JP2003293039A - Method for producing high strength steel plate and steel pipe with low content of coarse crystal grin and having excellent low temperature toughness - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing an ultra-high strength steel plate and steel pipe having excellent low temperature toughness and ≥900 MPa tensile strength by which fine-granulation of crystal grain is performed by further promoting recrystallization in γ-recrystallization zone rolling and the low temperature toughness having ≥200 J shelf-energy (Charpy energy) is obtained. <P>SOLUTION: In the method for producing the ultra-high strength steel plate having ≥900 MPa tensile strength and excellent low temperature toughness, after reheating a continuously cast slab containing a prescribed component and consisting essentially of bainite and martensite in the casting structure, a hot-rolling at ≥900°C rolling temperature in the recrystallization zone, ≥5% average rolling-reduction ratio per one pass of each rolling and ≥3 sec passing time of each rolling, is performed. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to tensile strength (T
The present invention relates to a method for producing an ultrahigh strength hot-rolled steel sheet and a steel pipe which have excellent low temperature toughness of S) of 900 MPa or more. Such ultra-high strength hot-rolled steel is further processed and welded to be widely used as a weldable steel material such as a line pipe for transporting natural gas / crude oil, a pressure vessel, and a welded structure.

[0002]

2. Description of the Related Art In recent years, steel plates for line pipes and steel plates for pumping water
Demand for high strength and high temperature low temperature toughness is increasing for (for example, penstock) or pressure vessel steel plates. For the steel sheet for line pipe, for example, the tensile strength is 950 MPa.
Much research has already been conducted on the production of ultra-high strength steel sheets (X100 in the API standard) and above (PCT / JP
96/00155, 00157). Ultra high strength steel sheets used as such steel sheets for line pipes are often required to have not only high strength but also good low temperature toughness. For example, steel sheets for line pipes have a low temperature toughness of 200 J or more at shelf energy. Is required.

Generally, in steels having a tensile strength of 900 MPa or more and an ultrahigh strength level, Mn, Ni and Cu are used as steel components.
Since it is necessary to add a relatively large amount of alloying elements with high hardenability such as, when producing such steel by continuous casting, ferrite formation in the cast structure is suppressed, and Coarse bainite and martensite single phase containing mixed composition of bainite and martensite of 90% or more as a casting structure and having a crystal grain size of 1 mm or more in terms of former austenite grain size (the bainite structure and martensite structure here) Is a structure having a lath structure and is difficult to distinguish by an optical microscope, and thus the expression “single phase” is used. The same applies hereinafter) or a main structure. When performing reheating for performing hot rolling using a slab having such a cast structure, even after the bainite and martensite structures are transformed into austenite, the crystal grain size is the crystal of the cast structure. The grain size becomes coarse to the same extent as the grain size (former austenite grain size), and then recrystallization is not sufficiently promoted even if normal hot rolling is performed,
It was easy for coarse crystal grains to remain on the steel sheet. When coarse crystal grains are present in a part of the steel sheet, it becomes a point of fracture and the Charpy energy is lowered even in the upper shelf region. This is because the tensile strength of ultra high strength hot rolled steel at 900 MPa or higher is low. It is a cause of lowering toughness.

As a method for improving the low temperature toughness of an ultra high strength steel sheet having a tensile strength of 900 MPa or more by suppressing the coarsening of the slab crystal grains during reheating in hot rolling,
For example, in JP-A-11-140580, a cooling rate during continuous casting is controlled to manufacture a slab containing 10% or more of intragranular transformation ferrite in the casting structure, and the slab is used for heat treatment. A method is disclosed in which a large amount of austenite is generated from the interface of ferrite by austenite transformation in reheating for hot rolling, and crystal grains are sized and refined. However, in this method, it is necessary to reduce the Al content to less than 0.004% in order to generate 10% or more of intragranular ferrite in the cast structure, and to control the cooling rate during continuous casting. There are disadvantages in terms of productivity and economy such as refining and casting time being long and cost being high.

On the other hand, as a method for improving the low temperature toughness of an ultra high strength steel sheet having a tensile strength of 900 MPa or more by using austenite recrystallization in the hot rolling step, for example, in Japanese Patent Publication No. 2001-511482, hot rolling is performed. Although a method for improving the low temperature toughness of the steel sheet by defining the reduction ratio in the austenite recrystallization temperature region and the reduction ratio in the austenite non-recrystallization temperature region is known,
Since the recrystallization conditions such as the time between rolling passes in the austenite recrystallized grain region and the rolling reduction per pass are not taken into consideration, when performing hot-hot rolling using ultra-high strength slab mainly composed of bainite and martensite. 2 with shelf energy
It was difficult to stably manufacture a steel sheet having a low temperature toughness of 00 J or more and having excellent low temperature toughness.

[0006]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention The present invention is intended for hot rolling using bainite and martensite single-phase or ultra-high-strength continuous cast slabs having a main structure, in which coarsening of austenite grains tends to occur during reheating. The rolling conditions, especially γ recrystallization zone rolling, the average rolling reduction per rolling 1 pass and each rolling pass time are specified to further refine the austenite recrystallization to make the grains finer, and with shelf energy. It is intended to provide a method for producing an ultrahigh-strength steel sheet and a steel pipe having a low temperature toughness of 200 J or more and an excellent low temperature toughness and a tensile strength of 900 MPa or more.

[0007]

The present invention is to solve the above technical problems, and the gist of the invention is as follows. (1) In mass%, C: 0.03 to 0.10%, Si:
0.01-0.6%, Mn: 1.7-2.5%, P:
0.015% or less, S: 0.003% or less, Ni: 0.
1 to 2.0%, Mo: 0.01 to 0.60%, Nb:
0.001-0.10%, Ti: 0.001-0.03
0%, Al: 0.001 to 0.040%, N: 0.00
01-0.006%, and further B: 0.0001
~ 0.003%, V: 0.01-0.10%, Cu:
0.01-1.0%, Cr: 0.01-0.6%, C
a: 0.0001 to 0.01%, REM: 0.0001
~ 0.02% and Mg: 0.0001-0.006%
Of one or more of the above, the balance consisting of iron and unavoidable impurities, and the recrystallized zone rolling after reheating a continuously cast slab containing 90% or more of bainite and martensite in the casting structure. At a rolling temperature of 900 ° C. or higher, and an average reduction rate of 5% or more per rolling pass, and a tensile strength of 900 M, which is characterized by performing hot rolling.
A method for producing an ultra-high strength steel sheet excellent in low temperature toughness of Pa or more. (2) C: 0.03 to 0.10% by mass%, Si:
0.01-0.6%, Mn: 1.7-2.5%, P:
0.015% or less, S: 0.003% or less, Ni: 0.
1 to 2.0%, Mo: 0.01 to 0.60%, Nb:
0.001-0.10%, Ti: 0.001-0.03
0%, Al: 0.001 to 0.040%, N: 0.00
01-0.006%, and further B: 0.0001
~ 0.003%, V: 0.01-0.10%, Cu:
0.01-1.0%, Cr: 0.01-0.6%, C
a: 0.0001 to 0.01%, REM: 0.0001
~ 0.02% and Mg: 0.0001-0.006%
Of one or more of the above, the balance consisting of iron and unavoidable impurities, and the recrystallized zone rolling after reheating a continuously cast slab containing 90% or more of bainite and martensite in the casting structure. The method for producing an ultra-high strength steel sheet having a tensile strength of 900 MPa or more and excellent low-temperature toughness, characterized in that hot rolling is performed at a rolling temperature of 900 ° C. or more and a rolling pass time of 3 seconds or more. (3) C: 0.03 to 0.10% by mass%, Si:
0.01-0.6%, Mn: 1.7-2.5%, P:
0.015% or less, S: 0.003% or less, Ni: 0.
1 to 2.0%, Mo: 0.01 to 0.60%, Nb:
0.001-0.10%, Ti: 0.001-0.03
0%, Al: 0.001 to 0.040%, N: 0.00
01-0.006%, and further B: 0.0001
~ 0.003%, V: 0.01-0.10%, Cu:
0.01-1.0%, Cr: 0.01-0.6%, C
a: 0.0001 to 0.01%, REM: 0.0001
~ 0.02% and Mg: 0.0001-0.006%
Of one or more of the above, the balance consisting of iron and unavoidable impurities, and the recrystallized zone rolling after reheating a continuously cast slab containing 90% or more of bainite and martensite in the casting structure. At a rolling temperature of 900 ° C. or higher, an average reduction rate of 5% or more per one pass of each rolling, and a hot rolling at a rolling pass time of 3 seconds or more. A method of manufacturing super high strength steel sheets with excellent heat resistance. (4) In the reheating of the continuously cast slab, the temperature at the time of inserting the slab into a heating furnace is 400 ° C. or lower, according to any one of (1) to (3) above. A method for producing an ultrahigh-strength steel sheet having a tensile strength of 900 MPa or more and excellent low temperature toughness. (5) In the reheating of the continuously cast slab, the heating temperature of the slab is 1100 to 1250 ° C., and the tensile strength according to any one of (1) to (4) above. A method for producing an ultra-high strength steel sheet excellent in low temperature toughness of 900 MPa or more. (6) Furthermore, the rolling temperature in the non-recrystallization region rolling is Ar
₃ or Bs to 850 [deg.] C. and a cumulative rolling reduction of 60% or more, excellent tensile strength of 900 MPa or more and low temperature toughness according to any one of (1) to (5) above. Ultra high strength steel sheet manufacturing method. (7) The tensile strength is characterized in that the steel sheet produced by the method for producing a steel sheet according to any one of (1) to (6) above is cold-formed into a tubular shape, and then seam welding is performed at a butt portion. A method for producing an ultra-high strength steel pipe excellent in low temperature toughness of 900 MPa or more.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The contents of the present invention will be described in detail below.

Usually, the cast structure of a continuously cast slab having a tensile strength of 800 MPa or less and a relatively small content of alloying elements is a mixed structure of ferrite and bainite or ferrite and pearlite. When this slab is reheated for hot rolling, a large amount of new austenite is generated mainly from the ferrite grain boundaries, and the average crystal grain size is about 20 μm at a heating temperature near 950 ° C. just above the Ac ₃ point. It becomes sized austenite. Then, when a steel sheet is manufactured by hot rolling thereafter, the grains are further refined by recrystallization to have a substantially uniform grain size control structure having an average austenite grain size of about 5 μm.

On the other hand, the microstructure of the continuously cast slab for ultra-high strength steel having a relatively high content of alloying elements such as Mn, Ni and Cu and having a tensile strength of 900 MPa or more has a microstructure of bainite and martensite. It is a coarse bainite and martensite single-phase or main structure having a mixed structure of 90% or more and a crystal grain size of 1 mm or more in terms of prior austenite grain size.

When this slab is reheated for hot rolling as it is, austenite is partially generated also from the former austenite grain boundary, but the retained austenite which is mostly present in the crystal grain (former austenite grain) is present. As a result of easy coalescence and growth, the reheating temperature is 900 ~
At 1000 ° C, the crystal grains of the slab (former austenite grains)
Coarse austenite grains with a diameter of 1 mm or more, which is almost the same as the diameter, may be formed. This is called abnormal ferrite-austenite transformation.

If such coarse austenite grains are generated in the steel during reheating, it becomes difficult to recrystallize in the subsequent hot rolling, and the grain size of the final product is 50-100 μm.
Coarse crystal grains of m remain, which causes a decrease in low temperature toughness.

The inventors of the present invention used a continuous cast slab of bainite and martensite single phase or a main structure in which coarsening of crystal grains is likely to occur due to abnormal ferrite / austenite transformation during reheating, Even in the case of rolling, the present inventors have earnestly studied a method for producing a steel sheet which can improve the low temperature toughness by refining crystal grains by promoting austenite recrystallization in hot rolling.

Furthermore, as a result of various experiments conducted by the inventors of the present invention, the hot rolling conditions for promoting the austenite recrystallization were examined. As a result, among the rolling conditions, particularly in the austenite recrystallization zone rolling at a predetermined temperature. It has been found that the average reduction of each rolling pass or the time between rolling passes is important for promoting austenite recrystallization.

FIG. 1 shows the relationship between the average rolling reduction per pass and rolling pass time in the austenite recrystallized grain region rolling at a rolling temperature of 900 ° C. and the fraction of coarse austenite crystal grains having a grain size of 10 μm or more. Show.

1 in austenite recrystallization grain area rolling
Under the condition that the average rolling reduction per pass is 5% or more, or the time between rolling passes in austenite recrystallization grain area rolling is 3 seconds or more, the grain size at which fracture occurs in the steel sheet structure: 10 μm or more The ratio of the coarse austenite crystal grains is reduced to 20% or less, and the crystal grains coarsened by the abnormal ferrite-austenite transformation during reheating can be refined by austenite recrystallization in hot rolling. Further, under the condition that the average rolling reduction per rolling in austenite recrystallization grain region rolling is 5% or more and the rolling pass time in austenite recrystallization grain region rolling is 3 seconds or more, coarse austenite with a grain size of 10 μm or more is used. It was found that the proportion of crystal grains was further reduced to 10% or less, and the crystal grains could be refined by further promoting the austenite recrystallization in hot rolling. Particle size: 10
Fraction of coarse austenite crystal grains of μm or more and -40 ° C
Shows the relationship with the Charpy absorbed energy at. -40
In order to stably obtain excellent low temperature toughness with Charpy absorbed energy at 200 ° C. of 200 J or more, the proportion of coarse austenite crystal grains having a grain size of 10 μm or more at which fracture occurs in the steel sheet structure is less than 20%. There is a need to.

Therefore, in the present invention, based on the above-mentioned examination results, the crystal grains coarsened by the abnormal ferrite-austenite transformation at the time of reheating are refined by performing austenite recrystallization of hot rolling, and the steel sheet structure -40 ° C. by decreasing the residual amount of coarse austenite crystal grains having a particle size of 10 μm or more at which fracture occurs at
In order to stably obtain excellent low temperature toughness with a Charpy absorbed energy of 200 J or more, the average rolling reduction per pass in austenite recrystallized grain region rolling is 5% or more, or rolling in austenite recrystallized grain region rolling is performed. It is performed under any condition that the time between passes is 3 seconds or more. Also,
In the present invention, in order to stably obtain a superior low temperature toughness of Charpy absorbed energy at −40 ° C. of 230 J or more, the average rolling reduction per pass in austenite recrystallization grain area rolling is 5% or more, and The time between rolling passes in austenite recrystallized grain region rolling is set to 3 seconds or more.

Further, in the present invention, when the rolling temperature in the recrystallization zone rolling is lower than 900 ° C., sufficient recrystallization of austenite cannot be achieved, and the average rolling reduction per rolling pass and the time between rolling passes are not appropriate. If it does not increase, the Charpy absorbed energy at −40 ° C. cannot obtain a sufficiently high low temperature toughness of 200 J or more, which causes equipment / operation restrictions and productivity deterioration, so the rolling temperature of the above recrystallization zone rolling Is 900 ° C. or higher.

In the present invention, as the manufacturing conditions for improving the low temperature toughness of the steel sheet, in particular, as described above, the rolling temperature in the recrystallization zone rolling, the average rolling reduction per one rolling pass, and each rolling pass time are set. It is important to specify the above in the above range, and in order to further stabilize the low temperature toughness by further controlling the grain size of the steel sheet and suppressing the retention of coarse crystal grains in the structure, heating as follows is performed. The insert temperature of the slab into the furnace, the heating temperature, and the rolling temperature in the non-recrystallization zone rolling are specified.

First, the reheating conditions of the cast piece will be described.

In the present invention, a continuously cast slab produced by continuous casting is targeted as a rolling material. When the slab is reheated in a heating furnace, the slab is inserted into the heating furnace. The austenite transformation behavior during reheating changes depending on the temperature, that is, the temperature at which the slab is inserted into the heating furnace.

Bainite or martensite transformation has not yet proceeded sufficiently in the cast structure of the slab with the heating furnace insertion temperature exceeding 400 ° C., and a large amount of retained austenite with the same crystal orientation exists in the former austenite grain boundaries. is doing. Retained austenite with the same crystal orientation has the property of easily coalescing and growing at a temperature directly above Ac ₃ ,
During the reheating process, a large amount of retained austenite existing in grain boundaries coalesces and grows, and as a result, austenite coarsened to almost the same size as the grain size of old austenite is likely to be formed (this is called abnormal ferrite-austenite transformation). Therefore, in the present invention, in order to suppress the formation of coarse austenite due to the abnormal ferrite-austenite transformation in the reheating process, the temperature for inserting the slab into the furnace is set to 40.
The slab is cooled to 0 ° C. or lower and cooled by the time the slab is inserted into a heating furnace, and the cast structure is made to have a bainite-martensite-based structure containing less than 90% of retained austenite and bainite and martensite.

By setting the heating temperature during reheating to 1100 ° C. or higher, which is higher than just above Ac ₃ , the generation of coarse austenite caused by abnormal ferrite-austenite transformation during reheating is suppressed, and austenite crystals are formed. Grains can be almost sized. This is because martensite in the cast structure is decomposed into cementite at a temperature of Ac ₁ point or lower in the reheating process to form a large amount of massive cementite at the lath interface in the former austenite grain boundary, and further, Ac ₃ point or lower. Austenite nucleates from this massive cementite at high temperatures. Austenite generated from massive cementite in grain boundaries has a higher C content than retained austenite and has a different crystal orientation, so that it is not absorbed and coalesced with retained austenite and tends to grow alone. Furthermore, it is considered that austenite is sized by the action of secondary recrystallization. This effect of sizing the austenite increases as the heating temperature rises to a certain degree, but when the heating temperature exceeds 1250 ° C., austenite absorption coalescence due to Ostwald growth and grain growth occur rapidly and the austenite becomes coarse. It

Therefore, in the present invention, in order to promote grain size control of austenite by the action of secondary recrystallization and suppress coarsening of austenite due to Ostwald growth, the heating temperature during reheating is 1100 to 1250. Specified in ° C.

Next, the conditions for rolling in the non-recrystallized region will be described.

In the non-recrystallization rolling condition of the present invention, the rolling temperature is set to 850 ° C. or less and the cumulative rolling reduction ratio is set in order to introduce more deformation zones in addition to the old austenite grain boundaries to further refine the crystal grain size. 60% or more. When the rolling temperature of the unrecrystallized region rolling is too low such that it is less than the Ar ₃ point or the Bs point and the rolling reduction is performed under high pressure with a cumulative reduction ratio of 60% or more, a worked structure is formed and low temperature toughness is improved. In order to cause deterioration, the lower limit of the rolling temperature in the non-recrystallization region rolling is set to the Ar ₃ point or Bs point temperature.

In the present invention, in order to obtain desired properties such as tensile strength: 900 MPa or more, low temperature toughness: shelf energy of 200 J or more, it is necessary to specify the components of the slab used for hot rolling together with the above-mentioned manufacturing conditions. There is.

As described above, the austenite nucleated from the lumpy cementite in the grain boundary during the reheating process is removed.
Secondary recrystallization promotes grain growth,
In order to suppress the austenite transformation, it is necessary to raise the heating temperature. However, as the temperature rises, austenite absorption coalescence and grain growth occur due to Ostwald growth, so raising the heating temperature is not preferable. In the present invention, in order to carry out sizing of austenite by secondary recrystallization at a heating temperature in a lower temperature range, it is necessary to reduce an intervening material such as a carbide or a nitrite and suppress a pinning action due to it. Based on the finding that is effective, elements such as Nb, V, and Ti that form carbides or nitriding substances are reduced as much as possible.

The reasons for limiting the components of the steel sheet of the present invention will be described below.

C is extremely effective in improving the strength and hardenability of the steel by forming a solid solution or carbonitride in the steel, and obtains the bainite and martensite-based microstructure and the target strength of the present invention. Therefore, the lower limit of the content is set to 0.03%. On the other hand, if the C content is too large, the low-temperature toughness of the steel material and the welded HAZ part deteriorates, or the on-site weldability such as the occurrence of cold cracking after welding remarkably deteriorates, so the upper limit of the content is 0.10. %.
Further, in order to improve the low temperature toughness, the upper limit of the C content is 0.0
It is preferably 7%.

Si has the effect of deoxidizing and improving the strength, and is added in an amount of 0.01% or more to obtain the effect. On the other hand, if too much is added, the weld HAZ toughness and field weldability are significantly deteriorated, so the upper limit of its content is 0.6%.
And In addition, Al and Ti in the steel of the present invention are also Si
Since it has a deoxidizing effect similarly to the above, the Si content is preferably adjusted by the contents of Al and Ti.

Mn is an indispensable element for ensuring a good balance of strength and low temperature toughness by making the microstructure of the steel of the present invention mainly of bainite and martensite, and the lower limit of its content is 1.7. %. On the other hand, M
If too much n is added, the hardenability increases and welding H
In addition to deteriorating the AZ toughness and on-site weldability, the upper limit of the content is set to 2.5% in order to promote center segregation in the continuously cast steel piece and deteriorate the low temperature toughness of the steel material.

P and S are unavoidable impurity elements, and P
Promotes center segregation of continuously cast steel, and improves low temperature toughness by intergranular fracture, and S reduces ductility and toughness due to MnS in steel drawn by hot rolling. Therefore, in the present invention, in order to further improve the low temperature toughness of the base material and HAZ, the upper limits of the respective contents of P and S are limited to 0.015% and 0.003%.

Ni is added to improve the properties of the low carbon steel of the present invention without deteriorating the low temperature toughness and the field weldability. That is, Ni can relatively reduce the formation of a hardened structure detrimental to the low temperature toughness in the structure of hot rolling (particularly the central segregation zone of the continuously cast steel slab) as compared with Mn, Cr, and Mo. Since the addition of a trace amount of 1% or more is effective in improving the weld HAZ toughness, the lower limit of the Ni content is set to 0.1.
%. Furthermore, in order to improve the weld HAZ toughness,
The lower limit of the Ni content is preferably 0.3% or more.
On the other hand, if the Ni content is too large, not only the economic efficiency is deteriorated due to the expensive Ni, but also the weld HAZ toughness and field weldability are deteriorated, so the upper limit of the content is 2.0%.
And

The addition of Ni is also effective in preventing surface cracks caused by Cu in continuous casting and hot rolling. When added for this purpose, the Ni content is preferably added at 1/3 or more of the Cu content.

Mo is added to improve the hardenability of steel and to obtain the desired microstructure mainly composed of bainite and martensite. In particular, in the case of B-added steel, the effect of improving the hardenability by adding Mo becomes remarkable. Also, M
The coexistence of o with Nb has the effect of suppressing recrystallization of austenite during controlled rolling and refining the austenite structure. In order to obtain the effect of adding Mo, the lower limit of the content is set to 0.01%. on the other hand,
If the content exceeds 0.60% and is excessively added, the manufacturing cost becomes high, and the weld HAZ toughness and field weldability deteriorate. Therefore, the upper limit of the content is set to 0.60%.

Nb coexists with Mo to suppress recrystallization of austenite during controlled rolling, refines the austenite structure due to the precipitation of carbonitrides, and contributes to the improvement of hardenability. Add to convert In particular, the effect of improving the hardenability by adding Nb is synergistically enhanced when it coexists with B. Since the effect due to the addition of Ni cannot be obtained when the content is less than 0.001%, the lower limit of the content is set to 0.001%. On the other hand, if the addition amount is too large, the weld HAZ toughness and field weldability deteriorate and the upper limit of the content is set to 0.10% in order to drag the abnormal ferrite-austenite transformation to the high temperature side during reheating. From the viewpoint of improving these effects, the upper limit of the content is preferably 0.04%.

Ti forms fine nitrides in steel and suppresses coarsening of austenite during reheating, and
In the case of B-added steel, it reduces harmful solute N which is harmful to the improvement of hardenability, and further improves hardenability. Further, when the Al content is as small as 0.005% or less, Ti forms an oxide in the steel, acts as an intragranular transformation generation nucleus in the weld HAZ, and also has the effect of refining the structure of the weld HAZ. Have.
The effect of adding Ti is that the content is 0.00
If it is less than 1%, it cannot be obtained. Therefore, the lower limit of the Ti content is 0.
It was 001%. In order to stably obtain the effect of forming the nitride and fixing the solid solution N, it is preferable that the lower limit of the Ti content is 3.4N in view of the relationship with the N content. On the other hand, if too much Ti is added, the low temperature toughness deteriorates due to the coarsening of the nitride and the precipitation hardening of the carbide, and in addition, the abnormal ferrite / austenite transformation is dragged to the high temperature side during reheating, so that its content is increased. The upper limit of the amount is 0.
It was set to 030%.

Al is added as a deoxidizer, and
Since it also has a function of refining the structure, the lower limit of the Al content is made 0.001% in order to obtain the effect. On the other hand, A
When the content of l exceeds 0.040%, the amount of non-metallic inclusions of Al oxide type increases and the cleanliness of steel is impaired, and steel and weld H
In order to deteriorate the AZ toughness, the upper limit of its content is 0.06.
%. In addition, Si and Ti in the steel of the present invention are also A
Since it has a deoxidizing effect like l, the Al content is Si
It is preferable to adjust the content depending on the contents of Ti and Ti.

Since N forms a nitride of TiN in the steel of the present invention and acts as an intragranular transformation nucleus in the steel material or the weld HAZ during reheating, it has the effect of suppressing coarsening of austenite and improving low temperature toughness. In order to obtain the action and effect, the lower limit of the content is set to 0.0001%. On the other hand, if too much is added to the steel, the surface flaws of the steel slab will be generated and the weld HAZ toughness will be deteriorated due to the solid solution N.
In order to impede the effect of improving the hardenability, the upper limit of the content is set to 0.006%.

The steel of the present invention contains the above-described components as basic components. However, the strength and toughness of the steel of the present invention can be further improved and the size of steel that can be manufactured can be increased without impairing the excellent characteristics of the steel of the present invention. B, V, Cu, C
One or more of r, Ca, REM and Mg are added with the following contents.

B is an effective element for obtaining the target structure of bainite and martensite, which is the object of the steel of the present invention, because the hardenability of steel is dramatically enhanced by the addition of an extremely small amount. Further, B remarkably enhances the effect of improving the hardenability of Mo of the steel of the present invention and synergistically promotes the effect of improving the hardenability by coexisting with Nb. Since these effects cannot be obtained when the content is less than 0.0001%,
The lower limit of the B content was 0.0001%. On the other hand, excessive addition of B promotes the formation of brittle particles such as Fe ₂₃ (C, B) ₆ and deteriorates the low temperature toughness, and may rather extinguish the hardenability improving effect of B. The upper limit of the content was 0.0030%.

V has almost the same action as Nb, and its effect is weaker than that of Nb, but the coexistence with Nb in the steel of the present invention makes the toughening effect of the steel more remarkable. Since the effect cannot be obtained when the V content is less than 0.001%, the upper limit of the content is set to 0.001%.
On the other hand, if the added amount is too large, the weld HAZ toughness and field weldability deteriorate and abnormal ferrite during reheating.
In order to drag the austenite transformation to the high temperature side, the upper limit of its content is set to 0.10%. In order to stably obtain the effect of the addition of V, the content thereof is 0.03 to
It is preferably 0.08%.

Cu and Cr are base metal and weld HAZ
In order to obtain the effect of improving the strength of
Include at least 1%. On the other hand, if the content is too large,
Weld HAZ To significantly deteriorate toughness and field weldability,
The upper limit of each of the Cu and Cr contents is 1.0%,
It was set to 0.6%.

Ca and REM are sulfides (Mn
S) has the effect of controlling the morphology and improving the low temperature toughness of steel (such as absorbed energy in the Charpy test). In order to obtain the effect, the lower limit of the Ca and REM contents is set to 0.0001%. . On the other hand, the amount of Ca is 0.006%,
CaO-CaS when REM is added over 0.02%
Alternatively, since a large amount of REM-CaS is generated and becomes large clusters and large inclusions, which impairs the cleanliness of steel and deteriorates the field weldability, the upper limits of the respective contents of Ca and REM are 0.006%, 0. It was set to 0.02%. In the case of an ultra-high-strength line pipe, the S and O contents in steel are further limited to 0.001% and 0.002% or less, respectively, and an index relating to the shape control of sulfide-based inclusions is used. A certain ESSP (relational expression: ESSP = (Ca)
[1-124 (O)] / 1.25S) from 0.5 to 10.
It is preferably within the range of 0.

Mg has a function of forming a finely dispersed oxide, suppressing coarsening of austenite grains of the welded HAZ and improving low temperature toughness, and the lower limit of its content is set to obtain the effect. The amount is 0.0001%. On the other hand, if its content exceeds 0.006%, coarse oxides are formed, and conversely the toughness is deteriorated. Therefore, the upper limit of the content is made 0.006%.

In the present invention, an ultra-high strength steel sheet excellent in low temperature toughness can be obtained by hot rolling under the steel components, reheating and rolling conditions described above. After cold forming, seam welding of the butt portions makes it possible to manufacture an ultra-high-strength steel pipe excellent in low-temperature toughness like hot-rolled steel sheets.

The ultrahigh strength excellent in low temperature toughness of the present invention can be manufactured by cold forming a steel sheet into a tube and then seam welding the butt portions, and the manufacturing conditions need not be specified in particular.
Normal conditions are fine.

[0049]

EXAMPLES Next, examples of the present invention will be described.

Steels containing the chemical components shown in Tables 1 and 2 (continued from Table 1) were continuously cast, and then the microstructure thereof was transformed into bainite and martensite by cooling to obtain steel pieces having a thickness of 240 mm. This steel slab was further reheated under the conditions shown in Table 3, Table 4, and Table 5 (Table 4 and Table 5 are continued from Table 3), then recrystallized in the temperature range, and then unrecrystallized in the range. A steel plate was manufactured by cooling with water to a temperature of 450 ° C. or lower.

After that, a test piece for investigating the mechanical properties was taken from the steel sheet and subjected to a tensile test and a Charpy test. The low temperature toughness of the steel material is -4 in the C direction and the notch position in the plate thickness direction according to the Charpy test result.
The absorbed energy at 0 ° C. was obtained and evaluated.

In Table 1, steels A to D and steels Q are steels whose component contents satisfy the range of the present invention, and steels E to P
The content of any of the components is out of the range of the present invention. In Table 2, the test No. Nos. 1 to 60 are invention examples in which the steel components and the manufacturing conditions both satisfy the scope of the present invention, and the test No. Nos. 61 to 72 are comparative examples in which the manufacturing conditions are out of the range of the present invention. 73-7
No. 5 is a comparative example in which the steel composition is out of the range of the present invention.

As is clear from the test results of Table 2, the test No. In each of the invention examples 1 to 60, an ultrahigh strength steel sheet having a base material having a tensile strength of 900 MPa or more and a Charpy absorbed energy at −40 ° C. of 200 J or more and excellent in low temperature toughness was obtained.

On the other hand, the test No. In Comparative Examples 61 to 72, the tensile strength (TS) or -40 of the base material is at least because the content of the steel component is out of the range of the present invention.
Charpy absorbed energy (vE-40) at 0 ° C decreased. In addition, the test No. 73 to 75, the content of the steel component is within the range of the present invention, but since the recrystallization rolling conditions are out of the range of the present invention, the Charpy absorbed energy (vE-40) of the base material at −40 ° C. Was less than 200 J and the low temperature toughness was deteriorated.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

[Table 3]

[0058]

[Table 4]

[0059]

[Table 5]

[0060]

INDUSTRIAL APPLICABILITY The present invention is applied to hot rolling using bainite and martensite single-phase or ultra-high-strength continuous cast slabs having a main structure, in which coarsening of austenite crystal grains easily occurs due to abnormal ferrite-austenite transformation during reheating. In addition, a steel sheet of coarse austenite grains that becomes a starting point of fracture by finely sizing and sizing crystal grains by appropriately controlling the average reduction rate per each rolling pass and each rolling pass time in the γ recrystallization zone rolling It is possible to reduce the residual amount in the structure as much as possible, to produce an ultrahigh-strength steel sheet and a steel pipe having a tensile strength of 900 MPa or more and a Charpy energy at −40 ° C. of 200 J or more and excellent low temperature toughness. Therefore, the safety of steel structures used in line pipes for transporting natural gas and crude oil, steel plates for pumping water, pressure vessels, welded structures, etc. can be greatly improved.

[Brief description of drawings]

FIG. 1 shows the average rolling reduction per pass, the time between rolling passes and the grain size of 10 in the austenite recrystallized grain region rolling.
It is a figure which shows the relationship with the fraction of the coarse austenite crystal grain of (micrometer) or more.

FIG. 2 is a diagram showing the relationship between the fraction of coarse austenite crystal grains having a grain size of 10 μm or more and the Charpy absorbed energy at −40 ° C.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C22C 38/00 301 C22C 38/00 301A 301Z 38/58 38/58 // B23K 101: 06 B23K 101: 06 (72) Inventor Toshihiko Ozeki 20-1 Shintomi, Futtsu-shi, Chiba Shin-Nippon Steel Co., Ltd. F-term in the Technical Development Division (Reference) 4E001 AA03 CA02 CC03 4E002 AA07 AD04 BC01 BC05 BC07 CB01 4E081 AA08 BA19 FA03 4K032 AA01 AA02 AA04 AA08 AA11 AA14 AA16 AA17 AA19 AA21 AA22 AA23 AA24 AA27 AA29 AA31 AA35 AA36 AA40 BA01 CA02 CA03 CB02

Claims (7)

[Claims] 1. C: 0.03 to 0.10% by mass%,
Si: 0.01-0.6%, Mn: 1.7-2.5%,
P: 0.015% or less, S: 0.003% or less, Ni:
0.1 to 2.0%, Mo: 0.01 to 0.60%, N
b: 0.001 to 0.10%, Ti: 0.001 to 0.
030%, Al: 0.001-0.040%, N: 0.
0001 to 0.006%, and further B: 0.00
01-0.003%, V: 0.01-0.10%, C
u: 0.01 to 1.0%, Cr: 0.01 to 0.6%,
Ca: 0.0001 to 0.01%, REM: 0.000
1-0.02% and Mg: 0.0001-0.006
%, One or two or more of which are contained, the balance being iron and unavoidable impurities, and the recrystallized region after reheating a continuously cast slab containing 90% or more of bainite and martensite in the casting structure. Rolling temperature in rolling is 900 ℃
Above, and the average rolling reduction per pass of each rolling is 5% or more, hot rolling is carried out at a tensile strength of 900
A method for producing an ultra-high strength steel sheet excellent in low temperature toughness of at least MPa. 2. C: 0.03 to 0.10% by mass%,
Si: 0.01-0.6%, Mn: 1.7-2.5%,
P: 0.015% or less, S: 0.003% or less, Ni:
0.1 to 2.0%, Mo: 0.01 to 0.60%, N
b: 0.001 to 0.10%, Ti: 0.001 to 0.
030%, Al: 0.001-0.040%, N: 0.
0001 to 0.006%, and further B: 0.00
01-0.003%, V: 0.01-0.10%, C
u: 0.01 to 1.0%, Cr: 0.01 to 0.6%,
Ca: 0.0001 to 0.01%, REM: 0.000
1-0.02% and Mg: 0.0001-0.006
%, One or two or more of which are contained, the balance being iron and unavoidable impurities, and the recrystallized region after reheating a continuously cast slab containing 90% or more of bainite and martensite in the casting structure. Rolling temperature in rolling is 900 ℃
A method for producing an ultra-high strength steel sheet excellent in low-temperature toughness with a tensile strength of 900 MPa or more, characterized in that hot rolling is performed for each rolling pass time of 3 seconds or more. 3. C: 0.03 to 0.10% by mass%,
Si: 0.01-0.6%, Mn: 1.7-2.5%,
P: 0.015% or less, S: 0.003% or less, Ni:
0.1 to 2.0%, Mo: 0.01 to 0.60%, N
b: 0.001 to 0.10%, Ti: 0.001 to 0.
030%, Al: 0.001-0.040%, N: 0.
0001 to 0.006%, and further B: 0.00
01-0.003%, V: 0.01-0.10%, C
u: 0.01 to 1.0%, Cr: 0.01 to 0.6%,
Ca: 0.0001 to 0.01%, REM: 0.000
1-0.02% and Mg: 0.0001-0.006
%, One or two or more of which are contained, the balance being iron and unavoidable impurities, and the recrystallized region after reheating a continuously cast slab containing 90% or more of bainite and martensite in the casting structure. Rolling temperature in rolling is 900 ℃
Above, ultra-high strength excellent in low-temperature toughness with a tensile strength of 900 MPa or more, characterized by performing hot rolling with an average rolling reduction of 5% or more per each rolling pass and each rolling pass time of 3 seconds or more. Steel plate manufacturing method. 4. The reheating of the continuously cast slab, wherein the temperature at which the slab is inserted into the heating furnace is 400 ° C. or lower, wherein the temperature is 400 ° C. or lower. A method for producing an ultra-high strength steel sheet excellent in low temperature toughness having a tensile strength of 900 MPa or more. 5. The reheating of the continuously cast slab, wherein the heating temperature of the slab is 1100 to 1250 ° C., and the tensile strength according to any one of claims 1 to 4 is 900 MPa. The above-mentioned method for producing an ultra-high strength steel sheet having excellent low temperature toughness. 6. The rolling temperature in the non-recrystallization region rolling is Ar ₃ or Bs to 850 ° C., and the cumulative rolling reduction is 6.
It is 0% or more, The manufacturing method of the ultra high strength steel plate excellent in the low temperature toughness of the tensile strength of 900 MPa or more of any one of Claim 1 to 5 characterized by the above-mentioned. 7. A steel sheet produced by the method for producing a steel sheet according to claim 1, after cold-forming into a tubular shape,
A method for producing an ultra-high strength steel pipe having a tensile strength of 900 MPa or more and excellent low-temperature toughness, characterized by performing seam welding at a butt portion.