JP5573003B2 - High tensile welded steel pipe for automobile parts - Google Patents

Description

  The present invention relates to a high-tensile welded steel pipe suitable for use in automobile members centered on automobile undercarriage members, automobile skeleton members, etc., in particular, improvement of low-temperature toughness of the base metal part and the welded part, and the vicinity of the welded part. It relates to improvement of material uniformity in the circumferential direction.

  In recent years, from the viewpoint of conservation of the global environment, reduction in fuel consumption of automobiles has been aimed at, and therefore, weight reduction of automobile bodies has been promoted. As a measure to reduce the weight of the car body, steel plates with increased strength have been used as materials. Also, for example, welded plate materials have been conventionally used in automobile structural members. However, in order to reduce the weight of the vehicle body, the use of steel pipes (hollow materials) having high rigidity in a closed cross section has been actively promoted. Yes. However, especially in the case of a welded steel pipe made of a high-strength steel sheet containing a large amount of C or strengthening element for high strength, the hardness of the welded portion is significantly increased compared to the base metal part, For this reason, there exists a problem that low temperature toughness falls remarkably in a welding part compared with a base material part. In addition, softening may occur in the weld heat affected zone (hereinafter also referred to as HAZ), and the target machining is completed in the HAZ softened zone, especially when applied to members that require hydroforming and tube expansion. There was a problem such as the occurrence of breakage.

  For such a problem, for example, in Patent Document 1, C: 0.10 to 0.65%, Si: 0.05 to 0.60%, Mn: 0.25 to 2.0%, Ti: 0.020 to 0.150%, Mo: 0.01 to 0.5%, Winding temperature of steel for pipes containing Nb: 0.01-0.1%, V: 0.01-0.1% during hot rolling: Winding at 400-600 ° C or 700-750 ° C, and heat treatment at 600-700 ° C after pipe forming A method for producing a high-strength ERW steel pipe excellent in HAZ softening resistance is described. According to the technique described in Patent Document 1, it is a soft material that can be molded at the time of pipe making, adjusting the heat treatment after the pipe making to express the precipitation effect of Ti and suppress the softening of the HAZ part, and welding It is said that a high-strength steel pipe having a uniform hardness distribution in the circumferential direction can be manufactured.

In Patent Document 2, C: 0.10 to 0.20%, Si: 0.15 to 0.50%, Mn: 1.3 to 2.5%, Al: 0.01 to 0.08%, Ti: 0.02 to 0.2%, B: 0.0010 to 0.0030%, A raw steel slab containing N: 0.002 to 0.005%, Cr: 0.3 to 0.7%, Mo: 0.3 to 1.0%, or Nb: 0.01 to 0.10% is hot-rolled at a finishing temperature of 950 ° C or less and above the _Ar3 transformation point. , Hot-stretched coil wound at 450-700 ° C, or high-strength electro-welding that is hard to soften the heat-affected zone by applying post-pipe strain relief annealing, normalizing, and cold drawing A method of manufacturing a steel pipe is described. The electric resistance welded steel pipe manufactured by the technique described in Patent Document 2 has a high strength of 100 to 130 kgf / mm ² in tensile strength, and the base material portion and the welded portion are uniform, and the balance of strength and ductility toughness is balanced. It is said to be an excellent ERW steel pipe.

Japanese Patent Laid-Open No. 04-311526 Japanese Patent Laid-Open No. 06-10046

  However, the technique described in Patent Document 1 has a problem that heat treatment after pipe forming is required, the process becomes complicated, productivity is lowered, and manufacturing costs are increased. In addition, the technique described in Patent Document 2 also requires heat treatment after pipe making, and similarly, there is a problem that the process becomes complicated, the productivity is lowered, and the manufacturing cost is increased.

  The present invention solves such problems of the prior art, has high strength, high toughness, excellent balance of strength and ductility, and has a small hardness difference between the base metal part and the welded part without performing post-tube-forming heat treatment. An object is to provide a high-tensile welded steel pipe having a homogeneous material in the direction.

“High strength” as used herein refers to the case where the tensile strength TS is 780 MPa or more, and “high toughness” refers to the fracture surface transition temperature of the Charpy impact test in the circumferential direction of the steel pipe. The case where Trs ₅₀ is −40 ° C. or lower shall be said. “Excellent strength and ductility balance” means that the tensile strength TS × elongation El obtained using JIS No. 12 B (arc-shaped) tensile specimen with the longitudinal direction of the pipe as the tensile direction is 11000 MPa ·% or more. It shall mean a certain case. Also, “the hardness difference between the base metal part and the welded part is small and the material is homogeneous in the circumferential direction” means that the hardness difference ΔHV between the base metal part and the welded part is measured at Vickers hardness HV (test force: 4.9 N) ), When the tensile strength TS is 780 MPa or more and less than 980 MPa, it means 30 points or less, and when the tensile strength TS is 980 MPa or more, it is 60 points or less. The hardness is measured at a pitch of 0.2 mm in the circumferential direction within a range of 5 mm on both sides around the weld. Then, the average value of the hardness in a range not affected by welding heat is set as the base material part hardness HVm, and the maximum hardness HVmax of the welded part and the minimum hardness HVmin of the softened part are obtained. The hardness difference ΔHV between the base metal part and the welded part is calculated using the following equation.
ΔHV = | HVmax−HVm | or = | HVm−HVmin |

  In order to achieve the above-mentioned object, the present inventors diligently studied various factors affecting strength and toughness. As a result, C: 0.02 to 0.20% low carbon, a composition containing Ti and further Mo and / or V, a fine ferrite phase having an average crystal grain size of 10 μm or less as a main phase, and an average grain By forming a microstructure in which composite carbide containing fine Ti with a diameter of 10 nm or less and Mo and / or V is precipitated, the tensile strength is high strength of 780 MPa or more, high toughness, and remarkable welded parts It was found that high-strength welded steel pipes with uniform material in the circumferential direction could be obtained without any significant hardening and significant softening of HAZ.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1 ) By mass%, C: 0.17 to 0.20%, Si: 0.001 to 1.0%, Mn: 0.01 to 2.0%, P: 0.1% or less, S: 0.01% or less, Al: 0.01 to 0.1%, Ti: 0.01 A composition containing Mo: 0.5% or less and / or V: 0.5% or less, the balance consisting of Fe and unavoidable impurities, and a ferrite phase having an average particle size of 10 μm or less as a main phase. And a second phase having an area ratio of 10% or less (including 0%), having an average particle size of Ti of 10 nm or less, and a structure in which composite carbide containing Mo and / or V is dispersed. , Tensile strength TS: 980 MPa or more, excellent low-temperature toughness, strength ductility balance TS × El is 11000 MPa% or more, and the hardness difference ΔHV between the welded part and the base metal part without post-heat treatment of the welded part is Vickers hardness A high-strength welded steel pipe for automobile parts, characterized in that it is 60 points or less.
(2) (1), in addition to the composition, by mass%, Nb: high tensile welded steel pipe for automotive member, characterized in that a composition containing 0.05% or less.
( 3 ) In (1) or ( 2) , in addition to the above-mentioned composition, in mass%, Cr: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, W: 0.05% or less A high-tensile welded steel pipe for automobile parts, characterized in that the composition contains one or more kinds.
( 4 ) In any one of (1) to ( 3 ), a high-strength welded steel pipe for automobile members, characterized in that, in addition to the above composition, the composition further contains, by mass%, Ca: 0.02% or less.

According to the present invention, the tensile strength TS has a high strength of 780 MPa or more, a toughness with a fracture surface transition temperature Trs ₅₀ of −40 ° C. or less in the Charpy impact test in the pipe circumferential direction, and TS × El Is a high-strength welded steel pipe that has an excellent balance between strength and ductility of 11000 MPa ·% or more, has a small hardness difference between the base metal part and the welded part, and has a homogeneous material in the circumferential direction without performing post-heat treatment. It can be manufactured easily and inexpensively, and has a remarkable industrial effect.

  First, the reasons for limiting the composition of the high-strength welded steel pipe of the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply expressed as%.

C: 0.17 to 0.20%
C is an element that contributes to increasing the strength of the steel by solid solution and is combined with carbide forming elements to precipitate as carbides and contributes to increasing the strength by precipitation strengthening, in order to ensure the desired tensile strength. In the present invention, a content of 0.02% or more is required. On the other hand, if the content exceeds 0.20%, ductility and toughness are lowered, and excessive hardness rise is caused in the welded part, resulting in lowered workability and toughness of the welded part. In the present invention , C is limited to 0.17 to 0.20%.

Si: 0.001 to 1.0%
Si is an element that contributes as a deoxidizer and has the effect of increasing the strength of steel by solid solution. In order to obtain such an effect, the content of 0.001% or more is required. On the other hand, the content exceeding 1.0% causes a remarkable deterioration of the surface properties and lowers the ductility to ensure a desired ductility. Is difficult and also reduces weldability. For this reason, Si was limited to the range of 0.001 to 1.0%. In addition, Preferably it is 0.001 to 0.3%.

Mn: 0.01-2.0%
Mn is an element that contributes to increasing the strength of the steel by solid solution, and needs to be contained in an amount of 0.01% or more in order to ensure a desired tensile strength. On the other hand, if the content exceeds 2.0%, the ductility is lowered and the desired ductility cannot be secured, and the weldability is lowered. For this reason, Mn was limited to the range of 0.01 to 2.0%. In addition, Preferably it is 0.5 to 1.5%.

P: 0.1% or less P is an element that increases the strength of steel. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.1% causes a drop in ductility. For this reason, P was limited to 0.1% or less. In addition, when the reinforcement | strengthening by P is not so required, it is preferable to limit to 0.05% or less.

S: 0.01% or less S is present as a non-metallic inclusion in steel and causes a decrease in ductility and toughness. Therefore, it is desirable to reduce S as much as possible. However, if it is reduced to 0.01% or less, the above-mentioned concern will be less affected. For this reason, S was limited to 0.01% or less. In addition, Preferably it is 0.005% or less.

Al: 0.01 to 0.1%
Al is an element that acts as a deoxidizer and has an action of suppressing coarsening of crystal grains. In the present invention, Al is contained by 0.01% or more. On the other hand, if the content exceeds 0.1%, oxide inclusions increase, the cleanliness decreases, and the ductility decreases. For this reason, Al was made 0.01 to 0.1%. In addition, Preferably it is 0.01 to 0.06%.

Ti: 0.01 to 0.5%, Mo: 0.5% or less, and / or V: 0.5% or less
Ti, Mo, and V are all elements that form carbides and contribute to increasing the strength of steel through precipitation strengthening, and are important elements in the present invention, and are contained in a composite. By containing Ti and Mo and / or V in a composite, composite carbides of these elements are formed and dispersed and precipitated as precipitates, ensuring the desired high strength in the base metal and significant hardening in the weld zone. Can be suppressed. In order to obtain such effects, it is necessary to contain Ti: 0.01% or more and Mo: 0.01% or more and / or V: 0.01% or more. On the other hand, if the content exceeds Ti: 0.5% and Mo: 0.5% and / or V: 0.5%, a hard phase is formed and ductility is lowered. For this reason, Ti is limited to the range of 0.01 to 0.5%, Mo: 0.5% or less, and V: 0.5% or less, respectively.

  The above-mentioned components are basic components. In the present invention, in addition to the basic composition, Nb: 0.05% or less and / or Cr: 0.5% or less, Cu: 1% or less, Ni: One or more selected from 1% or less, W: 0.05% or less, and / or Ca: 0.02% or less can be contained.

Nb: 0.05% or less
Nb is an element that increases the strength of the steel and contributes to the refinement of the structure, and can be contained as necessary. In order to acquire such an effect, it is desirable to contain 0.005% or more, but inclusion exceeding 0.05% reduces ductility. For this reason, it is preferable to limit Nb to 0.05% or less. More preferably, it is 0.005 to 0.02%.

One or more selected from Cr: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, W: 0.05% or less
Cr, Cu, Ni, and W are all elements that increase the strength of steel, and can be selected and contained as necessary.

  Cr is an element that dissolves in steel and increases the strength of the steel. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, when the content exceeds 0.5%, the ductility deteriorates remarkably. For this reason, when it contains, Cr is limited to 0.5% or less.

Cu, like Cr, is an element that dissolves in steel and increases the strength of the steel, and such an effect becomes significant when the content is 0.01% or more. On the other hand, when the content exceeds 0.5%, the ductility deteriorates remarkably. For this reason, when it contains, it is preferable to limit Cu to 0.5% or less.
Ni, like Cr and Cu, is an element that dissolves in steel and increases the strength of the steel. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, when the content exceeds 0.5%, the ductility deteriorates remarkably. For this reason, when it contains, it is preferable to limit Ni to 0.5% or less.
W, like Cr, Cu, and Ni, is an element that increases the strength of the steel by forming a solid solution in the steel. Such an effect becomes remarkable when the content is 0.01% or more. On the other hand, if the content exceeds 0.05%, the ductility and toughness are significantly lowered. For this reason, when contained, W is preferably limited to 0.05% or less.

Ca: 0.02% or less
Ca is an element that contributes to the improvement of moldability by controlling the shape of so-called inclusions, with the expanded inclusions being spherical inclusions, and suppressing the development of cracks and cracks during molding. Yes, it can be contained if necessary. In order to acquire such an effect, it is desirable to contain 0.0020% or more. On the other hand, if it exceeds 0.02%, the amount of non-metallic inclusions increases and the ductility decreases. For this reason, when it contains, it is preferable to limit Ca to 0.02% or less.
The balance other than the components described above consists of Fe and inevitable impurities. As unavoidable impurities, N: 0.01% or less and O: 0.01% or less are acceptable.

The high-strength welded steel pipe of the present invention has the above-described composition, and is composed of a ferrite phase having an average particle size of 10 μm or less as a main phase and a second phase having an area ratio of 10% or less, and an average particle size of 10 nm or less. The composite carbide containing Ti and Mo and / or V is dispersed. As a result, it has a high tensile strength of TS: 780 MPa or more, a toughness with a fracture surface transition temperature Trs ₅₀ of −40 ° C. or less in the Charpy impact test in the pipe circumferential direction, and TS × El of 11000 MPa · It is possible to produce a high-tensile welded steel pipe having a uniform material in the circumferential direction with a small hardness difference between the base metal part and the welded part and having a uniform material in the circumferential direction without performing post-heat treatment.

  The “main phase” here refers to a phase occupying 90% or more, preferably 95% or more in terms of area ratio. High strength and high ductility can be secured by making the main phase a fine ferrite phase having a low dislocation density with an average particle size of 10 μm or less. On the other hand, if the average particle size of the ferrite phase exceeds 10 μm, it is difficult to ensure both desired high strength and desired high toughness. For this reason, the main phase was a fine ferrite phase having an average particle size of 10 μm or less. The second phase other than the main phase is preferably pearlite, bainite, martensite, or a mixture thereof. The second phase has an area ratio of 10% or less, preferably 5% or less (including 0%). When the amount of the second phase exceeds 10% in area ratio, extreme softening occurs in the weld zone (welding heat affected zone). This is because hard martensite or the like changes to cementite at the welded portion (welding heat affected zone) and may be significantly softened at the welded portion (welded heat affected zone). For this reason, the second phase was limited to 10% or less (including 0%) in area ratio. The area ratio is preferably 5% or less.

  The composite carbide to be dispersed is a fine precipitate having an average particle size of 10 nm or less. When the composite carbide is coarsened with an average particle size exceeding 10 nm, the above-described increase in strength cannot be achieved sufficiently. The composite carbide to be dispersed is a composite carbide containing Ti and Mo and / or V, that is, a Ti—Mo based composite carbide, a Ti—Mo—V based composite carbide, and a Ti—V based composite carbide. Since all of these composite carbides precipitate finely, high strength can be ensured without degrading workability. Further, these composite carbides are thermally stable and are not easily coarsened even in the welded portion, and can suppress the softening of the welded portion.

Next, a preferred method for producing the high-strength welded steel pipe of the present invention will be described.
First, it is preferable that the steel material having the above composition is hot-rolled to form a steel pipe material as a hot-rolled steel strip (hot-rolled sheet), and the steel pipe material is made into a welded steel pipe through a pipe forming process.

  First, although the manufacturing method of the steel material is not particularly limited, the molten steel having the above composition is melted by a conventional melting method such as a converter or an electric furnace, and the continuous casting method, the ingot-bundling method, etc. It is preferable to use a rolling material such as a slab by a conventional method.

  In the hot rolling applied to the steel material having the above composition, the finish rolling finish temperature is preferably 850 ° C. or higher and lower than 950 ° C., and the winding temperature is preferably 500 ° C. or higher and lower than 700 ° C. More preferably, the coiling temperature is 550 ° C. or higher. When the finish rolling finish temperature is 950 ° C. or higher, the surface properties of the steel sheet deteriorate and adversely affect the fatigue characteristics. On the other hand, when the temperature is lower than 850 ° C., the crystal grains of the surface layer become coarse and the fatigue characteristics are deteriorated, and the rolling load is increased, which makes hot rolling difficult.

  The coiling temperature is set to 500 ° C. or higher in order to obtain a ferrite structure and to ensure the running stability of the steel sheet on the run-out table. On the other hand, when the temperature is 700 ° C. or higher, the amount of pearlite generated increases, making it difficult to secure a ferrite structure.

  In addition, the welded steel pipe uses the above-described steel pipe material, for example, after the steel pipe material is formed into an open pipe shape by roll forming or bending process in a cold or warm manner, the ends of the open pipe are butted together, It is preferable to manufacture by using an electric resistance welding method in which heating is performed at a temperature equal to or higher than the melting point using induction heating or the like and abutting joining is performed using a squeeze roll. Needless to say, the welded steel pipe is not limited to the above-described electric seam welding method, and may be manufactured by a forging method or other methods such as a solid-phase pressure welding method.

  Hereinafter, the present invention will be described in more detail based on examples.

  After molten steel having the composition shown in Table 1 was melted in a converter, a steel material (slab) was obtained by a continuous casting method. These steel materials were subjected to a hot rolling process under the conditions shown in Table 2 to obtain hot rolled sheets (hot rolled steel strip). Next, using these hot-rolled sheets as steel pipe materials, after forming into an open pipe shape by continuous forming using rolls, both ends of the open pipe are butted together, heated to the melting point or higher by induction heating, and abutted with a squeeze roll. It was a welded steel pipe (outer diameter 70 mmφ x wall thickness 2.6 mm).

Test pieces were collected from the obtained welded steel pipes, and subjected to a structure observation test, a tensile test, an impact test, and a hardness test. The test method is as follows.
(1) Microstructure observation test A specimen for microstructural observation was collected from the obtained welded steel pipe, polished so that the cross section perpendicular to the pipe axis direction became the observation surface, and subjected to Nital corrosion, and an optical microscope (magnification: 400). ) Or a scanning electron microscope (magnification: 2000 times), observe the structure, take an image, identify the main phase and second phase structures, and use an image analyzer to determine the crystal grain size and tissue fraction. And calculated. In addition, the average particle diameter of each phase measured the area of each particle | grain of each phase, converted into a circle equivalent diameter, made it the diameter of each particle | grain, and averaged them, and it was set as the average particle diameter of each phase of the steel pipe.

Moreover, the structure | tissue, the kind of deposit, and the particle size were measured using the transmission electron microscope. The type of precipitates was determined by identifying the contained elements with an energy dispersive X-ray analyzer (EDX) attached to a transmission electron microscope. The particle size of the precipitate is obtained by measuring the area of each precipitate, calculating the equivalent circle diameter from the area, setting the particle size of each particle, and arithmetically averaging them to obtain the particle size of the composite carbide (precipitate) ( Average).
(2) Tensile test From the obtained welded steel pipe, JIS No. 12 A test specimens were collected in accordance with the provisions of JIS Z 2201 so that the longitudinal direction of the pipe would be the tensile direction. Tensile tests were performed, and tensile properties (tensile strength TS, elongation El) were measured.
(3) Impact test A test material is collected from the obtained welded steel pipe and developed, and a V-notch test piece is set in accordance with the provisions of JIS Z 2242 so that the pipe circumferential direction is the length direction of the test piece. Samples were taken from the material part and the welded part (notch position: bond part), a Charpy impact test was performed, a fracture surface transition temperature Trs ₅₀ was determined, and low temperature toughness was evaluated.
(4) Hardness test From the obtained welded steel pipe, a hardness test piece including a base metal part and a welded part is sampled and polished so that a cross section perpendicular to the longitudinal direction of the pipe becomes a measurement surface, and a Vickers hardness tester is obtained. (Test force: 4.9 N) Vickers hardness HV was measured at a pitch of 0.2 mm in the circumferential direction within a range of 5 mm on each side from the center of the weld. Calculate the average hardness (base metal hardness) HVm, the maximum hardness HVmax and the minimum hardness HVmin of the weld zone, and ΔHV = | HVmax−HVm | or = | HVm−HVmin | did.

  The obtained results are shown in Table 3.

In all of the examples of the present invention, tensile strength TS: high strength of 780 MPa or more, high toughness with Trs ₅₀ of −40 ° C. or less in the pipe circumferential direction of the base metal part and the welded part, and a balance between strength and ductility TS × El Has a high strength / ductility balance of 11000 MPa% or more, and the hardness difference ΔHV between the base metal part and the welded part is 30 points (TS: 760 MPa or more and less than 980 MPa), 60 points or less (TS: 980 MPa or more) ), A high-tensile welded steel pipe having a small hardness difference ΔHV between the base metal part and the welded part and having a homogeneous material in the pipe circumferential direction.

  On the other hand, in the comparative example outside the scope of the present invention, either the strength, ductility, or low temperature toughness is reduced, or the hardness difference ΔHV between the base metal part and the welded part is large, and there is a material difference in the pipe circumferential direction. Has occurred.

Claims (4)

% By mass
C: 0.17 to 0.20%, Si: 0.001 to 1.0%,
Mn: 0.01 to 1.5%, P: 0.1% or less,
S: 0.01% or less, Al: 0.01 to 0.1%,
Ti: 0.01-0.5%
And a composition comprising Mo: 0.5% or less and / or V: 0.5% or less, the balance being Fe and inevitable impurities,
Average particle size as the main phase: Ferrite phase of 10 μm or less and second phase with area ratio of 10% or less (including 0%), Ti with an average particle size of 10 nm or less, Mo and / or V A structure in which a composite carbide containing is dispersed,
Tensile strength TS: 980 MPa or more, excellent low-temperature toughness, strength ductility balance TS x El is 11000 MPa% or more, and hardness difference ΔHV between welded part and base metal part without post-heat treatment of welded part A high-tensile welded steel pipe for automobile parts, characterized by having a Vickers hardness of 60 points or less. The high-strength welded steel pipe for automobile members according to claim 1, wherein the composition further contains Nb: 0.05% or less in mass% in addition to the composition. In addition to the above composition, the composition further contains, by mass%, one or more selected from Cr: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, W: 0.05% or less. The high-tensile welded steel pipe for automobile parts according to claim 1 or 2 . The high-tensile welded steel pipe for automobile members according to any one of claims 1 to 3 , wherein in addition to the composition, the composition further contains Ca: 0.02% or less by mass%.