JP4770485B2 - Cr steel for reinforcing steel with excellent mechanical properties and corrosion resistance in concrete - Google Patents

Description

  The present invention relates to a Cr steel for rebar excellent in corrosion resistance in concrete.

  Conventionally, steels such as SD345 steel and SD390 steel have been used as concrete reinforcing steel, but such steel reinforcing steel is used when the concrete is neutralized or when chlorine ions are mixed into the concrete. Since it is easily corroded, the danger of concrete collapse is pointed out, which is a big problem.

  As one of the countermeasures, application of Cr steel bars is being studied in Japan. For example, Patent Document 1 proposes Cr steel for reinforcing bars in which the Cr content is 5 to 15 mass% and the Co content is 0.01 to 1.0 mass% to improve the corrosion resistance in concrete. In addition, as a recycled aggregate with a high chlorine content and rebar for eco-cement, Patent Document 2 states that Cr steel has 4.0 to 10.0 mass% Cr to reduce the anode reaction in concrete and ensure the necessary corrosion resistance. Has been proposed.

  However, the Cr steels proposed in Patent Document 1 and Patent Document 2 are too hot to be rolled, so it is necessary to perform annealing after hot rolling, but there is a Cr-free layer on the steel surface during this annealing. Since it is formed, the corrosion resistance is extremely deteriorated and is not suitable for use as it is in hot rolling. Therefore, it is necessary to perform pickling to remove the Cr removal layer, resulting in extra costs.

As a method that does not require heat treatment after hot rolling, Patent Document 3 proposes a method for producing 13Cr stainless steel that secures corrosion resistance by setting the Cr amount to 12 to 14 mass%.
Japanese Patent Laid-Open No. 2002-212682 Japanese Patent Laid-Open No. 2004-10909 JP-A-4-26719

  However, the stainless steel described in Patent Document 3 cannot be used because the entire structure is composed of a high-strength and brittle martensite phase, so that the brittle transition temperature is relatively high and the toughness is insufficient in cold regions.

  On the other hand, stainless steel bars are partially used overseas as corrosion protection reinforcing bars for salt damage areas, but most of them are austenitic stainless steels such as SUS304 and SUS316, which are extremely excellent in corrosion resistance but expensive. There is a problem that the range of use is limited.

  The present invention was developed in view of the above circumstances, is cheaper than austenitic stainless steel, does not require annealing or pickling after hot rolling, and has mechanical properties such as toughness even in hot rolling. The purpose is to provide Cr steel for reinforcing bars with excellent corrosion resistance in concrete.

The inventors of the present invention have made extensive studies on Cr steel for reinforcing bars that are excellent in mechanical properties and corrosion resistance in concrete even in hot rolling, and obtained the following knowledge.
1) The component composition consisting of a ferrite phase + austenite phase during heating before hot rolling produces a ferrite phase with good toughness even without passing through annealing and other processes after hot rolling, and is inexpensive for reinforcing steel Sufficient toughness can be obtained as steel.
2) If the Cr amount is more than 10.0 mass% and less than 20.0 mass%, the structure is two phases of ferrite phase and martensite phase, and the volume fraction of ferrite phase is 30-90%, the strength and workability will be improved. As a result, it is possible to obtain a steel for reinforcing bars that is suitable for reinforcing bars and excellent in toughness and corrosion resistance in concrete.

The present invention has been made based on such findings, C: more than 0.005 mass%, less than 0.1 mass%, N: more than 0.005 mass%, less than 0.1 mass%, Mn: more than 0.05 mass%, less than 2.0 mass%, Si : More than 0.05mass%, less than 2.0mass%, P: less than 0.04mass%, S: less than 0.03mass%, Cr: more than 10.0mass%, less than 20.0mass%, balance Fe and inevitable impurities, ferrite phase and martens Cr steel for rebar as hot-rolled with excellent mechanical properties and corrosion resistance in concrete, which has a structure consisting of a site phase and the volume fraction of the ferrite phase is 30 to 90% provide.

Moreover, reinforcing bars for Cr steel of the present invention, M o to further: can contain 3.0 mass% less than.

  Furthermore, the Cr steel for reinforcing bars of the present invention is further Nb: less than 0.5 mass%, Ti: less than 0.5 mass%, Ta: less than 0.5 mass%, Zr: less than 0.5 mass%, Al: less than 0.5 mass%, B: At least one element selected from less than 0.005 mass% can be contained.

  According to the present invention, it is possible to obtain a Cr steel for reinforcing steel that is superior in mechanical properties such as toughness and corrosion resistance in concrete even in hot rolling as compared with conventional ordinary steel. In addition, the Cr steel for reinforcing bars of the present invention is not an austenitic stainless steel and can be manufactured at a lower cost because annealing and pickling after hot rolling are not required.

  The present invention is described in detail below.

1) Component composition
C: C is an element that forms an austenite phase and carbide. When the austenite phase is transformed into a martensite phase, the strength increases and the toughness decreases. Carbides and solute carbon also increase strength. However, if the amount of C is 0.005 mass% or less, not only the volume fraction of the ferrite phase exceeds 90%, but also the amount of carbides and solid solution carbon decreases, resulting in insufficient strength for reinforcing bars. On the other hand, when the amount of C is 0.1 mass% or more, the volume fraction of the ferrite phase becomes less than 30%, leading to a decrease in toughness. Therefore, the amount of C is limited to more than 0.005 mass% and less than 0.1 mass%.

  N: N is an element that forms an austenite phase and a nitride. When the austenite phase is transformed into a martensite phase, the strength increases and the toughness decreases. Nitride also increases strength. However, when the N content is 0.005 mass% or less, not only the volume fraction of the ferrite phase exceeds 90%, but also the amount of nitride is reduced, resulting in insufficient strength for reinforcing bars. On the other hand, if the N content is 0.1 mass% or more, the volume fraction of the ferrite phase becomes less than 30%, which causes a decrease in toughness. Therefore, the N amount is limited to more than 0.005 mass% and less than 0.1 mass%.

  Mn: Like C, Mn is an element that promotes transformation from the austenite phase to the martensite phase. When the amount of Mn is 0.05 mass% or less, the volume fraction of the ferrite phase exceeds 90%, so that the strength is insufficient for reinforcing bars. On the other hand, when the Mn amount is 2.0 mass% or more, the inclusions remaining in the steel increase and the corrosion resistance deteriorates. Therefore, the amount of Mn is limited to more than 0.05 mass% and less than 2.0 mass%.

  Si: Si is useful as a deoxidizer and is an element that produces a ferrite phase. However, if the Si amount is 0.05 mass% or less, a sufficient deoxidation effect cannot be obtained, and if it is 2.0 mass% or more, it hardens and causes deterioration of mechanical properties. Therefore, the amount of Si is limited to more than 0.05 mass% and less than 2.0 mass%.

  P: P is preferably as small as possible because it is an element harmful not only to mechanical properties such as toughness but also to corrosion resistance. In particular, when the P amount is 0.04 mass% or more, the adverse effect becomes significant, so the P amount is limited to less than 0.04 mass%.

  S: S combines with Mn to form MnS, which not only serves as a starting point for rust generation but also segregates at the grain boundaries to promote grain boundary embrittlement. In particular, when the S amount is 0.03 mass% or more, the adverse effect becomes significant, so the S amount is limited to less than 0.03 mass%.

  Cr: Cr is an important element in the present invention for improving the corrosion resistance and adjusting the volume ratio of the ferrite phase and the martensite phase. In order to ensure a level of corrosion resistance that does not cause corrosion even when used as a reinforcing bar in concrete for a long period of time, the Cr content must be at least over 10.0 mass%. On the other hand, when the Cr content is 20.0 mass% or more, the volume fraction of the ferrite phase exceeds 90%, and the strength becomes insufficient for reinforcing bars. Therefore, the Cr content is limited to more than 10.0 mass% and less than 20.0 mass%.

  The balance is Fe and inevitable impurities, but at least one of the following elements can be appropriately contained.

Mo: Mo is an extremely effective element for improving the corrosion resistance of Cr steel, but if it exceeds 3.0 mass%, the cost increases and the mechanical properties deteriorate, so the amount of Mo is limited to less than 3.0 mass%. To do. In addition, in order to exhibit the effect, it is preferable that it is more than 0.05 mass%.

  Nb, Ti, Ta, Zr: Nb, Ti, Ta, and Zr all have the function of reducing the precipitation of Cr carbonitride and improving the corrosion resistance. However, since mechanical properties deteriorate when both are 0.5 mass% or more, the amount of Nb, Ti, Ta, and Zr is limited to less than 0.5 mass% in both cases of single addition and composite addition. In addition, in order to exhibit the effect, it is preferable that it is more than 0.01 mass%, respectively.

  Al: Al is useful as a deoxidizer and is also an element that produces a ferrite phase. When deoxidation with Si is insufficient, deoxidation with Al is performed, but when the amount exceeds 0.5 mass%, inclusions increase and corrosion resistance deteriorates. Therefore, the amount of Al is limited to less than 0.5 mass%. In addition, in order to exhibit the effect, it is preferable that it is more than 0.01 mass%.

  B: When B is combined with N, it has the effect of reducing the precipitation of Cr nitride and improving the corrosion resistance. However, since the hot workability deteriorates when the amount is 0.005 mass% or more, the B amount is limited to less than 0.005 mass%. In addition, in order to exhibit the effect, it is preferable that it is more than 0.0005 mass%.

2) Structure The structure of the Cr steel for reinforcing bars according to the present invention needs to be composed of a ferrite phase and a martensite phase in order to ensure the strength and workability for reinforcing bars. At this time, the volume fraction of the ferrite phase needs to be 30% or more from the viewpoint of improving toughness and 90% or less from the viewpoint of securing strength.

  The Cr steel for reinforcing bars according to the present invention can be produced by an ordinary method, and does not require annealing or pickling after hot rolling. For example, molten steel obtained by decarburizing blast furnace hot metal while adding Cr in a converter, or melting Fe and Cr raw materials such as scrap in an electric furnace, decarburizing by VOD, etc. Ingot in bloom or ingot, this bloom or ingot is heated to 1100-1200 ° C, hot rolled or hot forged into a billet of about 50mm square, this billet is heated again to about 1100 ° C, Use a wire rolling mill to make a steel bar of about 15mmφ. At this time, a structure composed of a ferrite phase and a martensite phase is obtained by controlling the cooling rate after rolling to 30 to 150 ° C./h for the steel bar, and the volume ratio of the ferrite phase can be adjusted to 30 to 90%.

  A 50 kg steel ingot of steel a to z having the composition shown in Table 1 was vacuum-melted, and then the surface 5 mm was ground and then heated to 1170 ° C. to form a 50 mm square billet by hot forging. The billet was heated at 1100 ° C. for 1 h, converted into a 15 mmφ bar with a bar rolling mill, and cooled to room temperature at a cooling rate of 30 to 150 ° C./h. The following tests were carried out using the steel bar rebars produced in this way and commercially available SD390, SUS304, and SUS316 rebars.

(1) Impact test A notch with a depth of 3 mm was cut into the reinforcing bar, a metal material impact test specified in JIS Z 2242 was performed at -25 ° C, and the brittle fracture surface ratio after fracture was measured to evaluate toughness.

(2) Corrosion Test gypsum material about 1200 kg / m ³ in a concrete, Sunaaburazai about 800 kg / m ^3, Portland cement 300 kg / m ^3, water 200 kg / m ^3, the chloride ion amount 0.6 kg / m ³ Using concrete with a composition, reinforcing bars were embedded 30 mm from the surface and cured for 30 days to prepare reinforced concrete specimens. After spraying seawater on this specimen for 6 hours every day, the degree of cracking (no cracking, small cracking, large cracking) and corrosion area rate in the concrete were measured, and the corrosion resistance in the concrete was evaluated.

  The results are shown in Table 1. The Cr steels a to w for reinforcing bars according to the present invention were good in both toughness and corrosion resistance in concrete. The conventional SUS304 and SUS316 also have good toughness and corrosion resistance, but they have a large amount of Ni and are expensive.

Claims (3)

C: more than 0.005 mass%, less than 0.1 mass%, N: more than 0.005 mass%, less than 0.1 mass%, Mn: more than 0.05 mass%, less than 2.0 mass%, Si: more than 0.05 mass%, less than 2.0 mass%, P: Less than 0.04 mass%, S: less than 0.03 mass%, Cr: more than 10.0 mass%, less than 20.0 mass%, consisting of the balance Fe and inevitable impurities, having a structure consisting of a ferrite phase and a martensite phase, and the ferrite phase Cr steel for rebar as hot rolled with excellent mechanical properties and corrosion resistance in concrete, characterized by a volume ratio of 30-90%. Further to M o: 3.0 mass% less than the characterized in that it contains claim 1 hot-rolled rebar for Cr steel with excellent corrosion resistance in the mechanical properties and concrete in described. Furthermore, Nb: less than 0.5 mass%, Ti: less than 0.5 mass%, Ta: less than 0.5 mass%, Zr: less than 0.5 mass%, Al: less than 0.5 mass%, B: at least 1 selected from less than 0.005 mass% 3. The hot-rolled Cr steel for rebar excellent in mechanical properties and corrosion resistance in concrete according to claim 1 or 2, characterized by containing various elements.