JP7485920B2 - Intermediate bending roll steel pipe - Google Patents

Description

The present invention relates to intermediate bending roll steel pipes, and in particular to intermediate bending roll steel pipes with excellent circumferential weld metal toughness, which have a plate thickness of 40 mm or more and are circumferentially welded with three or more multi-pass welding.

Bending roll steel pipes have traditionally been used as steel pipe piles and steel pipe sheet piles in the civil engineering and construction fields.

In recent years, there has been a demand for bending roll steel pipes that have excellent toughness as well as strength. Specifically, for example, a toughness value of 27 J at 0°C in the Charpy impact test is required.

Bending roll steel pipes are made by roll forming steel plates, tack welding the butt joints, and then welding them in the longitudinal direction. The pipes are then manufactured to the desired length by performing intermediate circumferential welding. Therefore, it is also very important to ensure the toughness of the circumferential weld metal.

Patent Document 1 discloses a high-strength spiral steel pipe pile having a structure in which the main phase is a bainitic ferrite phase and a secondary phase containing one or more of the following phases in a volume fraction of 10% to less than 50% in total: martensite phase, bainite phase, and pearlite; a low yield ratio high strength of axial yield strength YS of 450 MPa or more, tensile strength TS of 570 MPa or more, and yield ratio YR of 90% or less; and a high toughness of absorbed energy vE0 of 27 J or more at a test temperature of 0°C in a Charpy impact test.

Patent Document 2 discloses a spiral steel pipe in which the combined ratio of tempered martensite and bainite in the weld metal structure is 80% or more.

JP 2016-47956 A JP 2011-161500 A

For SM570 material, which is commonly used for steel pipe piles, the Charpy absorbed energy at -5°C is specified as an index of toughness. However, the toughness of the circumferential weld metal can sometimes be inferior to the toughness of the base material.

The objective of the present invention is to provide a joint bending roll steel pipe with excellent toughness of the weld metal in circumferential welding.

The inventors conducted extensive research to solve the above problems and discovered that it is possible to improve the toughness of the weld metal by suppressing grain boundary ferrite and forming a weld metal structure that is mainly composed of acicular ferrite. The present invention was made as a result of further research, and the gist of the invention is as follows.

(1) A joint bending roll steel pipe having two base materials and a circumferential welded portion that joints the two base materials, the base material being a bending roll steel pipe having a plate thickness of 40 mm or more, the components of the weld metal of the circumferential welded portion are, in mass%, C: 0.030 to 0.150%, Si: 0.50% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Cu: more than 0.50% and 0.0010 to 0.0500%, Ti: 0.0020% to 0.0500%, N: 0.0100% or less, and O: 0.0150 to 0.0600%. A joint bending roll steel pipe characterized in that it contains Al/O, with the balance being Fe and impurities, satisfies 0.300≦Al/O≦1.000, α′ defined by α′=(1.5×(O−0.89Al)+3.4×N-Ti)×1000 is 0% or more and 60.0% or less, Ceq defined by Ceq=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 is 0.350% or more and 0.500% or less, and Pcm defined by Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B is 0.30% or less. Here, the element symbols in the above formulas indicate the content (mass%) of each element.

(2) The intermediate bending roll steel pipe of (1) characterized in that the structure of the weld metal contains, by area ratio, acicular ferrite: 75.0% or more, grain boundary ferrite: 15.0% or less, and island martensite: 3.0% or less, and the EBSD grain size is 10.0 μm or less.

(3) The intermediate bending roll steel pipe according to (1) or (2), characterized in that the components of the weld metal contain, in place of a portion of the Fe, one or more elements selected from the group consisting of Ni: 0-0.50%, Cr: 0-0.50%, Mo: 0-0.50%, V: 0-0.050%, Nb: 0-0.050%, B: 0-0.0100%, Mg: 0-0.010%, and Ca: 0-0.0060%.

(4) A bending roll steel pipe according to any one of (1) to (3), characterized in that the toughness of the circumferential weld at -5°C in a Charpy impact test is 100 J or more.

(5) A joint bending roll steel pipe according to any one of (1) to (4), characterized in that the components of the base material are, by mass%, C: 0.030 to 0.150%, Si: 0.55% or less, Mn: 0.50 to 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.100% or less, Ti: 0.005 to 0.030%, N: 0.0020 to 0.0060%, O: 0.0050% or less, with the balance being Fe and impurities.

(6) The intermediate bending roll steel pipe according to (5), characterized in that the components of the base material contain, in place of a portion of the Fe, one or more selected from the group consisting of Ca: 0-0.0050%, Ni: 0-0.50%, Cr: 0-0.50%, Cu: 0-0.50%, Mo: 0-0.50%, Nb: 0-0.100%, B: 0-0.0020%, V: 0-0.060%, and Mg: 0-0.0100%.

According to the present invention, it is possible to obtain a joint bending roll steel pipe having excellent toughness of the circumferential weld metal. More specifically, it is possible to obtain a joint bending roll steel pipe having a weld metal Charpy absorbed energy of preferably 100 J or more at -5°C.

FIG. 2 is a diagram for explaining the positions at which Charpy test pieces were taken.

The present invention will be described in detail below.

The present invention is a joint bending roll steel pipe in which two base materials are joined at a circumferential weld. The plate thickness of the two base materials is 40 mm or more, preferably 60 mm or more, and the circumferential weld is preferably multi-layered with three or more layers. There is no particular upper limit to the plate thickness, but a plate thickness of up to about 100 mm is preferable.

The jointed bending roll steel pipe of the present invention can be manufactured by groove-cutting the butt joint of the bending roll steel pipe and then submerged arc welding while rotating the steel pipe.

First, we will explain the chemical composition of the weld metal parts.

C: 0.030 to 0.150%
If the C content is less than 0.030%, the hardenability is low and the weld metal does not become a sufficiently hardened structure, so that a large amount of grain boundary ferrite is formed. In addition, the susceptibility to hot cracking is high. In particular, since circumferential welding is a multi-layer welding, the cooling rate is slowed down due to the preheating effect, so there is a high possibility of hot cracking. In addition, if the C content exceeds 0.150%, the hardenability becomes excessive, and the risk of cracking during cooling in the cooling process increases. In addition, if the C content is high, hot cracking is also a concern. Therefore, the C content is set to 0.030% or more and 0.150% or less.

Si: 0.50% or less Si is contained due to dilution of the base metal, welding wire, and flux. If the Si content exceeds 0.50%, coarse island martensite (MA) is formed, resulting in a decrease in the toughness of the weld metal. Therefore, the Si content is set to 0.50% or less.

Mn: 0.50 to 2.00%
Mn is an element necessary to ensure hardenability. If the Mn content is less than 0.50%, the hardenability is insufficient. If the Mn content exceeds 2.00%, the hardenability becomes excessive, and there is a risk of cracking during cooling in the cooling process. In addition, it becomes difficult to recover the toughness of the weld metal part after hardening. Furthermore, coarse MnS is formed, which becomes the starting point of fracture and reduces the toughness. Therefore, the Mn content is set to 0.50% or more and 2.00% or less.

P: 0.020% or less,
P is an impurity, and its content is set to 0.020% or less. The content may be 0. P is an element that promotes solidification cracking. If the P content exceeds 0.020%, the risk of solidification cracking increases, so the content is reduced in consideration of refining costs.

S: 0.010% or less S is an impurity, and its content is set to 0.010% or less. The content may be 0. S is an element that promotes solidification cracking together with P. If the S content exceeds 0.010%, the risk of solidification cracking increases, so the content is reduced in consideration of refining costs.

Cu: more than 0% and 0.50% or less Cu is an element that can improve the strength of the weld metal. The Cu content is preferably more than 0% and 0.50% or less.

Ni: 0 to 0.50%
Ni is an element that can improve the strength of the weld metal without reducing the toughness, and also improves the hardenability. Although not essential, it is preferable to include Ni in the range of 0 to 0.50%.

Cr: 0 to 0.50%
Cr is an element that can improve the strength of the weld metal and also improve the hardenability. Although not essential, it is preferable to include 0 to 0.50%.

Mo: 0 to 0.50%
Mo is an element that can improve the strength of the weld metal and also improve the hardenability. Although not essential, it is preferable to include Mo in the range of 0 to 0.50%.

V: 0 to 0.050%
V is an element that can improve the strength of the weld metal. Although not essential, it is preferable that V is contained in the range of 0 to 0.050%.

Nb: 0 to 0.050%
Nb is an effective element for improving strength and for providing dissolved B, which is effective in suppressing grain boundary ferrite. The inclusion of Nb is not essential. In order to prevent a decrease in toughness due to the formation of island martensite, the content is preferably 0 to 0.050%.

Al: 0.0010 to 0.0500% Al is an element necessary for controlling the amount of oxygen in order to disperse a large number of oxides that become acicular ferrite formation sites in the weld metal. Al is contained in the base metal, welding wire, and flux. If the Al content is less than 0.0010%, the above oxides are hardly obtained. If the Al content exceeds 0.0500%, large amounts of Al ₂ O ₃ are formed, which become the starting points of fracture and reduce toughness. Therefore, the Al content is set to 0.0010% or more and 0.0500% or less.

Ti: 0.0020% to 0.0500%
Ti is one of the constituent elements of oxides that become acicular ferrite formation sites, and promotes refinement of the weld metal structure. If the Ti content is less than 0.0020%, the above effect cannot be obtained. If the Ti content exceeds 0.0500%, the amount of dissolved Ti increases, forming carbides in the tempering process, and the toughness of the weld metal part decreases. Therefore, the Ti content is set to 0.0020% or more and 0.0500% or less.

B: 0 to 0.0100%
B has the effect of promoting the formation of acicular ferrite by suppressing the formation of grain boundary ferrite in the weld metal when in a solid solution state. Although B does not have to be contained, it is preferable to contain 0.0001% or more to obtain this effect. In order to prevent a decrease in toughness due to excessive strength, the content is set to 0.0100% or less.

N: 0.0100% or less N is an impurity. In order to prevent dissolved N that does not react with Ti and remains there, from reducing toughness, the content is set to 0.0100% or less.

O: 0.0150 to 0.0600%
O is to be 0.0150% or more in order to form oxides that become the nuclei of acicular ferrite, and is to be 0.0600% or less in order to suppress a decrease in toughness due to excessive formation, aggregation and coarsening of oxides.

Mg: 0 to 0.010%,
Mg is an element that acts as a deoxidizer to reduce the amount of oxygen in the weld metal and improve toughness. Although not essential, it is preferable to include it in the range of 0 to 0.010%.

Ca: 0 to 0.0060%,
Ca is an element effective for improving ductility and refining the structure by morphology control. The inclusion of Ca is not essential. In order to prevent a decrease in ductility and toughness due to coarsening of sulfides and oxides, the Ca content is preferably 0.0060% or less.

The remainder of the weld metal is Fe and impurities. Impurities are components that are mixed in during the welding process from the welding wire, flux, steel plate, surrounding atmosphere, etc., and are not intentionally included.

Specific examples include P, S, N, Sb, Sn, W, Co, As, Pb, Bi, and H. Of these, P, S, and N must be controlled to P: 0.020% or less, S: 0.010% or less, and N: 0.0100% or less, as described above.

As for other elements, Sb, Sn, W, Co, and As may be present as unavoidable impurities at levels of 0.1% or less, Pb and Bi at levels of 0.005% or less, and H at levels of 0.0005% or less. However, within normal ranges, there is no need to control them.

In the following explanation, "%X" in the formula means the content (mass%) of element X in the weld metal. Also, elements that are not added to the weld metal are calculated as zero.

%Al/%O: 0.300-1.000
%Al/%O is the ratio of the amount of Al to the amount of O, and is an index showing the oxygen potential after the completion of aluminum deoxidation. By controlling %Al/%O to 0.300 to 1.000, the amount of acicular ferrite produced can be improved.

If the %Al/%O ratio is less than 0.300, the amount of O will be excessive, and the dissolved oxygen that did not form Ti oxides will reduce the cleanliness of the steel, resulting in reduced toughness. On the other hand, if the %Al/%O ratio is more than 1.000, the amount of Al will be excessive, reducing the amount of O that bonds with Ti, decreasing the amount of Ti oxides that become acicular ferrite nuclei, and reducing toughness. Therefore, the %Al/%O ratio should be between 0.300 and 1.000.

α': 0-60.0%
α' is a parameter that indicates the effective acicular ferrite formation ability based on the stoichiometric ratio of Al, O, and Ti, N, and is defined as α' = (1.5 x (%O - 0.89%Al) + 3.4 x %N - %Ti) x 1000. By controlling α' within the range of 0 to 60, the acicular ferrite nucleation ability is improved.

When α' is less than 0%, either the amount of Al or Ti is excessive, or the amount of N and O is insufficient, so the nucleation ability of acicular ferrite is significantly reduced. When α' is more than 60.0%, either the amount of Al or Ti is insufficient, or the amount of N and O is excessive, so the nucleation ability of acicular ferrite is significantly reduced.

Ceq: 0.350 to 0.500%
The chemical composition of the weld metal needs to have a Ceq of 0.350 to 0.500%, defined as Ceq = %C + %Mn/6 + (%Cr + %Mo + %V)/5 + (%Ni + %Cu)/15.

Ceq is the sum of the hardening ability of each alloy element converted into the amount of C for the base metal due to the welding heat effect. In order for the weld metal to achieve the desired tensile strength, Ceq is controlled to 0.350-0.500%. Preferably, Ceq is 0.400-0.430%.

Pcm: 0.30% or less The chemical composition of the weld metal must be such that Pcm, defined as %C + %Si/30 + (%Mn + %Cu + %Cr)/20 + %Ni/60 + %Mo/15 + %V/10 + 5%B, is 0.30% or less.

Pcm is called weld sensitivity, and is a quantitative evaluation of the effect of the chemical components of steel on cold cracking. If Pcm exceeds 0.30%, cold cracking becomes more likely to occur, so the upper limit is set at 0.30%.

Next, we will explain the structure of the weld metal. In the following explanation, the structure ratio is the area ratio.

Acicular ferrite: 75.0% or more Acicular ferrite is an acicular ferrite structure with Ti-based oxides as its nucleus, and the greater the proportion of acicular ferrite, the finer the fracture units of the weld metal part. To obtain this effect, it is preferable that the acicular ferrite content is 75.0% or more. Most preferably, the proportion of acicular ferrite is 100%.

Grain boundary ferrite: 15.0% or less Grain boundary ferrite is one of the embrittlement phases, which becomes the starting point of fracture and causes a decrease in toughness. Therefore, the grain boundary ferrite content is set to 15.0% or less.

Island martensite: 3.0% or less
Island martensite is one of the embrittling phases, and because it has a very high hardness, it can become the starting point of fracture and cause a decrease in toughness. Therefore, the island martensite content is set to 3.0% or less.

The remainder of the structure may include intragranular ferrite, ferrite side plates, and upper bainite. These structures are the starting points of fracture and cause a decrease in toughness, so as mentioned above, it is preferable that the fracture units are refined by acicular ferrite.

The area ratio of the weld metal structure is measured as follows:

A test piece is taken from the surface of the weld metal, half the width of the weld bead at t/4 of the wall thickness, and after polishing, it is subjected to nital etching and Repera etching. The exposed structure is measured with an optical microscope in 10 fields of view, focusing on the structure observed in an area of 1000 μm x 1000 μm. The obtained image is subjected to image analysis, and the average area ratio of each structure is calculated.

EBSD grain size: 10.0 μm or less Furthermore, in the weld metal structure, the EBSD grain size must be 10.0 μm or less. The EBSD (Electron Back Scatter Diffraction) grain size is a crystal grain size that serves as a guide for fracture units. If the EBSD grain size is 10.0 μm or less, the fracture units are fine, and toughness at low temperatures can be ensured.

In the present invention, the EBSD grain size is the average grain size measured by EBSD analysis of 20 fields of view in an area of 500 μm x 500 μm, and divided into sections with a crystal orientation difference of 15°.

In the intermediate weld metal part of the bending roll steel pipe of the present invention, the composition of the weld metal can be controlled as described above by combining an appropriate flux and wire, and the welding heat input can be appropriately controlled to obtain the above-mentioned structure. Specifically, the above-mentioned structure can be obtained by appropriately adjusting the number of welding electrodes, preheat temperature, and heat input.

The welding method may be submerged arc welding, gas shielded arc welding, or both.

This makes it possible to obtain a spiral steel pipe with excellent toughness even in the weld metal zone. Specifically, it is possible to obtain a joint bending roll steel pipe in which the Charpy absorbed energy of the weld metal zone at -5°C is preferably 100 J or more.

Next, the chemical composition of the base material will be described. In the intermediate bending roll steel pipe of the present invention, the composition of the base material is not particularly limited as long as it is the composition of a general bending roll steel pipe. The following is an example of a chemical composition suitable for the base material of the intermediate bending roll steel pipe of the present invention.

C: 0.030 to 0.150%
C is effective in improving the strength of steel, and is contained at 0.030% or more in order to obtain the desired strength. If the C content is too high, the hardenability improves too much and the toughness of the base material decreases, so the C content is set to 0.150%, and preferably 0.060 to 0.080%.

Si: 0.55% or less Si is an element necessary for deoxidation. If the amount of Si is large, island martensite is easily formed, and low-temperature toughness is significantly deteriorated, so the amount of Si is less than 0.55%. It is preferably less than 0.35%. Since deoxidation can be performed with Al and Ti, the addition of Si is not essential.

Mn: 0.50 to 2.00%
Mn acts as an element for improving hardenability, and is contained in an amount of 0.50% or more to obtain this effect. If the Mn content is too high, the hardenability of the steel increases, deteriorating the HAZ toughness and weldability, and further promoting central segregation of continuously cast steel slabs and deteriorating the low-temperature toughness of the base material, so the Mn content is set to 2.00% or less, and preferably 1.00 to 1.80%.

P: 0.020% or less S: 0.010% or less Both P and S are impurities and are elements that deteriorate the toughness of joints. It is preferable that their contents are as low as possible, with P being 0.020% or less and S being 0.010% or less. Preferably, P is 0.010% or less. Preferably, S is 0.003% or less.

Al: 0.100% or less Al is usually used as a deoxidizer and is an element contained in steel materials. If the Al content is too high, the amount of Al-based nonmetallic inclusions increases, the cleanliness of the steel material decreases, and the toughness deteriorates, so the content is set to 0.100% or less.

Ti: 0.005 to 0.030%
Ti forms fine TiN in steel, and the TiN alone or in combination with Mg (MgAl ₂ O ₄ ) oxide acts as pinning particles. As a result, the coarsening of austenite grains in the HAZ is suppressed, the microstructure is refined, and low-temperature toughness is improved. To achieve this effect, Ti is contained at 0.005% or more. If the Ti content is too high, Ti oxides will aggregate and coarsen, deteriorating toughness, so the Ti content is set to 0.030% or less. The Ti content is preferably 0.008 to 0.020%.

N: 0.0020 to 0.0060%
N is an element that combines with Ti to form TiN, and is contained at 0.0020% or more. If the N content is large, the dissolved N that does not combine with Ti reduces toughness, so the N content is set to 0.0060% or less, preferably 0.0030 to 0.0050%.

O: 0.0050% or less O is an element that forms pinning particles. However, since the cleanliness of the steel decreases when O is contained, the less O the better, and the content is set to 0.0050% or less. Preferably, the content is 0.0030% or less.

Ca: 0 to 0.0050%
Ca is an element that controls the morphology of sulfide-based inclusions and improves low-temperature toughness. If the Ca content is large, CaO-CaS may become large clusters or inclusions, which may adversely affect toughness. Ca does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable Ca content is 0 to 0.0050%.

Ni: 0 to 0.50%
Ni is an element that can improve the strength of the base material without reducing the toughness. When the Ni content is large, the effect is saturated. Ni does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable Ni content is 0 to 0.50%.

Cr: 0 to 0.50%
Cr is an element that can improve the strength of the base material. When the Cr content is large, the effect is saturated. Cr does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable Cr content is 0 to 0.50%.

Cu: 0 to 0.50%
Cu is an element that can improve the strength of the base material. When the Cu content is large, the effect is saturated. Cu does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable Cu content is 0 to 0.50%.

Mo: 0 to 0.50%
Mo is an element that can improve the strength of the base material. If the amount of Mo increases, the effect becomes saturated and furthermore, the toughness decreases. Mo does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable amount of Mo is 0 to 0.50%.

Nb: 0 to 0.100%
Nb is an element that improves the strength of the base material. If the Nb content is large, island martensite is easily formed, and the toughness decreases. Nb does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable Nb content is 0 to 0.100%.

B: 0 to 0.0020%
B is an element effective in improving the hardenability of the base material and suppressing the formation of grain boundary ferrite. When the amount of B increases, the effect becomes saturated. B does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable amount of B is 0 to 0.0020%.

V: 0 to 0.060%
V is an element that improves the strength of the base material. If the V content is large, the yield ratio may increase due to precipitation hardening. V does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable V content is 0 to 0.060%.

Mg: 0 to 0.0100%
Mg is an element that forms inclusions such as MgAl ₂ O ₄ and MgS. MgAl ₂ O ₄ precipitates on TiN. These inclusions act as pinning particles, suppressing the coarsening of austenite grains in the HAZ, refining the microstructure, and improving low-temperature toughness. When the Mg content increases, the effect saturates. Mg does not necessarily need to be contained in the base material of the intermediate bending roll steel pipe, and the suitable Mg content is 0 to 0.0100%.

The remainder, other than what has been explained above, is Fe and impurities. Impurities are components that are contained in the raw materials or mixed in during the manufacturing process, and are not intentionally included in the steel.

Specific examples include P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H. Of these, it is preferable that P, S, and O are controlled to be within the preferred ranges described above.

As for other elements, Sb, Sn, W, Co, and As may be present as unavoidable impurities at levels of 0.1% or less, Pb and Bi at levels of 0.005% or less, and H at levels of 0.0005% or less. However, within normal ranges, there is no need to control them.

An embodiment of the present invention will be described. The conditions in the embodiment are an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

Steel materials with various chemical compositions were melted, and the refined molten steel was turned into slabs using the continuous casting method. After heating to 1200°C, the slabs were hot-rolled to produce steel plates with thicknesses of 40 to 100 mm, with a pre-hot-rolling finish temperature of 1000°C and a coiling temperature of 500 to 700°C. Table 1 shows the thickness, chemical composition, and tensile strength of the steel plates.

Next, the steel plates were roll-formed, the butt joints were tack welded, and then welded in the longitudinal direction. Then, intermediate circumferential welding was performed with a single electrode or two electrodes with a heat input in the range of 5-60 kJ/mm. For circumferential welding, single-electrode submerged arc welding was performed on the butt joints of steel pipes with V- or K-shaped grooves. The interpass temperature was controlled in the range of 100-250°C. The composition of the obtained weld metal is shown in Table 2.

After submerged arc welding, the area ratio (%) of the weld metal structure (total of acicular ferrite, grain boundary ferrite and island martensite), the EBSD grain size of the weld metal, the tensile strength of the weld metal, and the absorbed energy in the Charpy impact test were measured.

The results are shown in Table 3. The AF rate, GBF rate, and MA rate in Table 3 respectively indicate the area ratios of acicular ferrite, grain boundary ferrite, and island martensite in the weld metal structure.

The absorbed energy in the Charpy impact test was measured as follows:

As shown in Figure 1, a Charpy test piece was taken from the center of the weld metal part at a position 1/4 mmt from the surface of the steel plate, and a Charpy impact test was performed at -5°C in accordance with JIS Z2242 to measure the absorbed energy. The absorbed energy was calculated by averaging the results of three Charpy impact tests, and values less than 100 J were judged to have poor toughness.

The tissue area ratio was measured as follows:

A test piece was taken from the surface of the second pass, half the width of the weld bead at the t/4 position of the wall thickness, and after polishing, it was subjected to nital etching and Repera etching. The exposed structure was measured with an optical microscope in 10 fields of view for the structure observed in an area of 1000 μm x 1000 μm, and the obtained images were subjected to image analysis, and the average area ratio of each structure was calculated.

The EBSD grain size was calculated by EBSD analysis of 20 fields of view in an area of 500 μm x 500 μm, and the average grain size was calculated when the crystal orientation difference was divided by 15°.

As shown in Table 3, all of the invention examples that satisfied the weld joint composition of the present invention had a Charpy absorbed energy of 100 J or more at -5°C and had excellent weld metal toughness.

Claims (4)

A joint bending roll steel pipe having two base materials and a circumferential welded portion that joints the two base materials,
The base material is a bending roll steel pipe having a plate thickness of 40 mm or more,
The composition of the weld metal of the circumferential weld is, in mass%,
C: 0.030 to 0.150%,
Si: 0.50% or less,
Mn: 0.50 to 2.00%,
P: 0.020% or less,
S: 0.010% or less,
Cu: more than 0% and 0.50% or less,
Al: 0.0010 to 0.0500%,
Ti: 0.0020% to 0.0500%,
N: 0.0100% or less,
O: 0.0150 to 0.0600%,
and further comprising
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
V: 0 to 0.050%,
Nb: 0 to 0.050%,
B: 0 to 0.0100%,
Mg: 0 to 0.010%, and
Ca: 0 to 0.0060%
(excluding the case where the contents of Ni, Cr, Mo, V, Nb, B, Mg, and Ca are all 0), with the balance being Fe and impurities;
0.300≦Al/O≦1.000 is satisfied,
α' defined as α' = (1.5 × (O-0.89Al) + 3.4 × N-Ti) × 1000 is 0% or more and 60.0% or less,
Ceq defined as C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 is 0.350% or more and 0.500% or less,
A joint bending roll steel pipe characterized in that Pcm, defined as Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B, is 0.30% or less.
Here, the element symbols in the above formula indicate the content (mass %) of each element. The intermediate bending roll steel pipe according to claim 1, characterized in that the structure of the weld metal contains, in terms of area ratio, acicular ferrite: 75.0% or more, grain boundary ferrite: 15.0% or less, and island martensite: 3.0% or less , and an EBSD grain size, which is the average of crystal grain sizes when divided at crystal orientation differences of 15° in 20 fields of view EBSD analysis in an area of 500 μm × 500 μm, is 10.0 μm or less. The intermediate bending roll steel pipe according to claim 1 or 2 , characterized in that the toughness of the circumferential welded portion at -5°C in a Charpy impact test is 100 J or more. The composition of the base material is, in mass%,
C: 0.030 to 0.150%,
Si: 0.55% or less,
Mn: 0.50 to 2.00%,
P: 0.020% or less,
S: 0.010% or less,
Al: 0.100% or less,
Ti: 0.005 to 0.030%,
N: 0.0020 to 0.0060%,
O: 0.0050% or less, and further,
Ca: 0 to 0.0050%,
Ni: 0 to 0.50%,
Cr: 0 to 0.50%,
Cu: 0 to 0.50%,
Mo: 0 to 0.50%,
Nb: 0 to 0.100%,
B: 0 to 0.0020%,
V: 0 to 0.060%, and
Mg: 0 to 0.0100%
Contains one or more selected from the group consisting of (excluding the case where the contents of Ca, Ni, Cr, Cu, Mo, Nb, B, V, and Mg are all 0);
The intermediate bending roll steel pipe according to any one of claims 1 to 3 , characterized in that it contains 0.05% by weight of the steel, and the balance is Fe and impurities.