JP5011773B2 - Manufacturing method of low yield ratio ERW steel pipe with excellent low temperature toughness - Google Patents

Description

  The present invention relates to a method for producing a low yield ratio electric resistance welded steel pipe excellent in low temperature toughness, particularly suitable for pipelines of crude oil, gas, etc., water pipes, columns for construction and civil engineering.

Carbon steel pipes or low alloy steel pipes such as ERW steel pipes, spiral steel pipes, seamless steel pipes, UOE steel pipes, and press bend steel pipes can be manufactured in large quantities and stably. It is widely used as fluid transportation piping such as oil and gas line pipes and water pipes, or as pillars for construction and civil engineering.
However, when a large earthquake occurs, large tensile and compressive forces are repeatedly applied in the longitudinal direction of these steel pipes, causing local buckling in steel pipes with a large outer diameter / thickness ratio, and in some cases in the circumferential direction It can lead to cracking and fracture.

Generally, it is said that when cold working is applied to a steel material, the yield stress and tensile strength, particularly the yield stress, are increased by work hardening, the yield ratio is increased, and the plastic deformation absorption capacity is decreased. In particular, in the case of ERW steel pipe, the yield ratio in the coil plate width direction (circumferential direction of the pipe) is bent at the time of pipe making, and due to the Bauschinger effect due to the development on the flat plate during the final material evaluation test on the plate, Although it may be lower than the yield ratio of the material, a steel pipe with a low yield ratio cannot be obtained in the coil longitudinal direction (tube axis direction) because the Bauschinger effect cannot be expected.
As a method of manufacturing a steel pipe having a low yield ratio, for example, Patent Document 1 discloses a method of manufacturing a steel pipe for building to reduce the yield ratio, which is the ratio of yield stress and tensile strength, as earthquake resistance. The heat treatment process is complicated after re-heating, quenching, and tempering during pipe making after hot rolling.

Patent Document 2 discloses a method in which a material having a high carbon content with C: 0.10 to 0.18% is subjected to final finish hot rolling in a temperature range of 780 ° C. or higher and then cooled by air cooling. However, when steel with a large plate thickness of 12 mm or more and a high carbon content is cooled by air cooling, the steel structure becomes ferrite and a coarse pearlite structure because the cooling rate is slow, and low temperature toughness and sour resistance are achieved. It becomes an extremely poor material and cannot be applied to steel pipes that require high functionality such as line pipes.
JP-A-3-173719 JP-A-5-156357

  Accordingly, the object of the present invention is to solve the above-mentioned problems of the prior art, and has excellent low-temperature toughness, and is also capable of absorbing plastic deformation that is less likely to cause local buckling against axially acting compressive and bending stresses during large earthquakes. It is providing the manufacturing method of the low yield ratio electric resistance welded steel pipe which has these.

The gist of the present invention for solving the above problems is as follows.
[1] By mass%, C: 0.01 to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01 to 0.10%, Nb: 0.005 -0.10%, and Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5 so as to satisfy the following formula (1) % Or less, hot-rolled slab consisting of iron and inevitable impurities, and cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more. The low-yield ratio ERW steel pipe excellent in low temperature toughness, characterized in that it is retained in this temperature range for 10 minutes or more and then cooled to a temperature of less than 500 ° C. to form a hot-rolled steel sheet, and pipes the hot-rolled steel sheet Manufacturing method.
Mneq (%) = Mn + 0.26Si + 3.5P + 1.30Cr + 0.37Ni + 2.67Mo ≧ 2.0% (1)

[2] In the production method of [1] above, the slab is further mass%, Ti: 0.1% or less, V: 0.1% or less, Zr: 0.1% or less, or one or two A method for producing a low yield ratio ERW steel pipe excellent in low temperature toughness, characterized by containing at least a seed.
[3] In the production method of [1] or [2], the low-temperature toughness is characterized in that the slab contains, by mass%, Mo: 0.5% or less and Cr: 0.5% or less. A low yield ratio ERW steel pipe manufacturing method.
[4] In the production method according to any one of [1] to [3], a low temperature characterized by imparting a compressive strain of 10% or less or a tensile strain of 5% or less in the longitudinal direction of the tube during pipe forming. A method for producing low yield ratio electric resistance welded steel pipe with excellent toughness.

  ADVANTAGE OF THE INVENTION According to this invention, the low yield ratio ERW steel pipe which is excellent in low temperature toughness, especially a weld heat affected zone toughness, and has a high plastic deformation absorptivity can be manufactured. Therefore, by using the electric resistance welded steel pipe obtained by the method of the present invention, when a large earthquake occurs, line pipes and water pipes such as crude oil and gas are damaged and internal fluids flow out, or the pier column of the highway Disasters such as collapse due to breakage can be prevented, and the risk of brittle fracture can be avoided even when laid in a cold region.

As a result of intensive studies, the present inventors have been able to produce a low yield ratio ERW steel pipe excellent in low temperature toughness by optimizing the composition of steel and hot rolling conditions and controlling it to a specific steel structure. The present invention was completed.
That is, the manufacturing method of the ERW steel pipe of the present invention is, in mass%, C: 0.01 to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01 to 0.10%, Nb: 0.005 to 0.10%, and Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0 to satisfy the following formula (1) 0.5% or less, Cr: containing one or more of 0.5% or less,
Mneq (%) = Mn + 0.26Si + 3.5P + 1.30Cr + 0.37Ni + 2.67Mo ≧ 2.0% (1)
Further, if necessary, it contains one or more of Ti: 0.1% or less, V: 0.1% or less, Zr: 0.1% or less, and the balance is made of iron and inevitable impurities. The slab is hot-rolled, cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, wound up in this temperature range for 10 minutes or more, and then cooled to a temperature of less than 500 ° C. A hot rolled steel sheet having a structure in which the parent phase is bainitic ferrite, the martensite volume fraction is 3% or more, and the retained austenite volume fraction is 1% or more, if necessary. It is something to be tubed. Moreover, at the time of this pipe making, it is preferable to give a compressive strain of 10% or less or a tensile strain of 5% or less in the longitudinal direction of the tube.

By limiting the composition of steel and production conditions as described above, the present invention is less likely to cause local buckling against compressive and bending stress acting in the axial direction in the event of a large earthquake. In addition, it is possible to obtain a steel pipe excellent in earthquake resistance suitable for a pillar for construction and civil engineering.
The component composition and production conditions of the present invention will be specifically described below.

(1) About component composition Hereinafter, the reason for limitation of a component composition is demonstrated. In addition, all content of each element in a component composition means the mass%.
(C: 0.01-0.09%)
When the amount of C is less than 0.01%, the crystal grains become coarse, so that the amount of solid solution C increases and the yield ratio increases. On the other hand, if it exceeds 0.09%, coarse pearlite is easily generated in the steel structure, and the low-temperature toughness deteriorates. Therefore, the C amount is set to 0.01 to 0.09%.
(Si: 0.5% or less)
If the Si content exceeds 0.5%, oxides are likely to be generated during ERW welding, and the weld heat affected zone toughness deteriorates. Therefore, the Si content is 0.5% or less, preferably 0.3% or less.

(Mn: 2.5% or less)
Mn is an element effective for ensuring the strength of steel, but if added over 2.5%, the weld heat affected zone toughness deteriorates. For this reason, the amount of Mn is 2.5% or less. In order to more stably generate the low temperature transformation phase, the Mn content is preferably more than 1.15%.
(Al: 0.01-0.10%)
In addition to being used as a deoxidizer in the steelmaking stage, Al is an element effective for fixing N, which is the cause of strain aging, and needs to be added so as to contain 0.01% or more. However, the addition of more than 0.10% is not preferable because it unnecessarily raises the cost of molten steel, and also increases the oxide in the steel and lowers the toughness of the base metal and the ERW weld. Or cause nitridation of the surface layer in a heating furnace, leading to an increase in yield ratio. For this reason, the Al content is 0.01 to 0.10%, preferably 0.02 to 0.05%.

(Nb: 0.005-0.10%)
Nb precipitates NbC during hot rolling or after hot rolling and suppresses the recrystallization behavior and grain growth of steel, refines the crystal grains, turns into bainitic ferrite, promotes strength increase, and toughness It is an essential element because it is an effective element for ensuring the above. In order to exhibit the effect, it is necessary to contain 0.005% or more. However, the content exceeding 0.10% is not preferable because it increases the cost of molten steel and increases the deformation resistance of hot rolling. Moreover, the coarsening of the precipitate at the time of welding may be caused, and there exists a possibility of impairing the heat-affected zone toughness. Therefore, the Nb content is 0.005 to 0.10%, preferably 0.02 to 0.07%.

(Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less)
Mo, Cu, Ni and Cr are effective elements for increasing the strength of the steel sheet. In any case, addition exceeding 0.5% causes deterioration of weldability and weld heat-affected zone toughness. Therefore, at least one of Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, and Cr: 0.5% or less is added.
Of the above element groups, Cr contributes to the stabilization of the γ phase, facilitates the formation of low-temperature transformation products, and is an effective element for reducing the yield ratio. Mo is also an element that contributes to the formation of a low-temperature transformation product and a low yield ratio. Therefore, it is particularly preferable to add Mo: 0.5% or less and Cr: 0.5% or less, and more preferable addition amounts are Mo: 0.3% or less and Cr: 0.3% or less. These contents are desirably 0.05% or more, particularly preferably 0.10% or more for Mo, and 0.10% or more, particularly preferably 0.15% or more for Cr.
(Mneq (%) = Mn + 0.26Si + 3.5P + 1.30Cr + 0.37Ni + 2.67Mo: 2.0% or more)
The reason why Mneq (%) is set to 2.0% or more is that if it is less than 2.0%, martensite is generated after hot rolling, and a low yield ratio of 85% or less cannot be achieved. Therefore, in the present invention, Mneq (%) is set to 2.0% or more. Moreover, when it is desired to more surely make the yield ratio 85% or less, it is desirable to set it to 2.2% or more.

In the present invention, one or more of the following selective elements may be further contained in accordance with targets such as the strength and toughness of the steel pipe.
(Ti: 0.1% or less, V: 0.1% or less, Zr: 0.1% or less)
Ti, V, and Zr are elements that form fine carbonitrides and increase the toughness and strength of steel. However, addition exceeding Ti: 0.1%, V: 0.1%, Zr: 0.1% deteriorates the weld heat affected zone toughness. Accordingly, at least one of Ti: 0.1% or less, V: 0.1% or less, and Zr: 0.1% or less is added. Further, it is preferable that the content of each be 0.05% or less.

  In addition, P and S mixed as impurity elements are not particularly limited in the present invention, but P segregates at the grain boundaries to weaken the grain boundary strength and deteriorate the weld heat affected zone toughness. Therefore, the P amount is preferably 0.03% or less. Moreover, when S exceeds 0.01%, the weld heat-affected zone toughness is deteriorated. Therefore, the S content is 0.01% or less, preferably 0.005% or less.

(2) Manufacturing conditions, etc. Desirable manufacturing conditions are described below.
(Cooling rate after hot rolling: 5 ° C / s or more)
The reason why the cooling rate after hot rolling was set to 5 ° C./s or more is that when the cooling rate is less than 5 ° C./s, the γ → α transformation starts at a high temperature, and thus the ferrite structure to be formed becomes polygonal ferrite. This is because it becomes difficult to ensure toughness. Therefore, a cooling rate of 5 ° C./s or more is required to make the matrix structure bainitic ferrite to ensure strength and toughness.
However, if the cooling rate exceeds 45 ° C./s, two-phase separation of γ / α is not sufficiently performed in the cooling process, and it becomes difficult to obtain a desired low-temperature transformation product. For this reason, from the viewpoint of producing a low-temperature transformation product, the cooling rate is desirably 45 ° C./s or less.

(Taking-up temperature after hot rolling: 500 to 650 ° C.)
The reason for setting the coiling temperature after hot rolling to 500 to 650 ° C. is that when the coil is wound at less than 500 ° C., most of the low-temperature transformation product becomes a bainite structure, and it becomes difficult to ensure the strength and reduce the yield ratio. On the other hand, when the temperature exceeds 650 ° C., a pearlite structure which is not good in toughness is generated. Therefore, in order to avoid the formation nose of bainite and pearlite and to change the main low temperature transformation product to martensite, it is necessary to set the coiling temperature after hot rolling to 500 to 650 ° C. Moreover, a more preferable winding temperature is 520-630 degreeC.
(Cooling condition after winding: after winding, stay for 10 minutes or more and then cool to below 500 ° C)
The reason for cooling to a temperature of less than 500 ° C. after being retained for 10 minutes or more after winding is that the retention time of less than 10 minutes passes through the bainite generation nose and most of the low-temperature transformation product becomes a bainite structure. This is because martensite and retained austenite, which are advantageous for lowering the yield ratio, cannot be obtained.

(Hot rolled sheet structure)
In the hot-rolled sheet structure obtained in the present invention, the parent phase (phase with a volume ratio of more than 50%) is bainitic ferrite, and the remainder is a low-temperature transformation phase, and the volume fraction of martensite is the low-temperature transformation phase. 3% or more, and if necessary, the volume ratio of retained austenite is 1% or more.
Here, bainitic ferrite has a substructure with a lath-like structure with a high dislocation density in the bainite structure, and is clear from the bainite structure in that it does not have iron carbide in the structure. Different. Also, it is different from a polygonal ferrite structure having a substructure with little or no dislocation density, or a quasi-polygonal ferrite structure having a substructure such as fine subgrains.

  The parent phase is bainitic ferrite, and the volume fraction of martensite in the low-temperature transformation product is 3% or more, and if necessary, the volume fraction of retained austenite is 1% or more to ensure strength and toughness. For this reason, even when the volume ratio of martensite is less than 3%, or when the amount of strain during pipe making is large due to the addition of uniform elongation, a rapid increase in stress due to work hardening is suppressed, and the increase in yield ratio is reduced. If the volume ratio of retained austenite is less than 1%, it is difficult to achieve the desired low yield ratio. There is no particular problem even if some of the low temperature transformation products contain bainite. In addition, since excessive martensite impairs low temperature toughness, the volume ratio of martensite is preferably 20% or less, and preferably 10% or less. In addition, since the retained austenite is transformed into martensite due to tube-forming strain, the volume ratio of retained austenite is 10% or less, preferably 5% or less in order not to impair the low temperature toughness due to excessive martensite. It is preferable.

(Pipe making conditions)
The ERW steel pipe is basically manufactured by a usual method. That is, for example, a steel strip is formed by cage roll forming, electric resistance welding is performed, bead grinding of the inner and outer surfaces is performed, heat treatment is applied by a post-annealer, and sizing is performed.
Giving compression pre-strain for the purpose of preventing an increase in yield ratio during pipe making is an effective means for reducing the yield ratio using the Bauschinger effect, but forcibly exceeds 10%. If compressive strain or tensile strain exceeding 5% is applied, the yield ratio increases due to work hardening, so it is preferable to avoid applying such compressive strain or tensile strain. For this reason, it is desirable that the compressive strain applied in the longitudinal direction of the pipe during pipe making is 10% or less, preferably 5% or less. Similarly, the tensile strain is desirably 5% or less.
In addition, the manufacturing method of this invention can also be employ | adopted also for the method of various raw materials, such as not only an ERW steel pipe but a UOE steel pipe, a spiral steel pipe, and a press bend steel pipe.

Table 1 shows the composition of the steels of the present invention and the comparative steel. Tables 2 and 3 show that the steel pipe material is manufactured by each manufacturing method and made into an ERW steel pipe having an outer diameter of 18 inches, and then the pipe shaft in the circumferential direction of 180 ° from the welded portion (the center portion of the width of the material coil). The material characteristics in the direction (rolling direction of the material coil) are shown.
Tensile properties were measured using small round bar tensile specimens. Here, the yield ratio refers to the ratio of stress / tensile strength at 0.5% strain. The yield ratio is 85% or less as “good” and the yield ratio exceeding 85% as “bad”.

The microstructure of the steel sheet was investigated by observing a cross section in the rolling direction of the steel sheet with a scanning electron microscope. Using a cross-sectional structure photograph at a magnification of 3000 times, the area ratio of martensite existing in a 100 mm square area set arbitrarily by image analysis was determined and used as the volume ratio of martensite.
The amount of retained austenite was obtained by polishing the steel plate to the center plane in the plate thickness direction and measuring the diffraction X-ray intensity at the plate thickness center plane. MoKα rays are used as incident X-rays, and the diffracted X-ray intensity ratios of the {111}, {200}, {220}, and {311} surfaces of the retained austenite phase are determined, and the average value of these is calculated as the volume of retained austenite. Rate.

Base metal and weld heat affected zone toughness were evaluated by Charpy impact test using No. 4 test piece of JIS Z 2202, and indicated by fracture surface transition temperature indicated by JIS Z 2242. A fracture surface transition temperature of −50 ° C. or lower is defined as “good”, and a temperature above −50 ° C. is defined as “bad”.
As is apparent from Tables 1 to 3, steel pipes satisfying the conditions of the present invention have a low yield ratio and good weld heat affected zone toughness.
On the other hand, the comparative example is inferior in at least one of the yield ratio and the weld heat affected zone toughness.

Claims (4)

In mass%, C: 0.01 to 0.09%, Si: 0.50% or less, Mn: 2.5% or less, Al: 0.01 to 0.10%, Nb: 0.005 to 0.005. 10% is contained, and Mo: 0.5% or less, Cu: 0.5% or less, Ni: 0.5% or less, Cr: 0.5% or less so as to satisfy the following formula (1) A slab containing one or more of them, the balance being iron and unavoidable impurities is hot-rolled, cooled to 500 to 650 ° C. at a cooling rate of 5 ° C./s or more, and taken up at this temperature. A method for producing a low yield ratio ERW steel pipe excellent in low temperature toughness, characterized in that the hot rolled steel sheet is cooled by cooling to a temperature of less than 500 ° C. after being retained for 10 minutes or more in the range, and the hot rolled steel sheet is piped .
Mneq (%) = Mn + 0.26Si + 3.5P + 1.30Cr + 0.37Ni + 2.67Mo ≧ 2.0% (1)   The slab further contains, by mass%, one or more of Ti: 0.1% or less, V: 0.1% or less, Zr: 0.1% or less. 2. A method for producing a low yield specific electric resistance welded steel pipe excellent in low temperature toughness according to 1.   The slab contains, by mass%, Mo: 0.5% or less and Cr: 0.5% or less. The low yield specific electric resistance steel pipe excellent in low temperature toughness according to claim 1 or 2 Production method.   The low yield specific electricity excellent in low temperature toughness according to any one of claims 1 to 3, wherein a compressive strain of 10% or less or a tensile strain of 5% or less is imparted in the longitudinal direction of the tube during pipe forming. Manufacturing method of sewn steel pipe.