JP6398575B2 - Steel sheet with excellent toughness and method for producing the same - Google Patents

Description

  The present invention relates to a steel plate having excellent toughness and a method for producing the same. The steel plate produced by this method can be used in general for welded structures such as shipbuilding, bridges, buildings, marine structures, pressure vessels, tanks, line pipes, etc., and is particularly used at high temperatures of about 200 to 500 ° C., and It is effective for use in a pressure vessel that requires a toughness of about 27 J at −32 ° C. to a steel plate after post-weld heat treatment called PWHT (Post Weld Heat Treatment).

  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 a tensile strength of about 515 to 690 MPa at room temperature strength and various strengths at a high temperature of 200 to 500 ° C., and external inspection is required for inspection during production and periodic inspection during use. Because it is exposed to the same temperature as the environment, low temperature toughness at the installation environment temperature is also required. The toughness here refers to the toughness of the steel sheet after PWHT. As the thickness of the steel plate increases, the toughness at the center of the plate thickness decreases, so the steel plate has a large plate thickness, excellent high-temperature strength, and excellent toughness at the center of the plate thickness, particularly excellent toughness after PWHT. It is difficult to provide a steel plate.

  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 toughness is improved by adding Nb or Ca. However, with this method, a steel sheet with a large thickness and a small cooling rate during quenching does not sufficiently improve the hardenability, so the effect of improving toughness is small.

  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, in this technique, since the amount of Al added is high, the effect of improving toughness is small because re-oxidation of molten steel in the steelmaking process causes toughness reduction due to an increase in alumina clusters.

  In other words, the current technology cannot provide a steel sheet having a large thickness, excellent high-temperature strength, and excellent toughness at the center of the thickness.

Japanese Patent No. 2743765 JP 2014-95130 A

  An object of the present invention is to provide a steel plate having a large plate thickness, excellent high-temperature strength, and excellent toughness at the center of the plate thickness, and a method for producing the same.

The present invention provides 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, the gist of which is as follows.
(1) Steel is in mass%, C: 0.10% to 0.20%, Si: 0.02% to 0.80%, Mn: 0.30% to 1.00%, P : 0.0010% to 0.0150%, S: 0.0001% to 0.0035%, Cu: 0.00% to 0.50%, Ni: 0.010% to 0.500% , Cr: 0.00% to 1.80%, Mo: 0.00% to 0.80%, Ti: 0.012% to 0.030%, Al: 0.010% to 0.090 % Or less, B: 0.0005% or more and 0.0014% or less, N: 0.0005% or more and 0.0090% or less, with the balance being Fe and inevitable impurities, and the following formula (A X is 0.200 or more and the reciprocal of the square root of the effective crystal grain size is 9.0 m. In ^-1/2 or more, the steel sheet in the Charpy impact absorption energy at -32 ° C. the steel sheet after stress relief annealing at least 27 J, the tensile strength is equal to or less than 690MPa or more 515MPa, which is excellent in toughness.
X = 1.78Al + Ti-3.4N + 346B (A)
Here, Al: mass% of Al, Ti: mass% of Ti, N: mass% of N, and B: mass% of B.

(2) The reciprocal of the square root of the effective crystal grain size is 10.0 mm ^−1/2 or more, and the Charpy impact absorption energy at −32 ° C. of the steel sheet after stress relief annealing is 54 J or more, A steel sheet having excellent toughness as described in 1).

(3) Further, by mass%, Nb: 0.005% to 0.030%, V: 0.020% to 0.070%, Ca: 0.0003% to 0.0040%, Mg: 0 .0003% or more and 0.0040% or less, REM: 0.0003% or more and 0.0040% or less of any one or more, and the balance is a steel composition composed of Fe and inevitable impurities The steel plate excellent in toughness as described in said (1) or (2).

(4) Steel is in mass%, C: 0.10% to 0.20%, Si: 0.02% to 0.80%, Mn: 0.30% to 1.00%, P : 0.0010% to 0.0150%, S: 0.0001% to 0.0035%, Cu: 0.00% to 0.50%, Ni: 0.010% to 0.500% , Cr: 0.00% to 1.80%, Mo: 0.00% to 0.80%, Ti: 0.012% to 0.030%, Al: 0.010% to 0.090 % Or less, B: 0.0005% or more and 0.0014% or less, N: 0.0005% or more and 0.0090% or less, with the balance being Fe and inevitable impurities, represented by the following formula (A) The slab having a steel composition with X of 0.200 or more is set to 1100 ° C or higher and 1280 ° C or lower Heating is performed to perform rough rolling to a reduction ratio of 1.5 or more, and further to finishing rolling to have a reduction ratio of 1.3 or more and a temperature before the first pass of 800 ° C. or more, and then 880 ° C. or more and 960 ° C. perform quenching to water cooling after reheating below, then have rows tempering performing air cooling after heating to 670 ° C. or higher 770 ° C. or less, the inverse of the square root of the effective crystal grain size of 9.0 mm ^-1/2 or more A method for producing a steel sheet having excellent toughness, characterized in that the steel sheet after stress relief annealing has a Charpy impact absorption energy at −32 ° C. of 27 J or more and a tensile strength of 515 MPa or more and 690 MPa or less .
X = 1.78Al + Ti-3.4N + 346B (A)
Here, Al: mass% of Al, Ti: mass% of Ti, N: mass% of N, and B: mass% of B.

(5) The method for producing a steel sheet having excellent toughness as described in (4) above, wherein the time from the end of rough rolling to the start of finish rolling is 100 seconds or less.

(6) Further, by mass%, Nb: 0.005% to 0.030%, V: 0.020% to 0.070%, Ca: 0.0003% to 0.0040%, Mg: 0 .0003% or more and 0.0040% or less, REM: 0.0003% or more and 0.0040% or less of any one or more, and the balance is a steel composition composed of Fe and inevitable impurities The manufacturing method of the steel plate excellent in toughness as described in said (4) or (5).

  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 correlation of the reciprocal number of the square root of an effective crystal grain size, and an alloy component. It is a graph which shows the Charpy impact absorption energy of -32 degreeC. It is a graph which shows the relationship between the reciprocal number of the square root of an effective crystal grain size, and transport time.

The present invention will be described in detail.
The inventors have a steel plate called A387-11-2 and SA387-11-2 by ASTM and ASME, that is, containing about 1.25% of Cr and about 0.5% of Mo, and 200 to 500 ° C. Among steel plates with excellent high-temperature strength, the fracture surface of a Charpy impact test piece of a steel plate having a large thickness and low toughness after PWHT was investigated to investigate the cause of low toughness. As a result, it was found that low toughness steel sheets have large fracture surface units, and there are coarse carbides, resulting in low toughness. The fracture surface was cleaved. The above-mentioned form is the same in the steel plate and the steel plate after PWHT, and particularly in the steel plate after PWHT having low toughness, the tendency of coarsening of carbides was remarkable.

  The inventor examined various methods for improving the toughness of the steel sheet and the steel sheet after PWHT. As a result, we found that the fracture surface unit can be reduced by reducing the effective crystal grain size, that is, the unit of the structure surrounded by the large-angle grain boundary, resulting in improved toughness. To refine the matrix, it is effective to refine the heated austenite during quenching. However, in the case of A387-11-2 and SA387-11-2 steel plates having a large thickness, the hardenability is further reduced by refining the heated austenite at the time of quenching, and the ferrite fraction in the steel structure is reduced. Increase. Since the ferrite formed in such a thick steel plate grows and becomes coarse, even if the heated austenite is fine, the effective crystal grain size of the final structure is not reduced. For this reason, it is effective to refine the heating austenite grain size during quenching and to add B to increase the hardenability.

In order to make the hardenability by B function, it is useful to add Ti that fixes N. However, addition of excessive Ti lowers the toughness due to the formation of Ti carbide, so it cannot be fixed with Ti. In order to fix N, Al is added in combination. As a result of investigating the correlation between the effective crystal grain size and the alloy component and the correlation between the effective crystal grain size and the toughness for various combinations of Al, Ti, N, and B, the inventors obtained the relationship shown in FIGS. . That is, the following formula (A)
X = 1.78Al + Ti + 3.4N + 346B (A)
Here, Al: mass% of Al, Ti: mass% of Ti, N: mass% of N, B: mass% of B
When X represented by is 0.200 or more, the effective crystal grain size d ^−1/2 is 9.0 or more, and the toughness, that is, the Charpy impact absorption energy at −32 ° C. of the steel sheet after PWHT is excellent at 27 J or more. Value can be obtained. Therefore, the value of X represented by X = 1.78Al + Ti + 3.4N + 346B in the present invention is defined as 0.200 or more.

  In addition, although the upper limit of X is not particularly provided, 0.67 is the upper limit from the definition of the four alloy components. 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 the strength, its addition amount is set to 0.10% 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 0.20%.

  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 0.80% causes a decrease in toughness and weldability, so the upper limit is made 0.80%.

  Mn is an element effective for increasing the strength, and at least 0.30% of addition is necessary. On the contrary, if added over 1.00%, the temper embrittlement susceptibility becomes high and the toughness decreases. . Therefore, the addition amount of Mn is defined as 0.30% or more and 1.00% 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.

  If the Cu content exceeds 0.50%, the toughness decreases. Therefore, the addition amount of Cu is defined as 0.01% or more and 0.50% 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 0.010% or more in order to secure toughness, but if it exceeds 0.500%, the manufacturing cost will increase significantly. Therefore, the addition amount of Ni is defined as 0.010% or more and 0.500% or less.

  When Cr exceeds 1.80%, toughness and weldability are lowered. Therefore, the addition amount of Cr is defined as 1.80% or less. The lower limit is not particularly specified and may be 0.00%, but Cr is an element that effectively acts on oxidation resistance and high-temperature strength, and is preferably added at 1.00% or more.

  When Mo exceeds 0.80%, toughness and weldability are lowered. Therefore, the addition amount of Mo is defined as 0.80% or less. The lower limit is not particularly specified and may be 0.00%, but Mo is an element that effectively works for high-temperature strength, and it is preferable to add 0.30% or more.

  Ti is an important element in the present invention. It is an element effective for forming nitrides and ensuring the hardenability of B, and at least 0.012% should be added. Conversely, if added over 0.030%, the toughness decreases due to the formation of carbides. Therefore, the addition amount of Ti is defined as 0.012% or more and 0.030% or less. In addition, when the addition amount of Ti is 0.024% or less, a decrease in toughness is significantly suppressed. Therefore, desirably, the addition amount of Ti is defined as 0.012% or more and 0.024% or less.

  Al is an important element in the present invention. In addition to being effective as a deoxidizing material, it is an element effective for forming a nitride and ensuring the hardenability of B. At least 0.010% addition is required. Conversely, if added over 0.090%, the toughness is reduced through the formation of alumina clusters through molten steel reoxidation. Therefore, the addition amount of Al is defined as 0.010% or more and 0.090% or less. In addition, when the addition amount of Al is 0.045% or less, the toughness reduction is significantly suppressed. Therefore, the addition amount of Al is desirably 0.010% or more and 0.045% or less.

  B is an important element in the present invention. If it is less than 0.0005%, the effect of increasing hardenability cannot be obtained, and if it exceeds 0.0014%, the toughness decreases due to the formation of B carbonitride. Therefore, the addition amount of B is defined as 0.0005% or more and 0.0014% or less. In addition, when the addition amount of B is 0.0010% or less, a decrease in toughness due to the formation of B carbonitride is significantly suppressed. Therefore, the addition amount of B is desirably 0.0005% or more and 0.0010%. The following.

  If N is less than 0.0005%, the productivity decreases due to an increase in the refining load, and if it exceeds 0.009%, it becomes difficult to ensure the hardenability with B. Therefore, the addition amount of N is defined as 0.0005% or more and 0.009% or less. In addition, when the addition amount of N is 0.005% or less, it is easy to ensure the hardenability by B and improve toughness. Therefore, the addition amount of N is preferably 0.0005% or more and 0.005%. The following.

In the present invention, the following elements can be further added.
Nb is an element effective for securing strength. If the addition is less than 0.005%, the effect is small, and if it exceeds 0.030%, the toughness is reduced. Therefore, the amount of Nb added is specified to be 0.0005% or more and 0.030% or less.

  V is an element effective for ensuring the strength. If the addition is less than 0.020%, the effect is small, and if it exceeds 0.070%, the toughness is reduced. Therefore, the addition amount of V is defined as 0.020% or more and 0.030% or less.

  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. The steel sheet is manufactured by quenching and tempering after hot rolling a slab manufactured by continuous casting.

  First, hot rolling will be described. When the heating temperature of hot rolling exceeds 1280 ° C., austenite becomes coarse and toughness decreases. Further, if the heating temperature is less than 1100 ° C., the productivity is greatly reduced, and therefore 1100 ° C. or higher is preferable. 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. 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 significantly lowered. 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. When the heating temperature during quenching exceeds 960 ° C., the austenite becomes coarse, and when it is less than 880 ° C., it becomes two-phase region heating and the toughness is reduced. 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. Therefore, the heating temperature during tempering is defined as 670 ° C. or more and 770 ° C. or less. Air-cool after heating and holding during tempering.

  As described above, the steel sheet of the present invention has a range of alloy components defined in the scope of the invention, and excellent toughness can be obtained by manufacturing by general quenching and tempering, but between rough rolling and finish rolling. By controlling the time, the toughness can be further improved. 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 in the case where hot rolling, quenching, and tempering are in the above ranges. As a result, the transfer time can be shortened to suppress recovery during transfer, and the strain introduced in rough rolling and finish rolling can be effectively utilized for recrystallization, and toughness can be further improved through refinement of the effective crystal grain size. I found. As shown in FIG. 3, when the transfer time is set to 100 seconds or less, the effective crystal grain size d ^−1/2 becomes 10.0 or more. As shown in FIG. 2, the Charpy impact absorption energy at −32 ° C. 54J or more. Therefore, further excellent toughness can be obtained by setting the transfer time in the present invention to 100 seconds or less as required. 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.

  For steel plates having thicknesses of 50, 135, and 200 mm manufactured under various chemical components and manufacturing conditions, a tensile test and a Charpy impact test were performed on the steel plates subjected to heat treatment assuming PWHT. Table 1 shows the evaluation results of the chemical composition, the plate thickness, X, the reciprocal of the square root of the effective crystal grain size, the manufacturing method, and the characteristics of the steel plate. The heat treatment assuming PWHT was performed at 682 ° C. and held for 2005 minutes. The tensile test was performed based on the metal material tensile test method described in JIS Z 2241. 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 performed at room temperature, and the average value of the two samples was determined to be acceptable if it was 515 MPa or more and 690 MPa or less. 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 −32 ° C., and the average value of the three was determined to be 27 J or more. As shown in Examples 1 to 29 in Table 1, a steel sheet with excellent toughness was obtained by manufacturing a steel sheet with 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 29, which are steel materials manufactured according to the present invention, are steel materials having excellent toughness.

Claims (6)

Steel is mass%
C: 0.10% or more and 0.20% or less,
Si: 0.02% or more and 0.80% or less,
Mn: 0.30% or more and 1.00% or less,
P: 0.0010% or more and 0.0150% or less,
S: 0.0001% to 0.0035%,
Cu: 0.00% or more and 0.50% or less,
Ni: 0.010% to 0.500%,
Cr: 0.00% or more and 1.80% or less,
Mo: 0.00% or more and 0.80% or less,
Ti: 0.012% or more and 0.030% or less,
Al: 0.010% or more and 0.090% or less,
B: 0.0005% or more and 0.0014% or less,
N: 0.0005% or more and 0.0090% or less, the balance being a steel composition consisting of Fe and inevitable impurities, X represented by the following formula (A) being 0.200 or more, effective crystals The reciprocal of the square root of the grain size is 9.0 mm ^−1/2 or more, the Charpy impact absorption energy at −32 ° C. of the steel sheet after stress relief annealing is 27 J or more, and the tensile strength is 515 MPa or more and 690 MPa or less. A steel sheet with excellent toughness.
X = 1.78Al + Ti-3.4N + 346B (A)
Here, Al: mass% of Al, Ti: mass% of Ti, N: mass% of N, and B: mass% of B. The reciprocal of the square root of the effective crystal grain size is 10.0 mm ^−1/2 or more, and the Charpy impact absorption energy at −32 ° C. of the steel sheet after stress relief annealing is 54 J or more. Steel sheet with excellent toughness. In addition,
Nb: 0.005% or more and 0.030% or less,
V: 0.020% or more and 0.070% or less,
Ca: 0.0003% or more and 0.0040% or less,
Mg: 0.0003% or more and 0.0040% or less,
The toughness according to claim 1 or 2, characterized in that it contains any one or more of REM: 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: 0.10% or more and 0.20% or less,
Si: 0.02% or more and 0.80% or less,
Mn: 0.30% or more and 1.00% or less,
P: 0.0010% or more and 0.0150% or less,
S: 0.0001% to 0.0035%,
Cu: 0.00% or more and 0.50% or less,
Ni: 0.010% to 0.500%,
Cr: 0.00% or more and 1.80% or less,
Mo: 0.00% or more and 0.80% or less,
Ti: 0.012% or more and 0.030% or less,
Al: 0.010% or more and 0.090% or less,
B: 0.0005% or more and 0.0014% or less,
N: A slab having a steel composition containing 0.0005% or more and 0.0090% or less, the balance being Fe and unavoidable impurities, and having X of 0.200 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, further performing finish rolling to a reduction ratio of 1.3 or more and a temperature before the first pass of 800 ° C. or more, perform quenching to water cooling after reheated to 880 ° C. or higher 960 ° C. or less, have rows tempering performing air cooling after then heated to 670 ° C. or higher 770 ° C. or less, the inverse of the square root of the effective crystal grain size of 9.0mm ^A toughness characterized by obtaining a steel plate having a stress absorption energy at −32 ° C. of −27 ° C. or more and a tensile strength of 515 MPa or more and 690 MPa or less. A method for producing steel sheets with excellent properties.
X = 1.78Al + Ti-3.4N + 346B (A)
Here, Al: mass% of Al, Ti: mass% of Ti, N: mass% of N, and B: mass% of B.   The method for producing a steel sheet with excellent toughness according to claim 4, wherein the time from the end of rough rolling to the start of finish rolling is 100 seconds or less. In addition,
Nb: 0.005% or more and 0.030% or less,
V: 0.020% or more and 0.070% or less,
Ca: 0.0003% or more and 0.0040% or less,
Mg: 0.0003% or more and 0.0040% or less,
REM: 0.003% or more and 0.0040% or less of any one or more types, The balance is the steel composition which consists of Fe and an unavoidable impurity, The toughness of Claim 4 or 5 characterized by the above-mentioned. Excellent steel plate manufacturing method.