JP4585483B2 - High strength steel pipe with excellent weld toughness and deformability and method for producing high strength steel plate - Google Patents

Description

  The present invention is a steel pipe and steel plate having high strength of X100 (yield strength of 690 MPa or more and 840 MPa or less, tensile strength of 760 MPa or more and 990 MPa or less) according to the American Petroleum Institute (API) standard, and excellent weld toughness and deformability. It relates to the manufacturing method.

  Line pipes used for pipelines that transport crude oil and natural gas over long distances are (1) increased in tension due to improved transport efficiency under high pressure and (2) improved local welding efficiency due to thinner walls. Tend to. Up to now, line pipes up to X80 in the API standard have been put into practical use, but there is a need for higher-strength line pipes.

  Currently, high-strength line pipes of X100 or higher are studied based on the manufacturing method of X80 class line pipes (for example, see Non-Patent Document 1 and Non-Patent Document 2), but these line pipes are low-temperature toughness of welds. In particular, there is a problem in terms of HAZ toughness, and an innovative high-strength steel pipe that overcomes these problems is desired. Furthermore, in pipelines laid in discontinuous frozen land zones or earthquake-prone areas, the frozen soil partially melts and freezes, or the pipeline itself is distorted by the earthquake and has a deformability that can prevent the occurrence of ductile cracks. A large steel pipe excellent in safety is desired.

The HAZ toughness of the low alloy steel is as follows: (1) grain size, (2) mixture of martensite and austenite (MA), dispersed state of hardened phase such as upper bainite (Bu), (3) grain boundary It is governed by various metallurgical factors such as the presence or absence of embrittlement and (4) microsegregation of elements. Among them, the size of the HAZ crystal grains is known to have a great influence on the low temperature toughness, and many techniques for refining the HAZ structure have been developed and put into practical use.
For example, means for finely dispersing TiN and improving HAZ toughness during high heat input welding of 490 MPa class high-tensile steel is disclosed (for example, see Non-Patent Document 3). However, since these precipitates are exposed to a high temperature of 1400 ° C. or higher in the vicinity of the melting line, most of them are coarsened or dissolved, and the HAZ structure is coarsened to deteriorate the HAZ toughness.

In order to solve this problem, the Ti oxide is finely dispersed in the steel to generate intragranular acicular ferrite (hereinafter sometimes abbreviated as IGF) in the HAZ at the time of welding. Techniques for reducing the size and improving the HAZ toughness have been proposed (see, for example, Patent Document 1 and Patent Document 2).
However, when the strength is increased, the formation of MA becomes prominent, and the formation of IGF from Ti oxide alone cannot sufficiently refine the structure, and the HAZ toughness deteriorates. Therefore, the HAZ toughness of the X100 line pipe Improvement is strongly desired.

  On the other hand, regarding deformability, for example, Patent Document 3 discloses a steel pipe excellent in anti-buckling characteristics containing 10 to 50% of lower bainite in area fraction. For example, Patent Document 4 has an average aspect ratio. A steel pipe excellent in buckling resistance is disclosed, containing 2 to 15% of island-like martensite in an area fraction of 2 to 15. However, none of these are intended for X100 high-strength steel pipes, and the low-temperature toughness of welds is not satisfactory. Further, when the average aspect ratio is 2 or more, there is a problem that separation is generated on the fracture surface of the DWTT test piece and the absorbed energy in the DWTT test is reduced.

Further, for example, Patent Document 5 and Patent Document 6 disclose line pipes excellent in X60 to X100 grade deformability. However, the techniques described in Patent Document 5 and Patent Document 6 do not consider the deformability after coating heating, and do not consider HAZ toughness.
Japanese Patent Laid-Open No. 63-210235 Japanese Patent Laid-Open No. 1-15321 JP 11-279700 A JP-A-11-343542 JP 2003-293089 A JP 2005-15823 A “NKK technique (No. 138)” Nippon Steel Pipe Co., Ltd., 1992, pp. 24-31 “The 7th offshore Mechanics Arc Engineering”, The American Society of Mechanical Engineers, 198 pp. 179-185 "Iron and Steel" Japan Steel Association, June 1979, Vol. 65, No. 8, p. 1232

  The present invention provides an X100 high-strength steel pipe having good HAZ toughness and excellent deformability and a method for producing a steel sheet as a base material thereof.

The gist of the present invention is as follows.
(1)
In mass%, C: 0.03% to 0.08%, Si: 0.6% or less, Mn: 0.8 to 2.5%, P: 0.015% or less, S: 0.001 to 0 0.005%, Cr: 0.5 to 1.5%, Mo: less than 0.02%, Nb: 0.01 or more and less than 0.025%, Ti: 0.005 to 0.030%, Al: 0.0. 005% or less, N: 0.001 to 0.006%, O: 0.001 to 0.006%, Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2 %, V: 0.01 to 0.1%, B: 0.0003 to 0.002%, Mg: 0.0001 to 0.0050%, Ca: 0.0005 to 0.0050% It contains more than seeds, the balance consists of iron and inevitable impurities, Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V And the base material portion Pb value is in the range of 2.5 to 4.0 as defined,
In mass%, C: 0.035 to 0.08%, Si: 0.6% or less, Mn: 1.5 to 2.5%, P: 0.015% or less, S: 0.005% or less, Ni: 0.2-2.5%, Cr: 0.2-1.5%, Mo: 0.2-1.5%, Nb: 0.01-0.05%, Ti: 0.005- 0.03%, B: 0.0003 to 0.002%, Al: 0.05% or less, N: 0.001 to 0.01%, O: 0.015 to 0.045%, the balance Consisting of iron and inevitable impurities, Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1. The weld metal part having a Pw value defined by 0.27 to 0.35 Nb in the range of 0.2 to 0.35,
When heated to a temperature of 250 ° C. or less, the difference in yield strength in the tensile test in the tube axis direction of the base metal part before and after heating is 70 MPa or less. High strength excellent in weld toughness and deformability Steel pipe.

(2)
The weld metal part is further mass%, Cu: 0.1 to 1.0%, V: 0.01 to 0.1%, Mg: 0.0001 to 0.005%, Ca: 0.0005. The high-strength steel pipe having excellent weld toughness and deformability according to claim 1, comprising one or more of 0.005%.

(3)
The high-strength steel pipe excellent in weld toughness and deformability according to claim 1 or 2, wherein the metal structure of the base metal part contains 5 to 50% of ferrite having a particle size of 10 µm or less.

(4)
3. The welding according to claim 1, wherein the metal structure of the base material portion contains 2 to 7% of a mixture of martensite and austenite (M-A constituent) having an average aspect ratio of less than 2. 4. High strength steel pipe with excellent toughness and deformability.

(5)
In mass%, C: 0.03% to 0.08%, Si: 0.6% or less, Mn: 0.8 to 2.5%, P: 0.015% or less, S: 0.001 to 0 0.005%, Cr: 0.5 to 1.5%, Mo: less than 0.02%, Nb: 0.01 or more and less than 0.025%, Ti: 0.005 to 0.030%, Al: 0.0. 005% or less, N: 0.001 to 0.006%, O: 0.001 to 0.006%, Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2 %, V: 0.01 to 0.1%, B: 0.0003 to 0.002%, Mg: 0.0001 to 0.0050%, Ca: 0.0005 to 0.0050% It contains more than seeds, the balance consists of iron and inevitable impurities, Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V The cast slab Pb value defined in the range of 2.5 to 4.0,
After heating to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, and after rolling in the temperature range of 700 to 850 ° C., the temperature range from 650 to 800 ° C. is 5 ° C./second or more, 15 A method for producing a high-strength steel sheet for high-strength steel pipes excellent in weld toughness and deformability, characterized by cooling to a temperature of 250 ° C. to 400 ° C. at a cooling rate of less than ° C./second and then air cooling.

(6)
In mass%, C: 0.03% to 0.08%, Si: 0.6% or less, Mn: 0.8 to 2.5%, P: 0.015% or less, S: 0.001 to 0 0.005%, Cr: 0.5 to 1.5%, Mo: less than 0.02%, Nb: 0.01 or more and less than 0.025%, Ti: 0.005 to 0.030%, Al: 0.0. 005% or less, N: 0.001 to 0.006%, O: 0.001 to 0.006%, Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2 %, V: 0.01 to 0.1%, B: 0.0003 to 0.002%, Mg: 0.0001 to 0.0050%, Ca: 0.0005 to 0.0050% It contains more than seeds, the balance consists of iron and inevitable impurities, Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V The cast slab Pb value defined in the range of 2.5 to 4.0,
After heating to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, and after rolling in the temperature range of 700 to 850 ° C., the temperature range from 650 to 800 ° C. is 5 ° C./second or more, 15 For high-strength steel pipes with excellent weld toughness and deformability, which are cooled to a temperature of 400 ° C. or less at a cooling rate of less than ° C./second, then heated to 300 ° C. to 450 ° C. and then air-cooled. Manufacturing method of high strength steel sheet.

(7)
In mass%, C: 0.03% to 0.08%, Si: 0.6% or less, Mn: 0.8 to 2.5%, P: 0.015% or less, S: 0.001 to 0 0.005%, Cr: 0.5 to 1.5%, Mo: less than 0.02%, Nb: 0.01 or more and less than 0.025%, Ti: 0.005 to 0.030%, Al: 0.0. 005% or less, N: 0.001 to 0.006%, O: 0.001 to 0.006%, Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2 %, V: 0.01 to 0.1%, B: 0.0003 to 0.002%, Mg: 0.0001 to 0.0050%, Ca: 0.0005 to 0.0050% It contains more than seeds, the balance consists of iron and inevitable impurities, Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V The cast slab Pb value defined in the range of 2.5 to 4.0,
After heating to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, rolling is finished in a temperature range of 700 to 850 ° C., and then cooled to 5 ° C./second or more from a temperature range of 650 to 800 ° C. It is cooled to an arbitrary temperature of 400 ° C. or less at a rate, and then heated to 300 ° C. to 450 ° C. at a heating rate of 10 ° C./second or more. Manufacturing method of high strength steel sheet.

    By adopting a high-strength steel pipe (API standard X100 or higher) having excellent HAZ toughness and high deformability according to the present invention in the pipeline, the safety of the pipeline is remarkably improved and the transportation efficiency is drastically improved. It was.

Below, the manufacturing method of the high-strength steel pipe of this invention and the high-strength steel plate for high-strength steel pipes is demonstrated in detail.
The high-strength steel pipe of the present invention comprises a base metal part based on a low C-Mo free-high Cr-low Nb-Ti-low Al system, and a low C-Mn-Ni-Cr-Mo-B based weld metal. Is an X100 grade high strength steel pipe having good HAZ toughness, large uniform elongation of the base material, and small difference in yield strength before and after heating during coating heating.

Low temperature toughness of low alloy steel is governed by various metallurgical factors such as (1) crystal grain size, (2) dispersion state of hardened phase such as MA and upper bainite (Bu). Among these, HAZ crystal grain size and M-A are known to have a large effect on low-temperature toughness.
It is also known that the aging behavior during coating heating is greatly affected by the amount of solute C, the amount of solute N and the amount of strain (dislocation).

In HAZ of high-strength steel pipe, the amount of alloy element added is increased in order to satisfy the strength, and a large amount of MA that is harmful to low-temperature toughness is generated, so that HAZ toughness deteriorates. Therefore, the present inventor has intensively studied a component system for satisfying the target strength of X100 and reducing the amount of MA produced that is harmful to low temperature toughness.
And while the base material part does not substantially add Mo, the amount of Nb added is less than 0.025%, and by adding 0.5% or more of Cr, the intended strength is obtained. It was found that the amount of M-A produced was greatly reduced. In particular, when the amount of Cr added is large, reducing the amounts of C and Nb is extremely effective in reducing the amount of M-A produced. Furthermore, by generating intragranular ferrite (IGF) from γ grains in HAZ, the size of the MA produced with the refinement of HAZ crystal grains is reduced, so that HAZ toughness can be remarkably improved. Furthermore, the present inventors have found that the increase in yield strength due to aging can be suppressed even when heated to about 250 ° C. during coating by reducing the amount of C and Nb and substantially not adding Mo. The present invention has been reached.

  That is, by making the base material part a component system of low C—Mo free—high Cr—low Nb, sufficient target strength is achieved, and the amount of MA produced in HAZ is reduced, and Ti, By generating IGF from oxides and precipitates containing Mn and S, crystal grains and MA can be refined, so that HAZ toughness can be remarkably improved, and even when heated during coating, It is possible to suppress an increase in yield strength.

First, improvement of HAZ toughness will be described.
Mo is known as an element that improves hardenability and increases strength. However, since the amount of MA produced in HAZ of high-strength steel increases, the amount of Mo added needs to be reduced as much as possible, and it is preferable not to add as much as possible. Considering the amount inevitably mixed, the amount of Mo added to the base material portion needs to be less than 0.02%.

  Addition of Cr is effective to ensure strength without adding Mo. The addition of Cr reduces the amount of M-A produced compared to Mo. Furthermore, by making the C amount 0.08% or less and making the Nb amount less than 0.025%, even when the Cr addition amount is large, the MA production amount is reduced. In order to achieve the target strength, the amount of Cr added to the base material portion needs to be 0.5% or more. However, when there is too much addition amount, field weldability and HAZ toughness will deteriorate remarkably. For this reason, the upper limit of the Cr content is set to 1.5%. Further, the lower limit of 0.03% of the C amount of the base material part is an amount necessary to achieve the target strength. Nb not only suppresses recrystallization of ν during controlled rolling and refines the crystal grains, but also contributes to precipitation hardening and hardenability, and has the effect of strengthening the steel. In order to obtain this effect, it is necessary to add at least 0.01% of Nb.

  By using a complex of oxide and MnS as IGF production nuclei, crystal grains and MA are refined. The base material part contains 0.001 to 0.005% of S, and an oxide mainly composed of Ti, Al, Mg, and Ca and a complex of MnS generates IGF, and M -A and crystal grains are refined, and low temperature toughness is remarkably improved. At this time, when the S content is less than 0.001%, the production amount of MnS effective for the production of IGF is small, and the crystal grains are not refined. If the S content exceeds 0.005%, a large amount of MnS is generated, and the toughness deteriorates. Further, when the Al amount is 0.005% or less, the production of IGF becomes remarkable, so the upper limit of the Al amount is set to 0.005%.

  In order to increase the strength of steel, it is inevitably necessary to increase the amount of alloy element added, but the HAZ toughness deteriorates. Therefore, in order to obtain the target strength without greatly degrading the HAZ toughness, as a result of studying the appropriate amount of alloying elements constituting the base metal part, the value defined by the Pb value is limited to a predetermined range. It was found that the strength can be ensured by doing so. Moreover, about the addition amount of the alloy element in a weld metal part, in order to satisfy the target intensity | strength, without impairing the toughness of a weld metal largely by limiting the value defined by Pw value to a predetermined range. The amount of alloying element added was found.

  Next, improvement of deformability is described. In the pipeline laid in the discontinuous frozen land zone, it is said that about 3% strain is applied to the pipeline due to the melting and freezing of frozen soil. When such a strain is applied, by increasing the uniform elongation of the steel pipe base material and making it a low yield ratio, and further by making the yield strength of the circumferential weld zone higher than the yield strength of the steel pipe base material, Deformability, that is, the limit strain at which ductile cracking occurs increases. For this purpose, it is necessary to reduce the yield strength increase after coating heating to 70 MPa or less.

  Moreover, when using a steel pipe for a pipeline, although it paints from a viewpoint of corrosion prevention, when coating, it heats to about 250 degreeC. In general, a steel pipe heated to 250 ° C. has an increased yield strength after heating due to aging, and becomes larger than the yield strength of a circumferential weld, and its deformability is reduced. Further, when the SS curve of the tensile test shows a yield point due to aging, the buckling resistance limit strain becomes small, and the deformability decreases. Therefore, there is a need for a steel pipe that is small in aging even after being heated to about 250 ° C. Deformability can be maintained by setting the yield strength increase after coating heating to 70 MPa or less.

  In order to suppress the increase in yield strength to 70 MPa or less even after the coating is heated and to have a large deformability, it contains 5 to 50% of ferrite of 10 μm or less, or the metal structure of the base material part has an average aspect ratio of 2 It is desirable to contain 2-7% of less MA.

Moreover, this inventor discovered the following three methods as a manufacturing method of the high strength steel plate for high strength steel pipes, and came to this invention.
(1) After heating the slab to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, and rolling is finished in the temperature range of 700 to 850 ° C., and then from the temperature range of 650 to 800 ° C. A method of cooling to a temperature of 250 ° C. to 400 ° C. at a cooling rate of not less than 15 ° C./second and then cooling with air.

(2) After heating the slab to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, rolling is finished in a temperature range of 700 to 850 ° C., and then from the temperature range of 650 to 800 ° C. A method of cooling to 400 ° C. or lower at a cooling rate of at least 15 ° C./second and then heating to 300 ° C. to 450 ° C., followed by air cooling.
(3) After heating the slab to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, and rolling is finished in the temperature range of 700 to 850 ° C., and then from the temperature range of 650 to 800 ° C. A method of cooling to an arbitrary temperature of 400 ° C. or lower at a cooling rate of at least C ° C./second and then heating to 300 ° C. to 450 ° C. at a heating rate of 10 ° C./second or higher.

  In other words, the feature of the present invention is that it ensures good HAZ toughness by the component of the base material part of low C-Mo free-high Cr-low Nb-Ti-low Al system, and further ensures the deformability after heating the paint. There is to do. In addition, when applying the basic component system of the base material part, in order to ensure the target strength, the alloy element addition amount is limited to an appropriate range defined by the Pb value, and as a weld metal part, toughness In order to satisfy the target strength without impairing the deterioration of the alloy, the alloy element addition amount is limited to an appropriate range defined by Pw. Further, in order to ensure excellent deformability, the metal structure of the base material part contains 5 to 50% of ferrite having a particle size of 10 μm or less, and the metal structure of the base material part is composed of martensite and austenite having an average aspect ratio of less than 2. The purpose is to contain 2 to 7% of the mixture (MA constituent).

Below, the other component limitation reason of the base material part of a high-strength steel pipe is demonstrated.
Si is an element added for deoxidation and strength improvement, but if added in large amounts, the field weldability and the HAZ toughness deteriorate, so the upper limit was made 0.6%. For the deoxidation of steel, Ti alone is sufficient, and Si does not necessarily have to be added.
Mn is an indispensable element for securing strength and low temperature toughness, and its lower limit is 0.8%. However, if Mn is too much, not only the hardenability of the steel is increased and the on-site weldability and HAZ toughness are deteriorated, but also the center segregation of continuously cast steel pieces is promoted and the low temperature toughness is also deteriorated. 5%.

In the present invention, the amount of P which is an inevitable impurity is set to 0.015% or less. The main reason is to further improve the low temperature toughness of the base material and the HAZ. The reduction of the P content reduces the center segregation of the continuously cast slab, prevents the grain boundary fracture and improves the low temperature toughness.
Ti forms fine TiN, suppresses coarsening of ν grains of HAZ during reheating of the slab, refines the microstructure, and improves the low temperature toughness of the base material and HAZ. In addition, an oxide containing Ti is formed, and functions as an IGF production nucleus due to a combined effect with precipitation of MnS. In order to exhibit this function, addition of 0.005% or more is necessary. On the other hand, if it is too much, coarsening of TiN and precipitation hardening due to TiC occur and the low temperature toughness is deteriorated, so the upper limit value is limited to 0.030%.

N forms TiN and suppresses coarsening of ν grains of HAZ during reheating of the slab and improves the low temperature toughness of the base material and HAZ. The minimum amount required for this is 0.001%. However, if the amount of N is too large, HAZ toughness is deteriorated due to slab surface flaws or solute N, so the upper limit value must be limited to 0.006%.
O forms an oxide containing Ti, Mg, Ca, etc., and functions as an IGF transformation nucleus in HAZ. In order to exhibit these functions, 0.001% or more of O is necessary. However, if O exceeds 0.006%, a coarse oxide exceeding 10 μm is generated, which becomes a point of occurrence of brittle fracture in the base material or HAZ, so 0.006% was made the upper limit value.

  Next, the reason for adding Ni, Cu, V, B, Mg, and Ca will be described. The main purpose of adding these elements to the basic component is to improve the properties such as strength and low temperature toughness without impairing the characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be restricted by itself.

Ni improves the strength and low temperature toughness of the base material without adversely affecting weldability and HAZ toughness, but the effect is small at 0.1% or less. Moreover, since addition of 1.0% or more is not preferable for weldability, the upper limit value is set to 1.0%.
Cu has substantially the same effect as Ni, and is also effective in corrosion resistance, resistance to hydrogen-induced cracking, and the like, and it is necessary to add 0.1% or more. However, when added excessively, Cu-cracks are generated during precipitation hardening due to precipitation hardening and HAZ toughness, and the upper limit value was set to 1.2%.

V has substantially the same effect as Nb, but the effect is much weaker than Nb. In order to exhibit the effect, addition of 0.01% or more is necessary. Further, the upper limit is allowable up to 0.1% from the viewpoint of on-site weldability and HAZ toughness.
B is a very small amount, which dramatically enhances the hardenability of the steel and provides good strength and toughness. In order to exert this effect, 0.0003% or more must be added. Moreover, since HAZ toughness will deteriorate when there is too much, the upper limit was limited to 0.002%.

Mg forms fine Mg oxide, and TiN is complex-precipitated using this oxide as a nucleus, so that an excellent pinning effect of γ grains can be maintained even at a high temperature of 1400 ° C. or higher. In order to generate this TiN, 0.0001% or more of Mg needs to be added. If the amount of Mg added is too large, Mg-based oxides increase and low temperature toughness deteriorates, so the upper limit was limited to 0.0050%.
Ca controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test), and also exhibits a remarkable effect in improving sour resistance. If less than 0.0005%, the effect is weak, and if added over 0.0050%, a large amount of CaO-CaS is formed to form clusters and large inclusions, not only detracting from the cleanliness of the steel, but also on-site weldability. It also has an adverse effect. For this reason, the amount of Ca added is limited to 0.0005 to 0.0050%.

  Furthermore, in order to satisfy the target strength of X100 or more, it is necessary to optimize the addition amount of the alloy element constituting the base material part. That is, the Pb value defined by the equation Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V must be in the range of 2.5 to 4.0. If the Pb value is less than 2.5, the target strength of X100 cannot be secured. On the other hand, when the Pb value exceeds 4.0, the formation of MA becomes remarkable and the HAZ toughness deteriorates. For this reason, the range of Pb value was limited to 2.5-4.0.

Next, the reasons for limiting the components of the weld metal part will be described.
In order to prevent hot cracking of the weld metal, the C content needs to be 0.035% or more. If it is less than 0.035%, δ solidification occurs in the process of solidification after welding, and hot cracking occurs. However, if the C content exceeds 0.08%, the low temperature toughness of the weld metal deteriorates, so the upper limit value was made 0.08%.
Si is an element added for deoxidation and strength improvement, but if added in a large amount, the low temperature toughness and on-site weldability deteriorate, so the upper limit was made 0.6%.

Mn is an essential element for securing strength and low temperature toughness, and its lower limit is 1.5%. However, too much Mn increases the hardenability of the steel and degrades low temperature toughness and on-site weldability, so the upper limit was made 2.5%.
The purpose of adding Ni is to increase the strength without deteriorating the low temperature toughness and on-site weldability. However, if the addition amount is too large, not only the economy but also the low temperature toughness is deteriorated, so the upper limit was made 2.5% and the lower limit was made 0.2%.

Cr increases the strength, but if it is too much, the low temperature toughness and on-site weldability deteriorate significantly. For this reason, the upper limit of the Cr content is set to 1.5% and the lower limit is set to 0.2%.
The reason for adding Mo is to improve the hardenability of the steel. In order to obtain this effect, Mo needs to be at least 0.2%. However, excessive addition of Mo deteriorates low temperature toughness and on-site weldability, so the upper limit was made 1.5%.
Nb has an effect of strengthening steel and needs to be 0.01% or more. However, adding Nb in excess of 0.05% adversely affects on-site weldability and low temperature toughness, so the upper limit was made 0.05%.

Ti addition forms fine TiN and improves low temperature toughness. In order to exhibit such an effect of TiN, it is necessary to add at least 0.005% Ti. However, if the amount of Ti is too large, TiN coarsening and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit must be limited to 0.03%.
B is an element that greatly increases the hardenability of steel in a very small amount. In order to obtain such an effect, B must be at least 0.0003%. On the other hand, if added excessively, not only the low temperature toughness is deteriorated, but also the effect of improving the hardenability of B may be lost, so the upper limit was made 0.002%.

Al usually has an effect as a deoxidizing element. However, if the Al content exceeds 0.05%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.05%.
N forms TiN and improves low temperature toughness. The minimum amount required for this is 0.001%. However, if the amount is too large, the low temperature toughness is deteriorated, so the upper limit must be suppressed to 0.01%.
O forms an oxide in the weld metal, acts as a nucleus of intragranular transformed ferrite, and is effective in refining the structure. However, if the amount is too large, the low temperature toughness of the weld metal deteriorates and welding defects such as slag entrainment occur. For this reason, the lower limit of the amount of O is set to 0.015%, and the upper limit is set to 0.045%.

  Further, in the present invention, the amounts of impurity elements P and S are set to 0.015% or less and 0.005% or less, respectively. The main reason is to further improve the low temperature toughness. Reduction of the P content prevents grain boundary fracture and improves low temperature toughness. Moreover, reduction of the amount of S has the effect of reducing MnS and improving ductility.

Next, the reason why Cu, V, Mg, and Ca are added to the weld metal portion will be described.
The main purpose of adding these elements as necessary to the basic components of the weld metal part is to improve the properties of the weld metal part, such as strength and low temperature toughness, without impairing the excellent characteristics of the steel of the present invention. It is for measuring. Therefore, the amount of addition is a property that should be restricted by itself.
Cu, like Ni, increases strength without deteriorating low-temperature toughness and on-site weldability. However, if added in excess, the low temperature toughness deteriorates, so the upper limit was made 1.0%. The lower limit of 0.1% of Cu is the minimum value at which the effect on the material due to addition becomes remarkable.

V has almost the same effect as Nb, but the effect is weaker than that of Nb. V causes strain-induced precipitation and increases the strength. The lower limit is 0.01%, and the upper limit is acceptable up to 0.1% from the viewpoint of on-site weldability and low temperature toughness.
Mg controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test). However, if the amount of Mg is 0.0001% or less, there is no practical effect, and if added over 0.005%, coarse Mg oxide is generated and weld defects are generated. Therefore, the Mg addition amount is limited to 0.0001 to 0.005%.

  Ca, like Mg, controls the form of sulfide (MnS) and improves low-temperature toughness (increased absorbed energy in the Charpy test, etc.). However, when the Ca content is 0.0005% or less, there is no practical effect, and when it is added over 0.005%, a large amount of CaO—CaS is generated and weld defects are generated. For this reason, Ca addition amount was limited to 0.0005 to 0.005%.

  Furthermore, in order to satisfy the strength of X100 or more also in the weld metal part, it is necessary to optimize the addition amount of the alloy element. That is, the Pw value defined by Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb must be limited to a range of 0.2 to 0.35. When the Pw value is less than 0.2, the weld strength of X100 or more cannot be ensured. On the other hand, if the Pw value exceeds 0.35, the formation of MA becomes remarkable, the toughness is deteriorated, and low temperature cracking occurs. For this reason, the range of Pw value was limited to 0.2-0.35.

Next, the reason for limitation for obtaining high deformability will be described below.
In a pipeline laid in a discontinuous frozen land zone, a strain of about 3% is applied to the pipeline due to the melting and freezing of frozen soil, and in order to prevent the occurrence of ductile cracks, the base metal part is pulled in the axial direction. By increasing the uniform elongation in the test to a low yield ratio, and by making the yield strength of the circumferential weld zone higher than the yield strength of the steel pipe base metal, the deformability, that is, the limit strain at which ductile cracking occurs is reduced. It becomes possible to enlarge.
Moreover, when using a steel pipe for a pipeline, although it paints from a viewpoint of corrosion prevention, a steel pipe is heated to about 250 degreeC at the time of coating. Even if heated to 250 ° C, it suppresses the increase in yield strength due to aging and matches with the yield strength of the circumferential weld (the yield strength of the weld metal in the circumferential weld is higher than the yield strength of the base metal) Need to keep). By suppressing the increase in yield strength after coating heating to 70 MPa or less, the yield strength of the base metal part can be kept lower than the yield strength of the circumferential weld metal, so that large deformability can be maintained. Furthermore, it is necessary to keep the buckling resistance limit strain large by maintaining the SS curve in the tensile test in a round shape (a shape in which no yield point appears).

  In order to have a large deformability even after coating and heating, it is desirable to contain 5 to 50% of ferrite of 10 μm or less. This is because if it exceeds 10 μm, the toughness of the base material decreases. This is because when the ferrite fraction is less than 5%, the effect of improving uniform elongation cannot be obtained. Moreover, since sufficient intensity | strength is not obtained when it exceeds 50%, content of the ferrite fraction was limited to 5 to 50%.

  In order to maintain a large deformability by making SS in the tensile test round after coating and heating, the metal structure of the base material should contain 2 to 7% of MA with an average aspect ratio of less than 2. Is desirable. This is because the inclusion of M-A provides the effect of introducing movable dislocations into the metal structure to lower the yield ratio and increase the uniform elongation. In order to obtain this effect, it is necessary to contain 2% or more of MA. When the production rate of M-A exceeds 7%, the low temperature toughness decreases, so the upper limit value is set to 7%. When the aspect ratio is 2 or more, the low-temperature toughness decreases, so the average aspect ratio of MA is less than 2.

Moreover, as a manufacturing method of the high strength steel plate for high strength steel pipes, the following three methods can be mentioned, for example.
(1) After heating the slab to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, and rolling is finished in the temperature range of 700 to 850 ° C., and then from the temperature range of 650 to 800 ° C. A method of cooling to a temperature of 250 ° C. to 400 ° C. at a cooling rate of not less than 15 ° C./second and then cooling with air.

(2) After heating the slab to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, rolling is finished in a temperature range of 700 to 850 ° C., and then from the temperature range of 650 to 800 ° C. A method of cooling to 400 ° C. or lower at a cooling rate of at least 15 ° C./second and then heating to 300 ° C. to 450 ° C., followed by air cooling.
(3) After heating the slab to 950 to 1200 ° C., the reduction rate of 950 ° C. or less is set to 50% or more, and rolling is finished in the temperature range of 700 to 850 ° C., and then from the temperature range of 650 to 800 ° C. A method of cooling to an arbitrary temperature of 400 ° C. or lower at a cooling rate of at least C ° C./second and then heating to 300 ° C. to 450 ° C. at a heating rate of 10 ° C./second or higher.

  First, the reheating temperature is limited to a range of 950 to 1200 ° C. The reheating temperature must be 950 ° C. or higher in order to dissolve Nb precipitates, refine the structure during rolling, and obtain excellent low temperature toughness. However, when the reheating temperature exceeds 1200 ° C., the ν grains become extremely coarse and cannot be completely miniaturized even by rolling, so that excellent low temperature toughness cannot be obtained. For this reason, the upper limit of the reheating temperature was set to 1200 ° C.

  Furthermore, the cumulative rolling reduction at 950 ° C. or less must be 50% or more, and the rolling end temperature must be 700 to 850 ° C. This is because the ν grains refined by recrystallization zone rolling are stretched by low-temperature rolling, and the crystal grains are thoroughly refined to improve low-temperature toughness. If the cumulative rolling reduction is less than 50%, the extension of the ν structure is insufficient and fine crystal grains cannot be obtained. If the rolling end temperature exceeds 850 ° C., fine crystal grains cannot be achieved even if the cumulative rolling reduction is 50% or more, for example. Further, if the rolling temperature is too low, excessive ν / α2 phase rolling occurs and the low temperature toughness deteriorates, so the lower limit of the rolling end temperature was set to 700 ° C.

After rolling, it is essential to cool the steel plate at an accelerated rate. Accelerated cooling allows for increased strength and improved uniform elongation and low yield ratio based on microstructure control without compromising low temperature toughness. In particular, it is necessary to limit to the following three cooling conditions in order to suppress aging after heating the paint.
As conditions for the first accelerated cooling, after the rolling, it is cooled to a temperature of 250 ° C. or more and 400 ° C. or less at a cooling rate of 5 ° C./second or more and less than 15 ° C./second from a temperature range of 650 to 800 ° C. Then it must be air cooled.

  When the temperature at which cooling starts exceeds 800 ° C., the uniform elongation decreases. Moreover, when the temperature which starts cooling is less than 650 degreeC, sufficient intensity | strength cannot be obtained. Therefore, the temperature range at which cooling is started is limited to 650 to 800 ° C. By cooling at a cooling rate of less than 15 ° C./second, ferrite is generated during cooling, and a large uniform elongation and a low yield ratio are obtained. However, when the cooling rate is less than 5 ° C./second, sufficient strength cannot be obtained. Therefore, the cooling rate was limited to 5 ° C./second or more and less than 15 ° C./second. When the cooling stop temperature is less than 250 ° C., the amount of dissolved C increases, the yield strength increases due to aging after coating heating, the yield point appears in the SS curve in the tensile test, and the deformability decreases. To do. When cooling is stopped at a temperature exceeding 400 ° C., sufficient strength cannot be obtained. Therefore, the cooling stop temperature is limited to 250 ° C. or more and 400 ° C. or less.

  As the second accelerated cooling condition, cooling is performed from a temperature range of 650 to 800 ° C. to a temperature of 400 ° C. or less at a cooling rate of 5 ° C./second or more and less than 15 ° C./second, and then 300 ° C. or more and 450 ° C. After heating to the following, it must be air cooled.

  When the temperature at which cooling starts exceeds 800 ° C., the uniform elongation decreases. Moreover, when the temperature which starts cooling is less than 650 degreeC, sufficient intensity | strength cannot be obtained. Therefore, the temperature range which starts cooling was limited to 650-800 degreeC. By cooling at a cooling rate of less than 15 ° C./second, ferrite is generated during cooling, and an increase in uniform elongation and a low yield ratio are obtained. However, when the cooling rate is less than 5 ° C./second, sufficient strength cannot be obtained. Therefore, the cooling rate was limited to 5 ° C./second or more and less than 15 ° C./second. When cooling is stopped at a temperature at which the cooling stop temperature exceeds 400 ° C., sufficient strength cannot be obtained. When it is cooled to an arbitrary temperature of 400 ° C. or lower, the amount of dissolved C increases, and thereafter, it must be heated to 300 ° C. or higher and 450 ° C. or lower. By heating to 300 ° C. or higher, the amount of dissolved C can be reduced, and deterioration of deformability due to aging can be prevented. When heated above 450 ° C., a decrease in strength and a decrease in low temperature toughness occur.

  As conditions for the third accelerated cooling, cooling is performed from a temperature range of 650 to 800 ° C. to an arbitrary temperature of 400 ° C. or less at a cooling rate of 5 ° C./second or more, and then 300 ° C. at a heating rate of 10 ° C./second or more. As described above, it must be heated to 450 ° C. or lower.

  When the temperature at which cooling starts exceeds 800 ° C., the uniform elongation decreases. Moreover, when the temperature which starts cooling is less than 650 degreeC, sufficient intensity | strength cannot be obtained. Therefore, the temperature range at which cooling is started is limited to 650 to 800 ° C. After ferrite is formed in a temperature range of 650 to 800 ° C., the ferrite is cooled at a cooling rate of 5 ° C./second or more. This is because sufficient strength cannot be obtained when the cooling rate is less than 5 ° C. When cooling is stopped at a temperature at which the cooling stop temperature exceeds 400 ° C., sufficient strength cannot be obtained. When cooled to an arbitrary temperature of 400 ° C. or lower, the amount of solid solution C increases, and thereafter the amount of solid solution C is reduced by heating to 300 ° C. or higher and 450 ° C. or lower at a heating rate of 10 ° C./second or higher. . When heated at a temperature exceeding 450 ° C. after cooling, a decrease in strength and a decrease in DWTT characteristics occur. When the heating rate is less than 10 ° C./second, cementite is coarsened and the DWTT characteristics are deteriorated.

  The present invention is most preferably applied to a thick plate mill, but can also be applied to a hot coil (in this case, the steel sheet after rolling and cooling is wound and cooled). Moreover, since the steel plate manufactured by this method is excellent in low temperature toughness, it can be applied to a pressure vessel as well as a pipeline in a cold region.

"Example"
Next, examples of the present invention will be described.
The steel pipes of Experimental Examples 1 to 48 were manufactured using steel sheets manufactured from billets of various steel components by the converter-continuous casting method, and various properties were investigated. Tables 1 to 5 show the chemical components of the base metal part and the weld metal part of the high-strength steel pipe, Tables 6 to 7 show the steel sheet manufacturing conditions and the microstructure of the base metal part, and Tables 8 to 9 Table 10 shows the mechanical properties of the base material of the high-strength steel pipe, and Tables 10 to 11 show the mechanical properties and deformability of the steel pipe weld.

  The properties of the welded portion of the steel pipe were evaluated using Charpy test pieces taken from the 1/2 t portion of the steel pipe after performing one layer SAW (submerged arc welding) on the inner and outer surfaces. The notch positions were the center of the weld metal and HAZ (1 mm from the point where the weld metal of the inner surface welding and the outer surface welding intersected). In Tables 10 and 11, 1) shows a test with a notch in the center of the weld metal, and 2) shows a test with a notch in HAZ.

In addition, a tensile test in the circumferential direction of the steel pipe (“YS: yield strength” “TS: tensile strength”) was a round bar tensile test piece having a diameter of 12.7 mm and a gauge length of 50.8 mm. For the tensile test (“YS: yield strength” “TS: tensile strength”) in the axial direction of the steel pipe, an arc-shaped full thickness tensile test piece was used.
In order to investigate the tensile characteristics after the coating was heated, the steel pipe was cut into small pieces of 300 mm × 300 mm, heated to 250 ° C. with an induction heater and held for 5 minutes, and then a tensile test piece was collected from the air-cooled piece. The deformability of the steel pipe was evaluated by the tensile fracture strain of the specimen welded in the circumferential direction and the critical buckling strain of the bending buckling test of the steel pipe.

  When manufacturing steel sheets in the steel pipes of Experimental Examples 1 to 12 and Experimental Examples 15 to 43, the manufacturing process under the first accelerated cooling condition described above was assumed. When manufacturing steel sheets in the steel pipes of Experimental Example 13, Experimental Example 44, and Experimental Example 45, the manufacturing process under the second accelerated cooling condition described above was assumed. When manufacturing steel sheets in the steel pipes of Experimental Example 14 and Experimental Example 46 to Experimental Example 48, the manufacturing process under the third accelerated cooling condition described above was assumed.

  As is apparent from Tables 6 to 11, in Experimental Examples 1 to 14, which are steel pipes of the present invention, excellent strength (YS, TS), uniform elongation (uEl), low temperature toughness, weld toughness, steel pipe Has tensile fracture strain and critical buckling strain. On the other hand, in the steel pipes of Experimental Examples 15 to 48, the chemical components and the steel sheet manufacturing conditions to be provided are not appropriate, and any of the characteristics is inferior.

In Experimental Example 15, since the amount of C in the base material is small, the strength of the base material does not satisfy X100.
Since Experimental Example 16 has a small amount of S in the base material, the HAZ toughness is inferior.
In Experimental Example 17, HAZ toughness is inferior because the amount of Nb in the base material is too large, and the increase in yield strength after heating at 250 ° C. exceeds 70 MPa with respect to the yield strength before heating, and the yield point in the SS curve Therefore, tensile fracture strain and buckling limit strain are small.
Since Experimental Example 18 has a large amount of Al in the base material, the HAZ toughness is inferior.

In Experimental Example 19, the strength of the base material does not satisfy X100 because the amount of Cr in the base material is small.
In Experimental Example 20, since the amount of Mo in the base material is large, the HAZ toughness is inferior, the increase in yield strength after heating at 250 ° C. exceeds 70 MPa with respect to the yield strength before heating, and the yield point is the SS curve. In order to appear, the tensile fracture strain and the buckling limit strain are small.
In Experimental Example 21, since the Pb value of the base material is too low, the strength of the base material does not satisfy X100.
In Experimental Example 22, since the amount of Ni in the base material is large and the Pb value of the base material is too high, the strength of the base material becomes too high and the HAZ toughness is also inferior.
In Experimental Example 23, since the C amount of the weld metal is small, hot cracking of the weld metal occurs.
Since Experimental Example 24 has too much C amount of a weld metal, the low temperature toughness of a weld metal is inferior.

In Experimental Example 25, since the Pw value of the weld metal is too low, the strength of the welded portion is low.
In Experimental Example 26, the amount of Cu in the weld metal is large, and the Pw value is too high, so the toughness of the weld metal is inferior.

In Experimental Example 27, the uniform elongation is improved, but since the ferrite fraction of 10 μm or less is less than 5%, the buckling limit strain is slightly small.
In Experimental Example 28, since the ferrite fraction of 10 μm or less exceeds 50%, the strength of the base material is just below X100.
In Experimental Example 29, since the MA fraction with an average aspect ratio of less than 2 is less than 2%, the buckling limit strain is slightly small.
In Experimental Example 30, the MA fraction with an average aspect ratio of less than 2 exceeds 7%, so the low temperature toughness is slightly low.
Since Experimental Example 31 has an average aspect ratio of 2 or more, low temperature toughness is slightly low.

  In Experimental Example 32, since the increase in yield strength after heating at 250 ° C. exceeded 70 MPa with respect to the yield strength before heating, the critical fracture strain was small and the deformability was poor.

In Experimental Example 33, since the slab reheating temperature is 950 ° C. or lower, the strength is barely low and the low-temperature toughness is slightly low. In Experimental Example 34, since the slab reheating temperature exceeds 1200 ° C., the low temperature toughness is slightly low.
In Experimental Example 35, since the amount of reduction at 950 ° C. or less is less than 50%, the low temperature toughness is slightly low.
In Experimental Example 36, since the rolling end temperature exceeds 850 ° C., the low temperature toughness is slightly low. In Experimental Example 37, since the rolling end temperature is less than 700 ° C., the low temperature toughness is slightly low.
In Experimental Example 38, the uniform elongation is improved, but since the cooling start temperature exceeds 800 ° C., the buckling limit strain is slightly small. In Experimental Example 39, since the cooling start temperature is less than 650 ° C., the strength is almost limitless.

In Experimental Example 40, the uniform elongation is slightly high, but the cooling rate is 15 ° C./second or more, so the buckling limit strain is slightly small. In Experimental Example 41, the cooling rate is less than 5 ° C./second, so that the strength is almost limitless.
In Experimental Example 42, since the cooling stop temperature exceeds 400 ° C., the strength is barely enough. In Experimental Example 43, since the cooling stop temperature is less than 250 ° C., the increase in yield strength after heating at 250 ° C. exceeds 70 MPa with respect to the yield strength before heating, and the yield point appears in the SS curve. Small tensile fracture strain and buckling limit strain.

  In Experimental Example 44, since the heating temperature after cooling exceeds 450 ° C., the strength is barely low, and the low-temperature toughness is slightly low. In Experimental Example 45, since the heating temperature after cooling is less than 300 ° C., the increase in yield strength after heating at 250 ° C. exceeds 70 MPa with respect to the yield strength before heating, and a yield point appears in the SS curve. Therefore, the tensile fracture strain and the buckling limit strain are small.

In Experimental Example 46, since the heating rate after cooling is less than 10 ° C./second, the low temperature toughness is slightly low. In Experimental Example 47, since the heating temperature after cooling exceeds 450 ° C., the strength is extremely low. In Experimental Example 48, since the heating temperature after cooling is less than 300 ° C., the increase in yield strength after heating at 250 ° C. exceeds 70 MPa with respect to the yield strength before heating, and a yield point appears on the SS curve. Therefore, the tensile fracture strain and the buckling limit strain are small.

Claims (7)

% By mass
C: 0.03% to 0.08%,
Si: 0.6% or less,
Mn: 0.8 to 2.5%
P: 0.015% or less,
S: 0.001 to 0.005%,
Cr: 0.5 to 1.5%
Mo: less than 0.02%,
Nb: 0.01 or more and less than 0.025%,
Ti: 0.005 to 0.030%,
Al: 0.005% or less,
N: 0.001 to 0.006%,
O: 0.001 to 0.006%,
In addition, Ni: 0.1 to 1.0%,
Cu: 0.1 to 1.2%,
V: 0.01 to 0.1%
B: 0.0003 to 0.002%,
Mg: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%
One or more of the following, with the balance consisting of iron and inevitable impurities,
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V
A base material part having a Pb value defined in the range of 2.5 to 4.0,
% By mass
C: 0.035 to 0.08%,
Si: 0.6% or less,
Mn: 1.5 to 2.5%
P: 0.015% or less,
S: 0.005% or less,
Ni: 0.2 to 2.5%,
Cr: 0.2 to 1.5%
Mo: 0.2 to 1.5%
Nb: 0.01-0.05%
Ti: 0.005 to 0.03%,
B: 0.0003 to 0.002%,
Al: 0.05% or less,
N: 0.001 to 0.01%,
O: 0.015-0.045%
And the balance consists of iron and inevitable impurities,
A weld metal part having a Pw value defined by Pw = C + 0.11Si + 0.03Mn + 0.02Ni + 0.04Cr + 0.07Mo + 1.46Nb in a range of 0.2 to 0.35;
When heated to a temperature of 250 ° C. or less, the difference in yield strength in the tensile test in the tube axis direction of the base metal part before and after heating is 70 MPa or less. High strength excellent in weld toughness and deformability Steel pipe. The weld metal part is further in mass%,
Cu: 0.1 to 1.0%
V: 0.01 to 0.1%
Mg: 0.0001 to 0.005%,
Ca: 0.0005 to 0.005%
The high-strength steel pipe excellent in weld toughness and deformability according to claim 1, wherein one or more of them are contained.   The high-strength steel pipe excellent in weld toughness and deformability according to claim 1 or 2, wherein the metal structure of the base metal part contains 5 to 50% of ferrite having a particle size of 10 µm or less.   3. The welding according to claim 1, wherein the metal structure of the base material portion contains 2 to 7% of a mixture of martensite and austenite (M-A constituent) having an average aspect ratio of less than 2. 4. High strength steel pipe with excellent toughness and deformability. % By mass
C: 0.03% to 0.08%,
Si: 0.6% or less,
Mn: 0.8 to 2.5%
P: 0.015% or less,
S: 0.001 to 0.005%,
Cr: 0.5 to 1.5%
Mo: less than 0.02%,
Nb: 0.01 or more and less than 0.025%,
Ti: 0.005 to 0.030%,
Al: 0.005% or less,
N: 0.001 to 0.006%,
O: 0.001 to 0.006%,
In addition, Ni: 0.1 to 1.0%,
Cu: 0.1 to 1.2%,
V: 0.01 to 0.1%
B: 0.0003 to 0.002%,
Mg: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%
One or more of the following, with the balance consisting of iron and inevitable impurities,
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V
A slab having a Pb value defined in the range of 2.5 to 4.0,
After heating to 950-1200 ° C,
The rolling reduction at 950 ° C. or less is set to 50% or more, and after the rolling is finished at a temperature range of 700 to 850 ° C.,
Cool from a temperature range of 650 to 800 ° C. to a temperature of 250 ° C. to 400 ° C. at a cooling rate of 5 ° C./second or more and less than 15 ° C./second,
A method for producing a high-strength steel sheet for high-strength steel pipes with excellent weld toughness and deformability, which is then air-cooled. % By mass
C: 0.03% to 0.08%,
Si: 0.6% or less,
Mn: 0.8 to 2.5%
P: 0.015% or less,
S: 0.001 to 0.005%,
Cr: 0.5 to 1.5%
Mo: less than 0.02%,
Nb: 0.01 or more and less than 0.025%,
Ti: 0.005 to 0.030%,
Al: 0.005% or less,
N: 0.001 to 0.006%,
O: 0.001 to 0.006%,
In addition, Ni: 0.1 to 1.0%,
Cu: 0.1 to 1.2%,
V: 0.01 to 0.1%
B: 0.0003 to 0.002%,
Mg: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%
One or more of the following, with the balance consisting of iron and inevitable impurities,
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V
A slab having a Pb value defined in the range of 2.5 to 4.0,
After heating to 950-1200 ° C,
The rolling reduction at 950 ° C. or less is set to 50% or more, and after the rolling is finished at a temperature range of 700 to 850 ° C.,
Cool from a temperature range of 650 to 800 ° C. to a temperature of 400 ° C. or less at a cooling rate of 5 ° C./second or more and less than 15 ° C./second,
Then, after heating to 300 to 450 degreeC, it is air-cooled, The manufacturing method of the high strength steel plate for high strength steel pipes excellent in the weld part toughness and deformability characterized by the above-mentioned. % By mass
C: 0.03% to 0.08%,
Si: 0.6% or less,
Mn: 0.8 to 2.5%
P: 0.015% or less,
S: 0.001 to 0.005%,
Cr: 0.5 to 1.5%
Mo: less than 0.02%,
Nb: 0.01 or more and less than 0.025%,
Ti: 0.005 to 0.030%,
Al: 0.005% or less,
N: 0.001 to 0.006%,
O: 0.001 to 0.006%,
In addition, Ni: 0.1 to 1.0%,
Cu: 0.1 to 1.2%,
V: 0.01 to 0.1%
B: 0.0003 to 0.002%,
Mg: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%
One or more of the following, with the balance consisting of iron and inevitable impurities,
Pb = 2.7C + 0.4Si + Mn + 0.8Cr + 0.45 (Ni + Cu) + Mo + V
A slab having a Pb value defined in the range of 2.5 to 4.0,
After heating to 950-1200 ° C,
The rolling reduction at 950 ° C. or less is set to 50% or more, and after the rolling is finished at a temperature range of 700 to 850 ° C.,
Cool from a temperature range of 650 to 800 ° C. to an arbitrary temperature of 400 ° C. or less at a cooling rate of 5 ° C./second or more,
Then, the manufacturing method of the high strength steel plate for high strength steel pipes excellent in the weld part toughness and the deformability characterized by heating to 300 to 450 degreeC with a heating rate of 10 degree-C / sec or more.