JP2008030086A - Manufacturing method of high strength clad steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength clad steel plate having excellent strength and toughness in a base metal, and also having excellent weldability and high practicality. <P>SOLUTION: In the method for producing a clad steel plate obtained by cladding a C-Mn steel base metal and a clad plate, the C-Mn steel base metal whose carbon equivalent is controlled to ≤0.4 mass% and the clad plate are combined, the combination is heated to the range of 1,150 to 1,250°C, and is hot-rolled. The finish rolling is finished in the temperature range of 750 to 950°C, direct water quenching is performed, and thereafter, tempering is performed. The clad steel plate having higher strength satisfying the performance of the APIX 70 grade can be produced without changing the components of the clad steel plate produced by the conventional method. Further, a reheating step for quenching is eliminated, and the effects leading to the shortening of a construction period, reduction of production cost and energy consumption can be obtained. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a high-strength clad steel plate used for applications requiring high strength equivalent to APIX70 grade for a clad steel plate produced by quenching and tempering (hereinafter referred to as tempering).

  Since natural gas has fewer air pollution problems than petroleum energy, the demand for clean energy is on the rise in the future, and construction plans for natural gas pipelines are being greatly accelerated as an international economic policy. In the extraction of natural gas, in addition to the target gas, there are many cases where highly corrosive gases such as hydrogen sulfide, carbon dioxide and chlorine ions are included, and metals that can withstand use in highly corrosive environments. A clad steel plate and a clad pipe having excellent corrosion resistance and high strength as materials are used.

  With regard to clad steel sheet production produced by quenching and tempering (hereinafter referred to as tempering), high strength and excellent low temperature of APIX65 grade is achieved by adding trace alloy elements based on C-Mn steel and optimizing heat treatment conditions. It is possible to produce a clad steel plate having toughness and sufficient performance in the welded portion after welding (see, for example, Patent Document 1). On the other hand, in recent years, in order to improve the safety of line pipes and reduce costs by reducing the wall thickness, there has been an increasing demand for higher strength for materials. It is necessary to manufacture a steel plate.

In the production of clad steel plates, a direct quenching method after rolling is known as a method for obtaining high strength and toughness. An example of applying this direct quenching method in the production of a clad steel plate is proposed in Patent Document 2, for example. In Patent Document 2, a pre-cladding material in which a clad steel plate base material and a laminated material are combined is heated to 1000 ° C. or more. Then, after hot working and cladding, a method of quenching from a temperature of 800 ° C. or higher as it is after hot working and then tempering at a temperature below the _AC1 point of the base material is disclosed. ing. According to the example of the above-mentioned patent in Patent Document 2, a clad steel sheet having a tensile strength of about 100 kgf / mm ² and a fracture surface transition temperature of −50 to −100 ° C. is obtained.
JP 2004-149821 A JP 61-144284 A

According to the manufacturing method disclosed in Patent Document 2, clad steel having a relatively high strength can be obtained. However, in a preferred embodiment in Patent Document 2, the weld cracking sensitivity index for evaluating the cracking susceptibility of the welded portion is high, and the carbon equivalent used for evaluating the maximum hardness of the welded portion from the steel material component is also recently increased. It deviates from the range of C _eq ≦ 0.4 mass% and P _CM ≦ 0.2 mass% required from the customer, which is practically difficult. That is, in the specific example shown in Patent Document 2, it can be said that the weldability is inferior although high strength characteristics are obtained due to the high carbon equivalent.

  The present invention has been made against the background of the above circumstances, and can provide a clad steel sheet satisfying the performance of APIX 70 grade or higher, which is higher in strength than a clad steel sheet obtained by a normal tempering method, and has sufficient strength and toughness. In addition, an object is to provide a method for producing a clad steel sheet having excellent practicality by having good weldability.

  That is, in the method for manufacturing a high strength clad steel plate according to the present invention, the invention according to claim 1 is a method for producing a clad steel plate clad with a C-Mn steel base material and a laminated material. The C-Mn steel base material and the laminated material described below were combined and heated to a temperature in the range of 1150 ° C to 1250 ° C to perform hot rolling, and the finish rolling was finished in a temperature range of 750 ° C to 950 ° C. Tempering is performed after direct water quenching.

  The invention of the method for producing a high strength clad steel sheet according to claim 2 is characterized in that, in the invention according to claim 1, the total thickness of the clad steel sheet after rolling is 50 mm or less.

  Invention of the manufacturing method of the high intensity | strength clad steel plate of Claim 3 is the invention of Claim 1 or 2, In direct water quenching after rolling, after finishing rolling in the said temperature range, water temperature is 30 degreeC. It is performed using the water tank managed below, The cooling rate in 800 to 500 degreeC of the thickness center part of the clad steel plate at that time shall be 100 degrees C / min or more.

Invention of the method of producing a high strength clad steel according to claim 4 is the invention according to any one of claims 1 to 3, after quenching, the heating furnace of 500 ° C. to 650 ° C. or less A _c1 point using Tempering is performed.

The invention of the method for producing a high strength clad steel sheet according to claim 5 is the invention according to any one of claims 1 to 4, wherein the clad steel sheet base material after quenching and tempering has a tensile strength ≧ 564 MPa, 0.2%. It has the performance of proof stress ≧ 483 MPa, vE−40 ° C. ≧ 35 J, and the carbon equivalent (C _eq ) of the following formula (1) and the weld crack sensitivity index (P _CM ) of the formula (2) are C _eq ≦ 0 .4 mass% and P _CM ≦ 0.2 mass%.
Formula (1): C _eq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (mass%)
Formula (2): P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (mass%)

  The invention of the method for producing a high strength clad steel sheet according to claim 6 is the invention according to any one of claims 1 to 5, wherein the composition of the C-Mn steel base material is C: 0.03 by mass%. 0.10%, Si: 0.10 to 0.30%, Mn: 1.20 to 1.60%, P: 0.015% or less, S: 0.002% or less, Ni: 0.10 to 0 50%, Cr: 0.05 to 0.30%, Mo: 0.05 to 0.30%, Cu: 0.05 to 0.30%, V: 0.010 to 0.040%, Ti: It contains alloy elements in the range of 0.005% to 0.020%, Nb: 0.030 to 0.10%, N: 0.0030 to 0.0090%, Al: 0.01 to 0.10%. The balance is low alloy steel composed of Fe and inevitable impurities.

  That is, according to the present invention, by directly quenching in hot rolling, the strength and toughness of the base material are improved, and the corrosion resistance of the laminated material is also improved. Thereby, sufficient strength and toughness can be obtained even at a wall thickness of 50 mm or less. Below, each condition prescribed | regulated in this invention is demonstrated.

Heating conditions before hot rolling: 1150 to 1250 ° C
In the hot rolling, the combined C-Mn steel base material and the laminated material are heated to an appropriate temperature so that the hot rolling can be performed well, and the quenching effect of the base material and the solution treatment effect of the laminated material are obtained. . In addition, in the case of hot rolling, if the heating temperature of the combined C—Mn steel base material is less than 1150 ° C., there is a possibility that the rolling operation becomes difficult as the deformation resistance increases. Also, for the laminated material, unlike the conventional tempering method, in the production of the clad steel plate by direct quenching, the heating before rolling also serves as a solution heat treatment for the laminated material, so that the heating temperature is less than 1150 ° C. There is a risk that the carbides or precipitates at the time of manufacturing the material slab are not sufficiently solutionized and the corrosion resistance is deteriorated. On the other hand, when the temperature exceeds 1250 ° C., the base crystal grains become coarse and may adversely affect toughness. Therefore, the temperature is set to 1150 to 1250 ° C.

Finishing rolling finish temperature: 750-950 ° C
By regulating the temperature of the clad steel sheet that is directly subjected to quenching after hot rolling as the finish rolling finish temperature, the effect of solution treatment of the laminated material and the quenching effect of the base material can be sufficiently obtained, and austenite re- This is a temperature range in which the structure can be refined by processing in the crystal region and the structure can be refined by processing in the austenite non-recrystallized region. When the rolling finish temperature is less than 750 ° C., for example, when the laminated material is an austenitic stainless steel such as SUS316, the time to be exposed to the vicinity of the sensitization temperature during the rolling becomes long, and the corrosion resistance is deteriorated. The potential is great. On the other hand, when the finish temperature is 950 ° C. or higher, the base material is repeatedly rolled mainly at the recrystallization temperature range or higher, so that it is difficult to obtain a sufficiently refined structure, which may adversely affect toughness. Therefore, the rolling finish temperature is set to 750 to 950 ° C.

Cooling rate: water cooling A high strength is obtained in the base material by directly quenching from the rolling finish temperature by water cooling. In the water cooling, it is desirable that the cooling rate at 800 ° C. to 500 ° C. of the thickness center portion of the clad steel plate is 100 ° C./min or more. Adjustment of this cooling rate can be performed using the water tank which managed water temperature to 30 degrees C or less. Moreover, since the thickness of the clad steel plate affects the cooling rate, the cooling rate condition can be satisfied by regulating the thickness. As shown in FIG. 3, when the thickness of the clad steel plate is 50 mm or less, the cooling rate at the thickness center of the clad steel can be reliably kept within the above conditions by using the water tank. In the figure, T is the thickness, W.V. Q. Is water-quenched. C. Is blast cooling; C. Represents air cooling. In a natural gas pipeline or the like, in order to obtain a desired strength, a certain thickness is required, and a thickness of 5 mm or more is desirable.

Tempering temperature: 500 ° C. to 650 ° C. below A _c1 point
After quenching, the toughness of the base material can be improved by tempering. At this time, preferably performed tempering at a 500 ° C. to 650 ° C. or less _{A c1} point by using a heating furnace.

Carbon equivalent (C _eq ): 0.4% by mass or less The carbon equivalent represented by the following formula (1) needs to be 0.4% by mass or less in the base material in order to ensure good weldability. If it exceeds 0.4%, the weldability is lowered and the practicality is inferior.
Formula (1):
C _eq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (mass%)

Weld cracking susceptibility (P _CM ): 0.2% by mass or less By setting the weld cracking sensitivity represented by the following formula (2) to 0.2% by mass or less, occurrence of cracks during welding can be avoided. If the cracking sensitivity exceeds 0.2%, the practicality is inferior.
Formula (2): P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (mass%)

Base Material Material In the present invention, high strength and high toughness clad steel can be obtained by direct water quenching after hot rolling without specially regulating the base material composition. Furthermore, by optimizing the composition of the base material, it is possible to obtain performances of tensile strength ≧ 564 MPa, 0.2% proof stress ≧ 483 MPa, vE−40 ° C. ≧ 35 J. Examples of optimized base material compositions are shown below.
C: 0.03-0.10%, Si: 0.10-0.30%, Mn: 1.20-1.60%, P: 0.015% or less, S: 0.002% in mass% Hereinafter, Ni: 0.10 to 0.50%, Cr: 0.05 to 0.30%, Mo: 0.05 to 0.30%, Cu: 0.05 to 0.30%, V: 0.00. 0.10 to 0.040%, Ti: 0.005% to 0.020%, Nb: 0.030 to 0.10%, N: 0.0030 to 0.0090%, Al: 0.01 to 0.10 % Is a low alloy steel containing alloy elements in the range of% and the balance being Fe and inevitable impurities.
By applying the present invention to a clad steel plate having the above base material composition, a clad steel plate having high strength and high toughness can be obtained. Furthermore, by optimizing the composition within the range of the above-mentioned base material composition, it is possible to obtain performances of tensile strength ≧ 564 Mpa, 0.2% proof stress ≧ 483 MPa, and _V E- ₄₀ ≧ 35 J when the thickness is 50 mm or less. it can. The reasons for limiting the components in the base material composition are as follows. In addition, all content shown below is shown by the mass%.

C: 0.03-0.10%
C is an effective component for ensuring strength, but from the viewpoint of weldability, excessive addition of C leads to a decrease in HAZ toughness and an increase in the carbon equivalent and the weld cracking susceptibility index shown in claim 5. The amount added must be limited to a low level. However, the conventional tempered clad steel plate manufacturing method requires addition of at least 0.05% or more in order to ensure the strength equivalent to API X70 grade. In the high strength clad steel plate production method of the present invention, it is possible to produce a high strength clad steel plate with a lower C addition amount than before by applying direct quenching, and the lower limit of the addition amount is 0.03%. And The upper limit is set to 0.10% or less for the above reason.

Si: 0.10 to 0.30%
Si is an effective component for securing the strength of the base material, and it is necessary to add 0.10% or more. However, excessive addition causes hardening of the HAZ, so the upper limit was made 0.30%.

Mn: 1.20 to 1.60%
Mn is an effective component for securing the strength and toughness of the base material and needs to be added in an amount of 1.20% or more. Moreover, the upper limit was made 1.60% in consideration of the toughness and cracking of the weld.

P: 0.015% or less P is desirable as the content is small, but 0.015%, which can be industrially realized, was made the upper limit.

S: 0.002% or less S is preferable as the content is small, but 0.002%, which can be industrially realized, was set as the upper limit.

Ni: 0.10 to 0.50%
Ni is an effective component for strength and toughness, but considering the manufacturing cost, the upper limit of addition was set to 0.50% and the lower limit was set to 0.10%.

Cr: 0.05-0.30%
Cr is an effective component for strength and toughness, but excessive addition reduces the toughness in the HAZ part, so the upper limit was made 0.30% and the lower limit was made 0.05%.

Mo: 0.05-0.30%
Mo is a component that improves the strength and toughness of the base material after the solution heat treatment. If less than 0.05%, the effect cannot be obtained, and excessive addition impairs the toughness of the HAZ part. %, And the upper limit was 0.30%.

Cu: 0.05-0.30%
Cu is effective in improving the strength, but excessive addition increases the cracking sensitivity during welding, so the upper limit was made 0.30% and the lower limit was made 0.05%.

V: 0.010-0.040
V is an effective component for ensuring strength, but excessive addition impairs toughness, so the upper limit was set to 0.040% and the lower limit was set to 0.010%.

Al: 0.01-0.10%
Al is an element effective for crystal grain refinement, but if added too much, excessive precipitates may be produced, which may adversely affect the performance of the base material. Therefore, the upper limit of the addition amount is 0.10%, the lower limit The value is 0.010%.

Ti: 0.005% to 0.020%
Ti has the effect of refining the structure after the solution heat treatment, but the added amount is not effective if it is 0.005% or less, and if it exceeds 0.020%, the toughness deteriorates, so the upper limit is set to 0.020. %, And the lower limit was set to 0.005%.

Nb: 0.030 to 0.10%
Nb is a component that prevents coarsening of the austenite grains and contributes to the refinement of the structure, and at the same time, the strength can be improved by finely dispersing Nb carbide or the like in the base material. In order to obtain the effect, 0.030% or more of addition is necessary, but if it exceeds 0.10%, there is a risk of surface scratches occurring in the steel ingot, so the upper limit is 0.10%, The lower limit was set to 0.030%.

N: 0.0030 to 0.0090%
N is effective for improving the strength of the base metal by solid solution strengthening, but excessive addition increases the solid solution N and causes a decrease in HAZ toughness. Therefore, the upper limit is set to 0.0090% and the lower limit is set to 0.0030%.

In the present invention, a laminated material made of a material is appropriately selected according to the usage application of the clad steel sheet, and a laminated material having excellent corrosion resistance is used in an application where corrosion resistance is required. For example, non-ferrous metals such as stainless steel, high alloy steel such as Ni-base alloy, and Cu alloy can be used. In the present invention, the material of the laminated material is not limited to a specific material, and a material having characteristics according to the intended use can be used.

  As described above, according to the method for producing a high-strength clad steel plate of the present invention, in the method for producing a clad steel plate clad with a C-Mn steel base material and a laminated material, the carbon equivalent is 0.4 mass% or less. The C-Mn steel base material and the laminated material are combined and heated to a temperature in the range of 1150 ° C to 1250 ° C to perform hot rolling, and finish rolling in the temperature range of 750 ° C to 950 ° C is completed. Since tempering is performed after quenching, high strength and high toughness characteristics can be obtained as the base material performance, and good weldability can be obtained, and a clad steel excellent in practicality can be obtained. Without changing the components of the clad steel plate produced by the conventional method, it is possible to produce a clad steel plate satisfying the performance of APIX70 grade having higher strength. In addition, since the reheating step for quenching is omitted, there is an effect that leads to a reduction in work period, manufacturing cost, and energy consumption.

Hereinafter, an embodiment of the present invention will be described.
The base material for clad steel of the present invention preferably has a carbon equivalent of 0.4 mass% or less (calculated by Formula (1)) within the above-described component range, and a weld crack sensitivity index of 0.2 mass% or less (Formula (2). ), And can be melted by a conventional method or the like. As the base material, the material of the laminated material is selected depending on the application and the like, and the clad steel plate is formed by hot rolling. In corrosive applications such as those used in natural gas pipelines, for example, a high alloy such as Alloy 825 can be used as a bonding material. In addition, the base material thickness, the laminated material thickness, and the rolling reduction are set so that the thickness of the steel plate after cladding is preferably 50 mm or less.

  In the hot rolling, the combined C-Mn steel base material and the laminated material are combined and heated in a range of 1150 to 1250 ° C. In order to obtain two sets of clad steel, the combination can be sandwiched and hot rolled. In the hot rolling, rolling is performed at a desired reduction rate in one or a plurality of passes, and in finishing rolling, a finishing temperature is set to 750 to 950 ° C. In one pass of hot rolling, the pass is finish rolling.

  After finish rolling, water quenching is preferably performed directly using a water tank whose temperature is controlled to 30 ° C. or less. In this case, it is desirable to obtain a cooling rate of 100 ° C./min or more from 800 to 500 ° C. at the thickness center of the clad steel.

After the quenching, using a heating furnace, preferably below A _c1 point performing tempering heated to a temperature of 550 to 650 ° C.. Examples of the heating time during tempering include 0.5 to 1.0 hour.
The obtained clad steel sheet has excellent characteristics obtained by selecting appropriate components and direct water quenching after hot rolling with controlled temperature. In the base material characteristics, tensile strength ≧ 564 MPa, 0.2% proof stress It becomes possible to obtain the performance of ≧ 483 MPa, vE−40 ° C. ≧ 35 J. The clad steel plate may be used as a clad steel pipe in addition to the plate shape.

Below, the Example of this invention performed in order to confirm the performance of a clad steel base material is shown.
After 50 kg of test steel having the composition shown in Table 1 was melted using a high-frequency vacuum melting furnace, it was heated to 1200 ° C. and rolled to 25 mm by hot rolling. At that time, the rolling finishing temperature was 6 conditions between 650 ° C and 940 ° C. Immediately after rolling, water quenching was performed, followed by tempering at 580 ° C. × 2 h. In addition, for comparison, a steel plate that was subjected to finish rolling at 850 ° C., which was a conventional method, and then air-cooled to room temperature and quenched at 950 ° C. × 1 h and tempered at 580 ° C. × 2 h was produced.

  In FIG. 1, the tension test result of each steel plate prepared above is shown. From FIG. 1, when the rolling finish temperature is 750 ° C. or higher, the tensile strength (TS) and the 0.2% proof stress (0.2% Y.S.) are compared with the steel sheet obtained by the conventional tempering method. In both cases, an increase of 100 MPa or more was observed, and it was confirmed that the strength of X70 grade was sufficiently satisfied.

Furthermore, the Charpy impact test result of each steel plate is shown in FIG. Although the impact value of the direct-quenched material (DQT material) was inferior to that of the steel sheet obtained by the conventional tempering method, the result was as good as 100 J or more at -40 ° C under any conditions.
In the clad steel plate manufacturing method according to the invention method, a clad steel plate having a higher strength can be produced than in the conventional method, and the toughness has an impact value of −100 ° C. or more at −40 ° C. and has sufficient performance. It can be said that.

  One of the indexes for evaluating the weldability is the hardness of the welded portion. Table 2 shows the results of measuring the hardness of the welded portion when a welding test is performed on the clad steel plate manufactured according to the present invention. In addition, the value of Table 2 is an average value as a result of performing multipoint measurement. Moreover, 1 / 2t in the table | surface has shown the 1/2 depth of thickness. Since the hardness of the HAZ is about HV210 and no hardening due to thermal influence is observed, it is judged that the weldability of the clad steel plate manufactured according to the present invention is good.

It is a graph which shows the tension test result of the clad steel plate base material manufactured by the present invention in an example, and the clad steel plate base material manufactured by the conventional tempering method. It is a graph which shows the Charpy impact test result of the clad steel plate base material manufactured by this invention in an Example, and the clad steel plate base material manufactured by the conventional tempering method. It is a figure which shows the plate | board thickness in each cooling method, and the cooling rate between 800 degreeC and 500 degreeC.

Claims (6)

  In the manufacturing method of the clad steel plate which clad the C-Mn steel base material and the laminated material, the C-Mn steel base material and the laminated material having a carbon equivalent of 0.4% by mass or less are combined and 1150 ° C to 1250 ° C. A method for producing a high-strength clad steel plate, characterized by performing hot rolling by heating to a range of 950 ° C., finishing the finish rolling in a temperature range of 750 ° C. to 950 ° C., and performing tempering after direct water quenching .   The method for producing a high-strength clad steel plate according to claim 1, wherein the total thickness of the clad steel plate after rolling is 50 mm or less.   The direct water quenching after rolling is performed using a water tank whose water temperature is controlled to 30 ° C. or lower after the rolling is finished in the above temperature range, and at 800 ° C. to 500 ° C. at the center of the thickness of the clad steel plate at that time. The method for producing a high-strength clad steel sheet according to claim 1 or 2, wherein the cooling rate is 100 ° C / min or more. After quenching, the method of producing a high strength clad steel according to any one of claims 1 to 3, characterized in that the tempering at a 500 ° C. to 650 ° C. or less A _c1 point by using a heating furnace. The clad steel plate base material after quenching and tempering has the properties of tensile strength ≧ 564 MPa, 0.2% proof stress ≧ 483 MPa, vE−40 ° C. ≧ 35 J, and the carbon equivalent (C _eq ) of the following formula (1) And the weld cracking sensitivity index (P _CM ) of the formula (2) satisfies C _eq ≦ 0.4 mass% and P _CM ≦ 0.2 mass%. A manufacturing method of a strength clad steel plate.
Formula (1): C _eq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (mass%)
Formula (2): P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (mass%)   The composition of the C-Mn steel base material is C: 0.03 to 0.10%, Si: 0.10 to 0.30%, Mn: 1.20 to 1.60%, P: 0% by mass. 0.015% or less, S: 0.002% or less, Ni: 0.10 to 0.50%, Cr: 0.05 to 0.30%, Mo: 0.05 to 0.30%, Cu: 0.0. 05-0.30%, V: 0.010-0.040%, Ti: 0.005% -0.020%, Nb: 0.030-0.10%, N: 0.0030-0.0090 %, Al: alloy element in a range of 0.01 to 0.10%, the balance being a low alloy steel consisting of Fe and inevitable impurities. Manufacturing method of high strength clad steel sheet.

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