JP3387378B2 - High Mn steel slab, continuous casting method thereof, and method of manufacturing high tensile steel material - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Mn steel cast slab suitable for producing a high tensile steel having a tensile strength of 900 MPa or more and excellent in low temperature toughness, a casting method therefor, and a high tensile steel from the cast. To a method of manufacturing.

[0002]

2. Description of the Related Art A pipeline is used for transporting a large amount of natural gas or crude oil mined from a gas field or oil field over a long distance. This pipeline is often laid in remote areas, cold areas, and other places where natural environment conditions are poor. Further, the pipeline is constructed by a method in which a steel pipe for pipeline (line pipe) is transported to a laying site and welded there.

In the transportation of crude oil and the like by this pipeline, reduction of transportation cost is a major issue. As a solution, measures have been taken to improve transportation efficiency by increasing operating pressure.

In order to increase the operating pressure, the wall thickness of the pipe may be increased, and this method is the simplest. But,
If the wall thickness is made thick, it takes time to weld the pipes at the laying site, so that the welding work efficiency is significantly reduced. Further, since the weight of the pipe increases, there is a problem that the efficiency of the work of transporting the pipe to the site and the work efficiency of the pipe are reduced.

If the strength of the pipe material itself can be increased, not only can the operating pressure for transporting crude oil and the like be increased without the adverse effects of increased wall thickness and weight of the pipe, but also the above-mentioned welding. It is possible to prevent a decrease in efficiency in transportation, construction, etc. To this end, the American Petroleum Institute (A
In PI), a steel called X80 grade steel has been standardized and put into practical use. The X80 grade steel means a steel having a yield strength of 80 ksi (551 MPa) or more.

[0006] Recently, in preparation for future demand, the development of steel equivalent to X100 grade or steel exceeding X100 grade has been promoted based on the manufacturing technology of X80 grade steel. For example, as a high-strength steel exceeding X100 grade equivalent and a manufacturing method thereof, in order to secure strength, C
A method of utilizing the aging precipitation of u (for example, JP-A-8-10
4922, JP-A-8-209287, JP-A-8-2
No. 09288), and a method of setting the Mn content to 1.7% by weight or more (for example, JP-A-8-209290 and JP-A-8-209291).

However, the above-mentioned steel and its manufacturing method have the following problems. In the method utilizing the precipitation strengthening of Cu, since the ε-Cu precipitate is present in the matrix, the low temperature toughness, which is a problem when used in cold regions, is inferior, and the ε-Cu precipitate is not present. There is a drawback in that the hardness of the welded part is reduced and problems such as insufficient strength occur.

In general, when a steel having a Mn content of more than 1% by weight is cast by a continuous casting method, segregation of Mn in the center of the slab becomes remarkable, so that the steel material produced from the slab is , Low temperature toughness and hydrogen induced cracking resistance (HI resistance
It is known that characteristics such as (C property) deteriorate. Therefore, when the Mn content is as high as 1.7% by weight or more, such a problem becomes more prominent, so that a decrease in low temperature toughness and a decrease in weldability due to center segregation cannot be avoided. .

Alloying elements for adding Mn are relatively inexpensive. Therefore, if a high-Mn steel slab with a slight center segregation can be manufactured, a steel having high strength and excellent low temperature toughness can be manufactured at low cost.

As a measure for preventing center segregation when casting a cast product by a continuous casting method, a method of stirring unsolidified molten steel in the cast product drawn from the mold by an electromagnetic device before completion of solidification is used. well known. Further, one of the inventors of the present invention has proposed a method in which a slab pulled out from a mold is caused to undergo bulging and then a reduction corresponding to the bulging amount is applied before solidification is completed (Japanese Patent Laid-Open No. 9-57410). .

However, these center segregation prevention measures alone cannot sufficiently prevent center segregation when the Mn content is high. Therefore, the X100 grade equivalent or the tensile strength is 9
It has been difficult to manufacture steel for line pipes having performance exceeding the equivalent of X100 grade such as exceeding 100 MPa.

[0012]

DISCLOSURE OF THE INVENTION The present invention is directed to a high Mn steel slab suitable for producing a high strength steel having a slight Mn segregation in the central portion and a tensile strength of 900 MPa or more, and a continuous casting method for the slab. It is also an object of the present invention to provide a method for producing a high-strength steel material excellent in low temperature toughness and weldability using this cast slab.

[0013]

The gist of the present invention is to provide a high Mn steel slab (1), a continuous casting method for a high Mn steel slab (2), and a method for producing a high tensile steel product (3) described below. It is in.

(1)% by weight, C: 0.02 to 0.1%, Si: 0.03 to 0.6%, Mn: 0.8 to 2.5%, P: 0.015% or less , S: 0.003% or less, Ni: 0.3 to 1.2%, Nb: 0.01 to 0.1%, Ti: 0.005 to 0.03%, Al: 0.004 to 0. 1%, N: 0.001 to 0.006%, Cu: 0 to 0.6%, Cr: 0 to 0.8%, Mo: 0 to 0.6%, V: 0 to 0.1%, B: 0 to 0.0025%, Ca: 0 to 0.006%, the following formula is satisfied, and the balance has a chemical composition consisting of Fe and unavoidable impurities. ) / (Average Mn content of the slab), a high Mn steel slab having a Mn segregation degree of 3 or less.

[0015] 0.28 ≦ Vs ≦ 0.42 ····· where, Vs = C + 0.2Mn + 5P -0.1Ni- 0.07 Mo + 0.1Cu However, atomic symbol content of each element in the formula ( % By weight).

(2) A molten steel having the chemical composition of the above (1) is poured into a continuous casting mold, and one of the following (a) and (b) is applied to the cast piece withdrawn from the mold:
Alternatively, a method for continuously casting a high-Mn steel slab by adding both operations.

(A) Bulging is caused in the cast slab, and a reduction corresponding to the bulging amount is applied before the solidification of the cast slab is completed.

(B) Before solidification of the cast slab is completed, stirring is applied to the unsolidified molten steel by using an electromagnetic stirrer.

(3) The slab of (1) above or (2) above
The slab obtained by the method of is heated to a temperature of 1000 to 1250 ° C., hot rolled under the condition that the cumulative rolling reduction at 950 ° C. or less is 25% or more, and after the hot rolling is completed at 700 ° C. or more, A method for producing a high-strength steel material by cooling from 700 ° C or higher to a temperature range of 100 to 450 ° C at a cooling rate of 10 to 70 ° C / s. 500 more after cooling
You may give a tempering process at -675 degreeC.

In the cast slab of the present invention, by selecting the chemical composition described in the above (1) and particularly satisfying the formula,
Segregation of Mn in the central portion of the slab is suppressed. The formula is created by extracting elements that suppress Mn segregation and elements that promote Mn segregation, and considering the degree of influence of those elements on segregation, and is an expression in which the degree of Mn segregation is indexed. Is. By setting the Vs value, which is this index, to 0.42 or less, the center segregation of Mn is low even when a high Mn steel having a Mn content of 0.8 to 2.5 wt% is manufactured by the continuous casting method. The slab can be obtained.

Further, as in the method (2) above, when the above chemical composition is used in combination with casting conditions capable of reducing center segregation during continuous casting, the most stable M
It is possible to obtain a slab with a small n-center segregation.

The steel material obtained by the manufacturing method of the present invention (hereinafter simply referred to as the steel material of the present invention) has such Mn.
Using a slab with a slight center segregation of No. 3, the production was carried out under the above condition (3). Therefore, a high-strength steel material having a tensile strength of 900 MPa or more, which is a target of the present invention, can be manufactured even in commercial-scale production. The steel material of the present invention has a high tensile strength and, at the same time, an impact energy at −40 ° C. of 120 J or more and is excellent in low temperature toughness (hereinafter, simply referred to as toughness). The heat input is 3 to 10 kJ /
In the submerged arc welded part under the condition of mm, the tensile strength of the joint part is 900 MPa or more, and the weld heat affected zone (HA
Z) has a shock absorption energy of 7 at -20 ° C.
The strength and toughness of the welded portion are excellent as 0 J or more.

Further, since the steel material of the present invention is manufactured from a cast piece having a slight Mn segregation, the steel material of the present invention is less likely to undergo hydrogen-induced cracking due to MnS and the like,
It is possible to provide a feature that there is little adverse effect on weldability.

The term "steel material" as used in the present invention mainly means a steel sheet, especially a thick steel sheet having a thickness of about 15 to 30 mm, but hot-rolled steel sheets, shaped steels, forged steels and the like having a smaller thickness are also available. Contains.

[0025]

BEST MODE FOR CARRYING OUT THE INVENTION The cast piece of the present invention, the method for casting the same, and the method for manufacturing a steel material will be specifically described below. In addition,% indication regarding the content rate of an alloy element means weight%.

(A) Chemical composition The chemical composition of the steel in the present invention hardly changes between the molten steel, the cast piece and the steel material before casting. Therefore, the aforementioned chemical composition is intended for molten steel to steel materials. The range of the content rate of each alloy element and the reason for selecting the range are as follows.

C: 0.02 to 0.1% C is an element effective for securing the strength (hereinafter, simply referred to as strength) of steel such as tensile strength and yield strength, and obtains the effect. Therefore, it is necessary to contain 0.02% or more.
However, if it exceeds 0.1%, not only the toughness of the steel is lowered but also the weldability is remarkably deteriorated. Furthermore, since it promotes the segregation of Mn in the central portion of the slab, the upper limit was made 0.1%.

Si: 0.03 to 0.6% Si is an element effective in deoxidizing molten steel, and in order to obtain the effect, it is preferable to set it to 0.03% or more. But,
If it exceeds 0.6%, not only the toughness of the weld heat affected zone is deteriorated but also the hot workability is deteriorated, so the upper limit is 0.6.
%.

Mn: 0.8 to 2.5% Mn is an essential element for increasing the strength of the steel material of the present invention, and is required to be 0.8% or more. But,
When the Mn content increases, the central segregation of Mn becomes remarkable, and the toughness of the base material and the welded portion is reduced. Therefore, this M
The center segregation of n must be kept as low as possible when producing high-strength steel having a tensile strength of 900 MPa or more, which is the target value of the steel material of the present invention. From this point of view, the upper limit of the Mn content is 2.5%. Preferably 2
%, And more preferably less than 1.7%. In particular,
When producing a slab by a continuous casting method in which center segregation is more likely to occur than in the ingot making method, the content is preferably less than 1.7%.

P: 0.015% or less, S: 0.003% or less P and S are elements that significantly adversely affect the toughness of steel.
P itself segregates in the center of the slab, and Mn
Has the effect of promoting center segregation of. This center segregation reduces the toughness of the steel. Further, S becomes MnS and precipitates in the steel, and this MnS is stretched by rolling and adversely affects toughness.

Therefore, it is preferable that the amounts of these elements are as small as possible. Considering productivity on a commercial scale, P
Was 0.015% or less and S was 0.003% or less.

Ni: 0.3 to 1.2% Ni is an element effective for improving the strength. Also,
It has the effect of increasing the toughness of steel and improving the property of stopping the propagation of brittle cracks, and also has the function of suppressing the segregation of Mn. In order to exert these effects, 0.3
% Or more must be contained. On the other hand, the Ni content is 1.
If it exceeds 2%, it is not possible to obtain the performance improvement of steel commensurate with the addition of expensive Ni. From the viewpoint of preventing an increase in the manufacturing cost of steel materials, the upper limit was made 1.2%.

Nb: 0.01 to 0.1% Nb is an effective element for refining austenite crystal grains when a steel material is manufactured by the method described later. In order to obtain the effect, it is necessary to contain 0.01% or more. However, if it exceeds 0.1%, the toughness of the steel decreases, and the weldability deteriorates and the welding work efficiency at the laying site decreases like Ni, so the upper limit was made 0.1%.

Ti: 0.005-0.03% Ti has the function of refining the austenite crystal grains when the slab is heated. To obtain this effect, 0.005% or more is necessary. Particularly, in the case of the slab of the present invention containing Nb as described above, in order to prevent cracks that are likely to occur on the surface of the continuously cast slab due to Nb, a small amount of Ti of about 0.005% or more is used. It is effective to include. On the other hand, if the Ti content exceeds 0.03%, TiN produced by the reaction with N in steel becomes coarse and the austenite crystal grain refining effect disappears, so the upper limit was made 0.03%.

Al: 0.004 to 0.1% Al is usually used as a deoxidizing agent for molten steel. Further, since it has the effect of refining the structure of the steel material, it is also effective in improving the toughness of the steel. However, in the present invention, Al is an element that is indispensable as an element that improves the strength and toughness of the fusion line portion (bond portion) of the welded portion in addition to these effects. In the case of the present invention, T
Since it contains i, TiN existing in the base material is decomposed into Ti and N at the bond portion, and the generated free N easily reacts with B in the steel to form BN. When BN is formed, the solid solution B is reduced, so that the hardenability is deteriorated. As a result, the strength and toughness of the welded joint are likely to decrease. In order to fix the generated free N as AlN and prevent the reaction with B, the Al content must be 0.004% or more.

On the other hand, if the Al content exceeds 0.1%, coarse cluster-like alumina is formed and the cleanliness of the steel is impaired, so the upper limit was made 0.1%.

From the viewpoint of ensuring the strength and toughness of the welded portion, the preferable lower limit of the Al content is 0.01%,
More preferably, it is 0.02%.

N: 0.001 to 0.006% N forms TiN precipitates by reacting with Ti. This precipitate, during heating and welding of the slab for rolling,
It has an effect of suppressing coarsening of austenite crystal grains. To obtain this effect, N needs to be contained in an amount of 0.001% or more. On the other hand, the N content is 0.006%
If it exceeds 1.0, lateral cracking or the like will occur in the cast piece and the quality will be deteriorated, and the solute N will increase, so that the toughness of the fusion line part will be deteriorated. Therefore, the upper limit of the N content is 0.
It was set to 006%.

Cu, Cr, Mo, V and B: These elements have the function of increasing the strength of the base material, and are elements added as necessary.

The contents necessary to bring out the effect of each element are 0.2% or more for Cu, 0.3% or more for Cr, and Mo.
Is 0.3% or more, V is 0.01% or more, B is 0.000
4% or more is desirable. On the other hand, if the Cu and Mo contents exceed 0.6% and the Cr, V, and B contents exceed 0.8%, 0.1%, and 0.0025%, respectively, Since the toughness decreases, the upper limit is preferably set to the above content rate or less.

Therefore, the contents of Cu, Cr, Mo, V and B are 0 to 0.6%, 0 to 0.8%, 0 to 0.
6%, 0-0.1%, 0-0.0025%. When added, the preferable contents are 0.2 to 0.6, respectively.
%, 0.3 to 0.8%, 0.3 to 0.6%, 0.01 to
0.1% and 0.0004 to 0.0025%. More preferable Cu, Cr, Mo and V contents are 0.2 to 0.4%, 0.3 to 0.7% and 0.3 to respectively.
It is 0.5% and 0.01 to 0.06%.

Ca: 0 to 0.006% Ca has the function of preventing the form of MnS from becoming elongated by rolling, and therefore improves the toughness of the rolled steel sheet in the direction perpendicular to the rolling direction. It is an element that is effective in adding and is added in the present invention as needed.

In order to obtain the effect, it is desirable to contain 0.001% or more. However, the content rate is 0.00
If it exceeds 6%, the amount of non-metallic inclusions in the base material increases, causing internal defects. Therefore, the Ca content is 0 to 0.
006%, the preferred content when added is 0.001
It is good to be set to 0.006%.

Vs: 0.28 to 0.42% In the present invention, in addition to the above-mentioned regulation of the content of each alloying element, in order to reduce the segregation of Mn at the center of the cast slab, the following formula is used. It is characterized in that the Vs value, which is an index showing the susceptibility of the center segregation of Vs, is calculated and the Vs value is limited.

[0045] Vs = C + 0.2Mn + 5P- 0.1Ni- 0.07 Mo + 0.1Cu However, element symbol in the formulas represents the content of each element (% by weight).

When the Vs value exceeds 0.42%, the center segregation of Mn becomes remarkable when a cast piece is cast by the continuous casting method. If the Vs value is 0.42% or less, the center segregation is slight, so that a high-strength steel of 900 MPa or more can be obtained, and the toughness is hardly reduced.

On the other hand, when the Vs value is less than 0.28%,
Since the strength and toughness of the base material, which are the goals of the present invention, cannot be obtained, Vs was set to 0.28% or more.

(B) Mn Segregation Degree The cast slab of the present invention needs to have a Mn segregation degree of 3 or less. When the Mn segregation degree exceeds 3, the toughness of the base material tends to be significantly deteriorated. A preferable Mn segregation degree is 1.5 or less.

The segregation degree of Mn is (M at the center of the slab)
It is a value expressed as (n content) / (average Mn content of cast slab), and Ladle Mn analysis value is preferably used for the average Mn content of slab. The Mn content in the central part of the slab can be determined by performing Mn analysis on at least 5 points in the central part of the slab and calculating the average value.

In the cast slab of the present invention, as described above, since the Vs value is limited in addition to the regulation of the content of each element, the Mn segregation degree can be controlled within the above range relatively easily. it can. It is more preferable to employ a combination with the continuous casting method described below.

(C) Continuous casting method When the cast piece of the present invention is cast by the continuous casting method,
It is most suitable for one of the present inventors to adopt the technique disclosed in Japanese Patent Laid-Open No. 9-57410 as a continuous casting method for reducing the center segregation of the continuously cast slab.

That is, the molten steel adjusted to the above chemical composition is poured into the mold of a vertical or curved continuous casting machine,
This is a method in which bulging is once caused to the slab that has been pulled out of the mold, and the slab corresponding to the amount of bulging is applied to the slab by a reduction roll immediately before the completion of solidification.
By this operation, in addition to Mn, center segregation of elements such as P that easily cause segregation can be significantly reduced.

In order to cause bulging in the slab,
By gradually increasing the interval in the thickness direction of the slab of the guide rolls arranged on the downstream side of the mold, bulging occurs at the position where the solid fraction at the center of the slab is 0.1 or less. It is better to let them do it. As for the bulging amount, the thickness of the slab is 2
In the case of about 00 to 300 mm, it is appropriate to make the amount 20 to 100 mm thicker than the thickness of the slab (length of the short side of the mold).

It is appropriate that the reduction corresponding to the bulging amount is performed at a position where the solid fraction of the slab center is less than 0.8, that is, just before the solidification completion point. The number of reduction rolls may be one pair or plural pairs, and the reduction amount per pair of reduction rolls is preferably 20 mm or more. The final thickness of the cast piece after reduction is preferably the target thickness of the cast piece (the length of the short side of the mold).

The solid phase ratio means the ratio (volume ratio) of the solid phase in the unsolidified portion consisting of the liquid phase and the solid phase, and this solid phase ratio is one-dimensional unsteady in the thickness direction of the cast piece. It can be obtained by heat transfer analysis. That is, the solidification of molten steel begins at the liquidus temperature and the latent heat is released, and when the solidus temperature is reached, solidification ends and the latent heat is no longer released. The rate can be calculated.

In the continuous casting method of the present invention, in addition to the above-mentioned center segregation reducing method, it is effective to use an electromagnetic stirrer to stir the unsolidified molten steel in the slab. When this agitation is added, it is most effective to perform it in the range of the solid fraction of 0.05 to 0.7.

(D) Method for Manufacturing Steel Material When manufacturing a high-strength steel material such as a steel plate from the high Mn steel slab of the present invention, the following method is preferred.

First, the slab is heated to 1000 to 1250 ° C. When the heating temperature is less than 1000 ° C., Nb does not sufficiently form a solid solution in the matrix, so that recrystallization of austenite cannot be suppressed in the next hot rolling. For this reason, not only the refinement of the metal structure after the martensite and bainite transformation becomes insufficient, but also the precipitation of Nb (C, N) during the tempering during the transformation and the subsequent hardening and the hardening due thereto become insufficient. Therefore, the target high tensile strength cannot be obtained. Also, the heating temperature is 12
If it exceeds 50 ° C., the austenite crystal grains become coarse during heating of the slab, and the toughness of not only the central portion of the plate thickness but the entire base material decreases. For this reason, the heating temperature of the slab is 1000-
It was 1250 ° C.

After the hot rolling, in order to refine the martensite structure and the lower bainite structure formed during cooling, 9
25% cumulative rolling reduction from 50 ° C or lower to rolling end temperature
Roll under the above conditions. The reason for rolling under such conditions is as follows.

If the temperature is 950 ° C. or lower, the recrystallization of austenite will be significantly delayed in the slab of the present invention containing Nb. Therefore, if rolling is performed in the unrecrystallized austenite region at 950 ° C. or lower, the effect of working can be accumulated, and thus working strain for refining the martensite and the lower bainite structure can be accumulated. Although there is no particular limitation on the upper limit of the cumulative reduction rate, if the cumulative reduction rate exceeds 90%, it may be difficult to finish the shape of the steel material into a target shape due to, for example, poor flatness. The following is desirable.

The cumulative rolling reduction at 950 ° C. or lower is {(thickness of rolled material at 950 ° C.−thickness of rolled material after rolling) / thickness of rolled material at 950 ° C. } Is said.

The rolling end temperature is preferably 700 ° C. or higher. This is because if the temperature is lower than 700 ° C., the deformation resistance of the steel increases and it is difficult to finish the shape of the steel material after rolling into the target shape. The upper limit of the rolling end temperature is 25% cumulative rolling reduction.
In order to secure the above, it is desirable to set the temperature to 850 ° C.

After hot rolling, the material is cooled from 700 ° C. or higher to a temperature range of 100 to 450 ° C. at an average cooling rate of 10 to 70 ° C./s.

The cooling start temperature of 700 ° C. or higher is
If the temperature is less than 700 ° C, it will take some time to start cooling after rolling.
This is because, depending on the steel, the hardenability is deteriorated during the subsequent cooling and the toughness cannot be secured. The upper limit of the cooling start temperature is preferably about 850 ° C. from the viewpoint of ensuring a cumulative reduction of 25% or more.

When the average cooling rate from 100 to 450 ° C. (hereinafter, simply referred to as cooling rate) is less than 10 ° C./s, an upper bainite structure accompanied by coarse carbides is likely to be formed, so that the steel material is particularly preferable. A tensile strength of 900 MPa or more in the central part (a central part of the plate thickness of a steel plate) cannot secure good strength. On the other hand, the cooling rate is 70 ℃
If it exceeds / s, quenching tends to occur near the surface layer portion of the steel material, so the toughness of the surface layer portion may decrease. Therefore,
The cooling rate is preferably 10 to 70 ° C./s.

The cooling rate is practically controlled by measuring the surface temperature of the steel material and controlling the temperature of the surface layer of the steel material.

The cooling stop temperature at the above cooling rate is set to 100.
The reason why the temperature is set to ~ 450 ° C is as follows.

The cooling stop temperature is 10 at the temperature of the surface layer of the steel material.
If the temperature is lower than 0 ° C, dehydrogenation by slow cooling using the heat inside the steel material and flatness correction by a leveler during warming cannot be sufficiently performed. The reason why dehydrogenation by slow cooling is required is to prevent hydrogen defects which are likely to occur in high-strength steel. The cooling rate during slow cooling is preferably 10 to 50 ° C./hr. On the other hand, when the cooling stop temperature exceeds 450 ° C., the tensile strength cannot be ensured because the martensite structure and the like are not sufficiently generated not only in the central portion of the steel material but also in the surface layer portion.

After the cooling is stopped, the steel material may be tempered. The tempering treatment may be performed after gradually cooling to room temperature or before cooling to room temperature. The tempering temperature is preferably 500 to 675 ° C. If the temperature is lower than 500 ° C., the carbide precipitated from martensite formed in the cooling process after rolling remains in a flaky shape and has a large diameter across which the toughness of the steel material cannot be ensured. On the other hand, the tempering temperature is 6
If it exceeds 75 ° C., cohesive coarsening of carbides precipitated from martensite, dislocation density decrease, etc. occur, and tensile strength cannot be secured.

When tempering is performed, dehydrogenation proceeds during tempering, so that gradual cooling in the previous step is not necessary. Therefore, it is effective in shortening the production time.

[0071]

EXAMPLES Eight kinds of steels having the chemical composition defined in the present invention shown in Table 1 (Examples of the present invention, steel Nos. 1 to 8 ) and eight kinds of steels having a chemical composition outside the range specified in the present invention (comparison) example, the molten steel steel No. X1~X 8), width 2000 mm, and cast into slabs having a thickness of 200mm by a continuous casting method capable of suppressing the center segregation. The continuous casting method capable of suppressing center segregation means a casting method in which a slab drawn from a mold is caused to undergo bulging and a reduction corresponding to the bulging amount is applied immediately before the completion of solidification. Table 2 shows the continuous casting conditions in each test.

With regard to steel No. 8, instead of the above-mentioned countermeasure for preventing center segregation causing bulging, a method using an electromagnetic stirrer (conditions of test No. 8 in Table 2 ) was used. After casting, the effectiveness of the chemical composition defined in the present invention for suppressing center segregation was confirmed.

[0073]

[Table 1]

[0074]

[Table 2]

The segregation degree of Mn of the obtained slab was first investigated. The segregation degree of Mn is the value of the analysis value of the center of the slab with respect to the Ladle Mn analysis value (Mn content in Table 1), and the analysis value of the center of the slab was determined by the following method.
5 in the width direction of the slab across the center line of the slab's cross section
mm, also from the area of 50 mm in the length direction, 5 m in length and width
A total of 10 m samples were sampled, the Mn content of each sample was analyzed by optical emission spectroscopy, and the average value was calculated.

Table 1 also shows the Mn segregation degree and the Vs value.

These cast pieces were made into a piece having a width of 1600 mm and a thickness of 15
It was hot-rolled to a steel plate having a thickness of -30 mm to prepare a steel material for a test. Table 3 shows the hot rolling conditions of the slab and the cooling conditions of the steel material after rolling. Steel materials obtained by the manufacturing method specified by the present invention (Test Nos. 1 to 8 ; hereinafter referred to as steel materials of the present invention examples) and steel materials obtained by a manufacturing method out of the conditions specified by the present invention (Test No. . 9-2 1. hereinafter, targeting referred) and the steel of the comparative example were examined strength and toughness of the base material. Furthermore, the strength, toughness and HIC resistance of the welded joint were investigated.

The base material was evaluated for strength by a tensile test and toughness by a Charpy impact test. For the tensile test, JIS cut out from the thickness center of thick steel plate
The No. 4 test piece specified by A2201 was used, and the No. 4 test piece specified by JIS Z2202 (with a 2 mmV notch) was used for the Charpy impact test. The tensile test and the Charpy impact test are performed according to JIS Z224, respectively.
1. The test was carried out according to JIS Z2242. The test temperature of the Charpy impact test is -40 for the base material.
C. and the welded part was -20.degree.

With respect to the welded joint, the strength of the joint was evaluated by a tensile test, the toughness of the joint was evaluated by a Charpy impact test, and the likelihood of cracking due to hydrogen was evaluated by a HIC resistance test. The welded joint is subjected to submerged arc welding (heat input 4 kJ / mm) of a V-groove single-sided 4 layer for a tensile test and a rectangular groove single-sided 4 layer for a Charpy impact test on a steel plate having a thickness of 25 mm. It was produced by The welding flux and wire are 100
A commercially available product for Kilohiten was used.

As the tensile test piece, the No. 1 test piece specified in JIS Z3121 taken from this welded joint was used. For the Charpy impact test piece, JIS Z3 was sampled from the 1/2 position of the plate thickness so that the notch bottom position coincides with the fusion line appearing by macro etching.
The test piece specified in 128 was used.

The HIC resistance is measured by the TM0177 solution (H 2 S saturated-5% Na) at a temperature of 25 ° C. specified by NACE.
(Cl-0.5% acetic acid solution), using a 4-point bending support, dipping for 96 hours in a bent state so that 80% of the nominal proof stress was applied to the center of the strip-shaped test piece, and then the cracking rate was measured. It was evaluated by measuring.

In addition to the above test, in order to evaluate the welding workability in the field when the steel material manufactured by the method of the present invention is used as a line pipe or the like, a y-type weld crack test (JIS Z3158) I went. The weld bead was prepared by placing a weld bead without preheating (at an air temperature of 25 ° C.) using a commercially available hand-weld rod for 100 kilo high tension steel. The hydrogen content of the weld bead was 1.2 CC / 100 as a result of analysis by gas chromatography.
It was g.

Table 3 summarizes the rolling and cooling conditions of steel materials and the above-mentioned investigation results.

[0084]

[Table 3]

As is clear from Table 3, test No. 1 which is a steel material of the present invention example. For 1 to 8 , the chemical composition of steel, V
Since the s value and the Mn segregation degree satisfy the conditions specified in the present invention, the strength (tensile strength), toughness (Charpy impact energy) and field weldability of the base material were excellent. As for welded parts, in addition to strength, toughness and field weldability,
It was confirmed that the HIC resistance was excellent. In particular, the tensile strength exceeds 900 MPa for both the base metal and the weld,
It was found that the strength targeted by the present invention was sufficiently obtained.

Among these steel materials of the examples of the present invention, test N
o. No. 8 is an example in which bulging was caused in the slab during continuous casting of molten steel, and thereafter, no treatment for rolling down was taken.
Test No. that caused bulging Compared with Nos. 1 to 7, test No. in which electromagnetic stirring was performed. 8 is slightly HIC resistant
There was a tendency to be inferior . Therefore, during continuous casting,
It has been found that the continuous casting method of the present invention, in which the slab is caused to undergo bulging and the amount of reduction corresponding to the bulging is applied before the completion of solidification, is the most excellent.

On the other hand, the test No. of the comparative example. 9 to 1
No. 3 is a steel material manufactured based on the slab of the present invention. However, since the manufacturing conditions of the steel material do not satisfy the conditions specified in the present invention, among the strength and toughness of the base material and the welded portion, Was inferior to at least one property of.

In addition, the test No. of the comparative example. 1 4 to 2 1 are
It is a result regarding a steel material obtained based on a slab in which one or two of the chemical composition, the Vs value and the Mn segregation degree is out of the range specified in the present invention. Also in this case, both the base metal and the welded part had particularly poor toughness. Also, HIC resistance,
It was confirmed that many of them had inferior field weldability, and a steel material excellent in strength and toughness, which is the target of the present invention, could not be obtained.

Incidentally, the test No. of the comparative example. Slab used in 9-2 1, after causing bulging in slab was cast by continuous casting method before solidification completing addition of reduction of bulging significant amount. The inferior properties of these steel materials are due to the test No. 9-1 3 production conditions for machining the steel from slab, Test No. 1 4-2 1 the chemical composition of the slab, Mn
It is considered that the degree of segregation is out of the range specified in the present invention.

From these results, it is not until the steel material is manufactured by the manufacturing method of the present invention by using the high Mn steel slab of the present invention or the slab obtained by the method of manufacturing the high Mn steel slab of the present invention. It was proved that the steel material excellent in strength and toughness targeted by the present invention can be obtained.

[0091]

The high Mn steel cast piece of the present invention and the cast piece obtained by the continuous casting method for the high Mn steel cast piece of the present invention are M
The center segregation of n is mild. Therefore, based on these cast pieces, the steel material obtained by the manufacturing method of the present invention,
Both the base metal and welded part have high strength exceeding 900 MPa, and also have excellent properties such as low temperature toughness, field weldability, and HIC resistance. Therefore, the steel material obtained by the production method of the present invention is extremely suitable as a material for steel pipes used in pipelines laid in places with poor natural environmental conditions such as remote areas and cold regions.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C22C 38/58 C22C 38/58 (72) Inventor Masahiko Hamada 4-53-3 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd. In-company (72) Inventor Yuichi Komizo 4-533 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture Sumitomo Metal Industries, Ltd. (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38 / 60

Claims (4)

(57) [Claims] 1. By weight%, C: 0.02-0.1%, Si: 0.03-0.6%, Mn: 0.8-2.5%, P: 0.015% or less, S: 0.003% or less, Ni: 0.3 to 1.2%, Nb: 0.01 to 0.1%, Ti: 0.005 to 0.03%, Al: 0.004 to 0.1 %, N: 0.001 to 0.006%, Cu: 0 to 0.6%, Cr: 0 to 0.8%, Mo: 0 to 0.6%, V: 0 to 0.1%, B : 0 to 0.0025%, Ca: 0 to 0.006%, the following formula is satisfied, and the balance has a chemical composition consisting of Fe and unavoidable impurities (Mn content in the slab center). High M characterized by a Mn segregation degree of 3 or less expressed as / (average Mn content of cast slab)
n Steel slab. Here 0.28 ≦ Vs ≦ 0.42 ·····, Vs = C + 0.2Mn + 5P-0.1Ni- 0.07 Mo + 0.1Cu However, atomic symbol content of each element in the formula (wt%) Represents 2. A molten steel having the chemical composition according to claim 1 is poured into a casting mold for continuous casting, and one of the following (a) and (b) is applied to the slab that has been withdrawn from the casting mold: Alternatively, a method for continuously casting a high-Mn steel slab is characterized by adding both operations. (A) Bulging is caused in the slab, and a reduction corresponding to the bulging amount is applied before the solidification of the slab is completed. (B) Stirring is applied to the unsolidified molten steel using an electromagnetic stirrer before the solidification of the slab is completed. 3. The slab according to claim 1 or the slab obtained by the method according to claim 2 is heated to a temperature of 1000 to 1250 ° C., and a cumulative rolling reduction at 950 ° C. or less is 25% or more. After performing hot rolling and finishing the hot rolling at 700 ° C. or higher, cooling is performed from 700 ° C. or higher to a temperature range of 100 to 450 ° C. at a cooling rate of 10 to 70 ° C./s. Method of manufacturing tensile steel. 4. The method for producing a high-strength steel material according to claim 3, further comprising tempering in a temperature range of 500 to 675 ° C.