JP2006037207A - Stock for hot dip plated electric resistance welded tube having excellent bead cutting property in tube making - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stock for an electric resistance welded tube which has a smooth weld zone surface after the cutting away of a weld bead, and is worked into a product shape having no occurrence of cracks by tube shrinking, tube extending or the like. <P>SOLUTION: In the stock, a Ti-added extra low carbon steel sheet to which, by mass, 0.0003 to 0.0050% B is added is used as a base, and a hot dip galvanizing layer, an alloyed hot dip galvanizing layer, a hot dip zinc-aluminum alloy plating layer or a hot dip zinc-aluminum-magnesium alloy plating layer is provided. In the Ti-added extra low carbon steel sheet, the content of C is 0.001 to 0.025%, the content of N is ≤0.01%, and the content of Ti is controlled to the range of [(48/12×C+48/32×S+48/14N)+0.01] to 0.10%, by mass. The stock is produced through a hot rolling stage, a cold rolling stage, reduction heating and a hot dip plating stage, and, in the reduction heating prior to the hot dip plating, heating temperature is set to the range of 800 to 900°C, and cooling velocity is set to the range of 10 to 50°C/s. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is a material for an electric-resistance-welded steel pipe that is used for automobile parts, various machine structure parts, and the like that are scheduled to be processed such as contraction and expansion, and has excellent formability and bead cutability during pipe making, and a method for manufacturing the same. About.

Ti-added ultra-low carbon steel sheets are widely used when steel sheets used for various mechanical structures including automobile parts require excellent deep drawability and stretchability. Various steel types from mild steel class to high-tensile steel class have been developed for Ti-added ultra-low carbon steel sheets.
In automotive parts and machine structural members that are often used in highly corrosive environments, plated steel sheets with improved corrosion resistance by hot-plating base steel sheets such as low carbon steel sheets are used (patents) Reference 1).
JP-A-60-165366

  By the way, in order to give strength to automobile parts, machine structural members, etc., an electric resistance welded steel pipe is often used. ERW steel pipes are manufactured by forming a cut plate cut into a predetermined size into an open pipe shape on a pipe making line, and then welding the width direction end by high frequency induction heating, etc. It is formed. The outer bead is usually cut to ensure the outer diameter of the steel pipe, but the inner bead may be cut if the inner diameter guarantee is required. For excision of the weld bead, running cutting is employed in which a cutting tool such as a cutting tool fixed to the rear of the welding machine is pressed against a running welded steel pipe in a high temperature state.

Ti-added ultra-low carbon steel sheet is excellent in deep drawability, stretchability, etc., but because it is inferior in bead cutability, it will be made into an electric-welded steel pipe used as a material for automobile parts, machine structural members, etc. Has a problem.
When an ERW steel pipe made of Ti-added ultra-low carbon steel is bead cut on a pipe making line, the cut weld bead is cut and cannot be collected as continuous chips, and the surface of the material after the bead cut is uneven. Prone to wrinkles. The width of the bead cut mark is uneven and the outer diameter accuracy is out of tolerance, and the cut inner bead is entangled with the cutter, making it difficult to cut or break the cutter. This phenomenon is not possible. The low bead cutability is a cause of lowering the production yield of the electric-welded steel pipe made of Ti-added ultra-low carbon steel, and the advantages of the Ti-added ultra-low carbon steel sheet excellent in workability cannot be fully utilized.

  On the other hand, with the reduction in weight and the complexity of part shapes, there is a strong tendency to form ERW steel pipes into a predetermined shape by contraction, expansion, etc. It is desired to provide ERW steel pipes with excellent workability. In this regard, if the bead cutting property of the Ti-added ultra-low carbon steel ERW steel pipe can be improved, an ERW steel pipe suitable as a material for automobile parts and various mechanical structural members that can withstand processing such as contraction and expansion is obtained. .

  In order to meet such demands, the present invention improves the component design of Ti-added ultra-low carbon steel while ensuring workability, and in particular, by adjusting the hardness of the weld bead by adding an appropriate amount of B, pipe making An object of the present invention is to provide a material for an electric resistance welded steel pipe with improved bead cutability and improved corrosion resistance by a zinc-based hot-dip plated layer.

The material for an electric resistance welded steel pipe of the present invention is based on a Ti-added ultra-low carbon steel sheet to which an appropriate amount of B is added, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip zinc-aluminum alloy plated layer or a hot-dip zinc- An aluminum-magnesium alloy plating layer is provided.
Underlying Ti-added ultra-low carbon steel sheet is C: 0.001 to 0.025 mass%, Si: 1.5 mass% or less, Mn: 0.1 to 2.5 mass%, P: 0.15 mass% Hereinafter, S: 0.01 mass% or less, sol.Al: 0.01-0.10 mass%, Ti: [(48/12 × C + 48/32 × S + 48 / 14N) +0.01] -0.10 mass %, N: 0.01 mass% or less, B: 0.0003 to 0.0050 mass%. If necessary, Nb: 0.01 to 0.10% by mass, V: 0.01 to 0.10% by mass, Zr: 0.01 to 0.10% by mass, or two or more of them may be included. good.

  The ERW steel pipe material is manufactured through a hot rolling process, a cold rolling process, a reduction heating process, and a hot dipping process in which manufacturing conditions are controlled. In the hot rolling step, the slab heated to 1100 to 1250 ° C. is hot-rolled at a finishing temperature of 880 to 940 ° C. and wound into a coil at a winding temperature of 680 to 760 ° C. In the cold rolling process, the rolling rate is set in the range of 70 to 85%. In the reduction heating prior to hot dipping, the heating temperature is set to 800 to 900 ° C., and the cooling rate is set to 10 to 50 ° C./second. The steel strip subjected to reduction annealing is introduced from a reduction annealing furnace through a snout to a hot dip galvanizing bath, a hot dip zinc-aluminum alloy plating bath, or a hot dip zinc-aluminum-magnesium alloy plating bath.

Effects and best mode of the invention

It is presumed that the Ti-added ultra-low carbon steel electric resistance welded steel pipe has an Ac ₃ transformation point as high as about 900 ° C. and a relatively low high-temperature strength, so that the bead cut property is poor. The present inventors examined a method for improving the bead cutting property by appropriately hardening the weld bead based on such a premise. As a result, when adding an appropriate amount of B under a completely non-aging component design that does not cause deterioration in properties due to aging, and securing a solid solution B amount effective for hardening the weld bead, a Ti-added ultra-low carbon steel It has been clarified that the bead cut property of the electric resistance welded steel pipe is improved, and the material for the electric resistance welded steel pipe that can utilize the excellent workability of the Ti-added ultra-low carbon steel sheet is obtained.
Hereinafter, various conditions defined in the present invention will be described individually.

(Ingredient design)
C: 0.001 to 0.025% by mass
The Ti-added ultra-low carbon steel sheet used for the base is a steel type aimed at complete non-aging by fixing C as a fine precipitate in the steel with a carbonitride-forming element. Necessary addition of carbonitride-forming elements In order to reduce the amount and prevent deterioration of workability due to an increase in fine precipitates, the C content is suppressed to 0.025% by mass or less. However, extremely low carbonization requires a long-time decarburization treatment and causes an increase in manufacturing cost, so the lower limit of the C content was set to 0.001 mass%.

・ Si: 1.5% by mass or less Although it is a solid solution strengthening element that is effective for improving room temperature strength and high temperature strength, it has an adverse effect on workability and surface properties of plated steel sheets, and non-plating occurs as the Si content increases. It tends to occur. In order to suppress such adverse effects, the Si content is regulated to 1.5% by mass or less.
Mn: 0.1 to 2.5% by mass
In order to suppress cracking during continuous casting, 0.1% by mass or more of Mn is required. Further, it exhibits a solid solution strengthening effect similar to that of Si, adversely affects workability and the surface properties of the plated steel sheet, and non-plating tends to occur as the Mn content increases. Therefore, the Mn content is selected to be 0.1 to 2.5% by mass.

-P: 0.15% by mass or less Although it is an effective component for increasing strength, excessive addition exceeding 0.15% by mass causes deterioration of workability and causes frequent secondary processing cracks after shrink tube forming It becomes.
・ S: 0.01 mass% or less This element is harmful to hot workability and cold workability, and hot brittleness caused by MnS frequently occurs during continuous casting. If it exceeds 0.01 mass%, the formability is extremely high. To drop. S consumes Ti added for improving workability as TiS, and also reduces the effective Ti amount. Therefore, it is preferable to reduce the S content as much as possible.

・ Sol.Al: 0.01 to 0.10% by mass
Al is an alloy component added as a deoxidizer in the steelmaking stage, and in order to obtain a sufficient deoxidation effect, 0.01 mass% or more is required as sol.Al. However, excessive addition exceeding 0.10 mass% increases Al inclusions in the steel, and cracks are likely to occur during processing such as contraction and expansion. Therefore, the Al addition amount is selected so that sol.Al is in the range of 0.01 to 0.10% by mass.

Ti: [(48/12 × C + 48/32 × S + 48 / 14N) +0.01] to 0.10% by mass
It is a component necessary for fixing C and N to be completely non-aged, and also exhibits an effect of fixing S as sulfide. Sulfide, together with Ti-based carbonitrides, improves the workability of the ERW steel pipe material. Therefore, the Ti addition amount is set to a value that is 0.01 mass% greater than the amount necessary for fixing C, N, and S. The property improvement effect becomes remarkable as the Ti content increases, but it saturates at 0.10% by mass, and adding more than that only increases the steel material cost.

・ N: 0.01 mass% or less It becomes completely non-aged by being fixed in carbon as a carbonitride by adding Ti as in C, but the carbonitride produced increases when the N content is high. Reduce workability. Although the upper limit of the N content is set to 0.01% by mass, it is preferable to secure the workability to reduce the N content as long as the manufacturing cost allows.

B: 0.0003-0.0050 mass%
When an appropriate amount of B having quenching and hardening action is added, an ERW steel pipe material exhibiting excellent bead cutting properties during pipe making is obtained, and the surface of the bead cut ERW steel pipe material is smoothed. The effect of improving the bead cutability by adding B is considered to be due to the fact that the weld bead is appropriately hardened by the heat history during pipe making and the hardness of the weld bead is ensured even in a high temperature region during bead cut. When solid solution B is secured in the steel, an effect of improving the bead cut property is observed, but B fixed as BN or the like does not have a positive influence on the bead cut property. Moreover, excessive addition of B is not preferable for the formability of the material, and promotes processing defects such as buckling during the molding of the material for the ERW steel pipe. Therefore, the B content is selected in the range of 0.0003 to 0.0050 mass% (preferably 0.0003 to 0.0003 mass%).

・ Nb, V, Zr: 0.01-0.10 mass% for all
Nb, V, and Zr are alloy components that are added as necessary, exhibiting the effect of improving workability necessary for contraction and expansion, and also for secondary cracks after contraction or expansion. It is valid. In any case, the effect of addition is observed at 0.01% by mass or more, but excessive addition exceeding 0.10% by mass not only saturates the effect of addition but also increases the production cost.

(Hot rolling)
After the molten steel adjusted to a predetermined composition is cast into a slab according to a conventional method, it is hot-rolled under controlled production conditions to improve the workability of the Ti-added ultra-low carbon steel sheet and ERW steel pipe material. In the heat treatment prior to hot rolling, the slab is heated to a temperature range of 1100 to 1250 ° C. where a Ti-based carbonitride effective for improving workability is precipitated. In hot rolling, the finishing temperature is set to 880 ° C or higher in order to suppress the generation and growth of textures that are harmful to workability, but setting the finishing temperature to 940 ° C or lower will cause excessive energy consumption and damage to furnace materials. To avoid. The hot-rolled steel strip is wound at a high temperature above 680 ° C. in order to prevent re-dissolution and secure the necessary amount of Ti-based carbonitride, but the pickling property is remarkably high at a winding temperature exceeding 760 ° C. Shows a tendency to deteriorate. Preferably, the heating temperature is selected in the range of 1150 to 1250 ° C, the finishing temperature is 880 to 920 ° C, and the winding temperature is 680 to 720 ° C.

(Cold rolling)
The workability of the Ti-added ultra-low carbon steel sheet is also affected by the rolling rate during cold rolling. If the rolling rate is too high, a sub-oriented texture that is not preferable for improving workability is formed, so the upper limit of the rolling rate is set to 85%. However, if the rolling rate is less than 70%, the growth of the cold-rolled texture is slow due to the effect of adding B, and the necessary mechanical properties cannot be obtained.

[Reduction annealing during hot dipping]
When hot-rolling Ti-added ultra-low carbon steel cold-rolled material, the steel sheet surface is activated by heating in a reducing atmosphere at a temperature range of 800 to 900 ° C. Heating to 800 ° C. or higher provides a completely recrystallized structure and necessary mechanical properties. However, heating at a temperature higher than 900 ° C. requires a large amount of energy consumption and causes an increase in manufacturing cost. In the cooling process after reduction heating, the amount of solute B effective for improving bead cutability is affected by the cooling rate. When the cooling rate does not reach 10 ° C./second, the amount of dissolved B tends to decrease, and the appearance of the hot dipped product may be partially damaged. The reason why the amount of solute B decreases is not necessarily clear, but it is presumed that solute B reacts with N in the reducing atmosphere near the steel sheet surface layer and is consumed as BN. Conversely, a cooling rate exceeding 50 ° C./second is a difficult condition due to the configuration of the hot dipping line, and may cause equipment trouble.

[Hot plating]
In order to impart corrosion resistance, the Ti-added ultra-low carbon steel sheet, which has been annealed by reduction, is introduced into a hot dip galvanizing bath, hot dip zinc-aluminum alloy plating bath, hot dip zinc-aluminum-magnesium alloy plating bath, and attached by gas wiping, etc. The amount of plating basis weight is adjusted by the amount control means. It is also possible to pass the steel strip after hot-dip plating through an alloying furnace to form an alloyed hot-dip galvanized layer. The maximum plate temperature at the time of hot dipping is about 500 ° C., and does not adversely affect the amount of Ti-carbonitride required for workability and the amount of solute B required for bead cutability.
In hot dipping, hot-dip galvanized layers such as Zn-0.1% Al and Zn-0.15% Al, hot-dip Zn-Al alloy plated layers such as Zn-5% Al and Zn-55% Al, Zn-6 A molten Zn—Al—Mg alloy plating layer such as% Al—3% Mg is formed.

  A slab having the composition shown in Table 1 was hot-rolled at a heating temperature of 1230 ° C., a finishing temperature of 920 ° C., and a winding temperature of 690 ° C. to obtain a hot-rolled steel strip having a thickness of 4.2 mm. The hot-rolled steel strip was annealed and pickled, and then cold-rolled to a thickness of 1.0 mm.

Furnace temperature: Each cold-rolled steel strip is passed through an in-line reduction annealing furnace at 850 ° C. at a line speed of 100 m / min, cooled at the cooling rate shown in Table 2 after reduction annealing, introduced into a hot dip galvanizing bath, and melted. Wiping gas was sprayed onto the steel strip pulled up from the galvanizing bath, and the basis weight of plating per side was adjusted to 45 g / m ² .

The manufactured hot-dip galvanized steel sheet was sent to a pipe making line and welded by high frequency induction heating to obtain an ERW steel pipe having an outer diameter of 31.8 mm. In an actual pipe making line, the weld bead is cut and removed by a cutting bit arranged behind the welder. In this embodiment, the inner bead was manually cut to investigate the bead cutability.
The obtained electric resistance welded steel pipe was examined for mechanical strength, bead cutability, and workability.

In the mechanical test, JIS Z2201 No. 5 test piece cut out from the ERW steel tube was subjected to a tensile test at room temperature, and 0.2% proof stress, tensile strength and total elongation were measured, and the Rankford value was calculated.
In the bead cutting test, an inner bead of an ERW steel pipe was cut using a laboratory-level cutting machine equipped with a standard tool. Visually observe the inner surface of the welded part after bead cutting, ◎ for specimens with a smooth surface, ◯ for specimens with slight chattering, and specimens with chattering that can be removed by paper polishing The bead cutability was evaluated by △, and x for the test piece in which chatter wrinkles that required cutting and grinding occurred.

  In the processing test, the electric resistance welded steel pipe from which the inner surface bead was cut and removed was subjected to three-stage press working to reduce the pipe contraction ratio at 60% or at a pipe expansion ratio of 170%, and investigated the pipe contractibility and pipe expandability. The testability was evaluated with ◯ for a test piece that had no cracks and processed into the target shape, △ for a test piece with a small amount of microcracks that could be observed with a magnifying glass, and X for a test piece that could be detected by visual observation. .

As can be seen from the results of the investigation in Table 3, the present invention example in which an appropriate amount of B was added and the cooling rate after reduction annealing was maintained in the range of 10 to 50 ° C./second was excellent in workability and bead cut. Showed sex. On the other hand, in No. 4 in which B was not added and the cooling was too slow, many chatter cracks were detected on the inner surface of the welded portion after the inner surface bead was removed by cutting, and work cracks frequently occurred after pipe expansion. In No. 9 in which an excessive amount of B was added and cooled at 20 ° C./second after reduction annealing, although the bead cutability was good, a work crack was detected in the reduced welded steel pipe.
From this comparison, it can be seen that when an appropriate amount of B is dissolved in the underlying steel sheet, the bead cutability is improved and the excellent workability of the Ti-added ultra-low carbon steel sheet can be utilized.

  As explained above, by using a Ti-added ultra-low carbon steel sheet with an appropriate amount of B added to the base of the hot-dip plated steel sheet, the weld bead is appropriately hardened, resulting in a conventional Ti-added ultra-low carbon steel sheet. Trouble at the time of cutting which was apt to be avoided is avoided, and a smooth weld surface is obtained after the bead is removed by cutting. In addition, the excellent workability inherent to the Ti-added ultra-low carbon steel sheet is utilized, and the occurrence of defects such as work cracks can be suppressed even after processing such as contraction and expansion. Furthermore, since the base steel plate is coated with a zinc-based hot-dip coating layer, it is excellent in corrosion resistance. For this reason, it is used as a material suitable as a member for various mechanical structures including automobile parts, which tend to be lighter and more complicated.

Claims (3)

  C: 0.001 to 0.025 mass%, Si: 1.5 mass% or less, Mn: 0.1 to 2.5 mass%, P: 0.15 mass% or less, S: 0.01 mass% or less , Sol.Al: 0.01 to 0.10% by mass, Ti: [(48/12 × C + 48/32 × S + 48 / 14N) +0.01] to 0.10% by mass, N: 0.01% by mass or less , B: 0.0003 to 0.0050 mass%, with the balance being a steel sheet having a substantially Fe composition, as a base, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip zinc-aluminum alloy plated layer or hot-dip zinc -A material for ERW steel pipes with excellent bead cutability during pipe making, which is provided with an aluminum-magnesium alloy plating layer.   The base steel sheet further comprises one or more of Nb: 0.01 to 0.10 mass%, V: 0.01 to 0.10 mass%, and Zr: 0.01 to 0.10 mass%. The material for ERW steel pipe described.   The slab having the composition according to claim 1 or 2 is hot-rolled at a heating temperature of 1100 to 1250 ° C, a finishing temperature of 880 to 940 ° C, a winding temperature of 680 to 760 ° C, and a rolling rate of 70 to 85%. After cold rolling and reduction annealing at a heating temperature of 800 to 900 ° C. and a cooling rate of 10 to 50 ° C./second, a hot dip galvanizing bath, a hot dip zinc-aluminum alloy plating bath or a hot dip zinc-aluminum-magnesium alloy plating bath A method for producing a material for an ERW steel pipe excellent in bead cutability during pipe making, which is characterized in that it is introduced into the steel.