JP5053213B2 - High-strength steel with excellent corrosion resistance during painting in the coastal area and its manufacturing method - Google Patents

Description

  TECHNICAL FIELD The present invention relates to steel for high-strength bolts and nuts for friction joining used in various structures in the field of construction and civil engineering, and in particular, corrosion resistance at the time of painting used in corrosive environments involving flying salt such as structures in coastal areas. The present invention relates to a high-strength steel having an excellent tensile strength of 780 MPa to 1400 MPa and a method for producing the same.

  In structures such as bridges that require long-term corrosion resistance, weatherproof steel is generally used. Accordingly, steel having weather resistance has been applied to high strength bolts and nuts used for joints in structures. For example, in Patent Document 1, Patent Document 2, etc., high strength in which the progress of corrosion is significantly delayed by covering the surface with highly corrosion-resistant stable rust enriched with effective elements such as Cu and Cr. Steel is disclosed. These high-strength steels are premised on being subjected to quenching and tempering treatment and used at a strength of 1000 MPa or more.

  However, it is known that in areas with a large amount of incoming salt, such as the coastal area, the above weather-resistant material is unlikely to form a stable rust layer made of Cu or Cr. Therefore, in recent years, steel materials for high-strength bolts that can be used barely even in coastal areas have been developed and disclosed in Patent Literature 3, Patent Literature 4, Patent Literature 5, Patent Literature 6, and the like. This is characterized in that Ni is added in an amount of 1% or more, and further Mo is added in an amount of 0.25% or more as required, and the Di value is defined in order to ensure the necessary quenching strength. Although the above-mentioned invention certainly shows excellent corrosion resistance, it has a drawback that the cost of the material becomes high because it contains expensive elements such as Ni and Mo. Furthermore, bridges are generally designed on the premise of painting, and it is not very reasonable to use bolt steel that can withstand expensive bare specifications.

  Therefore, in recent years, for example, the inventions of Patent Literature 7, Patent Literature 8, Patent Literature 9, and Patent Literature 10 have been made as steels that speak of high corrosion resistance steel premised on painting. These steels contain 10% or less of Cr, and by adding Al or more of 0.3% or more, a base metal layer, such as Zn, is applied or plated in an aqueous solution. This is a technology that can drastically improve the corrosion resistance. However, when these techniques are applied to bolts, there is a problem that the strength is low. When a bridge is manufactured using these steel materials, there is a problem that there is no similar material and bolt connection cannot be performed.

Further, martensitic stainless steel has been developed as a steel having high corrosion resistance and high strength. For example, Patent Document 11 is an oil-based martensitic stainless steel excellent in corrosion resistance containing Cr: 7 to 15%, Cu: 0.25 to 5%, and Patent Document 12 is Cr: 7 to 15%. High corrosion resistance martensitic stainless steel containing Ni: 0.7-8.0%, Al: 0.001-0.2%, Patent Document 13: Cr: 4-11%, Al: 0.001- Machining comprising 0.1%, Ca: 0.0005 to 0.02%, O: 0.0005 to 0.01%, and containing a predetermined amount of Ca-containing sulfide. A martensitic stainless steel part having excellent properties and a method for producing the same are disclosed. However, such martensitic stainless steel cannot be used for a long time in the beach area where a large amount of Cl ions are generally present, and there is a concern that foreign metal corrosion may occur between the base metal to be fastened. Therefore, it is generally not applicable in civil engineering applications such as bridges.
Japanese Patent Publication 53-24890 JP 56-20350 JP 2000-63992 A JP2000-198114A JP 2000-198115 A JP 2001-1017190 A JP-A-6-212227 JP-A-6-280048 JP-A-1-350086 JP 2004-162120 A JP 11-310855 A WO99 / 04052 JP 2000-282185 A

  In order to solve the above-mentioned problems, the present invention provides means capable of producing high-strength bolts and nuts that can improve corrosion resistance during painting even in coastal areas.

  In order to achieve the above object, the present inventors have studied from various viewpoints in order to develop a steel having excellent corrosion resistance in a corrosive environment such as a seawater corrosive environment and having a strength of 780 MPa or more. If the tensile strength is too high, there is a risk of delayed fracture, so the upper limit was set to 1400 MPa for development. Also, considering the productivity at the time of bolt processing, we also considered that there were no defects such as cracks.

  As a result, we focused on the corrosion resistance in the coastal environment when painting containing Zn, which is commonly used, and as a result of examining the manufacturing conditions to satisfy the tensile strength at the same time, the appropriate component system should be satisfied The metallographic structure and heat treatment temperature conditions for realizing it were found.

The gist of the present invention made based on the above findings is as follows.
(1) In mass%,
C: 0.01 to 0.25%
Si: 0.01 to 1.0%
Mn: Less than 0.1 to 3% P: 0.03% or less S: 0.01% or less Cr: 3 to 7%
Al: 0.2-2%
N: 0.02% or less, the balance being Fe and inevitable impurities, the metal structure being martensite of 90% or more, high strength excellent in corrosion resistance during coating in the coastal area steel.

(2) In mass%,
Cu: 0.1 to 2%
Ni: 0.1 to 2%
Mo: 0.005 to 1%
V: 0.005-0.1%
Nb: 0.005 to 0.050%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005 to 0.05%
REM: 0.001 to 0.1%
The high-strength steel excellent in corrosion resistance at the time of painting in the coastal area according to (1), characterized by containing one or more of the above.

(3) After heating the steel having the composition described in (1) to a temperature of 950 ° C. or more and 1200 ° C. or less at a surface temperature, hold it for at least 30 minutes or more and 120 minutes or less, and then from a temperature of 850 ° C. or more to 150 ° C. A method for producing high-strength steel excellent in corrosion resistance at the time of painting in the coastal region, characterized by quenching to a temperature at an average cooling rate of 1 ° C./sec or more.
(4) After heating the steel having the composition described in (1) and (2) to a surface temperature of 950 ° C. or more and 1200 ° C. or less, hold the steel for at least 30 minutes or more and 120 minutes or less, and then from a temperature of 850 ° C. or more. A method for producing high-strength steel excellent in corrosion resistance at the time of painting in the coastal region, characterized by quenching to a temperature of 150 ° C. at an average cooling rate of 1 ° C./sec or more.

  The present invention can provide steel for high-strength bolts having a tensile strength of 780 MPa to 1400 MPa for construction of structures such as bridges in the coastal area, and can be used as a high-strength bolt for friction bonding with excellent corrosion resistance. Yes, it is extremely useful industrially.

The composition of the high strength steel according to the present invention will be described below.
C: C is an element that improves the strength and at the same time is an element that can increase the hardness of the hard phase of the base material. Therefore, addition of 0.01% or more is necessary, but 0.25 If the addition exceeds 50%, the content is excessively cured and the processability is inferior, so the content range is 0.01% to 0.25%.

  Si: It is effective to add Si as a deoxidizer and strengthening element to steel containing 2% or more of Cr. However, if the content is less than 0.01%, the deoxidation effect is not sufficient. If it exceeds 0%, it will be excessively cured, so the content range is limited to 0.01% or more and 1.0% or less.

  Cr: Cr needs to contain 3% or more, preferably 4.5% or more in order to ensure corrosion resistance, but if it contains more than 7%, it causes an excessive increase in strength and processability. Therefore, the range is made 3 to 7%, preferably 4.5 to 6.5%.

  Al: Al is an important element along with Cr in order to ensure corrosion resistance in the present invention, and the content of Al is required to be 0.2% or more from the viewpoint of ensuring corrosion resistance, If added over 2%, the transformation point will be increased, and it will be difficult to secure a quenching temperature for obtaining an appropriate strength, resulting in a decrease in strength. Therefore, its content should be 0.2% or more and 2% or less. limit.

  Mn: In the present invention, Mn mainly acts as an element for improving strength and austenite, lowering the transformation point increased by Cr and Al added from the viewpoint of corrosion resistance, and ensuring hardenability. To be added. That is, as is well known, Cr and Al are ferrite-forming elements, and when they are added in large amounts, they undergo a transformation from solidification to room temperature and become a ferrite single-phase structure. Inhibits manufacturability, such as inducing cracks. As a result, it is necessary to add Mn in an amount of 0.1% or more. However, if the addition amount is 3% or more, an excessive increase in strength and workability are reduced, so the addition is made less than 3%.

  N: When N is added in a large amount to the steel sheet, the hardness at the time of quenching increases and the tensile strength exceeds the upper limit, so the upper limit content is 0.02%.

  P: If P is present in a large amount, defects such as cracks at the time of bolting are generated. Therefore, a smaller amount is desirable, and the upper limit content is 0.03%.

  S: If S is also present in a large amount, the pitting corrosion resistance is lowered, so the smaller one is desirable, and the upper limit content is 0.01%. S, like P, should have the unavoidable amount of contamination as small as possible.

Furthermore, in the present invention, the following elements can be selected and added.
Cu, Ni: Both Cu and Ni have the effects of improving strength and suppressing the formation of ferrite in the same manner as Mn. In particular, Ni increases the hardenability and is effective in improving the strength during quenching. Both of these effects require the addition of 0.1% or more, but even if both are added in excess of 2%, the effect is saturated. And

  Mo: In steel added with Cr and Al, when 0.005% or more is added, the effect of suppressing the occurrence and growth of pitting corrosion is recognized, but Mo exceeds 1.0%. Since the effect is saturated, the range is made 0.005% to 1.0%.

  Nb: Nb is an element that improves the strength without impairing the corrosion resistance, and its effect is recognized from 0.005%, but if it exceeds 0.05%, coarse precipitates are generated and become the starting point of cracking. The range is 0.005% to 0.05%.

  V: V is an element that improves the strength without impairing the corrosion resistance, and an effect is recognized at 0.005% or more. However, since an excessive amount of added strength is increased, the upper limit is 0.1%. And

  Ti: Ti is an element that contributes to refinement of the crystal grain size at high temperatures through the formation of nitrides, and is effective in improving workability and delayed fracture. Therefore, addition of 0.005% or more is necessary. However, if it exceeds 0.03%, coarse precipitates are generated, which may be the starting point of cracking. Therefore, the range is set to 0.005% to 0.03% for both elements.

  Ca, Mg: Ca and Mg are elements that can improve corrosion resistance in steels containing Cr and Al. At present, there are many unclear points in the mechanism, but the corrosion resistance is further improved when both of them are 0.0005% or more, and as the amount is increased, the corrosion resistance is improved, but added exceeding 0.05%. Then, not only does the corrosion resistance improving effect saturate, but also a tendency for the toughness to decrease is clarified, and the addition amount is limited to 0.0005 to 0.05% or less.

  REM: Furthermore, in the present invention, even if a rare earth element (REM) is added as appropriate, the corrosion resistance of the base material can be improved without impairing its corrosion resistance. The addition amount needs to be 0.001% or more, but addition of a large amount inhibits toughness and the like, so the upper limit is made 0.1%.

  In addition, when manufacturing this invention steel, after manufacturing as a steel ingot by the ingot-splitting method or the continuous casting method and other methods, it manufactures as a steel plate or a round bar by hot rolling or hot forging. Then, in order to obtain a predetermined martensite structure fraction, a quenching process is performed.

  A quenching process is implemented at the temperature of 950 degreeC or more and 1200 degrees C or less. This condition depends on the transformation point depending on the composition, but in order to obtain the necessary amount of austenite during heating (depending on the temperature condition, the austenite single phase or a mixed structure of austenite (90% or more) and the ferrite phase). 950 ° C. is necessary, and if it exceeds 1200 ° C., the structure becomes coarse and at the same time, δ ferrite is generated and the quenching strength is lowered, so the upper limit value is made less than 1200 ° C. Further, during the quenching, the holding time is required for 30 minutes or more from the homogenization of the inside of the raw material, but if it exceeds 120 minutes, the structure becomes coarse, so the holding time is 30 minutes or more and 120 or less. In cooling at the time of quenching, it is necessary that the quenching start temperature is from 850 ° C. or higher to 150 ° C. at an average cooling rate of 1 ° C./sec or higher. This is because when the quenching start temperature is less than 850 ° C., quenching is insufficient and an appropriate hardened structure cannot be obtained, and when the average cooling rate up to 150 ° C. is less than 1 ° C./sec, quenching is still performed. There is a concern of shortage.

The steel thus obtained has a martensite structure of 90% or more. In this component range, if the martensite fraction is 90% or more, a tensile test strength of 780 MPa class can be realized, but if the martensite fraction is less than 90%, the strength decreases, and the high strength bolt and nut The specified characteristics cannot be satisfied.
The quenching process is carried out after rolling or forging into a round bar, and in some cases, after rolling into a steel plate, and then forming into a bolt or nut shape by bolt forming / screw rolling or machining.
Examples of the present invention will be described below.

After melting the component steels shown in Table 1, hot rolled into a steel plate with a thickness of 30 mm and a round bar with a diameter of 25 mm, and using that as a raw material, bolting with a diameter of 20 mm was performed, followed by quenching (mainly water quenching) And partly oil quenching). The conditions during quenching and bolt processing were observed, and defects such as cracks were recorded. Details of the production method at that time are shown in Table 2. The tensile strength of the base material was evaluated by a tensile test in accordance with JIS B 1186 after bolt processing. Further, the martensite fraction was obtained from the area ratio by observing 20 visual fields at x500 magnification with an optical microscope. Corrosion resistance evaluation is the ratio of the area of rust on the surface when exposed for about 1.5 years at an exposure field at a distance of 100 m from the coast after coating with a primer containing 60% inorganic Zn after bolt processing. Area / evaluation target area).
Table 2 also shows the results of the test.

Steel A to steel K are all within the scope of the present invention, and steel L to steel V have component systems that depart from the scope of the present invention.
That is, the steel has a composition of steel A to steel K and the heat treatment conditions are within the scope of the present invention (numbers: 1, 2, 3, 4, 5, 6, 8, 9, 11, 12, 14 ) Has no problems during production, has a tensile strength in the range of 825 to 1324 MPa, and also exhibits good corrosion resistance.

  On the other hand, even if the component system is within the scope of the present invention, Nos. 7, 10, 13 and 15 whose quenching temperatures deviate from the scope of the present invention are examples that do not satisfy the scope of the present invention. That is, No. 7 has a quenching start temperature of 820 ° C., which is lower than the range of the present invention, and No. 10 is an example in which the quenching temperature exceeds 1250 ° C., the upper limit of the range of the present invention. No. 13 has a cooling rate of 0.5 ° C./sec, which is below the range of the present invention, and No. 15 has a quenching temperature below the lower limit of the range of the present invention. Therefore, in all cases, the martensite fraction is less than 90%, and the tensile strength is lower than the desired 780 MPa.

On the other hand, all of L steel to V steel are comparative steels deviating from the component range of the present invention.
That is, No. 16 (L steel) has a low tensile strength because C deviates from the lower limit of the range of the present invention.
No. 17 (M steel) has the disadvantage that tensile strength exceeds 1400 MPa because of excessive addition of Mn, and cracking occurs during processing.
No. 18 (N steel) still contains cracks during processing because P is excessively contained.
No. 19 (O steel) contains S in excess, and a decrease in corrosion resistance is observed.
In No. 20 (P steel) and No. 21 (Q steel), Cr and Al, which are important for corrosion resistance, are added below the lower limit of the present invention, and the corrosion resistance is reduced.
In No. 22 (R steel), Al was added excessively, the martensite fraction was as low as 55%, and the tensile strength did not satisfy the predetermined strength.
No. 23 (S steel) is an example in which N is excessively added, and the tensile strength is higher than 1400 MPa.
No. 24 (T steel) is an example in which Mn falls below the lower limit of the present bright range, the martensite fraction is as low as 60%, and the tensile strength is reduced.
No. 25 (U steel) is Cr exceeding the upper limit of the present invention. In this case, the tensile strength exceeds 1400 MPa.
In No. 26 (V steel), C exceeds the upper limit of the present invention, and the tensile strength is also higher than 1400 MPa.

Claims (4)

% By mass
C: 0.01 to 0.25%
Si: 0.01 to 1.0%
Mn: Less than 0.1 to 3% P: 0.03% or less S: 0.01% or less Cr: 3 to 7%
Al: 0.2-2%
N: 0.02% or less, the balance being Fe and inevitable impurities, the metal structure being martensite of 90% or more, high strength excellent in corrosion resistance during coating in the coastal area steel. % By mass
Cu: 0.1 to 2%
Ni: 0.1 to 2%
Mo: 0.005 to 1%
V: 0.005-0.1%
Nb: 0.005 to 0.050%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005 to 0.05%
REM: 0.001 to 0.1%
The high strength steel excellent in corrosion resistance at the time of painting in the coastal area according to claim 1, further comprising one or more of the following. % By mass
C: 0.01 to 0.25%
Si: 0.01 to 1.0%
Mn: Less than 0.1 to 3% P: 0.03% or less S: 0.01% or less Cr: 3 to 7%
Al: 0.2-2%
N: 0.02% or less, and the steel having a composition composed of Fe and unavoidable impurities in the balance is held at a surface temperature of 950 ° C. or more and 1200 ° C. or less, and held for at least 30 minutes or more and 120 minutes or less, Then, the manufacturing method of the high strength steel excellent in the corrosion resistance at the time of the coating in the coastal area characterized by quenching from the temperature of 850 degreeC or more to the temperature of 150 degreeC with the average cooling rate of 1 degreeC / sec or more. During mass composition of steel,
Cu: 0.1 to 2%
Ni: 0.1 to 2%
Mo: 0.005 to 1%
V: 0.005-0.1%
Nb: 0.005 to 0.050%
Ti: 0.005 to 0.03%
Ca: 0.0005 to 0.05%
Mg: 0.0005 to 0.05%
REM: 0.001 to 0.1%
The method for producing high-strength steel excellent in corrosion resistance at the time of painting in the coastal area according to claim 3, further comprising one or more of the following.