JP6365210B2 - Steel sheet excellent in high temperature strength and toughness and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate excellent in high-temperature strength and toughness and a method for producing the same. The steel plate manufactured by this manufacturing method can be used in general for welded structures such as shipbuilding, bridges, buildings, marine structures, pressure vessels, tanks, line pipes, etc. In particular, post-weld heat treatment called PWHT (Post Weld Heat Treatment). It is effective for use in a pressure vessel that requires a tensile strength of 470 MPa or more at 452 ° C. and a Charpy impact absorption energy of 150 J or more at −40 ° C. at the center of the later steel plate.

  A pressure vessel used in a desulfurization tower of a crude oil refining process tends to be large for improving productivity, and a steel plate having a large thickness, for example, a steel plate having a thickness of 100 to 200 mm may be used. Such steel plates for pressure vessels are required to have high temperature strength and low temperature toughness. In order to increase production efficiency, the demand for high-temperature strength tends to increase. For example, 470 MPa or more is often required at a tensile strength of 454 ° C. The high temperature strength here refers to the high temperature strength of the steel sheet after PWHT. On the other hand, the demand for low-temperature toughness is also increasing. For example, 150 J or more is often required with a Charpy impact absorption energy of −40 ° C. The toughness here refers to the toughness of the steel sheet after PWHT. Generally, as the plate thickness of a steel plate increases, the high-temperature strength and toughness at the center of the plate thickness decrease, so that a steel plate having excellent high-temperature strength and low-temperature toughness after PWHT is provided at the center of the plate thickness. It is difficult.

  In response to such problems, techniques for improving the toughness of steel plates for pressure vessels have been proposed. Patent Document 1 discloses an invention in which high-temperature strength is ensured by adding V. However, with this method, sufficient toughness cannot be obtained with a thick steel plate.

  Patent Document 2 proposes an invention in which the structure is made bainite and tempered martensite and refined, and the toughness is improved by suppressing the coarsening of carbides. However, this technology cannot provide sufficient high-temperature strength.

  In other words, with the current technology, it is not possible to provide a steel plate having a large plate thickness and excellent in high-temperature strength and toughness in the central portion of the plate thickness after PWHT.

JP-A-4-183842 JP 2014-95130 A

  An object of the present invention is to provide a steel plate having a large plate thickness and excellent in high-temperature strength and toughness in a plate thickness central portion after PWHT and a method for producing the same.

The present invention provides a steel plate having a large plate thickness and excellent in high-temperature strength and toughness in the plate thickness central portion after PWHT, and a method for producing the same, and the gist thereof is as follows.
(1) Steel is in mass%, C: 0.13% to 0.15%, Si: 0.02% to 0.10%, Mn: 0.40% to 0.60%, P : 0.0010% to 0.0150%, S: 0.0001% to 0.0035%, Cu: 0.00% to 0.20%, Ni: 0.01% to 0.25% , Cr: 2.00% to 2.50%, Mo: 0.90% to 1.10%, Nb: 0.005% to 0.070%, V: 0.295% to 0.350 %: Ti: 0.005% to 0.030%, Al: 0.020% to 0.080%, B: 0.0005% to 0.0020%, N: 0.0005% to 0 .0090% or less, the balance being a steel composition consisting of Fe and inevitable impurities, In represented by X is not less 22.6 or more, a tensile strength at 454 ° C. after stress relief annealing is 470MPa or more, the reciprocal of the square root of the effective crystal grain size of ^{8.5 mm -1/2} or more, the stress A steel sheet excellent in high-temperature strength and toughness, wherein the steel sheet after removal annealing has a Charpy impact absorption energy at -40 ° C of 150 J or more.
X = 4Cr + 11Mo + 10V (A)
Here, Cr: mass% of Cr, Mo: mass% of Mo, and V: mass% of V.

(2) Further, by mass%, Ca: 0.0003% or more and 0.0040% or less, Mg: 0.0003% or more and 0.0040% or less, REM: 0.0003% or more and 0.0040% or less The steel sheet excellent in high-temperature strength and toughness as described in (1) above, wherein the steel composition contains a seed or more, and the balance is Fe and inevitable impurities.

(3) Steel is mass%, C: 0.13% to 0.15%, Si: 0.02% to 0.10%, Mn: 0.40% to 0.60%, P : 0.0010% to 0.0150%, S: 0.0001% to 0.0035%, Cu: 0.00% to 0.20%, Ni: 0.01% to 0.25% , Cr: 2.00% to 2.50%, Mo: 0.90% to 1.10%, Nb: 0.005% to 0.070%, V: 0.295% to 0.350 %: Ti: 0.005% to 0.030%, Al: 0.020% to 0.080%, B: 0.0005% to 0.0020%, N: 0.0005% to 0 .0090% or less, the balance being Fe and inevitable impurities, represented by the following formula (A) A slab having a steel composition with X of 22.6 or more is heated to 1100 ° C. or more and 1280 ° C. or less to perform rough rolling with a reduction ratio of 1.5 or more, and the time from the end of rough rolling to the start of finish rolling Is 100 seconds or less, finish rolling is performed such that the reduction ratio is 1.3 or more and the temperature before the first pass is 800 ° C. or more, and then re-heated to 880 ° C. or more and 1040 ° C. or less, and then quenched with water. then have line tempering for performing air cooling after heating to 670 ° C. or higher 770 ° C. or less, and a tensile strength at 454 ° C. after stress relief annealing is 470MPa or more, the inverse of the square root of the effective crystal grain size of 8.5 mm ^{- A} method for producing a steel sheet excellent in high-temperature strength and toughness, characterized by obtaining a steel sheet having a Charpy impact absorption energy at -40 ° C. of 150 J or more at −40 ° C. Law.
X = 4Cr + 11Mo + 10V (A)
Here, Cr: mass% of Cr, Mo: mass% of Mo, and V: mass% of V.

(4) Further, by mass%, Ca: 0.0003% or more and 0.0040% or less, Mg: 0.0003% or more and 0.0040% or less, REM: 0.0003% or more and 0.0040% or less The method for producing a steel sheet having excellent high-temperature strength and toughness as described in (3) above, wherein the steel composition contains at least a seed and the balance is Fe and inevitable impurities.

  According to the present invention, it is possible to provide a steel plate having a large plate thickness, excellent high-temperature strength, and excellent toughness at the central portion of the plate thickness, and a method for producing the same. .

It is a graph which shows the relationship between the tensile strength in 454 degreeC, and X. FIG. It is a rough showing the relationship between the inverse of the square root of the effective crystal grain size and toughness. It is a graph which shows the relationship between the reciprocal number of the square root of an effective crystal grain size, and transfer time.

The present invention will be described in detail.
The inventors have steel plates called A542D-4A and SA542D-4A by ASTM and ASME, that is, containing about 2.25% of Cr and about 1.0% of Mo, at a high temperature of about 200 to 550 ° C. Among steel plates with excellent strength, various methods for improving the high-temperature strength and toughness of the steel plates after PWHT were investigated for steel plates having a large thickness of about 100 to 200 mm.
As for the high-temperature strength, as a result of various investigations on alloy components for satisfying the condition that the tensile strength at 454 ° C. is 470 MPa or more, as shown in FIG.
X = 4Cr + 11Mo + 10V (A)
Here, Cr: mass% of Cr, Mo: mass% of Mo, V: mass% of V
It was found that the condition was satisfied when the value of X expressed by Therefore, the value of X in the present invention is defined as 22.6 or more. Although there is no particular upper limit for the value of X, the upper limit of X is 25.6 due to the upper limit of the amount of Cr, Mo, V added.

  Regarding toughness, various investigations were made on manufacturing conditions for satisfying the condition that Charpy impact absorption energy at a test temperature of −40 ° C. is 150 J or more. As a result, even when steel sheets are produced by quenching and tempering, it is important to refine the structure before quenching, and it is effective to refine austenite before transformation as much as possible by recrystallization during hot rolling. I found out. The temperature range of rough rolling and finish rolling in the present invention is mostly the recrystallization temperature range because the plate thickness may be large. By refining as much as possible to refine the austenite, the final effective grain size Can reduce toughness.

The inventor investigated the influence of the transfer time between rough rolling and finish rolling. As a result, as shown in Fig. 2 and Fig. 3, the transfer time is shortened to suppress recovery during transfer, and the strain introduced in rough rolling and finish rolling is effectively utilized for recrystallization. It has been found that toughness can be improved through refinement of crystal grain size. As shown in FIG. 2, by setting the transfer time to 100 seconds or less, the reciprocal d ^−1/2 of the square root of the effective crystal grain size becomes 8.5 mm ^−1/2 or more, and as shown in FIG. The Charpy impact absorption energy at 40 ° C. can be 150 J or more. Therefore, the transfer time in the present invention is defined as 100 seconds or less. Here, the transfer time indicates the time required when the final pass of rough rolling is used as the starting point and the first pass of finish rolling is used as the end point.

  In the present invention, the effective crystal grain size refers to a particle size measured by EBSD. For example, in EBSD, software attached to EBSD, such as OIM-Analysis, is defined as grain boundaries where the orientation difference is 15 ° or more based on data measured at an interval of 0.2 μm / step for an area of 160 μm × 160 μm. A histogram is created using spreadsheet software based on the data of the area of each crystal grain calculated using, and the average area of each section is multiplied by the percentage of the total area. Based on the sum of these values for all sections, the value calculated as the equivalent circle diameter is the effective crystal grain size.

The range of alloy elements of the steel sheet is specified below.
Since C is an element essential for ensuring strength, its addition amount is set to 0.13% or more. However, on the other hand, an increase in the amount of C causes a decrease in toughness due to the formation of coarse precipitates, so the upper limit is made 0.15%.

  Since Si is an element essential for ensuring strength, its addition amount is set to 0.02% or more. However, on the other hand, addition of Si exceeding 0.10% causes a decrease in toughness and weldability, so the upper limit is made 0.10%.

  Mn is an element effective for increasing the strength, and at least 0.40% or more must be added. Conversely, if added over 0.60%, temper embrittlement susceptibility increases and toughness decreases. . Therefore, the amount of Mn added is defined as 0.40% or more and 0.60% or less.

  If P is less than 0.0010%, the productivity is greatly reduced due to an increase in the refining load, which is not preferable. Moreover, when it exceeds 0.0150%, toughness will fall by temper embrittlement. Therefore, the addition amount of P is defined as 0.0010% or more and 0.0150% or less.

  If S is less than 0.0001%, productivity is greatly reduced due to an increase in the refining load, which is not preferable. Moreover, when it exceeds 0.0035%, toughness will fall. Therefore, the addition amount of S is defined as 0.0001% or more and 0.0035% or less.

  When Cu exceeds 0.20%, toughness decreases. Therefore, the addition amount of Cu is specified to be 0.20% or less. The lower limit is not particularly specified and may be 0.00%. However, it is preferable to add 0.01% or more in order to ensure the strength.

  Ni needs to be added in an amount of 0.01% or more in order to ensure toughness, but if it exceeds 0.25%, the manufacturing cost will increase significantly. Therefore, the addition amount of Ni is defined as 0.01% or more and 0.25% or less.

  Cr needs to be added in an amount of 2.00% or more due to the strength and oxidation resistance at high temperatures, but if added over 2.50%, the toughness and weldability deteriorate. Therefore, the addition amount of Cr is defined as 2.00% or more and 2.50% or less.

  Mo needs to be added in an amount of 0.90% or more because of its high temperature strength, but if it exceeds 1.10%, the toughness and weldability are lowered. Therefore, the addition amount of Mo is defined as 0.90% or more and 1.10% or less.

  Nb is an element effective for securing strength. If the addition is less than 0.005%, the effect is small, and if it exceeds 0.070%, the toughness is lowered. Therefore, the addition amount of Nb is defined as 0.0005% or more and 0.070% or less.

  V is an element effective for securing the strength. If the addition is less than 0.295%, the effect is small, and if it exceeds 0.350%, the toughness and weldability are reduced. Therefore, the addition amount of V is defined as 0.295% or more and 0.350% or less.

  Ti is an element effective for improving toughness. If the addition is less than 0.005%, the effect is small, and if it exceeds 0.030%, the toughness is reduced. Therefore, the addition amount of Ti is defined as 0.005% or more and 0.030% or less.

  Al is an element that is effective as a deoxidizing material and is effective in forming nitrides and ensuring the hardenability of B. A minimum addition of 0.020% is required. Conversely, if added over 0.080%, the toughness is reduced through the formation of alumina clusters through molten steel reoxidation. Therefore, the addition amount of Al is defined as 0.020% or more and 0.080% or less.

  B is an important element for improving hardenability. If it is less than 0.0005%, the effect of increasing hardenability cannot be obtained, and if it exceeds 0.0020%, the toughness decreases due to the formation of B carbonitride. Therefore, the addition amount of B is defined as 0.0005% or more and 0.0020% or less.

If N is less than 0.0005%, the productivity decreases due to an increase in the refining load, and if it exceeds 0.0090%, the toughness decreases. Therefore, the addition amount of N is defined as 0.0005% or more and 0.0090% or less.
In the present invention, the following elements can be further added.

  Ca is an element effective for preventing nozzle clogging. If the addition is less than 0.0003%, the effect is small, and if the addition exceeds 0.0040%, the toughness is reduced. Therefore, the addition amount of Ca is defined as 0.0003% or more and 0.0040% or less.

  Mg is an element effective for improving toughness. If the addition is less than 0.0003%, the effect is small, and if the addition exceeds 0.0040%, the toughness is reduced. Therefore, the addition amount of Mg is defined as 0.0003% or more and 0.0040% or less.

  REM is an element effective for improving toughness. If the addition is less than 0.0003%, the effect is small, and if the addition exceeds 0.0040%, the toughness is reduced. Therefore, the amount of REM added is specified to be 0.0003% or more and 0.0040% or less.

  In manufacturing steel sheets and welding materials, Zn, Sn, Sb, etc., which can be mixed as raw materials including additive alloys or unavoidable impurities eluted from furnace materials during melting, are less than 0.002%. If it is mixed, the effect of the present invention is not impaired.

  Next, the manufacturing method of the steel plate of this invention is described. A steel plate is manufactured by hot rolling a slab manufactured by continuous casting and then quenching, tempering, or normalizing, quenching, and tempering.

  First, hot rolling will be described. When the heating temperature of hot rolling exceeds 1280 ° C., austenite becomes coarse and toughness decreases. Further, when the heating temperature is lower than 1100 ° C., the productivity is significantly reduced. Therefore, the heating temperature at the time of hot rolling is defined as 1100 ° C. or more and 1280 ° C. or less. The holding time after heating is not particularly defined. However, from the viewpoint of uniform heating and ensuring productivity, the holding time at the heating temperature is preferably 2 hours or more and 10 hours or less.

After heating, rough rolling and finish rolling are performed. When the rolling reduction ratio of the rough rolling is less than 1.5, recrystallization does not proceed sufficiently, the structure becomes coarse and the toughness decreases. Therefore, the rolling ratio in rough rolling is defined as 1.5 or more. Further, as described above, in order to reduce the effective crystal grain size and improve the toughness, the transfer time between rough rolling and finish rolling is set to 100 seconds or less. 2 and 3, the reciprocal d ^−1/2 of the square root of the effective crystal grain size is 8.5 mm ^−1/2 or more, and the Charpy impact absorption energy at −40 ° C. can be 150 J or more. In finish rolling, when the reduction ratio is less than 1.3, recrystallization does not proceed sufficiently, the structure becomes coarse and the toughness decreases. Therefore, the reduction ratio in finish rolling is defined as 1.3 or more. Further, when the temperature before the first pass of finish rolling is less than 800 ° C., the productivity is greatly reduced. Therefore, the temperature before the first pass is preferably 800 ° C. or more. Here, the reduction ratio is a value obtained by dividing the plate thickness before the start of rolling by the plate thickness after the end of rolling in each rolling of rough rolling and finish rolling. Further, the temperature before one finishing pass refers to the temperature measured on the surface of the steel plate immediately before the final pass of finishing rolling.

  Next, quenching will be described. If the heating temperature during quenching exceeds 1040 ° C., the austenite becomes coarse, and if it is less than 880 ° C., two-phase heating occurs and the toughness decreases. Therefore, the heating temperature during quenching is defined as 880 ° C. or more and 1040 ° C. or less. Perform water cooling after heating and holding during quenching. Here, it is preferable to cool to 100 ° C. or lower.

  Next, tempering will be described. If the heating temperature during tempering exceeds 770 ° C, the strength is insufficient, and if it is less than 670 ° C, the toughness decreases. Air-cool after heating and holding during tempering. In order to stabilize the material, normalization can be performed prior to quenching. The heating temperature at this time is preferably 900 ° C. or higher and 1000 ° C. or lower.

  High-temperature tensile tests and Charpy impact tests were carried out on steel plates having a thickness of 50, 100, and 200 mm manufactured under various chemical components and manufacturing conditions and subjected to heat treatment assuming PWHT. Table 1 shows the evaluation results of the chemical composition, thickness, X, reciprocal of the square root of the effective crystal grain size, production method, and characteristics of the steel sheet. The heat treatment assuming PWHT was performed at 719 ° C. and held for 1920 minutes. A round bar tensile specimen was used for the high temperature tensile test. The test specimens were collected so that the longitudinal direction of the test specimens was perpendicular to the rolling direction at a site that entered the steel sheet surface by a half of the thickness. Two tests were conducted at a test temperature of 454 ° C., and the average value of the two was determined to be 470 MPa or more. The Charpy impact test is performed by placing a full-size test piece of 2 mm V notch test piece in a position where the length of the test piece is perpendicular to the rolling direction, at a site that is half the thickness of the steel sheet. The line connecting the leading edges of the samples was taken so as to be parallel to the thickness direction. Three tests were conducted at a test temperature of −40 ° C., and the average value of the three was determined to be 150 J or more. As shown in Examples 1 to 26 in Table 1, a steel sheet having excellent high-temperature strength and toughness was obtained by manufacturing a steel sheet using the components and manufacturing method defined in the present invention.

  From the above examples, it is clear that the steel plates of Invention Examples 1 to 34, which are steel materials manufactured according to the present invention, are steel materials excellent in high temperature strength and toughness.

Claims (4)

Steel is mass%
C: 0.13% to 0.15%,
Si: 0.02% or more and 0.10% or less,
Mn: 0.40% or more and 0.60% or less,
P: 0.0010% or more and 0.0150% or less,
S: 0.0001% to 0.0035%,
Cu: 0.00% or more and 0.20% or less,
Ni: 0.01% or more and 0.25% or less,
Cr: 2.00% to 2.50%,
Mo: 0.90% to 1.10%,
Nb: 0.005% or more and 0.070% or less,
V: 0.295% or more and 0.350% or less,
Ti: 0.005% or more and 0.030% or less,
Al: 0.020% or more and 0.080% or less,
B: 0.0005% or more and 0.0020% or less,
N: 0.0005% or more and 0.0090% or less, the balance being a steel composition composed of Fe and inevitable impurities, X represented by the following formula (A) is 22.6 or more, and stress relief The tensile strength at 454 ° C. after annealing is 470 MPa or more, the reciprocal of the square root of the effective crystal grain size is 8.5 mm ^−1/2 or more, and the Charpy impact absorption energy at −40 ° C. of the steel plate after stress relief annealing is A steel sheet excellent in high-temperature strength and toughness, characterized by being 150 J or more.
X = 4Cr + 11Mo + 10V (A)
Here, Cr: mass% of Cr, Mo: mass% of Mo, and V: mass% of V. In addition,
Ca: 0.0003% or more and 0.0040% or less,
Mg: 0.0003% or more and 0.0040% or less,
REM: High temperature strength and toughness according to claim 1, characterized in that it contains any one or more of 0.0003% or more and 0.0040% or less, and the balance is a steel composition composed of Fe and inevitable impurities. Excellent steel plate. Steel is mass%
C: 0.13% to 0.15%,
Si: 0.02% or more and 0.10% or less,
Mn: 0.40% or more and 0.60% or less,
P: 0.0010% or more and 0.0150% or less,
S: 0.0001% to 0.0035%,
Cu: 0.00% or more and 0.20% or less,
Ni: 0.01% or more and 0.25% or less,
Cr: 2.00% to 2.50%,
Mo: 0.90% to 1.10%,
Nb: 0.005% or more and 0.070% or less,
V: 0.295% or more and 0.350% or less,
Ti: 0.005% or more and 0.030% or less,
Al: 0.020% or more and 0.080% or less,
B: 0.0005% or more and 0.0020% or less,
N: A slab having a steel composition containing 0.0005% or more and 0.0090% or less, the balance being Fe and inevitable impurities, and having X of 22.6 or more represented by the following formula (A) at 1100 ° C. Heating to 1280 ° C. or less, rough rolling to a reduction ratio of 1.5 or more, finishing from the end of the rough rolling to the start of finish rolling to 100 seconds or less, with a reduction ratio of 1.3 or more Perform finish rolling so that the temperature before one pass is 800 ° C or higher, then reheat to 880 ° C or higher and 1040 ° C or lower, quench with water, and then heat to 670 ° C or higher and 770 ° C or lower, and then air-cool There line tempering, and the tensile strength at 454 ° C. after stress relief annealing is 470MPa or more, the inverse of the square root of the effective crystal grain size by ^{8.5 mm -1/2} or more, the steel sheet after stress relief annealing -40 At ℃ A method for producing a steel sheet excellent in high-temperature strength and toughness, characterized by obtaining a steel sheet having Charpy impact absorption energy of 150 J or more .
X = 4Cr + 11Mo + 10V (A)
Here, Cr: mass% of Cr, Mo: mass% of Mo, and V: mass% of V. In addition,
Ca: 0.0003% or more and 0.0040% or less,
Mg: 0.0003% or more and 0.0040% or less,
REM: High-temperature strength and toughness according to claim 3, characterized in that it contains any one or more of 0.0003% or more and 0.0040% or less, and the balance is a steel composition composed of Fe and inevitable impurities. Steel sheet manufacturing method with excellent performance.