JP6693186B2 - Method for producing low-temperature nickel-containing steel sheet excellent in tensile strength and toughness - Google Patents

Description

The present invention relates to a method for producing a low-temperature nickel-containing steel sheet having excellent tensile strength and toughness.

  The steel plate manufactured by this manufacturing method can be generally used for welded structures such as shipbuilding, bridges, buildings, offshore structures, pressure vessels, tanks, line pipes, etc., but especially at low temperatures of about -196 ° C to -160 ° C. It is effective in use in a low temperature tank that requires fracture toughness and tensile strength of 690 MPa or more and 830 MPa or less.

  Along with the tightening of environmental regulations, the development of LNG fueled ships that drive the engine by LNG instead of heavy oil and proceed is underway. As a material for the LNG tank mounted on the LNG fuel ship, it is considered that, in addition to the austenitic stainless steel plate, ferritic low temperature steel plate such as 9% Ni steel and 6% Ni steel can be used.

  However, since the ferrite-containing low-temperature nickel-containing steel sheet for low temperature shows a decrease in toughness due to strain aging, overcoming this is a key to practical use. In order to maintain excellent toughness after strain aging, it is desirable that the minimum value of the Charpy impact absorbed energy at -196 ° C at the time of the base material is about 150J. At the current level, most steel sheets achieve this, but it is not easy to achieve with all steel sheets.

Inclusions may be involved in the low value of the Charpy impact absorbed energy at 196 ° C. of the nickel-containing ferritic nickel-containing steel sheet at a low probability. In the steel slab manufactured by continuous casting, inclusions of several μm remain without being floated and separated, but if the cleanliness is normal, such independent inclusions are Charpy at −196 ° C. The impact on shock absorption energy is minor. However, when the inclusions of several μm form an aggregated cluster, the Charpy impact absorption energy at −196 ° C. may decrease to less than 150 J. The main inclusion is alumina (Al ₂ O ₃ ).

  Cross rolling is a method for reducing the harmful effects of inclusions, for example, elongated inclusions such as MnS. The cross rolling is a hot rolling for forming the shape of a steel sheet, which is a rolling that is usually performed only in the longitudinal direction of the steel sheet, and a part of the rolling is performed in the width direction of the steel sheet. In the case of, the MnS expansion in the longitudinal direction of the steel sheet is suppressed, so that the Charpy impact absorption energy is improved in the Charpy test using the test piece in which the longitudinal direction of the test piece is parallel to the rolling width direction.

  For example, in Patent Literature 1 and Patent Literature 2, bending workability and low temperature toughness are improved by performing widthwise rolling when performing cross rolling in a non-recrystallization temperature range. However, when the width direction rolling in the non-recrystallization temperature region is performed, the austenite grain size before reduction is to be performed in the non-recrystallization region while the grain size is large, and the toughness often decreases, and in this method, I cannot achieve my purpose.

  Further, in Patent Document 3, a steel plate having high isotropy is defined by defining a reduction ratio of widthwise rolling and longitudinal rolling when performing cross rolling. Regarding the control of inclusions, although this method is effective, the effective crystal grain size, which is the factor that most affects Charpy impact absorbed energy, cannot be reduced only by specifying the reduction ratio, so this method achieves the above object. Can not.

  In addition to the above-mentioned low temperature toughness, the LNG tank also requires high strength. In order to construct a large tank, a tensile strength of 690 MPa or more is required, while a weldability constraint of 830 MPa or less is required. That is, the current technology cannot provide a low-temperature nickel-containing steel sheet having excellent tensile strength and toughness.

Japanese Patent No. 4897125 JP, 2005-226080, A JP 2002-161341 A

The problem to be solved by the present invention is to provide a method for producing a low-temperature nickel-containing steel sheet having excellent tensile strength and toughness.

It is a subject matter of the present invention is as shown below.

(1) Steel, in mass%, C: 0.03% or more and 0.10% or less, Si: 0.02% or more and 0.40% or less, Mn: 0.30% or more and 1.20% Hereinafter, Ni: 8.0% or more and 9.5% or less, Al: 0.010% or more and 0.080% or less, TO: 0.0001% or more and 0.0030% or less, and P : 0.0010% or more and 0.0100% or less, S: 0.0001% or more and 0.0035% or less, N: 0.0005% or more and 0.0070% or less, and the balance is Fe and inevitable impurities. After heating the slab to 900 ° C. or more and 1270 ° C. or less, hot rolling is performed, the total reduction ratio of the hot rolling is 0.65 or more, and the reduction ratio of cross rolling performed in the rolling width direction among the hot rolling is 0.1 or more and 0.6 or less, the temperature range of cross rolling is 800 ° C. or more and 100 0 ° C or less, the temperature before one pass for finishing is 600 ° C or more and 850 ° C or less, air cooling is performed after rolling, and further quenching is performed by heating the steel sheet to 760 ° C or more and 880 ° C or less and then water cooling, and subsequently 500 ° C or more and After tempering by heating to 650 ° C or lower, tempering is performed, and a SEM secondary electron beam image, a fracture surface photograph of a Charpy impact test piece is taken at a magnification of 10 times, and the total length of the separation-like vertical fracture surface is measured (unit: Alumina cluster index defined as a value obtained by dividing the measured value by the fracture surface area (80 mm ² ) is 0.030 or less, the effective crystal grain size is 7.0 μm or less, and the average tensile strength is Nitrogen for low temperature with excellent tensile strength and toughness, characterized in that a steel sheet having a value of 690 MPa or more and 830 MPa or less and a Charpy impact absorption energy at -196 ° C. of 150 J or more is obtained. Manufacturing method of Le-containing steel sheet.

(2) Further, Cr: 0.05% or more and 2.0% or less, Mo: 0.02% or more and 1.0% or less, Cu: 0.1% or more and 3.0% or less, Nb: 0. 0.005% to 0.100%, V: 0.010% to 0.500%, Ti: 0.005% to 0.500%, Ca: 0.0001% to 0.0050%, Mg : 0.0001% to 0.0050%, REM: 0.0001% to 0.0100%, Zr: 0.0001% to 0.0100%, B: 0.0002% to 0.0030% 1 or 2 or more are contained, The manufacturing method of the nickel containing steel plate for low temperature excellent in the tensile strength and toughness as described in said (1) characterized by the above-mentioned.

  According to the present invention, it is possible to provide a low-temperature nickel-containing steel sheet excellent in tensile strength and toughness and a method for producing the same, and it can be said that the invention has a high industrial value.

It is a graph which shows the relationship between an alumina cluster index and a cross rolling reduction. It is a graph which shows the relationship between Charpy impact absorption energy and an alumina cluster index. It is a graph which shows the relationship between an effective crystal grain size and cross rolling temperature. It is a graph which shows the relationship between an effective crystal grain size and the total rolling reduction. 6 is a graph showing the relationship between toughness and effective crystal grain size.

  The present invention will be described in detail. Among the low-temperature nickel-containing steel sheets, the inventors have found that alumina clusters, that is, alumina having a major axis of several μm, continuously affect the toughness of steel sheets having a Ni content of 8.0% or more and 9.5% or less in the rolling direction. In order to clarify the effect of aggregates distributed on the surface, the fracture surface of the Charpy impact test specimen was investigated.

  As a result, when the longitudinal direction of the Charpy test piece is parallel to the width direction of the steel sheet, the continuous distribution in the rolling direction of the alumina clusters is parallel to the surface to be the fracture surface of the Charpy test piece. It was newly found that a separation-like coarse void coalescence type fracture surface is formed, which causes reduction of ductile crack growth resistance and reduction of Charpy impact absorption energy through transition from ductile crack growth to brittle fracture. The inventors have conceived that reducing the number of alumina on the plane parallel to the fracture surface of the Charpy test piece, that is, the plane perpendicular to the width direction of the steel sheet is effective for improving the toughness, and variously studied the method. As a result, during hot rolling of steel sheets, it was found that it is necessary to strictly define various conditions when performing cross rolling in which a part of rough rolling is rolled in the width direction of the steel sheet. The details will be described below.

The effect of cross rolling, in which a part of the rough rolling is rolled in the width direction of the steel sheet, was confirmed by experiments. Here, the cross rolling refers to rolling performed in a direction substantially perpendicular to the final longitudinal direction of the steel sheet, and is performed during rough rolling. After the cross rolling, the steel strip is rotated about 90 ° in order to roll the steel sheet in the longitudinal direction. By cross-rolling, the densely packed alumina will be spread and distributed in the width direction of the steel sheet, and the number of alumina particles on the plane perpendicular to the steel sheet width direction, that is, the plane parallel to the fracture surface of the Charpy test piece. Is relatively reduced. The distribution of alumina clusters was measured from the information on the fractured surface of the Charpy test piece, which was almost parallel to the plane perpendicular to the steel sheet width direction. A SEM (scanning electron microscope) secondary electron image, a fracture surface photograph of a Charpy impact test piece was taken at a magnification of 10 times, and the total length (unit: mm) of the separation-like vertical crack fracture surface was measured. A value obtained by dividing this by the fracture surface area (80 mm ² ) was defined as an alumina cluster index. The unit is 1 / mm. The relationship between the alumina cluster index and the cross rolling reduction is shown in FIG. The cross rolling reduction is a value obtained by dividing the total reduction amount of cross rolling and straight rolling by the thickness of the billet before hot rolling . The alumina cluster index tends to decrease with an increase in the cross rolling reduction.

  The relationship between the Charpy impact absorption energy and the alumina cluster index is shown in FIG. The Charpy impact absorbed energy increases as the alumina cluster index decreases. In order to improve the toughness, that is, to set vE-196 to 150 J, the alumina cluster index needs to be 0.030 or less, which requires the rolling reduction of the cross rolling to be 0.1 or more. When the rolling reduction of the cross rolling exceeds 0.6, the upper limit of the rolling reduction of the cross rolling is set to 0.6 for the operational reason that the width of the steel sheet becomes large and the subsequent hot rolling becomes difficult.

  In order to obtain a steel sheet having excellent toughness, it is necessary to reduce the effective grain size in addition to ensuring the reduction ratio that regulates the cross rolling reduction ratio as described above. For this purpose, it is necessary to specify the temperature at which the cross rolling is performed and the total rolling reduction of the hot rolling. The details will be described below.

  Defining the temperature range for cross rolling is important for making the effective grain size finer. When the cross rolling temperature is more than 1000 ° C., austenite is coarsened by grain growth after recrystallization, and the effective crystal grain size of the structure mainly composed of martensite after transformation is coarsened. On the contrary, even when the cross rolling temperature is lower than 800 ° C., rolling is mainly performed in the non-recrystallized temperature range where recrystallization hardly occurs, so that the effective crystal grain size becomes coarse. The relationship between effective grain size and cross rolling temperature is shown in FIG. Here, the effective crystal grain size refers to an average crystal grain size calculated by performing EBSD on the structure after transformation and defining an orientation difference of 15 ° or more as a grain boundary.

  Prescribing the total reduction ratio of hot rolling is also important for refining the effective grain size. The effective grain size becomes smaller by increasing the total reduction ratio of hot rolling. This is because the refinement of austenite through recrystallization and the introduction of dislocations into austenite through reduction in the unrecrystallized region refines the structure mainly composed of martensite after transformation. FIG. 4 shows the relationship between the effective crystal grain size, the total rolling reduction, and the toughness when the cross rolling temperature is 800 ° C. or higher and 1000 ° C. or lower. By setting the total rolling reduction to 0.65 or more, the effective crystal grain size can be reduced to 7.0 μm or less. Here, the total reduction is a value obtained by dividing the value obtained by subtracting the steel plate thickness after hot rolling from the steel plate thickness before hot rolling by the steel plate thickness before hot rolling. The hot rolling in the present invention is a part of which is a cross rolling which is performed in a direction substantially perpendicular to the longitudinal direction of the steel sheet, and the rest is performed in a direction substantially parallel to the longitudinal direction of the final steel sheet. Straight rolling is performed, and the total reduction is a value obtained by dividing the total reduction amount of cross rolling and straight rolling by the thickness of the billet before hot rolling.The cross rolling reduction is the reduction amount of cross rolling. It is the value divided by the thickness of the steel slab before hot rolling.

  FIG. 5 shows the relationship between the toughness and the effective crystal grain size when the effective crystal grain size is refined by defining the temperature range of the cross rolling and the total rolling reduction of the hot rolling. In order to secure the toughness, that is, vE-196 to 150 J or more, the effective crystal grain size needs to be 7.0 μm or less. For this purpose, the cross rolling temperature is 800 ° C. or more and 1000 ° C. or less, and the total rolling reduction is Must be 0.65 or more.

The range of alloy elements of the steel sheet is specified below.
C is an essential element for ensuring a tensile strength of 690 MPa or more and further ensuring a Charpy impact absorption energy of 150 J or more at -196 ° C, so the addition amount is made 0.03% or more. On the other hand, however, if the amount of C is increased, the toughness decreases and it becomes difficult to secure 150 J or more at Charpy impact absorbed energy at -196 ° C, and the tensile strength exceeds 830 MPa, so the upper limit is 0.10. %.

  Since Si is an essential element for ensuring a tensile strength of 690 MPa or more, its addition amount is set to 0.02% or more. However, on the other hand, addition of Si in excess of 0.40% lowers toughness and makes it difficult to secure 150 J or more at Charpy impact absorbed energy at -196 ° C, so the upper limit is made 0.40%.

  Mn is an element effective for ensuring a tensile strength of 690 MPa or more, and at least 0.30% or more is required to be added, but conversely, if it is added in excess of 1.20%, it is susceptible to temper embrittlement. Becomes higher, the toughness decreases, and it becomes difficult to secure 150 J or more with Charpy impact absorbed energy at -196 ° C, and the tensile strength exceeds 830 MPa. Therefore, the amount of Mn added is specified to be 0.30% or more and 1.20% or less.

  The lower limit of Ni is to ensure toughness, that is, to secure 150 J or more in Charpy impact absorption energy at -196 ° C, so Ni must be added at least 8.0% or more. Although the upper limit is not particularly specified, if it exceeds 9.5%, the manufacturing cost greatly increases, so 9.5% or less is preferable. Therefore, the addition amount of Ni is set to 8.0% or more and 9.5% or less.

  Al is an element effective for deoxidation, and at least 0.010% or more needs to be added. Addition of less than 0.010% or conversely more than 0.080% makes it difficult to secure 150 J or more at a Charpy impact absorbed energy of -196 ° C because the volume fraction of inclusions increases and the toughness decreases. Become. Therefore, the addition amount of Al is specified to be 0.010% or more and 0.080% or less.

  The lower limit of T-O is not particularly specified, but if it is less than 0.0001%, the productivity decreases due to an increase in the refining load, so 0.0001% or more is preferable. If added in excess of 0.0030%, the toughness will decrease due to the formation of alumina clusters, and it will be difficult to secure 150 J or more at Charpy impact absorbed energy at -196 ° C, so the upper limit is made 0.0030%. Therefore, the amount of T-O added is set to 0.0001% or more and 0.0030% or less.

  The lower limit of P is not particularly specified, but if it is less than 0.0010%, the productivity is greatly reduced due to an increase in the refining load, so 0.0010% or more is preferable. If it exceeds 0.0100%, the toughness decreases due to temper embrittlement, and it becomes difficult to secure 150 J or more at Charpy impact absorbed energy at -196 ° C, so the upper limit is made 0.0100%. Therefore, the amount of P added is set to 0.0010% or more and 0.0100% or less.

  The lower limit of S is not particularly specified, but if it is less than 0.0001%, the productivity is greatly reduced due to an increase in the refining load, so 0.0001% or more is preferable. Further, if it exceeds 0.0035%, the toughness decreases, and it becomes difficult to secure 150 J or more at the Charpy impact absorbed energy at -196 ° C, so the upper limit is made 0.0035%. Therefore, the addition amount of S is set to 0.0001% or more and 0.0035% or less.

  The lower limit of N is not particularly specified, but if it is less than 0.0005%, the productivity decreases due to an increase in the refining load, so 0.0005% or more is preferable. However, if added in excess of 0.0070%, the toughness decreases and it becomes difficult to secure 150 J or more at Charpy impact absorbed energy at -196 ° C, so the upper limit is made 0.0070%. Therefore, the amount of N added is set to 0.0005% or more and 0.0070% or less.

In the present invention, the following elements can be further added.
Cr is an element effective in ensuring hardenability, and it is necessary to add at least 0.05%, but if it is added in excess of 2.0%, toughness and weldability are deteriorated. Therefore, the addition amount of Cr is specified to be 0.05% or more and 2.0% or less.

  Mo is an element effective in reducing temper embrittlement, and it is necessary to add 0.02% at a minimum. Conversely, if it is added in excess of 1.0%, toughness and weldability deteriorate. Therefore, the addition amount of Mo is specified to be 0.02% or more and 1.0% or less.

  Cu needs to be added at least 0.1% or more to secure the strength, but if it exceeds 3.0%, the toughness decreases. Therefore, the additive amount of Cu is specified to be 0.1% or more and 3.0% or less.

  Nb is an element effective in securing strength. If it is less than 0.005%, the effect is small, and if it exceeds 0.100%, the toughness is lowered. Therefore, the amount of Nb added is specified to be 0.005% or more and 0.100% or less.

  V is an element effective for ensuring strength. If it is less than 0.010%, the effect is small, and if it exceeds 0.500%, the toughness is lowered. Therefore, the addition amount of V is specified to be 0.010% or more and 0.500% or less.

  Ti is an element effective for ensuring strength. If it is less than 0.005%, the effect is small, and if it exceeds 0.500%, the toughness is lowered. Therefore, the addition amount of Ti is specified to be 0.005% or more and 0.500% or less.

  Ca is an element effective in preventing nozzle clogging. If it is less than 0.0001%, the effect is small, and if it exceeds 0.0050%, the toughness is lowered. Therefore, the amount of Ca added is specified to be 0.0001% or more and 0.0050% or less.

  Mg is an element effective in improving toughness. If it is less than 0.0001%, the effect is small, and if it exceeds 0.0050%, the toughness is lowered. Therefore, the addition amount of Mg is defined as 0.0001% or more and 0.0050% or less.

  REM is an element effective in improving toughness. If it is less than 0.0001%, its effect is small, and if it is more than 0.0100%, toughness is lowered. Therefore, the amount of REM added is specified to be 0.0001% or more and 0.0100% or less.

  Zr is an element effective in improving toughness. If it is less than 0.0001%, its effect is small, and if it is more than 0.0100%, toughness is lowered. Therefore, the added amount of Zr is specified to be 0.0001% or more and 0.0100% or less.

  B is an element effective for improving hardenability. If it is less than 0.0002%, its effect is small, and if it exceeds 0.0030%, the toughness decreases. Therefore, the addition amount of B is specified to be 0.0002% or more and 0.0030% or less.

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

  Next, a method for manufacturing the steel sheet of the present invention will be described. The steel sheet is produced by a method of hot rolling the slab produced by continuous casting by the method described above, but in addition to the above, the following is generally performed to refine the structure mainly composed of martensite. Conditions are also required. When the heating temperature of the steel slab is 1270 ° C. or higher, the toughness decreases due to the coarsening of the structure mainly composed of martensite after transformation due to grain growth of austenite, and the Charpy impact absorption energy at −196 ° C. is 150 J or higher. Since it becomes difficult to secure the temperature and hot rolling becomes difficult at less than 900 ° C, the temperature is set to 900 ° C or more and 1270 ° C or less. If the temperature before finishing 1 pass in the rolling performed after the cross rolling exceeds 850 ° C, the reduction in the unrecrystallized region becomes small, and the structure mainly composed of martensite after transformation becomes coarse. Since rolling becomes difficult, the temperature before finishing one pass is set to 600 ° C or more and 850 ° C or less.

After the rolling, air cooling is performed, the steel sheet is further heated to 760 ° C. or higher and 880 ° C. or lower, and then quenched by water cooling, and subsequently tempered. If the tempering temperature is less than 500 ° C., temper embrittlement occurs, and if it exceeds 650 ° C., toughness decreases due to formation of fresh martensite, and it becomes difficult to secure 150 J or more in Charpy impact absorption energy at −196 ° C. The heating temperature during tempering is set to 500 ° C. or higher and 650 ° C. or lower.
By the heat treatment, a structure mainly composed of tempered martensite that has been tempered to the optimum temperature can be formed, and the balance can be a stable austenite structure, and the toughness is improved.

  Tensile tests and Charpy impact tests were carried out on steel plates with thicknesses of 20 and 50 mm manufactured under various chemical components and manufacturing conditions. Table 1-1 and Table 1-2 (continued from Table 1-1) show the evaluation results of the chemical composition of the steel sheet, alumina cluster index, effective crystal grain size, sheet thickness, hot rolling conditions, heat treatment conditions, and mechanical properties. .. The tensile test was performed based on the metallic material tensile test method described in JIS Z2241. The test piece was sampled so that the longitudinal direction of the test piece was perpendicular to the rolling direction at a portion that entered from the surface of the steel plate by 1/4 of the plate thickness. Two tests were conducted at room temperature, and an average tensile strength value of 690 MPa or more and 830 MPa or less was regarded as acceptable. The Charpy impact test is performed by notching the full-size test piece of the 2 mm V notch test piece so that the longitudinal direction of the test piece is perpendicular to the rolling direction at a portion that enters from the steel plate surface by 1/4 of the plate thickness. Sampling was performed so that the line connecting the front edges of was parallel to the plate thickness direction. Three tests were conducted at a test temperature of -196 ° C, and the average value of the three tests was 150 J or more. As shown in Examples 1 to 38, the steel sheet having excellent tensile strength and toughness was obtained by producing the steel sheet with the components and the production method specified in the present invention.

  From the above examples, it is clear that the steel sheets of Examples 1 to 38 produced according to the present invention are steel sheets excellent in tensile strength and toughness.

Claims (2)

Steel, in mass%,
C: 0.03% or more and 0.10% or less,
Si: 0.02% or more and 0.40% or less,
Mn: 0.30% or more and 1.20% or less,
Ni: 8.0% or more and 9.5% or less,
Al: 0.010% or more and 0.080% or less,
TO: containing 0.0001% or more and 0.0030% or less,
P: 0.0010% or more and 0.0100% or less,
S: 0.0001% or more and 0.0035% or less,
N: 0.0005% or more and 0.0070% or less is contained, and the balance is composed of Fe and unavoidable impurities. The slab is heated to 900 ° C. or more and 1270 ° C. or less and then hot-rolled. Total rolling reduction is 0.65 or more, rolling reduction of hot rolling in the width direction is 0.1 or more and 0.6 or less, temperature range of cross rolling is 800 ° C or more and 1000 ° C or less, finishing 1 pass After rolling at a pre-temperature of 600 ° C or higher and 850 ° C or lower, air-cooling is performed after rolling, and then steel plate is heated to 760 ° C or higher and 880 ° C or lower and then water-cooled, followed by heating to 500 ° C or higher and 650 ° C or lower After cooling and tempering, a SEM secondary electron image, a photograph of a fracture surface of a Charpy impact test piece was taken at a magnification of 10 times, and the total length of the separation-like vertical fracture surface was measured (unit: mm). To Alumina cluster index defined as divided by the fracture area (80 mm ²⁾ it is 0.030 or less, the effective crystal grain size of 7.0μm or less, or more the average value of the tensile strength is 690 MPa 830 MPa or less, and - A method for producing a low-temperature nickel-containing steel sheet having excellent tensile strength and toughness, characterized in that a steel sheet having a Charpy impact absorption energy at 196 ° C of 150 J or more is obtained . further,
Cr: 0.05% or more and 2.0% or less,
Mo: 0.02% or more and 1.0% or less,
Cu: 0.1% or more and 3.0% or less,
Nb: 0.005% or more and 0.100% or less,
V: 0.010% or more and 0.500% or less,
Ti: 0.005% or more and 0.500% or less,
Ca: 0.0001% or more and 0.0050% or less,
Mg: 0.0001% or more and 0.0050% or less,
REM: 0.0001% or more and 0.0100% or less,
Zr: 0.0001% or more and 0.0100% or less,
B: 0.0002% or more and 0.0030% or less of 1 type or 2 types or more are contained , The manufacturing method of the low temperature nickel containing steel plate excellent in the tensile strength and toughness of Claim 1 characterized by the above-mentioned .

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