JP2000144247A - Production of high strength reinforcing bar - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain excellent yield extensibility without requiring large scale rolling equipment, when heating a steel having a specified compsn. and next subjecting it to hot rolling to produce a reinforcing bar by controlling the rough rolling temp., intermediate rolling temp. and rolling finishing temp. to the specified ones. SOLUTION: This is a method for subjecting steel composed of, by weight, 0.20 to 0.50% C, 0.15 to 1.50% Si, 0.50 to 1.80% Mn, 0.02 to 0.50% Cu, 0.02 to 0.50% Cr, 0 to 0.50% Ni, 0.05 to 0.40% V, 0.01 to 0.10% Nb, 0.005 to 0.02% N, and the balance Fe or the like to heating in a heating furnace and hot rolling, and the rough rolling temp. is controlled to 830 to 900 deg.C, the intermediate rolling temp. to 890 to 950 deg.C, and the rolling finishing temp. to >950 to 1050 deg.C so that the yield elongation percentage of the reinforcing bar [the ratio (ε-εy)/εy between the strain εy at the starting point of yielding and the strain (ε-εy) till stress furthermore reaches 755 N/mm2 therefrom] is controlled to the prescribed value, preferably to >=5.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high-strength rebar used as a straight bar for a main bar having a yield stress of 685 N / mm ² or more, which is excellent in yield elongation and bending workability.

[0002]

^2. Description of the Related Art A method for manufacturing a high-strength rebar for a main rebar having a yield stress of 685 N / mm ² or more is described in JP-A-9-324215. This production method has a C: 0.25
0.40% (the same applies to weight% or less), Si: 0.25 to 2.
0%, Mn + Cr: 1.0 to 3.0% (including Mn = 0% or Cr = 0%), V: 0.2 to 0.5%, Nb:
0.010 to 0.10%, Al: 0.005 to 0.10
%, N: 0.002 to 0.020%, balance: Fe and steel satisfying inevitable impurities, heating temperature: 1000 to 11
Rolling is performed at 50 ° C. and a rolling end temperature of more than 850 to 950 ° C.

[0003] In this manufacturing method, finish roll rolling is performed at a low temperature which is advantageous for refining the metallographic structure, thereby improving the yield elongation of the reinforcing steel. However, finishing roll rolling at low temperature,
Since the rolling resistance is inevitably increased, large-scale rolling equipment having a large load capacity is required, and the cost is increased. or,
A steel material for high-strength shear reinforcement comprising substantially the same steel as the above components and a method for producing the same are described in JP-A-10-121200. However, since the shear reinforcing bar has a smaller diameter than the main reinforcing bar and is used by welding in a coil shape, the bending property and the weldability are important, but the yield elongation is not so required. Therefore, the rolling temperature is also set from the bendability and the weldability, and the yield elongation is not considered.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a low-cost method of producing a high-strength rebar having excellent yield elongation for a main rebar by finishing roll rolling in a high-temperature region which does not require large-scale rolling equipment. Is to provide. The inventor of the present application
C: 0.20 to 0.50% (the same applies to weight% or less), Si:
0.15 to 1.50%, Mn: 0.50 to 1.80%,
Cu: 0.02-0.50%, Cr: 0.02-0.5
0%, Ni: 0 to 0.50%, V: 0.05 to 0.40
%, Nb: 0.010 to 0.10%, N: 0.005 to
A test for producing a high-strength rebar having a yield stress of 685 N / mm ² or more by hot rolling steel containing 0.02% and the balance: Fe and unavoidable impurities has been repeatedly performed. As a result, it has been found that even when the finish roll rolling is performed in a high temperature region where the rolling resistance is relatively small, the yield elongation of the reinforcing bar can be significantly improved.

[0005]

Means for Solving the Problems In order to achieve the above-mentioned objects, means adopted by the present invention is as follows: C: 0.20 to 0.50
% (The same applies to weight% or less), Si: 0.15 to 1.50%,
Mn: 0.50 to 1.80%, Cu: 0.02 to 0.5
0%, Cr: 0.02 to 0.50%, Ni: 0 to 0.5
0%, V: 0.05 to 0.40%, Nb: 0.010 to
0.10%, N: 0.005 to 0.02%, balance: Fe
And a steel comprising inevitable impurities is heated in a heating furnace and then hot-rolled to produce a rebar. In a method for producing a rebar, the yield elongation of the rebar (strain ε _{y at the} yield starting point, and further stress of 755 N / mm until it reaches mm ² (ε-
Ratio _{(ε-ε y) / ε} y) is a predetermined value of the epsilon _y), for example, as a 5.0 or higher, a rough rolling temperature 830～
900 ° C, an intermediate roll rolling temperature of 890 to 950 ° C, and a rolling end temperature of more than 950 to 1050 ° C.
Here, preferably, the atmosphere temperature in the heating furnace is more than 1150 to 1250 ° C. It is not necessary to forcibly cool the rebar after rolling, and it is also possible to leave it at room temperature and cool it naturally.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that the steel composition and hot rolling conditions are specified.
The reasons for setting the requirements specified in the present invention will be described. First, basic chemical components will be described. C: 0.20 to 0.50% by weight C is an element indispensable for securing the yield stress of the reinforcing bar, and therefore, it must be contained at least 0.20% by weight or more. However, if the amount of C is too large, ductility and bendability deteriorate, so the upper limit must be suppressed to 0.50% by weight or less. Si: 0.15 to 1.50% by weight Si is an element that effectively acts on deoxidation at the time of melting steel material and also contributes to improvement of yield stress as a solid solution strengthening element. In order to effectively exert such an effect, the content must be at least 0.15% by weight or more. However, if the content is too large, ductility and bending workability decrease, so the upper limit is 1.5% by weight.

Mn: 0.50 to 1.80% by weight Mn is an element that lowers the pearlite transformation temperature during the cooling process after rolling, thereby contributing to increasing the strength of the rolled material. The yield stress of the rolled material is 685 N / mm
^In order to secure ² or more, it is necessary to contain at least 0.50% by weight or more in total. However, when added in a large amount, the formation of a ferrite structure is remarkably suppressed, and the yield elongation required for the reinforcing steel is reduced. Therefore, the upper limit is 1.80% by weight. Cr: 0.20 to 0.50% by weight Cr is an element having the same effect as Mn. But M
Since n is inexpensive, but Cr is an expensive element, it is preferable to add Mn as much as possible than Cr and to keep the amount of Cr naturally contained in the raw material in consideration of economy.

V: 0.05 to 0.40% by weight V is an element effective as a precipitation strengthening element. In order to secure the intended yield stress of 685 N / mm @ 2 in the present invention, it is necessary to add at least 0.05% by weight or more. In order to enhance the precipitation strengthening effect by the addition of V, it is necessary to increase the amount of solid solution V at the time of rolling and heating. However, if a large amount of V is added, it becomes difficult for V carbide to form a solid solution at the time of rolling and heating. Cannot effectively exert the effect of strengthening the precipitated steel. Therefore, the upper limit is set to 0.40% by weight. Nb: 0.010 to 0.10% by weight Nb has an effect of refining crystal grains after rolling and improving ductility of a rolled material. In order to effectively exert such an effect, it is necessary to add 0.010% by weight or more. However, even if it is added in excess of 0.10% by weight, its action is saturated and is economically useless.

N: 0.005 to 0.02% by weight N forms a carbonitride in ferrite with V and Nb,
It has the effect of increasing the strength and refining the crystal grains to make them tougher. However, if the content is less than 0.005% by weight, there is no effect, and if it is 0.02% by weight or more, the toughness is reduced. Cu: 0.02 to 0.50% by weight Cu has the effect of increasing the strength of the rolled material. If the content is less than 0.02% by weight, there is no effect, and if it exceeds 0.50% by weight, the effect is saturated. Ni: 0 to 0.50% by weight Ni enhances the strength of the rolled material, but if it exceeds 0.50% by weight, the effect is saturated and the cost increases.

Next, the rolling conditions will be described. Heating temperature: 1000 ° C. or higher, preferably more than 1150 to 1
250 ° C. The ambient temperature in the heating furnace in the preceding step plays an important role in strength and yield elongation. If the temperature is lower than 1000 ° C., satisfactory results cannot be obtained. Preferably, the temperature is higher than 1150 to 1250 ° C., and rolling is started in a state where Nb is sufficiently dissolved in the austenitic matrix.

As a result of the test, the inventor of the present invention found that in the three-stage rolling of the steel comprising the above components, the rough rolling and the intermediate rolling before the finish rolling were performed at the A3 transformation point (950).
° C) to the unrecrystallized γ-range rolling in the temperature range from the Ar3 transformation point, and the finishing roll rolling is also performed at a high temperature of 950 ° C or higher at a recrystallization rolling temperature of 950 ° C or more. The present inventors have found that a rebar having excellent extensibility can be obtained and completed the present invention. The temperature range is as follows. Coarse roll rolling temperature: 830 to 900 ° C Intermediate roll rolling temperature: 890 to 950 ° C Finish roll rolling finishing temperature: more than 950 to 1050 ° C After rolling is completed, the roll is cooled to 400 ° C at 30 to 80 ° C / min. This cooling can be by natural cooling left at room temperature.

The metal structure of the produced reinforcing bar has an area ratio of precipitated ferrite of 30 to 60%, a maximum particle size of 25 μm or less, and an average particle size of 15 μm or less. The reinforcement of the reinforcing bar is realized by the refinement of the grain size, and the high yield elongation of the reinforcing bar is realized by the value of the ferrite area ratio. As described above, the production method of the present invention performs finishing roll rolling at a high rolling resistance at a high temperature of more than 950 ° C., so that a large-scale rolling facility is unnecessary,
Equipment and operating costs are greatly reduced.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described based on a tensile test of a reinforcing bar. The reinforcing bars used in the test were the same as those in Example NO. 1. Example N
O. 2, Comparative Example NO. 3, Comparative Example NO. 4 types. The example and the comparative example are reinforcing bars of the same size manufactured from the same steel material, and the component range of the steel material is as follows.

C: 0.27 to 0.36% by weight Si: 0.27 to 0.68% by weight Mn: 1.36 to 1.59% by weight Cr: 0.14 to 0.20% by weight V: 0 0.20 to 0.38% by weight Nb: 0.02 to 0.06% by weight N: 0.005 to 0.02% by weight Remainder: Fe and unavoidable impurities Examples are
The rolling conditions are conventional. Reinforcing bars of Examples and Comparative Examples
At the ambient temperature in the heating furnace shown in the table below.
Hot rolling at the rolling end temperature shown in the table below.
It was manufactured. After manufacturing, each rebar is a tensile tester
, And a stress-strain curve shown in FIG. 1 was drawn.
From the stress-strain curve, the stress was 755 from the yield starting point.
N / mm ^Two Strain to reach (ε-ε_y) And surrender
Starting strain ε_yYield elongation (ε-ε_y) /
ε_yI asked. Yield elongation is as shown in the table below.
Was.

Reinforcing bar NO. NO. 1 NO. 2 NO. 3 NO. 4 types Example Example Example Comparative example Comparative example Atmospheric temperature in heating furnace ° C 1150 1200 1050 1150 Rolling end temperature ° C 1000 1050 850 950 Yield stress (N / mm ² ) 715 710 690 691 Yield elongation rate 6.1 5.8 4 from _{.5 5.2 (ε-ε y)} / ε y this table, rebar produced by the method of the present invention, yield stress 685N / mm ² or more is a high strength reinforcing bar, compared with the conventional method, the yield It turns out that elongation is remarkably excellent.

[0016]

As described above, the method for producing a high-strength rebar used as a main rebar from steel containing V and Nb according to the present invention is a method of producing a high-strength rebar having a large rolling load by lowering the rolling resistance to 950.degree. Unlike the conventional one that required a large-scale rolling equipment to perform at a temperature, the rough roll rolling and the intermediate roll rolling with a small rolling load are performed at 950 ° C.
Although it is carried out at the following low temperature, finishing roll rolling with a large rolling load has a relatively small rolling resistance of more than 950 to 1050 ° C.
Since large-scale rolling equipment is not required, it is possible to produce low-cost rebar with excellent yield elongation by using small and medium-sized rolling equipment with low equipment costs and operating costs. It has excellent effects.

[Brief description of the drawings]

FIG. 1 Stress-strain curve diagram

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Toshio Sekiguchi 520 Yokokura Nitta, Oyama City, Tochigi Prefecture F-term (reference) 4K032 AA05 AA11 AA14 AA16 AA21 AA22 AA23 AA31 AA32 AA36 BA02 CC04 CD05

Claims (3)

[Claims] 1. C: 0.20 to 0.50% (the same applies to weight% or less), Si: 0.15 to 1.50%, Mn: 0.50
１1.80%, Cu: 0.02 to 0.50%, Cr:
0.02 to 0.50%, Ni: 0 to 0.50%, V:
0.05 to 0.40%, Nb: 0.01 to 0.10%,
N: 0.005 to 0.02%, balance: Fe and unavoidable impurities are heated in a heating furnace, and then hot-rolled to produce a reinforcing bar. The strain ε _{y at the} yield start point and the strain (ε−) until the stress further reaches 755 N / mm ^2.
ratio of _{ε y) (ε-ε y} ) / ε y) such that a predetermined value or more, the rough rolling temperature eight hundred thirty to nine hundred ° C., the intermediate rolling temperature eight hundred ninety to nine hundred fifty ° C., the rolling end temperature 95
A method for producing a high-strength rebar, wherein the temperature is higher than 0 to 1,050 ° C. 2. The method according to claim 1, wherein the predetermined value of the yield elongation is 5.0 or more. 3. The method for producing a high-strength reinforcing bar according to claim 1, wherein the reinforcing bar after the rolling is allowed to cool naturally at room temperature.