JP2022506661A - High-strength structural steel and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength structural steel and a method for manufacturing the same. According to the present invention, in terms of weight%, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and less than 1.5%. , Cr: 0.5% or more and less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and 0.1 %, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, the balance is Fe and inevitable It is composed of impurities, and the microstructure is 20% or more of bainite, less than 80% of polygonal ferrite and acicular ferrite in total, and less than 10% of pearlite and island martensite as other phases. It is a feature. [Selection diagram] Fig. 1

Description

The present invention relates to high-strength structural steel and its manufacturing method, and more specifically, in an environment where corrosion is significantly accelerated by seawater, such as coastal building structural steel or ballast tanks inside ships and related accessories. The present invention relates to a high-strength structural steel having excellent corrosion resistance and a method for producing the same.

Corrosion of metals is generally promoted in an environment containing a large amount of ionic inorganic substances that dissolve well in water, such as salt, and in particular, has the property of promoting corrosion such as chloride ion ( ^Cl- ). In the presence of ions, corrosion occurs very quickly. Therefore, in seawater containing an average of 3.5% NaCl, metals are corroded at a very high rate, so that under various conditions such as structures adjacent to seawater and ships operating in seawater environment. Corrosion is a problem.
Along with this, some kinds of anticorrosion treatments have been proposed to suppress corrosion. However, since the anticorrosion period of such anticorrosion treatment is only about 20 to 30 years, maintenance and repair costs are constantly incurred if the corrosion resistance of the material itself is not ensured. That is, in order to increase the durability of the structure over a long period of 50 years or more and reduce various anticorrosion costs during the operation period of the structure, it is essential to enhance the corrosion resistance of the material itself.

Chromium (Cr) and copper (Cu) are the most effective elements for improving the seawater resistance of steel materials. Chromium and copper play different roles depending on the corrosive environment, and when added in an appropriate ratio, excellent corrosion resistance can be exhibited even in an environment where corrosion is accelerated by seawater. However, in the case of chromium, it cannot exert a great effect in an acidic environment, and in the case of copper, it induces casting cracks in the casting process, so that there is a problem that expensive nickel needs to be added above a certain level. However, chromium has the effect of improving corrosion resistance in most environments other than strong acids, and recent developments in continuous casting technology have reduced the minimum amount of nickel added to prevent casting defects in copper-added steel, resulting in an expensive amount of nickel added. It has become possible to reduce the cost of products.

On the other hand, as a prior art, Patent Documents 1, 2 and 3 have been proposed for steel materials having excellent seawater resistance. Patent Document 1 presents that the fine structure of a steel sheet is controlled by devising the constituent components and manufacturing conditions, but when the content of the temperature structure is small (less than 20%), it is difficult to secure the strength. Since the Ni content is specified to be 0.05% or less, a large amount of casting defects may occur during casting. In the case of Patent Document 2, 0.1% or more of Al is added to form coarse oxidizing inclusions in the steelmaking process, and the inclusions during rolling are crushed to generate long-running stretched inclusions, which is empty. There is a problem that pore formation is promoted and the resistance to local corrosion is hindered. Further, when W is added as in the case of Patent Document 3, there is a risk of continuous casting defects, there is a risk of galvanic corrosion due to the formation of coarse precipitates, and further, the structure is coarsened by air cooling. There is a risk of a decrease in strength due to the above.
Therefore, it is difficult for the structural steels according to Patent Documents 1 to 3 to secure seawater corrosion resistance and strength by themselves.

Korean Patent Application Publication No. 10-2011-0076148 Korean Patent Application Publication No. 10-2011-0065949 Korean Patent Application Publication No. 10-2004-0054272

An object of the present invention is to control the corrosion characteristics and microstructure of the surface of a steel sheet by optimizing the components and manufacturing conditions, improve the strength characteristics of the steel sheet, minimize the corrosion rate, and adjust the seawater environment of the steel sheet itself. It is an object of the present invention to provide a steel sheet having excellent corrosion resistance and a method for manufacturing the steel sheet.
On the other hand, the subject of the present invention is not limited to the above contents. The subject of the present invention can be understood from the whole contents of the present specification, and it is difficult for a person having ordinary knowledge in the technical field to which the present invention belongs to understand the additional subject of the present invention. There is no.

The high-strength structural steel of the present invention has C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and 1.5 in weight%. %, Cr: 0.5% or more and less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and 0 .1% or less, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, the balance is Fe And unavoidable impurities, the microstructure is bainite 20% or more in area fraction, polygonal ferrite and acicular ferrite are less than 80% in total, and pearlite and island martensite as other phases are less than 10%. It is characterized by.

C in the high-strength structural steel can be contained in an amount of 0.03% or more and less than 0.09%.
The Si in the high-strength structural steel may be contained in an amount of 0.2% or more and less than 0.8%.
The Cu content in the high-strength structural steel is preferably 0.1% or more and less than 0.45%.
The high-strength structural steel can have a yield strength of 500 MPa or more and a tensile strength of 600 MPa or more.

The method for producing high-strength structural steel of the present invention is C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more in weight%. Less than 1.5%, Cr: 0.5% or more and less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01 % Or more and less than 0.1%, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, The balance is a step of reheating a slab composed of Fe and unavoidable impurities at a temperature of 1000 ° C. or higher and 1200 ° C. or lower, a step of hot rolling the reheated slab at a finish rolling temperature of 750 ° C. or higher and 950 ° C. or lower, and rolling. It is characterized by including a step of cooling the rolled steel sheet at a cooling rate of 10 ° C./sec or more from a cooling start temperature of 750 ° C. or higher to a cooling end temperature of 400 to 700 ° C.

According to the present invention, the high-strength structural steel of the present invention is a structural steel sheet having excellent corrosion resistance in a seawater atmosphere environment, a yield strength of 500 MPa or more, and a tensile strength of 600 MPa or more. Can be provided.
The diverse and meaningful advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of explaining specific embodiments of the present invention.

It is a photograph of the invention steel 4 observed with a microscope, in which (a) is a photograph of the surface, (b) is a photograph of a portion 1/4 t in the thickness direction, and (c) is a photograph of a portion observed in the thickness direction 1 / 2t. ..

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the invention can be transformed into several other embodiments, and the scope of the invention is not limited to the embodiments described below. Also, embodiments of the invention are provided to more fully explain the invention to those with average knowledge in the art.
As a result of deep research on a method for improving the corrosion resistance of the structural steel itself, the present inventor appropriately controls the content of chromium, copper, etc., and reheats the temperature, the finish rolling temperature, the cooling end temperature, and the cooling rate. By optimizing the manufacturing conditions such as, it was confirmed that the fine structure can be controlled and excellent seawater resistance and strength characteristics can be ensured, and the present invention has been completed.

Hereinafter, the high-strength structural steel according to one aspect of the present invention will be described in detail.
[High-strength structural steel]
First, the constituent components of the high-strength structural steel of the present invention will be described.
The high-strength structural steel of the present invention has C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and 1.5 in weight%. %, Cr: 0.5% or more and less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and 0 .1% or less, Ni: 0.05% or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, the balance is Fe And unavoidable impurities. Hereinafter, the unit of each alloying element is% by weight.

Carbon (C): 0.03% or more and less than 0.1% C is an element added to improve the strength, and when the content thereof is increased, the hardenability is improved and the strength is improved. However, as the addition amount increases, the resistance to total corrosion is inhibited and the precipitation of carbides and the like is promoted, so that the resistance to local corrosion is also partially affected. In order to improve the resistance to total corrosion and local corrosion, it is necessary to reduce the C content, but if the content is less than 0.03%, sufficient strength as a material for structural steel will be obtained. It is difficult to secure, and if it is 0.1% or more, the weldability is deteriorated, which is not preferable as steel for welded structures. Therefore, it is preferable to limit the C content in the present invention to 0.03% or more and less than 0.1%. On the other hand, from the viewpoint of corrosion resistance, the C content is more preferably less than 0.09%, and in some cases, the C content is less than 0.08% in order to further improve casting cracks and reduce carbon equivalent. It is more preferable to have. On the other hand, the lower limit of the C content is preferably 0.035%. Further, the upper limit of the C content is more preferably 0.06%, further preferably 0.054%.

Silicon (Si): 0.1% or more and less than 0.8% Si is an element that not only acts as a deoxidizing agent but also increases the strength of steel, and its effect is exhibited. Requires 0.1% or more. Further, since Si contributes to the improvement of the resistance to total corrosion, it is advantageous to increase the content, but when the Si content is 0.8% or more, the toughness and weldability are impaired, and during rolling. It makes it difficult to peel off the scale and induces surface defects due to the scale. Therefore, it is preferable to limit the Si content in the present invention to 0.1% or more and less than 0.8%. In some cases, it is more preferable to limit the Si content to 0.2% or more in order to improve the corrosion resistance. Therefore, the lower limit of the Si content is more preferably 0.2%, further preferably 0.27%. The upper limit of the Si content is preferably 0.5%, more preferably 0.44%.

Manganese (Mn): 0.3% or more and less than 1.5% Mn is an effective component that does not reduce toughness but increases strength by strengthening solid solution. However, if it is added excessively, the corrosion resistance may be lowered by increasing the electrochemical reaction rate on the surface of the steel material during the corrosion reaction. When Mn is added in an amount of less than 0.3%, there is a problem that it is difficult to secure the durability of the structural steel material. On the other hand, when the Mn content increases, the hardenability increases and the strength increases, but when 1.5% or more is added, the segregated portion greatly develops at the center of the thickness during slab casting in the steelmaking process. However, there is a problem that the weldability is lowered and the corrosion resistance of the steel plate surface is lowered accordingly. Therefore, it is preferable to limit the Mn content in the present invention to 0.3% or more and less than 1.5%. The lower limit of the Mn content is more preferably 0.4%, further preferably 0.5%. The upper limit of the Mn content is preferably 1.4%, more preferably 0.9%.

Chromium (Cr): 0.5% or more and less than 1.5% Cr is an element that forms an oxide film containing Cr on the surface of a steel material in a corrosive environment to increase corrosion resistance. In the seawater environment, it is necessary to contain 0.5% or more in order for the effect of corrosion resistance due to the addition of Cr to fully appear. However, if the Cr content is excessively 1.5% or more, the toughness and weldability are adversely affected. Therefore, it is preferable to limit the content to 0.5% or more and less than 1.5%. On the other hand, the lower limit of the Cr content is more preferably 0.6%, further preferably 1.2%. Further, it is more preferable that the upper limit of the Cr content is 1.4%. That is, it is most preferable that the structural steel according to one aspect of the present invention has a Cr content of 1.2% or more and 1.4% or less (that is, 1.2 to 1.4%).

Copper (Cu): 0.1% or more and less than 0.5% When Cu is contained in an amount of 0.1% or more like Ni, the elution of Fe is delayed, so that the resistance to total corrosion and local corrosion is improved. It is valid. However, when a Cu content of 0.5% or more is added, Cu in a liquid state during slab production melts into the grain boundaries and induces a hot shortness phenomenon that causes cracks during hot working. The Cu content in the present invention is preferably limited to 0.1% or more and less than 0.5%. In particular, the lower limit of the Cu content is more preferably 0.2%, most preferably 0.28%.
On the other hand, surface cracks generated during slab production interact with the contents of C, Ni, and Mn, so the frequency of surface cracks may differ depending on the content of each element. Regardless of the content, in order to minimize the possibility of surface cracking, the Cu content is more preferably less than 0.45%, and the Cu content is preferably 0.43% or less. Most preferred.

Aluminum (Al): 0.01% or more and less than 0.08% Al is an element added for deoxidation and reacts with N in steel to form AlN, resulting in finer austenite crystal grains. It is an element that improves toughness. Al is preferably contained in a dissolved state in an amount of 0.01% or more for sufficient deoxidation. On the other hand, the lower limit of the Al content is preferably 0.02%, more preferably 0.022%. On the other hand, when Al is excessively contained in an amount of 0.08% or more, coarse oxide inclusions are formed in the steelmaking process, and due to the characteristics of the aluminum oxide system, they are crushed during rolling and continue for a long time. Form stretching inclusions. The formation of such stretched inclusions promotes the formation of pores around the inclusions, which act as a starting point for local corrosion and thus may impair the resistance to local corrosion. be. Therefore, the Al content in the present invention is preferably limited to less than 0.08%. On the other hand, the upper limit of the Al content is preferably 0.05%, more preferably 0.034%.

Titanium (Ti): 0.01% or more and less than 0.1% When Ti is added in an amount of 0.01% or more, it combines with carbon in steel to form TiC, thereby improving the strength by the precipitation strengthening effect. Fulfill. On the other hand, when the Ti content is 0.1% or more, the effect of improving the strength against the increase in the content is not great. Therefore, the Ti content in the present invention can be limited to 0.01% or more and less than 0.1%. The lower limit of the Ti content is more preferably 0.015%. Further, the upper limit of the Ti content is more preferably 0.05%, further preferably 0.028%.

Nickel (Ni): 0.05% or more and less than 0.1% Ni is effective in improving the resistance to total corrosion and local corrosion when it is contained in an amount of 0.05% or more like Cu. The lower limit of the Ni content is more preferably 0.07%. Further, when added together with Cu, it has the effect of suppressing the formation of a Cu phase having a low melting point by reacting with Cu and suppressing hot shortness. Therefore, in most Cu-added steels, Ni is 1 times the Cu content. It is common to add the above. When the content of elements related to carbon equivalent such as C and Mn is low and the Cr content is high as in the present invention, shortness can be sufficiently prevented even if the content is less than half of the Cu content. Since Ni is an expensive element, it is preferable to limit the upper limit of the Ni content to less than 0.1% in consideration of the relative input effect. Further, the upper limit of the Ni content is more preferably 0.09%.

Niobium (Nb): 0.002% or more and less than 0.07% Nb is an element that plays a role of precipitation strengthening by binding to carbon in steel to form NbC like Ti, and is 0.002. When added in% or more, the strength is effectively improved. However, when the content is 0.07% or more, the effect of improving the strength against the increase in the content is not so great. Therefore, the Nb content in the present invention is preferably limited to 0.002% or more and less than 0.07%. The lower limit of the Nb content is more preferably 0.01%, most preferably 0.017%. Further, the upper limit of the Nb content is more preferably 0.05%, further preferably 0.044%.

Phosphorus (P): 0.03% or less P is present as an impurity in steel, and when its content exceeds 0.03%, not only the weldability is significantly reduced, but also the toughness is reduced. descend. Therefore, it is preferable to limit the P content to 0.03% or less. Further, the upper limit of the P content is preferably 0.02%, more preferably 0.018%. On the other hand, since P is an impurity, it is advantageous to reduce its content, and therefore its lower limit is not particularly limited.

Sulfur (S): 0.02% or less S exists as an impurity in the steel, and if the content exceeds 0.02%, there is a problem that the ductility, impact toughness and weldability of the steel are deteriorated. Therefore, it is preferable to limit the S content in the present invention to 0.02% or less. In particular, S easily reacts with Mn to form stretched inclusions like MnS, and the pores existing at both ends of the stretched inclusions can be the starting point of local corrosion, so the upper limit of the content is 0.01. % Or less is more preferable, and 0.008% or less is most preferable. On the other hand, since S is an impurity, it is advantageous to reduce its content, and therefore its lower limit is not particularly limited.

In the high-strength structural steel of the present invention, in addition to the above alloying elements, the rest is an iron (Fe) component. However, in the normal manufacturing process, impurities unintended from the raw materials and the surrounding environment may be inevitably mixed, and this cannot be excluded. All of these impurities are not mentioned in detail, as any ordinary technician will know them.

On the other hand, the high-strength structural steel according to one aspect of the present invention has bainite 20% or more as a microstructure, polygonal ferrite and acicular ferrite less than 80% in total, and pearlite and MA (other phases). Island-like martensite) can be less than 10%.

The high-strength structural steel according to one aspect of the present invention can have a surface integral of bainite of 20% or more, more preferably 30% or more, and most preferably 51% or more in the fine structure. preferable.
On the other hand, the high-strength structural steel according to one aspect of the present invention can have a surface integral of bainite of 78% or less in the fine structure.
The high-strength structural steel according to one aspect of the present invention can have a surface integral of bainite of 68% or more and 71% or less in the fine structure.

Further, in the high-strength structural steel according to one aspect of the present invention, the total surface integral of the polygonal ferrite and the needle-like ferrite in the microstructure can be less than 80%, and more preferably 45% or less. preferable.
The high-strength structural steel according to one aspect of the present invention can have a total surface integral of 10% or more of the polygonal ferrite and the needle-like ferrite in the fine structure, and more preferably 19% or more.
The high-strength structural steel according to one aspect of the present invention can have a total surface integral of 25% or more and 30% or less of polygonal ferrite and acicular ferrite in the microstructure, and is 27% or more and 30% or less. It is more preferable to have.

Further, the high-strength structural steel according to one aspect of the present invention can have an area fraction of pearlite and MA (island-like martensite) as the other phase of the fine structure of less than 10%, and is 5% or less. It is preferably abundant, more preferably 4% or less, still more preferably 2% or less.

In order to use it as a material for high-strength structural steel, it is necessary to secure the strength of thick materials of at least 500 MPa, universally 600 MPa or more, and for this reason, bainite 20% or more and other polygonals in the microstructure must be secured. And / or a structure centered on needle-shaped ferrite was formed. Further, in the case of pearlite and MA, which are other phases, if 10% or more is contained, the low temperature toughness and corrosion resistance may be insufficient in the environment where the structural steel according to the present invention is used, so the upper limit is less than 10%. Limited to.
The high-strength structural steel according to one aspect of the present invention can have a yield strength of 500 MPa or more and a tensile strength of 600 MPa or more by satisfying the above-mentioned constituent component system and microstructure.

Next, a method for manufacturing a high-strength structural steel according to another aspect of the present invention will be described in detail.
[Manufacturing method of high-strength structural steel]
The method for producing high-strength structural steel according to one aspect of the present invention comprises the processes of slab reheating-hot rolling-cooling, and the detailed conditions for each production stage are as follows.

Slab reheating step First, a slab composed of the above components is prepared, and the slab is reheated in a temperature range of 1000 to 1200 ° C. It is more preferable to set the reheating temperature to 1000 ° C. or higher in order to dissolve the carbonitride formed during casting to a solid solution, and to heat it to 1050 ° C. or higher in order to sufficiently dissolve the carbonitride. On the other hand, when reheated at an excessively high temperature, austenite may be formed coarsely, so the reheating temperature is preferably 1200 ° C. or lower.

Hot rolling stage Hot rolling including rough rolling and finish rolling is performed on the reheated slab. At this time, the finish rolling is preferably completed at a finish rolling temperature of 750 ° C. or higher. If the finish rolling temperature is less than 750 ° C., there is a problem that a large amount of air-cooled ferrite is generated. On the other hand, if the finish rolling temperature exceeds 950 ° C., the strength and toughness may decrease due to the coarsening of the structure. Therefore, the finish rolling temperature in the present invention is preferably limited to 750 to 950 ° C.

Cooling stage Forced cooling by water cooling is performed on the steel material that has been hot-rolled. In the present invention, it is a core technique to secure high strength even for thick materials by sufficient cooling, and in order to prevent the coarsening of the structure, it is necessary to cool to a temperature of 700 ° C. or less at a cooling rate of 10 ° C./s or more. be. Further, the cooling can be started from a cooling start temperature of 750 ° C. or higher. However, when cooling to a temperature of less than 400 ° C, fine cracks may be induced in the central part due to the quenching process, which induces material deviation between the product surface and the central part and material deviation at the front / rear end of the product. Therefore, it is preferable to finish the cooling at a temperature of 400 ° C. or higher. That is, in the cooling step, it is preferable to cool the rolled steel sheet at a cooling rate of 10 ° C./s from a cooling start temperature of 750 ° C. or higher to a cooling end temperature of 400 to 700 ° C. In particular, the range of the cooling end temperature is more preferably 500 to 650 ° C, further preferably 522 to 614 ° C.

On the other hand, the upper limit of the cooling rate is mainly related to the equipment capacity, and if the temperature is 10 ° C./s or higher, no significant change is observed in the strength even if the cooling rate is increased. Therefore, the upper limit of the cooling rate is separately set. It does not have to be limited. On the other hand, the lower limit of the cooling rate is more preferably 20 ° C / s, further preferably 25 ° C / s, and most preferably 30 ° C / s.

Hereinafter, the present invention will be described in more detail with reference to examples. However, it should be noted that the following examples are merely intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of rights of the present invention. This is because the scope of rights of the present invention is determined by the matters described in the claims and the matters reasonably inferred from them.

(Example)
First, molten steel having the constituent components shown in Table 1 below was prepared, and then a slab was manufactured by continuous casting. After that, the slab was reheated, hot-rolled, and cooled under the production conditions shown in Table 2 below to produce a steel sheet.
The microstructure of the manufactured steel plate was observed with an optical and electron microscope, the area fraction of each phase was measured, and then the yield strength and tensile strength were measured by a tensile test and shown in Table 3. In addition, as a seawater resistance evaluation, after soaking in a 3.5% NaCl solution as a substitute for seawater for one day, the test piece was washed with an ultrasonic cleaner together with a 50% HCl + 0.1% Hexamethylene tetramine solution to reduce the weight. It was measured. In order to calculate the corrosion rate by dividing this weight loss amount by the surface area of the initial test piece and compare the corrosion rate of the comparative steel and the invention steel, the relative corrosion rate is compared and evaluated with the corrosion rate of the comparative steel 1 as a reference. The results are also shown in Table 3.

As can be seen from Table 1 above, the invention steels 1 to 4 all satisfy the component range specified in the present invention, whereas the comparative steels 1 to 3 have a component range of Cr, Cu, Ni or Mn of the present invention. Was out of range.
As a result, the invented steels 1 to 4 have a microstructure having a bainite 20% or more low temperature structure (on the substrate) in a ferrite system, and have a high yield strength of 500 MPa or more and a tensile strength of 600 MPa or more. It was found that it has sufficient material for structural steel. Further, it was confirmed that by satisfying the component range specified in the present invention, the corrosion rate was slower than that of the comparative steel 1, and the life was sufficient in a seawater-resistant atmosphere.
On the other hand, the comparative steels 1 to 3 were manufactured by the manufacturing method satisfying the manufacturing conditions of the present invention because the component range of Cr, Cu, Ni or Mn deviated from the range of the present invention. As can be seen from Table 3, it was found that the relative corrosion rate showed a high corrosion rate of 100 or more, and as a result, it was not possible to have a sufficient life in a seawater resistant atmosphere.

On the other hand, in the case of the invention steels 1 and 2 containing Cr in an amount of 1.2% or more and 1.4% or less, compared with the invention steels 3 and 4 in which Cr is not contained in an amount of 1.2% or more and 1.4% or less. It was confirmed that it showed a slow corrosion rate.
As a result, it was confirmed that the structural steel according to one aspect of the present invention has the best life characteristics in a seawater-resistant atmosphere by containing Cr in an amount of 1.2% or more and 1.4% or less.

Although the examples have been described above, a skilled ordinary engineer in the art has various modifications of the present invention within the scope of the idea and domain of the present invention described in the claims below. And can understand that changes are possible.

Claims (6)

By weight%, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and less than 1.5%, Cr: 0.5% or more Less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05 % Or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, the balance consists of Fe and unavoidable impurities.
The microstructure is characterized by an area fraction of 20% or more bainite, less than 80% total of polygonal ferrite and acicular ferrite, and less than 10% of pearlite and island martensite as other phases. Strength structural steel. The high-strength structural steel according to claim 1, wherein C is contained in an amount of 0.03% or more and less than 0.09%. The high-strength structural steel according to claim 1, wherein the Si is contained in an amount of 0.2% or more and less than 0.8%. The high-strength structural steel according to claim 1, wherein the Cu is contained in an amount of 0.1% or more and less than 0.45%. The high-strength structural steel according to claim 1, wherein the high-strength structural steel has a yield strength of 500 MPa or more and a tensile strength of 600 MPa or more. By weight%, C: 0.03% or more and less than 0.1%, Si: 0.1% or more and less than 0.8%, Mn: 0.3% or more and less than 1.5%, Cr: 0.5% or more Less than 1.5%, Cu: 0.1% or more and less than 0.5%, Al: 0.01% or more and less than 0.08%, Ti: 0.01% or more and less than 0.1%, Ni: 0.05 % Or more and less than 0.1%, Nb: 0.002% or more and less than 0.07%, P: 0.03% or less, S: 0.02% or less, the balance is a slab consisting of Fe and unavoidable impurities at 1000 ° C or more. The stage of reheating at a temperature of 1200 ° C or lower,
The step of hot rolling the reheated slab at a finish rolling temperature of 750 ° C. or higher and 950 ° C. or lower, and 10 ° C./sec from the cooling start temperature of 750 ° C. or higher to the cooling end temperature of 400 to 700 ° C. The stage of cooling at the above cooling rate,
A method for producing high-strength structural steel, which comprises.

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