JPH048486B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a reinforced steel bar with excellent low-temperature toughness. (Prior Art) Since safety is important as a storage container for liquefied natural gas, containers with a structure covered with concrete on the outside are preferred. The main material properties required for reinforced steel bars for concrete reinforcement are yield strength of 42Kgf/ ^mm2 or higher, -120℃
Shyalpy impact value is 10Kgfm/ ^cm2 or more,
Furthermore, it has excellent corrosion resistance. Low-temperature steel materials used at around -120℃ are mainly thick plates made of Ni-based alloy steel, such as 3.5% Ni or 6% Ni steel, which are heat-treated by normalizing, quenching and tempering. .
However, when applied to long and slender reinforcing steel bars, bending is inevitable, requiring a separate straightening process, and the manufacturing cost increases significantly. One of the simple heat treatment methods for such reinforcing steel bars is to rapidly cool red-hot steel material in the temperature range of the austenitic structure immediately after hot rolling, and quench only the surface layer, as shown in Japanese Patent Application Laid-Open No. 135413/1983. A method of curing has been proposed. This method causes less bending due to heat treatment and is a relatively inexpensive manufacturing method. However, with this method, the inside was not hardened, and therefore sufficient toughness could not be obtained because the amount of C was increased to improve strength. (Problems to be Solved by the Invention) A steel bar structure produced by the conventional method of quench hardening only the surface portion has a martensite structure on the surface and a ferrite/pearlite structure inside. It is possible to obtain a sharpy impact value of 10 Kgfm/cm ² or higher in the martensite part of the surface by adding expensive Ni, but sufficient toughness cannot be obtained in the ferrite/pearlite part inside. The present invention provides a method for manufacturing a reinforcing steel bar having sufficient toughness and excellent low-temperature toughness at low cost. In other words, the present inventors investigated various surface hardening methods for ordinary low carbon steel, and found that bainite + ferrite steel consisting mainly of bainite with some ferrite structure mixed in is sufficient over the entire cross section. It was found that it exhibits excellent strength and toughness. Based on the above findings, the present invention provides a method for producing a reinforcing steel bar with extremely excellent low-temperature toughness. (Means and effects for solving the problems) That is, the gist of the present invention is as follows: (1) C: 0.02 to 0.10%, Si: 0.5% or less, Mn:
1.50-2.00%, P: 0.010% or less, S: 0.010
% or less, Al: 0.010~0.080%, N: 0.0020~
Using a steel material containing 0.0070% Fe and the remainder consisting of Fe and unavoidable impurities, rolling in the finish rolling process was performed at a processing interval of 5 seconds or less and a cumulative reduction rate of 30%.
or more and the temperature ranges from 750°C to 650°C. Immediately thereafter, the surface of the steel material is cooled at a cooling rate of 30°C/sec or more and 150°C/sec or less,
A method for producing a reinforced steel bar with excellent low-temperature toughness having a bainite + ferrite structure containing 20% or more of bainite structure, and (2) C: 0.02 to 0.10%, Si: 0.5% or less, Mn:
1.50-2.00%, P: 0.010% or less, S: 0.010%
Below, Al: 0.010~0.080%, N: 0.0020~
Using a steel material containing 0.0070% and further containing 0.01 to 0.30% of one or more of Nb or V, with the remainder consisting of Fe and unavoidable impurities, rolling in the finish rolling process is performed at a processing interval of 5 seconds or less and with a cumulative reduction. rate 30
% or more and end at a temperature range of 750℃~650℃. Immediately after that, the surface of the steel material is heated at 30℃/sec or more.
The present invention provides a method for producing a reinforcing steel bar having excellent low-temperature toughness and having a bainite + ferrite structure containing 20% or more of bainite structure by cooling at a cooling rate of 150° C./sec or less. The reason why the component range, manufacturing method, etc. were determined as described above in the present invention will be described. C: 0.02 to 0.10% is because C is an element with high hardenability and is effective in increasing strength, but if it is less than 0.02%, a bainitic structure cannot be obtained even by rapid cooling after rolling, resulting in a decrease in strength and toughness. do. Also 0.10
%, a martensitic structure and a large amount of carbides will precipitate and the toughness will deteriorate. Si is used as a deoxidizing agent. However, in consideration of corrosion resistance, it is desirable to have a smaller amount. Si becomes a tetravalent + ion on the steel surface and easily combines with - oxygen ions. As a result, the iron oxide on the surface layer is reduced and the iron is ionized, which reacts with chlorine ions and the oxygen that has penetrated into the concrete layer. It is oxidized and a rust layer is formed in a chain reaction. For this reason, deoxidizing
When using Al, it is desirable to reduce the amount as much as possible in order to suppress the formation of a rust layer. Like C, Mn is an element that increases the hardenability of steel materials and is effective in increasing strength.At the same time, Mn improves strength and toughness through grain refinement because it lowers the Ar ₃ transformation temperature. However, if it is less than 1.5%, a bainitic structure cannot be obtained even by rapid cooling, and the strength and toughness co-deteriorate. Moreover, if it exceeds 2.00%, a martensitic structure will appear and the toughness will deteriorate, further increasing the cost. P and S segregate at grain boundaries and significantly impede low-temperature toughness. In particular, S becomes MnS in steel and tends to become a starting point for rust. Therefore, the upper limit of these elements was set at 0.010%. Al is used as a deoxidizing element, and AlN
It precipitates, suppresses the growth of recrystallized grains, has the effect of refining crystal grains, and improves low-temperature toughness. For this, a minimum of 0.010% or more is required, and 0.080%
If it exceeds 0.0%, alumina-based inclusions will increase in addition to increasing the content, degrading the material, so the upper limit was set at 0.080%. N has a large effect on steel materials that have been aged, especially on steel materials that have been rapidly cooled after rolling, and deteriorates the bending workability and toughness of reinforcing bars, so a lower N content is desirable. However, a minimum content of 0.0020% or more is required for AlN formation, and if it exceeds 0.0070%, bending workability and toughness will significantly deteriorate. Nb has the effect of increasing the recrystallization temperature of steel. Therefore, it is used to increase the amount of processing in the non-recrystallized temperature range, refine the crystal grains, and improve toughness. V precipitates carbonitrides in the ferrite phase and grain boundaries, improving both strength and toughness through strengthening and grain refinement. For these reasons, both Nb and V are required to be at least 0.01%. Also 0.30%
If it exceeds this, the cost becomes high compared to the effectiveness, so the upper limit was set at 0.30%. Next, the reason for limiting the rolling conditions will be described. The lower the finish rolling temperature, the finer the crystal grains and the better the strength and toughness. Therefore, the upper limit temperature was set to 750°C in order to obtain the target Shalpy impact value vE _-120 ≧10Kgfm. However, below 650°C, the load on the rolling mill increases, further causing work hardening and deterioration of toughness. Toughness also greatly affects the rolling reduction during the finish rolling process. In order to obtain the target toughness, a reduction ratio of at least 30% is required. This 30
A reduction rate of % or more may be achieved in one pass, but
When performing several passes, the processing interval should be 5 seconds or less.
Exceeding this will reduce the cumulative effect. Cooling of the steel after rolling is necessary to achieve the desired structure and grain refinement. In order to obtain the 20% or more bainitic structure necessary to ensure the target strength and toughness, a cooling rate of at least 30°C/sec or higher is required, and at a cooling rate of 150°C/sec or higher, a martensitic structure that is harmful to toughness appears. do. The cooling may be carried out for a long period of time and the material may be kept in a water-cooled state, or the cooling may be stopped after several seconds and tempering may be performed by recuperating heat from the center. (Example) Next, an example of the present invention will be described. Table 1 shows the types and chemical components of the test materials.
In addition to the present invention steels a, b, and c as test materials, two steel types d and e having different compositions from the present invention steel were used as comparison steels. Each test piece was melted in a converter or test furnace, formed into a 120 mm square by blooming or forging, and then welded together to prepare six billets.
Heated to 1020°C. Next, as shown in Table 2, the steel of the present invention was rolled into a reinforcing bar of different diameter D25 under manufacturing conditions according to the invention and those other than the invention, and the comparative steel was rolled under manufacturing conditions according to the invention only. .
In other words, the finishing temperature of the steel of the present invention is heated to 850 and 720°C, and the reduction rate is 15% (in 1 pass) and 45% (in 3 passes).
), and the machining interval for 45% is

【table】

[Table] Tests were conducted at two levels: 1.5 and 6.3 seconds. Furthermore, after the rolling was completed, the surface layer portion was cooled from 280°C to 150°C. In this case, the cooling rate up to 500°C, at which the bainite transformation was completed, was carried out at three levels: 15, 134°C, and 210°C/sec. On the other hand, in the case of comparative steel, the reduction rate was 45
%, a processing interval of 1.5 sec, and a cooling rate of 134°C, and the rolling was completed at 720°C. For material testing, a tensile test was conducted with the different diameter shape intact, and for a Charpy impact test, a JIS No. 4 (2 mm V notch) test piece was cut from the middle portion and conducted at -120°C. Furthermore 20×40×2mm
Cut out a test piece with a thickness of 0.2 and adjust the PH value to 12.5.
A voltage was applied in a Ca(OH) ₂ aqueous solution containing % NaCl, and salt resistance was evaluated based on the magnitude of the applied voltage at which corrosion current was generated. Table 2 shows the material test results. Even if the steel of the present invention is used, if the finish rolling temperature exceeds 750°C, the toughness will be particularly low even if other manufacturing conditions are within the range of the present invention, making it impossible to satisfy the target (manufacturing method: A ₁ , B ₁ , _C1 ). Also, the finish rolling temperature is 750℃
Even if the following is within the scope of the present invention, the rolling reduction rate is outside the scope of the present invention (A ₂ ) or the processing interval is outside the scope of the present invention (A ₆ , B ₅ , C ₅ )
In this case, toughness is also insufficient. Furthermore, even if the rolling temperature, reduction rate, and working interval are within the range of the present invention, if the cooling rate is low (A ₃ , B ₂ , C ₂ ), a bainite structure cannot be obtained, resulting in insufficient strength and toughness. . When the cooling rate is high (A ₅ , B ₄ , C ₄ ), a martensitic structure is generated, which strengthens the reinforcing steel but lacks toughness. On the other hand, the strength and toughness of the reinforced steel bar manufactured by the method of the present invention fully satisfy the target values, and the toughness is particularly excellent at low temperatures. On the other hand, comparative steels are significantly lacking in toughness even when produced by the method of the present invention. Corrosion resistance is greatly influenced by the ingredients, and the steel of the present invention exhibits good corrosion resistance no matter which method it is produced. Comparative steel has inferior corrosion resistance because corrosion current flows even at a low voltage. (Effects of the Invention) As described above, the present invention provides a method for producing high-strength steel bars that are low-cost and have unprecedented low-temperature toughness and corrosion resistance. It is an extremely useful invention that compensates for the deficiencies of the law.

Claims (1)

[Claims] 1 Contains C: 0.02 to 0.10% Si: 0.5% or less Mn: 1.50 to 2.00% P: 0.010% or less S: 0.010% or less Al: 0.010 to 0.080% N: 0.0020 to 0.0070% with the remainder Using a steel material consisting of Fe and unavoidable impurities, the finishing rolling process is carried out at a processing interval of 5 seconds or less, a cumulative reduction rate of 30% or more, and a temperature of 750°C.
Termination ranges from ℃ to 650℃. A method for producing a reinforcing steel bar with excellent low-temperature toughness and having a bainite + ferrite structure containing 20% or more of bainite structure, by immediately cooling the surface of the steel material at a cooling rate of 30°C/sec or more and 150°C/sec or less. . 2 Contains C: 0.02-0.10% Si: 0.5% or less Mn: 1.50-2.00% P: 0.010% or less S: 0.010% or less Al: 0.010-0.080% N: 0.0020-0.0070%, and further contains 0.01-0.30% Using a steel material containing at least one type of Nb or V, with the remainder consisting of Fe and unavoidable impurities, rolling in the finish rolling process is carried out at a processing interval of 5 seconds or less, a cumulative reduction rate of 30% or more, and a temperature of 30% or more.
Finishes in the range of 750℃ to 650℃. A method for producing a reinforcing steel bar with excellent low-temperature toughness and having a bainite + ferrite structure containing 20% or more of bainite structure, by immediately cooling the surface of the steel material at a cooling rate of 30°C/sec or more and 150°C/sec or less. .