JPS6268694A - Production of heat-treated steel pipe - Google Patents

Abstract

PURPOSE:To obtain a steel pipe having high strength in a weld zone and excellent low-temp. toughness by limiting a steel pipe stock, welding wire, welding flux and heat treatment conditions in the stage of making the pipe. CONSTITUTION:A steel plate consisting, by weight, of 0.05-0.15% C, <=0.4% Si, 0.5-2.5% Mn, 0.5-9.5% Ni, <=0.1% Al and the balance Fe and inevitable impurities is used. The steel plate is made into the pipe by submerged arc welding using the welding wire consisting, by weight, of 0.05-0.15% C, <=0.2% Si, 0.5-2.5% Mn, <=11.5% Ni, <=0.2% Al and the balance Fe and impurities and the flux having >=1.5 basicity Bx determined by a mathematical expression, contg. 6-60% CaF2 by the weight of the flux except the CO2 component and contg. the gaseous CO2 component of the carbonate ore in the flux at <=5% of the weight of the flux except the gaseous CO2 component in terms of the weight of the gaseous CO2. The steel pipe is then heated for 1-30min at Ac3-1,050 deg.C and is then cooled; thereafter the steel pipe is tempered for 5-30min at 500 deg.C - the temp. just below the Ac1.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for producing a heat-treated steel pipe that provides necessary strength and toughness by heat-treating a steel pipe produced by V'i latent arc welding.

(Conventional technology) Recently, due to the depletion of energy resources, the developed areas are
The trend toward polarization and deepening of the ocean is remarkable, and the characteristics required of the steel pipes used are becoming even higher in toughness and strength.

From the standpoint of safety, this requirement cannot be met by the conventional as-welded or stress-relief annealing methods. Conventionally, as a technique for achieving high toughness by heat treating welded steel pipes, Japanese Patent Application Laid-Open No. 50-33946,
In Japanese Patent No. 56-77334, etc., lowering the oxygen content of weld metal is proposed as an effective method for increasing toughness. these are,
This is intended to prevent a decrease in toughness due to an increase in the amount of oxide inclusions and an upper bainite structure when the amount of oxygen increases, by reducing the oxygen content of the weld metal.

However, sufficient hardenability cannot be obtained only by reducing oxygen, and there are also problems with flux workability (defect resistance). On the other hand, Japanese Patent Application Laid-Open No. 59-4993 proposes a technique that uses a large amount of alloying elements, claiming that even if the amount of oxygen in the weld metal is large, high toughness can be obtained as long as the hardenability is sufficient. However, since the alloy components are large, there are problems in terms of low temperature cracking with hydrogen and the performance of the weld metal that can be achieved. Furthermore, in Japanese Patent Publication No. 60-28885, the austenitizing temperature is changed depending on the carbon equivalent of the weld metal, and quenching is performed.
It is proposed that if this is carried out, a weld metal with excellent toughness after heat treatment can be obtained.

Since the as-welded microstructure of this weld metal is that of so-called acicular ferrite, it cannot be applied to weld metals with a tensile strength of 70 kg/m or more, which is the object of the present invention. High toughness could not be obtained at quenching temperatures below the quenching temperature where ferrite and austenite coexist.In this way, with conventional technology, there are issues with weld metal performance, low-temperature cracking, flux workability, etc. Ta.

(Problem to be solved by the invention) The invention is to reduce the amount of oxygen in the weld metal and at the same time reduce the amount of Nl.
, At, and B to provide a method for manufacturing a high-tensile steel pipe in which the tensile strength of weld metal is 70 kgf/rxti2 or more.

(Means for Solving the Problems) As a result of various studies regarding the manufacture of this steel pipe, the present inventors decided to limit the steel pipe material, welding wire, welding flux, and their heat treatment conditions during pipe manufacturing bath welding. It was discovered that high strength and high toughness steel pipes can be effectively obtained by using C: 0.05 to 0.15% by weight.
, Si: 0.4% or less.

Mn: 0.5-0.15%, Nl
: 0.5 to 9.5%, At: 0.1% or less, and if necessary, B: 0.0020% or less, MQ: 1.0% or less, Cr: Q, 6% or less,
'rl: A steel plate containing 0.06% or less of one or more types, with the balance being Fe and unavoidable impurities.
05-0.15%, S1: 0.2% or less, Mn:
Q, 5-0.15 qb, Ni: 11.5 qb or less,
At: 0.2% or less.

Further, if necessary, Mo: 1.0%
below.

A welding wire containing one or more of Cr: 0.6% or less, T1: 0.06% or less, and the balance consisting of Fe and unavoidable impurities, and a basicity Bx given by the following formula is 1.5 or more. , and contains CaF'z: 6 to 60% with respect to the flux weight excluding CO2 gas components, and the CO2 gas component of the carbonate mineral in the slack is converted to the amount of CO□ gas, excluding the CO2 gas component. A steel pipe made by submerged arc welding using a flux containing 5% or less of the flux weight is heated at hcs to 1050°C for 1 to 30 minutes and then cooled to 500°C to ka.

The method of manufacturing a heat-treated steel pipe includes tempering the pipe for 5 to 30 minutes at a temperature just below that of the present invention.

However, Bx=6゜50 NB10 + 6.05 N(
a□ + 4.8 NMaO+ 4.0 NMIO+
3.4 NF, O+ 5. I NC&F2+ 0.3
NZrO2-0,2Nht2o3-2.2 NTiO
The amount of 2-6,3Ns is 02, and Nk is the mole fraction of the component.

The present invention will be explained in detail below.

(Function) The steel pipe targeted by the present invention has a tensile strength of 70 kg/+
It has a high strength of more than m''.First, we will explain the reasons for limiting the composition of the steel plate that is the material for the steel pipe.

First, if C is less than 0.05%, hardenability is insufficient and the desired hardness cannot be obtained. However, if it exceeds 0.015%, low temperature toughness decreases. From this point of view, the amount of C in the steel plate is limited to 0.05 to 0.15%.

Next, Si is added to the steel sheet for the purpose of deoxidizing and ensuring strength. However, if the content exceeds 0.4%, it is harmful to the toughness, and this also applies when it is transferred to the weld metal, so the upper limit of the amount of 31 in the steel sheet is set at 0.4%.

Furthermore, Mn is an element commonly added to obtain strength, but if it is less than 0.5%, the strength will be insufficient, and if it exceeds 0.15%, the low temperature toughness will decrease. The same holds true when moving to weld metal. Therefore, the amount of 0Mn in the steel sheet is 0.5 to 0.
It shall be within the range of 15%.

Furthermore, the reason why the range for Nl is set to 0.5 to 9.5% is that the present invention is based on HT7 containing about 0.5% or more of Nl.
This is because the targets range from high-strength materials with a strength of 0 or more to extremely low-temperature (-196° C.) materials containing approximately 9.5% Ni.

AA reduces inclusions in the base metal, increases the hardenability of B, and forms nitrides, contributing to microstructural refinement, so it is 0.1
% is added. However, if it exceeds 0.1%, the toughness deteriorates. Therefore, it is limited to 0.1% or less.

The above is the basic component system of the steel plate, but in addition to this, B, Mo, and Cr are added to improve the strength, toughness, etc. of the steel plate.

One or more types of Ti can be added.

First, B is an element that increases the hardenability of steel when added in small amounts in combination with At, which is added from the wire during welding, but if it exceeds 0.002%, it is unnecessary and actually affects the hardenability. It is harmful. Therefore, it is limited to 0.002% or less.

Furthermore, Mo is added for the purpose of ensuring the strength of low-alloy high-strength steel sheets with HT70 or higher, but addition of more than 1.0 Mo leads to a decrease in toughness. Therefore, i is limited to 0% or less.

Further, Cr is sometimes added for the same purpose as Mo, but in this case, if it exceeds 0.6%, the toughness decreases, so it is limited to 0.6% or less.

Ti is an element that increases the tensile strength and toughness of steel sheet materials when added in small amounts, but it forms nitrides, carbides, or carbonitrides and deteriorates toughness, so it is necessary to limit the amount to an extremely small amount. It should be 0.06% or less.

The above are the reasons for limitations regarding the composition of steel sheets. Next, the reason for determining the composition of the welding wire as described above will be described.

If C is less than 0.05%, the weld metal obtained when welding the above-mentioned steel plates may have a Ci of 0.03%, resulting in insufficient hardenability and the desired strength cannot be obtained. do not have. On the other hand, if the Ci of the wire exceeds 0.15%, the C% may exceed 0.15% when transferred to the weld metal, resulting in decreased low-temperature toughness. For the above reasons, the Ci content of the welding wire was set to 0.05 to 0.15%.

Regarding Si, when welding is performed in combination with the above-mentioned steel plates, if the Si content of the wire exceeds 0.2%, the 81st grade of the weld metal may exceed 0.4%, which adversely affects the toughness. Therefore, the slB of the wire was set to 0.2% or less.

Regarding Mn, as in the case of steel sheets, if less than 0.05 mm is added, the required strength may not be obtained;
．． Addition of more than 15 modulus lowers low temperature toughness. Therefore, the range of Mn in the wire was set to 0.5 to 0.15%.

Furthermore, regarding Ni, the steel to be welded is Ntz9. s% or less, it is difficult to obtain a matching weld metal and to harden the weld metal.
Considering that the strength is slightly inferior to that of the base material, the N of the wire is
The upper limit of the liter amount was set to 11.5 inches.

Next, regarding kl, At has the effect of fixing free N as AlN during heat treatment after welding and the effect of suppressing coarsening of austenite grains. In order for the target steel type in the present invention to have a strength of 70 kg/llllm2 or more,
The structure after the austenite-ferrite transformation becomes a lower bainite + martensite structure. Therefore, when the heat treatment temperature after welding becomes high, the grains of the weld metal become noticeably coarser and the toughness decreases.

In addition, since At is the element with the strongest deoxidizing power in the weld metal, it plays a role of protecting B, which is added in a small amount from the steel plate side to improve hardenability. To avoid this, it is necessary to add up to 0.2% At into the welding wire. However, if it exceeds 0.2%, the amount of ALN transferred into the weld metal increases, leading to excessive precipitation of ALN and deterioration of toughness. Furthermore, if the welded metal is left unwelded, the toughness of the welded metal will be significantly reduced, and there is a risk that it will break during handling until heat treatment.

The above is the basic component system of the wire, but in addition to this, in order to improve the strength, toughness, etc. of the weld metal, Mo, Cr, Ti
One or more of these can be added to the wire.

First, Mo is an element necessary to obtain the strength of the weld metal like a steel plate, but when the modulus exceeds 1.0, the toughness decreases. Therefore, the Mo content of the welding wire is set to be 1.0 or less.

Next, Cr is the same as MO, and if it exceeds 0.6%, the toughness decreases, so the upper limit of the amount added is 0.6%.
%.

Furthermore, TI is used for the purpose of grain refinement, but if it exceeds 0.06 inch, the toughness decreases and the hardness increases significantly.
06 section or below.

The above is the composition of the welding wire. Although impurity elements are not particularly specified, the following can be said regarding the properties of the weld metal obtained by the above combination of steel and wire.

First, the lower the oxygen content, the better the toughness of the weld metal after heat treatment in the austenitic region. Figure 1 shows a tensile strength of 70
Using kl/m" grade steel, corresponding welding wires, and buried welding fluxes with greatly different basicities, the heat input was 5.
This figure shows the relationship between the oxygen content of the weld metal and the toughness (%5 value) at -50°C when submerged arc welding is performed at 3 kJ/crn. oxygen -12001111]1 and beyond that, @
It can be seen that the toughness is drastically reduced. Also, the amount of oxygen is 2
If it exceeds 00p1m, the toughness cannot be improved even if At is added. Therefore, the amount of oxygen in the bath metal is 200 pI.
In order to keep it below n, it is necessary to increase the basicity of the welding flux, which determines the amount of oxygen in the weld metal, as described later. Note that the amount of oxygen in the steel plate and wire has little effect on the amount of oxygen in the weld metal. However, P and S are elements that are inevitably mixed as impurity elements, and it is known that the lower the amount, the better the low-temperature toughness, but it is sufficient if the amount is 0.015 or less.

Furthermore, the lower the N content, the higher the toughness, but since kt is added from the wire during welding,
If the amount of N in the steel plate is about 1004 or less, the N will be fixed in the form of AtN in the weld metal and will suppress the coarsening of austenite grains, so there will not be much of an adverse effect.

Regarding hydrogen, since the weld metal targeted by the present invention has a high tensile strength of 70yuf 7mm” or more, H
If the content is high, cold cracking is likely to occur in the weld metal or the weld heat affected zone around it, and it is necessary to reduce this. Diffusible hydrogen amount (measured according to JIS Z-3116)
If it is ICC/100g or less, the occurrence of cold cracking can be prevented without requiring excessive preheating or postheating.

Next, the welding flux used in the present invention will be described.

As mentioned above in the case of Figure 1, it is effective to reduce the amount of oxygen in the weld metal to 200 p- or less in order to obtain the toughness of the heat-treated weld metal to vTra (-50℃). When welding, the amount of oxygen can be reduced if a highly basic flux is used. Figure 2 shows the welding heat input using 1.5% Mn wire and molten flux with different basicities on mild steel. 50kJ/
This figure shows the relationship between basicity BX and the amount of oxygen in the weld metal when submerged arc welding is performed at m, but as seen in the figure, when the amount of oxygen in the weld metal is 200 or less, In order to achieve this, it is necessary to set the basicity of the flux to 1.5 or more.

However, in general, increasing the basicity of flux makes welding defects such as deterioration of bead shape, slag entrainment, and undercuts more likely to occur, and as the amount of oxygen in the weld metal decreases, nitrogen Because of the problem of increased oxygen content, conventionally the limit was to reduce the oxygen content of the weld metal by about 300 ml. As a result of considering the above-mentioned problems regarding such highly basic fluxes, we found that by using CaFz 'E-process carbonate, sound welding can be achieved without increasing the amount of oxygen, nitrogen, and hydrogen in the weld metal. It has been found to be particularly effective in obtaining metals.

That is, s Cm)'2 is an effective component for making the flux highly basic. It is also an effective component when blended in an appropriate amount since it lowers the viscosity and softened melting temperature of slacks. Furthermore, CaF2 stabilizes the shape of the melt penetration,
Prevents slag entrainment. It also reduces the amount of nitrogen and hydrogen in the weld metal. For these purposes, especially to prevent slag entrainment, it is necessary to
2 amounts of a F are required. This CaF2 can also reduce oxygen and hydrogen in the weld metal, but CaF2
When 2 exceeds 60, pockmarks and herrings begin to occur on the bead surface due to the gasification of CaF2. Therefore, the amount of CaF Z is 6.0% or more, 6
It is necessary to keep it below 0%. Furthermore, increasing the amount of CO2 in the flux is effective in reducing nitrogen and hydrogen in the weld metal. However, as the amount of CO□ increases, the amount of oxygen in the weld metal also increases, so the amount of oxygen should be reduced to 200p.
If the amount of CO2 exceeds 5% of the amount of flux X, the object of the present invention cannot be achieved.

As mentioned above, when the oxygen in the weld metal is less than 200p due to CaF2 and carbonate in the flux, nitrogen is reduced to 20p.
The amount of diffusible hydrogen can be controlled from Icc/100g to 100g or less.

In addition, as a CO2 gas component source, CaCO32MgCO
3゜(Ca, Mg)CO3. BaCO3, MnC
O5, FeCO3, Na2CO3, etc., but among these, natural MgCO5 and (Ca, Mg)COx contain water in their crystals, BaCO5 is toxic, and MnC0+
FeCO3 significantly increases the amount of oxygen in the weld metal and
JL 2 CO3 has problems such as hygroscopicity, so C
The use of aCO3 is preferred.

The above-mentioned CaF2 and CO2 gas source components are the basic components of the flux used, but in addition to these, in consideration of bead shape control and prevention of various welding defects, other main components include CaO. , MgO, A
When t203.8102 is blended and the other four components are taken as 100, CaO: 10 to 60%, MgO: 30 or less,
At203: 20 to 70%, 8102: 3 to 40%, and the total amount of 7n, etc. is preferably 35% or more of the total weight of the flux excluding CO2.

Finally, the heat treatment conditions for steel pipes welded by submerged arc welding using the steel materials and welding materials described above will be described.

In order to obtain a bath-welded metal with high toughness, it is not sufficient to limit the components of the A plate and the welding material alone; it is necessary to limit the heat treatment conditions. In the present invention, high toughness is achieved through the use of lower bainite and tempered martensite, so a faster quenching rate is desirable. However, as long as the steel material and welding material specified in the present invention are used, cooling by water cooling that is normally performed is sufficient. Next, the present inventors obtained 70 kg/mn as shown in Table 1 specified in the present invention.
Class 2 steel materials and welding wires in Table 5 and F in Table 4
The welded part was submerged arc welded using a highly basic flux under the welding conditions shown in Table 2.
After performing heat treatment while changing the temperature between 00°C, a test piece was taken from the center of the plate thickness, and the impact value at -50°C was determined. In addition, the composition of the weld metal is shown in flat 23 of Table 6. The relationship between the toughness thus obtained and the quenching temperature is shown in FIG. As is clear from the figure, approximately 820
v'FJ-sac is 10 kg- in the range of ~1050℃
High toughness of m or more can be obtained. The Ac3 point of this weld metal was 822°C, indicating that it was necessary to heat the weld metal above the austenite temperature, so the lower limit of the quenching temperature was set as Ac3. On the other hand, the upper limit is 10 because this weld metal is intended to use tempered martensite, does not use ferrite structure, and is thought to have little dependence on austenite grain size.
The temperature was 50°C. From the above points, the austenitizing temperature is A
c3 to 1050°C.

As for the heating time, there is no point in making it longer than 30 minutes.
Rather, the toughness tends to deteriorate due to coarsening of austenite grains, and if it is longer than 1 minute, the complete austenitization time is short (segregating elements such as C and B are not sufficiently uniformed).

1 to 30 minutes.

The reason why the tempering temperature was limited to 500°C to just below Ac is that if it is too low than 500°C, it will take time to obtain the tempering effect, and a sufficient tempering effect will not be obtained. If it exceeds Ae+, softening is significant and the desired strength cannot be obtained, and at the same time, ferrite-austenite transformation occurs locally and high C martensite, which is harmful to toughness, precipitates, resulting in a decrease in toughness.

Furthermore, if the tempering time is shorter than 5 minutes, a sufficient effect will not be obtained, and if it is longer than 30 minutes, carbonitride formation and strength reduction will occur, so the tempering time is limited to 5 to 30 minutes.

The effects of the present invention will be shown below in more detail with reference to Examples.

The heating method is not particularly limited, and may be either continuous heating using high frequency waves or heating using a normal heating furnace method.

(Example) Table 1 shows the components of the steel pipe material. These steel plates are HT7
0. It is equivalent to HT80.9%N1 steel. The welding conditions and heat treatment conditions adopted in this example are shown in Tables 2 and 3.
Shown below. Furthermore, Table 4 compares the components and mechanical properties of the welding flux used, the bath welding wire, and the obtained weld metal with those of the conventional method.

The details are shown in Tables 5 and 6. 1 to 5 in Table 6.
7 to 11 show the results of the conventional method for HT70 to 80 class, but because the combination of steel plate, wire, and flux or heat treatment conditions were not suitable, it was difficult to obtain vTrs (-50℃ toughness) in the weld metal. It has not been done.

On the other hand, in welding by the method of the present invention (Tables CM4 to CM26), the combination of steel plate, wire, and flux and heat treatment conditions are appropriate, so the oxygen content of the weld metal is low, and therefore sufficient hardenability is obtained and vTrs ( -50° C.Although 6 in Table 6 has a sufficient toughness value, welding workability is poor and it cannot be adopted.

Furthermore, 12.13 in Table 6 is 9mm Ni obtained by the conventional method.
% welding results, but the toughness of the weld metal at 196°C is very low. In contrast, the results obtained by the method of the present invention (27 to 29 in Table 6) show that the weld metal exhibits high toughness at 1-t-196°C.

(Effects of the Invention) As is clear from the above examples, according to the method of the present invention, steel pipes for cryogenic pipelines, steel pipes for large offshore structures, and steel pipes for cryogenic use with high weld strength and excellent low-temperature toughness. etc. can be manufactured with high efficiency, and its industrial effects are extremely remarkable.

[Brief explanation of drawings]

Figure 1 shows the relationship between the Charpy impact value (vE-50C) and the amount of oxygen in the weld metal, Figure 2 shows the relationship between the amount of oxygen in the weld metal and the basicity of the bath flux, and Figure 3 The figure is a diagram showing the relationship between Charpy impact value (vE-50C) and quenching temperature. Figure 1: Oxygen content (pprn) in metals
Bx Figure 3

Claims (2)

[Claims] (1) C: 0.05-0.15%, Si: 0.
4% or less, Mn: 0.5-2.5%, Ni: 0.5-9
．． 5%, Al: 0.1% or less, balance Fe and inevitable impurities, C: 0.05-0.15%, S
i: 0.2% or less, Mn: 0.5 to 2.5%, Ni: 1
Welding wire consisting of 1.5% or less, Al: 0.2% or less, balance Fe and unavoidable impurities, basicity Bx given by the following formula is 1.5 or more, and flux weight excluding CO_2 component On the other hand, using a flux containing 6 to 60% of CaF_2 and in which the CO_2 gas component of the carbonate mineral in the flux is 5% or less of the weight of the flux excluding the CO_2 gas component, converted to the amount of CO_2 gas. Ac_3~105 steel pipes made by submerged arc welding
After heating at 0°C for 1 to 30 minutes, cooling, and then heating at 500°C to Ac
A method for producing a heat-treated steel pipe, characterized by tempering at a temperature just below ___1 for 5 to 30 minutes. Bx=6.50N_B_a_O+6.05N_C_a_
O+4.8N_M_n_O+4.0N_M_g_O+3
．． 4N_F_a_O+5.1N_C_a_F__2+0
．． 3N_Z_r_O___2-0.2N_A_l___2O
___3-2.2N_T_i_O___2-6.3N_S_
i_O___2 where Nk is the mole fraction of component k (2) C: 0.05-0.15%, Si: 0.
4% or less, Mn: 0.5-2.5%, Ni: 0.5-9
．． 5%, Al: 0.1% or less, and B: 0.0020%
Below, Mo: 1.0% or less, Cr: 0.6% or less, Ti
A steel plate containing 0.06% or less of one or more types, with the balance consisting of Fe and unavoidable impurities, C: 0.05 to 0.15
%, Si: 0.2% or less, Mn: 0.5 to 2.5%, N
i: 11.5% or less, Al: 0.2% or less, and M
o: 1.0% or less, Cr 0.6% or less, Ti: 0.06
% or less, with the balance consisting of Fe and unavoidable impurities, and a basicity B given by the following formula:
x is 1.5 or more, and contains 6 to 60% of CaF_2 based on the weight of the flux excluding the CO_2 component, and the CO_2 gas component of the carbonate mineral in the flux is CO
A steel pipe made by submerged arc welding using a flux containing 5% or less of the flux weight excluding CO_2 gas components in terms of _2 gas amount is heated at Ac_3 to 1050°C for 1 to 30 minutes and then cooled. , and then tempering at a temperature of 500°C to just below Ac_1 for 5 to 30 minutes. Bx=6.50N_B_a_O+6.05N_C_a_
O+4.8N_M_n_O+4.0N_M_g_O+3
．． 4N_F_e_O+5.1N_C_a_F__2+0
．． 3N_Z_r_O__2-0.2N_A_l__2_
O__3-2.2N_T_i_O__2-6.3N_S
_i_O__2 where Nk is the mole fraction of component k.