JP2005146395A - Steel material to be hydroformed, electric resistance welded pipe to be hydroformed, and method for manufacturing them - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material to be hydroformed which has superior hydroformability, superior weldability and a tensile strength of 390 MPa or higher. <P>SOLUTION: The steel material to be hydroformed has a chemical composition comprising 0.02-0.35% C, 0.01-1.0% Si, 0.01-2.5% Mn, 0.005-0.10% P, S≤0.0100%, 0.001-0.1% Al, 0.005-0.020% Ti, 0.0004-0.0100% N, and the balance Fe with impurities; includes crystallization-type TiN-based particles with particle diameters of 0.3 μm or larger in an amount of 50 to 50,000 particles per 1 mm<SP>2</SP>of a cross section, while the whole crystallization-type TiN-based particles have an average particle diameter of 7 μm or less; and includes such ferrite as to occupy 50% or more by an area rate in a structure and have an average diameter of 3 to 30 μm. The steel material may include one or more groups among the following groups (a) to (d): (a) one or more elements of 0.1% or less Nb and 0.2% or less V; (b) one or more elements of 1.0% or less Mo, 1.0% or less Ni and 1.0% or less Cu; (c) one or more elements of 1.0% or less Cr and 0.0005-0.003% B; and (d) 0.0002-0.01% Ca, 0.0002-0.01% Mg and 0.0002-0.01% REM. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  TECHNICAL FIELD The present invention relates to a steel material for hydrofoam, an electric sewing tube for hydrofoam, and a method for producing them. In particular, the present invention is suitable for use as a material for automobile structural members, suspension members, and the like, and as a material for a hydroform electric sewing tube. The present invention relates to a steel material excellent in weldability with few cracks in a welded portion and a method for manufacturing the same, and an electric-welded pipe for hydroform made of the steel material and a method for manufacturing the same.

  In recent years, particularly from the viewpoint of protecting the global environment, it has been studied to reduce the mass of a vehicle body by reducing the strength and thickness of various members of an automobile to improve fuel consumption and to regulate the emission of carbon dioxide gas and the like.

  For this reason, recently, an attempt has been made to form and process a complex part such as an automobile from a high-strength steel pipe by a hydroform method. According to this hydroform method, the number of parts and the number of welded flanges can be reduced, so that it is possible to meet demands for not only weight reduction but also cost reduction of automobiles.

  In addition, if the hydroforming method is used as a molding method for parts with complex shapes, it is expected that there will be significant advantages such as cost reduction and increased design flexibility. In order to make full use of the advantages, a material (processed material) suitable for this molding method is required.

  Patent Documents 1 to 3 disclose steel pipes suitable for the hydroforming method. The steel pipes proposed in these patent documents are characterized in that the structure is a ferrite main body or a ferrite single phase structure in order to improve the hydroformability.

  That is, in Patent Document 1, if there is a phase (structure) such as pearlite, martensite, and cementite, which is a hard second phase, in addition to ferrite, soft ferrite and hard at a relatively early stage of plastic deformation during hydroforming. Therefore, it has been proposed that the structure of the steel pipe is a ferrite-based structure because cracks are generated from the interface of the second phase. In Patent Documents 2 and 3, it is proposed that the structure of the steel pipe is a structure mainly composed of ferrite so as not to deteriorate the ductility.

  Therefore, in order to obtain a material with improved hydroformability, it can be presumed that a structure mainly composed of ferrite is desirable.

  However, if the proportion of ferrite in the structure is increased in order to improve the hydroformability, it becomes difficult to ensure the strength. Therefore, in order to ensure strength in a ferrite-based structure, it is necessary to strengthen ferrite by solid solution strengthening or precipitation strengthening, but in the case of solid solution strengthening, the alloy element content is increased compared to precipitation strengthening. It is necessary and disadvantageous in cost. For this reason, it is desirable to strengthen ferrite by precipitation strengthening, and it is particularly desirable to strengthen ferrite by precipitation strengthening action of Ti, Nb, V, or the like. When the precipitation strengthening action such as Ti, Nb, and V is used, the hard second phase that adversely affects the hydroformability is reduced. This is because C is necessary to form TiC, NbC, VC, and the like that contribute to strengthening, and the amount of solid solution C decreases because of precipitation of C due to precipitation strengthening. This is because the formation of two phases is reduced.

  Of the above Ti, Nb, and V, Ti is the cheapest element, and the amount of increase in strength relative to the content is larger than that of Nb, V, etc., so Ti is usually used as a precipitation strengthening element. Yes.

  However, when precipitation strengthening by Ti is used, not only fine TiC contributing to strengthening but also coarse crystallized TiN-based particles are generated. And while TiC that contributes to precipitation strengthening is a fine precipitate with a size of several tens of nanometers, crystallized TiN-based particles are coarse because they crystallize at high temperatures with oxides as nuclei. It does not contribute to the strength increase at all. Further, the coarse crystallization type TiN-based particles deteriorate the hydroformability.

  On the other hand, most of the steel materials for hydroforming are processed into electric sewing tubes, and thereafter, the obtained electric sewing tubes are hydroformed to obtain parts of various shapes. In order to process into an electric sewing tube, after forming a steel material into a cylindrical tube, the end surface and the end surface are butted and welded. Therefore, in general, excellent butt resistance weldability (hereinafter simply referred to as “weldability”) is required for the steel material for hydroform.

  However, so far, no technology has been found that combines good hydroformability and excellent weldability in Ti precipitation strengthened steel.

JP-A-2001-32034

Japanese Patent Laid-Open No. 2000-111981 Japanese Patent Laid-Open No. 2002-69584

  The present invention has been made in view of the above situation, and its purpose is suitable as a material for automobile structural members and suspension members, and has excellent hydroformability and excellent weldability. An object of the present invention is to provide a manufacturing method thereof, and an electro-sewn tube for hydroform and a manufacturing method thereof. Specifically, the crystallized TiN-based particles, which are usually coarse, are refined and present in the steel material in a large amount while utilizing the precipitation strengthening effect of Ti, while ensuring high strength and good hydroformability. Another object of the present invention is to provide a hydroform steel material having excellent weldability and a tensile strength of 390 MPa or more and a method for producing the same, and an electroformed tube for hydroform and a method for producing the same.

  The gist of the present invention is as follows. Steel materials for hydrofoam shown in the following (1) to (8), electric sewing tubes for hydrofoam shown in (9), methods for producing steel materials for hydrofoam shown in (10) to (14), and It exists in the manufacturing method of the electric sewing pipe for hydroforms shown in (15).

(1) By mass%, C: 0.02 to 0.35%, Si: 0.01 to 1.0%, Mn: 0.01 to 2.5%, P: 0.005 to 0.10% , S: 0.0100% or less, Al: 0.001-0.1%, Ti: 0.005-0.20% and N: 0.0004-0.0100%, the balance being Fe and impurities The crystallized TiN-based particles having a particle size of 0.3 μm or more and 50 to 50,000 crystallized TiN-based particles per 1 mm ² in cross-sectional area, and the average particle size of the crystallized TiN-based particles is 7 μm or less. A steel material for hydroform, wherein the area ratio of ferrite to the structure is 50% or more and the average particle size is 3 to 30 μm.

  (2) The steel material for hydroform as described in the above item (1), which contains one or more of Nb: 0.1% or less and V: 0.2% or less in mass% instead of part of Fe.

  (3) In place of a part of Fe, in mass%, the composition contains one or more selected from Mo: 1.0% or less, Ni: 1.0% or less, and Cu: 1.0% or less ( Steel material for hydroforms as described in 1) or (2).

  (4) In place of a part of Fe, by mass%, containing at least one of Cr: 1.0% or less and B: 0.0005-0.003% from (1) to (3) above Steel material for hydrofoam as described in any one.

  (5) Instead of a part of Fe, in mass%, Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, and REM (rare element): 0.0002 to 0 Steel material for hydrofoams in any one of said (1) to (4) containing 1 or more types selected from 0.01%.

  (6) The average particle size of crystallized TiN-based particles having a particle size of 0.3 μm or more is 0.3 to 3.0 μm, according to any one of (1) to (5) above Steel material for hydroform.

  (7) Crystallized TiN particles having a particle size of 2 μm or more and hard particles having a particle size of 2 μm or more in the plate thickness center region in the range of 15% of the plate thickness from the plate thickness center toward the plate surface. The steel material for hydrofoam according to any one of (1) to (6) above, wherein an average particle interval with the second phase is 10 to 50 μm.

  (8) The intergranular hardness of ferrite in Vickers hardness and the area ratio of ferrite in the structure satisfy the following formula (1), and the remaining structure other than ferrite is selected from martensite, pearlite, bainite, and austenite 1 The steel material for hydrofoam according to any one of (1) to (7) above, which is a seed or more.

A ≦ (B + 200) /1.5 (1).
Here, A in the formula (1) is the intragranular hardness of the ferrite in terms of Vickers hardness, and B is the area ratio (%) that the ferrite occupies in the structure.

  (9) A hydroform electric sewing tube made of the steel material for hydrofoam according to any one of (1) to (8) above.

  (10) A method for producing a steel material for hydroform, which is a liquidus line of molten steel when continuously casting a molten steel having the chemical composition according to any one of (1) to (5) above to form a steel ingot. The manufacturing method of the steel material for hydroforms which includes the process which makes the average cooling rate of the cross section perpendicular | vertical to the casting direction of the said steel ingot in the temperature range of 1300 degreeC from temperature to 0.4-7 degreeC / second in a manufacturing process.

  (11) The method for producing a steel material for hydroform as described in (10) above, wherein the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot is 2 to 7 ° C / second.

  (12) A steel ingot cooled at an average cooling rate of a cross section perpendicular to the casting direction of the steel ingot described in (10) or (11) above is set to a finishing temperature of “Ar3 point−100 ° C.” or more and 1050 ° C. or less. Hot rolling, then cooling to a temperature of 730 ° C. or less at an average cooling rate of 10 ° C./second or more, and then winding the steel material for hydroform as described in (10) or (11) above Production method.

  (13) A steel ingot cooled at an average cooling rate of a cross section perpendicular to the casting direction of the steel ingot described in (10) or (11) above is set to a finishing temperature of “Ar3 point−100 ° C.” or more and 1050 ° C. or less. After hot rolling, it is cooled to a temperature range of 730 to 600 ° C. at an average cooling rate of 10 ° C./second or more, then air-cooled for 2 to 15 seconds, and then 600 ° C. at an average cooling rate of 15 ° C./second or more. The method for producing a steel material for hydrofoam according to the above (10) or (11), wherein the material is cooled to a temperature of less than 1, and then wound.

  (14) When the molten steel is cast into a steel ingot, the unsolidified layer of the steel ingot is subjected to reduction or electromagnetic stirring on the portion where the thickness of the steel ingot is 30% or less. (10) to (13) The manufacturing method of the steel material for hydroforms in any one.

  (15) A method for producing a hydroform electric sewing tube, wherein the hydroform steel material obtained by the production method according to any one of (10) to (14) is formed into a tubular shape, and then a butt portion Manufacturing method of electroformed pipe for hydroforming.

  The “steel material for hydroforming” as used in the present invention refers to a steel material for forming or processing by a hydroforming method, that is, a liquid pressure. The “hydroforming electric sewing tube” refers to an electric sewing tube for molding or processing by the hydroforming method.

  “TiN-based particles” are particles containing Ti and N produced in the steelmaking stage, in molten steel, in the solidification process of slabs, in hot rolling and in the subsequent cooling process, and in the hot winding process. There are a type and a precipitation type, including those expressed as so-called (Ti, Nb) N when Nb is contained in steel.

  Among the TiN-based particles, the “crystallized TiN-based particles” are produced by using an Al-based oxide or a Ca-based oxide when Ca is contained in steel as a nucleus. This can be easily distinguished from “precipitation-type TiN-based particles” whose “particle size” is usually about 30 nm at most. Of the “crystallized TiN-based particles”, those having a “particle size” of about 0.1 to 20 μm are defined as “crystallized TiN-based particles” in the present invention.

  The “ferrite” in the present invention includes so-called “bainitic ferrite”. Note that “bainitic ferrite” has a lath-like structure as a substructure, but unlike a normal bainite structure, a structure in which cementite does not exist or a dislocation density that does not have a clear subgrain structure. It means high ferrite.

  The “hard second phase” in the present invention refers to pearlite, bainite, martensite, and cementite among phases (structures) other than the “ferrite” described above. However, the cementite here does not include cementite that forms pearlite.

  “Particle size” refers to a value defined by “1/2 of the sum of minor axis and major axis” of crystallized TiN-based particles and ferrite that are individual particles. The arithmetic mean of the above “particle diameter”.

  Specifically, the crystallized TiN-based particles and ferrite can be observed using an optical microscope, a scanning electron microscope, and a transmission electron microscope having an acceleration voltage of 100 to 200 kV. The obtained image is subjected to image analysis to measure the minor axis and the major axis, and the grain size of each crystallized TiN-based particle or ferrite can be obtained from 1/2 of the sum. On the other hand, an arithmetic average of the particle diameters of individual particles obtained by observing 100 visual fields as described above is defined as “average particle diameter”.

  In addition, the shortest interval between each crystallized TiN-based particle and each hard second phase is defined as the individual particle interval, and 100 fields of view are observed using an optical microscope, a scanning electron microscope, a transmission electron microscope, or the like. An arithmetic average of the obtained individual particle intervals is referred to as “average particle interval between crystallized TiN-based particles and the hard second phase”.

  The area ratio of ferrite occupying the structure refers to the area ratio of ferrite to the area of 100 visual field observations obtained by the same observation method as described above.

The number of crystallized TiN-based particles per 1 mm ² cross-sectional area refers to the number obtained by converting the number with respect to the area for 100 field observations obtained by the same observation method as described above to 1 mm ² cross-sectional area.

  The “intragranular hardness of ferrite” refers to an arithmetic average of values obtained by measuring the hardness of 50 ferrite grains at a test force of 0.009807 to 0.09807N, for example, using a micro Vickers hardness tester. .

  “REM (rare earth element)” is a general term for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of the above elements.

  “Average cooling rate of the cross section perpendicular to the casting direction of the steel ingot” means the steel ingot when it is called a steel ingot including the case where a solidified shell is formed in the mold or continuous casting machine and the inside is in a molten state. The average value of the cooling rate in the whole area | region from the surface part in the cross section perpendicular | vertical to the casting direction of this to the center part.

  Moreover, the temperature of the steel material hot-rolled refers to the temperature at the surface of the steel material, and the average cooling rate of the steel material refers to the temperature difference between before and after cooling at the surface of the steel material divided by the cooling time.

  “Air cooling” refers to air cooling and forced air cooling.

  Hereinafter, it relates to the invention relating to the steel material for hydroform (1) to (8), the invention relating to the electroformed tube for hydroform (9), and the method for producing the steel material for hydroform (10) to (14). The inventions relating to the invention and the invention relating to the method for producing the hydroformed electric pipe of (15) are referred to as inventions (1) to (15), respectively.

  The steel material for hydroform of the present invention is an inexpensive Ti precipitation strengthening type steel material having a tensile strength of 390 MPa or more, and has both good hydroformability and excellent weldability. It can be used as a material for rotating parts. Moreover, this steel material for hydroforming can be utilized as a material for the electroformed tube for hydroforming.

  In order to solve the above-described problems, the present inventors first performed a basic study on plastic deformation behavior during forming of a steel pipe by the hydroform method, and confirmed the following matters.

  (A) The bulging deformation | transformation in shaping | molding by a hydroforming method is plane distortion deformation | transformation in which the deformation | transformation of the longitudinal direction of the steel pipe was restrained.

  (B) In plane strain deformation, triaxial stress is generated from the initial stage of deformation. For this reason, if there are many hard particles such as TiN and hard phases such as martensite and pearlite, cracks occur relatively early in the deformation stage from the interface between them and ferrite.

  Next, in order to easily evaluate the hydroformability of steel pipes, tensile test specimens of various shapes were collected from various directions of the steel pipe and from various directions of the plate that was cut open and flattened. did. As a result, the following knowledge was obtained.

  (C) In a plate flattened by cutting a steel pipe, a tensile test piece having a specific shape taken from a direction corresponding to the pipe circumferential direction of the original steel pipe, that is, a “tensile test piece with R” shown in FIG. Elongation has a good positive correlation with the hydroforming properties of steel pipes.

  (D) The elongation value of the tensile test piece with R taken in the direction perpendicular to the rolling direction of the steel sheet is the direction corresponding to the pipe circumferential direction of the original steel pipe of the plate that was cut and flattened after the steel pipe was made into a steel pipe. There is a good positive correlation with the elongation value of the tensile specimen with R taken from

  Said knowledge (c) and (d) shows that hydroform property can be evaluated by the elongation value of a tensile test piece with R. Therefore, “steel material with good hydroformability” means “steel material having a high elongation value with a tensile test piece with R”, and “elongation value with a tensile test piece with R taken from a steel plate”. When the steel pipe is made of the steel plate as a raw material, it has been clarified that a “steel pipe having good hydroformability” can be obtained.

  Then, in order to achieve both high strength and good hydroformability, various studies were made on the hydroformability of Ti precipitation strengthened steel that can be expected to increase in strength even when the hard second phase is reduced.

  As a result, it became clear that the size and amount of crystallized TiN-based particles have an effect on the hydroformability, and the following new findings were obtained.

  (E) At the time of plane strain deformation, coarse crystallized TiN-based particles cause cracks at a relatively early stage of deformation from the interface with ferrite in the same manner as martensite and pearlite, which are hard phases.

  (F) If there are many hard materials such as coarse crystallized TiN-based particles and hard phases such as martensite and pearlite in the steel, cracks generated from the interface with the ferrite are connected, and the steel Lead to breakage.

  (G) When the crystallized TiN-based particles become fine, cracks do not occur in the initial stage of plane strain deformation from the interface between the crystallized TiN-based particles and the ferrite. Therefore, the hydroformability is improved.

  (H) When shearing represented by shear cutting or slitting is performed, if there are many fine crystallized TiN-based particles in the steel, the fracture surface property of the shearing end face of the steel Improved. Therefore, when welding (butt resistance welding) is performed by butting the end faces of steel materials that produce an electric resistance welded tube, the adhesion of the steel materials is improved at the welded portion, resulting in a reduction in weld metal cracking at the welded portion. To do.

  (I) When a large amount of fine crystallized TiN-based particles are present in the steel material, the reason for the improved fracture surface properties of the sheared end surface of the steel material is to sew small voids generated by the fine TiN during the shearing process. This is because the final shearing end face is formed substantially in a straight line because the crack propagates.

  The present inventions (1) to (15) have been completed based on the above findings.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".

(A) Chemical composition of steel C: 0.02 to 0.35%
C is an element necessary for precipitation strengthening by TiC and securing strength by the second phase other than ferrite. In order to ensure the tensile strength of 390 MPa or more using the precipitation strengthening action by TiC, the C content needs to be 0.02% or more. However, if the content of C exceeds 0.35%, the hard second phase increases and the hydroformability deteriorates. Therefore, the content of C is set to 0.02 to 0.35%. Note that the C content is preferably 0.02 to 0.15%, and more preferably 0.02 to 0.10%.

Si: 0.01 to 1.0%
Si is an element that improves the strength of a steel material by solid solution strengthening. However, when the content is less than 0.01%, it is difficult to obtain the above effect. On the other hand, when the Si content exceeds 1.0%, Si-based oxides increase in the weld metal portion during welding, and the Si-based oxides serve as nuclei and easily cause weld metal cracking. Therefore, the Si content is set to 0.01 to 1.0%. Note that the Si content is preferably 0.01 to 0.5%, and more preferably 0.01 to 0.2%.

Mn: 0.01 to 2.5%
Mn is an element effective for increasing the strength of steel, but if its content is less than 0.01%, sufficient strength cannot be obtained. On the other hand, if it exceeds 2.5%, the area ratio of the hard second phase increases and the hydroformability is lowered. Therefore, the Mn content is set to 0.01 to 2.5%. Note that the Mn content is preferably 0.2 to 2.0%, and more preferably 0.5 to 1.5%.

P: 0.005-0.10%
P is an element having a solid solution strengthening action and is effective for increasing the strength. However, if the content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, since P is an element that easily segregates, if added in a large amount, it causes a decrease in workability. In particular, if its content exceeds 0.10%, the segregation becomes remarkable and the workability decreases. Becomes extremely large. Therefore, the content of P is set to 0.005 to 0.10%. The P content is preferably 0.005 to 0.05%, and more preferably 0.005 to 0.02%.

S: 0.0100% or less Since S generates sulfides that lower the hydroformability, it should be reduced as much as possible. However, in the present invention, the hydroformability is improved by adding other component elements. In consideration of the degree and further the process cost in steelmaking, the upper limit of the content was set to 0.0100%. If the cost is not taken into consideration, the upper limit of the S content is preferably 0.0050%, and more preferably 0.0020%. The S content is extremely preferably 0.0010% or less.

Al: 0.001 to 0.1%
Al is an element useful for deoxidation of steel. In order to obtain the effect, a content of at least 0.001% is necessary. However, if the content exceeds 0.1%, coarse alumina inclusions increase and the hydroformability deteriorates. Furthermore, Al-based oxides increase in the weld metal portion during welding, and the Al-based oxides serve as nuclei and are liable to cause weld metal cracking. Therefore, the Al content is set to 0.001% to 0.1%. Note that the Al content is preferably 0.001 to 0.06%, and more preferably 0.01 to 0.05%.

Ti: 0.005 to 0.20%
Ti is the most important element in the present invention. It is a precipitation strengthening element and combines with N to form crystallized TiN-based particles. When the Ti content is less than 0.005%, the amount of TiC having a precipitation strengthening action is small due to the crystallization type TiN-based particles, and the effect of increasing the strength cannot be obtained. Moreover, even if Ti is added in an amount of 0.2% or more, the above effect is saturated and the cost is increased. Therefore, the content of Ti is set to 0.005 to 0.2%. Note that the Ti content is preferably 0.01 to 0.15%, and more preferably 0.02 to 0.10%.

N: 0.0004 to 0.0100%
N forms crystallized TiN-based particles in the steelmaking and casting processes. Coarse crystallized TiN-based particles reduce the hydroformability, but fine crystallized TiN-based particles, when subjected to shearing, improve the fracture surface properties of the shearing end surface of the steel material, There is an effect of improving weldability. In order to obtain fine crystallized TiN-based particles, N in a content of 0.0004% or more is necessary. On the other hand, if the N content exceeds 0.0100%, a large amount of coarse crystallized TiN-based particles are produced, and the hydroformability is lowered. Therefore, the content of N is set to 0.0004 to 0.0100%. Note that the N content is preferably 0.0010 to 0.0050%, and more preferably 0.0010 to 0.0030%.

  The chemical composition of the steel material for hydroform according to the invention of (1) is composed of the above elements C to N, the balance being Fe and impurities.

  The chemical composition of the steel material for hydrofoam according to the invention of (2) is replaced with a part of Fe of the steel material of the invention of (1) above for the purpose of further increasing the strength by precipitation strengthening. One or more of 1% or less and V: 0.2% or less are included.

  Since both Nb and V have the effect of further increasing the strength by precipitation strengthening, Nb and V may be contained alone or in combination within the range described below. .

Nb: 0.1% or less, V: 0.2% or less Nb and V are elements that increase the strength by precipitation strengthening like Ti. In order to reliably obtain this effect, it is desirable that both Nb and V have a content of 0.01% or more. However, when Nb and V are added excessively, especially when Nb exceeds 0.1% and V exceeds 0.2%, ductility is lowered, and the alloy cost is also lower than that of Ti. The cost of raw materials will increase significantly. Therefore, when Nb and V are added, their contents are preferably 0.1% or less and 0.2% or less, respectively. In addition, when adding, it is preferable that the upper limit of the content of Nb and V is 0.07% and 0.15%, respectively, and more preferably 0.05% and 0.10%, respectively.

  The chemical composition of the steel material for hydroform according to the invention of (3) is replaced with a part of Fe of the steel material of the invention of (1) or (2) for the purpose of further increasing the strength by solid solution strengthening. , Mo: 1.0% or less, Ni: 1.0% or less and Cu: 1.0% or less selected from 1.0% or less.

  Since any element from Mo to Cu has the effect of further increasing the strength by solid solution strengthening, each element from Mo to Cu may be contained alone within the range described below. More than one species may be contained in combination.

Mo: 1.0% or less, Ni: 1.0% or less Mo and Ni are effective elements for increasing the strength by solid solution strengthening. In order to reliably obtain this effect, it is desirable that both Mo and Ni have a content of 0.05% or more. However, since both Mo and Ni have high alloy costs, the addition of a large amount is economically disadvantageous. Furthermore, the addition of a large amount of Mo and Ni deteriorates the ductility. In particular, when both elements contain more than 1.0%, the ductility is significantly reduced. Therefore, when adding Mo and Ni, the content is preferably 1.0% or less. In addition, when adding, the upper limit of the content of Mo and Ni is preferably 0.5% for both, and more preferably 0.3% for both.

Cu: 1.0% or less Cu is also an element effective for increasing the strength by solid solution strengthening. Cu also has an effect of increasing fatigue resistance. Furthermore, it has the effect | action which precipitates as (epsilon) -Cu by heat processing and raises an intensity | strength. In order to reliably obtain these effects, it is preferable that Cu is contained in an amount of 0.05% or more. On the other hand, even if the content exceeds 1.0%, the effect described above is saturated and the cost is increased. Therefore, when adding Cu, the content is preferably 1.0% or less. Note that the upper limit of the Cu content in the case of addition is preferably 0.5%, and more preferably 0.3%.

  The chemical composition of the steel material for hydrofoam according to the invention of (4) is the Fe of the steel material of any one of the inventions (1) to (3) for the purpose of improving the hardenability and further increasing the strength. In place of a part of Cr, one or more of Cr: 1.0% or less and B: 0.0005-0.003% are included.

  Since both Cr and B have the effect of improving the hardenability and further increasing the strength, Cr and B may be contained alone or in combination within the range described below. May be.

Cr: 1.0% or less Cr is an element effective for improving the hardenability and producing a desired structure, and effective for increasing the strength. In order to reliably obtain this effect, the Cr content is preferably 0.05% or more. However, if the content exceeds 1.0%, the area ratio of the hard second phase increases and the hydroform property is lowered. Therefore, when adding Cr, the content is preferably 1.0% or less. The upper limit of the Cr content when added is preferably 0.5%, and more preferably 0.3%.

B: 0.0005 to 0.003%
B is an element that improves hardenability in a small amount and is effective in increasing strength. In order to reliably obtain this effect, the B content is preferably 0.0005% or more. However, when the content exceeds 0.003%, the area ratio of the hard second phase increases, and the hydroforming property is deteriorated. Therefore, when adding B, the content is good to be 0.0005 to 0.003%. When B is added, the content of B is preferably 0.0005 to 0.002%, and more preferably 0.0005 to 0.0015%.

  The chemical composition of the steel material for hydrofoam according to the invention of (5) described above is to form an oxide as a nucleus of crystallized TiN-based particles in molten steel, finely disperse the crystallized TiN-based particles, and hydroform In order to further improve the weldability by further improving the fracture characteristics of the sheared end face and further improving the weldability, the steel material of any one of the inventions (1) to (4) above Instead, one containing at least one selected from Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, and REM (rare earth element): 0.0002 to 0.01% It is.

  Any of the above elements from Ca to REM forms an oxide that becomes the core of crystallized TiN-based particles in molten steel, finely disperses the crystallized TiN-based particles, and further enhances hydroformability. In addition, since it has the effect of improving the fracture surface properties of the sheared end face and further improving the weldability, the elements from Ca to REM may be contained alone or in the range described below. The above may be combined and contained.

  Here, as described above, REM indicates a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM indicates the total content of the above elements as already described.

Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, REM (rare earth element): 0.0002 to 0.01%
Ca, Mg, and REM all form oxides that form the core of crystallized TiN-based particles in molten steel, finely disperse crystallized TiN-based particles, further enhance hydroformability, and shear processing It has the effect of further improving the weldability by improving the fracture surface properties of the end face. In order to reliably obtain this effect, it is preferable that the content of Ca, Mg, and REM is 0.0002% or more. However, if the content of each of the above elements exceeds 0.01%, the oxides of these elements increase in the weld metal part during welding, and the oxides serve as nuclei and are liable to cause weld metal cracking. Become. Therefore, when adding Ca, Mg, and REM, the contents are all preferably 0.0002 to 0.01%.

  In addition, Ca, Mg and REM, when included in the above-mentioned appropriate range, all have the property of changing the properties of MnS, and form inclusions that do not easily expand during hot rolling to prevent deterioration of workability. Has the effect of

  In addition, when adding, content of Ca, Mg, and REM is preferably set to 0.0002 to 0.0050% for all, and more preferably 0.0002 to 0.0030% for all.

  In addition, Zr can be added in order to prevent the above-mentioned deterioration of workability by MnS. In order to reliably obtain the effect, it is preferable that Zr has a content of 0.0002% or more. The above effect of Zr is also saturated at a content of 0.01%.

(B) Steel structure In addition to the chemical composition of steel described in the above section (A), the present invention actively refines crystallized TiN-based particles and optimizes the amount thereof. The area ratio and its size are also optimized to improve the hydroformability of the Ti precipitation strengthened steel and improve the weldability.

That is, the steel material for hydrofoam according to the inventions of (1) to (5) includes 50 to 50,000 crystallization type TiN-based particles having a particle size of 0.3 μm or more per 1 mm ² in cross-sectional area, and The average particle size of the crystallized TiN-based particles must be 7 μm or less, and the area ratio of ferrite to the structure should be 50% or more and the average particle size should be 3 to 30 μm.

  Here, as already described, “crystallized TiN-based particles” as used in the present invention are those that are produced with an Al-based oxide or a Ca-based oxide in the case where Ca is contained in steel as a nucleus. The “particle size” is about 0.1 to 20 μm. “Ferrite” includes so-called “bainitic ferrite”.

  “Particle size” refers to a value defined by “1/2 of the sum of the minor axis and major axis” of crystallized TiN-based particles and ferrite, which are individual particles. It refers to an arithmetic average of particle sizes of individual particles obtained by observation.

  The crystallized TiN particles and ferrite can be observed using an optical microscope, a scanning electron microscope, and a transmission electron microscope having an acceleration voltage of 100 to 200 kV. Image analysis is performed to measure the minor axis and the major axis, and the particle size of each crystallized TiN-based particle or ferrite can be obtained from 1/2 of the sum.

As in the case of the “average particle size”, the area ratio of ferrite in the structure indicates the area ratio of ferrite with respect to the area for 100 field observations, and the number of crystallized TiN-based particles per 1 mm ^{2 in} cross-sectional area. However, as already mentioned, it indicates the number obtained by converting the number for the area for 100 visual field observations per 1 mm ² of the cross-sectional area.

  Among the “crystallized TiN-based particles” according to the present invention defined above, those having a particle size of less than 0.3 μm, that is, particles having a particle size of less than 0.1 μm to less than 0.3 μm The degree of stress concentration in the steel is low, cracks do not occur from the interface with the ferrite, and no fine voids are formed. Therefore, the fracture surface property of the shearing end surface of the steel material is not improved, and therefore the weldability cannot be improved. . For this reason, the particle size of the crystallized TiN-based particles needs to be 0.3 μm or more.

If the number of crystallized TiN-based particles is 0.3 μm or more and the number of cross-sectional areas per mm ² is less than 50, the number of fine voids generated from the interface with ferrite during shearing Therefore, the fracture surface property of the shearing end face of the steel material is not improved, and the weldability is not improved. On the other hand, if the number per 1 mm ² cross-sectional area exceeds 50,000, the number of fine voids generated from the interface with the ferrite when molding by the hydroform method is too large, and the steel material breaks early because it is connected. Hydroformability is not improved.

Even when 50 to 50,000 crystallized TiN particles having a particle size of 0.3 μm or more are contained per 1 mm ² in cross-sectional area, if the average particle size of the crystallized TiN particles exceeds 7 μm, Since the void generated from the interface with the ferrite becomes coarse at the time of molding by the foam method, it breaks early in the initial stage of plastic deformation by hydroforming, and the hydroformability is lowered.

  Even in the case where the stipulation regarding the crystallized TiN-based particles is satisfied, when the area ratio of ferrite in the structure is less than 50%, the ratio of the hard second phase, that is, martensite, pearlite, bainite, and Since the ratio of cementite increases and the steel material is prematurely fractured at the initial stage of molding by the hydroforming method, cracking occurs at the interface with the ferrite, so that good hydroforming properties cannot be ensured.

  Further, even if the area ratio of ferrite in the structure is 50% or more, if the average particle diameter of ferrite is less than 3 μm, the yield ratio becomes too high and the hydroformability deteriorates. Furthermore, when the average particle diameter of ferrite exceeds 30 μm, the structure anisotropy in the steel material becomes too strong, and the hydroformability deteriorates.

Therefore, in the steel material for hydrofoam according to the inventions of the above (1) to (5), “including 50 to 50000 crystallization type TiN-based particles having a particle size of 0.3 μm or more per 1 mm ² in cross-sectional area, and The average particle size of the crystallized TiN-based particles is 7 μm or less, and the area ratio of ferrite to the structure is 50% or more and the average particle size is 3 to 30 μm ”.

  Note that the lower limit of the average particle diameter of the crystallized TiN-based particles may be 0.3 μm, similarly to the lower limit of the individual particle diameters of the crystallized TiN-based particles. The ferrite only needs to have an average particle size of 3 to 30 μm, and the particle size of each ferrite need not be specified. However, in order to prevent local non-uniform deformation at the time of hydroforming, the particle diameter of each ferrite is preferably about 1 to 50 μm. In the present invention, the upper limit of the area ratio of ferrite to the structure is about 99%.

  When the average particle size of the crystallized TiN particles having the particle size of 0.3 μm or more is 0.3 to 3.0 μm, the hydroformability is further improved by refining the crystallized TiN particles. In addition, the weldability is further improved because the improvement of the fracture surface properties of the sheared end face of the steel material is promoted.

  Therefore, in the steel material for hydrofoam according to the invention of (6), the average particle size of the crystallized TiN-based particles having a particle size of 0.3 μm or more is set to 0.3 to 3.0 μm.

  The central portion of the steel ingot has a slower cooling rate than the surface portion, and in addition, the concentration of the molten steel component (so-called “center segregation”) tends to occur. For this reason, as the steel material is closer to the center of the plate thickness, there are more coarse crystallized TiN-based particles and coarse hard second phase. In the vicinity of the center of the plate thickness, especially in the plate thickness center region in the range from the plate thickness center to the plate surface up to 15% of the plate thickness, the grain size distribution control of the crystallization type TiN particles and the hard By controlling the area ratio of the second phase, it is possible to ensure better hydroformability and weldability.

  That is, in the above region, when the average particle interval between the crystallized TiN-based particles having a particle size of 2 μm or more and the hard second phase having a particle size of 2 μm or more is 10 to 50 μm, the hydroformability and welding The properties become even better.

  Therefore, in the steel material for hydrofoam according to the invention of (7), a crystallization having a grain size of 2 μm or more in the plate thickness center region in the range of 15% of the plate thickness from the plate thickness center toward the plate surface. The average particle spacing between the type TiN-based particles and the hard second phase having a particle size of 2 μm or more was defined as 10 to 50 μm.

  As described above, the “hard second phase” refers to pearlite, bainite, martensite, and cementite. Also, the shortest interval between each crystallized TiN-based particle and each hard second phase is defined as the individual particle interval, and 100 fields of view are observed using an optical microscope, a scanning electron microscope, a transmission electron microscope, or the like. As described above, the arithmetic average of the individual particle intervals obtained in this way is called “average particle interval between crystallized TiN-based particles and the hard second phase”.

  The balance structure other than ferrite is at least one selected from martensite, pearlite, bainite, and austenite, and the hardness in the ferrite grains with respect to the ferrite area ratio is optimized, specifically, ferrite with Vickers hardness By making the intragranular hardness and the area ratio of the ferrite occupying the structure satisfy the above formula (1), that is, “A ≦ (B + 200) /1.5”, the ferrite and the hard second phase Before the crack occurs at the interface, the soft ferrite takes charge of local elongation and suppresses the crack at the interface, so that the hydroformability can be made extremely good.

  Therefore, in the steel material for hydroform according to the invention of (8), the intergranular hardness of ferrite in Vickers hardness and the area ratio occupied by ferrite in the structure satisfy the above formula (1), and the remaining structure other than ferrite is It was decided to be at least one selected from martensite, pearlite, bainite and austenite.

  In addition, said A represents the intragranular hardness of the ferrite in a Vickers hardness, and measured the hardness in a ferrite grain 50 places, for example by making test force 0.009807-0.09807N using a micro Vickers hardness meter. And B represents the area ratio (%) of the ferrite occupied in the structure.

  As already described, when a steel pipe is manufactured using a steel material having a good hydroforming property, a steel pipe having a good hydroforming property can be obtained. Therefore, the electroformed pipe for hydrofoam according to the invention of (9) is defined as being made of the steel material for hydrofoam according to any of the inventions (1) to (8) having good hydroform properties. did.

(C) Manufacturing method The structure of the steel material having the chemical composition described in the item (A) is described in the item (B) only by changing the conditions of hot rolling or hot rolling and the subsequent cold rolling. In particular, 50 to 50,000 crystallized TiN particles having a particle size of 0.3 μm or more should be included per 1 mm ² in cross-sectional area, and the average particle size of the crystallized TiN particles should be 7 μm or less. I can't. However, for example, by controlling the cooling rate in the temperature range of 1300 ° C. from the liquidus temperature of the molten steel that is the material of the steel material, it is possible to obtain the above structure. .

  That is, for example, when casting molten steel into a steel ingot, the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature of the molten steel to 1300 ° C is 0.4 to 7 ° C / By including in the manufacturing process a process that is at least 2 seconds, the structure of the steel material having the chemical composition described in the item (A) is described in the item (B). Can be secured.

  Among the crystallized TiN-based particles, most of the crystallized TiN-based particles observed between the dendrite primary arm or secondary arm trees are in the region above the liquidus temperature of the molten steel, or It is generated near the solid-liquid coexistence temperature of molten steel, and adversely affects hydroformability and weldability.

  However, when the molten steel is cast into a steel ingot, the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature of the molten steel to 1300 ° C. is 0.4 to 7 ° C./second or more. By doing so, crystallization type TiN-based particles having an appropriate size and amount are dispersed in the steel, and the hydroformability and weldability are improved.

In addition, when the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the above temperature range is less than 0.4 ° C./second, the crystallized TiN-based particles become coarse and the cross-sectional area is 1 mm ^2. The number of hits may be less than 50 or the average particle size may exceed 7 μm. On the other hand, when the average cooling rate exceeds 7 ° C./second, the crystallization type TiN-based particles become small, but are too finely dispersed and the number per 1 mm ² cross-sectional area exceeds 50000. There is.

  Therefore, in the invention of (10), when casting molten steel to form a steel ingot, the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature of the molten steel to 1300 ° C. is set to 0. The manufacturing process includes a step of 4 to 7 ° C./second or more.

  As already mentioned, the “average cooling rate of the cross section perpendicular to the casting direction of the steel ingot” means that the steel ingot including the case where a solidified shell is formed in the mold or continuous casting machine and the inside is in a molten state. The average value of the cooling rate in the whole area | region of the center part from the surface part in the cross section perpendicular | vertical to the casting direction of a steel ingot in the case of calling.

  If the average cooling rate in the temperature range from the liquidus temperature of the molten steel to 1300 ° C. is 2 to 7 ° C./second, the average particle size of the crystallized TiN-based particles having the above-mentioned particle size of 0.3 μm or more. Can be easily adjusted to 0.3 to 3.0 μm, and it is possible to ensure better hydroformability and weldability.

  Therefore, in the invention of (11), the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot is defined as 2 to 7 ° C./second.

  In order to set the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature of the molten steel to 1300 ° C. to 0.4 ° C./second or more, for example, a secondary spray in a continuous casting machine For example, the cooling zone may be subjected to forced cooling by increasing the pressure or the amount of water, or by reducing the mold thickness or reducing the slab thickness by reducing the unsolidified layer of the slab in the continuous casting machine. Moreover, the said average cooling rate can be easily suppressed to 7 degrees C / sec by adjusting the water quantity of the secondary spray in a continuous casting machine.

  A steel ingot obtained by adjusting the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot in the temperature range from the liquidus temperature of the molten steel to 1300 ° C. when casting the molten steel as described above. Is hot-rolled at a finishing temperature of “Ar3 point−100 ° C.” or more and 1050 ° C. or less, then cooled to a temperature range of 730 ° C. or less at an average cooling rate of 10 ° C./second or more, and then wound up. preferable.

  A finishing temperature exceeding 1050 ° C. is not realistic because it is necessary to increase the plate passing speed in terms of equipment, and enormous costs are required for equipment investment. On the other hand, by ensuring a finishing temperature of “Ar 3 point−100 ° C.” or more, the generation of non-uniformly processed ferrite is reduced.

  On the other hand, if the average cooling rate after finish rolling is less than 10 ° C./second, or if the coiling temperature exceeds 730 ° C., coarse ferrite may be generated and the hydroform property may be lowered.

  Therefore, in the invention of (12), the steel ingot obtained by adjusting the average cooling rate of the cross section perpendicular to the casting direction of the steel ingot described above is finished at a finishing temperature of “Ar3 point−100 ° C.” or more and 1050 ° C. or less. Then, the steel sheet was hot-rolled, cooled to a temperature range of 730 ° C. or lower at an average cooling rate of 10 ° C./second or higher, and then wound up.

  In addition, after finishing hot rolling in the said temperature range, it cools to the temperature range of 730-600 degreeC with an average cooling rate of 10 degree-C / sec or more, and then air-cools for 2-15 seconds, and also 15 degreeC / Winding may be performed after cooling to below 600 ° C. at an average cooling rate of at least 2 seconds. This is because the ratio of ferrite and bainite, martensite, austenite, etc. can be adjusted by this treatment.

  Therefore, in the invention of (13), a steel ingot obtained by adjusting the average cooling rate is hot-rolled at a finishing temperature of “Ar 3 points-100 ° C.” or higher and 1050 ° C. or lower, and then 10 ° C./second or higher. Then, it was cooled to a temperature range of 730 to 600 ° C. with an average cooling rate of 2 to 15 seconds, then air-cooled for 2 to 15 seconds, and further cooled to less than 600 ° C. with an average cooling rate of 15 ° C./second or more and then wound. .

  As already described, the temperature of the steel material that is hot-rolled refers to the temperature at the surface of the steel material, and the average cooling rate of the steel material refers to the temperature difference between the surface of the steel material before and after cooling divided by the cooling time. Air cooling refers to air cooling and forced air cooling.

  When casting the molten steel into a steel ingot, so-called "unsolidified layer reduction" can increase the cooling rate in the vicinity of the steel ingot center by reducing the thickness of the steel ingot. It is also possible to discharge the molten steel. Further, when the “electromagnetic stirring” treatment is performed, the concentrated molten steel can be discharged.

  For this reason, when casting molten steel into a steel ingot, it is preferable to apply “unsolidified layer pressure” or electromagnetic stirring, and in particular, the unsolidified layer of the steel ingot has become 30% or less of the thickness of the steel ingot. It is preferable to apply pressure reduction or electromagnetic stirring to the site. This treatment prevents the coarsening of crystallized TiN-based particles and the increase of the hard second phase in the plate thickness center region in the range of 15% of the plate thickness from the plate thickness center toward the plate surface. This is because the average particle interval between the crystallized TiN-based particles having a particle size of 2 μm or more and the hard second phase having a particle size of 2 μm or more can be relatively easily set to 10 to 50 μm.

  Therefore, in the invention of (14), when molten steel is cast into a steel ingot, the unsolidified layer of the steel ingot is subjected to reduction or electromagnetic stirring on the portion where the thickness of the steel ingot is 30% or less. .

  In the above reduction, it is preferable to add a reduction amount of 5% or more of the steel ingot thickness to the unsolidified layer, and it is more preferable to add a reduction amount corresponding to the thickness of the unsolidified layer. In addition, as for the ratio with respect to the thickness of the steel ingot of the unsolidified layer of the steel ingot in the case of performing reduction or electromagnetic stirring, it is desirable that it is 1% or more.

  As already described, when a steel pipe is manufactured using a steel material having a good hydroforming property, a steel pipe having a good hydroforming property can be obtained. And the steel material for hydroforms manufactured by the method according to any one of the inventions (10) to (14) has good hydroformability and weldability. Therefore, after forming the steel material for hydrofoam produced by the method according to any one of the inventions (10) to (14) into a tubular shape and then welding the butt portion to produce an electric sewing tube, hydroform It is possible to obtain an electric sewing tube excellent in properties.

  For this reason, the manufacturing method of the hydroformed electric sewn tube according to the invention of (15) is obtained by tubularly forming the steel material for hydroform obtained by the manufacturing method according to any of the inventions from (10) to (14). After molding, the butt portion was defined as being welded.

  In addition, it is not necessary to prescribe | regulate the conditions at the time of welding a butt | matching part, What is necessary is just to be the conditions normally performed by manufacture of an electric resistance welded tube.

  In addition, the steel material for hydrofoam which concerns on invention of (1)-(5) is easily obtained by the manufacturing method which concerns on invention of (10)-(14), for example.

  The steel material for hydrofoam according to the invention of (6) is easily obtained by, for example, the production method according to the inventions of (11) to (14).

  The steel material for hydrofoam according to the invention of (7) is easily obtained by the production method according to the invention of (14), for example.

  The steel material for hydrofoam according to the invention of (8) is easily obtained by the production method according to the inventions of (12) and (13), for example.

  The electroformed tube for hydrofoam according to the invention of (9) can be easily obtained by the manufacturing method according to the invention of (15), for example.

  When the steel material for hydroforming is obtained by cold rolling, the hot rolled steel material obtained as described above may be cold-rolled by a usual method. In addition, it is desirable that the rolling reduction during cold rolling is 40% or more, and annealing treatment is performed after cold rolling. This annealing process may be performed by a normal method. That is, the holding may be performed for 10 seconds or more at a temperature equal to or higher than the Ac1 point, and then cooled by a normal method.

  Note that the reduction ratio in the above “% unit” means {“thickness of the material to be rolled before cold rolling−thickness of the material to be rolled after cold rolling” / “of the material to be rolled before cold rolling”. Thickness "} means a value represented by x100.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Various steels having the chemical composition shown in Table 1 are continuously cast under the conditions shown in Table 2 and Table 3 into slabs having a width of 1200 mm and a thickness of 70 to 250 mm, and each slab is adjusted to 1100 to 1300 ° C. After heating, it was hot-rolled under the conditions shown in Tables 2 and 3 to finish a hot-rolled steel sheet having a thickness of 2.0 mm.

  The slab was cast by a test continuous casting machine having a mold width of 1200 mm and a mold thickness of 100 to 250 mm, and for each steel type, average cooling of the cross section perpendicular to the slab casting direction from the liquidus temperature to 1300 ° C. The morphology control of the crystallized TiN-based particles was performed at different speeds.

  The change of the average cooling rate of the cross section perpendicular to the casting direction of the slab was mainly performed by changing the amount of secondary cooling water and changing the slab thickness by reducing the unsolidified portion of the slab in the test continuous casting machine. The average cooling rate of the cross section perpendicular to the casting direction of the slab at 1300 ° C. from the liquidus temperature was calculated by measuring the dendrite secondary arm interval at a pitch of 5 mm from the slab surface to the center.

  In the hot rolling, the finishing temperature was set to 1010 to 780 ° C., and after finishing rolling, the steel was cooled at an average cooling rate of 50 to 90 ° C./second and wound at 120 to 750 ° C. For some, intermediate air cooling during cooling was also performed.

  For some steel types, hot-rolled steel sheets having a thickness of 2.0 mm were produced by hot rolling under the conditions shown in Tables 2 and 3, and the hot-rolled steel sheets were cooled at a reduction rate of 10 to 50%. Cold rolling was also performed by hot rolling and then annealing. The details of the cold rolling conditions are as shown in Table 8 below.

  In addition, each hot-rolled steel sheet having a thickness of 2.0 mm and each cold-rolled steel sheet having a thickness of 1.0 to 1.8 mm obtained as described above were sheared into a slit shape, and a roughness meter was used. The fracture surface roughness (average roughness Ra) of the cut end face was measured.

  Next, after each steel plate sheared into the slit shape was formed into a tubular shape by a normal method, the butt portion was resistance-welded, and an electric sewing tube having an inner diameter of 60.5 mm (hereinafter referred to as a 60.5φ electric sewing tube). Also manufactured).

For each hot rolled steel sheet having a thickness of 2.0 mm, the cross-sectional structure of the steel sheet thickness was observed using an optical microscope and a scanning electron microscope. Further, the individual particle diameters of the observed crystallized TiN-based particles were measured, and for the crystallized TiN-based particles having a particle diameter of 0.3 μm or more, the number per 1 mm ² cross-sectional area and the average particle diameter were determined. Asked.

  Moreover, while specifying the phase (structure | tissue) in a steel plate structure | tissue, the area ratio of the ferrite which occupies for a structure | tissue was calculated | required, and also the particle size of each ferrite was measured and the average particle size was calculated | required.

  In the plate thickness center region in the range of up to 15% of the plate thickness from the plate thickness center to the plate surface, crystallized TiN particles having a particle size of 2 μm or more and a hard second phase having a particle size of 2 μm or more. (In other words, the individual particle interval was determined from the shortest distance from pearlite, bainite, martensite, and cementite), and the above-mentioned individual particle interval obtained by observing 100 fields of view was arithmetically averaged to obtain “crystallized TiN-based particles. Average particle spacing between the second phase and the hard second phase ”.

  Furthermore, the test force was set to 0.01961N, and the value obtained by measuring the hardness in 50 ferrite grains was arithmetically averaged to obtain the ferrite intragranular hardness.

  Similarly, for each cold-rolled steel sheet having a thickness of 1.0 to 1.8 mm, an optical microscope and a scanning electron microscope are used to observe the cross-sectional structure of the steel sheet thickness, and the phase (structure) in the steel sheet structure is specified. did.

  The mechanical properties of each hot-rolled steel sheet having a thickness of 2.0 mm and each cold-rolled steel sheet having a thickness of 1.0 to 1.8 mm were investigated by the following method.

  That is, a No. 5 tensile test piece described in JIS Z 2201 was cut out from a direction perpendicular to the rolling direction of the steel sheet, and a tensile test was performed at room temperature to measure tensile strength (TS) and elongation (El). Further, from a direction perpendicular to the rolling direction of the steel sheet, a tensile test piece with R having a length of 180 mm, a width of 40 mm, and a radius R = 10 mm on both sides of the central portion in the length direction was collected. The gauge distance (GL) was set to 2 mm, that is, a marking line with an interval of 2 mm was drawn at the center, a tensile test was performed at room temperature, and elongation (local elongation) was obtained from the following equation (2).

  Elongation of tensile test piece with R (%) = {“Scribing line interval after test (mm) −Scribing line interval before test (mm)” / “Scribing line interval before test (mm)”} × 100 ... (2).

  About each said 60.5 (phi) ERW pipe, the mechanical property, hydroform property, and weldability were investigated with the following method.

  That is, the No. 12B tensile test piece specified in JIS Z 2201 was sampled from the pipe axis direction of each 60.5φ ERW pipe, and the tensile test was performed at room temperature to measure the tensile strength (TS) and elongation (El). did.

  Furthermore, each ERW pipe was cut open and flattened, and processing with a radius R = 10 mm was performed on both sides of the central portion in the length direction shown in FIG. 1 from the direction corresponding to the pipe circumferential direction of the original ERW pipe. Tensile specimens with R were collected and subjected to a tensile test at room temperature with a gauge distance (GL) of 2 mm, and elongation (local elongation) was determined from the above equation (2).

  Moreover, using the upper and lower molds as shown in FIG. 2, a hydroform molding test was performed on each of the above 60.5φ ERW pipes 6 to obtain a tube expansion rate at the time of hydroforming molding.

  That is, the upper mold 1, the lower mold 2, and the seal portion 3 that form the mold space 4 hold the 60.5φ electric sewing tube 6, and hydraulic pressure (hydraulic pressure) is applied to the electric sewing tube through the injection portion 5. The electric sewing tube was expanded into the mold space 4. Next, the circumference of the ERW pipe at the bulging deformation cracked portion was measured, and the pipe expansion rate at the time of hydroforming was calculated from the following formula (3).

  Tube expansion ratio (%) = {"peripheral length after deformation-element tube periphery" / "element tube periphery"} × 100 (3).

  Weldability was evaluated by performing a bending test on each of the 60.5φ electric resistance welded pipes until the bending radius, that is, the inner radius was 121 mm, until 90 °, and the crack occurrence rate at the welded portion at that time.

  Tables 4 to 6 show the structure and mechanical properties of a hot-rolled steel sheet having a thickness of 2.0 mm. Table 7 shows the fracture surface roughness (Ra) of the cut end face of the hot-rolled steel sheet having a thickness of 2.0 mm. Table 8 shows the mechanical properties, pipe expansion rate, and crack occurrence rate in the welded portion of the ERW pipe made of a hot-rolled steel sheet having a thickness of 2.0 mm. Table 8 shows the coldness of the hot-rolled steel sheet having a thickness of 2.0 mm. Rolling conditions, mechanical properties of cold-rolled steel sheet, structure, fracture surface roughness (Ra) of cut end surface, mechanical properties of ERW pipe made of cold-rolled steel sheet, pipe expansion rate, and crack occurrence rate in welds Indicates.

In the case of test numbers 1 to 23 relating to hot rolled steel sheets and ERW pipes made of hot rolled steel sheets, the number of crystallized TiN-based particles having a particle size of 0.3 μm or more per 1 mm ² cross-sectional area is 50 to 47000. The average particle diameter of the crystallized TiN-based particles satisfying the provision of the present invention of 50 to 50,000 particles and having a particle diameter of 0.3 μm or more is 0.5 to 6.8 μm and 7 μm as defined in the present invention. It is as follows. Furthermore, in the case of each of the above test numbers, the area ratio of the ferrite occupying the structure satisfies 62% to 98% of the present invention, and the average particle diameter of the ferrite is 3 to 28 μm. In the range of 3 to 30 μm.

  For this reason, the elongation of the tensile test piece with R of the hot-rolled steel sheet and the elongation of the tensile test specimen with R of the electric resistance welded tube made of the hot-rolled steel sheet are both as good as 90% or more. The expansion rate was 17% or more, and the hydroforming property was excellent. Furthermore, the fracture surface roughness (Ra) of the cut end surface when the hot-rolled steel sheet is sheared into a slit shape is also excellent at 4.5 μm or less. Cracks did not occur in the weld zone and the weldability was excellent.

  Among the above test numbers, a test in which the intragranular hardness of ferrite in Vickers hardness and the area ratio of ferrite in the structure satisfy the above formula (1), that is, “A ≦ (B + 200) /1.5” In the case of Nos. 1 to 8, Test No. 10, Test Nos. 12 to 16, Test Nos. 18 to 20, Test No. 22 and Test No. 23, the expansion ratio of the electric resistance welded tube is 20% or more and the hydroform property is even better. there were. Among the above test numbers, test number 2, test number 3, test number 5, test in which the average particle size of crystallized TiN-based particles having a particle size of 0.3 μm or more is 0.5 to 3.0 μm. In the case of No. 7, Test No. 8, Test No. 10, Test No. 16, Test No. 18 and Test No. 20, or the plate thickness center region in the range from the plate thickness center to the plate surface up to 15% of the plate thickness. In the case of test number 4 in which the average particle interval between crystallized TiN-based particles having a particle size of 2 μm or more and pearlite which is a hard second phase having a particle size of 2 μm or more is 30 μm, The average particle diameters of crystallized TiN-based particles of 0.3 μm or more are 0.6 μm and 2.3 μm, and the center of the plate thickness is in the range of 15% of the plate thickness from the plate thickness center to the plate surface. In the region, crystallized TiN-based particles having a particle diameter of 2 μm or more and grains In the case of Test No. 13 and Test No. 14 in which the average particle distance between the bainite or pearlite, which is a hard second phase of 2 μm or more, is 48 μm and 24 μm, the expansion ratio of the ERW tube is 30% or more and the hydroform The properties were extremely good.

On the other hand, in the case of test numbers 24 to 28, the average particle size of crystallization type TiN-based particles having a particle size of 0.3 μm or more is 7.1 to 7.6 μm, which exceeds 7 μm. The elongation of the piece is 46 to 74%, and the elongation of the R-stretched tensile test piece made of hot-rolled steel sheet is 44 to 70%, which is low. Therefore, the expansion ratio of the ERW tube is 9 to 9%. It was inferior to hydroform property at 14%. In the case of test number 25 and test number 26 among the above test numbers, the number of crystallized TiN-based particles having a particle size of 0.3 μm or more per 1 mm ² cross-sectional area is small at 35 and 48, When the hot-rolled steel sheet is sheared into slits, the fracture surface roughness (Ra) of the cut end surface is 5.5 μm and 5.3 μm, respectively. The crack occurrence rate of the welded part increased to 40% and 30%, and the weldability was inferior.

In the case of test number 29 and test number 30, the number of crystallized TiN-based particles having a particle diameter of 0.3 μm or more per 1 mm ² in cross-sectional area is 53,000 and 51,000, respectively. Elongation of tensile test pieces with R is 62% and 65%, and the elongation of tensile test pieces with R of hot-rolled steel sheets is 60% and 63%, both of which are low. The tube expansion ratio of 12 and 13% was inferior in hydroformability.

  In the case of Test No. 31 and Test No. 34, the average grain size of ferrite exceeds 31 μm and 32 μm, which is 30 μm, which is the upper limit prescribed in the present invention. Moreover, the elongation of the tensile test piece with R of the electric resistance welded tube made from a hot-rolled steel sheet was 68% and 60%, both of which were low. Therefore, the expansion ratios of both electric resistance welded pipes were 14% and 13%, and the hydroform property was inferior.

  In the case of test number 32, the average grain size of ferrite is 2 μm, which is less than 3 μm, which is the lower limit of the present invention. Therefore, the elongation of the tensile test piece with R of the hot-rolled steel sheet is 60%, and the hot-rolled steel sheet is used as the material. The elongation of the tensile test piece with R of the electric resistance welded tube was as low as 58%. Therefore, the expansion ratio of the ERW pipe was 12%, which was inferior to the hydroform property.

  In the case of test number 33, the area ratio of ferrite occupying the structure is 48%, which is less than 50% which is the lower limit of the present invention. Therefore, the elongation of the tensile test piece with R of the hot-rolled steel sheet is 45%. The elongation of the tensile test piece with R of the electric resistance welded tube as a raw material was as low as 43%. Therefore, the expansion ratio of the ERW pipe was as small as 9%, which was inferior in terms of hydroformability.

In the case of test numbers 35 to 38, when the number of crystallized TiN-based particles having a particle size of 0.3 μm or more per 1 mm ² in cross-sectional area is as small as 40 to 45, the hot-rolled steel sheet was sheared into a slit shape The fracture surface roughness (Ra) of the cut end face was as rough as 5.0 to 5.3 μm. For this reason, when the electric resistance welded tube made of hot-rolled steel sheet is subjected to a bending test, the crack occurrence rate of the welded portion is 10 to 20%, and the weldability is inferior.

  In the case of test numbers C1 to C11 related to cold-rolled steel sheets and ERW pipes made of cold-rolled steel sheets, hot-rolled steel sheets that satisfy the conditions defined in the present invention, such as hot-rolled steel sheet manufacturing symbols H1, are used. Therefore, the elongation of the tensile test piece with R of the cold-rolled steel sheet is 100% or more and the elongation of the tensile test piece with R of the electric-welded pipe made of the cold-rolled steel sheet is 100% or more. The tube expansion rate was 20% or more, and the hydroformability was excellent. Furthermore, the fracture surface roughness (Ra) of the cut end surface when the cold-rolled steel sheet is sheared into a slit shape is also excellent at 4.5 μm or less, and even when an electric resistance welded tube made of a cold-rolled steel sheet is subjected to a bending test Cracks did not occur in the weld zone and the weldability was excellent.

  On the other hand, in the case of test numbers C12 to C16, since the hot-rolled steel sheet deviated from the conditions specified in the present invention, such as the hot-rolled steel sheet manufacturing symbol H24, is used as the material, the tensile test with R of the cold-rolled steel sheet The elongation of the piece is 52 to 75%, and the elongation of the R-stretched test piece made of cold-rolled steel sheet is 52 to 75%, which is low. Therefore, the expansion ratio of the ERW tube is 9 to 9%. Hydroformability was inferior at 13%.

In addition, Test No. C13 and Test No. C14 indicate that a hot-rolled steel sheet in which the number of crystallized TiN-based particles having a particle size of 0.3 μm or more per cross-sectional area of 1 mm ² is less than 50 is a cold-rolled steel sheet material. Therefore, the fracture surface roughness (Ra) of the cut end face when the cold-rolled steel sheet was sheared into a slit shape was as rough as 5.2 to 5.4 μm, and an electric resistance welded tube made of the cold-rolled steel sheet was subjected to a bending test. In this case, the crack occurrence rate of the welded portion was 20 to 30%, which was inferior in weldability.

  Since the steel material for hydroform of the present invention has a tensile strength of 390 MPa or more and is excellent in hydroformability and weldability, it can be used as a material for structural members and suspension members of automobiles. Moreover, this steel material for hydroforming can be utilized as a material for the electroformed tube for hydroforming. The steel material for hydrofoam and the electric sewing tube for hydrofoam of the present invention can be manufactured relatively easily by the method of the present invention.

It is a figure explaining "a tensile test piece with R". It is a figure explaining the hydroform shaping | molding test of an electric sewing pipe.

Explanation of symbols

1: Upper mold,
2: Lower mold 2,
3: Seal part
4: Mold space 5: Injection part,
6: 60.5φ ERW pipe.

Claims (15)

In mass%, C: 0.02-0.35%, Si: 0.01-1.0%, Mn: 0.01-2.5%, P: 0.005-0.10%, S: A chemistry containing 0.0100% or less, Al: 0.001-0.1%, Ti: 0.005-0.20% and N: 0.0004-0.0100%, with the balance being Fe and impurities The composition includes 50 to 50000 crystallized TiN-based particles having a particle size of 0.3 μm or more per 1 mm ² in cross-sectional area, and the average particle size of the crystallized TiN-based particles is 7 μm or less. A steel material for hydrofoam characterized by having an area ratio of 50% or more in the structure and an average particle size of 3 to 30 μm.   The steel material for hydroforms according to claim 1, which contains at least one of Nb: 0.1% or less and V: 0.2% or less in mass% instead of a part of Fe.   It replaces with a part of Fe and contains 1 or more types selected from Mo: 1.0% or less, Ni: 1.0% or less, and Cu: 1.0% or less by mass%. Steel material for hydroforming as described in 1.   The hydro described in any one of claims 1 to 3, which contains one or more of Cr: 1.0% or less and B: 0.0005-0.003% in mass% instead of a part of Fe. Steel for foam.   Instead of a part of Fe, by mass%, Ca: 0.0002 to 0.01%, Mg: 0.0002 to 0.01%, and REM (rare element): 0.0002 to 0.01% The steel material for hydrofoam according to any one of claims 1 to 4, comprising one or more selected from the group consisting of:   The steel material for hydrofoam according to any one of claims 1 to 5, wherein the average particle size of crystallized TiN-based particles having a particle size of 0.3 µm or more is 0.3 to 3.0 µm.   In the plate thickness center region in the range of up to 15% of the plate thickness from the plate thickness center to the plate surface, crystallized TiN particles having a particle size of 2 μm or more and a hard second phase having a particle size of 2 μm or more. The steel material for hydrofoam according to any one of claims 1 to 6, wherein an average particle interval is 10 to 50 µm. The intergranular hardness of ferrite in Vickers hardness and the area ratio occupied by ferrite satisfy the following formula (1), and the remaining structure other than ferrite is one or more selected from martensite, pearlite, bainite and austenite. The steel material for hydrofoam according to any one of claims 1 to 7, wherein the steel material is a hydroforming steel material.
A ≦ (B + 200) /1.5 (1)
Here, A in the formula (1) is the intragranular hardness of the ferrite in terms of Vickers hardness, and B is the area ratio (%) that the ferrite occupies in the structure.   A hydroforming electric sewing pipe made of the steel material for hydrofoam according to any one of claims 1 to 8.   It is a manufacturing method of the steel material for hydroforming, Comprising: When the molten steel which has the chemical composition in any one of Claim 1-5 is continuously cast to make a steel ingot, the temperature of 1300 degreeC from the liquidus temperature of a molten steel The manufacturing method of the steel material for hydrofoams which includes the process which makes the average cooling rate of the cross section perpendicular | vertical to the casting direction of the said steel ingot in a range to 0.4-7 degreeC / second in a manufacturing process.   The manufacturing method of the steel material for hydrofoams of Claim 10 whose average cooling rate of the cross section perpendicular | vertical to the casting direction of a steel ingot is 2-7 degrees C / sec.   A steel ingot cooled at an average cooling rate of a cross section perpendicular to the casting direction of the steel ingot according to claim 10 or 11, is hot-rolled at a finishing temperature of "Ar3 point-100 ° C" or higher and 1050 ° C or lower, and then It cools to the temperature of 730 degrees C or less with an average cooling rate of 10 degrees C / sec or more, and winds up after that, The manufacturing method of the steel materials for hydroforms of Claim 10 or 11 characterized by the above-mentioned.   After hot rolling the steel ingot cooled at an average cooling rate of a cross section perpendicular to the casting direction of the steel ingot according to claim 10 or 11, at a finishing temperature of "Ar3 point-100 ° C" or higher and 1050 ° C or lower, Cool to a temperature range of 730 to 600 ° C. at an average cooling rate of 10 ° C./second or more, then air cool for 2 to 15 seconds, and then further cool to a temperature of less than 600 ° C. at an average cooling rate of 15 ° C./second or more. The method for producing a steel material for hydrofoam according to claim 10 or 11, wherein the steel material is wound after being wound.   The hydrostatic material according to any one of claims 10 to 13, wherein when the molten steel is cast into a steel ingot, the unsolidified layer of the steel ingot is subjected to reduction or electromagnetic stirring on a portion where the thickness of the steel ingot is 30% or less. Manufacturing method of steel for foam. It is a manufacturing method of the electric sewing pipe for hydrofoams, Comprising: After forming the steel material for hydrofoams obtained by the manufacturing method in any one of Claim 10-14 in the shape of a tube, for hydroforms where a butt part is welded A method for manufacturing an electric resistance tube.

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