JP2001262275A - High tensile strength seamless steel pipe excellent in toughness, ductility and weldability and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a seamless steel pipe having high tensile strength and excellent also in toughness, ductility and weldability. SOLUTION: This high tensile strength steel pipe excellent in toughness, ductility and weldability has a composition comprising by mass, 0.10 to 0.40% C, <=0.8% Si, 0.5 to 1.3% Mn, 0.03 to 0.20% V, <=0.05% Al, >0.01 to 0.03% M and, if required, Cu, Cr, Mo, Ti, Nb, or the like, and the balance Fe with inevitable impurities, in which Ceq defined by the following formula satisfies the condition of 0.40 to 0.55 and has tensile strength of >=590 MPa, Charpy absorbed energy at -20 deg.C of >=30 J and elongation of >=20%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is excellent in toughness, ductility and weldability applied to hydraulic cylinders and cylinder rods used in construction machines, etc., raw materials for automobile airbags and cylinders for cylinders. The present invention relates to a high-tensile seamless steel pipe and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a seamless steel pipe applied to a hydraulic cylinder or the like, a carbon steel pipe of a heat treatment specification or a cold working finish specification has been frequently used. However, the presence of these additional steps inevitably increases the cost, and there is a need for a technology that can obtain the same properties as a heat-treated material or a cold-processed material while hot working. However, carbon steel pipes that can be manufactured as they are hot worked are, for example, J
Tensile strength 540MP as seen in IS G3473
The upper limit is about a, and this level of strength cannot meet the demand for higher strength exceeding the recent tensile strength of 590 MPa.

On the other hand, if high strength is to be obtained while hot rolling is performed, alloying elements may be added and strengthened. However, in the above-mentioned application fields, not only strength but also weldability, toughness and workability are required. Therefore, the degree of freedom in alloy design is actually extremely narrow.

[0004] Under such circumstances, technical development has been carried out to provide a seamless steel pipe which is not heat-treated, has high strength, and is excellent in workability and weldability. For example, JP-A-5-20
As disclosed in Japanese Patent No. 2447, in addition to the precipitation hardening action of V and the strengthening of the matrix of Mn and Cr, Al, T
A technique for obtaining a non-heat-treated and high-strength seamless steel pipe by adding an appropriate amount of i and N has been proposed. In this technique, strength and ductility are sufficient, but toughness is not at a satisfactory level, and there is a problem in weldability due to high carbon equivalent.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a technique for overcoming the above-mentioned problems, and it is possible to optimize the conditions of steel components and the conditions in a rolling process to obtain generally contradictory characteristics. It is an object of the present invention to produce a seamless steel pipe having both high tensile strength, toughness, ductility, and weldability by a Mannesmann-type rolling method which is excellent in economical efficiency, in principle, as-rolled. In the present invention, as a specific target based on market requirements, a tensile strength (hereinafter, TS) of 590 MPa or more, a Charpy absorbed energy at −20 ° C. of 30 J or more, and an elongation of 20% or more are defined as high tension, high toughness, and high ductility, respectively. The purpose is to set it as a characteristic, and to have good weldability so as not to cause cold cracking without performing preheating and post-heating treatments.

[0006]

As a result of studying alloy design to achieve the above-mentioned object, first, TS ≧ 59
In addition to obtaining high strength of 0 MPa, it is customary to utilize precipitation hardening to minimize the increase in carbon equivalent (Ceq) as an index of weldability, but sufficient toughness is obtained by simple precipitation strengthening. It was not obtained, and it was concluded that an element that also contributes to grain refinement was necessary. As precipitation elements, V, Nb, Ti, etc. are known. As a result of research on various steel component systems, not only the strength is enhanced, but also the toughness and ductility are improved through the effect of grain refinement, and the present invention is also improved. Has found that the most suitable precipitate for the desired Mannesmann rolling method is vanadium nitride (VN). Furthermore, in order to maximize the effect of VN, the process conditions before and after the reheating step inherent in the Mannesmann rolling method, which affects the precipitation behavior, have been optimized.

The present invention is based on such knowledge, and the gist is as follows. (1) In mass%, C: 0.10 to 0.40%, Si: ≤ 0.8%, Mn: 0.5 to 1.3%, V: 0.03 to 0.20%, Al: ≦ 0.05%, N: more than 0.01% to 0.03%, the balance being Fe and unavoidable impurities,
Furthermore, Ceq defined by the following formula (1) satisfies the condition of 0.40 or more and 0.55 or less, and the tensile strength is 590 MPa.
As described above, the Charpy absorbed energy at −20 ° C. is 3
A high-tensile seamless steel pipe excellent in toughness, ductility, and weldability, characterized by having 0 J or more and elongation of 20% or more. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 (1) (each component is mass%) (2) mass% Further, Ni: ≤ 1.0%, Cu: ≤ 1.0%, Cr: ≤ 1.0%, Mo: ≤ 1.0%, Ti: ≤ 0.05%, Nb: ≤ 0.05% A high-tensile seamless steel pipe excellent in toughness, ductility, and weldability according to the above (1), which comprises one or more of the following. (3) In mass%, C: 0.10 to 0.40%, Si: ≤ 0.8%, Mn: 0.5 to 1.3%, V: 0.03 to 0.20%, Al: ≦ 0.05%, N: more than 0.01% to 0.03%, the balance being Fe and unavoidable impurities,
Further, the present invention is a method of forming a steel slab having a composition satisfying a condition that Ceq defined by the following formula (1) is 0.40 or more and 0.55 or less by a Mannesmann type hot rolling method, comprising: The tube is heated from a temperature below Ar3 -30 ° C to 900 ° C.
After being reheated to the above temperature, finish rolling is performed, whereby the tensile strength is 590 MPa or more, the Charpy absorbed energy at −20 ° C. is 30 J or more, and the elongation is 20% or more, toughness, ductility, and weldability. Method for producing high-strength seamless steel pipe with excellent performance. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 (1) (each component is mass%) (4) mass% Further, Ni: ≤ 1.0%, Cu: ≤ 1.0%, Cr: ≤ 1.0%, Mo: ≤ 1.0%, Ti: ≤ 0.05%, Nb: ≤ 0.05% The method for producing a high-tensile seamless steel pipe excellent in toughness, ductility, and weldability according to the above (3), wherein one or two or more of these are contained. (5) The high-tension seam excellent in toughness, ductility and weldability according to the above (3) or (4), wherein after the finish rolling, the steel pipe is heated to a temperature of 900 ° C. or more and air-cooled. Manufacturing method of steelless pipe.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the steel components (% by mass) of the present invention and the reasons for the limitation will be described below. C: C is an indispensable element for securing the strength, but too much content lowers the toughness, ductility, and weldability, so the content range is 0.10 to 0.40%.

Si: Si is indispensable as a deoxidizing element and is an element that remains after being added in the steelmaking process. However, if its content exceeds 0.8%, its effect is saturated, and from the viewpoint of toughness, the effect is saturated. Since the addition of more than 8% is not preferable, the upper limit is 0.8%.
And The lower limit is not specified.

Mn: Mn is also an effective element for improving the strength, but if it is less than 0.5%, the strengthening effect is insufficient, and if it exceeds 1.3%, a brittle phase such as bainite is formed and the toughness is deteriorated. In addition to reducing ductility and weldability,
The appropriate content range was 0.5 to 1.3%.

V: V is an element indispensable for enjoying the effect of precipitation strengthening and grain refinement of VN together with N. If the content is less than 0.03%, sufficient precipitation cannot be obtained, and even if the content exceeds 0.20%, a sufficient effect cannot be obtained unless the N content is increased according to the stoichiometric coefficient of VN. The upper limit was set to 0.20%. The desirable content is 0.04 to 0.1
4%.

Al: The content of Al must be regulated as an element inhibiting VN precipitation, and the upper limit is 0.05%. The lower limit is not specified.

N: N is a constituent element of VN like V.
If the content is 0.01% or less, the addition of V of 0.03% or more cannot be utilized, so this is set as the minimum addition amount. Further, if the content exceeds 0.03%, casting defects and deterioration in weldability occur, so the upper limit is made 0.03%. In addition, as a desirable content, 0.013-0.022%
Range.

Ceq: Carbon equivalent defined by the following formula (1) is an index of weldability. If this value exceeds 0.55, toughness, ductility, and weldability decrease. On the other hand, if it is less than 0.40, sufficient strength cannot be obtained. Therefore, the appropriate range is 0.40
-0.55. In addition, the content (% by mass) of the alloy element contained in the formula (1) must satisfy the above range and comprehensively satisfy the condition of Ceq: 0.40 to 0.55. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 ・ ・ ・ ・ (1)

In the following, Ni, C to be contained as required
u, Cr, Mo, Ti, and Nb will be described. Ni,
Cu, Cr, Mo: Since Ni, Cu, Cr, and Mo have an effect of improving the strength, they may be contained as needed within a range that satisfies the condition of the above formula (1), but generally expensive elements Therefore, the upper limit of the content was set to 1%.

Ti, Nb: Since Ti and Nb are more stable nitride-forming elements than VN, if they are contained in the same manner as Al, they inhibit the deposition of VN. However, it contributes to the grain refining effect and is effective in improving toughness and ductility, and may be contained at an upper limit of 0.05%.

Next, the tensile strength, elongation,
The definition of the Charpy absorbed energy will be described. The tensile strength and elongation referred to in the present invention are measured by an arc test specimen specified in JIS Z2201 according to the tensile test method of JIS Z2241. The Charpy absorbed energy was measured according to JIS Z220 so that the direction perpendicular to the rolling direction and the pipe thickness direction was the propagation direction of the fracture crack.
JIS using 2 specified V-notch Charpy test pieces
It is measured using the method of Z2242 with -20 ° C as the test temperature. Although the required values of these properties are determined based on market requirements, these material properties do not depend solely on the above-mentioned component requirements, but rather optimize the effect of VN in the seamless steel pipe rolling process described below. It can only be obtained in combination with the design.

Next, the conditions of the manufacturing method in the present invention will be described. The rolling method of the Mannesmann system referred to in the present invention is hot rolling performed in the production of ordinary seamless steel pipes. As illustrated in FIG. Or after drilling by the Mannesmann drilling method, if necessary, stretched by an inclined rolling machine called elongator, plug mill or mandrel mill, if necessary, by adjusting the wall thickness by rolling with a reeler mill, polishing pipe, then This is a general term for a continuous rolling process in which pipes are formed by equalizing the entire length of a pipe in a reheating furnace and then adjusting the dimensions by a finisher such as a sizer mill or a stretch reducer.

The reheating step included in this process is unique to the seamless steel tube rolling process, and is the largest difference from the rolling of a section steel or a plate. In order to ensure this, it is important to define the conditions of the tube temperature immediately before reheating (hereinafter, the lowest point temperature) and the tube temperature in the reheating furnace.

As described above, the present invention basically utilizes the effects of strengthening and grain refinement by precipitation of VN. If VN precipitates during the cooling process after final rolling, it greatly contributes to strengthening, but if it precipitates before reheating, precipitation strengthening decreases. The VN precipitation start temperature is about 800 ° C., and the lowest point temperature may be lower than 800 C under normal operating conditions. For this reason, the VN deposited before the reheating furnace is inserted must be dissolved again, and it is necessary to secure a raw tube temperature of 900 ° C. or more in the reheating furnace.

[0021] However, this alone may not be able to industrially secure the target toughness, and further refinement is required. For this purpose, it is effective to lower the tube temperature to the Ar3 point or lower before reheating and utilize the γ → α transformation. In this case, VN is almost precipitated at the lowest temperature, but is re-heated to 900 ° C. or higher, so that VN is solid-dissolved again, and desired VN precipitation occurs in a cooling process after final rolling. . The steel pipe thus obtained is strengthened and refined by VN precipitation, and refined by transformation, so that the toughness contrary to strength can be secured stably. Although there is no particular upper limit for the reheating temperature, the upper limit is preferably 1000 ° C. or less in order to suppress surface roughness due to oxidation in the reheating furnace.

In the present invention, emphasis is placed on economy and the main object is to obtain desired characteristics in principle as rolled. However, for the purpose of adding higher toughness, ductility and weldability, off-line heat treatment is performed. May be. As a heat treatment method in this case, normalizing treatment is desirable. As the heating temperature at that time,
Similar to the reason for limiting the temperature of the raw tube in the reheating furnace, the temperature is preferably 900 ° C. or more at which VN solid-dissolves again. In this case, since the VN precipitation and grain refinement required for the product can be secured by normalizing, the rolling conditions of the raw tube need not necessarily satisfy the above conditions. The heating method and the upper limit temperature of the normalizing treatment are not particularly limited, but a short-time treatment by high-frequency induction heating at 1100 ° C. or lower is desirable from the viewpoint of suppressing roughening due to oxidation.

[0023]

EXAMPLES The present invention will be described more specifically based on examples. The continuous cast bloom having a composition shown in Table 1 and having a cross section of 290 × 290 mm was rolled into a seamless steel pipe having an outer diameter of 244 mm and a wall thickness of 12 mm in accordance with the rolling method shown in FIG. At this time, the tube temperature in the reheating furnace was set to 850 to
The temperature was changed in the range of 980 ° C. In some test levels, normalizing treatment was performed by high-frequency induction heating. Specimens were collected from these pipes and subjected to a tensile test and a Charpy test. In addition, the pipe was subjected to groove processing, subjected to arc welding under the conditions of no preheating, no post-heating, and a heat input of about 20 kJ / cm, and evaluated for the presence or absence of cold cracking. The test results are also shown in Table 1.

The weldability in Table 1 indicates the presence or absence of cold cracking (○: no cracking, ×: cracking). In Table 1, No. In the present invention of Nos. 1 to 6, the T
High tensile strength of S ≧ 590 MPa, good ductility of e1 (elongation) ≧ 20%, and excellent toughness of vE-20 ≧ 30J are obtained.
No cold cracking after welding was observed, and good weldability was also achieved.

On the other hand, in Comparative Example No. In Nos. 7 to 13, satisfactory results were not obtained in any of strength, ductility, toughness, and weldability because the components or Ceq deviated from the range of the present invention. Comparative Example No. Component requirements of Nos. 14 and 15 of the present invention Same as 1, but the lowest point temperature,
Since the rolling conditions of the temperature of the raw tube in the reheating furnace are out of the range of the present invention, the excellent characteristics that should be originally obtained are reduced. In Comparative Example No. No. 16 of the present invention. The composition is the same as that of No. 2, and the results are satisfactory except for the toughness. However, since the lowest point temperature is out of the range of the present invention, the result is slightly below the toughness target value.

[0026]

[Table 1]

[0027]

According to the present invention, a seamless steel pipe having high tensile strength and excellent in toughness, ductility and weldability can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a rolling process of a seamless steel pipe of the Nessmann system.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/50 C22C 38/50 F term (Reference) 4K032 AA01 AA05 AA11 AA14 AA16 AA19 AA21 AA22 AA23 AA31 AA35 AA36 BA03 CB02 CD05 CF03 4K042 BA01 BA02 BA11 CA09 CA10 CA12 CA13 DA04 DB01 DC02 DC03 DD05 DE03

Claims (5)

[Claims] 1. Mass%, C: 0.10 to 0.40%, Si: ≤ 0.8%, Mn: 0.5 to 1.3%, V: 0.03 to 0.20%, Al: ≦ 0.05%, N: more than 0.01% to 0.03%, the balance being Fe
And Ceq defined by the following formula (1) satisfying a condition of 0.40 or more and 0.55 or less, a tensile strength of 590 MPa or more, a Charpy absorption energy at −20 ° C. of 30 J or more, 20% growth
A high-tensile seamless steel pipe excellent in toughness, ductility and weldability, characterized by having the above. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 (1) (each component is mass%) 2. In mass%, C: 0.10 to 0.40%, Si: ≦ 0.8%, Mn: 0.5 to 1.3%, V: 0.03 to 0.20%, Al: ≦ 0.05%, N: more than 0.01% to 0.03%, Ni: ≦ 1.0%, Cu: ≦ 1.0%, Cr: ≦ 1.0%, Mo : ≦ 1.0%, Ti: ≦ 0.05%, Nb: ≦ 0.05%, one or two or more of which are composed of Fe and unavoidable impurities. Ceq defined by is 0.40 or more and 0.5
5 or less, and the tensile strength is 590 MPa or more,
A toughness characterized by having a Charpy absorbed energy at −20 ° C. of at least 30 J and an elongation of at least 20%,
High tensile seamless steel pipe with excellent ductility and weldability. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 (1) (each component is mass%) C: 0.10 to 0.40%, Si: ≦ 0.8%, Mn: 0.5 to 1.3%, V: 0.03 to 0.20% by mass%, Al: ≦ 0.05%, N: more than 0.01% to 0.03%, the balance being Fe
And a steel slab composed of unavoidable impurities and having a Ceq defined by the following formula (1) satisfying a condition of 0.40 or more and 0.55 or less, is formed into a seamless steel pipe by a Mannesmann hot rolling method. In which the tube is made of Ar3
After reheating from a temperature of −30 ° C. or less to a temperature of 900 ° C. or more, and performing finish rolling, the tensile strength is 590.
A method for producing a high-tensile seamless steel pipe having excellent toughness, ductility, and weldability, wherein the Charpy absorbed energy at -20 ° C or more is 30 J or more and the elongation is 20% or more. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 (1) (each component is mass%) 4. Mass%, C: 0.10 to 0.40%, Si: ≦ 0.8%, Mn: 0.5 to 1.3%, V: 0.03 to 0.20%, Al: ≦ 0.05%, N: more than 0.01% to 0.03%, Ni: ≦ 1.0%, Cu: ≦ 1.0%, Cr: ≦ 1.0%, Mo : ≦ 1.0%, Ti: ≦ 0.05%, Nb: ≦ 0.05%, and the balance consists of Fe and unavoidable impurities. A method wherein a steel slab having a composition that satisfies the condition that a defined Ceq satisfies the condition of 0.40 or more and 0.55 or less is made into a seamless steel pipe by a hot rolling method of the Mannesmann method, wherein the raw pipe is Ar3 -30 ° C or less After reheating to a temperature of 900 ° C. or more from the temperature of above, finish rolling is performed, so that the tensile strength is 590 MPa or more and at −20 ° C. Charpy absorbed energy at least 30 J, toughness, ductility, process for producing a high tensile seamless steel pipe having excellent weldability elongation, characterized in that 20% or more. Ceq = C + Mn / 6 + Si / 24 + (V + Cr + Mo + Cu + Ni + Ti + Nb) / 5 (1) (each component is mass%) 5. The high-tensile seamless steel with excellent toughness, ductility and weldability according to claim 3, wherein after finishing rolling, the steel pipe is heated to a temperature of 900 ° C. or more and air-cooled. Manufacturing method of steel pipe.