JP6809502B2 - Forge welding steel pipe manufacturing method - Google Patents

Description

The present invention relates to a method for producing a forge-welded steel pipe, and in particular, a method for producing a forge-welded steel pipe capable of producing a forge-welded steel pipe having excellent aging resistance and excellent formability that is hard to crack even when subjected to strong processing such as flaring. Regarding.

There are various methods for connecting pipes, one of which is a pipe end flange joint. The pipe end flange joint is subjected to so-called flare processing (flaring processing) in which the pipe end is expanded outward to the connected pipe, and the flared pipes are sandwiched between flanges or the like. It is a method of fixing. Pipe end flange joints have been widely used in recent years because they do not require welding and are easy to install.

As a pipe to which a pipe end flange joint is applied, an electric resistance sewn steel pipe having a performance that is hard to crack even if a strong processing such as flaring is applied has been generally used. However, in recent years, there has been an increasing demand for using forge welded steel pipes, which are cheaper than electric resistance welded steel pipes, even in pipes to which pipe end flange joints are applied.

A forge-welded steel pipe is a steel pipe manufactured by pressure-welding the widthwise ends of a steel strip and performing diffusion welding (forge welding). Generally, a hot-rolled steel strip is heated in a heating furnace to form a forge welder. It is manufactured by forming it into a tubular shape, forge welding it, and then hot drawing and rolling it.

As a technique related to a forge welded steel pipe in consideration of flaring, for example, Patent Document 1 can be mentioned. Patent Document 1 proposes a method of suppressing the occurrence of cracks due to flaring by controlling the heating conditions at the end of a steel strip during the production of a forge-welded steel pipe and the atmosphere during forge welding.

Japanese Unexamined Patent Publication No. 2014-0798005

According to the method proposed in Patent Document 1, cracking in the forge welded portion during flaring can be suppressed. However, since flaring is a strong process in which the tube expansion ratio reaches 160%, cracks may occur not only in the forge welded portion but also in the base metal portion other than the forge welded portion. Therefore, there is a demand for forge-welded steel pipes that do not crack even after being subjected to severe flaring.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for manufacturing a forge-welded steel pipe having excellent aging resistance and excellent formability that is hard to crack even when subjected to strong processing such as flaring. And.

The present inventors investigated the causes of cracks generated during flaring, and obtained the following findings (1) and (2).

(1) Since flaring is usually performed on-site at the time of pipe construction, not at the time of steel pipe manufacturing, aging hardening occurs between the production of steel pipe and flaring, resulting in deterioration of moldability. Cracks occur during processing. In particular, forge-welded steel pipes are often used after hot-dip galvanizing, and it is considered that age hardening also progresses due to the heat during hot-dip plating. Therefore, if the aging resistance of the forge welded steel pipe is improved, cracking during flaring can be suppressed.

(2) Age hardening is a phenomenon in which hardness increases as C and N that have been solid-solved in steel precipitate over time. Therefore, by controlling the amount of C in the steel and the winding temperature after hot rolling within a specific range, age hardening (C aging) caused by solid solution C can be suppressed. In addition, by controlling the amount of Al and N in the steel and the winding temperature after hot rolling within a specific range, coarse AlN is precipitated during the production of hot-rolled steel strips, and age hardening due to solid melt N ( N aging) can be suppressed, ferrite grain growth during forge-welded steel pipe production can be promoted, and formability of forge-welded steel pipe can be improved.

The present invention is based on the above-mentioned novel findings, and the gist structure thereof is as follows.

1. 1. By mass%
C: 0.02 to 0.06%,
Si: 0.10 to 0.30%,
Mn: 0.10 to 0.50%,
P: 0.030% or less,
S: 0.014% or less,
Al: 0.01 to 0.10%,
N: Including 0.0100% or less
A steel slab having a component composition in which the balance is composed of iron and unavoidable impurities is hot-rolled to form a steel strip.
The steel strip is wound at a winding temperature of 600 to 700 ° C.
The steel strip is heated in a heating furnace to a heating temperature of 1250 to 1300 ° C.
The heated steel strip is formed into a cylindrical shape and
The widthwise end of the molded steel strip is forge welded.
A method for manufacturing a forge-welded steel pipe, which is then hot-rolled.

2. 2. The method for manufacturing a forge-welded steel pipe according to 1 above, wherein the inlet temperature is 850 to 1100 ° C. and the outlet temperature is 750 to 1000 ° C. in the hot drawing rolling.

3. 3. The method for producing a forge-welded steel pipe according to 1 or 2 above, wherein the component composition further contains Ti: 0.03% or less in mass%.

According to the present invention, it is possible to manufacture a forge-welded steel pipe having excellent aging resistance and excellent formability that is hard to crack even when subjected to strong processing such as flaring. The forge-welded steel pipe produced by the method of the present invention is not easily damaged during flaring, is excellent in workability, and is extremely effective in improving the reliability of the pipe.

It is a graph which shows the relationship between Al content and winding temperature, and yield strength after aging treatment.

Hereinafter, embodiments of the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited thereto.

[Steel slab]
In the method for producing a forge-welded steel pipe of the present invention, first, a steel slab having the above-mentioned predetermined component composition is hot-rolled to obtain a steel strip (hot-rolled steel strip). The steel slab can be produced by any method such as an ingot forming method or a continuous casting method. However, from the viewpoint of stabilizing moldability by homogenizing the components, it is preferable to use a steel slab manufactured by a continuous casting method.

Hereinafter, the component composition of the steel slab will be described. Unless otherwise specified, "%" representing the content of each component shall represent "mass%".

C: 0.02 to 0.06%
C is a component added to obtain the desired strength. If the C content is less than 0.02%, the cementite precipitated by cooling during the production of the forge-welded steel pipe becomes sparse, and the aging resistance is lowered. Therefore, the C content is set to 0.02% or more. On the other hand, if the C content exceeds 0.06%, the moldability deteriorates, and it becomes unsuitable for severe molding such as flaring. Therefore, the C content is 0.06% or less, preferably 0.05% or less.

Si: 0.10 to 0.30%
Si is a component having an effect of improving the plating property of a steel pipe. In order to obtain the above effect, the Si content is 0.10% or more, preferably 0.15% or more. On the other hand, if the Si content exceeds 0.30%, the moldability is lowered. Therefore, the Si content is 0.30% or less, preferably 0.25% or less.

Mn: 0.10 to 0.50%
Mn is a component having an effect of suppressing the precipitation of FeS, which is a cause of lowering moldability, by binding with S. In order to obtain the above effect, the Mn content is set to 0.10% or more, preferably 0.30% or more. On the other hand, if Mn is excessive, the moldability is rather lowered. Therefore, the Mn content is 0.50% or less, preferably 0.40% or less.

P: 0.030% or less When the P content exceeds 0.030%, the moldability deteriorates. Therefore, the P content is set to 0.03% or less, preferably 0.024% or less. On the other hand, the lower the P content, the better, so the lower limit is not particularly limited and may be 0%, but even in that case, it is permissible to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in manufacturing cost, the P content is preferably 0.001% or more.

S: 0.014% or less S binds to Mn and precipitates as MnS. However, if S is excessive, S that does not become MnS becomes FeS, which lowers moldability. Therefore, the S content is 0.014% or less, preferably 0.005% or less. On the other hand, the lower the S content, the better, so the lower limit is not particularly limited and may be 0%, but even in that case, it is permissible to contain it as an unavoidable impurity. However, since excessive reduction causes an increase in manufacturing cost, the S content is preferably 0.0005% or more.

Al: 0.01 to 0.10%
Al is a component having an effect of combining with N to precipitate AlN and improving aging resistance. Further, by adding an appropriate amount of Al, coarse AlN can be precipitated and the number of AlNs that hinder the growth of ferrite grains can be reduced. In order to obtain the above effect, the Al content is 0.01% or more, preferably 0.02% or more. On the other hand, if Al is excessive, a large amount of oxide is generated during forge welding, and the quality of the forge welded portion becomes unstable. Therefore, the Al content is 0.10% or less, preferably 0.05% or less.

N: 0.0100% or less When N is present in the steel as a solid solution N, the formability is lowered by aging. Further, N is bonded to Al and precipitated as AlN, but AlN hinders the growth of ferrite grains and lowers the moldability. Therefore, in order to ensure moldability, the N content is set to 0.0100% or less, preferably 0.0060% or less.

In one embodiment of the present invention, a steel slab having a component composition consisting of each of the above elements, the remaining Fe, and unavoidable impurities can be used.

Further, in another embodiment of the present invention, the component composition may further optionally contain Ti: 0.03% or less.

Ti: 0.03% or less Ti combines with N and precipitates as TiN. Therefore, by adding Ti, the aging resistance can be further improved. When the Ti content is in the range of atomic equivalent to the N content, the aging resistance is improved by adding Ti. However, when Ti is excessive, Ti that has not been bonded to N forms a precipitate with C, and as a result of increasing the strength, the moldability is rather lowered. Therefore, when Ti is added, the Ti content is 0.03% or less, preferably 0.02% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but is preferably 0.005% or more.

In the present invention, Nb and V are not intentionally added. Nb and V have a weaker bond with N than Ti, and have a low effect of improving aging resistance. Further, when Nb and V are added, the strength is increased, and as a result, the moldability is lowered. It is permissible that one or both of Nb and V are contained in the steel slab as unavoidable impurities. The amount of Nb contained as an unavoidable impurity is preferably 0.02% or less. Similarly, the amount of V contained as an unavoidable impurity is preferably 0.02% or less.

In the present invention, a steel strip is manufactured from the above steel slab, and a forge welded steel pipe is manufactured from the obtained steel strip. Hereinafter, the steel strip manufacturing process will be described first.

[Slab heating]
The steel slab is heated to a desired temperature (heating temperature) and then rolled. The heating can be performed using, for example, a heating furnace. The molten steel slab may be heated once after being cooled to room temperature, or may be heated without being cooled to room temperature. It is preferable to cool the slab to some extent before heating so that the coarse AlN precipitated in the cooling stage of the steel slab is not completely dissolved by heating. Specifically, it is preferable to cool the steel slab to a cooling temperature of 500 ° C. or lower prior to heating for hot rolling.

The heating temperature in the above heating is not particularly limited, but it is preferably 1100 ° C. or higher in order to reduce the hot deformation resistance and perform heat spreading. On the other hand, in order to prevent the solid solution of AlN due to the heating, the heating temperature is preferably 1180 ° C. or lower.

[Hot rolling]
Hot rolling can be performed under any conditions without particular limitation. The hot rolling can be composed of rough rolling and finish rolling, like general hot rolling. In order to make the temperature of the steel uniform and improve the uniformity of the material in the final steel pipe, it is preferable to heat the steel (sheet bar) after rough rolling before the start of finish rolling. In the heating (heating after rough rolling), the entire seat bar may be heated, or only both ends in the width direction of the seat bar where the temperature tends to decrease may be heated. Further, a plurality of sheet bars may be welded and finish rolling may be continuously performed.

The finishing temperature in the hot rolling is not particularly limited, but is preferably 780 to 830 ° C. By setting the finishing temperature within the above range, it is possible to promote the precipitation of coarse AlN, further improve the aging resistance, and further enhance the effect of improving the moldability by growing ferrite grains. Here, the "finishing temperature" refers to the "finishing rolling output side temperature".

[Winding]
Winding temperature: 600-700 ° C
Next, the steel strip obtained by the hot rolling is wound into a coil. The temperature at the time of winding (winding temperature) is 600 ° C. or higher, preferably 640 ° C. or higher from the viewpoint of AlN precipitation. On the other hand, if the winding temperature is too high, cementite is coarsely precipitated, and the amount of solid solution C is increased, so that the aging resistance is lowered. Therefore, the winding temperature is 700 ° C. or lower, preferably 680 ° C. or lower. Here, the "winding temperature" refers to the surface temperature of the steel strip immediately before the winding.

The steel strip after being wound into a coil is preferably cooled to room temperature. The cooling can be performed by, for example, one or both of air cooling and water cooling. When water cooling is performed, it is preferable that the water cooling start temperature is 500 ° C. or lower, which is lower than the precipitation temperature of AlN.

Next, the process of manufacturing a forge welded steel pipe from the obtained steel strip will be described. After being heated, the steel strip is formed into a cylindrical shape, and the ends are forge-welded and then hot-rolled to obtain a forge-welded steel pipe.

Prior to heating, the steel strip may be slit if necessary so that the width matches the diameter of the forge welded steel pipe to be manufactured. The slit strip may be selected so that the directions of the burrs and sagging are aligned in consideration of the influence of the burrs and sagging on the unevenness at the time of forge welding. Further, after the slit, the edge portion may be cut with an edge mirror or the like.

[heating]
Heating temperature: 1250 to 1300 ° C
Next, the steel strip is heated to a heating temperature of 1250 to 1300 ° C. in a heating furnace. This heating heats the entire steel strip. The heating temperature is set to 1250 ° C. or higher, preferably 1260 ° C. or higher in order to enable molding into a cylindrical shape. On the other hand, when the heating temperature exceeds 1300 ° C., the AlN deposited on the hot-rolled steel strip is solid-solved, and the aging resistance of the forge-welded steel pipe is lowered. Further, when the heating temperature exceeds 1300 ° C., fine AlN is precipitated during the production of the forge welded pipe, and the fine AlN hinders the growth of ferrite grains, resulting in a decrease in formability of the forge welded steel pipe. Therefore, the heating temperature is set to 1300 ° C. or lower, preferably 1290 ° C. or lower. Here, the heating temperature refers to the temperature of the steel sheet immediately after it comes out of the heating furnace.

In the heating, it is preferable to further heat both ends of the steel strip in the width direction. The heating of both ends can be performed using, for example, an induction heating device. The heating temperature at both ends is preferably 1350 ° C. or higher and 1450 ° C. or lower for forge welding. Further, in order to adjust the shape of the steel strip edge portion before heating, edge forming may be performed.

[Molding]
Next, the heated steel strip is formed into a cylindrical shape. The method for performing the molding is not particularly limited, and may be performed according to a conventional method. Usually, the molding may be performed by roll molding.

Next, the widthwise end of the formed steel strip is forge welded. The method for performing the molding is not particularly limited, and may be performed according to a conventional method. The molding and forge welding can be continuously performed using a general molding and forge welding machine. Further, before performing the forge welding, air, oxygen, or air having an oxygen concentration increased to 22 to 30% may be blown onto the end of the steel strip.

[Hot drawing rolling]
After the above forge welding, hot drawing rolling is further performed. In the hot drawing, the inlet temperature may be 850 ° C. or higher and 1100 ° C. or lower, and the outlet temperature may be 750 ° C. or higher and 1000 ° C. or lower in order to coarsely precipitate AlN so as not to hinder the growth of ferrite grains. preferable. It is more preferable that the outlet temperature is 800 ° C. or higher and 950 ° C. or lower.

By manufacturing the forge-welded steel pipe by the above process, it is possible to manufacture a forge-welded steel pipe having excellent aging resistance and excellent formability that is hard to crack even if strong machining such as flaring is performed.

Next, the present invention will be described in more detail based on Examples. The following examples show a preferred example of the present invention, and the present invention is not limited to this example.

(Example 1)
A forge-welded steel pipe was manufactured by the following procedure, and the characteristics of the obtained forge-welded steel pipe were evaluated.

First, a steel slab having the composition shown in Table 1 was produced by a continuous casting method. The obtained steel slab was once air-cooled to 300 ° C. or lower, then heated to the slab heating temperature shown in Table 2 and hot-rolled to obtain a steel strip. The finishing temperature in the hot rolling was as shown in Table 2. Then, the obtained steel strip was wound into a coil at the winding temperature shown in Table 2.

Next, the obtained steel strip was slit, and then a forge welded steel pipe having a nominal diameter of 15A to 100A was manufactured under the conditions shown in Table 2. That is, the entire steel strip was heated to the heating temperature shown in Table 2 in the heating furnace, and both ends in the width direction of the steel strip were further heated to 1380 to 1420 ° C. by the induction heating device. Then, it was formed into a cylindrical shape by roll forming with a forming and forge machine, and forge welding was performed while blowing a mixed gas of oxygen and air whose oxygen concentration was adjusted to 28% to the end portion in the width direction of the steel strip with a nozzle. After the forge welding, hot drawing rolling with a drawing ratio of 1 to 83% was performed to obtain a forge welded steel pipe.

A tensile test piece was cut out in the longitudinal direction from the base metal portion (the portion not the forge welded portion) of the obtained forge welded steel pipe. Using the tensile test piece as it is, a tensile test was carried out according to JIS Z2241 to evaluate the yield strength (YS) and the tensile strength (TS). The measurement results are shown in Table 3.

Further, in order to evaluate the aging resistance of the forge-welded steel pipe, the tensile test piece was subjected to an aging treatment (heat treatment) at 450 ° C. for 60 seconds, air-cooled to room temperature, and then the yield strength was evaluated. The conditions for the aging treatment simulate the conditions for hot-dip galvanizing. The measurement results are also shown in Table 3. Here, if the yield strength after the aging treatment is 310 MPa or less, it can be said that cracks do not occur even in severe flaring.

(Example 2)
Next, in order to evaluate the influence of the Al content and the winding temperature, forge welded steel pipes were manufactured at various Al contents and the winding temperature, and the yield strength after the aging treatment was evaluated. The content of components other than Al in the steel slab used was controlled within the range shown below (unit: mass%) so as to be substantially constant, and the balance was Fe and unavoidable impurities.
C: 0.028 to 0.042%
Si: 0.18 to 0.22%
Mn: 0.33 to 0.37%
P: 0.009 to 0.012%
S: 0.003 to 0.004%
N: 0.0029 to 0.0038%

Other conditions were also controlled so as to be substantially constant for each forge welded steel pipe. Specifically, the following conditions were set.
Slab heating temperature: 1150 to 1170 ° C
Finishing temperature: 790 to 810 ° C Finishing temperature Steel strip heating temperature: 1268 to 1279 ° C
Hot drawing rolling inlet temperature: 1050-1070 ° C
Hot drawing rolling Out side temperature: 940-970 ° C
Nominal diameter of forge welded steel pipe: 65A

The other points and the method for evaluating the yield strength after the aging treatment were the same as in Example 1. Those having a yield strength of 310 MPa or less after the aging treatment were evaluated as ◯ (OK), and those having a yield strength of more than 310 MPa were evaluated as × (NG). The evaluation result is shown in FIG.

As can be seen from the results shown in FIG. 1, when the Al content and the winding temperature satisfy the conditions of the present invention, the yield strength after the aging treatment is suppressed to 310 MPa or less, which makes it suitable for strong molding such as flare processing. A suitable forge welded steel pipe can be obtained.

As can be seen from the results of Examples 1 and 2 above, according to the method of the present invention, a forge welded steel pipe having excellent aging resistance and excellent formability that is hard to crack even when subjected to strong processing such as flaring is produced. be able to. On the other hand, the forge-welded steel pipe obtained under the conditions of the comparative example which does not satisfy the conditions of the present invention has a high yield strength after the aging treatment and is not suitable for strong machining such as flare.

Claims (3)

By mass%
C: 0.02 to 0.06%,
Si: 0.10 to 0.30%,
Mn: 0.10 to 0.50%,
P: 0.030% or less,
S: 0.014% or less,
Al: 0.01 to 0.10%,
N: Including 0.0100% or less
A steel slab having a component composition in which the balance is composed of iron and unavoidable impurities is hot-rolled to form a steel strip.
The steel strip is wound at a winding temperature of 600 to 700 ° C.
The steel strip is heated in a heating furnace to a heating temperature of 1250 to 1300 ° C.
The heated steel strip is formed into a cylindrical shape and
The widthwise end of the molded steel strip is forge welded.
A method for manufacturing a forge-welded steel pipe, which is then hot-rolled. The method for manufacturing a forge-welded steel pipe according to claim 1, wherein the inlet temperature is 850 to 1100 ° C. and the outlet temperature is 750 to 1000 ° C. in the hot drawing rolling. The method for producing a forge-welded steel pipe according to claim 1 or 2, wherein the component composition further contains Ti: 0.03% or less in mass%.

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