Thick steel plate and method for manufacturing thick steel plate

Abstract

The present invention provides a thick steel sheet having both excellent fatigue crack propagation resistance and total elongation, and also excellent elongation in the sheet thickness direction. A thick steel sheet having a composition consisting of, in mass%, C: 0.01 to 0.16%, Si: 1.00% or less, Mn: 0.50 to 2.00%, P: 0.030% or less, S: 0.020% or less, Al: 0.06% or less, and the remainder consisting of Fe and unavoidable impurities, having a microstructure containing, in area fraction, 75 to 97% of bainite and 3 to 25% of pearlite, the bainite having an average equivalent circle diameter of grain of 18 μm or less, the pearlite having an average equivalent circle diameter of grain of 10 μm or less, and a reduction of area in the sheet thickness direction of 30% or more.

Description

Technical Field

[0001] This invention relates to a thick steel plate, and more particularly to a thick steel plate that possesses excellent resistance to fatigue crack propagation and total elongation, as well as excellent elongation in the thickness direction. The thick steel plate of this invention can be suitable for use in structures requiring high structural safety, such as ships, marine structures, bridges, buildings, and tanks. Furthermore, this invention relates to a method for manufacturing the aforementioned thick steel plate. Background Technology

[0002] Thick steel plates are widely used in ships, marine structures, bridges, buildings, tanks, and other structures. In addition to requiring excellent mechanical properties such as strength and toughness, as well as weldability, these thick steel plates also require good fatigue resistance.

[0003] When using structures like the ones described above, these structures are subjected to repeated loads such as vibrations caused by wind and earthquakes. Therefore, for thick steel plates, fatigue characteristics that ensure the safety of the structure even under such repeated loads are required. In particular, to prevent ultimate failure such as component fracture, the fatigue crack propagation resistance of thick steel plates must be improved.

[0004] Therefore, various studies have been conducted to improve the fatigue crack propagation resistance of steel plates.

[0005] For example, Patent Document 1 discloses a steel plate for oil tankers that exhibits excellent resistance to fatigue crack propagation in a humid hydrogen sulfide environment. This steel plate has a mixed microstructure consisting of ferrite as the first phase and bainite and / or pearlite as the second phase. Furthermore, in this steel plate, the average grain size of the ferrite is 20 μm or less.

[0006] Furthermore, Patent Document 2 also proposes a steel plate with excellent resistance to fatigue crack propagation. The steel plate is characterized by having a microstructure consisting of hard and soft portions, wherein the hardness difference between the hard and soft portions is 150 or more on a Vickers hardness scale.

[0007] Patent Document 3 proposes a dual-phase steel with a microstructure consisting of bainite and ferrite comprising 38–52% by area. In the technology proposed in Patent Document 3, fatigue crack propagation resistance is improved by controlling the Vickers hardness of the ferrite phase and the density of the boundary between the ferrite and bainite phases.

[0008] Here, thick-walled steel plates are typically manufactured by rolling large steel ingots produced using the casting method into billets, and then hot-rolling the resulting billet plates. However, in this casting-billing process, the thick segregated portion at the riser and the negative segregated portion at the bottom of the ingot need to be discarded. Therefore, the yield rate does not increase, the manufacturing cost rises, and the production cycle is lengthened.

[0009] On the other hand, the aforementioned problems do not occur when manufacturing thick-walled steel plates using a process that uses continuously cast slabs as billets. However, continuously cast slabs are thinner than those manufactured using ingot casting, resulting in a reduction in the amount of material rolled down to the product thickness, which can lead to the problem of not being able to press out center porosity. If center porosity cannot be pressed out, the elongation caused by stretching in the thickness direction of the plate deteriorates. Furthermore, as mentioned above, thick steel plates are used in structures and the like, thus requiring high strength. Therefore, the amount of alloying elements added to ensure the required strength increases, resulting in new problems such as center porosity caused by center segregation and deterioration of internal quality due to enlargement.

[0010] To address this problem, a technique has been proposed to improve the characteristics of the central segregation region within the steel plate by reducing the porosity of the press-fit center during the manufacturing of thick steel plates from continuously cast slabs.

[0011] Patent document 4 proposes a technique for forging before hot rolling when manufacturing thick-walled steel plates with a cumulative reduction rate of less than 70% from continuously cast slabs.

[0012] Patent document 5 proposes that when manufacturing an extremely thick steel plate by forging and thick plate rolling of a continuously cast slab, the center of the slab thickness is held at a temperature of 1200°C or higher for more than 20 hours before the forging, and then forging is performed with a reduction rate of 16% or higher.

[0013] Patent document 6 proposes a technique of hot rolling after cross-forging of continuously cast slabs.

[0014] Patent document 7 proposes a technique for producing extremely thick steel plates by forging and thick plate rolling of continuously cast slabs after holding them at a temperature above 1200°C for more than 20 hours. In this technique, the reduction rate of the forging and the total reduction rate of the forging and thick plate rolling are set within specific ranges, and the thick plate rolling is followed by quenching and tempering under specific conditions.

[0015] Existing technical documents

[0016] Patent documents

[0017] Patent Document 1: Japanese Patent Application Publication No. 06-322477

[0018] Patent Document 2: Japanese Patent Application Publication No. 07-242992

[0019] Patent Document 3: Japanese Patent Application Publication No. 08-225882

[0020] Patent Document 4: Japanese Patent Application Publication No. 07-232201

[0021] Patent Document 5: Japanese Patent Application Publication No. 2002-194431

[0022] Patent Document 6: Japanese Patent Application Publication No. 2000-263103

[0023] Patent Document 7: Japanese Patent Application Publication No. 2006-111918 Summary of the Invention

[0024] However, in the prior art described in Patent Documents 1 to 7, the following problems (1) to (4) exist.

[0025] (1) For steel used in structures such as ships, marine structures, bridges, buildings, and tanks, the standard usually specifies the total elongation value. Therefore, even steel plates with excellent resistance to fatigue crack propagation are required to meet the standard value for total elongation.

[0026] However, fatigue crack propagation resistance and total elongation are opposite properties, so the existing technologies described in Patent Documents 1 to 3 cannot simultaneously achieve excellent fatigue crack propagation resistance and total elongation.

[0027] In other words, the technologies proposed in Patent Documents 1-3 do not consider total elongation. In reality, the steel plates proposed in Patent Documents 1-3 all possess a microstructure consisting of ferrite as a soft phase and bainite or martensite as a hard phase, improving resistance to fatigue crack propagation by widening the hardness difference between the soft and hard phases. However, when the hardness difference between the soft and hard phases is large, the microstructure becomes heterogeneous, resulting in a decrease in the total elongation of the steel plate.

[0028] (2) In addition, from the point of view of ensuring the safety of the structure, for thick steel plates, excellent resistance to fatigue crack propagation in the thickness direction is required.

[0029] In general structures, steel plates are welded from various directions, resulting in fatigue cracks that propagate in a variety of directions. However, at welded locations with angled joints, fatigue crack formation is unavoidable due to the structural characteristics, and these cracks tend to propagate primarily towards the plate thickness. Therefore, to prevent structural collapse caused by fatigue cracks, it is crucial to suppress their propagation towards the plate thickness.

[0030] (3) Furthermore, it is difficult to control the manufacturing conditions for conventional steel sheets with the aforementioned microstructure. Specifically, in the case of manufacturing such steel sheets using an online process, in order to obtain the desired microstructure, it is necessary to accelerate cooling from the ferrite-austenite dual-phase region and lower the cooling stop temperature during the cooling process after hot rolling. At this time, the area fraction of the soft and hard phases in the final microstructure varies significantly depending on the temperature at the start of cooling. Therefore, in the manufacturing of the aforementioned conventional steel sheets, it is necessary to strictly control the cooling conditions in order to obtain the desired microstructure.

[0031] (4) The techniques described in Patent Documents 4 to 7 are effective in reducing central porosity and improving central segregation bands, but require a hot forging process. In addition, by simply hot rolling, the processing of the central part of the plate thickness becomes insufficient, central porosity remains, and the tensile properties in the thickness direction may deteriorate.

[0032] The present invention was made in view of the above circumstances, and its object is to provide a thick steel plate with excellent total elongation, elongation in the thickness direction and resistance to fatigue crack propagation.

[0033] Specifically, the aim is to provide a thick steel plate that possesses the excellent characteristics of (1) to (4) below.

[0034] (1) It has both excellent resistance to fatigue crack propagation and total elongation.

[0035] (2) Regarding the above-mentioned fatigue crack propagation resistance, the fatigue crack propagation resistance in the thickness direction is excellent, which is particularly important in ensuring the safety of the structure.

[0036] (3) It can be manufactured without the need for high-level cooling control in the two-phase region.

[0037] (4) In hot-rolled manufacturing, the elongation based on the thickness direction of the plate is also excellent.

[0038] In order to solve the above-mentioned problems, the inventors conducted research and obtained the following insights.

[0039] (a) Even if the hardness difference between the soft and hard phases in the microstructure is not as large as in Patent Documents 1-3, sufficient resistance to fatigue crack propagation can still be obtained.

[0040] (b) By using bainite as the first phase, the fatigue crack propagation resistance is improved compared to the past.

[0041] (c) By fabricating a thick steel plate that contains bainite as a soft phase and pearlite as a hard phase in a specific area fraction, and the grains of bainite and pearlite are microstructures within a specific range, it is possible to obtain a thick steel plate that has both excellent resistance to fatigue crack propagation and total elongation.

[0042] (d) Thick steel plates with the above-described microstructure can be manufactured by controlling manufacturing conditions, particularly the conditions during hot rolling and subsequent accelerated cooling. These thick steel plates use bainite as the first phase, and therefore are suitable for manufacturing using in-line processes compared to conventional steel plates.

[0043] (e) Furthermore, in hot rolling, defects such as casting defects are rendered harmless by rolling with a reduction ratio of 3 or more, and for at least 2 of the final 3 passes, the reduction rate of each pass is set to 10% or more to achieve overall granulation of the steel plate, suppressing the presence of abnormally large particles, thereby improving the tensile properties in the thickness direction of the plate.

[0044] This invention is based on the above insights, and its main points are as follows.

[0045] 1. A thick steel plate having the following composition (in mass percent): C: 0.01-0.16%, Si: less than 1.00%, Mn: 0.50-2.00%, P: less than 0.030%, S: less than 0.020%, Al: less than 0.06%.

[0046] The remainder consists of Fe and unavoidable impurities.

[0047] The thick steel plate has the following microstructure:

[0048] In terms of area fraction,

[0049] It contains 75-97% bainite and 3-25% pearlite.

[0050] Bainite grains are less than 18 μm in average equivalent circle diameter.

[0051] Pearlite grains have an average equivalent circle diameter of less than 10 μm.

[0052] The reduction of area in the thickness direction is over 30%.

[0053] 2. The thick steel plate according to claim 1 above, wherein the above-mentioned composition further comprises, in mass percent, an ingredient selected from Cr: 0.01–1.00%, Cu: 0.01–2.00%, Ni: 0.01–2.00%, Mo: 0.01–1.00%, Co: 0.01–1.00%, Sn: 0.005–0.200%, Sb: 0.005–0.200%, Nb: 0.005–0.200%. One or more of the following: 0.005–0.200%, V: 0.005–0.200%, Ti: 0.005–0.050%, B: 0.0001–0.0050%, Zr: 0.005–0.100%, Ca: 0.0001–0.020%, Mg: 0.0001–0.020%, and REM: 0.0001–0.020%.

[0054] 3. A method for manufacturing a thick steel plate, comprising heating a steel billet having the composition described in 1 or 2 above to a heating temperature of 1000°C to 1300°C.

[0055] The above-mentioned heated steel billet is hot-rolled into a hot-rolled steel sheet under the conditions of a reduction ratio of 3 or more and a reduction rate of 10% or more in the final three passes of at least two passes.

[0056] The hot-rolled steel plate was subjected to accelerated cooling under the following conditions: cooling start temperature above Ar3, cooling stop temperature of 300-650℃, and average cooling rate on the surface of the steel plate from the start of cooling to the end of cooling of 20-60℃ / s.

[0057] Invention Effects

[0058] The thick steel plate of the present invention exhibits excellent resistance to fatigue crack propagation and total elongation, with particularly superior resistance to fatigue crack propagation in the thickness direction. Furthermore, the reduction of area in the thickness direction is over 30%, and the elongation due to tension in the thickness direction is excellent. Moreover, the aforementioned excellent properties can be achieved in the thick steel plate of the present invention even without the addition of large amounts of alloying elements such as Cr and Sn, thus offering cost advantages. Additionally, the thick steel plate of the present invention does not require highly controlled cooling in the two-phase region, enabling stable manufacturing. Attached Figure Description

[0059] Figure 1 It is a diagram showing the shape and dimensions of a single-sided notched, simple tensile fatigue test specimen used in the evaluation of fatigue crack propagation characteristics in the thickness direction of a plate. Detailed Implementation

[0060] The present invention will now be described in detail. It should be noted that the present invention is not limited to this embodiment.

[0061] [Ingredients]

[0062] First, the composition of the thick steel plate of the present invention will be described. Unless otherwise specified, the "%" indicating the content of each component refers to "mass %".

[0063] C: 0.01~0.16%

[0064] C is an element that improves strength. Furthermore, C promotes the formation of the pearlite phase, which is beneficial for fatigue resistance. If the C content is below 0.01%, the desired strength and resistance to fatigue crack propagation cannot be obtained. Therefore, the C content is set to 0.01% or more. On the other hand, when the C content exceeds 0.16%, excessive pearlite formation or coarsening occurs, thus deteriorating the total elongation and toughness. Therefore, the C content is set to 0.16% or less, preferably 0.15% or less, and more preferably 0.10% or less.

[0065] Si: below 1.00%

[0066] Si is an element that also has a deoxidizing effect and further improves strength. In addition, Si has the effect of inhibiting the formation of excessive cementite. However, if the Si content exceeds 1.00%, weldability and toughness deteriorate, and the formation of the pearlite phase, which is beneficial to fatigue resistance, is suppressed. Therefore, the Si content is set to 1.00% or less, preferably 0.50% or less. On the other hand, there is no particular limitation on the lower limit of the Si content, but from the viewpoint of improving the effect of Si addition, it is preferable to set the Si content to 0.01% or more, more preferably 0.10% or more.

[0067] Mn: 0.50~2.00%

[0068] Mn is an element that improves hardenability, resulting in increased strength in thick steel plates. To achieve this effect, the Mn content is set to 0.50% or more, preferably 0.80% or more. On the other hand, when the Mn content exceeds 2.00%, the hardenability becomes excessively high, thereby inhibiting the formation of the pearlite phase, which is beneficial to fatigue resistance. Furthermore, if the Mn content exceeds 2.00%, the total elongation and toughness decrease. Therefore, the Mn content is set to 2.00% or less, preferably 1.65% or less.

[0069] P: below 0.030%

[0070] Polymer (P) degrades toughness. Therefore, the P content is set to 0.030% or less. On the other hand, the lower the P content, the better. Therefore, there is no particular limitation on the lower limit of the P content, and the P content can be 0% or more, or even more than 0%. However, excessive reduction will increase manufacturing costs. Therefore, from the point of view of manufacturing costs, it is preferable to set the P content to 0.001% or more, and more preferably 0.002% or more.

[0071] S: below 0.020%

[0072] Sulfur (S) is an element contained as an impurity in thick steel plates, which deteriorates toughness. Therefore, the S content is set to 0.020% or less, preferably 0.010% or less. On the other hand, the lower the S content, the better. There is no particular limitation on the lower limit of the S content; the S content can be 0% or more, or even exceed 0%. However, excessive reduction will increase manufacturing costs. Therefore, from the viewpoint of manufacturing costs, it is preferable to set the S content to 0.0005% or more, more preferably 0.001% or more.

[0073] Al: below 0.06%

[0074] Al acts as a deoxidizer in the steel deoxidation process. Additionally, Al fixes nitrogen (N) in steel as AlN, contributing to improved toughness of the base metal. However, if the Al content exceeds 0.06%, the toughness and total elongation of the base metal (thick steel plate) decrease, and Al mixes into the weld metal during welding, deteriorating the weld's toughness. Therefore, the Al content is set to 0.06% or less, preferably 0.05% or less. On the other hand, there is no particular limitation on the lower limit of the Al content, but from the viewpoint of maximizing the effect of Al addition, it is preferable to set the Al content to 0.01% or more.

[0075] The thick steel plate of one embodiment of the present invention may have a composition comprising the above-mentioned elements, with the remainder consisting of Fe and unavoidable impurities.

[0076] Furthermore, the composition of the thick steel plate in other embodiments of the present invention may further arbitrarily contain at least one of the elements listed below. By adding these arbitrary additive elements, the strength, toughness, weldability, weather resistance, and coating weather resistance of the thick steel plate can be further improved.

[0077] Cr: 0.01~1.00%

[0078] Cr is an element that further enhances strength. Furthermore, Cr promotes cementite formation and the formation of the pearlite phase, which is beneficial for fatigue resistance. When Cr is added, to achieve the above-mentioned effects, the Cr content is set to 0.01% or more, preferably 0.10% or more. On the other hand, when the Cr content exceeds 1.00%, weldability and toughness are impaired. Therefore, when Cr is present, the Cr content is set to 1.00% or less, preferably 0.80% or less, and more preferably 0.50% or less.

[0079] Cu: 0.01–2.00%

[0080] Cu is an element that further increases strength through solid solution treatment. To achieve the aforementioned effect, the Cu content is set to 0.01% or more when Cu is added. On the other hand, when the Cu content exceeds 2.00%, weldability is impaired, and defects are easily generated during the manufacture of thick steel plates. Therefore, when Cu is present, the Cu content is set to 2.00% or less, preferably 0.70% or less, and more preferably 0.60% or less.

[0081] Ni: 0.01~2.00%

[0082] Ni is an element that improves low-temperature toughness. Furthermore, Ni mitigates hot brittleness when Cu is added. To achieve these effects, the Ni content is set to 0.01% or more when Ni is added. On the other hand, when the Ni content exceeds 2.00%, weldability is compromised, and steel costs increase. Therefore, when Ni is present, the Ni content is set to 2.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

[0083] Mo: 0.01~1.00%

[0084] Mo is an element that further enhances strength. To achieve this effect, the Mo content is set to 0.01% or more when Mo is added. On the other hand, weldability and toughness are compromised when the Mo content exceeds 1.00%. Therefore, the Mo content is set to 1.00% or less, preferably 0.70% or less, and more preferably 0.40% or less.

[0085] Co: 0.01~1.00%

[0086] Co is an element that increases the hardness of the matrix phase. To achieve this effect, the Co content is set to 0.01% or more, preferably 0.35% or more, when Co is present. However, if the Co content exceeds 1.00%, the effect saturates, and the alloy cost increases. Therefore, when Co is added, the Co content is set to 1.00% or less, preferably 0.50% or less.

[0087] Sn: 0.005~0.200%

[0088] Sn is an element that increases the hardness of the base phase. When adding Sn, to achieve the above-mentioned effect, the Sn content is set to 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, when the Sn content exceeds 0.200%, it leads to a deterioration in the ductility and toughness of the steel. Therefore, when adding Sn, the Sn content is set to 0.200% or less, preferably 0.100% or less, and more preferably less than 0.050%.

[0089] Sb: 0.005~0.200%

[0090] Sb is an element that increases the hardness of the base phase. When adding Sb, to achieve the aforementioned effect, the Sb content is set to 0.005% or more, preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, when the Sb content exceeds 0.200%, it leads to a deterioration in the ductility and toughness of the steel. Therefore, when adding Sb, the Sb content is set to 0.200% or less, preferably 0.150% or less, and more preferably 0.100% or less.

[0091] Nb: 0.005~0.200%

[0092] Nitrogen (Nb) is an element that inhibits the recrystallization of austenite during hot rolling, resulting in finer grains. Furthermore, Nb precipitates during air cooling after accelerated cooling, further enhancing strength. To achieve these effects, the Nb content is set to 0.005% or more when Nb is added. On the other hand, if the Nb content exceeds 0.200%, hardenability becomes excessive, and martensite formation becomes significant. This results in the inability to obtain the desired microstructure and reduced toughness. Therefore, the Nb content is set to 0.200% or less, preferably 0.050% or less.

[0093] V: 0.005~0.200%

[0094] V is an element that precipitates during air cooling after accelerated cooling, further increasing strength. To achieve this effect, the V content is set to 0.005% or more when V is added. On the other hand, weldability and toughness decrease when the V content exceeds 0.200%. Therefore, the V content is set to 0.200% or less, preferably 0.050% or less.

[0095] Ti: 0.005~0.050%

[0096] Ti is an element that can further increase strength and improve the toughness of the weld. To achieve these effects, the Ti content is set to 0.005% or more when Ti is added. On the other hand, the cost increase becomes significant when the Ti content exceeds 0.050%. Therefore, the Ti content is set to 0.050% or less, preferably 0.030% or less, and more preferably 0.020% or less.

[0097] B: 0.0001~0.0050%

[0098] Boron (B) is an element that improves hardenability, resulting in further increased strength. To achieve this effect, the B content is set to 0.0001% or more when B is added. However, when the B content exceeds 0.0050%, hardenability becomes excessive, and martensite formation becomes significant. This results in an undesirable microstructure and reduced weldability. Therefore, the B content is set to 0.0050% or less, preferably 0.0030% or less.

[0099] Zr: 0.005~0.100%

[0100] Zr is an element that further enhances strength. To fully achieve this effect, a Zr content of 0.005% or higher is required. Therefore, when adding Zr, the Zr content is set to 0.005% or higher. On the other hand, when the Zr content exceeds 0.100%, its strength-enhancing effect saturates. Therefore, when containing Zr, the Zr content is set to 0.100% or lower.

[0101] Ca: 0.0001~0.020%

[0102] Ca is an element that controls the form of sulfides, resulting in a further improvement in toughness. To achieve this effect, the Ca content is set to 0.0001% or higher when Ca is added. However, the effect saturates when the Ca content exceeds 0.020%. Therefore, the Ca content is set to 0.020% or less.

[0103] Mg: 0.0001~0.020%

[0104] Mg is an element that further improves toughness through grain refinement. To achieve this effect, the Mg content is set to 0.0001% or more when Mg is added. However, if the Mg content exceeds 0.020%, its effect saturates. Therefore, the Mg content is set to 0.020% or less.

[0105] REM: 0.0001~0.020%

[0106] REM (rare earth metals) is an element that further enhances toughness. To achieve this effect, the REM content is set to 0.0001% or higher when REM is added. However, its effect saturates when the REM content exceeds 0.020%. Therefore, the REM content is set to 0.020% or lower.

[0107] [Microstructure]

[0108] Next, the microstructure of the thick steel plate will be described. The thick steel plate according to one embodiment of the present invention has the following microstructure: comprising 75-97% bainite and 3-25% pearlite in terms of area fraction, wherein the bainite grains are 18 μm or less in terms of average equivalent circle diameter, and the pearlite grains are 10 μm or less in terms of average equivalent circle diameter. It should be noted that the microstructure of the present invention refers to the microstructure at 1 / 4 position (1 / 4t position) of the thickness t of the thick steel plate. The area fraction and grain size of each microstructure can be determined by etching a section parallel to the rolling direction at 1 / 4 depth from the surface of the thick steel plate with nitric acid and ethanol, and then observing the result. More specifically, the area fraction and grain size can be determined by the method described in the examples.

[0109] Area fraction of bainite: 75–97%

[0110] In this invention, bainite is the first phase in the aforementioned microstructure, functioning as a soft phase. Ferrite is a representative soft phase found in steel billets, but bainite exhibits a higher crack propagation inhibition effect compared to ferrite. Therefore, by ensuring that the area fraction of bainite is 75% or more, fatigue crack propagation can be suppressed. If the area fraction of bainite is less than 75%, the desired resistance to fatigue crack propagation can be suppressed. The area fraction of bainite is preferably 80% or more. On the other hand, if the area fraction of bainite exceeds 97%, pearlite becomes insufficient, resulting in the inability to suppress fatigue crack propagation. Therefore, the area fraction of bainite is 97% or less.

[0111] Bainite grain size: below 18 μm

[0112] The bainite grain size is set to 18 μm or less, based on the average equivalent circle diameter. By refining the bainite, the desired toughness and total elongation characteristics can be obtained. When the bainite grain size exceeds 18 μm, the desired toughness cannot be obtained. On the other hand, there is no particular limit to the lower limit of the bainite grain size, but excessive refinement makes manufacturing difficult. Therefore, in actual manufacturing, it is preferable to set the bainite grain size to 5 μm or more.

[0113] It should be noted that the bainite of the present invention includes upper bainite, acicular ferrite and granular bainite.

[0114] Pearlite area fraction: 3–25%

[0115] In this invention, pearlite is the second phase in the aforementioned microstructure, functioning as a hard phase. When a fatigue crack propagating in bainite reaches the pearlite, which is the hard phase, the crack stops or bends at the interface between bainite and pearlite. Furthermore, as a result, crack propagation is suppressed. To achieve this effect, the area fraction of pearlite is set to 3% or more, preferably 5% or more. On the other hand, when the area fraction of pearlite exceeds 25%, the total elongation decreases. Therefore, the area fraction of pearlite is 25% or less, preferably 20% or less.

[0116] Pearlite grain size: below 10 μm

[0117] The pearlite grain size is set to be 10 μm or less, based on the average equivalent circle diameter. By refining the pearlite, the desired toughness and total elongation characteristics can be obtained. When the pearlite grain size exceeds 10 μm in average equivalent circle diameter, the desired toughness cannot be obtained. On the other hand, there is no particular limitation on the lower limit of the pearlite grain size; it can be 1 μm or more, or even 2 μm or more.

[0118] It should be noted that the pearlite of the present invention includes pearlite and pseudo-pearlite.

[0119] (Other organizations)

[0120] The thick steel plate of one embodiment of the present invention may have a microstructure composed of bainite and pearlite. However, the aforementioned microstructure may further arbitrarily include other structures. Hereinafter, structures other than bainite and pearlite will be referred to as "other structures". The aforementioned other structures may be, for example, one or both of martensite and ferrite. Here, the aforementioned martensite includes island martensite, lath martensite, and lenticular martensite.

[0121] When the microstructure of thick steel plates includes other microstructures, the area fraction (total area fraction) of these other microstructures is not particularly limited. However, when martensite is present in excess, high-hardness regions are formed locally, increasing strength but worsening total elongation and potentially reducing toughness. Furthermore, when ferrite is present in excess, fatigue crack propagation rate deteriorates, and soft regions are formed locally, potentially worsening total elongation due to the widening hardness difference. Therefore, a lower area fraction of these other microstructures is preferred, but if it is below 5%, the effect can be ignored. Therefore, it is preferable to set the total area fraction of microstructures other than bainite and pearlite to 5% or less.

[0122] In other words, the thick steel plate in one embodiment of the present invention has a microstructure consisting of 75-97% bainite, 3-25% pearlite, and 0-5% bainite and other structures.

[0123] (plate thickness)

[0124] The thickness of the steel plate in this invention is not particularly limited, and is generally 6 mm or more. However, central porosity is prone to occur in steel plates with a thickness of 25 mm or more; therefore, this invention is particularly suitable for steel plates with a thickness of 25 mm or more. Therefore, the thickness of the steel plate is preferably set to 25 mm or more. On the other hand, there is no particular limit to the upper limit of the plate thickness, but fatigue damage is prone to occur in steel plates with a thickness of 100 mm or less; therefore, this invention is particularly suitable for steel plates with a thickness of 100 mm or less. Therefore, the thickness of the steel plate is preferably set to 100 mm or less, and more preferably 80 mm or less.

[0125] (Cross-sectional contraction)

[0126] The thick steel plate of the present invention is manufactured under the conditions described later, thereby pressing together a centrally porous structure, resulting in excellent elongation due to stretching in the thickness direction. Specifically, the reduction of area (RA) in the thickness direction of the thick steel plate of the present invention is 30% or more. The reduction of area is preferably 35% or more, more preferably 40% or more. A higher reduction of area indicates better elongation in the thickness direction. It should be noted that in the present invention, the reduction of area refers to the reduction of area in the thickness direction measured according to JIS G3199 using a test piece with the parallel portion including the center of the thickness, and more specifically, it can be measured using the method described in the examples.

[0127] (Tensile strength)

[0128] The thick steel plate of the present invention has the above-described composition and microstructure, resulting in excellent tensile strength (TS). The value of TS is not particularly limited, but is preferably 500 MPa or more, more preferably 530 MPa or more, and even more preferably 550 MPa or more. On the other hand, there is no upper limit to the value of TS, but it can be, for example, 720 MPa or less, 700 MPa or less, 640 MPa or less, or 620 MPa or less.

[0129] (Yield stress)

[0130] The yield stress (YS) of the thick steel plate of the present invention is not particularly limited, and can be 420 MPa or more, 430 MPa or more, or 440 MPa or more. In addition, YS can be 560 MPa or less, 530 MPa or less, or 520 MPa or less.

[0131] (toughness)

[0132] The thick steel plate of the present invention has the above-described composition and microstructure, resulting in excellent toughness. The toughness of the thick steel plate of the present invention is not particularly limited, but it is preferable that the Charpy impact absorption energy vE0 at 0°C, one of the indicators of toughness, is set to 100 J or more, more preferably 130 J or more, even more preferably 150 J or more, and most preferably 200 J or more. On the other hand, there is no upper limit to vE0; for example, it can be 400 J or less, 300 J or less, or 270 J or less. It should be noted that vE0 can be measured using the method described in the examples.

[0133] (Total elongation)

[0134] The thick steel plate of the present invention has the above-described composition and microstructure, resulting in excellent total elongation (EL). The value of EL is not particularly limited, but is preferably 21% or more, more preferably 22% or more, further preferably 23% or more, and most preferably 26% or more. There is no particular limitation on the upper limit of EL, but it can be 36% or less. It should be noted that EL can be measured using the method described in the examples.

[0135] (Resistance to fatigue crack propagation)

[0136] The thick steel plate of the present invention has the above-described composition and microstructure, resulting in excellent resistance to fatigue crack propagation in the thickness direction. The fatigue crack propagation rate (da / dN) can be used as an indicator of fatigue crack propagation resistance. The value of the fatigue crack propagation rate is not particularly limited.

[0137] It should be noted that the fatigue crack propagation rate in the thickness direction (Z direction) preferably satisfies the conditions in (a) and (b) below.

[0138] (a) Stress amplification factor range ΔK: 15MPa / m 1 / 2 The fatigue crack propagation rate under the given conditions was 8.75 × 10⁻⁶. －9 (m / cycle) or less

[0139] (b) Stress amplification factor range ΔK: 25MPa / m 1 / 2 The fatigue crack propagation rate under the given conditions is 4.25 × 10⁻⁶. －8 (m / cycle) and below

[0140] It should be noted that, as mentioned above, fatigue cracks initially tend to propagate towards the thickness direction. Therefore, suppressing fatigue crack propagation, particularly in the thickness direction (Z direction), becomes especially important in improving fatigue crack propagation resistance. However, after the crack propagates in the Z direction, it sometimes further propagates in the rolling direction (L direction) or the width direction (C direction). Therefore, from the viewpoint of further improving fatigue crack propagation resistance, it is preferable that either the fatigue crack propagation rate in the rolling direction (L direction) or the fatigue crack propagation rate in the width direction (C direction) satisfies the following conditions (c) and (d), and more preferably that both satisfy conditions (c) and (d).

[0141] (c) Stress amplification factor range ΔK: 15MPa / m 1 / 2 The fatigue crack propagation rate under the given conditions is 1.75 × 10⁻⁶. －8 (m / cycle) or less

[0142] (d) Stress amplification factor range ΔK: 25MPa / m 1 / 2 The fatigue crack propagation rate under the given conditions is 8.50 × 10⁻⁶. －8 (m / cycle) and below

[0143] [Manufacturing Conditions]

[0144] Next, the manufacturing method of the thick steel plate of the present invention will be described. The thick steel plate of one embodiment of the present invention can be manufactured by sequentially performing the following steps (1) to (3) on a steel billet having the above-described composition.

[0145] (1) Heating

[0146] (2) Hot rolling

[0147] (3) Accelerate cooling

[0148] The conditions for each process are described below. It should be noted that, unless otherwise specified, temperature refers to the surface temperature of the workpiece (steel billet or hot-rolled steel plate).

[0149] (steel billet)

[0150] As for the aforementioned steel billet, any material can be used as long as it has the above-mentioned composition. The final thick steel plate has the same composition as the steel billet used. The aforementioned steel billet can be, for example, one or both of slabs and steel ingots. Examples of slabs include continuously cast slabs and cast ingot slabs.

[0151] (1) Heating

[0152] Heating temperature: 1000~1300℃

[0153] First, the steel billet is heated to a temperature of 1000°C to 1300°C. This heating causes the precipitates in the microstructure to dissolve, homogenizing the grains. However, if the heating temperature is below 1000°C, the precipitates will not dissolve sufficiently, thus failing to obtain the desired properties. Therefore, the heating temperature is set to 1000°C or higher, preferably 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1300°C, the material deteriorates due to grain coarsening, and excessive energy is required, reducing productivity. Therefore, the heating temperature is 1300°C or lower, preferably 1250°C or lower.

[0154] (2) Hot rolling

[0155] Next, the heated steel billet is hot-rolled to produce a hot-rolled steel sheet. At this time, in order to manufacture a thick steel sheet that meets the conditions of the present invention, the reduction ratio during the hot rolling process needs to meet the following conditions.

[0156] [Reduction ratio in hot rolling: 3 or higher]

[0157] When the reduction ratio in hot rolling is less than 3, the improvement in tensile properties in the thickness direction due to the loose center of the press cannot be obtained. Furthermore, when the reduction ratio in hot rolling is less than 3, the promotion of recrystallization and the resulting granulation effect from rolling become insufficient, leaving coarse austenite grains. This results in deterioration of properties such as strength and toughness. Therefore, a reduction ratio of 3 or more is preferred, preferably 4 or more, and more preferably 5 or more. On the other hand, the upper limit of the reduction ratio is not particularly limited, but it is preferably 50 or less. This is because when the reduction ratio exceeds 50, the anisotropy of mechanical properties becomes significantly greater. Here, the reduction ratio in hot rolling is defined as "the thickness of the billet / the thickness of the rolled sheet".

[0158] [Number of passes with a reduction rate of 10% or higher out of the final 3 passes: 2 or more]

[0159] In this invention, it is important that at least two of the final three passes of the hot rolling process have a reduction rate of 10% or more. In other words, at least two of the final three passes of the hot rolling process involve a reduction rate of 10% or more. This allows for the reliable neutralization of defects (such as casting defects) present in the steel billet through pressing, and also enables the overall granulation of the steel sheet to prevent the retention of abnormally large grains. Specifically, it enables the number of bainite grains with an equivalent circle diameter of 100 μm or more to be [amount missing] per unit area (1 mm²). 2 (Three or fewer.) Furthermore, the result is that the reduction of area in the thickness direction of the plate can be 30% or more. Without meeting the above conditions, a reduction of area of ​​30% or more cannot be guaranteed. Here, the aforementioned rolling rate refers to the reduction rate in each pass.

[0160] From the above perspective, it is preferable to set the number of passes with a reduction rate of 10% or more in the final three passes of hot rolling to be 3. In other words, it is preferable to perform reduction with a reduction rate of 10% or more in all three final passes of hot rolling. On the other hand, from the viewpoint of preventing crimping defects, a higher reduction rate is better, so there is no particular upper limit on the reduction rate of each of the final three passes. However, if the reduction rate is increased, the rolling load increases, so from the perspective of equipment constraints, it is preferable to set the reduction rate of each of the final three passes to 30% or less.

[0161] (3) Accelerate cooling

[0162] Next, the hot-rolled steel sheet obtained in the above hot rolling process is subjected to accelerated cooling. The conditions for the accelerated cooling are as follows.

[0163] Cooling start temperature: above Ar3

[0164] When the cooling start temperature in the aforementioned accelerated cooling process is lower than the Ar3 point, excessive precipitation of ferrite and coarse pearlite occurs, reducing strength and resistance to fatigue crack propagation. Therefore, the aforementioned cooling start temperature is set to be above the Ar3 point. On the other hand, there is no particular upper limit to the aforementioned cooling start temperature, but from the viewpoint of ensuring the cumulative reduction rate in the temperature region above the Ar3 point, it is preferable to set it to 870°C or lower.

[0165] Here, point Ar3 can be obtained using the following formula.

[0166] Ar3(℃)=910-310C-80Mn-20Cu-15Cr-55Ni-80Mo

[0167] In the above formula, the element symbol represents the content (mass%) of the element in the steel billet, and is set to zero if the element is not present in the steel billet.

[0168] Furthermore, a cooling start temperature above Ar3 necessarily means that the rolling end temperature in the aforementioned hot rolling process is above Ar3. If the rolling end temperature is below Ar3, it becomes two-phase rolling, resulting in a deterioration of the total elongation. However, if the rolling end temperature is above Ar3, rolling is performed in the austenitic single-phase region, thus preventing a deterioration of the total elongation.

[0169] Cooling stop temperature: 300~650℃

[0170] To transform the unmodified austenite phase into the hard phase (pearlite), the cooling stop temperature in the aforementioned accelerated cooling is set to 650°C or lower, preferably 600°C or lower. When the cooling stop temperature is higher than 650°C, excessive ferrite and coarse pearlite are formed, thus failing to obtain the desired resistance to fatigue crack propagation and strength. On the other hand, when the cooling stop temperature is lower than 300°C, the amount of martensite formation increases, resulting in an undesirable microstructure and reduced toughness and total elongation. Furthermore, pearlite formation becomes insufficient, thus failing to obtain the desired resistance to fatigue crack propagation. Therefore, the cooling stop temperature is 300°C or higher, preferably 350°C or higher, and more preferably over 400°C.

[0171] Average cooling rate: 20–60℃ / s

[0172] The average cooling rate in the aforementioned accelerated cooling is 20°C / s or higher. When the average cooling rate is lower than 20°C / s, ferrite is formed, and the desired microstructure cannot be formed, thus reducing resistance to fatigue crack propagation. Furthermore, due to reduced toughness, the desired total elongation cannot be obtained. On the other hand, when the average cooling rate exceeds 60°C / s, residual stress caused by cooling strain and excessive martensite are generated, resulting in deterioration of both total elongation and toughness. Additionally, pearlite formation becomes insufficient, thus failing to achieve the desired resistance to fatigue crack propagation. Therefore, the aforementioned average cooling rate is 60°C / s or lower, preferably 50°C / s or lower. It should be noted that the aforementioned average cooling rate refers to the average cooling rate of the steel plate surface from the start of accelerated cooling to the cessation of accelerated cooling.

[0173] There are no particular limitations on the methods used for the above-mentioned accelerated cooling; any method can be used, but water cooling is preferred.

[0174] There are no particular limitations on the treatment after the accelerated cooling described above. For example, the thick steel plate can be allowed to cool naturally in an atmosphere after accelerated cooling. In the natural cooling described above, it can be cooled to room temperature, for example. In addition, after the accelerated cooling described above, the warping of the thick steel plate can be corrected arbitrarily using a hot straightening machine.

[0175] It should be noted that the temperature of the steel plate drops immediately after hot rolling. Therefore, the thick steel plate of the present invention is preferably manufactured by an online process using equipment equipped with a rolling device and an accelerated cooling device on a conveyor line.

[0176] Example

[0177] The effects and functions of the present invention will be described below using examples. It should be noted that the present invention is not limited to the following examples.

[0178] Follow these steps to manufacture thick steel plates.

[0179] First, steel billets (steel billet materials) with the composition shown in Table 1 are produced by converter-continuous casting.

[0180] Next, the steel billet is heated to the heating temperature shown in Table 2, and then hot-rolled to produce a hot-rolled steel sheet using the reduction ratio shown in Table 2. The reduction ratio, rolling end temperature, and final thickness of the hot-rolled steel sheet for the final three passes in the hot rolling process are listed in Table 2. Then, the hot-rolled steel sheet is accelerated cooled under the conditions shown in Table 2 to obtain a thicker steel sheet. The thickness of the resulting thicker steel sheet is the same as the final thickness.

[0181] For the obtained thick steel plates, the microstructure, mechanical properties, and fatigue crack propagation characteristics were evaluated. The evaluation methods are described below. The results of each evaluation are shown in Table 3.

[0182] (Microstructure)

[0183] First, samples for microstructure observation were collected from a section 1 / 4t along the thickness direction of the thick steel plate, with the longitudinal section serving as the observation surface. Here, the longitudinal section refers to a section perpendicular to the width direction of the thick steel plate. Next, the surface of the samples was etched with nitric acid and ethanol, and the microstructure was imaged using optical microscopes at 100x and 400x magnification and scanning electron microscopes (SEM) at 2000x magnification. Using the images, the present microstructures were identified, and the images were analyzed to determine the area fractions of bainite, pearlite, and the total area fraction of other microstructures. It should be noted that the identification of pearlite was performed using SEM images, while the determination of the area fractions of each microstructure was performed using optical microscope images.

[0184] (Bainite grains)

[0185] Then, using the aforementioned microstructure observation sample, the bainite grains were determined. In this determination, the surface of the sample was first mirror-polished, and the crystal orientation was determined using an Electron Back-Scattering Pattern (EBSP) device attached to the SEM based on the electron backscatter diffraction pattern. Measurements were performed at 0.3 μm intervals within a 200 μm square region. Regions enclosed by boundaries where the crystal orientation difference from adjacent grains was greater than 15° were defined as grains, and the equivalent circle diameter of each grain was calculated. The average value of the obtained equivalent circle diameters was taken as the bainite grain.

[0186] (Number density of coarse B grains)

[0187] In addition, optical microscope images were obtained by photographing the nitric acid-ethanol etched surface of the sample used for microstructure observation at 100x magnification. Bainite grains were observed as white particles in the optical microscope images. Therefore, image analysis was performed on the optical microscope images to calculate the number density of bainite grains with an equivalent circle diameter of 100 μm or more, i.e., per 1 mm. 2 The number of.

[0188] (Pearlite grains)

[0189] When observing the nitric acid-etched surface of the sample under a 400x optical microscope, the areas that appeared black were identified by SEM as pearlite with lamellar structure. Then, using image analysis software (Image-J), the area of ​​the black region in the optical microscope image was calculated from the number of pixels and converted into the average equivalent circle diameter of the pearlite. The obtained average equivalent circle diameter was considered as the pearlite grain size.

[0190] (Mechanical properties)

[0191] Tensile test specimens were collected from the width direction (C direction) of the thick steel plate. Using these tensile test specimens, tensile tests were performed based on JIS Z 2241 to determine the yield strength (YS), tensile strength (TS), and total elongation (EL). It should be noted that the type of test specimen used in the above determinations was selected according to JIS Z 2241. Specifically, firstly, tensile tests were performed using JIS No. 4 test specimens. For Examples No. 4 and 6, where the tensile strength was less than 570 MPa and the final plate thickness was less than 50 mm, tensile tests were performed again using JIS No. 1A test specimens, and the results of the tensile tests using JIS No. 1A test specimens were adopted.

[0192] The reduction of area (RA) in the thickness direction based on tensile testing was evaluated according to JIS G3199. In the determination of the RA, Type A test pieces collected from the aforementioned thick steel plate were used. The test pieces were collected such that the center of the thickness of the steel plate was included in the parallel portion of the test piece.

[0193] In addition, Charpy impact test pieces were collected from the center of the thickness of the aforementioned thick steel plate, parallel to the rolling direction (L direction), and Charpy impact tests were conducted at 0°C according to JIS Z 2202 to determine the absorbed energy vE0.

[0194] (Resistance to fatigue crack propagation)

[0195] As an indicator of fatigue crack propagation resistance, the stress amplification factor range ΔK: 15MPa / m was used. 1 / 2 and 25MPa / m 1 / 2 The fatigue crack propagation rate (da / dN) in the thickness direction (Z direction), rolling direction (L direction), and width direction (direction perpendicular to the rolling direction, C direction) was measured under two conditions. In the above measurement, fatigue crack propagation tests were carried out based on the crack measurement method to determine the fatigue crack propagation rate.

[0196] In the determination of fatigue crack propagation rate in the thickness direction (Z direction), the following methods are used: Figure 1 The test specimen shown is a simple tensile fatigue test piece with a side notch. The specimen was taken from a thick steel plate, and the fatigue crack propagation rate along the thickness direction was measured.

[0197] The fatigue crack propagation rate in the rolling direction (L direction) was determined using test specimens taken from thick steel plates with the load direction coinciding with the rolling direction. Similarly, the fatigue crack propagation rate in the width direction (C direction) was determined using test specimens taken from thick steel plates with the load direction coinciding with the width direction. The test specimens were compact tension test specimens according to ASTM E647.

[0198] As shown in Table 3, the thick steel plate satisfying the conditions of the present invention possesses extremely superior properties that satisfy all of the following conditions. In particular, it exhibits excellent resistance to fatigue crack propagation and total elongation, and also excellent resistance to fatigue crack propagation in the thickness direction. Therefore, the thick steel plate of the present invention is extremely suitable for use as a material for structures with strong structural safety requirements, such as ships, marine structures, bridges, buildings, and tanks. In contrast, the thick steel plate of the comparative example that does not satisfy the conditions of the present invention does not satisfy at least one of the following conditions.

[0199] • Bainite grains with an equivalent circle diameter of 100 μm or more per 1 mm 2 Number of items: three or less

[0200] TS: Above 500MPa

[0201] • EL: 21% or higher (using JIS 1A test piece)

[0202] EL: ≥23% (using JIS No. 4 test piece)

[0203] •RA: 30% or higher (JIS G3199 Type A test sample)

[0204] ·vE0: 100J or more

[0205] • Fatigue crack propagation rate in the Z direction:

[0206] ΔK: at 15MPa / m 1 / 2 Under the condition of 8.75×10 －9 (m / cycle) and below

[0207] ΔK: at 25MPa / m 1 / 2 Under the given conditions, it is 4.25 × 10 －8 (m / cycle) and below

[0208] Furthermore, the thick steel plate that meets the conditions of the present invention also meets the following conditions: excellent resistance to fatigue crack propagation in both the rolling direction (L direction) and the width direction (C direction).

[0209] • Fatigue crack propagation rates in the L and C directions:

[0210] ΔK: at 15MPa / m 1 / 2 Under the given conditions, it is 1.75 × 10 －8 (m / cycle) and below

[0211] ΔK: at 25MPa / m 1 / 2 Under the given conditions, it is 8.50 × 10 －8 (m / cycle) and below

[0212]

[0213]

[0214]

Claims

1. A thick steel plate having the following composition (by mass%): C: 0.01-0.16%, Si: less than 1.00%, Mn: 0.50-2.00%, P: less than 0.030%, S: less than 0.020%, Al: less than 0.06%, with the remainder consisting of Fe and unavoidable impurities. The thick steel plate has the following microstructure: Based on area fraction, it contains 75-97% bainite and 3-25% pearlite. Bainite grains are less than 18 μm in average equivalent circle diameter. Pearlite grains have an average equivalent circle diameter of less than 10 μm. The plate thickness is 25mm or more. The reduction of area in the thickness direction is over 30%.

2. The thick steel plate according to claim 1, wherein, The composition further comprises, by mass%, one or more of the following: Cr: 0.01-1.00%, Cu: 0.01-2.00%, Ni: 0.01-2.00%, Mo: 0.01-1.00%, Co: 0.01-1.00%, Sn: 0.005-0.200%, Sb: 0.005-0.200%, Nb: 0.005-0.200%, V: 0.005-0.200%, Ti: 0.005-0.050%, B: 0.0001-0.0050%, Zr: 0.005-0.100%, Ca: 0.0001-0.020%, Mg: 0.0001-0.020%, and REM: 0.0001-0.020%.

3. A method for manufacturing thick steel plates, specifically a method for manufacturing thick steel plates with a thickness of 25 mm or more. The steel billet having the composition described in claim 1 or 2 is heated to a heating temperature of 1000°C to 1300°C. The heated steel billet is hot-rolled to produce a hot-rolled steel sheet under the conditions that the reduction ratio is 3 or more and the reduction rate in the final three passes is 10% or more in two or more passes. The hot-rolled steel plate is subjected to accelerated cooling under the following conditions: cooling start temperature above Ar3, cooling stop temperature of 300~650℃, and average cooling rate on the surface of the steel plate from the start of cooling to the end of cooling of 20~60℃ / s.

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