JP3869735B2 - Direct quenching type high tensile steel plate with excellent arrestability - Google Patents

Description

【００３４】
【００３５】
【表３】

【００３６】
【発明の効果】
本発明によれば、アレスト性に優れた９５０Ｎ／ｍｍ^２級の高張力厚鋼板を提供することが可能であり、その意義は極めて大きい。
【図面の簡単な説明】
【図１】 ＥＳＳＯ試験片の脆性き裂の破面形態を示す模式図である。
【図２】 １／４ｔ部と１／２ｔ部の引張強さの差とアレスト靭性値（Ｋｃａ）の関係を示す図である。
【図３】 本発明鋼の圧延方向と板幅方向および１／４ｔ部と１／２ｔ部における４種類の引張強さの関係を示す図である。
【図４】 圧延方向の１／４ｔ部における降伏比とアレスト靭性値（Ｋｃａ）の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel plate for a welded structure that requires brittle crack propagation stopping characteristics (arrestability) of a high-strength steel, particularly a steel plate base material.
[0002]
[Prior art]
With the increase in size of welded structures, higher strength has been demanded for steel plates used in these structures, and the hydraulic iron pipes (Penstock) used in pumped-up hydroelectric power stations are also in 1999. 950N / mm ² steel was adopted for the first time in the world at the Cluesson-Dixen hydropower plant in Switzerland. However, the hydraulic iron pipe fracture accident at Cleeson-Dixence, which occurred on November 13, 2000, triggered the brittle cracking property from the welded part in 950 N / mm ² steel, and the brittleness of the steel plate base material. The importance of crack propagation arresting properties (arrestability) is now attracting attention.
[0003]
There are already several inventions for 950 N / mm ² steel with a plate thickness of less than 25-100 mm.
For example, Japanese Patent Laid-Open No. 59-100214 discloses that the material in the longitudinal direction and the plate thickness direction is uniform by rapidly cooling simultaneously with an appropriate water density according to the plate thickness after rolling the steel plate and then tempering. There is disclosed a method for producing a thick high-strength steel.
Japanese Patent Laid-Open No. 63-266023 discloses a yield ratio of 90% by the direct quenching method, characterized in that a low yield ratio is obtained by omitting direct quenching and tempering within 150 seconds after rolling in an unrecrystallized region. The following methods for producing high toughness, low yield ratio, high strength steel sheets are disclosed.
Japanese Patent Laid-Open No. 2-133521 is characterized by having a tensile strength of 97 kgf / mm ² or more and an impact transition temperature of −60 ° C. or less by utilizing a synergistic effect of Nb addition and twice-quenching. A method for producing a tempered high-tensile steel sheet having excellent toughness is disclosed.
According to these inventions, 950 N / mm ² steel can be manufactured, but there is no description about arrestability, and it is unclear what arrestability these steel plates have.
[0004]
[Problems to be solved by the invention]
The object of the present invention is to satisfy Ceq ≦ 0.59 and Pcm ≦ 0.29 when the plate thickness is 50 mm or less, Ceq ≦ 0.62 and Pcm ≦ 0.33 when the plate thickness exceeds 50 mm, and yield stress of 885N. / Mm ² or more, tensile strength 950 to 1130 N / mm ² , Charpy impact energy 47 J or more (test temperature is −55 ° C. when plate thickness is 50 mm or less, −60 ° C. when plate thickness is more than 50 mm), 0 in temperature gradient type ESSO test Arrest toughness (Kca value) at 300 ° C. is 300 N / mm ² · m ^1/2 or more, preferably 400 N / mm ² · m ^1/2 or more, and has an arrestability of plate thickness of less than 25 to 100 mm. It is to provide an excellent direct-quenching type high-tensile thick steel plate.
[0005]
[Means for Solving the Problems]
In general, the brittle crack propagation stop property can be evaluated by the arrest toughness value (Kca, Crack Arrest Toughness), and it is known that the arrest toughness value has a correlation with the base material toughness (WES3003-1990 commentary). However, even with the same base metal toughness, there is variation in the arrest toughness value, and it is estimated that the controlling characteristics other than the base metal toughness are also affecting the arrestability, but there is little knowledge about this.
Therefore, the present inventors have made various studies in order to achieve such an object, paying attention to the relationship between the brittle crack propagation stopping property and the tensile property measured by the temperature gradient type ESSO test.
[0006]
About the sampling direction of the test piece and the plate thickness position for evaluating the tensile properties, since 950 N / mm ² steel is not a JIS standard product, the tensile strength for welding structure described in JIS G 3128 is lower than this. Based on the yield point steel plate, a method of sampling from a 1/4 t portion in the plate width direction is common.
[0007]
However, since the direction in which a brittle crack can propagate in penstock is assumed to be the sheet width direction, the present inventors are concerned with the tensile properties in the rolling direction, which is the load-loading direction when a steel sheet is torn by a brittle crack. Also examined. Until now, there have been few examples of examining the tensile properties in the rolling direction of 950 N / mm ² steel, and only a few are described in JP-A-2-133521 and JP-A-2-141528. In addition, as for the plate thickness position, the plate thickness is less than 25 to 100 mm and is relatively thick as a high-strength material. Therefore, in order to confirm the influence of the tensile properties at the plate thickness position, The department was also investigated.
[0008]
As a result of the examination, the tensile strength at the 1/4 t part in the rolling direction is higher than the 1/2 t part in the rolling direction by (0.5 t) MPa or more, the rolling direction and the sheet width direction, and the 1/4 t part and 1/2 t part. Directly characterized in that the tensile strength at the ¼ t part in the rolling direction is the highest among the four types of tensile strengths in, or the yield ratio at the ¼ t part in the rolling direction is further greater than 97.0%. It was found that the quenching type 950 N / mm ² steel significantly improved the arrestability by the ESSO test. Here, the yield ratio is the ratio of the yield stress to the tensile strength.
[0009]
FIG. 1 shows a schematic diagram of a fracture surface form of a brittle crack in an ESSO test of a conventional material and steel of the present invention. In the conventional material, the length of the brittle crack in the 1 / 2t part and the 1 / 4t part is almost the same, and the brittle crack is U-shaped, but in the steel of the present invention, the 1 / 4t part is more than the 1 / 2t part. However, the length of the brittle crack is short and the form of the brittle crack is V-shaped. From this result, when the steel of the present invention has the above-described tensile properties, it is presumed that the arrestability is improved by suppressing the progress of the brittle crack particularly at the 1/4 t portion from the surface.
[0010]
In addition, since the steel of the present invention is manufactured by directly quenching immediately after rolling in the non-recrystallized region, the tensile strength is increased by the work strain introduced by rolling at the 1/4 t portion in the rolling direction. It is thought that the crystal grain size is refined due to the deformation zone caused by the above, and the yield ratio is also increased.
As described above, steel sheets that satisfy certain conditions of tensile strength difference and yield ratio at the plate thickness position show that a remarkable arrest effect appears especially for brittle cracks propagating in the plate width direction. Since it was discovered for the first time, the present invention was filed.
[0011]
That is, the present invention is a direct-quenching type high-tensile steel plate produced by direct quenching and tempering immediately after rolling in a non-recrystallized region, in weight%,
C: 0.07 to 0.20%
Si: 0.01-0.30%,
Mn: 0.40 to 1.50%,
Ni: 1.0 to 3.5%
Cr: 0.10 to 1.0%,
Mo: 0.10 to 1.0%,
V: 0.01 to 0.10%,
Al: 0.01 to 0.10%,
B: 0.0003 to 0.0030%,
N: 0.0010 to 0.0070%
Cu: 0.05 to 0.50%,
Nb: 0.005 to 0.030%,
Ti: 0.005-0.020%,
Ca: 0.001 to 0.008%
The other is composed of Fe and inevitable impurities, and the tensile strength at the 1/4 t part in the rolling direction when the plate thickness is t (unit: mm) is (0) than the 1/2 t part in the rolling direction. .5t) MPa or higher, the tensile strength at the 1 / 4t portion in the rolling direction is the highest among the four types of tensile strength at the rolling direction, the sheet width direction, and the 1 / 4t portion and 1 / 2t portion. It is a direct quenching type high-tensile thick steel plate excellent in arrestability, characterized in that the yield ratio at the 1/4 t portion is greater than 97.0%.
[0012]
[0013]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the reason for limiting the component range is as follows. In this specification, “%” means “% by weight” unless otherwise specified.
C is necessary for securing hardenability and as a precipitate-forming element, and requires at least 0.07% addition. However, if it exceeds 0.20%, the base metal toughness and weldability deteriorate, so the amount of C Was 0.07 to 0.20%.
[0014]
Si is a deoxidation effect and a solid solution strengthening element, and at least 0.01% is necessary as the amount that the effect can be expected. However, if it exceeds 0.30%, the weldability is deteriorated and the heat affected zone is also affected. In order to make the island-like martensite structure appear and reduce the toughness remarkably, the Si content is set to 0.01 to 0.30%.
[0015]
Mn has an effect of fixing S and preventing intergranular cracking during hot rolling and ensuring hardenability, and a minimum of 0.40% is necessary. In order to reduce toughness and weldability, the amount of Mn is set to 0.40 to 1.50%.
[0016]
Ni is an important element indispensable for ensuring hardenability and toughness of the matrix, and at least 1.0% is necessary. However, if it exceeds 3.5%, the economic efficiency is greatly reduced. 0.0 to 3.5%.
[0017]
Cr is an essential element for ensuring hardenability, and it is necessary to add at least 0.10%, but if added over 1.0%, the weldability is remarkably lowered, so the Cr amount is 0.00%. It was 10 to 1.0%.
[0018]
Mo needs to be added at a minimum of 0.10% from the viewpoint of matrix strengthening by precipitation strengthening and solid solution strengthening and ensuring hardenability, but if it exceeds 1.0%, weldability and hot workability are reduced. The amount of Mo was 0.10 to 1.0%.
[0019]
V is an essential element for improving the hardenability and strengthening precipitation by carbonitride, and at least 0.01% is necessary, but if added over 0.10%, the base metal and joint toughness are remarkably lowered and weldability is increased. Therefore, the V amount is set to 0.01 to 0.10%.
[0020]
Al is not only important as a deoxidizing element, but also binds with N to form AlN and suppress the coarsening of austenite grains, so it is necessary to add at least 0.01%. When exceeding 10%, precipitation of coarse AlN is caused and the toughness is remarkably lowered, so the Al amount is set to 0.01 to 0.10%.
[0021]
B is an important element that segregates at the austenite grain boundaries and suppresses the formation of ferrite, thereby ensuring hardenability. Therefore, B must be at least 0.0003%, but if added over 0.0030%, weldability and Since the toughness is lowered, the B content is set to 0.0003 to 0.0030%.
[0022]
N is necessary for preventing the austenite grains from coarsening by forming AlN, and should be contained in an amount of 0.0010% or more. Since it is necessary to control to 0070% or less, the N amount is set to 0.0010 to 0.0070%.
[0023]
Moreover, the reason for limiting the component range for the following elements is as follows.
Cu is an element added for the purpose of securing strength by precipitation strengthening, and it is necessary to add 0.05% or more. However, if added over 0.50%, the base metal toughness and weldability are deteriorated. 0.05 to 0.50%.
[0024]
Nb is an element added for the purpose of making the austenite grains finer and strengthening the precipitation of the matrix by being present in the austenite as fine precipitates. If 0.005% is not added at least, the effect does not appear, but if adding over 0.030%, the base metal and joint toughness are lowered, so the Nb content was made 0.005 to 0.030%.
[0025]
Ti is an element added for the purpose of improving the toughness of the weld zone by forming a stable nitride even at a high temperature and preventing coarsening of the weld heat affected zone. In order to obtain this effect, it is necessary to add at least 0.005%, but if added over 0.020%, the toughness of the base material is greatly reduced by the nitride, so the Ti amount is 0.005 to 0.020%. did.
[0026]
Ca is an element added for the purpose of improving the base material toughness and reducing the anisotropy by making the form of the nonmetallic inclusions spherical. In order to obtain this effect, it is necessary to add at least 0.001%, but if added over 0.008%, the amount of inclusions increases and the toughness of the base metal is greatly reduced, so the Ca content is 0.001%. -0.008%.
[0027]
Further, the reason for limiting the mechanical characteristics will be described. Here, JIS Z 2201 No. 4 test piece (round bar) is used as the tensile test piece.
The difference in tensile strength between the 1/4 t part and the 1/2 t part in the rolling direction is one of the important features related to the basis of the present invention, which is different from the conventional 950 N / mm ² steel. As the plate thickness increases, the difference in tensile strength tends to increase as the difference in cooling rate between the 1/4 t part and 1/2 t part during direct quenching increases. FIG. 2 shows the relationship between the arrest toughness value (Kca) and the difference in tensile strength between the 1 / 4t part and the 1 / 2t part expressed as a ratio to the sheet thickness. In the thickness range from 25 to 100 mm, when the difference in tensile strength between the ¼ t part and the ½ t part is 0.5 t or more, the effect of improving the arrest property appears. The difference in tensile strength between the part and the 1/2 t part was (0.5 t) MPa or more.
[0028]
The relationship between the four types of tensile strength in the rolling direction and the sheet width direction and in the 1 / 4t part and the 1 / 2t part is one of the important features of the present invention that is different from the conventional 950 N / mm ² steel. In FIG. 3, the schematic diagram about the relationship between the rolling direction of this invention steel, a board width direction, and four types of tensile strength in a 1 / 4t part and a 1 / 2t part is shown. Conventional materials often have the highest tensile strength at ¼ t in the sheet width direction and the lowest tensile strength at ½ t in the rolling direction, but the steel of the present invention has a tensile strength at ¼ t in the rolling direction. The strongest. The reason why the steel of the present invention can ensure the difference in tensile strength between the 1/4 t part and the 1/2 t part in the rolling direction is because the tensile strength of the 1/4 t part in the rolling direction is the highest in this way. In addition, as a condition where the effect of improving the arrestability appears, the greatest feature is that the tensile strength at the 1/4 t portion in the rolling direction is the highest.
[0029]
The yield ratio at the 1/4 t portion in the rolling direction is one of the important features of the present invention. FIG. 4 shows the relationship between the yield ratio and the arrest toughness value (Kca) at the 1/4 t portion in the rolling direction. In order to stably obtain excellent arrestability of 400 N / mm ² · m ^1/2 or more, it is necessary to make the yield ratio larger than 97.0%.
[0030]
A manufacturing method will be described. From the test results so far, when the slab was heated to the recrystallization zone, rolled in the non-recrystallization zone and then directly quenched, and tempered at a temperature below the Ac1 point, 1/4 t and 1 / the difference in tensile strength of 2t part at least 0.5 t, but the tensile strength of 1 / 4t of the rolling direction is most 950 N / mm ² steel obtained in the case of reheating quenching characteristics of the Since the 950 N / mm ² steel that can be obtained cannot be obtained, the production method was limited to the direct quenching type.
As an example of specific manufacturing conditions, it is heating temperature 900-1040 degreeC, finishing temperature 750-900 degreeC, and tempering temperature 550-650 degreeC.
[0031]
【Example】
Table 1 shows chemical components, carbon equivalent formulas, and Ac1 temperatures in the ladle of the present invention steel and comparative steel.
A steel ingot having these chemical components is heated to 900 to 1040 ° C., hot rolled at a finishing temperature of 750 to 900 ° C., directly tempered or reheated and tempered at 550 to 650 ° C., and a thickness of 30 ˜95 mm 950 N / mm ² steel was produced.
[0032]
Table 3 shows the slab heating temperature, finish rolling start temperature, quenching method, tempering temperature, sheet thickness, tensile test result, impact test result, and ESSO test result.
Inventive steel F has a tensile strength at 1/4 t part in the rolling direction higher than the 1/2 t part in the rolling direction by (0.5 t) MPa or more, the rolling direction and the sheet width direction, and the 1/4 t part and 1/2 t. Steel plate having the highest tensile strength at the 1/4 t portion in the rolling direction and the yield ratio at the 1/4 t portion in the rolling direction is greater than 97.0%, and the Kca value is A particularly excellent arrestability of 400 N / mm ² · m ^1/2 or more was exhibited.
The comparative steels G to J all have a chemical composition outside the scope of the claims of the present invention, and the quenching method is reheating quenching, and the tensile strength at the 1/4 t part in the rolling direction is higher than the 1/2 t part in the rolling direction. (0.5 t) MPa or less, the highest of the four types of tensile strength in the rolling direction and the sheet width direction, and in the 1/4 t part and the 1/2 t part, is the 1/4 t part in the sheet width direction. The yield ratio at the 1/4 t portion was 97.0% or less, and the Kca value was less than 300 N / mm ² · m ^1/2 .
[0033]
[Table 1]

[0034]
[0035]
[Table 3]

[0036]
【The invention's effect】
According to the present invention, it is possible to provide a high-strength thick steel plate of 950 N / mm ² class excellent in arrestability, and its significance is extremely great.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a fracture surface form of a brittle crack of an ESSO test piece.
FIG. 2 is a diagram showing a relationship between a difference in tensile strength between a 1 / 4t part and a 1 / 2t part and an arrest toughness value (Kca).
FIG. 3 is a diagram showing the relationship between the rolling direction and the sheet width direction of the steel of the present invention, and the four types of tensile strength at 1/4 t and 1/2 t parts.
FIG. 4 is a diagram showing a relationship between a yield ratio and an arrest toughness value (Kca) at a 1/4 t portion in a rolling direction.

Claims (1)

It is a direct quenching type high-tensile thick steel plate manufactured by directly quenching and tempering immediately after rolling in the non-recrystallized region,
C: 0.07 to 0.20%
Si: 0.01-0.30%,
Mn: 0.40 to 1.50%,
Ni: 1.0 to 3.5%
Cr: 0.10 to 1.0%,
Mo: 0.10 to 1.0%,
V: 0.01 to 0.10%,
Al: 0.01 to 0.10%,
B: 0.0003 to 0.0030%,
N: 0.0010 to 0.0070%
Cu: 0.05 to 0.50%,
Nb: 0.005 to 0.030%,
Ti: 0.005-0.020%,
Ca: 0.001 to 0.008%
The other is composed of Fe and inevitable impurities, and the tensile strength at the 1/4 t part in the rolling direction when the plate thickness is t (unit: mm) is (0) than the 1/2 t part in the rolling direction. .5t) MPa or higher, the tensile strength at the ¼t portion in the rolling direction is the highest among the four types of tensile strength at the rolling direction, the sheet width direction, and the ¼t portion and ½t portion. A direct-quenching type high-tensile thick steel plate excellent in arrestability, characterized in that the yield ratio at a 1/4 t portion is greater than 97.0%.

Images