JP2008248291A - Manufacturing method of thick steel plate with excellent low temperature toughness of base metal and weld heat affected zone - Google Patents

Abstract

A cooling method capable of controlling the yield strength of a thick steel plate is excellent in low-temperature toughness of a base metal having a low yield ratio of 80% or less and a tensile strength of 500 MPa class and a thickness of 10 to 30 mm and a weld heat affected zone. A method for producing a thick steel plate is provided.
When a thick steel plate is produced by rolling a cast slab having a predetermined composition and then cooling it with a water cooling device having a plurality of water cooling zones capable of independently adjusting the water density in the rolling direction. Density (Wi), final water density (Wf), water cooling start temperature (Tcs) and water cooling stop temperature (Tcf), and plate passing speed (R) are set as the following production conditions, and after 3 to 15 seconds after starting water cooling. It is characterized by performing air cooling for ˜20 s. Wi = 0.2 to 1.5 (m ³ m ⁻² min ⁻¹ ), Wf = 0.5 to 2.0 (m ³ m ⁻² min ⁻¹ ), Tcs = 700 to 790 (° C.), Tcf ≦ 450 (° C.), R = 10 to 60 (m / min)
[Selection] Figure 1

Description

  The present invention relates to a method for producing a thick steel plate having a yield ratio YR ≦ 80%, a tensile strength TS ≧ 500 MPa, and excellent in toughness of a base material and a weld heat-affected zone, and particularly having a thickness of 10 to 30 mm. A method of manufacturing a thick steel plate having a thickness of 10 to 30 mm excellent in low temperature toughness of a base metal having a tensile strength of 500 MPa class with a low yield ratio of 80% or less and a yield ratio of 80% or less by controlling cooling conditions of the thick steel plate It is about.

  Large-scale building structures such as medium- and high-rise buildings are required to have a low yield ratio (YR) for steel for construction due to recent demands for seismic resistance, and 80% or less for such building structures. There is a need for low yield ratio building steel. Further, due to the increasing energy demand in recent years, low-temperature steel materials are required as steel materials for LPG ships, which are in great demand. In some cases, ammonia is mixedly loaded on the LPG ship, and the upper limit of the yield strength of the steel material is severely restricted from the viewpoint of preventing stress corrosion cracking. Thus, in recent years, there is an increasing demand for structures in which the upper and lower limits of the yield ratio and yield strength are stricter.

  Then, the invention of 600 Mpa class steel excellent in the low yield ratio characteristic and super-high heat input welding joint toughness is proposed as a construction steel material used for large-sized building structures such as middle-high-rise buildings (for example, see Patent Document 1). In this invention, in order to achieve the target YR (yield strength / tensile strength) ≦ 80% and TS ≧ 590 MPa, the cooling rate after the hot rolling is finished is 4 to 7.5 ° C./sec, preferably It is described that the microstructure of the steel is required to be a ferrite fraction of 25 to 75% in two phases of ferrite and bainite as 5 to 6.5 ° C./sec. However, in this method, the microstructure of the steel is composed of two phases of ferrite and bainite according to the regulation of the cooling rate after completion of hot rolling (4 to 7.5 ° C./sec, preferably 5 to 6.5 ° C./sec). Although it is described that the ferrite fraction needs to be 25 to 75%, there is no detailed description of the cooling rate in each temperature range.

  Similarly, it is shown that low YR can be obtained by the definition of chemical components such as B and the specification of the bainite fraction (see, for example, Patent Document 2). However, this invention essentially requires the addition of B as an alloy element, and a certain degree of HAZ toughness deterioration is unavoidable, and there is a need to establish a method for producing a thick steel plate that does not depend on the alloy element.

  Moreover, after the hot rolling is completed, a two-phase structure of ferrite and bainite is used at room temperature by using a method in which water-quenching is performed from the Ar3 temperature or higher, and then re-heating to a ferrite + austenite two-phase region temperature and quenching again. To achieve a low yield ratio (see, for example, Non-Patent Document 1). As in this method, after the hot rolling is completed, water-cooled quenching is performed from the Ar3 temperature or higher, and then reheating to the ferrite + austenite two-phase region temperature and quenching again is performed. It is possible to obtain a low yield ratio by obtaining a two-phase structure, and to produce a steel that satisfies a low yield ratio, high tension, and joint toughness even with a steel plate having a thickness of 70 mm or more. However, since reheating and tempering are performed after hot rolling, there is a problem that productivity deteriorates and an increase in manufacturing cost cannot be avoided.

Furthermore, as a controlled cooling method for low yield ratio thick steel plates that suppresses the occurrence of a hardness difference between the surface and center of the thick steel plate, a slow cooling zone and a rapid cooling that can be controlled independently on the exit side of the thick plate rolling mill. Using a steel plate controlled cooling device in which strips are provided in order, 238 / t ^1.2 ° C./s or more and 713 / t ^1.2 ° C./s in the slow cooling zone for a steel plate having a thickness of t (mm). An invention of a controlled cooling method for a thick steel sheet is proposed in which cooling is performed at a cooling rate of s or less and then cooling is performed at a cooling rate of 1425 / t ^1.2 ° C./s or more in the rapid cooling zone. (For example, see Patent Document 3). However, in the case of a thick steel plate, there is a problem that it is difficult to control a predetermined low yield ratio simply by defining the cooling rate.

JP 2002-256377 A JP 2005-336541 A JP 2005-313223 A Yukio Tomita et al., “Effects of various processes on the yield point of architectural HT60”, CAMP-ISIJ, vol. 1 (1988), p. 88

  Until now, cooling after rolling has been actively used to control the material as accelerated cooling, but for cooling control of thick steel plates, the cooling characteristics in each temperature range during cooling are studied in detail. However, only a value representative of the entire cooling process is shown, and an effective cooling method capable of controlling the yield strength of the thick steel plate has not yet been provided. In view of such a situation, the present invention is a cooling method that can control the yield strength of a thick steel plate without using the cooling rate as an index when cooling the thick steel plate, and the tensile strength at a low yield ratio of 80% or less. It is an object of the present invention to provide a method for producing a thick steel plate having a thickness of 10 to 30 mm, which has a 500 Mpa class and is excellent in toughness of a base material and a weld heat affected zone.

  In particular, the inventors of the present invention have prepared a structure by appropriately controlling the cooling process after completion of rolling, and have earnestly studied a manufacturing method for reducing the yield ratio of a thick steel plate having a thickness of 10 to 30 mm to 80% or less. In the process of manufacturing the steel sheet, cooling is performed by a water cooling device having a plurality of water cooling zones that can adjust the water density independently in the rolling direction of the steel sheet, and the water density at the time of cooling is the water density at the start of water cooling Wi (hereinafter, initial water quantity) From the density) to the water density Wf at the end of water cooling (referred to as final water density) to stop the low-temperature cooling, and in the meantime, by performing air cooling for 3 to 15 seconds after the start of water cooling for 5 to 20 seconds, It was found that the yield strength of steel can be controlled. Furthermore, as a result of repeated detailed investigations on the chemical composition of the steel sheet, the present inventor has an appropriate strength as a structural steel, and has a yield ratio of 80% or less and a tensile strength of 500 MPa class base metal and a weld heat affected zone. The present inventors have completed the present invention by finding the conditions under which a thick steel plate having a thickness of 10 to 30 mm excellent in toughness can be obtained.

  The gist of the present invention is as follows.

(1) In mass%,
C: 0.05 to 0.12%,
Si: 0.05 to 0.25%,
Mn: 0.4 to 2.0%,
P: 0.02% or less,
S: 0.02% or less,
Nb: 0.01 to 0.05%,
Al: 0.005 to 0.04%,
A steel slab having a plurality of water-cooling zones each having a plurality of water-cooling zones capable of adjusting the water density independently of the rolling direction. In the manufacturing method, the initial water density (Wi), the final water density (Wf), the water cooling start temperature (Tcs), the water cooling stop temperature (Tcf), and the sheet passing speed (R) are set as the following manufacturing conditions. It is excellent in low temperature toughness of a base metal having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a thickness of 10 to 30 mm and a weld heat affected zone, characterized by performing air cooling for 3 to 15 seconds and after 5 to 20 seconds. Manufacturing method of thick steel plate.
Wi = 0.2 to 1.5 (m ³ m ⁻² min ⁻¹ ), Wf = 0.5 to 2.0 (m ³ m ⁻² min ⁻¹ ), Tcs = 700 to 790 (° C.), Tcf ≦ 450 (° C.), R = 10 to 60 (m / min)

(2) The cast slab is further mass%,
Cu: 0.1 to 0.23%,
Ni: 0.1 to 0.45%,
The base material having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a thickness of 10 to 30 mm as described in (1) above, and a steel plate excellent in low-temperature toughness of the weld heat affected zone Method.

(3) The cast slab is further mass%,
Ti: 0.005 to 0.03%,
Excellent in the low temperature toughness of the base material having a plate thickness of 10 to 30 mm with a yield strength of 80 MPa or less and a tensile strength of 500 MPa as described in (1) or (2) Manufacturing method of thick steel plate.

(4) The cast slab is further mass%,
Mo: 0.01 to 0.6%,
Cr: 0.01 to 0.6%,
B: 0.0003 to 0.003%
Low-temperature toughness of the base material having a plate thickness of 10 to 30 mm with a yield strength of 80% or less and a tensile strength of 500 MPa class according to any one of the above (1) to (3) A method for producing thick steel plates with excellent resistance.

  INDUSTRIAL APPLICABILITY The present invention can provide a steel plate having a thickness of 10 to 30 mm having excellent base material and weld heat-affected zone toughness, yield ratio YR ≦ 80%, and tensile strength TS ≧ 500 MPa. The effects brought to the industrial field such as large building structures are extremely great, and the contribution to society is also very large in terms of the safety of the structures.

  Hereinafter, the present invention will be described in detail.

  The inventors of the present invention adjust the structure by appropriately controlling the cooling process after the end of rolling of the thick steel plate, the manufacturing method for reducing the yield ratio of the steel plate having a plate thickness of 10 to 30 mm to 80% or less, and appropriate yielding. As a steel component for obtaining a tensile strength and toughness of a strength of 500 MPa class, it is necessary to add an appropriate alloy element. The appropriate range of these elements was examined and the present invention was made.

  First, a method of adjusting the structure by appropriately controlling the cooling process after the rolling of the thick steel plate having a thickness of 10 to 30 mm and setting the yield ratio of the steel thick plate to 80% or less will be described.

  FIG. 1 schematically shows the relationship between the cooling behavior and the continuous cooling transformation curve.

  In general, when producing thick steel plates by controlled cooling, water cooling is performed after the end of rolling by a water cooling device provided after the rolling line. In this case, the water density is almost constant from the start to the end of cooling. is doing.

  However, the present inventors cannot reduce the yield ratio by performing water cooling at an almost constant water density from the start to the end of cooling, and in order to reduce the yield ratio, 700 ° C. is required for cooling after rolling. It is necessary to obtain an appropriate amount of soft ferrite (hereinafter referred to as “pre-deposited ferrite”) generated by transformation from austenite at a high temperature.

  FIG. 1 is a diagram schematically showing the relationship between the cooling behavior of steel and the continuous cooling transformation curve. In order to obtain an appropriate amount of pro-eutectoid ferrite, it is necessary to moderate the cooling rate in the temperature range of approximately 750 to 650 ° C. where pro-eutectoid ferrite is generated, as shown in FIG. It was found that the amount of pro-eutectoid ferrite precipitated by the pro-eutectoid ferrite transformation can be increased by air cooling (cooling pass 1b in FIG. 1) after the first cooling pass 1a). Furthermore, the cooling rate at 650 ° C. or lower was increased (cooling pass 1c in FIG. 1) to obtain a bainite structure, and it was found that the required tensile strength can be obtained by this hard bainite structure. In FIG. 1, the air-cooled portion (1b) of the cooling curve crosses the pro-eutectoid ferrite transformation start temperature. However, even if the air-cooling starts after the pro-eutectoid ferrite transformation starts, there is a problem in the effect if the predetermined air-cooling time is maintained. Absent.

  In contrast, when the cooling path is not properly taken, the pro-eutectoid ferrite transformation starts at a lower temperature and passes through the pro-eutectoid ferrite transformation region as in the slow cooling path (cooling path 2a in FIG. 1). Since the time is short, the formation of pro-eutectoid ferrite is not sufficient, and the yield rate is reduced even if the cooling rate at 650 ° C. or lower is increased (cooling path 2b in FIG. 1) to obtain a bainite structure effective for generating movable dislocations. It is not possible.

  The reason why the yield ratio of the steel sheet can be reduced by obtaining the proper amount of pro-eutectoid ferrite is that the pro-eutectoid ferrite formed at high temperature is soft in the first place, and the transformation strain of the bainite transformation that has been transformed subsequently is the first. By introducing movable dislocations in the precipitated ferrite, the movable dislocations bring about an effect of reducing the yield point, and it is not necessary to obtain a bainite structure simply by increasing the cooling rate below 650 ° C. This is because the effect of suppressing the disappearance of the introduced movable dislocation is also brought about.

  For this reason, for example, if the cooling rate after the formation of the subunit of the bainite transformation is moderate as in the cooling path having a constant cooling rate in FIG. 1 (cooling path 3a in FIG. 1), it is assumed that a bainite structure is obtained. However, since the movable dislocations are reduced during the cooling process and the yield stress reduction effect is reduced, it is necessary to increase the cooling rate in the temperature range where the bainite transformation proceeds at 650 ° C. or lower. That is, if the cooling rate is not increased after the pro-eutectoid ferrite is precipitated, the movable dislocations are reduced during the cooling process, and the effect of reducing the yield stress is reduced.

  However, when the thickness of the steel plate is thin, there is almost no temperature difference at the time of cooling between the steel plate surface and the central portion. However, in the case of cooling a thick steel plate having a thickness of 10 to 30 mm, the cooling after hot rolling is performed. At the start and at the end of cooling, the temperature difference between the steel plate surface and the central part is small, but as the cooling proceeds, a temperature difference occurs between the steel plate surface and the central part during the cooling, so the structure control is performed based on the temperature of the steel plate surface Since the structure is not the same between the surface part and the center part of the steel sheet, it is difficult to control the structure of the thick steel sheet only with the cooling rate.

  Therefore, in the present invention, focusing on the water density pattern during cooling, which has a great influence on the yield strength and tensile strength, the structure control of a thick steel plate having a thickness of 10 to 30 mm was studied.

  That is, the water-cooling zone of the water-cooling device installed after the rolling line is divided into a plurality, and the hot-rolled steel sheet is cooled by a water-cooling density pattern in which the water-cooling density for each water-cooling zone is variously changed during cooling. Control [Proeutectoid ferrite + hard second phase (bainite, martensite, etc.)] was carried out. As a result, it was found that the yield strength of the steel sheet varies depending on the water density pattern and the plate thickness, and the condition of the water density pattern for obtaining a thick steel plate having a plate thickness of 10 to 30 mm with a yield strength of 80% or less and a tensile strength of 500 MPa. I found.

  In addition, in this invention, the water cooling apparatus provided with the some normal water cooling zone which can adjust a water quantity density as a water cooling apparatus is applicable.

According to the water density pattern found in the present invention as a production condition capable of obtaining a base material having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a steel plate having a thickness of 10 to 30 mm excellent in the toughness of the weld heat affected zone. The manufacturing conditions are the initial water density (Wi), the final water density (Wf), the water cooling start temperature (Tcs), the water cooling stop temperature (Tcf), and the plate feed speed (R) as the following manufacturing conditions. Air cooling is performed for 5 to 20 seconds after 3 to 15 seconds.
Wi = 0.2 to 1.5 (m ³ m ⁻² min ⁻¹ ), Wf = 0.5 to 2.0 (m ³ m ⁻² min ⁻¹ ), Tcs = 700 to 790 (° C.), Tcf ≦ 450 (° C.), R = 10 to 60 (m / min)

  Thus, the initial water density (Wi), the final water density (Wf), the water cooling start temperature (Tcs), the water cooling stop temperature (Tcf), the sheet passing speed (R), and the water cooling start 3-15 s after the start of water cooling 5-20 s. The reason why the air cooling is limited will be described.

Initial water density (Wi): 0.2 to 1.5 (m ³ m ⁻² min ⁻¹ ),
In order to reduce the yield strength of the steel sheet, it is necessary to obtain sufficient pro-eutectoid ferrite, and as shown in FIG. 1, the temperature range in which pro-eutectoid ferrite is generated (pro-eutectoid ferrite transformation start to bainite transformation initiation region). Therefore, it is necessary to slowly cool the steel sheet to produce a large amount of pro-eutectoid ferrite, so that the initial water density (Wi) should be 1.5 (m ³ m ⁻² min ⁻¹ ) or less to cool slowly. However, when the initial water density (Wi) is less than 0.2 (m ³ m ⁻² min ⁻¹ ), it is not possible to achieve cooling below an appropriate water cooling stop temperature (Tcf), which will be described later. It is necessary to be 0.2 (m ³ m ⁻² min ⁻¹ ) or more. The water density at the time of this slow cooling is the initial water density.

Final water density (Wf): 0.5 to 2.0 (m ³ m ⁻² min ⁻¹ ),
In order to form a bainite structure effective for the generation of movable dislocations, the final water density (Wf) needs to be 0.5 (m ³ m ⁻² min ⁻¹ ) or more. However, if the final water density (Wf) exceeds 2.0 (m ³ m ⁻² min ⁻¹ ), the yield strength becomes excessive due to the effect of refinement of the bainite structure, so 2.0 (m ³ m ^{− 2} min ⁻¹ ) or less is necessary. The water density at the time of cooling for forming this bainite structure is the final water density.

Air cooling for 5-20 s (seconds) after 3-15 s (seconds) after the start of water cooling,
In order to reduce the yield strength, it is necessary to sufficiently generate a pro-eutectoid ferrite with a low yield strength by taking a sufficiently long cooling time before cooling. When 3 seconds or more have elapsed after the start of water cooling, as shown in the cooling path 1a of FIG. 1, the cooling curve crosses the pro-eutectoid ferrite transformation start temperature during air cooling, or pro-eutectoid ferrite transformation starts during the first water cooling time. Therefore, a sufficient amount of pro-eutectoid ferrite can be generated. If it is less than 3 seconds after the start of water cooling, a sufficient amount of pro-eutectoid ferrite as in such a case cannot be produced. On the other hand, if it exceeds 15 s after the start of water cooling, the steel sheet is cooled too much, and as shown in the cooling path 2a or 3a in FIG. 1, the pro-eutectoid ferrite transformation start temperature is lowered and sufficient pro-eutectoid ferrite is generated. become unable. Therefore, in this invention, it was set as 3-15 s after the water cooling start. In addition, the air cooling time is set to 5 to 20 s. In order to sufficiently grow the pro-eutectoid ferrite necessary for reducing the yield strength, an air cooling time of 5 s or more is required as shown in the cooling path 1b of FIG. It is. However, if the air cooling time exceeds 20 s, proeutectoid ferrite becomes excessive and the tensile strength decreases, so the upper limit was set to 20 s.

Water cooling start temperature (Tcs): 700 to 790 (° C.)
When the water cooling start temperature (Tcs) is lower than 700 ° C., the ferrite transformation amount that proceeds before the start of water cooling is increased and coarse ferrite is formed, and the strength and toughness are significantly reduced. However, if the temperature exceeds 790 ° C, it is necessary to set the rolling end temperature high, and the toughness of the base material is impaired. Therefore, the upper limit is set to 790 ° C.

Water cooling stop temperature (Tcf): ≦ 450 (° C.)
In the present invention, it is necessary to maintain the movable dislocations of the bainite structure that occurs mainly during cooling after tc. For this purpose, it is necessary to keep Tcf low so that dislocation recovery does not occur after the completion of cooling, and the upper limit is 450 ° C. It was.

Plate passing speed (R): R = 10 to 60 (m / min),
Unless an appropriate sheet feeding speed (R) at the time of cooling according to the water density is adopted, appropriate cooling conditions necessary in the present invention cannot be obtained. If R is less than 10 (m / min), the steel material is excessively cooled. On the other hand, if R exceeds 60 (m / min), the cooling conditions necessary for the water cooling stop temperature and bainite transformation described later cannot be obtained. m / min) or less.

  As described above, the present invention appropriately controls the cooling process after the end of rolling, sufficiently generates proeutectoid ferrite by performing air cooling at the time of cooling before cooling, and causes bainite transformation by rapid cooling at the latter stage. Since the low-temperature cooling stop is performed above, the movable dislocation due to the pro-eutectoid ferrite is maintained by the bainite transformation.

  Next, the component range of the steel sheet used in the present invention will be described.

  C is an effective component for improving the tensile strength of steel, and is a component necessary for lowering the transformation temperature and promoting the formation of bainite effective in reducing the yield strength by introducing movable dislocations. In order to achieve 80% or less, addition of 0.05% or more is necessary. Further, excessive addition significantly lowers the low temperature toughness, weldability, HAZ toughness, etc. of the steel material, so the upper limit was made 0.12%.

  Si is a component necessary for securing the tensile strength of the base material, deoxidation, etc., and it is necessary to add 0.05% or more. However, the upper limit was made 0.25% in order to prevent the toughness from being lowered by the hardening of the HAZ.

  Mn needs to be added in an amount of 0.4% or more as an effective component for ensuring the strength and compatibility of the base material, but the upper limit is set to 2.0% within an allowable range such as toughness and crackability of the weld.

  P is a component inevitably contained as an impurity, and the smaller the content, the better. However, in order to reduce this industrially, it takes a great deal of cost, so 0.02% was made the upper limit.

  S is a component inevitably contained as an impurity, and the smaller the content, the better. However, in order to reduce this industrially, it takes a great deal of cost, so 0.02% was made the upper limit.

  A1 is an important deoxidizing element, and the lower limit was set to 0.005%. In addition, when A1 is present in a large amount, the surface quality of the slab deteriorates, so the upper limit was made 0.04%.

  Cu is effective for improving the tensile strength of the steel material and needs to be 0.1% or more. However, if it exceeds 1.0%, the HAZ toughness is lowered, so 1.0% was made the upper limit. In addition, although Cu may be contained in steel as an impurity from a raw material, in this invention, it is permissible to contain less than 0.1% of Cu as an impurity.

  Ni is effective for improving the strength and toughness of the steel material and needs to be 0.1% or more. However, an increase in the amount of Ni increases the manufacturing cost, so 1.0% was made the upper limit.

  Since Ti combines with N to form Ti nitride, it improves the HAZ toughness during high heat input welding, so 0.005% or more is added. However, since the HAZ toughness decreases as the amount of dissolved Ti increases, the upper limit is set to 0.03%.

  Nb is an effective element for improving the tensile strength and toughness of steel by improving the burnability, and is required to be 0.003% or more, but excessive addition in the HAZ part significantly reduces toughness. The upper limit was 0.05%.

  Mo is an element effective for improving the tensile strength and toughness of steel by improving the hardenability, and is effective when added in an amount of 0.01% or more, but excessive addition in the base material and HAZ reduces the toughness. In order to reduce significantly, 0.6% was made the upper limit.

  Cr is an element effective for improving the tensile strength and toughness of steel by improving the hardenability like Mo, and is effective when added in an amount of 0.01% or more, but is excessively added in the base material and HAZ. Has an upper limit of 0.6% in order to significantly reduce toughness.

  B is an element effective in improving the tensile strength and toughness of steel by improving the hardenability even in a small amount, and is effective when added in an amount of 0.0003% or more, but excessive addition in the base material and HAZ is tough. In order to significantly reduce the content, the upper limit was made 0.003%.

  As described above, in the present invention, by appropriately adjusting the components of the steel material and appropriately controlling the cooling process after completion of rolling, the base material having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a weld heat affected zone are obtained. A thick steel plate having a thickness of 10 to 30 mm and excellent in toughness can be produced.

  Hereinafter, the present invention will be described in detail based on examples.

  A 500 MPa grade steel plate was manufactured using the manufacturing method shown in Table 1 and the test steel of chemical composition shown in Table 2. In Table 1, no. 1 to No. No. 25 is an example of the present invention, No. 25. 26 to No. Reference numeral 37 is a comparative example. The prototype steel is melted in the converter and deoxidized during the vacuum degassing treatment of RH. After casting into a 280 mm thick slab by continuous casting, after heating at the heating temperature shown in Table 1, rough rolling to a plate thickness twice the final product plate thickness, then finish rolling under the conditions shown in Table 1 It was. Then, water cooling was performed on the conditions shown in Table 1, and the steel plate was manufactured.

  A full thickness tensile test piece was collected from the steel sheet, a tensile test was conducted, and a Charpy impact test piece was taken and tested in order to investigate the low temperature toughness of the base material. Regarding tensile properties, a tensile strength of 500 MPa or more and a yield ratio of 80% or less were accepted, and a base material toughness was accepted as 27 J or more at −68 ° C.

  In addition, in order to investigate the toughness of the weld heat-affected (HAZ) part, welding was performed by multi-layered submerged arc welding at a heat input of 48 kJ / cm, and Charpy impact test pieces were collected from the bond part and used for the test. The test temperature was −68 ° C., and the case where an absorption energy of 27 J or more was obtained was regarded as acceptable. Table 3 shows the test results.

  As shown in Table 3, Example No. of the present invention satisfying the component range and production condition range defined in the present invention. 1 to No. No. 25 had properties of a yield ratio of 80% or less and a tensile strength of 500 MPa or more, and was excellent in toughness of the base material and the weld heat affected zone.

  In contrast, Comparative Example No. Although Table 26 satisfies the component range defined in the present invention as shown in Table 2, since the cooling conditions in the manufacturing process were uniformly cooled as in the conventional case as shown in Table 1, the yield ratio was 80 % Exceeded. Comparative Example No. 27 to Comparative Example No. As shown in Table 2, 37 is a steel type outside the component range of the present invention.

  The properties for these steel types are shown in Table 3. That is, Comparative Example No. No. 27 had a low C content and insufficient base material strength. Comparative Example No. In No. 28, the C content was excessive and the base metal toughness and weld zone heat effect (HAZ) toughness were insufficient. Comparative Example No. No. 29 had a low Si content and lacked the strength and toughness of the base material. Comparative Example No. No. 30 had an excessive Si content, and the strength and toughness of the base material were insufficient. Comparative Example No. No. 31 had a low Mn content, and the base material strength was insufficient. Comparative Example No. In No. 32, the Mn content was excessive and the base metal toughness and weld zone heat effect (HAZ) toughness were insufficient. Comparative Example No. No. 33 had an excessive amount of impurity element P, and the base metal toughness and weld zone heat effect (HAZ) toughness were insufficient. Comparative Example No. No. 34 had an excess of impurity element S, and the base metal toughness and weld zone heat effect (HAZ) toughness were insufficient. Comparative Example No. No. 35 had an excess of Nb, and the weld zone heat effect (HAZ) toughness was insufficient. Comparative Example No. In No. 36, Al was excessive, the yield ratio was high, and slab cracking occurred. Comparative Example No. No. 37 had low Al and lacked the base material toughness.

  As is clear from the above examples of the present invention and comparative examples, according to the present invention, the plate thickness of 10 to 10 excellent in the toughness of the base metal and the weld heat affected zone, yield ratio YR ≦ 80%, and tensile strength TS ≧ 500 MPa. It turns out that a 30 mm thick steel plate is obtained.

It is a figure which shows typically the relationship between the cooling behavior of steel, and a continuous cooling transformation curve.

Claims (4)

% By mass
C: 0.05 to 0.12%,
Si: 0.05 to 0.25%,
Mn: 0.4 to 2.0%,
P: 0.02% or less,
S: 0.02% or less,
Nb: 0.01-0.05%
Al: 0.005 to 0.04%,
A steel slab having a plurality of water-cooling zones each having a plurality of water-cooling zones capable of adjusting the water density independently of the rolling direction. In the manufacturing method, the initial water density (Wi), the final water density (Wf), the water cooling start temperature (Tcs), the water cooling stop temperature (Tcf), and the sheet passing speed (R) are set as the following manufacturing conditions. It is excellent in low temperature toughness of a base metal having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a thickness of 10 to 30 mm and a weld heat affected zone, characterized by performing air cooling for 3 to 15 seconds and after 5 to 20 seconds. Manufacturing method of thick steel plate.
Wi = 0.2 to 1.5 (m ³ m ⁻² min ⁻¹ ), Wf = 0.5 to 2.0 (m ³ m ⁻² min ⁻¹ ), Tcs = 700 to 790 (° C.), Tcf ≦ 450 (° C.), R = 10 to 60 (m / min) The casting slab is further in mass%,
Cu: 0.1 to 0.23%,
Ni: 0.1 to 0.45%,
A base material having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a thickness of 10 to 30 mm according to claim 1, and a method for producing a thick steel plate excellent in low-temperature toughness of the weld heat affected zone . The casting slab is further in mass%,
Ti: 0.005 to 0.03%,
The base material having a yield ratio of 80% or less and a tensile strength of 500 MPa class and a thickness of 10 to 30 mm according to claim 1 or 2, wherein the steel plate is excellent in low temperature toughness of the weld heat affected zone. Production method. The casting slab is further in mass%,
Mo: 0.01 to 0.6%,
Cr: 0.01 to 0.6%,
B: 0.0003 to 0.003%
The base material having a plate thickness of 10 to 30 mm with a yield strength of 80 MPa or less and a tensile strength of 500 MPa as described in any one of claims 1 to 3 and excellent in low temperature toughness of a weld heat affected zone Manufacturing method of thick steel plate.

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