JP5556632B2 - Low yield ratio steel material excellent in corrosion resistance and method for producing the same - Google Patents

Description

  The present invention relates to a low yield ratio steel material and a method for producing the same, and more particularly to a low yield ratio steel material excellent in corrosion resistance that can be suitably used as a building material and a method for producing the same.

  The steel material of the present invention is mainly intended for plate-like materials having a thickness exceeding 16 mm, that is, “steel plates”. For this reason, in the following, “steel material” may be described as “steel plate”.

  Moreover, as a strength class of the steel material of this invention, the thing whose tensile strength is 490 Mpa or more becomes object.

A thick steel plate used for a large structure is manufactured by heating a slab to an austenite temperature range, that is, Ac ₃ point or higher, rolling to a predetermined thickness, and cooling.

  The characteristics of the thick steel plate are determined by the steel composition, heating temperature condition, rolling condition, cooling condition, etc., and by adjusting these conditions as appropriate, it is possible to produce a thick steel sheet with high added value.

  The heating temperature in the production of thick steel plates has been generally performed at a relatively high temperature of about 1150 ° C. even in the austenite temperature range. This is to reduce the rolling load in the subsequent rolling process due to the softening action of the slab by high temperature heating.

  By the way, in the manufacture of steel sheets, a reduction in energy intensity is always required. However, due to the recent increase in the price of energy resources, a further reduction in energy intensity has been required. In addition, in recent years, due to environmental considerations, there is a need for a technique for producing steel sheets without producing greenhouse gases such as carbon dioxide as much as possible.

  In the manufacture of thick steel plates, it is preferable that the slab is heated and the temperature is made uniform up to the center of the slab as a heating step. For this reason, a large amount of energy is required in the heating process. Therefore, if the heating temperature is lowered, for example, the heating temperature is less than 1000 ° C. and a thick steel plate can be manufactured, the above-described requirements can be satisfied.

  The manufacturing method of a thick steel plate is disclosed by patent documents 1-3, for example.

That is, Patent Document 1 discloses an invention that includes an example in which the heating temperature is defined as Ac ₃ point or higher and heated at a temperature of less than 1000 ° C.

Patent Document 2 discloses an invention including an example in which the heating temperature is defined as Ac ₃ point or higher and 1200 ° C. or lower and heated at 950 ° C.

  Further, Patent Document 3 discloses an invention including an example in which the heating temperature is defined as 950 ° C. or higher and the heating is performed at 975 ° C.

  However, the techniques disclosed in these Patent Documents 1 to 3 are positively applied at a low heating temperature of less than 1000 ° C., as is clear from the fact that there are many descriptions of heating temperatures of 1000 ° C. or more in the examples. It is not a technology for manufacturing thick steel plates.

  In particular, steels, thick steel plates, or steel materials that are also conscious of architectural uses are disclosed in Patent Documents 4 to 9, for example.

  That is, Patent Document 4 discloses an invention related to steel composed of a structure mainly composed of ferrite or bainite having an average crystal grain size of 2.5 μm or less.

  Patent Documents 5 to 7 disclose inventions related to steel composed of a structure mainly composed of ferrite or bainite having an average crystal grain size of 3 μm or less.

  Patent Document 8 discloses an invention relating to steel comprising a structure composed of ferrite having an average crystal grain size of 20 μm or less and bainite or / and martensite as a hard second phase.

In the above Patent Documents 4 to 8, as an invention of a manufacturing method, steel, a steel material, a steel material or a steel piece is heated to Ac ₃ points or more, and then the steel or a thick steel plate is manufactured through a certain process. It is disclosed. And a part of what was described as the invention example in these patent documents is what heated steel, steel materials, the raw material of steel, or a steel piece to the temperature below 1000 degreeC.

  However, the techniques disclosed in these Patent Documents 4 to 8 are also positively 1000 ° C., as is clear from the fact that there are many descriptions of invention examples produced at a heating temperature of 1000 ° C. or more in the examples. It is not a technique for producing a thick steel plate at a heating temperature of less than.

  Patent Document 9 discloses an invention relating to a steel material composed of a structure composed of ferrite and pearlite having a maximum crystal grain size of 40 μm or less and an average crystal grain size of 25 μm or less.

In the above Patent Document 9, unlike the above Patent Documents 4 to 8, as the invention of the production method, the steel is heated to 950 ° C. or higher, not the Ac ₃ point or higher which is the theoretical value of the minimum heating temperature of the steel. Manufacturing of the steel material is also disclosed, and many examples of steel materials heated at a temperature of less than 1000 ° C. are also described.

  However, the invention described in Patent Document 9 is not a technique for positively producing a steel material at a heating temperature of less than 1000 ° C.

  On the other hand, steel structures are often used in an environment with a large amount of incoming salt, such as a beach area or an area where snow melting salt is spread, or in a seawater spray environment.

  Generally, when a weather-resistant steel material is exposed to an atmospheric corrosive environment, a protective rust layer is formed on the surface, and subsequent steel material corrosion is suppressed. Therefore, weather-resistant steel is used for structures as minimum maintenance steel that can be used as it is without being painted.

  However, a protective rust layer is formed on the surface of weathering steel not only in the beach area but also in inland areas where there is a large amount of incoming salt, such as areas where snowmelt salt and antifreeze are sprayed. Since it is hard to be done, the effect which suppresses corrosion is hard to be exhibited. Therefore, in these regions, it is not possible to use bare weatherproof steel, and ordinary steel is used by painting on ordinary steel. However, in the case of using such ordinary steel for coating, it is necessary to repaint every 10 years because of coating deterioration due to corrosion, and therefore the cost required for maintenance becomes enormous.

In recent years, weathering steel standardized by Japanese Industrial Standards (JIS) (JIS G 3114: weathering hot rolled steel for welded structures) has an incoming salt content of 0.05 mg / dm ² / day (0. 05mdd) and higher areas, for example, beach areas, where the loss of corrosion is large due to the occurrence of scale-like rust, layered rust, etc., so it cannot be used without painting (Ministry of Construction, Public Works Research Institute, Steel Club) Japan Bridge Construction Association: Joint Research Report on the Application of Weatherproof Steel to Bridges (XX)-Design and Construction Guidelines for Uncoated Weatherproof Bridges (Ref. Rev. 1993.3)).

  In this way, in a salty environment such as a beach area, ordinary steel materials are usually coated. However, structures built on the coastal area near the estuary and roads such as mountainous areas where snow-melting salt is eroded are extremely corroded and must be repainted. Since these repainting takes a lot of man-hours, there is a strong demand for steel materials that can be used without painting.

  Recently, a Ni-based high weathering steel to which about 1 to 3% of Ni has been added has been developed. However, it has been found that in regions where the amount of incoming salt exceeds 0.3 to 0.4 mdd, it is difficult to apply to steel materials that can be used without coating with such addition of Ni alone.

  Since corrosion of steel materials increases as the amount of flying salt increases, weathering steel according to the amount of flying salt is required from the viewpoint of corrosion resistance and economy. Moreover, the corrosive environment of steel materials is not the same depending on the place and part used. For example, some parts are exposed to rain, condensed water and sunshine, and some parts are exposed to condensed water but not exposed to rain. In general, in an environment where the amount of incoming salt is large, it is said that the latter part is more corroded than the former part.

  In addition, in an environment where snow melting salt or an antifreezing agent is spread on the road, the salt is wound up on a running car and adheres to the steel structure, resulting in a severe corrosive environment. Furthermore, the eaves under the eaves a little away from the coast are also exposed to severe salt damage environments, and in such areas, the amount of incoming salt becomes a severe corrosive environment with 1 mdd or more.

  In order to cope with such problems, the development of steel materials that prevent corrosion in an environment with a large amount of incoming salt has been underway.

  For example, Patent Document 10 proposes a weather-resistant steel material with an increased chromium (Cr) content, and Patent Document 11 proposes a weather-resistant steel material with an increased nickel (Ni) content. .

  However, although the weathering steel material with the increased chromium (Cr) content proposed in Patent Document 10 can improve the weathering resistance in a region where the salt content is below a certain level, it is severer than that. In a salt environment, the weather resistance is deteriorated.

  Moreover, in the case of the weathering steel material which increased nickel (Ni) content proposed by the said patent document 11, although weather resistance will be improved to some extent, the cost of steel material itself will become high. In order to avoid this, if the Ni content is reduced, the weather resistance will not be improved so much, and if the amount of incoming salt is high, layered peeling rust will form on the surface of the steel material, corrosion will be remarkable, and it will be used for a long time. The problem of being unbearable arises.

JP-A-6-299237 JP-A-8-60239 JP 2004-2934 A JP-A-11-140584 JP-A-11-181543 JP 11-181544 A JP 11-181546 A JP 2003-3229 A JP-A-10-280088 JP-A-9-176790 Japanese Patent Laid-Open No. 5-118011

  In recent years, the design philosophy of building structures is that the structure should be a rigid body. From the philosophy of “allowable stress design type”, the structure is allowed to be plastically deformed, but the structure collapses from the viewpoint of lifesaving. This has changed to the “ultimate strength type” idea of preventing it. Specifically, the ultimate strength design concept is that even if a huge earthquake exceeding seismic intensity 7 occurs, a part of the structure is damaged and seismic energy is absorbed. It is to prevent collapse.

  The strength of the structure depends on the yield strength of the steel used. Recently, research and research on the influence of the steel yield ratio (hereinafter referred to as “YR”) on the destruction of steel structures has progressed. There is a growing demand for lower yield ratios for steel.

  Already, for steel for construction steel frames, a draft standard that incorporates a “lower yield ratio” as a YR regulation in the JIS standard has already been studied. Prior to that, a steel material that satisfied the YR regulation was manufactured and manufactured. It is used. More specifically, a technique for producing a low yield ratio steel using on-line or off-line quenching from a two-phase region of austenite and ferrite has been put into practical use for steel for architectural steel frames.

  On the other hand, as described above, from the viewpoint of the increase in the price of energy resources and the prevention of greenhouse gas emissions, it is required to manufacture a steel material by heating a slab in a low temperature range.

  Furthermore, in a steel structure used in an environment with a large amount of incoming salt, the coating peel resistance becomes a big problem. That is, as shown above, in a coastal environment where a large amount of chloride comes in or an environment where a snow melting agent or an antifreezing agent is sprayed, the coating peels off early and corrosion progresses. Therefore, it is necessary to repaint the paint every few to a few dozen years. In addition, when repainting is performed, as a pre-process, it is necessary to assemble a scaffold on a once corroded steel structure and perform a reblast treatment, which is very expensive. And even if it re-blasts, it is difficult to remove rust completely, but even if it paints again on the steel material which has not removed rust completely, the coating life will become remarkably short. The paint peel resistance is largely due to the characteristics including the corrosion resistance of the steel material as the base.

  Therefore, it has been strongly desired to extend the service life of the coating and greatly extend the interval between repair coatings. That is, there is a high demand for minimizing the life cycle cost, and extending the coating life is very important in considering the life cycle management of steel structures.

  Therefore, an object of the present invention is to provide a low-yield ratio steel material excellent in economy that can be manufactured at low cost while suppressing soaring energy costs, and a method for manufacturing the same. Another object of the present invention is to provide a low yield ratio steel material that is environmentally friendly and a method for manufacturing the same because of its low energy consumption. Furthermore, the present invention provides corrosion resistance in a high chloride environment (including that the coating does not peel off and that corrosion at the coating defect is suppressed and corrosion resistance is maintained (including coating peeling resistance) and weather resistance when no coating is applied). Another object of the present invention is to provide an excellent low yield ratio steel material and a method for producing the same.

Specifically, the low yield ratio steel material of the present invention has a yield ratio of 80% or less, a tensile strength (hereinafter referred to as “TS”) of 490 MPa (50 kgf / mm ² ) or more, and a steel material thickness of 20 mm. In the case of exceeding, in the Charpy impact test using the 10 mm V notch test piece described in JIS Z 2242 (2005), and in the case where the thickness of the steel material is 20 mm or less, the width described in JIS Z 2242 (2005) is 7 In a Charpy impact test using a sub-size V-notch test piece of .5 mm, the fracture surface transition temperature (hereinafter referred to as “vTrs”) at which the ductile fracture surface ratio is 50% satisfies −20 ° C. or less. .

  The present inventors have made various studies in order to solve the above problems. As a result, it has been clarified that a low yield ratio steel material excellent in desired corrosion resistance can be produced by appropriately controlling the chemical composition and microstructure of the steel material.

In particular, with respect to corrosion resistance, the present inventors have examined corrosion in an environment with a large amount of incoming salt, and as a result, repeated drying and wetting of the FeCl ₃ solution became an essential condition for corrosion under such an environment, and Fe ³⁺ It has been found that corrosion is accelerated when pH decreases due to decomposition and Fe ³⁺ acts as an oxidizing agent.

  The corrosion reaction at this time is as follows.

As the cathode reaction, the following reaction mainly occurs.
Fe ³⁺ + e ⁻ → Fe ²⁺ (reduction reaction of Fe ³⁺ )

In addition to this reaction, the following cathode reaction also occurs.
2H ₂ O + O ₂ + 2e ⁻ → 4OH ⁻ ,
2H ⁺ + 2e ⁻ → H ₂

On the other hand, the following anodic reaction occurs with respect to the above Fe ³⁺ reduction reaction.
Anode reaction: Fe → Fe ²⁺ + 2e ⁻ (Fe dissolution reaction)

Therefore, the overall reaction of corrosion is as shown in the following equation (A).
2Fe ³⁺ + Fe → 3Fe ²⁺ (A) formula

Fe ²⁺ generated by the reaction of the above formula (A) is oxidized to Fe ³⁺ by air oxidation, and the generated Fe ³⁺ acts again as an oxidant to accelerate corrosion. At this time, the reaction rate of air oxidation of Fe ²⁺ is generally slow in a low pH environment, but is accelerated in a concentrated chloride solution, and Fe ³⁺ is easily generated. It has been found that due to such a cyclic reaction, in an environment where the amount of incoming salt is very large, Fe ³⁺ is always supplied, corrosion of steel is accelerated, and corrosion resistance is significantly deteriorated.

  As a result of examining the influence of various alloy elements on the weather resistance based on the mechanism of corrosion in such a salt environment, the present inventors have obtained the knowledge shown in the following (a) to (c).

(a) Sn dissolves as ^{Sn 2+,} by lowering the concentration of ^{Fe 3+} by ^{2Fe 3+ + Sn 2+ → 2Fe 2+} + Sn 4+ comprising reaction, suppressing a reaction of the formula (A). Sn also has an effect of suppressing anodic dissolution.

  (b) Cu is an element that has conventionally been regarded as the basis for the effect of improving the corrosion resistance in an environment with a large amount of flying salt, and the effect of improving the corrosion resistance is seen in an environment with a relatively long wetting time. However, an environment where the chloride concentration is further increased and the pH is locally lowered, for example, a relatively dry environment in which salt is attached and the humidity is changed, resulting in repeated drying and wetting to produce β-FeOOH. Then, it was found that Cu rather promotes corrosion.

  (c) Thus, this steel material can be expected to have high corrosion resistance. In addition, since the corrosion resistance is high, even if the steel material is painted, there is little peeling of the paint due to the corrosion of the steel material, and the corrosion of the coating defect part is suppressed, but the anticorrosive effect by the coating film can also be expected. In addition, a further effect of corrosion resistance can be expected. Therefore, in addition to the corrosion resistance, the service life of the coating can be extended and the repair coating interval can be greatly extended.

  The gist of the present invention resides in the low yield ratio steel materials shown in the following (1) to (3) and the low yield ratio steel material manufacturing method shown in (4).

(1) By mass%, C: 0.05-0.20%, Si: 0.10-0.50%, Mn: 1.0-2.0%, P: 0.05% or less, S: Contains 0.02% or less, Nb: 0.005% or less , Al: 0.003-0.050% and Sn: 0.03-0.50%, with the balance being Fe and impurities. In addition, the microstructure has a ferrite phase having an average crystal grain size of more than 3 μm and not more than 20 μm, a hard phase having an average aspect ratio of less than 10, and an unavoidably formed phase (excluding a ferrite phase and a hard phase) . made), and the percentage of the ferrite phase is at 40% or more or less proportion of 5% of the unavoidable formation phase, the yield ratio YR is 80% or less, a tensile strength TS of more than 490 MPa, toughness vTrs is - excellent corrosion resistance, characterized in der Rukoto 20 ° C. or less Low yield ratio steel for construction.

(2) chemical composition, in mass%, further, Cu: containing less than 0.05% to 0.2%, and wherein the Cu / Sn ratio is 10 or less, the above (1) Low yield ratio steel for construction with excellent corrosion resistance.

(3) The chemical composition is selected from mass%, and Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.2% or less, and V: 0.05% or less. The low yield ratio steel for construction excellent in corrosion resistance according to the above (1) or (2), characterized by containing one or more elements.

(4) The slab having the chemical composition according to any one of the above (1) to (3) is sequentially processed in the following steps (a) to (d), (1) to (3) The manufacturing method of the low yield ratio steel materials for construction excellent in the corrosion resistance in any one of .
Step (a): The slab is heated to a temperature of Ac ₃ point or higher and lower than 1000 ° C.
Step (b): Rolling is performed so that the cumulative reduction rate is 30% or more in the austenite non-recrystallization temperature range.
Step (c): The rolling is completed at a temperature of Ar ₃ or higher.
Step (d): Cool to a temperature of 500 ° C. or lower at a cooling rate of 5 to 40 ° C./s.

  The “impurities” in the remaining “Fe and impurities” are components that are mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing steel materials. Therefore, it means that it is allowed as long as it does not adversely affect the present invention.

  The microstructure defined in the present invention refers to that in the center of the thickness of the thick steel plate, and the “hard phase” in the microstructure refers to bainite, martensite, and island martensite (hereinafter referred to as “MA”). means.

  The “inevitable formation phase” means a phase inevitably formed other than the ferrite phase and the hard phase, specifically a phase such as pearlite.

  The heating temperature of the slab defined in the present invention refers to the temperature at the slab surface. Similarly, the temperature at which rolling is performed and the temperature at which rolling is completed also refer to the temperature at the surface of the material to be rolled. Furthermore, the temperature at which cooling is finished at a temperature of 500 ° C. or lower also refers to the surface temperature of the steel material that has been rolled.

  And the cooling rate prescribed | regulated by this invention points out the value calculated | required from the surface temperature of the steel materials which completed rolling.

In addition, "cumulative rolling reduction" in the austenite non-recrystallization temperature range is
[(Thickness of slab before rolling-final thickness of rolled material by rolling in austenite non-recrystallization temperature range) / thickness of slab before rolling] × 100
The value represented by.

  The low yield ratio steel material of the present invention has the characteristics that YR is 80% or less, TS is 490 MPa or more, and vTrs is −20 ° C. or less. It is possible to prevent collapse of an object, and it can be suitably used as a building material based on a design concept of ultimate strength type. In addition, this low yield ratio steel material can be easily manufactured at an industrial scale at a low cost while suppressing the rising energy cost, and the energy consumption at the time of manufacture may be small. There is also an effect that the release of the effect gas can be suppressed. Furthermore, this low yield ratio steel material has good corrosion resistance in a high chloride environment and can be used effectively as a building material for construction.

  Hereinafter, the constituent requirements of the present invention will be described in detail. In addition, "%" display of the content of each component element means "mass%".

(A) About chemical composition:
C: 0.05-0.20%
C is an element necessary for increasing the strength of steel. For this reason, 0.05% or more of C is contained. On the other hand, if the C content exceeds 0.20%, the so-called “strength-toughness balance”, weldability and toughness are adversely affected. Therefore, the C content is 0.05 to 0.20%. The lower limit of the C content is preferably 0.08%. Further, the upper limit of the C content is preferably 0.17%.

Si: 0.10 to 0.50%
Si has an increase in strength of steel and a deoxidizing action. However, when the Si content is less than 0.10%, particularly the required strength cannot be ensured. On the other hand, if the Si content exceeds 0.50%, the weldability is lowered, and the toughness of the weld heat affected zone (hereinafter referred to as “HAZ”) is also deteriorated. Therefore, the content of Si is set to 0.10 to 0.50%. The lower limit of the Si content is preferably 0.20%. Further, the upper limit of the Si content is preferably 0.40%.

Mn: 1.0-2.0%
Mn has an effect of improving toughness as the strength of steel increases. However, if the Mn content is less than 1.0%, these effects are small. On the other hand, if the Mn content exceeds 2.0%, the weldability deteriorates and the toughness of the HAZ deteriorates. Furthermore, the center segregation of the continuously cast slab is also promoted. Therefore, the Mn content is 1.0 to 2.0%. The lower limit of the Mn content is preferably 1.2%. The upper limit of the Mn content is preferably 1.45%.

P: 0.05% or less P is an element present as an impurity in steel. If the P content increases and exceeds 0.05% in particular, it not only segregates at the grain boundaries and lowers the toughness, but also causes the HAZ toughness to deteriorate. Therefore, the P content is 0.05% or less. In addition, since content of P is so preferable that there is little, content of a minimum is not prescribed | regulated in particular.

S: 0.02% or less S, like P, is an element present as an impurity in steel. If the S content increases and exceeds 0.02% in particular, the center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and the HAZ deteriorate. Therefore, the content of S is set to 0.02% or less. In addition, since content of S is so preferable that there is little, content of a minimum is not prescribed | regulated in particular.

Nb: 0.01% or less Nb forms Nb carbide in the slab. If this Nb carbide does not dissolve in the matrix, the toughness of HAZ is deteriorated. And when the content of Nb increases, especially when it exceeds 0.01%, in the case of a slab heating temperature as low as less than 1000 ° C., the amount of undissolved Nb carbides increases so much that the HAZ toughness deteriorates. It becomes remarkable. Therefore, the Nb content is 0.01% or less. The preferable Nb content is 0.005% or less, and the smaller the content, the better.

Al: 0.003 to 0.050%
Al has an effect of improving toughness due to deoxidation of steel and precipitation as AlN. However, when the Al content is less than 0.003%, these effects are small. On the other hand, if the Al content exceeds 0.050%, the cleanliness of the steel deteriorates. Therefore, the content of Al is set to 0.003 to 0.050%.

Sn: 0.03-0.50%
Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly to reduce the Fe ^3+, by having an effect of reducing Fe ³⁺ concentration as oxidizing agent, since inhibit corrosion promoting effect of Fe ^3+, thereby improving the weather resistance in high airborne salt environments. Moreover, Sn has the effect | action which suppresses the anodic dissolution reaction of steel and improves corrosion resistance. These effects are obtained by containing 0.03% or more of Sn, and saturate when it exceeds 0.50%. Therefore, the Sn content is 0.03 to 0.50%. In addition, the minimum of content of preferable Sn is 0.05%, and an upper limit is 0.30%.

  One of the low yield ratio steel materials according to the present invention has a chemical composition in which the balance is composed of Fe and impurities in addition to the above elements.

  Another low yield ratio steel material according to the present invention is a chemistry containing one or more elements selected from Cu, Ni, Cr, Mo and V as optional elements in addition to the above elements. You may have a composition.

  That is, since said Cu, Ni, Cr, Mo, and V have the effect | action which raises an intensity | strength, when you want to ensure a bigger intensity | strength, you may contain these elements. Hereinafter, these elements will be described in detail.

Cu: Less than 0.2% and Cu / Sn ratio: 1.0 or less Cu can be contained as necessary. When Cu is contained, the strength can be improved. That is, when Cu is contained, the strength of the steel can be improved while suppressing an increase in the yield ratio. Therefore, Cu may be contained to obtain the above effect. However, when there is much content of Cu, weldability will fall. Further, in steel containing Sn, the corrosion resistance is significantly lowered due to the inclusion of Cu. Furthermore, when manufacturing steel materials, it becomes a cause of the rolling crack by inclusion of Cu. For this reason, Cu content in the case of making it contain is less than 0.2%, and (Cu / Sn ratio) ratio of Cu content with respect to Sn content shall be 1.0 or less.

  In addition, in order to stably express the above-described effects due to Cu, it is preferable to contain 0.01% or more of Cu. The lower limit of the Cu content when it is contained is more preferably 0.05%. Further, the upper limit of the Cu content is more preferably 0.15%.

Ni: 0.5% or less Ni can be contained as necessary. When Ni is contained, the strength can be improved. That is, when Ni is contained, the strength of the steel can be improved while suppressing an increase in the yield ratio, and further, the toughness can be increased. Therefore, Ni may be contained to obtain the above effect. However, if the Ni content exceeds 0.5%, the weldability is lowered, and in addition, there is a risk of meaningless cost increase. For this reason, when Ni is contained, the content of Ni is set to 0.5% or less. The upper limit of the Ni content in the case of inclusion is preferably 0.4%.

  In order to stably express the above-described effects due to Ni, it is preferable to contain Ni in an amount of 0.01% or more. More preferably, the lower limit of the Ni content is 0.15%.

Cr: 0.5% or less Cr can be contained as necessary. When Cr is contained, the strength can be increased. That is, when Cr is contained, the strength of the steel can be improved while suppressing an increase in yield ratio. Therefore, in order to acquire said effect, you may contain Cr. However, if the Cr content exceeds 0.5%, the weldability is lowered, and in addition, there is a possibility of causing a meaningless cost increase. For this reason, when Cr is contained, the content of Cr is set to 0.5% or less. The upper limit of the Cr content when contained is preferably 0.3%.

  In order to stably express the above-described effects due to Cr, it is preferable to contain 0.01% or more of Cr. The lower limit of the Cr content when contained is more preferably 0.10%.

Mo: 0.2% or less Mo can be contained as needed. When Mo is contained, the strength can be increased. That is, when Mo is contained, the strength of the steel can be improved while suppressing an increase in the yield ratio. Therefore, you may contain Mo in order to acquire said effect. However, if the Mo content exceeds 0.2%, the toughness of HAZ may be deteriorated. For this reason, content of Mo in the case of making it contain shall be 0.2% or less. The upper limit of the Mo content in the case of inclusion is preferably 0.18%.

  Although the above effect can be obtained even if the content of Mo is very small, it is preferable to contain 0.02% or more in order to stably exhibit the effect. The lower limit of the Mo content in the case of inclusion is more preferably 0.03%.

V: 0.05% or less V can be contained as necessary. If V is contained, the strength can be increased. That is, when V is contained, the strength of the steel can be improved by precipitation hardening, and the toughness can also be increased. Therefore, you may contain V in order to acquire said effect. However, if the V content exceeds 0.05%, the toughness may deteriorate. For this reason, when V is contained, the content of V is set to 0.05% or less. The upper limit of the V content when contained is preferably 0.045%.

  In addition, in order to make the said effect by V stably express, it is preferable to contain V 0.02% or more. The lower limit of the V content when it is contained is more preferably 0.03%.

  In addition, said Cu, Ni, Cr, Mo, and V can be contained only in any 1 type in them, or 2 or more types of composites. Note that the total content of these elements is preferably 4.6% or less.

In addition, it is preferable that the low yield ratio steel material of the present invention is such that the value of Ceq represented by the following formula (1) is 0.30 to 0.43%.
Ceq = C + Mn / 6 + Cu / 15 + Ni / 15 + Cr / 5 + Mo / 5 + V / 5 (1)
If the value of Ceq is within this range, the above-mentioned goal, i.e.
・ YR: 80% or less,
-TS: 490 MPa or more,
VTrs: −20 ° C. or lower,
Can be met. The preferred lower limit for the value of Ceq is 0.33. The preferable upper limit of the value of Ceq is 0.42.

  In addition, [Ceq] in the above formula (1) is a “carbon equivalent” generally used as an index for evaluating the hardenability and weldability of steel materials, and C, Si, Mn in the formula (1) , Ni, Cr, Mo, and V represent the content of each element in mass%.

(B) Microstructure:
In order to satisfy the above-mentioned target of the mechanical properties of YR, TS and vTrs in the low yield ratio steel material of the present invention, the microstructure of the steel material is a ferrite phase having an average crystal grain size of more than 3 μm and not more than 20 μm. The ratio of the hard phase having an average aspect ratio of less than 10 and the inevitable formation phase, the ratio of the ferrite phase being 40% or more, and the ratio of the inevitable formation phase being 5% or less is necessary. is there.

  In the low yield ratio steel material of the present invention, it is preferable that the inevitable formation phase does not exist in the microstructure (the ratio is 0%), but the characteristics of the low yield ratio steel material of the present invention are not impaired. A degree of inevitable formation phase is acceptable. Specifically, if the proportion of the inevitable formation phase in the microstructure is 5% or less, the characteristics of the low yield ratio steel material of the present invention will not be impaired.

  Hereinafter, the microstructure will be described in detail.

(B-1) Regarding the phase constituting the microstructure:
The phases constituting the microstructure of the low yield ratio steel material of the present invention must be a ferrite phase and a hard phase except for the above-mentioned proportion of inevitable forming phases that do not impair the characteristics of the low yield ratio steel material of the present invention. .

  As already mentioned, the “hard phase” means bainite, martensite and MA. The “inevitable formation phase” refers to a phase inevitably formed in addition to the ferrite phase and the hard phase, specifically a phase such as pearlite.

  By setting the ratio of the ferrite phase in the microstructure to 40% or more, YR of 80% or less can be secured in the low yield ratio steel material of the present invention.

  Since the microstructure has a great influence on the strength, the ratio of the ferrite phase in the microstructure is preferably 90% or less in order to secure TS of 490 MPa or more.

  Note that when a steel material with a YR of 80% or less (TS: 490 to 610 MPa, yield strength (hereinafter referred to as “YS”): 325 MPa or more) low yield ratio steel material is desired, the ratio of the ferrite phase in the microstructure Is preferably 80 to 90%.

  Further, when it is desired to obtain a 590 MPa class (TS: 590 to 740 MPa, YS: 325 MPa or more) low yield ratio steel material having a YR of 80% or less, the ratio of the ferrite phase in the microstructure is preferably 40 to 80%. .

  What is necessary is just to measure the area ratio by the normal observation means of a microstructure about the ratio of the specific phase which occupies for a microstructure. This is because it is known that the volume ratio of the phase in the microstructure is equal to the area ratio.

  An example of a specific method for measuring the proportion of a specific phase in the microstructure is shown in the examples described later.

(B-2) Regarding the average grain size of ferrite:
When the average crystal grain size of the ferrite phase in the above microstructure exceeds 20 μm, the toughness decreases. On the other hand, when the average crystal grain size of the ferrite phase in the microstructure is 3 μm or less, YS rises greatly, leading to an increase in YR, and YR of 80% or less cannot be secured.

  Accordingly, the average crystal grain size of the ferrite phase constituting the microstructure of the low yield ratio steel material of the present invention is more than 3 μm and not more than 20 μm. The lower limit of the average grain size of the ferrite phase is preferably 5 μm or more, and the upper limit is preferably 17 μm or less.

  The crystal grain size of the ferrite phase can be obtained by image analysis of an image obtained by a normal microstructure observation means, and the average crystal grain size of the ferrite phase is derived from the crystal grain size of each ferrite phase. Can do.

  An example of a specific method for deriving the average grain size of the ferrite phase is shown in the examples described later.

(B-3) About average aspect ratio of hard phase:
When the average aspect ratio of the hard phase in the above microstructure is 10 or more, the hard phase becomes a band shape, so that the plastic deformability decreases. And since the fall of plastic deformability causes the rise of YR, 80% or less of YR cannot be ensured. In addition, a hard phase having a large average aspect ratio tends to be a starting point at the time of fracture, leading to deterioration of toughness. In particular, a hard phase having an average aspect ratio of 10 or more greatly reduces the toughness. vTrs cannot be secured. Therefore, the average aspect ratio of the hard phase constituting the microstructure of the low yield ratio steel material of the present invention is less than 10.

  In particular, when it is desired to obtain a 590 MPa class low yield ratio steel material, the average aspect ratio of the hard phase is preferably 5 or less.

  Here, the “aspect ratio” refers to a value obtained by dividing the major axis of a crystal grain by the minor axis.

  As in the case of derivation of the average crystal grain size of ferrite described above, the aspect ratio of each hard phase is obtained from the major axis and minor axis of the hard phase obtained by image analysis of an image obtained by an ordinary microscopic observation means, The average aspect ratio of the hard phase can be derived from the aspect ratio of the individual hard phases. An example of a specific method for deriving the average aspect ratio of the hard phase is shown in the examples described later.

  Since the anisotropy is smaller as the aspect ratio is closer to 1, the ideal aspect ratio of the hard phase is 1. Therefore, the ideal average aspect ratio is also 1.

  For example, by adopting the manufacturing method of the present invention, the above-described microstructure of the steel material having the chemical composition described in the item (A) is as described above, that is, the average crystal grain size is more than 3 μm and 20 μm or less. It consists of a ferrite phase, a hard phase having an average aspect ratio of less than 10 and an inevitable formation phase, and the proportion of the ferrite phase is 40% or more, and the proportion of the inevitable formation phase is 5% or less. be able to.

(C) About manufacturing conditions:
The production conditions of the present invention described in detail below are one of the methods for economically realizing the low yield ratio steel material of the present invention on an industrial scale, and the technical scope of the low yield ratio steel material itself. Is not defined by this manufacturing condition.

  The low yield ratio steel material according to the present invention can be manufactured, for example, by sequentially treating the slab having the above-described chemical composition in the following steps (a) to (d).

  In addition, about manufacture of the slab in the case of processing sequentially by process (a)-(d), it is not necessary to specify the casting conditions in particular. This is because the microstructure can be controlled by sequentially performing the steps (a) to (d).

  However, the average aspect ratio and the average crystal grain size tend to depend on the rolling reduction and the rolling temperature. For this reason, as a manufacturing guideline for manufacturing the low yield ratio steel material according to the present invention, it is preferable to set a starting material, that is, a slab as a rolling material, to a certain size in advance. This is because if the rolling reduction is large in the rolling process, the average aspect ratio of the low yield ratio steel material, which is the final product, may increase to 10 or more. Therefore, it is preferable that the slab as the rolling material is thin. Specifically, it is preferable to use a slab having a thickness of 300 mm or less as a rolling material for producing the steel material according to the present invention. On the other hand, a reduction of a specific ratio or more is required to make the crystal grains finer. For this reason, it is preferable to use a slab having a thickness of 220 mm or more as a rolling material. In order to manufacture such a thin slab, the thick slab may be processed thinly in advance, and this slab may be used as a rolled material of the steel material according to the present invention.

(C-1) Heating step:
In the step (a) as the heating step, the slab as the rolling material for producing the low yield ratio steel material of the present invention is heated to a temperature of Ac ₃ point or higher and lower than 1000 ° C.

The reason why the slab is heated to Ac ₃ point or higher is that it is austenite transformed to form a uniform structure. On the other hand, the reason why the slab heating temperature is less than 1000 ° C. is to prevent coarsening of crystal grains and to suppress the average crystal grain size of the ferrite phase of the steel material as the final product to 20 μm or less. On the other hand, from another viewpoint, the temperature is set to less than 1000 ° C. in consideration of reduction of energy consumption and consideration for the global environment. The upper limit of the slab heating temperature is preferably less than 975 ° C, more preferably less than 950 ° C.

  In addition, in order to make temperature uniform to the center part of a slab, it is preferable that the heating time of the slab in the said temperature range shall be 3 hours or more. However, for the purpose of the present invention, the upper limit of the heating time is preferably about 12 hours.

(C-2) Rolling process:
The slab heated in the step (a) is rolled (step (b)) in the austenite non-recrystallization temperature range so that the cumulative reduction ratio is 30% or more, and the rolling is completed at a temperature of Ar ₃ point or more (step) (C)).

  Such rolling is performed by rolling at a rolling reduction of 30% or more in the austenite non-recrystallization temperature range, thereby ensuring a YR of 80% or less and making the grains sufficiently fine. This is because sufficient toughness can be secured.

  The cumulative rolling reduction in the austenite non-recrystallization temperature region is preferably 50% or more. The upper limit of the cumulative rolling reduction in the step (b) is preferably 70% in order to prevent excessive grain refinement.

  The slab immediately after being extracted by the heating furnace is in the austenite recrystallization temperature range. The present invention does not necessarily start rolling after the slab temperature falls to the austenite non-recrystallization temperature range, and such slab may be subjected to constant rolling in the austenite recrystallization temperature range.

All rolling is completed at a temperature of Ar ₃ or higher. This is because when the reduction is performed in a low temperature range lower than the Ar ₃ point, the YR rises due to work hardening, so that a YR of 80% or less cannot be secured.

(C-3) Cooling step:
After the steps (b) and (c) as the rolling step, it is cooled to a temperature of 500 ° C. or less at a cooling rate of 5 to 40 ° C./s (step (d)).

  In addition, as a method for obtaining a cooling rate of 5 ° C./s or higher after steps (b) and (c), for example, water cooling can be cited. Therefore, hereinafter, “cooling” will be described using “water cooling”. To do.

  If the cooling rate at the time of water cooling is less than 5 ° C./s, the effect of quenching cannot be obtained, and the strength of 490 MPa or more cannot be secured by TS. Even when water cooling is performed at a cooling rate of 5 ° C./s or higher, the effect of quenching cannot be obtained unless the water is cooled to a temperature of 500 ° C. or lower.

  On the other hand, when water cooling is performed at a cooling rate exceeding 40 ° C./s, the steel is excessively burned and the amount of the hard phase in the steel material increases, leading to an increase in YR, and a YR of 80% or less cannot be secured.

  The water cooling at the cooling rate of 5 to 40 ° C./s may be stopped at any temperature as long as the temperature is 500 ° C. or less. And after stopping water cooling, you may cool to room temperature, for example by standing to cool in air | atmosphere. Of course, water cooling may be continued to near room temperature.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

Steels 1 to 15 having the chemical composition shown in Table 1 were melted and continuously cast by a normal method to form slabs having a thickness of 250 to 300 mm. In Table 1, the Ceq value represented by the above formula (1) and the Ac ₃ point (° C.) and Ar ₃ point (° C.) values estimated from the chemical composition of the steel are also shown.

  Steels 1 to 7 in Table 1 are comparative steels whose chemical compositions deviate from the conditions defined in the present invention. On the other hand, Steels 8 to 15 are steels whose chemical compositions are within the range defined by the present invention.

  Using these various steel slabs, steel materials having a thickness of 16 to 100 mm were manufactured based on the manufacturing conditions shown in Table 2. The cooling after the completion of rolling was performed by water cooling, the water cooling was stopped at the “cooling stop temperature” shown in Table 2, and after the water cooling was stopped, it was allowed to cool in the air and cooled to room temperature.

  First, the microstructure of each steel material obtained as described above was examined.

  That is, a test piece is taken from the center of the thickness of each steel material so that a so-called “L cross section” that is a plane parallel to the rolling surface becomes the test surface, and then the test piece is embedded in a resin and mirror polished. Then, it was corroded with nital (nitric acid + ethanol), and the microstructure in the center of the thickness was investigated.

  Specifically, the surface corroded by nital (nitric acid + ethanol) is observed with 100 optical fields using an optical microscope at a magnification of 500 times, and the phases present in each field are investigated, and images obtained by the observation are imaged. The ratio of the ferrite phase occupying the entire area of each visual field was calculated, and the ratio of the ferrite phase occupying the microstructure was calculated by arithmetically averaging the area ratio of the ferrite phase for all 100 visual fields.

  Further, the image obtained by the above observation was subjected to image analysis, and the short diameter and the long diameter of each ferrite phase were measured and arithmetically averaged to obtain the crystal grain size of each ferrite phase. Next, the above investigation was performed for each ferrite phase in 100 fields of view, and the average grain size of the ferrite phase was obtained by arithmetically averaging the crystal grain size.

  Similarly, the image obtained by the above optical microscope observation was subjected to image analysis, and the aspect ratio of each hard phase was determined from the major axis and minor axis of each individual hard phase. Subsequently, the above investigation was performed on each hard phase in 100 visual fields, and the aspect ratio was arithmetically averaged to obtain the average aspect ratio of the hard phase.

  Next, the obtained steel materials were examined for tensile properties, toughness, and corrosion resistance as mechanical properties.

  For tensile properties, a tensile test piece according to JIS Z 2201 (1998) was sampled in the C direction centering on the thickness 1/4 position of the steel material, and was pulled at room temperature by the method described in JIS Z 2241 (1998). A test was conducted to investigate and YS (MPa) and TS (MPa) were obtained. YS was determined from the yield point, and when a clear yield point did not appear, YS was defined as 0.2% proof stress.

  Note that the target of tensile properties was TS of 490 MPa or more and YR of 80% or less.

  The toughness is JIS Z 2242 (2005) when the steel thickness is more than 20 mm, and JIS Z 2242 (2005) when the steel thickness is 20 mm or less. The sub-size V-notch test piece with a width of 7.5 mm described is sampled in the L direction centered on the steel thickness 1/4 position and 1/2 position, and subjected to a Charpy impact test. ° C). The target of toughness was that vTrs was −20 ° C. or lower.

And corrosion resistance evaluated the test piece obtained from the obtained steel materials by SAE (Society of Automotive Engineers) J2334 test. SAE J2334 test is wet: 50 ° C., 100% RH, 6 hours, salt adhesion: 0.5% NaCl, 0.1% CaCl ₂ , 0.075% NaHCO ₃ aqueous solution, 0.25 hour, dry: 60 It is an accelerated test with 1 cycle (total 24 hours) at ℃, 50% RH, 17.75 hours, and the corrosion form is said to be similar to the atmospheric exposure test (Hiroo Nagano, Masato Yamashita, Hitoshi Uchida) : Environmental Materials Science, Kyoritsu Shuppan (2004), p.74). This test is a test that simulates a severe corrosive environment in which the amount of incoming salt exceeds 1 mdd.

  After 120 cycles of the SAE J2334 test, the rust layer on the surface of each test piece was removed, and the thickness reduction was measured. Here, the “plate thickness reduction amount” is an average plate thickness reduction amount of the test piece, and is calculated using the weight reduction before and after the test and the surface area of the test piece.

In addition, in order to investigate the anti-peeling resistance, a test piece with a size of 150 x 70 mm was coated with a modified epoxy paint (Banno 200: made in China) by air spray to a dry film thickness of 150 μm, and the steel substrate After making a crosscut at a depth reaching, the SAE J2334 test was also evaluated.
In the above corrosion resistance test, the reduction in sheet thickness was set to 0.25 mm and the peel area ratio was 30% or less.

  Table 3 shows the microstructure (ferrite phase ratio (α fraction), ferrite phase average crystal grain size (α grain size) and average aspect ratio (hard phase aspect ratio)), tensile properties (YS), and toughness. The investigation results of (vTrs at the thickness 1/4 position and 1/2 position of the steel material) and corrosion resistance (plate thickness reduction amount and peel area ratio) are collectively shown. In the properties column of the table, “O” indicates that all of the tensile properties, toughness and corrosion resistance were achieved simultaneously, and “X” indicates that any of the tensile properties, toughness and corrosion resistance was not achieved. Show.

  From Table 3, test numbers 1-1, 2-1, 3-1, 4-1, 5-1, 7-1, 8-2, 9-2 of comparative examples deviating from the conditions specified in the present invention, In the case of steel materials of 11-2 and 14-2, it is clear that not all of the above-mentioned tensile property, toughness and corrosion resistance goals can be achieved simultaneously. In addition, about the test number 6-1, since a crack arises at the time of rolling, measurement, such as a tensile characteristic, was not carried out.

  On the other hand, test numbers 8-1, 9-1, 10-1, 11-1, 12-1, 13-1, 14-1, and 15-1 of the examples of the present invention that satisfy the conditions defined in the present invention. In the case of this steel material, the target of TS and YR as tensile properties and the target of vTrs as toughness could all be achieved simultaneously.

  In addition, the comparison between test numbers 8-1 and 8-2 using steel 8 whose chemical composition is within the range defined in the present invention, the comparison between test numbers 9-1 and 9-2 using steel 9, By comparing the test numbers 11-1 and 11-2 using the steel 11 and the test numbers 14-1 and 14-2 using the steel 14, the present manufacturing method can be used. It is clear that the low yield ratio steel of the invention can be easily obtained.

  As described above, the low yield ratio steel material of the present invention has the characteristics that YR is 80% or less, TS is 490 MPa or more, and vTrs is −20 ° C. or less. In addition, it is possible to prevent the collapse of the building structure, and it can be suitably used as a building material based on the ultimate strength type design concept. In addition, this low yield ratio steel material can be easily manufactured at an industrial scale at a low cost while suppressing the rising energy cost, and the energy consumption at the time of manufacture may be small. There is also an effect that the release of the effect gas can be suppressed. Furthermore, it has excellent corrosion resistance in a high chloride environment, and can reduce the life cycle cost by extending the repair coating interval.

Claims (4)

In mass%, C: 0.05 to 0.20%, Si: 0.10 to 0.50%, Mn: 1.0 to 2.0%, P: 0.05% or less, S: 0.02 %, Nb: 0.005% or less , Al: 0.003 to 0.050% and Sn: 0.03 to 0.50%, with the balance being a chemical composition comprising Fe and impurities, The structure is composed of a ferrite phase having an average crystal grain size of more than 3 μm and not more than 20 μm, a hard phase having an average aspect ratio of less than 10, and an unavoidably formed phase (excluding a ferrite phase and a hard phase). becomes, and, the ratio of the ferrite phase is at 40% or more or less proportion of 5% of the unavoidable formation phase, the yield ratio YR is 80% or less, a tensile strength TS of more than 490 MPa, toughness vTrs is -20 ° C. or less excellent building in corrosion resistance characterized by der Rukoto Low yield ratio steel. Chemical composition, in mass%, further, Cu: containing less than 0.05% to 0.2%, and wherein the Cu / Sn ratio is 10 or less, the corrosion resistance of claim 1 Excellent low yield ratio steel for construction . The chemical composition is one or more selected by mass%, and further selected from Ni: 0.5% or less, Cr: 0.5% or less, Mo: 0.2% or less, and V: 0.05% or less. The low yield ratio steel material for building excellent in corrosion resistance according to claim 1 or 2 , characterized by containing the above elements. The slab having the chemical composition according to any one of claims 1 to 3 is sequentially processed in the following steps (a) to (d) , according to any one of claims 1 to 3. A method of manufacturing low yield ratio steel for construction with excellent corrosion resistance .
Step (a): The slab is heated to a temperature of Ac ₃ point or higher and lower than 1000 ° C.
Step (b): Rolling is performed so that the cumulative reduction rate is 30% or more in the austenite non-recrystallization temperature range.
Step (c): The rolling is completed at a temperature of Ar ₃ or higher.
Step (d): Cool to a temperature of 500 ° C. or lower at a cooling rate of 5 to 40 ° C./s.