JP2020509156A - Low yield ratio type ultra-high strength steel material and its manufacturing method - Google Patents

Abstract

In the present invention, carbon (C): 0.05 to 0.09 mass%, silicon (Si): 0.1 to 0.4 mass%, manganese (Mn): 1.8 to 2.5 mass%, aluminum (Al): 0.01 to 0.06 mass%, nickel (Ni): 0.1 to 0.5 mass%, copper (Cu): 0.1 to 0.5 mass%, titanium (Ti): 0 0.01 to 0.05% by mass, niobium (Nb): 0.01 to 0.07% by mass, chromium (Cr): 0.1 to 0.5% by mass, molybdenum (Mo): 0.1 to 0. 6% by mass, vanadium (V): 0.01 to 0.05% by mass, phosphorus (P): 0.01% by mass or less (excluding 0% by mass), sulfur (S): 0.01% by mass or less ( 0% by mass), boron (B): 5 to 30 mass ppm, nitrogen (N): 20 to 60 mass ppm, calcium (Ca): 50 mass ppm or less ( Mass ppm is excluded), cobalt (Co): comprises 10 to 500 ppm by weight, wherein the balance being iron (Fe) and other unavoidable impurities. [Selection diagram] FIG.

Description

  The present invention relates to a low-yield-ratio ultrahigh-strength steel material and a method for producing the same, and more particularly, to a low-yield-ratio ultrahigh-strength steel material having a low yield ratio and a high tensile strength, which can be suitably used as a construction steel material. The present invention relates to a high-strength steel material and a method for manufacturing the same.

  In recent years, as structures such as buildings and bridges in Japan and overseas have become ultra-high-rise and long-span, the development of ultra-thick and high-strength steel materials has been required. When high-strength steel is used, it has a high allowable stress, so it is possible to rationalize and lighten the structure of buildings and bridges, and not only economical construction is possible, but also the thickness can be reduced. In addition, machining and welding operations such as cutting and drilling are facilitated.

  On the other hand, when the strength of the steel material is increased, the yield ratio (yield strength / tensile strength), which is the ratio between the tensile strength and the yield strength, often increases, but when the yield ratio increases, the point at which plastic deformation occurs (the yield point) ) To the point at which the failure occurs is not large, so there is not much room for the building to absorb the energy due to the deformation and prevent the damage, and to ensure safety when a huge external force such as an earthquake acts. There is a problem that is difficult to guarantee. Therefore, structural steel materials must satisfy both high strength and low yield ratio.

  Generally, the yield ratio of a steel material is such that a soft phase such as ferrite is a main structure in a metal structure of the steel material, and a hard phase such as bainite or martensite is suitable in a metal structure of the steel material. It is known that the cost can be reduced by realizing a distributed organization.

  In order to obtain a structure in which a hard phase is appropriately dispersed in a soft phase-based microstructure described above, Patent Document 1 discloses an appropriate quenching in a dual phase region of ferrite and austenite. And a method of lowering the yield ratio by tempering. However, in the above method, the number of heat treatment steps is added in addition to the rolling production step, so that there is a problem that not only productivity is lowered but also production cost is unavoidably increased.

  Therefore, there is a need for the development of a low-yield-ratio type ultra-high-strength steel material capable of solving all problems such as a decrease in productivity and an increase in manufacturing cost, and capable of securing an ultra-high strength and a low yield ratio, and a method of manufacturing the same. That is the fact.

JP-A-55-97425

  An aspect of the present invention is to provide a low yield ratio type ultra-high strength steel material and a method for manufacturing the same. More specifically, an object of the present invention is to provide a low-yield-ratio type ultra-high-strength steel material capable of securing an ultra-high strength and a low yield ratio without lowering productivity and increasing manufacturing cost, and a method for manufacturing the same.

  On the other hand, the subject of the present invention is not limited to the above contents. The objects of the present invention can be understood from the entire contents of the present specification, and those having ordinary knowledge in the technical field to which the present invention pertains will understand the additional problems of the present invention. There is no difficulty.

  One aspect of the present invention is as follows: carbon (C): 0.05 to 0.09 mass%, silicon (Si): 0.1 to 0.4 mass%, manganese (Mn): 1.8 to 2.5 mass. %, Aluminum (Al): 0.01 to 0.06% by mass, nickel (Ni): 0.1 to 0.5% by mass, copper (Cu): 0.1 to 0.5% by mass, titanium (Ti) ): 0.01 to 0.05 mass%, niobium (Nb): 0.01 to 0.07 mass%, chromium (Cr): 0.1 to 0.5 mass%, molybdenum (Mo): 0.1 0.6% by mass, vanadium (V): 0.01 to 0.05% by mass, phosphorus (P): 0.01% by mass or less (excluding 0% by mass), sulfur (S): 0.01% by mass % Or less (excluding 0 mass%), boron (B): 5 to 30 mass ppm, nitrogen (N): 20 to 60 mass ppm, calcium (Ca): 50 mass pp Or less (0 wt ppm is excluded), cobalt (Co): 10 to 500 ppm by weight, comprises, to a low yield ratio ultrahigh strength steel balance being iron (Fe) and other unavoidable impurities.

In the present invention, carbon (C): 0.05 to 0.09% by mass, silicon (Si): 0.1 to 0.4% by mass, and manganese (Mn): 1.8 to 2.5% by mass. , Aluminum (Al): 0.01 to 0.06 mass%, nickel (Ni): 0.1 to 0.5 mass%, copper (Cu): 0.1 to 0.5 mass%, titanium (Ti) : 0.01 to 0.05 mass%, niobium (Nb): 0.01 to 0.07 mass%, chromium (Cr): 0.1 to 0.5 mass%, molybdenum (Mo): 0.1 to 0.6% by mass, vanadium (V): 0.01 to 0.05% by mass, phosphorus (P): 0.01% by mass or less (excluding 0% by mass), sulfur (S): 0.01% by mass The following (excluding 0 mass%), boron (B): 5 to 30 mass ppm, nitrogen (N): 20 to 60 mass ppm, calcium (Ca): 50 mass ppm Under (0 ppm by weight excluding), cobalt (Co): it comprises 10 to 500 ppm by weight, and heating the slab balance being iron (Fe) and other unavoidable impurities at a temperature of 1050 to 1200 ° C.,
Rough-rolling the heated slab to obtain a bar;
Hot-rolling the bar at a finish rolling temperature of 700 to 950 ° C. to obtain a hot-rolled steel sheet;
A primary cooling step of cooling the hot-rolled steel sheet to Ar 3 or less at a cooling rate of 10 to 30 ° C./s,
And a secondary cooling step of cooling the primary-cooled hot-rolled steel sheet to Bs or less at a cooling rate of 30 to 70 ° C./s.

  It should be noted that the means for solving the above problems do not list all the features of the present invention. The various features of the invention and the advantages and advantages associated therewith can be more fully understood with reference to the following specific embodiments.

  ADVANTAGE OF THE INVENTION According to this invention, the effect which can provide the low-yield-ratio type ultra-high-strength steel material which can ensure an ultra-high strength and a low yield ratio, without lowering productivity and a rise in manufacturing cost, and the manufacturing method thereof can be provided. is there.

It is a photograph showing a CSL grain boundary and a general grain boundary. It is the photograph which image | photographed the fine structure of the test number 7 which is an invention example by an electron backscattering diffraction (EBSD) apparatus. It is the photograph which image | photographed the fine structure of the test number 4 which is a comparative example with an electron backscattering diffraction (EBSD) apparatus.

  Hereinafter, a preferred embodiment of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are also provided to more completely explain the present invention to those having average knowledge in the art.

  Hereinafter, the low yield ratio type ultra-high strength steel material according to the present invention will be described in detail.

  The low-yield-ratio ultra-high-strength steel material according to the present invention comprises carbon (C): 0.05 to 0.09 mass%, silicon (Si): 0.1 to 0.4 mass%, and manganese (Mn): 1. 8 to 2.5 mass%, aluminum (Al): 0.01 to 0.06 mass%, nickel (Ni): 0.1 to 0.5 mass%, copper (Cu): 0.1 to 0.5 Mass%, titanium (Ti): 0.01 to 0.05 mass%, niobium (Nb): 0.01 to 0.07 mass%, chromium (Cr): 0.1 to 0.5 mass%, molybdenum ( Mo): 0.1 to 0.6% by mass, vanadium (V): 0.01 to 0.05% by mass, phosphorus (P): 0.01% by mass or less (excluding 0% by mass), sulfur (S ): 0.01 mass% or less (excluding 0 mass%), boron (B): 5 to 30 mass ppm, nitrogen (N): 20 to 60 mass ppm, calcium ( a): 50 ppm by mass or less (0 wt ppm is excluded), cobalt (Co): comprises 10 to 500 ppm by weight, the remainder being iron (Fe) and other unavoidable impurities.

Carbon (C): 0.05 to 0.09 mass%
C is an important element that forms bainite or martensite and determines the size and fraction of bainite or martensite formed.

  When the C content exceeds 0.09% by mass, the low-temperature toughness is reduced, and when the C content is less than 0.05% by mass, the formation of bainite or martensite is hindered and the strength is reduced. Therefore, the C content is preferably 0.05 to 0.09 mass%.

  On the other hand, the upper limit of the C content is more preferably 0.085% by mass. In the case of a plate used as a steel structure for welding, the upper limit of the C content is set to 0.08% by mass for better weldability. More preferably,

Silicon (Si): 0.1 to 0.4 mass%
Si is used as a deoxidizing agent and is an element that improves strength and toughness.

  When the Si content exceeds 0.4% by mass, not only low-temperature toughness and weldability are reduced, but also a thick scale is formed on the surface of the sheet material, causing poor gas cutting properties and other surface cracks. On the other hand, if the Si content is less than 0.1% by mass, the deoxidizing effect becomes insufficient. Therefore, the Si content is 0.1-0.4% by mass.

  Further, a more preferred upper limit of the Si content is 0.35% by mass, and a still more preferred lower limit is 0.15% by mass.

Manganese (Mn): 1.8 to 2.5 mass%
Since Mn is a useful element for improving the strength by solid solution strengthening, it needs to be added by 1.8% by mass or more. However, if the Mn content exceeds 2.5% by mass, the toughness of the weld may be significantly reduced due to an excessive increase in hardening ability. Therefore, the Mn content is preferably 1.8 to 2.5% by mass.

Aluminum (Al): 0.01 to 0.06 mass%
Al is an element that can deoxidize molten steel at low cost and stabilizes ferrite. When the Al content is less than 0.01% by mass, the above-mentioned effects become insufficient. On the other hand, if the Al content exceeds 0.06% by mass, nozzle clogging occurs during continuous casting. Therefore, the Al content is preferably 0.01 to 0.06% by mass.

  Further, a more preferred upper limit of the Al content is 0.05% by mass, and a still more preferred lower limit is 0.015% by mass.

Nickel (Ni): 0.1 to 0.5% by mass
Ni is an element that simultaneously improves the strength and toughness of the base material. In order to sufficiently exhibit the above effects in the present invention, it is preferable to add 0.1% by mass or more. However, since Ni is an expensive element, if the addition amount exceeds 0.5% by mass, economic efficiency and weldability may be reduced. Therefore, the Ni content is preferably 0.1 to 0.5% by mass.

Copper (Cu): 0.1-0.5% by mass
Cu is an element that increases the strength while minimizing the decrease in the toughness of the base material. In order to sufficiently obtain the above effects, it is preferable to add 0.1% by mass or more. However, if the Cu content exceeds 0.5% by mass, the surface quality of the product may be greatly impaired. Therefore, the Cu content is preferably 0.1 to 0.5% by mass.

Titanium (Ti): 0.01-0.05% by mass
Ti is added in an amount of 0.01% by mass or more in order to suppress the growth of crystal grains during reheating and greatly improve low-temperature toughness. However, an excessive addition of 0.05% by mass or more causes clogging of the continuous casting nozzle. In addition, problems such as a decrease in low-temperature toughness due to crystallization at the center and the like may occur. Therefore, the upper limit of Ti is preferably 0.05% by mass.

Niobium (Nb): 0.01 to 0.07% by mass
Nb is an important element in the production of TMCP steel, and precipitates in the form of NbC or NbCN, and greatly improves the strength of the base material and the weld. Further, Nb dissolved as a solid solution at the time of reheating at a high temperature has an effect of suppressing recrystallization of austenite and transformation of ferrite or bainite to make the structure finer. Furthermore, in the present invention, when the slab is cooled after the rough rolling, not only does bainite form at a low cooling rate, but also increases the stability of austenite even at the time of cooling after final rolling. It also plays a role in promoting production.

  In order to sufficiently obtain the above-described effects, the Nb content is preferably 0.01% by mass or more. However, when the Nb content exceeds 0.07% by mass, brittle cracks occur at the edges of the steel material. Therefore, the Nb content is preferably 0.01 to 0.07% by mass.

Chromium (Cr): 0.1 to 0.5% by mass
Cr is an element added to secure strength, and also plays a role in increasing hardenability. In order to obtain the above effects sufficiently, it is necessary to add 0.1% or more. However, if the Cr content exceeds 0.5%, the hardness of the welded portion is excessively increased, and the toughness may be impaired. Therefore, the Cr content is preferably 0.1 to 0.5%.

Molybdenum (Mo): 0.1 to 0.6 mass%
Mo has the effect of greatly improving the curability even with a small amount of addition, and can greatly improve the strength. Therefore, it is necessary to add Mo in an amount of 0.1% by mass or more. However, there is a possibility that the hardness of the welded portion is excessively increased and the toughness is impaired. Therefore, the content of Mo is preferably 0.1 to 0.6% by mass.

Vanadium (V): 0.01 to 0.05% by mass
V has a lower solid solution temperature than other fine alloys, and has the effect of preventing a decrease in strength by depositing in the heat affected zone. In order to sufficiently obtain the above effects, it is preferable to add 0.01% by mass or more. However, if the V content exceeds 0.05% by mass, the toughness is rather reduced. Therefore, the V content is preferably 0.01 to 0.05% by mass.

Phosphorus (P): 0.01% by mass or less (excluding 0% by mass)
P is an element that is advantageous for strength improvement and corrosion resistance, but it may significantly impair impact toughness, so it is advantageous to keep P as low as possible. Therefore, the upper limit is preferably set to 0.01% by mass.

Sulfur (S): 0.01% by mass or less (excluding 0% by mass)
Since S is an element that forms MnS and the like and greatly impairs impact toughness, it is advantageous to keep S as low as possible. Therefore, the upper limit is preferably set to 0.01% by mass.

Boron (B): 5 to 30 mass ppm
B is a very inexpensive additive element, has a strong hardening ability, and is a useful element that greatly contributes to the formation of bainite even in low-speed cooling after rough rolling.

Since the strength can be greatly improved even by adding a small amount, 5 mass ppm or more can be added. However, when the B content exceeds 30 mass ppm, Fe ₂₃ (CB) ₆ is formed, and the hardening ability is rather lowered, and the low-temperature toughness is also greatly reduced. Therefore, the B content is preferably 5 to 30 ppm by mass.

Nitrogen (N): 20 to 60 ppm by mass N is preferably controlled to 60 ppm by mass or less in order to increase strength and greatly reduce toughness. However, since controlling the N content to less than 20 ppm by mass increases the steelmaking load, the lower limit of the N content is preferably 20 ppm by mass.

Calcium (Ca): 50 mass ppm or less (excluding 0 mass ppm)
Ca is mainly used as an element for suppressing nonmetallic inclusions of MnS and improving low-temperature toughness. However, when Ca is excessively added, it reacts with oxygen contained in steel to generate CaO, which is a nonmetallic inclusion, and therefore the upper limit is preferably 50 mass ppm.

Cobalt (Co): 10 to 500 mass ppm
Co is an element that can form a passivation film to secure corrosion resistance and enhance high-temperature strength. When the Co content is less than 10 ppm by mass, the above-mentioned effects become insufficient. However, Co is an expensive element, and if added in a large amount, the economic efficiency is reduced. Therefore, the upper limit is preferably 500 ppm by mass.

  The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintended impurities may be inevitably mixed from the raw material or the surrounding environment, and this cannot be excluded. Since these impurities are known to any person skilled in the art of ordinary manufacturing processes, their contents are not specifically described in the present specification.

  Although the steel material having the above-described advantageous steel composition of the present invention can obtain a sufficient effect only by containing the alloying elements in the above-described content ranges, Sn: 5 to 50 ppm by mass, W: 0.01 to 0 ppm. By further containing one or more of 0.5 mass% and Sb: 0.01 to 0.05 mass%, properties such as strength and toughness of the steel material, toughness of the weld heat affected zone, and weldability are further improved. be able to.

Tin (Sn): 5 to 50 mass ppm
Sn is an element useful for ensuring corrosion resistance. From the viewpoint of ensuring corrosion resistance, Sn is preferably added at 5 mass ppm or more. However, when the Sn content exceeds 50 mass ppm, there is a problem that more defects such as scale swelling or cracking like a water bubble are generated on the surface of the steel material than the effect of improving the corrosion resistance. Further, Sn can increase the strength of steel, but since it reduces elongation and low-temperature impact toughness, the upper limit is preferably 50 ppm by mass.

Tungsten (W): 0.01 to 0.5% by mass
W is an element useful for improving the hardening ability of steel and ensuring corrosion resistance. From the viewpoint of ensuring corrosion resistance, W is preferably added at 0.01% by mass or more. However, W is a very expensive element, and has a large specific gravity and tends to be unevenly distributed. Therefore, the upper limit is preferably 0.5% by mass.

Antimony (Sb): 0.01 to 0.05% by mass
Antimony is an element useful for securing the cut resistance and the corrosion resistance by increasing the adhesion between the scale generated on the surface of the steel and the base material. From the viewpoint of severability, Sb is preferably added at 0.01% by mass or more. However, if Sb is added in excess of 0.05% by mass, the scale formed on the surface of the steel material is difficult to peel off, and subsequent operations such as painting become difficult, so the upper limit is 0.05% by mass. It is preferred that

  In addition, the microstructure of the steel material of the present invention can include bainitic ferrite and granular bainite as main phases, and MA (island martensite) as a secondary phase.

  Since bainitic ferrite contains many high-angle boundaries in grains while maintaining the initial austenite grain boundaries, it is useful for improving the strength and impact toughness due to the grain refinement effect.

  Granular bainite maintains initial austenite crystal grains like bainitic ferrite, but has a secondary phase such as M-A within grains or at grain boundaries. Although there is no high-angle grain boundary in the grains, the impact toughness is somewhat disadvantageously affected, but the strength is slightly increased due to the presence of many low-angle grain boundaries such as intragranular dislocations.

  By containing bainitic ferrite and granular bainite as main phases, a low yield ratio and high strength can be ensured.

  At this time, in terms of area fraction, the bainitic ferrite is 60 to 90%, the granular bainite is 10 to 30%, and the MA (Martensitic-Austenite) is 5% or less (including 0%).

  If the area fraction of bainitic ferrite is less than 60%, it is difficult to secure a high tensile strength, the fraction of CSL (Coincidence Site Lattice) grain boundaries is reduced, the impact absorption energy value is low, and the corrosion properties and strength are high. And it is difficult to suppress crack propagation. On the other hand, when the area fraction of bainitic ferrite exceeds 90%, there is a problem that the yield ratio increases.

  If the area fraction of granular bainite is less than 10%, not only the tensile strength but also the yield strength increases, and a low yield ratio cannot be secured. On the other hand, if it exceeds 30%, coarse initial austenite crystal grains cannot be effectively refined, the tensile strength is inferior, the fraction of CSL (coincidence site lattice) grain boundaries decreases, and the impact absorption energy value decreases. And the corrosion properties and strength are poor, and crack propagation is suppressed and difficult.

  A secondary phase such as MA is a microstructure useful for realizing a low yield ratio, and preferably has an area fraction of 5% or less. If the area fraction of MA exceeds 5%, the yield ratio decreases, but it may act relatively as a starting point for cracks due to external stress, and it is difficult to ensure a high tensile strength. become.

  On the other hand, the steel material according to the present invention satisfies the above-mentioned fraction of the microstructure, so that the grain boundary azimuth difference angle is 15 ° or more, but the energy of the grain boundary is low, and the fraction of the CSL (Coincidence Site Lattice) grain boundary is obtained. 20% or more can be secured.

  The CSL grain boundary means a grain boundary in which the arrangement of metal atoms has repetition depending on a specific plane and an angle, as shown by a thin solid line in FIG. 1, and is also called a special grain boundary. For example, some grain boundaries have a specific orientation relationship such as twin grain boundaries (twin). On the other hand, the dotted line and the thick solid line in FIG. 1 indicate general grain boundaries.

  Even with the same component system, the fraction of the CSL grain boundary varies depending on the manufacturing conditions, and is closely related to the final microstructure. The fraction of CSL grain boundaries can be measured using an electron backscattering diffraction (EBSD) device.

  Since the CSL grain boundary has a regular orientation relationship, the energy value itself is lower than that of a random general grain boundary (generally, a high-angle grain boundary), and the corrosion characteristic is lower than that of a general grain boundary. Excellent. Further, since the grain boundary azimuth difference angle is as large as 15 ° or more like a general grain boundary, in addition to the effect of increasing the strength by making the grains finer, it plays a role of obstructing the propagation of cracks and improves the toughness.

  Such CSL grain boundaries are more advantageous in corrosion and strength than ordinary high-angle grain boundaries due to the low energy of the grain boundaries. That is, when the energy of a high-angle grain boundary having a random orientation relationship is set to 100, the energy of a CSL grain boundary is about 20 to 80. In the case of a twin grain boundary, the energy is particularly high among the CSL grain boundaries. Low.

  If the fraction of CSL grain boundaries is less than 20%, the impact absorption energy value is low, the corrosion characteristics and strength are poor, and it is difficult to suppress crack propagation.

  Further, the steel material according to the present invention has a yield ratio of 0.85 or less and can secure a tensile strength of 800 MPa or more, and thus can be preferably used as a construction steel material.

  Further, the steel material according to the present invention has an impact absorption energy at −5 ° C. of 150 J or more.

  On the other hand, the thickness of the steel material according to the present invention is 100 mm or less.

  Since the steel material according to the present invention can ensure high strength and a low yield ratio, machining and welding such as cutting and drilling are facilitated. Therefore, the thickness of the steel material is preferably 100 mm or less. It is more preferably at most 80 mm, even more preferably at most 60 mm. Although the lower limit does not need to be particularly limited, it is necessary to be 15 mm or more in order to use it as a steel material for construction structures.

  Hereinafter, the method for producing a low yield ratio type ultra-high strength steel material of the present invention will be described in detail.

  The method for manufacturing a low-yield-ratio ultra-high-strength steel material according to the present invention includes heating a slab having the above-described alloy composition at a temperature of 1050 to 1200 ° C, and rough-rolling the heated slab to form a bar. And hot-rolling the bar at a finish rolling temperature of 700 to 950 ° C. to obtain a hot-rolled steel sheet, and cooling the hot-rolled steel sheet to Ar 3 or less at a cooling rate of 10 to 30 ° C./s. A primary cooling step of cooling the hot-rolled steel sheet to a temperature of Bs or less at a cooling rate of 30 to 70 ° C./s.

Slab heating step A slab having the above alloy composition is heated at a temperature of 1050 to 1200C.

Rough rolling step The heated slab is roughly rolled to obtain a bar.

  At this time, the rough rolling is performed in a temperature range of 950 to 1050C. If the rough rolling temperature is lower than 950 ° C., the surface temperature of the slab is relatively low and the rolling load increases. As a result, effective deformation does not occur up to the center in the thickness direction of the slab, and defects such as pores may not be removed, or the effect of miniaturizing particles may be reduced. On the other hand, if the rough rolling temperature exceeds 1050 ° C., recrystallization takes place and grains grow at the same time, and austenite grains may become coarse.

Hot rolling step The bar is hot-rolled at a finish rolling temperature of 700 to 950 ° C to obtain a hot-rolled steel sheet.

  If the finish rolling temperature is lower than 700 ° C., the temperature of the sheet material is low and a load is applied to the rolling mill, and it may not be possible to perform rolling to the final thickness. If it exceeds 950 ° C., recrystallization occurs during rolling. May occur.

  At this time, the rolling reduction of the hot rolling is 50 to 80%.

  When the finish rolling reduction is less than 50%, the rolling load per pass acting on the material during rolling increases and there is a risk of equipment accident. When the finishing rolling reduction exceeds 80%, the number of rolling passes increases and the rolling is increased. There is a possibility that the final thickness cannot be secured up to the end temperature.

Cooling step The hot rolled steel sheet is multi-stage cooled in two stages. This is to secure both bainitic ferrite and granular bainite and to secure a high fraction of CSL grain boundaries.

Primary cooling step The hot-rolled steel sheet is primarily cooled to Ar3 or less at a cooling rate of 10 to 30C / s.

  If the primary cooling rate is less than 10 ° C./s, it is difficult to secure a tensile strength of 800 MPa or more due to generation of soft ferrite. If the primary cooling rate is more than 30 ° C./s, particles of phase transformation from austenite to granular bainite There is a problem that the yield is small and a low yield ratio cannot be secured.

  Therefore, the cooling rate in the primary cooling stage is preferably 10 to 30C / s, more preferably 15 to 25C / s.

  When the cooling end temperature exceeds Ar3, there is a problem that a phase transformation to granular bainite does not occur because a fine structure exists only as an austenite single phase. That is, when the microstructure existing as an austenitic single phase undergoes a phase transformation immediately after secondary cooling, the granular bainite fraction of the final microstructure phase is too small, less than 10%, and the low yield ratio cannot be satisfied. On the other hand, the lower limit of the cooling end temperature is Ar3-50 ° C. in consideration of the following secondary cooling stage.

Secondary cooling step The primary cooled hot rolled steel sheet is secondarily cooled to Bs or less at a cooling rate of 30 to 70 ° C / s.

  If the secondary cooling rate is less than 30 ° C./s, bainitic ferrite, which is the main structure, cannot sufficiently undergo phase transformation during cooling, and there is a problem that the tensile strength of 800 MPa or more cannot be satisfied. . On the other hand, when the secondary cooling rate exceeds 70 ° C./s, the probability that martensite as a low-temperature transformation structure is generated increases, and not only the tensile strength but also the yield strength increases, and the yield strength becomes 0.85 or less. It is difficult to satisfy the ratio.

  If the secondary cooling end temperature exceeds Bs, bainitic ferrite and granular bainite cannot be sufficiently phase transformed, and the strength cannot be secured.

  Hereinafter, the present invention will be described more specifically with reference to examples. However, it should be noted that the following examples are for illustrating the present invention in more detail and not for limiting the scope of the present invention. The scope of the present invention is determined by the matters described in the appended claims and matters reasonably inferred therefrom.

  A slab satisfying the component system shown in Table 1 was heated at a temperature of 1160 ° C., rough-rolled at a temperature of 1000 ° C., and then subjected to hot rolling and cooling to meet the production conditions shown in Table 2, thereby obtaining a steel material. . The microstructure, CSL fraction and mechanical properties of the steel material were measured and are shown in Table 3.

  The yield strength and the tensile strength were measured using a universal tensile tester, and the impact absorption energy value was measured by performing a Charpy impact test at −5 ° C.

  The microstructure was observed with an optical microscope after the steel material was mirror-polished and chemically corroded.

  The fraction of a CSL (Coincidence Site Lattice) grain boundary, which is a grain boundary having a low energy even though the grain boundary misorientation angle is 15 ° or more, was measured using an electron back scattering diffraction (EBSD) apparatus. .

  In Table 1, the units of B, N, Ca, and Co contents indicated by * are ppm by mass, and the units of the remaining element contents are% by mass.

  In Table 3, BF: bainitic ferrite, GB: granular bainite, MA: island martensite, AF: acicular ferrite, M: martensite, and the unit is area%.

  The microstructure of the invention example satisfying the alloy composition and the production conditions of the invention is as follows: bainitic ferrite is 60 to 90%, the granular bainite is 10 to 30%, and MA is 5% or less (including 0%). ) Was satisfied.

  This shows that a CSL grain boundary of 20% or more can be secured, and a low yield ratio of 0.85 or less and a tensile strength of 800 MPa or more can be secured. The impact absorption energy value at -5 ° C was 150 J or more.

  On the other hand, Test Nos. 4, 5, 6, 9, 13, 14, 18, and 19, which are comparative examples that satisfied the alloy composition of the present invention but did not satisfy the manufacturing conditions, satisfied the microstructure presented in the present invention. Did not.

  This confirms that the fraction of the CSL grain boundary is less than 20%, the impact absorption energy value at −5 ° C. is less than 150 J, the yield ratio is more than 0.85, or the tensile strength is less than 800 MPa. it can.

  When the microstructures of Test No. 7 of the invention example and Test No. 4 of the comparative example were photographed with an electron backscattering diffraction (EBSD) apparatus, and FIGS. It was confirmed that there was a remarkable difference in the fraction of CSL grain boundaries, which are grain boundaries in which the arrangement of atoms has repeatability depending on the specific plane and angle.

  In addition, in the case of the comparative steel that does not satisfy the alloy composition of the present invention, it can be confirmed that the yield ratio exceeds 0.85 or the tensile strength is less than 800 MPa even when the manufacturing conditions of the present invention are satisfied.

  Although described above with reference to the embodiments, those skilled in the art can make various modifications to the present invention without departing from the spirit and scope of the present invention described in the following claims. It can be understood that modifications and changes can be made.

Claims (12)

  Carbon (C): 0.05 to 0.09 mass%, silicon (Si): 0.1 to 0.4 mass%, manganese (Mn): 1.8 to 2.5 mass%, aluminum (Al): 0.01 to 0.06 mass%, nickel (Ni): 0.1 to 0.5 mass%, copper (Cu): 0.1 to 0.5 mass%, titanium (Ti): 0.01 to 0 0.05% by mass, niobium (Nb): 0.01 to 0.07% by mass, chromium (Cr): 0.1 to 0.5% by mass, molybdenum (Mo): 0.1 to 0.6% by mass, Vanadium (V): 0.01 to 0.05% by mass, phosphorus (P): 0.01% by mass or less (excluding 0% by mass), sulfur (S): 0.01% by mass or less (0% by mass Excluding), boron (B): 5 to 30 mass ppm, nitrogen (N): 20 to 60 mass ppm, calcium (Ca): 50 mass ppm or less (0 mass pp Is excluded), cobalt (Co): 10 to 500 comprise a mass ppm, low yield ratio ultrahigh strength steel, characterized in that the balance being iron (Fe) and other unavoidable impurities.   2. The steel material according to claim 1, further comprising at least one of Sn: 5 to 50% by mass, W: 0.01 to 0.5% by mass, and Sb: 0.01 to 0.05% by mass. 2. A low-yield-ratio ultra-high-strength steel material according to item 1.   The ultra-high-strength steel with a low yield ratio according to claim 1, wherein the microstructure of the steel material includes bainitic ferrite and granular bainite as main phases and MA as a secondary phase.   In terms of area fraction, the bainitic ferrite is 60 to 90%, the granular bainite is 10 to 30%, and the MA is 5% or less (including 0%). The low yield ratio type ultrahigh strength steel material according to claim 3.   2. The low yield ratio type super according to claim 1, wherein the steel material has a grain boundary misorientation angle of 15 ° or more and a CSL, which is a fraction of grain boundaries having low energy, of 20 area% or more. 3. High strength steel.   The low yield ratio type ultrahigh strength steel material according to claim 1, wherein the steel material has a yield ratio of 0.85 or less and a tensile strength of 800 MPa or more.   The low yield ratio type ultra-high strength steel material according to claim 1, wherein the steel material has an impact absorption energy value at -5 ° C of 150 J or more.   The low yield ratio type ultra-high strength steel material according to claim 1, wherein the steel material has a thickness of 100 mm or less. Carbon (C): 0.05 to 0.09 mass%, silicon (Si): 0.1 to 0.4 mass%, manganese (Mn): 1.8 to 2.5 mass%, aluminum (Al): 0.01 to 0.06 mass%, nickel (Ni): 0.1 to 0.5 mass%, copper (Cu): 0.1 to 0.5 mass%, titanium (Ti): 0.01 to 0 0.05% by mass, niobium (Nb): 0.01 to 0.07% by mass, chromium (Cr): 0.1 to 0.5% by mass, molybdenum (Mo): 0.1 to 0.6% by mass, Vanadium (V): 0.01 to 0.05% by mass, phosphorus (P): 0.01% by mass or less (excluding 0% by mass), sulfur (S): 0.01% by mass or less (0% by mass Excluding), boron (B): 5-30 mass ppm, nitrogen (N): 20-60 mass ppm, calcium (Ca): 50 mass ppm or less (0 mass pp Is excluded), cobalt (Co): comprises 10 to 500 mass ppm, and heating the slab balance being iron (Fe) and other unavoidable impurities at a temperature of from 1,050 to 1,250 ° C.,
Rough-rolling the heated slab to obtain a bar;
Hot-rolling the bar at a finish rolling temperature of 700 to 950 ° C. to obtain a hot-rolled steel sheet;
A primary cooling step of cooling the hot-rolled steel sheet to Ar 3 or less at a cooling rate of 10 to 30 ° C./s,
A secondary cooling step of cooling the primary-cooled hot-rolled steel sheet to Bs or less at a cooling rate of 30 to 70 ° C./s, the method comprising the steps of:   The slab further comprises at least one of Sn: 5 to 50 mass ppm, W: 0.01 to 0.5 mass%, and Sb: 0.01 to 0.05 mass%. 2. The method for producing a low-yield-ratio ultrahigh-strength steel material according to item 1.   The method of claim 9, wherein the hot rolling is performed at a reduction of 50 to 80%.   The method of claim 9, wherein the rough rolling is performed in a temperature range of 950 to 1050 ° C. 11.

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