The region of temperature over the Ac3 transformation point, or over (ACi + 50 ° C) or over 750 ° C, corresponds to the two-phase region of austenite-rich austenite-ferrite, or a single-phase austenite region . Therefore, it is difficult to obtain a ferritic texture or a texture based on ferrite per worked. further, the effect of producing fine crystal grains by ferritic work can not be fully exhibited. If the tempering is to be carried out at a temperature higher than 750 ° C, the ferrite grains grow considerably after recrystallization so that it becomes difficult to obtain fine grains. In the case where the tempering is carried out at a temperature lower than 400 ° C, on the other hand difficulties are encountered in carrying out the tempering because the temperature interval corresponds to the region of blue fragility, or a decrease in temperature. the ductility and tenacity due to the fact that the work tensions tend to remain due to insufficient recrystallization. Therefore, the tempering temperature is adjusted to a temperature not "lower than 400 ° C but not higher than the Ac3 transformation point, or a temperature not higher than (Acx + 50 ° C), and preferably at a temperature not greater than 750 ° C. More preferably, the temperature range is from 560 to 720 ° C, and much more preferably from 600 to 700 ° C. The reduction is performed in the temperature range described above, and under conditions satisfying equation (1), where di (μm), represents the average ferrite crystal diameter as observed in the cross section perpendicular to the longitudinal direction of the base steel tube; ? m (° C) represents the average boring temperature in tempering; and Tred (%) represents the proportion of total reduction. 5 In the case where di,? And Tred do not satisfy the relationship expressed by equation (1), the ferrite crystals of the resulting product tube can not be microgranulated so as to provide an average diameter (diameter as observed in the cross section perpendicular to the direction
longitudinal of the steel tube) of 1 μm or less. Similarly, the resulting high strength of the steel tube can not provide micrograins as such that have an average diameter (diameter observed in the cross section perpendicular to the longitudinal direction of the steel tube) of
2 μm or less. Product steel tubes that differ in diameter are produced by boring a base steel tube equivalent to JIS STKM 13A (having an ODi of 60.3 mm and a wall thickness of 3.5 mm) when using a boring apparatus consisting of from
22 stations connected in series of a 4-roll mill and under conditions of an output speed of 200 m / min, an average bore temperature of 550 to 700 ° C. Figure 6 shows the influence of the ratio of total reduction in the diameter and average diameter of the steel tube
basis on the glass grain diameter of the finished product tube. The conditions shown by the region with diagonals satisfies the relationship expressed by equation (1), and the base steel tubes with conditions found in this region are capable of providing product tubes comprising crystal grains of 1 or less diameter. After boring, a product tube 16 is preferably cooled to a temperature of 300 ° C or lower. The cooling can be carried out by cooling with air, but with the aim of suppressing grain growth as much as possible, any of the cooling methods known in the art can be applied, for example, cooling with water, cooling with fog or forced air cooling, when using a rapid cooling apparatus 24. The cooling rate is 1 ° C / sec or more, and preferably 10 ° C / sec or more. In the present invention, a cooling apparatus 26 can be installed on the inlet side of a bore apparatus 21, or in the middle part of a bore apparatus 21 for controlling the temperature. In addition, an oxide removal apparatus 23 may be provided on the inlet side of the boring apparatus 21. The base steel tube for use as the starting material in the present invention can be any steel tube selected from the seamless steel tube, welded steel tube, a steel tube welded by forging, a welded steel tube. by solid pressure, and the like. In addition, the superfine granular steel tube production line according to the present invention can be connected to the production line for a base steel tube described in the above. An example of connecting the production line to the production line of the solid-pressure-welded steel tube is shown in Figure 5. A flat strip 1 comes out from an unwinder 14 and is connected to a preceding ring when using the apparatus 15, and after being preheated by a preheating furnace 2 via a nipple 17, is worked in an open tube 7 when using a forming apparatus 3 formed of a plurality of forming rollers. The edge portion of the open tube 7 is thus obtained and heated to a region of temperature lower than the melting point by an induction heating apparatus 4 of edge preheating and an induction heating apparatus 5 the edge, and butt weld when using a press roll 6 to obtain a base steel tube 8. Then, as described above, the base steel tube 8 is heated or homogenized at a predetermined temperature. using a homogenizing furnace 22, the oxide is removed by the rust removing apparatus 23, bordered when using the boring apparatus 21, cut by a cutter and straightened by a tube straightening apparatus 19 to finally provide the product tube 16. The temperature of the steel tube is measured by using a thermometer 20. Similarly in the case of tempering, as described in the above, the boring is preferably carried out under lubrication. Therefore, according to the production method described above, a steel tube can be obtained consisting of superfine ferrite grains of 1 μm or less in an average crystal grain size, as seen in the section cut cross section perpendicular to the longitudinal direction of the steel material. In addition, the above production method is effective for producing steel tubes, such as steel tubes welded by sewing, steel tubes welded by forging, steel tubes welded by pressure in solid, having a uniform hardness in the portion of steel. sewing It is also possible to produce, din an intermediate annealing, a high strength steel tube having a texture, which comprises ferrite and a second phase different from the ferrite which constitutes more than 30% in proportion of area, and which still consists of Superfine ferrite grains of 2 μm or less in the average crystal grain size, as seen in a cross section cut perpendicular to the longitudinal direction of the steel material.
(EXAMPLE 1)
The base steel tubes whose chemical composition is shown in table 1 were each heated to the temperatures provided in table 2 by using an induction heating coil, and by using three-roll structure laminators, and were andrilated under conditions shown in table 2 to provide the product tubes. In table 2, a steel tube welded by pressure in the solid state is obtained by preheating to 2.6 mm in thickness of a flat strip hot-drilled at 600 ° C, continuously forming the resulting flat strip in an open tube by using a plurality of rollers, preheat both edge portions of the tube open at 1000 ° C by means of induction heating, and additional heating of both edge portions to the non-melting temperature region of 1450 ° C by an induction furnace, in which both ends are put on "when using a press roll, where the solid phase pressure welding is carried out, so that a steel tube of 42.7 mm in diameter and 2.6 mm in thickness is obtained. A seamless steel tube is produced by heating a continuously cast billet, followed by the production of a tube when using a Mannesmann mandrel type rolling mill. The structure and properties of the products are investigated and the results are given in table 2. The tensile properties were measured in a JIS No. 11 test piece. The elastic limit is obtained by taking the lower yield point in the case in that the elasticity phenomenon is clearly observed, but 0.2% of PS was used for the other cases. For the elongation value, a reduced value is obtained according to the following equation when taking the size effect of the test piece under consideration:
El = E10 x ((aO / a) 0.4
(where, E10 represents the observed elongation, aO is a value equivalent to 292 mm2, and a represents the cross-sectional area of the sample (mm2)). The collision impact properties were obtained by performing high-speed tensile tests at a drawing speed of 2,000 s "1. Then, the energy absorbed to a stretch of 30% is obtained from the stretch-deformation curve observed for Use as collision impact absorption energy, par evaluation The collision impact property is presented by a deformation energy of a material at a deformation speed from 1,000 to 2,000 s "1 that corresponds to the collision of a car, and it's higher for a higher value.
From Table 2, it can be understood that the samples within the scope of the present invention (numbers 1 to 16 and numbers 19 to 22) show excellent balance in ductility and strength. In addition, the high tensile strength is observed for these samples having a high strain rate, and these samples are also high in impact absorption energy by collision. On the other hand, samples that fall outside the scope of the claims according to the present invention, ie, Comparative Examples Number 17, Number 18 and Number 23, present low values for either ductility or strength. These samples suffer not only from a poor balance in strength and ductility but also a low collision impact property. Comparative examples Numbers 17 and 18 further provide a reduction ratio that are outside the range according to the present invention, showing coarseness in the ferrite grains, and have a poor balance in resistance-ductility and a low Impact absorption energy by collision.
(EXAMPLE 2)
The base steel tubes whose chemical composition is shown in table 3, where each one is heated to the temperatures indicated in table 4 when using an induction heating coil, and by the use of three roll structure laminators, were bored under conditions shown in Table 4 to provide the product tubes. The base steel tubes are produced in the same procedure as that described in example 1. The tensile properties, collision impact properties and structures of the product tubes were investigated in the same manner as in the example, and the results are given in Table 4. From Table 4, it can be understood that the samples within the scope of the present invention
(numbers 2-1 to 2-3, numbers 2-6 to 2-8 and numbers 2-10 to numbers 2-14) show excellent balance in ductility and strength. In addition, a high tensile strength is observed for these samples with a higher degree of deformation, and these samples are also high in terms of energy "impact absorption by collision." On the other hand, samples that are outside the range of According to the present invention, ie, Comparative Examples Number 2-4, Number 2-5 and Number 2-9, have low values for either ductility or strength.These samples not only present a poor balance in resistance-ductility but also a low collision impact property.
The present invention provides steel tubes that have not only a good balance never achieved before in terms of ductility and strength, but excellent impact resistance properties by collision. In addition, the steel tubes according to the present invention show superior properties in secondary working, for example, local expansion by internal hydraulic pressure such as hydro-forming, and are therefore suitable for use in local expansion by internal hydraulic pressure. Among the steel tubes according to the present invention, the welded steel tubes (welded steel tubes) and the solid phase pressure welded steel tubes were subjected to cooling of the seam which provides a sewing portion. hardened having a hardness at the same level as that of the original pipe after boring, and showing an additional distinguished improvement in local dilation by internal hydraulic pressure.
(EXAMPLE 3)
The base steel tubes whose chemical composition is shown in table 5 were each heated to the temperatures given in table 6 when using an induction heating coil, and when using three-roll structure laminators, they were bored under the conditions shown in table 6 to provide the product tubes. The steel tubes of 110 mm diameter and 4.5 mm thickness were produced from hot-rolled steel sheet produced by controlled boring and controlled cooling. The tensile properties, collision impact properties, structures of the product tubes and the resistance to fractures by sulfide stress corrosion were investigated, and the results are given in Table 6. In a similar manner to Example 1, the Tension properties were measured on a JIS No. 11 test piece. For elongation, a reduced value is obtained according to the following equation when considering the effect of the size of the test piece under consideration: E 1 - E10 X ( • (a? / A) ° '4 (where, E10 represents the observed elongation, aO is a value equivalent to 292 mm2, it already represents the cross-sectional area of the sample (mm2).) Similar to example 1 again, collision impact properties were obtained by performing high-speed stress tests at a deformation speed of 2,000 s "1. Afterwards, the absorbed energy was obtained up to a deformation of 30% from the bout curve. erzos and deformations to be used in the collision impact absorption energy for evaluation. The collision impact property is represented by a deformation energy of a material at a degree of deformation of 1,000 to 2,000 s "1 that corresponds almost to the collision of a car, and is higher for a higher value. resistance to sulfide stress corrosion fracture in a C-ring test sample shown in Figure 7. Therefore, a tensile stress was applied which corresponds to 120% of the apparent limit of elasticity to the sample in a bath NACE (containing 0.5% acetic acid and 5% brine water, saturated with H2S and at a temperature of 25 ° C and a pressure of 1 atm) to investigate whether fractures generated or not during the 200-h test period The C-ring samples were cut from the stem of the product tube in the T direction (the circumferential direction) The test was carried out in 2 pieces each under the same conditions., it can be understood that the samples that are within the scope of the present invention (numbers 3-1 to 3-3, numbers 3-5 to 3-8, numbers 3-10 and numbers 3-12) show an excellent balance in ductility and resistance. In addition, a high tensile strength is observed for these samples having a higher degree of deformation, and these samples also show a high value of impact absorption energy per collision. In addition, they have excellent resistance against stress corrosion fractures of sulfur and are therefore superior when used in line or pipeline tubes. On the other hand, samples that are out of range according to the present invention, ie, Comparative Examples Nos. 3-4, Numbers 3-9 and Numbers 3-11, have low values for ductility or strength. These samples show not only a poor balance in resistance-ductility, but also a low impact property due to collision. In addition, it has been found that rupture of these samples occurs in the NACE bath, which shows degradation in fracture corrosion resistance by sulfide stress. Comparative example number 3-4 provides a reduction ratio that is outside the range according to the present invention, shows thickness formation in the ferrite grains, exhibits a poor balance in the resistance-ductility and a low absorption energy of impact by collision, and shows a damaged resistance to stress corrosion fracture of sulfides. Comparative examples numbers 3-9 and numbers 3-11 are produced at a bore temperature 'which is outside the range according to the present invention. Therefore, they show thick formation in the ferrite grains, exhibit a poor balance in the resistance-ductility and low energy of impact absorption by collision, and show a damaged resistance in the stress corrosion fracture of sulfides.
(EXAMPLE 4)
The base steel tubes whose chemical composition is shown in table 7, were each heated to the temperatures indicated in table 8 when using an induction heating coil, and by using three-roll structure laminators, they were bored under conditions shown in table 8 to provide the product tubes. The base steel tubes for use in the present example were produced by first forming a hot rolled ring using a plurality of forming rolls to obtain open tubes. Then, welded steel tubes were produced by 110 mm diameter and 2.0 mm thick seams by welding both ends of each of the resulting open tubes using induction heating. Otherwise, seamless tubes of 110 mm diameter and 3.0 mm thickness were produced by heating the casting billets continuously and then tubes were produced from them using a Mannesmann mandrel mill. The tensile properties, collision impact properties, structure and fatigue resistance properties of the product tubes were investigated, and the results are given in Table 8. The tensile properties, collision impact properties and the structure were evaluated in the same way as in example 1.
For the fatigue properties, the product tubes were used as well as the test samples to which a cantilever type oscillation fatigue test was carried out
(oscillation speed: 20 Hz). Therefore, resistance to fatigue is obtained. From Table 8, it can be understood that the samples within the scope of the present invention (number 4-1, number 4-3 and numbers 4-6 to 4-9) show an excellent balance in ductility and resistance. In addition, a high tensile strength was observed for these samples with a higher degree of deformation, and these samples also have a high value in collision impact absorption energy. In addition, they produced excellent fatigue resistance properties suitable for use as steel tubes with this fatigue resistance. On the other hand, the samples that are outside the scope of the claims according to the present invention, that is to say, the comparative examples numbers 4-2, number 4-4 and number 4-5, present low values for the resistance to fatigue. Comparative Example Number 4-2 is produced without applying the bore according to the present invention. Comparative Example Number 4-5 of yields with a reduction ratio that is outside the claimed range, and Comparative Example Number 4-4 is bored at a temperature range outside the claimed range. Therefore, they show coarse formation in the ferrite grains, exhibit a poor balance in resistance-ductility and low energy of impact absorption by collision, and show damaged properties in terms of resistance to fatigue.
(EXAMPLE 5)
An initial steel material Al, whose chemical composition is shown in table 9, was hot spun to provide a flat strip 4.5 mm thick. When using the production line shown in Figure 5, the flat strip 1 was preheated to 600 ° C in a preheating furnace 2, and formed continuously in an open tube when using the forming apparatus 3 of a plurality of groups of forming rollers. The edge portions of each of the open tubes 7 obtained in this manner were heated to 1000 ° C by an induction heating apparatus 4 of edge preheating, and then heated to 1450 ° C by using an apparatus 5 heating by induction of edge heating, where they were butted and welded under pressure in solid phase by using the rollers 6 to obtain tubes 8 of base steel having a diameter of 88.0 mm and a thickness of 4.5 mm., each of the base steel tubes was subjected to seam cooling, and heated or homogenized to a predetermined temperature shown in Table 10 when using a tube heating apparatus 22, and a tube was produced therefrom. of product having a predetermined outside diameter when using the boring apparatus 21 consisting of a plurality of three-roll structured mills. The number of stations varies based on the outside diameter of the product tube; that is, 6 stations were used for a product tube having an outer diameter of 60.3 mm, while 16 stations were used for those having an outer diameter of 42.7 mm. In the previous boring step, the product pipe of number 5-2 was subjected to boring with lubrication by using a boring oil based on mineral oil mixed with a synthetic ester. The product tubes were cooled by air after boring. The crystal grain diameter, the tensile properties and the impact resistance properties were investigated for each of the product tubes obtained in this way, and the results are given in Table 10. The crystal grain diameter is obtained by microscopic observation under a 5,000-fold magnification of at least 5 fields of vision taken in a cross section (cross section C) perpendicular to the longitudinal direction of the steel tube, thereby measuring the average glass grain diameter of Ferrite grains. The tensile properties were measured on a JIS No. 11 test piece. For elongation, a reduced value was obtained according to the following equation when taking into consideration the effect of the size of the test piece: El = E10 x ( V (aO / a) ° -4 (where, E10 represents the observed elongation, aO is a value equivalent to 100 mm2, already represents the cross-sectional area of the sample (mm2)) Impact properties (tenacity) they were evaluated when submitting the real tube to the Charpy impact tests, and when using a ductile rupture ratio in cross section C at a temperature of -150 ° C. The Charpy impact test on a real tube was made when applying impact to a real tube with a V-notch by 2 mm in a direction perpendicular to the longitudinal direction of the tube, and the ductile rupture ratio is obtained from it.From Table 10, it can be understood that the samples that are within the "scope of the present invention (Number 5-2, Numbers 5-4 to 5-7, Numbers 5-9 to 5-11 and Number 5-13) consist of fine ferrite grains of 1 μm or less in the average crystal diameter , and that have a high elongation and tenacity, and that show an excellent balance in resistance, tenacity and ductility. In the case of Number 5-2 notch subjected to bore with lubrication, a slight fluctuation in the crystal grains was observed along the direction of the thickness of the tube. On the other hand, samples that are out of reach in accordance with the present invention, ie, the Comparative Examples (Number 5-1, Number 5-3, Number 5-8 and Number 5-12) show crystal grains with formation of coarse and present degradation in the ductility and tenacity. It has been found that the texture of the product tubes which are within the scope of the claims of the present invention consist of ferrite and pearlite grains, ferrite and cementite grains, or ferrite and bainite grains.
(EXAMPLE 6)
A steel material Bl, whose chemical composition is shown in table 9, was melted in a converter, and billets were formed by continuous casting. The resulting billets were heated, and obtained from the same 110.0 mm diameter and 6.0 mm thick seamless tubes, using a Mannesmann mandrel mill. The seamless tubes obtained in this manner are reheated to temperatures shown in table 11 when using induction heating coils and were produced from the same product tubes having an outer diameter shown in table 11 by using of a three-roll structured mill The number of stations varied based on the outer diameter of the product pipe, ie 18 stations were used for a product pipe having an outside diameter of 60.3 mm, 20 stations were used for a product tube of 42.7 mm in diameter, and 24 stations were used for a product tube of 31.8 mm in diameter, and 28 stations were used for those with an outer diameter of 25.4 mm. characteristics of the product tubes that were investigated for each one.Therefore, investigations were conducted in the same way as in example 5 to the structure, crystal grain size, tensile properties and tenacity. From Table 11, it can be understood that samples within the scope of the present invention (Number 6-1, Number 6-3, Number 6-6, Number 6-7 and Number 6-9) consist of of fine ferrite grains of 1 μm or less in average crystal diameter, have high elongation and tenacity and show an excellent balance in terms of strength, toughness and ductility. On the other hand, samples that are out of range according to the present invention, ie, the Comparative Examples (Numbers 6-2, Number 6-4, Number 6-5 and Number 6-8), show grains of thick formation glass and present degradation in ductility and tenacity. It has been found that the texture of the product tubes which are within the scope of the claims of the present invention consist of ferrite and pearlite grains, ferrite and cementite grains or ferrite and bainite grains.
(EXAMPLE 7)
The initial steel materials whose chemical composition is shown in Table-12 were each heated to a temperature indicated in Table 13 by the use of an induction heating coil and, by using the three-structure laminator. rolls, were bored under conditions shown in Table 13 to provide the product tubes. The number of stations varied
based on the type of tube; that is, 24 stations were used for seamless pipes; while 16 stations were used for solid-phase pressure welded tubes and welded tubes. In table 13, a welded steel tube is obtained by
solid-state pressure by preheating a 2.3-mm thick hot-andrilated strip at 600 ° C, continuously forming a resultant flat strip in an open tube by using a plurality of rollers, preheating both edge portions of the tube open to
1000 ° C by means of induction heating, additional heating of both edge portions by induction furnace at a temperature of 1450 ° C, that is, at a temperature below the melting, in which both ends were brought into contact by using a presser cylinder, and the
solid phase pressure welding. Therefore, steel tubes having a predetermined outside diameter are obtained. On the other hand, seamless steel tubes were produced by heating a continuously cast billet, and seamless tubes of 110 mm diameter and 4.5 mm thickness were produced therethrough by using the Mannesmann mandrel mill. The characteristic properties of the product tubes of each one were investigated and shown in table 13. Therefore,, investigations were carried out in the same manner as in Example 1 regarding the structure, crystal grain size, tensile properties and toughness. From Table 13, it can be understood that the samples within the scope of the present invention consist of fine ferrite grains of 1 μm or less in average crystal diameter, have a high elongation and toughness, and show a excellent balance in strength, tenacity and ductility. It has been found that the structure of the product tubes which are within the scope of the claims of the present invention consist of ferrite and pearlite grains, or of ferrite, pearlite and bainite grains or ferrite and cementite grains, and grains. of ferrite and martensite.
(EXAMPLE 8)
Each of the steel starting materials whose chemical composition is shown in Table 14 was hot-bored to provide a flat strip 4.5 mm thick. By using the production line shown in Figure 5, the flat strip 1 was preheated to 600 ° C in a preheating furnace 2 and formed continuously in an open tube by using the forming apparatus 3 consisting of a plurality of groups of shaping rollers. The edge portions of each of the open tubes 7 obtained in this manner were heated to 1,000 ° C by an edge preheating induction heating apparatus 4, and then heated to 1,450 ° C by the use of an apparatus 5. of induction heating of edge heating, then they were brought into contact and welded under pressure in solid phase by using the "cylinders 6 pressers to obtain tubes 8 of base steel having a diameter of 110.0 mm and a thickness of 4.5 mm Then, each of the base steel tubes was subjected to seam cooling, and heated or homogenized at a predetermined temperature shown in Table 15 by using a tube heating apparatus 22, and produced from there a product tube having a predetermined outer diameter when using a boring apparatus 21 consisting of a plurality of structured rolling mills of three rollers. The number of stations varied based on the outer diameter of the product tube; that is, 6 stations were used for a product tube having an outer diameter of 60.3 mm, while 16 stations were used for those having an outer diameter of 42.7 mm. In the previous boring step, the product tube number 1-2 was subjected to bore with lubrication by using a boring oil based on mineral oil mixed with a synthetic ester. The product tubes are cooled in the air after boring. The crystal grain diameter and tensile properties were investigated for each of the product tubes obtained in this way, and the results are given in table 15. The crystal grain diameter was obtained by microscopic observation under an extension 5,000 times at least 5 fields of vision taken in a cross section (cross section C) perpendicular to the longitudinal direction of the steel tube, thereby measuring the average crystal grain diameter of the ferrite grains. The tensile properties were measured on a JIS No. 11 test piece. For elongation, a reduced value was obtained according to the following equation when taking into consideration the effect of the size of the test piece: El = E10 x ( • / "(aO / a) ° -4 (where E10 represents the observed elongation, aO is a value equivalent to 100 mm2, it already represents the cross-sectional area of the sample (mm2)) From the table 15, it can be understood that samples within the scope of the present invention (Number 1-2, Nos. 1-4 to 1-7 and Number 1-10) consist of fine grains of 2 μm or less in diameter of average glass, have high elongation and tenacity, provide a tensile strength of 600 MPa or greater and show an excellent balance in strength, toughness and ductility, in the case of sample Number 1-2 subjected to bore by lubrication, observed a small fluctuation in crystal grains l along the direction of the tube thickness. On the other hand, samples that are out of range according to the present invention, ie, the Comparative Examples (Number 1-1, Number "l-3, Number 1-8 and Number 1-9), show grains of coarse forming glass and exhibit degradation in ductility It has been found that the texture of the product tubes which are within the scope of the claims of the present invention comprise ferrite and cementite which constitutes more than 30% in proportion of area as a second phase.
(EXAMPLE 9) Each of the base steel tubes whose chemical composition is shown in Table 16 was reheated by an induction heating coil at temperatures shown in Table 17, and were obtained from the same tubes. of product, each has the outer diameter shown in table 17 when using a three-roll structure rolling apparatus. The number of stations used in the laminator was 16. t The characteristic properties of the product tubes of each were investigated and shown in Table 17. Therefore, investigations were conducted in the same manner as in Example 8 Regarding texture, crystal grain size and tensile properties. From Table 17, it can be understood that the samples (Numbers 2-1 to 2-6) which are within the scope of the present invention, consist of fine ferrite grains of 2 μm or less in crystal diameter average, they provide a tensile strength of 600 MPa or higher, have a high elongation and show an excellent balance in terms of strength and ductility. On the other hand, samples that are out of reach according to the present invention, ie, the Comparative Examples (Number 2-7 and Number 2-8) show coarse-grained crystal grains and exhibit degradation in strength. so that the objective tensile strength is not obtained.
It has been found that the texture of the product tubes that are within the scope of the present invention comprise ferrite, and a second phase contains pearlite, cementite, bainite or martensite, which constitutes more than 30% in area ratio. As described in the above, the present invention provides high strength steel tubes considerably improved in terms of their ductility and strength balance.
In addition, steel tubes according to the present invention show superior properties in secondary working, for example, local expansion by internal hydraulic pressure such as hydro-forming. Therefore, they are particularly suitable for use in local dilation by internal hydraulic pressure. Among the steel tubes according to the present invention, the welded steel tubes and the solid-state pressure-welded steel tubes subjected to seam cooling provide a hardened seam portion having a hardness at the same level as the mother tube. after boring, and show an additional distinguished improvement in local dilation by internal hydraulic pressure.
Table 1
C?
Table 2-1 (continued)
Table 2-1 (continued)
to.
12-2 (continuous)
Without reduction mandrel
Table 3
C? -4
Table 4 (continued)
C? vo
Table 4 (continued)
or Note) *: ementta,: anta,: artensta,: er ta ** Without boring reduction
Tab 6 (continue)
- t
Table 6 (continued)
)
oa eia,: ainia; sandy,; eria e ** No bore reduction **** No break O, with break?
Table 8 (continued)
c
Table 9
Table 1 (continued)
co
Table 10 (continued)
: represents r, represents er
Table 1 1 (continued)
co o
Tabb 11 (continue)
00
Table 12
00 Ni
Table 13 (continued)
CD
Table 13 (continued)
co • t >
TsrWal4
00 - tp
Tabta 15 (continued)
c c
Table 15 (continued)
or
Table 16
c 0
0
Table 17 (continued)
< rO O ** 0.2% SP
Applicability in the Industry:
According to the present invention, high strength steel tubes have excellent ductility and impact resistance properties and can be obtained with high productivity and by a simple process. Therefore, the present invention extends the field of application of steel tubes and is therefore particularly effective in the industry. In addition, the present invention reduces the use of alloying elements and allows the production at low cost of steel pipes with high strength and high ductility improved in terms of fatigue resistance properties., or high strength and high tenacity steel tubes for use in line or pipe tubes improved in resistance to fracture by stress corrosion. In addition, a high-strength steel material is produced which contains superfine glass grains of 1 μm or less in size with a superior quality in toughness and ductility, thus expanding the use of steel materials. A steel material containing superfine glass grains of 2 μm or less in size, which provides a tensile strength of 600 MPa or greater, and excellent toughness and ductility, is also readily available without intermediate annealing.
Claims (28)
1. A method for producing a steel tube characterized in that it comprises heating or homogenizing a base steel tube having an outside diameter of ODi (mm) and having ferrite grains with an average glass diameter of di (μm) in the section cross section perpendicular to the longitudinal direction of the steel tube, followed by application of reduction to an average bore temperature of? m (° C) and a ratio of total reduction Tred (%) to obtain a product tube having a diameter of ODf (mm), the reduction comprises performing it in a temperature range of 400 ° C or more, but not greater than the heating temperature or homogenization, and in such a way that the average crystal diameter of di (μm), the temperature average bore (? C) and the total reduction ratio Tred (%) are in a ratio that satisfies equation (1) as follows: di * (2 .65 - .003 x? m) x? o < < 0-008 + ßm / 500 °° > x Tred > - - - ( 1 ) where, di represents the average glass diameter of the steel tube base (μm), *? m represents the average bore temperature (° C) (= (? i +? f) / 2; where? i is the temperature Initial bore (° C) and? f is the finished bore temperature (° C)); and Tred represents the ratio of total reduction (%) (= ODi - ODf x 100 / ODi; where, ODi is the outer diameter of the steel tube base (mm) and ODf is the outer diameter of the product tube (mm) ).
2. The method for producing a steel tube, according to claim 1, characterized in that the cross section perpendicular to the longitudinal direction of the steel tube after the reduction contains superfine ferrite grains having an average crystal grain size of 1. μm or less.
3. The method for producing a steel tube, according to claim 1, characterized in that the steel tube structure after the reduction consists of ferrite alone or ferrite together with a second phase other than ferrite constituting 30% or less in proportion of area, and the cross section perpendicular to the longitudinal direction of the steel tube after reduction contains superfine ferrite grains having an average crystal grain size of 3 μm or less.
4. The method for producing a steel tube, according to claim 1, characterized in that the steel tube structure after the reduction consists of ferrite alone or ferrite together with a second phase other than ferrite constituting 30% or less in proportion of area, and the cross section perpendicular to the longitudinal direction of the steel tube after reduction contains superfine ferrite grains having an average crystal grain size of 1 μm or less.
5. The method for producing a steel tube, according to claim 1, characterized in that the steel tube structure after the reduction consists of ferrite together with a second phase other than ferrite constituting more than 30% in proportion of area, and the cross section perpendicular to the longitudinal direction of the steel tube after tempering contains superfine ferrite grains having an average crystal grain size of 2 μm or less.
6. The method for producing "a steel tube, according to claim 1, characterized in that the steel tube structure after the reduction consists of ferrite together with a second phase other than ferrite constituting more than 30% in proportion of area , and the cross section perpendicular to the longitudinal direction of the steel tube after tempering contains superfine ferrite grains having an average glass grain size of 1 μm or less.
7. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the tempering is carried out in a temperature range from the Ac3 transformation point to 400 ° C.
8. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the method comprises heating the base zero tube in the temperature range from the Ac3 transformation point to 400 ° C before the reduction, and then perform the reduction in a temperature range from the Ac3 transformation point to 400 ° C.
9. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the method comprises heating the base steel tube in a temperature range from 400 ° C to 750 ° C before reduction, and then perform the reduction in a temperature range from 400 ° C to 750 ° C.
10. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the reduction is carried out under lubrication.
11. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the method comprises at least one boring step with a reduction ratio per step of 6% or more.
12. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the cumulative reduction ratio in tempering is 60% or more.
13. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the reduction is carried out on a base steel tube containing, by weight, 0 -.005 to 0.30% C, 0.01 at 3.0% Si, 0.01 to 2.0% Mn, 0.01 to 0.10% Al, and the rest of Fe with unavoidable impurities.
14. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the tempering is carried out in a base steel tube containing, by weight, 0.005 to 0.30% C, 0.01 to 3, 0% of Si, 0.01 to 2.0% of Mn, 0.001 to 0.10% of Al, and which also contains at least one or more types that are selected from the group consisting of 0.5% or less of Cu, 0.5% or less than Ni, 0.5% or less of Cr and 0.5% or less of Mo; or in addition one or more that are selected from the group consisting of 0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti and 0.004% or less of B; or additionally, one or more selected from the group consisting of 0.02% or less of REM and 0.01% or less of Ca; and the rest of Faith with inevitable impurities.
15. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the tempering is carried out in a steel tube of base containing, by weight, more than 0.30% to 0.70% C, 0.01 to 2.0% Si, 0.01 to 2.0% Mn, 0.001 to 0.10% Al, and the rest of Fe with unavoidable impurities.
16. The method for producing a steel tube, according to any of claims 1 to 6, characterized in that the tempering is carried out in a steel tube of base containing, by weight, more than 0.30% to 0.70% C, 0.01 to 2.0% Si, 0.01 to 2.0% Mn, 0.001 'to 0.10% Al, and which also contains at least one or more types that are selected from the group consisting of 0.5% or less of Cu , 0.5% or less of Ni, 0.5% or less of Cr and 0.5% or less of Mo; or in addition one or more that are selected from the group consisting of 0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti and 0.004% or less of B; or additionally, one or more selected from the group consisting of 0.02% or less of REM and 0.01% or less of Ca; and the rest of Faith with inevitable impurities.
17. A superfine granular steel tube that has a composition that contains, by weight, 0.005 to 0.30% C, 0.01 to 3.0% Ci, 0.01 to 2.0% Mn, 0.001 to 0.10% Al and the rest of Fe with impurities unavoidable, which is produced in a method for producing a steel tube, which comprises heating or homogenizing a base steel tube having an outer OD diameter. (mm) and having ferrite grains with an average crystal diameter of di (μm) in the cross section perpendicular to the longitudinal direction of the steel tube, and then apply reduction at an average bore temperature of? m (° C) and a total reduction ratio Tred (%) to obtain a product tube having a diameter of ODf (mm), wherein, the reduction comprises perform it in a temperature range of 400 ° C or more, but not greater than the heating or homogenization temperature, and in such a way that the average crystal diameter of di (μm), the average bore temperature of? m (° C) and the ratio of total reduction Tred (%) are in a relation that satisfies equation (1) as follows. * di = (2 .65 - .003 x? m) x 10 ((0'008 *? ra / 5000 ° > * Tred > - - - (1) where, di represents the average crystal diameter of the steel tube base (μm); ? m represents the average bore temperature (° C) (= (? i +? f) / 2; where? i is the initial bore temperature (° C) and? f is the finish bore temperature (° C) )); and Tred represents the ratio of total reduction (%) (= ODi - ODf x 100 / ODi; where ODi is the outer diameter of the steel tube base (mm) and ODf is the outer diameter of the product tube (mm)) .
18. The superfine granular steel pipe, according to claim 17, characterized in that the composition system of the steel pipe is such that it contains, by weight, 0.005 to 0.10% C, 0.01 to 0.5% Si, 0.01 to 1.8% of Mn, 0.001 to 0.10% of Al and the rest of Fe with unavoidable impurities.
19. The superfine granular steel tube, according to claim 17, characterized in that the composition system of the steel tube is such that it contains, by weight, 0.06 to 0.30% C, 0.01 to 1.5% Si, 0.01 to 2 ' ! 0% of Mn, 0.001 to 0.10% of Al and the rest of Fe with unavoidable impurities.
20. The superfine granular steel tube according to any of claims 17 to 19, characterized in that the cross section perpendicular to the longitudinal direction of the steel tube after the reduction contains superfine ferrite grains having an average crystal grain size. of 1 μ or less.
21. The superfine granular steel tube, according to any of claims 17 to 19, characterized in that the steel tube structure after reduction consists of ferrite alone or ferrite together with a second phase other than ferrite constituting 30% or less in proportion of area, and the cross section perpendicular to the longitudinal direction of the steel tube after reduction contains superfine ferrite grains having an average crystal grain size of 3 μm or less.
22. The superfine granular steel pipe, according to any of claims 17 to 19, characterized in that the steel tube structure after tempering consists of ferrite alone or ferrite together with a second phase other than ferrite constituting 30% or less in proportion of area, and the cross section perpendicular to the longitudinal direction of the steel tube after reduction "" contains superfine ferrite grains having an average crystal grain size of 1 μm or less.
23. The superfine granular steel tube, according to any of claims 17 to 19, characterized in that the steel tube structure after the reduction consists of ferrite together with a second phase other than ferrite constituting more than 30% in proportion to area, and the cross section perpendicular to the longitudinal direction of the steel tube after reduction contains superfine ferrite grains having an average crystal grain size of 2 μm or less.
24. The superfine granular steel tube, according to any of claims 17 to 19, characterized in that the steel tube structure after the reduction consists of ferrite together with a second phase other than ferrite constituting more than 30% in proportion to area, and the cross section perpendicular to the longitudinal direction of the steel tube after reduction contains superfine ferrite grains having an average crystal grain size of 1 μm or less.
25. A high strength steel tube, with improved working capacity, characterized in that it has a composition containing, by weight, more than 0.30% at 0.70% C, 0.01 at 2.0% Si, 0.01 at 2.0% Mn, 0.001 at 0.10% of Al and the rest of Fe with unavoidable impurities, and a structure consisting of ferrite and a second different phase of ferrite that constitutes more than 30% in proportion in area, with the cross section perpendicular to the longitudinal direction of the tube of steel containing superfine ferrite grains having an average crystal grain size of 2 μm or less.
26. The high strength steel tube, according to claim 25, characterized in that the tempering is performed using a steel tube of base having a composition containing, by weight, more than 0.30% at 0.70% C, 0.01 a 2.0% of Si, 0.01 to 2.0% of Mn, 0.001 to 0.10% of Al, and which also contains at least one or more types that are selected from the group consisting of 0.5% or less of Cu, 0.5% or less than Ni, 0.5% or less of Cr and 0.5% or less of Mo; or in addition one or more that are selected from the group consisting of 0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti and 0.004% or less of B; or additionally, one or more selected from the group consisting of 0.02% or less of REM and 0.01% or less of Ca; and the rest of Faith with inevitable impurities.
27. A high resistance steel tube, with improved working capacity and characterized in that it has a composition containing, by weight, more than 0.30% at 0.7"0% C, 0.01 to 2.0% Si, 0.01 to 2.0% Mn , 0.001 to 0.10% of Al, and which also contains at least one or more types that are selected from the group consisting of 0.5% or less of Cu, 0.5% or less of Ni, 0.5% or less of Cr and 0.5 % or less of Mo; or in addition one or more that are selected from the group consisting of 0.1% or less of Nb, 0.1% or less of V, 0.1% or less of Ti and 0.004% or less of B, or additionally, one or more selected from the group consisting of 0.02% or less of REM and 0.01% or less of Ca, and the rest of Fe with unavoidable impurities; and having a structure consisting of ferrite and a second, different ferrite phase constituting more than 30% in proportion in area, with the cross section perpendicular to the longitudinal direction of the steel tube containing superfine ferrite grains having a size of glass grain average of 2 μm or less; in which it occurs when performing reduction in a temperature range of 400 ° C or more, but not greater than the temperature of heating or homogenization, and such that the average glass diameter of di (μm), the temperature of boring average of? m (° C) and the proportion of total reduction Tred (%) are in a relation that satisfies equation (1) as follows: di ^ (2 .65 - 0. 003 X? m) X 10 < (0- oofl + ß »/ soooo) * network} - - - ( 1 ) where, di represents the average crystal diameter of the steel tube base (μm); ? m represents the average bore temperature (° C) (= (? i +? f) / 2; where? i is the initial bore temperature (° C) and? f is the finish bore temperature (° C) )); and Tred represents the ratio of total reduction (%) (= ODi - ODf x 100 / ODi; where ODi is the outer diameter of the steel tube base (mm) and ODf is the outer diameter of the product tube (mm)) .
28. The high strength steel tube, according to any of claims 25 to 27, characterized in that it has a structure consisting of ferrite and a second different phase of ferrite constituting more than 30% in proportion in area, with the cross section perpendicular to the longitudinal direction of the steel tube containing superfine ferrite grains having an average crystal grain size of 1 μm or less.
Applications Claiming Priority (6)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP9/170790 | 1997-06-26 | ||
| JP9/196038 | 1997-07-22 | ||
| JP9/223315 | 1997-08-20 | ||
| JP9/228579 | 1997-08-25 | ||
| JP9/240930 | 1997-09-05 | ||
| JP10/133933 | 1998-05-15 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| MXPA99001850A true MXPA99001850A (en) | 1999-09-20 |
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