JPS61124524A - Manufacture of bar steel for steel reinforced concrete - Google Patents

Abstract

PURPOSE:To enlarge yield point elongation and improve workability, by rolling and heat treating slab contg. prescribed ratios of C, Mn, Mo, Nb, Al, etc., under prescribed conditions. CONSTITUTION:Steel consisting of, by weight 0.1-0.5% C, <=0.5% Si, 1.8-5% Mn, if necessary, further >=1 kind among 0.05-0.8% Mo, 0.01-0.1% Nb, 0.01-0.1% Al, 0.05-0.3% Cu, 0.05-1.2% Ni, etc., is melted. Slab of the steel is heated to stable austenitic temp. range, then rolled at said temp. range by >=80% draft. Then,the plate is cooled in air, and tempered at 450-700 deg.C. The bar steel has >=4% yield point elongation, small load stress against various plastic deformation received during working and superior workability.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a steel bar having a large elongation at yield point at room temperature, particularly a steel bar for reinforced concrete.

(Prior Art) In recent years, the scope of application of high-strength steel bars for reinforced concrete has been increasing more and more, and yield point elongation has been attracting attention as a required property for steel bars for reinforced concrete.

Here, the yield point elongation is the plateau area (
In the figure, it refers to the elongation value corresponding to the area between point A and point 8).

The steel bars for reinforced concrete currently in practical use are JIS
Its chemical composition and mechanical properties are specified in G3112, and these are used in a wide range of applications. However, this steel bar for reinforced concrete according to the JIS standard is subjected to normal hot rolling (after heating at 1100 to 1250°C,
It is manufactured by finishing rolling at approximately 000°C) and is used as-rolled. Based on the JIS standard (for steel bars for reinforced concrete, the elongation at yield point is about 1% at most, and it was impossible to increase this value even if heat treatment was performed after hot rolling.

(Problems to be Solved by the Invention) By the way, steel bars for reinforced concrete are generally subjected to various types of plastic deformation before construction. A typical example of this is bending, where local deformation of more than 1% is often applied. Therefore, the fact is that steel bars for reinforced concrete according to the JIS standard, which have a small elongation at yield point as mentioned above, undergo significant work hardening during bending, which increases the load required for bending and significantly impairs workability. Met. In particular, in the current situation where high-strength reinforcing bars of 5D40.5D50 are beginning to be used in large quantities, there is an urgent need to develop new reinforcing bars with excellent plastic workability, as typified by bending.

(Means for solving the problem) From the above-mentioned viewpoint, the present inventors conducted research and development on steel bars for reinforced concrete with a large elongation at yield point, which is expected to be in increasing demand in the future, and developed the steel bars as described below. After gaining knowledge and discovering that the elongation at yield point can be dramatically increased by combining the chemical composition of steel, rolling conditions, and heat treatment method, the present invention was completed.

Tanamitsuri 1 a) In order to increase the elongation at yield point, the microstructure of the steel should be changed like that of conventional JIS reinforcing bars (ferrite + pearlite).
A necessary condition is to change from a mixed structure to a (ferrite + bainite) structure.

b) In order to achieve the above a), it is essential to combine and adjust the chemical composition of the steel and the hot rolling conditions, especially the rolling reduction in the austenite region.

C) By tempering the mixed structure (ferrite + bainite) obtained in b) above in a predetermined temperature range, the elongation at yield point can be significantly increased.

d) By performing hot rolling while restricting the combination of steel hardenability and hot rolling conditions to specific ones, it is possible to obtain a mixed structure of ferrite and bainite as hot rolled. However, as hot-rolled, a large number of mobile dislocations exist in the bainite phase, and the yield strength is extremely low, and the yield point does not show any elongation. In this regard, the inventors
By subjecting the ferrite and bainite obtained by hot rolling to tempering at a temperature range of 450 to 700°C, mobile dislocations in the bainite are fixed by C, yield strength increases, and the ferrite phase is Due to the coexistence of the bainite phase with fixed mobile dislocations, plastic deformation starts locally from the interface of both phases during a tensile test of a real reinforcing bar.
It was discovered that the local deformation propagates in the longitudinal direction of the reinforcing steel as the tensile test progresses, and that such local deformation gives the yield point elongation.

In other words, in this range of yield point elongation, steel bars for reinforced concrete do not work harden even when subjected to strong processing such as bending, and by allowing local plastic deformation to propagate in the longitudinal direction of the reinforcing bars. Processing becomes easier, and workability is therefore significantly improved.

The present invention has been made based on the above findings, and the gist thereof is as follows: In weight%, C: o, io ~ 0.50%, Si: 0.50% or less, Mn: 1.80. % to 5.00% or less, and if necessary, further Mo: 0.05 to
0.80%, Nb: 0.010-0.100%,
Al: 0.010-0.100%, Cu: 0.
05-0.30%, Ni: 0.05-1.20%,
Cr: 0.05-1.20%, Ti: 0.0
1-0.05%, and B: 0.0005-0.
A steel slab containing one or more of 0.0030% and the balance consisting of Fe and unavoidable impurities is heated to a stable austenite temperature range, and then subjected to a rolling reduction of 80% or more in the austenite temperature range. This is a method for producing a steel bar for reinforced concrete with a large elongation at yield point, which is characterized by completing rolling, then allowing it to cool to room temperature in the atmosphere, and then tempering in a temperature range of 450 to 700°C.

However, when adding Al as a deoxidizing agent instead of Si, the deoxidizing agent is added in an amount of 0.02% or less, but the total amount with Al added as an alloy component is 0.010% to 0.1%.
00%, and generally 0.100% or less.

That is, according to another aspect of the present invention, C: 0.
10-0.50%, Al: 0.02% or less, Mn
: more than 1.80% and 5.00% or less, and if necessary, further contains Mo: 0.05 to 0.80
%, Nb: 0.010-0.100%, Al: 0.100% in total. % or less, Cu: 0.05-0.3
0%, Ni: 0.05-1.20%, Cr:
0.05-1.20%, Ti: 0.01-0.05
%, and B: 0.0005 to 0.0030%, after heating a steel slab with a composition consisting of Fe and inevitable impurities to a stable austenite temperature range, austenite is formed. High yield point elongation characterized by completing rolling with a reduction rate of 80% or more in a temperature range, then cooling in the atmosphere to room temperature, and then tempering in a temperature range of 450 to 700'C. This is a method for producing steel bars for reinforced concrete.

(Function) Next, in the method for manufacturing a steel bar for reinforced concrete according to the present invention, the reason why the component composition ratio of the steel and the hot rolling and heat treatment conditions are numerically limited as described above will be explained.

■Xiabosubunkuyu: C (Carbon) C is contained in order to impart a predetermined strength to the steel bar, but if its content is less than 0.10%, the desired strength cannot be obtained. Furthermore, C has the effect of significantly increasing the hardenability of steel, and in the present invention, in order to obtain a (ferrite + bainite) structure by air cooling after hot rolling, addition of 0.10% or more is required. is necessary. On the other hand, 0.50
If the C content exceeds 0.5%, the corrosion resistance of the steel bar for reinforced concrete when exposed to various environments will deteriorate significantly. Therefore, in the present invention, the upper limit of the C content is set to 0.50%.

Si (Silicon) The Si component is an element effective in deoxidizing steel, and is usually used as a deoxidizing agent in an amount of 0.15% or more, generally 0.15 to 0°3.
The amount of addition is approximately 5%. however,
When deoxidizing with Al, it is not necessarily necessary to add Si; on the other hand, if it is added in excess of 0.5%, hot workability will be adversely affected. For this reason, in the present invention, the Si content is set at 0.5% or less.

Mn (Manganese) The Mn component is an element necessary for desulfurization of steel, and also has the effect of improving the strength of the steel material by forming a solid solution in the base material of the steel and imparting a predetermined hardenability to the steel material. In order to produce a mixed structure of ferrite and heinite and give the steel a predetermined yield point elongation, which is a requirement of the present invention, 1
．． It is necessary to have a Mn content of more than 80%, while 5.
If the Mn content exceeds 0.00%, Mn segregation becomes significant and toughness and weldability deteriorate, so the Mn content was set to be more than 1.80% and 5.00% or less.

Mo (Molybdenum) The Mo component is an extremely effective element for imparting strength without impairing the toughness of steel, and is an optional component that can be added as desired. In addition, in the case of the method of the present invention, it is also an effective element for adjusting the hardenability of steel and obtaining a mixed structure of ferrite and bainite as rolled, and the above-mentioned C,
It can be added in addition to Mn if necessary. Mo
If the Mo content is less than 0.05%, the above effects will not be fully exhibited, and on the other hand, if the content exceeds 0.80%, the above effects will be saturated. 0
．． It was set at 80%.

Nb (niobium) Wb is an element generally used to refine the microstructure of steel, but it also has the effect of increasing the hardenability of steel by partially solidifying it during heating in the austenite region. Therefore, it is an effective element for obtaining the mixed structure of ferrite and bainite of the present invention. In this way, Nb is an element that can improve strength through refinement of the structure and improvement of hardenability, and can be added as necessary.

However, even when added, the Nb content is o, oto%
If it is less than 0.1, the above effect will not be fully exhibited;
If the Nb content exceeds 0.00%, excessive Nb carbonitride will be formed and hot workability will be inhibited, so the upper limit was set at 0.100%.

Al (Aluminum) In the case of the present invention, Al may be added as a deoxidizing agent in place of Si in an amount of about 0.02% or less, but if it exists in austenite as a partially solid solution Al, it can be added to the steel. It also has the effect of increasing the hardenability of. Therefore, in the case of the present invention, aluminum is also an optional additive component that can be added not only as a mere deoxidizing agent but also to improve strength as necessary.

In addition, in order to fully exhibit the effect of improving strength, the temperature should be 0.
It is necessary to add 0.010% or more. On the other hand, 0°100%
Since the hot workability of steel deteriorates if the Al content exceeds 0.00010% to 0.100%.

However, when added as a deoxidizer instead of St, Al
is the total amount from 0.010 to 0.100%, generally 0.1
00% or less.

Cu (Copper) The Cu component has almost no adverse effect on the toughness of the steel (it has the effect of increasing the strength. In particular, when tempering is performed after hot rolling as in the present invention, Cu is It can be precipitated during tempering to increase the strength.In the case of the present invention, Cu is an optionally added component.This precipitation of Cu
Since Cu occurs in either ferrite or bainite, addition of Cu is also effective in the mixed structure of ferrite and bainite of the present invention. However, if the content is less than 0.05%, the desired effect cannot be obtained, while if the content exceeds 0.30%, it will impair the hot workability of steel. amount from 0.05
It was set at 0.30%.

Since the Ni component is an element that is particularly effective in improving the low-temperature toughness of steel, it is used when further improvement in low-temperature toughness is required.
It can be added in an amount of 0.05% or more to make the effect noticeable. Ni is also effective in improving the hardenability of steel, and is also effective in improving strength, so it is particularly effective in obtaining a mixed structure of ferrite and heinite in large-diameter pJ steel for reinforced concrete. However, 1.
If the Ni content exceeds 20%, the cost of the steel material will increase, and the rate of occurrence of hydrogen-induced defects such as white spots will increase during manufacturing.
~1°20%.

Cr (Chromium) Since the Cr component has the effect of increasing the strength of steel, it is included when higher strength is required, but if the content is less than 0.05%, the desired effect may not be achieved. On the other hand, if Cr content exceeds 1.20%, hot workability deteriorates.
The content was determined to be 0.05 to 1.20%.

Ti (Titanium) When steel is heated to an austenite state, Ti partially forms a solid solution in austenite and has the effect of increasing the hardenability of the steel and thereby improving its strength.

Therefore, it can be used to obtain a mixed structure of ferrite and bainite as hot-rolled, and can be added as necessary. Further, when tempering is performed after hot rolling as in the present invention, Ti also has the effect of forming Ti carbonitrides and increasing the strength of the steel. In the case of the present invention, Ti is also a desired additive element. In order to demonstrate the above 91 results, 0.01
It is necessary to add more than %. On the other hand, if Ti is added in an amount exceeding 0.05%, the hot workability of the steel deteriorates, so when Ti is added, the Ti content is set at 0.01 to 0.05%.

B (Poron) Component B has the effect of improving the hardenability of steel when added in small amounts, so it is added as an optional component when it is necessary to further increase the strength of steel materials. If the B content is less than 0.0005%, the desired effect cannot be obtained, while if the B content exceeds 0.0030%, hot workability will deteriorate. ,o
It was set as oos~0.0030%.

In addition, in the present invention, the above-mentioned desired additive component 0
, Nb, , Al, , Cu, , Ni, , Cr, Ti, and B all ultimately contribute to improving the strength of the material, and they are necessary to obtain the high-strength steel bars that are required today. At least one kind is added.

Rolling treatment treatment raw A raw: The heating conditions before rolling are a normal stable austenite region heating temperature, for example, 1100 to 1250'c, and are not particularly limited. Even if the steel billet is heated to a temperature range below 1100°C or above 1250°C due to the required performance other than elongation at yield point, the effect of the present invention will not be impaired as long as the heating is in the stable austenite range. .

The total rolling reduction in the austenite region needs to be 80% or more. If the rolling reduction is less than 80%, the ferrite grains and bainite grains obtained when allowed to cool to room temperature after rolling become coarse, making it impossible to obtain a sufficient elongation at yield point even after tempering. be.

Next, regarding the tempering conditions, in the present invention, tempering is performed at 450°C.
This is carried out in a temperature range of ~700°C, which is an essential condition for obtaining a reinforcing bar with a large elongation at yield point, which is the objective of the present invention. That is, when the tempering temperature is less than 450°C,
The fixation of mobile dislocations in the bainite phase by C in the steel is incomplete, and no local plastic deformation occurs in the tensile test of reinforcing bars, making it impossible to obtain yield point elongation. On the other hand, the tempering temperature is 700
When the temperature exceeds ℃, the bainite phase recovers and recrystallizes,
As the mobile dislocations disappear, the structure becomes substantially a single layer of ferrite, resulting in a significant decrease in elongation at yield point. Therefore, the tempering temperature range is 450 to 700℃.
And so.

Next, the effects achieved by the present invention will be explained using specific examples.

First, steel having the composition shown in Table 1 was melted using a conventional melting method, and thereby a billet of 1601 squares was manufactured. This billet was heated to 1200°C and rolled for 16 passes to obtain a D32 steel bar for reinforced concrete. The reduction ratio due to this hot rolling was 97%. As-rolled D
The microstructure of the No. 32 reinforced steel bar was investigated using an optical microscope.

Further, the as-rolled reinforcing steel bar was tempered at 600°C for 11 hours and then allowed to cool to room temperature.

The reinforcing bars thus obtained were subjected to actual tensile tests to determine yield strength, tensile strength, total elongation, and elongation at yield point. The total elongation is due to a gage distance of 160 mm. Further, the elongation at yield point was determined by setting an extensometer with a span of 1601 on an actual reinforcing bar, recording a load-displacement curve, and using the chart.

The results obtained are listed in Table 1. Steels 1 to 26 have chemical compositions that satisfy the requirements of the present invention, and in combination with the rolling conditions and tempering treatment according to the present invention, yield point elongation values of 4% or more are obtained. On the other hand, steel 27~
Comparative steel No. 31 has the same rolling and heat treatment conditions as the inventive example, but has a chemical composition that deviates from the requirements of the present invention. The yield point elongation of these steels is 1% or less, and the yield point elongation value is significantly lower than that of Steels 1 to 26. In other words, it can be seen that even if the rolling and tempering conditions according to the present invention are applied, if the chemical components do not meet the requirements of the present invention, the desired performance cannot be obtained.

Using the ml shown in Table 1, the effects of the rolling reduction during hot rolling and the tempering conditions were investigated. The steel billet size is 16
Four types were prepared: 0o++m square, 90fflI11 square, 6311I11 square, and 59R111 square. After heating each steel piece to 1200°C, it was rolled into a D32 reinforced concrete bar. The rolling reduction rate is steel billet size 160mm square,
90++uw angle, 63mn+angle and 59IIl111
97%, 90%, 80% and 7 corresponding to the corners respectively
It is 7%. The as-rolled reinforcing bars were then heated to 430°C and 450°C.
℃, 600℃, 700℃, and 720℃ after lhr tempering, the actual reinforcing bars were subjected to a tensile test in the same manner as in Example 1. The results are summarized in Table 2.

As can be seen from the results in Table 2, the yield point elongation is zero at any rolling reduction ratio when as rolled. At three levels of rolling reduction of 80.90.97%, the elongation at yield point increases as the tempering temperature increases, but at 430°C tempering, the elongation at yield point is less than 1%, which is the object of the present invention. A large elongation at yield point was not obtained. Tempering temperature is 450
Within the range of ~700°C, the elongation at yield point increases significantly and reaches a value of 4% or more. However, the tempering temperature is 700℃
If it exceeds 1%, the yield point elongation becomes 1% or less again. On the other hand, if the rolling reduction is less than 80%, the yield point elongation is less than 1% regardless of the tempering temperature, and the performance aimed at by the present invention cannot be obtained.

γ pg in Table 2) Actual] Itsu 1 Using a 160 mm square piece of steel 1, heat the piece at a temperature of 125.
0.1200.1100.1000.900 '5 of C
The standard was selected and hot rolled into a D32 reinforced concrete bar. After rolling, it was subjected to a tempering treatment of 600'C for 11 hours, and a tensile test was conducted in the same manner as in Example 1. The results are summarized in Table 3.

As is clear from Table 3, a yield point elongation of 4% or more was obtained at any heating temperature in the stable austenite temperature range of 1250 to 900°C, and the billet heating temperature is not particularly limited in the present invention. It is clear that there is no need.

Table 3 (Note) F: Ferrite, B: Paynite (Effect) As described above, reinforced concrete having the chemical composition specified in the present invention and obtained under the rolling and heat treatment conditions according to the present invention. -1. The steel for use has a yield point elongation of 4% or more, and is a product with extremely excellent workability, with a small load compared to the various plastic deformations that Keyaki steel for reinforced concrete IJ-G undergoes during construction.

[Brief explanation of the drawing]

The accompanying drawing is a graph showing a stress-strain curve for explaining elongation at yield point.

Claims (2)

[Claims] (1) In weight%, C: 0.10 to 0.50%, Si: 0.50% or less, M
n: more than 1.80% and 5.00% or less, and if necessary, further contains Mo: 0.05-0.80%, Nb: 0.010-0.
100%, Al: 0.010-0.100%, Cu: 0
．． 05-0.30%, Ni: 0.05-1.20%, C
r: 0.05-1.20%, Ti: 0.01-0.05
%, and B: 0.0005 to 0.0030%, after heating a steel slab with a composition consisting of Fe and inevitable impurities to a stable austenite temperature range, austenite is formed. Rolling is completed by applying a rolling reduction of 80% or more in a temperature range, then allowed to cool to room temperature in the atmosphere, and then tempered in a temperature range of 450 to 700°C, which has a large yield point elongation. Method for producing steel bars for reinforced concrete. (2) In weight%, C: 0.10 to 0.50%, Al: 0.02% or less, M
n: more than 1.80% and 5.00% or less, and if necessary, further contains Mo: 0.05-0.80%, Nb: 0.010-0.
100%, Al: 0.100% or less in total, Cu: 0.
05-0.30%, Ni: 0.05-1.20%, Cr
:0.05~1.20%, Ti:0.01~0.05%
, and B: 0.0005 to 0.0030%, after heating a steel piece with a composition consisting of Fe and unavoidable impurities to a stable austenite temperature range, the austenite temperature Reinforced concrete with a high elongation at yield point, characterized by completing rolling with a reduction rate of 80% or more at a temperature range of 80% or more, then cooling to room temperature in the atmosphere, and then tempering at a temperature range of 450 to 700°C. Method for producing steel bars for industrial use.