JPS62287012A - Production of low-temperature reinforcing bar having excellent weldability - Google Patents

Abstract

PURPOSE:To produce a low-temp. reinforcing bar having excellent weldability by specifying respective conditions for heating, finish rolling, and cooling of steel products contg. specific ratios of C, Si, Mn, Ni, Ti, N, Al, and B. CONSTITUTION:The steel products contg., by weight %, 0.02-0.12% C, 0.05-0.5% Si, 0.5-2.0% Mn, 0.2-1.5% Ni, 0.04-0.15% Ti, <=0.0005% N, 0.01-0.07% Al, and 0.0005-0.0030% B, and consisting of the balance substantially Fe are prepd. Such steel products are heated to 850-1,050 deg.C and are rolled. The rolled steel products are finished at 780-680 deg.C finishing temp. and >=15% finish draft and are cooled at >=10 deg.C/sec down to 500 deg.C. The low-temp. reinforcing bar having martensite and bainite structure is thereby obtd. and the deterioration of the low-temp. toughness in a weld heat affected zone is prevented.

Description

[Detailed Description of the Invention] 3. Detailed Description of the Invention [Industrial Application G1 The Japanese invention relates to a method for manufacturing a low-temperature reinforcing bar with excellent weldability,
More specifically, the present invention relates to a method for producing a low-temperature reinforcing bar with excellent weldability and excellent low-temperature toughness in the weld heat-affected zone.

``Prior art'' Conventionally, reinforcing bars used at very low temperatures have a C0.02~
0.17wt%, Si 0.7wt% or less, Mn 0.6
~2. Owt%, containing A I 0.01 to 0.07wt% j7, the remainder substantially consisting of Fe 850 to 1
Rolling is performed after heating to a temperature of 050°C, and the finishing temperature is 75°C.
After finishing at a temperature of 0°C ± 30°C, it is allowed to cool or forcedly cooled, or it is rapidly cooled after rolling to prevent the formation of bainite and pearlite, and then tempered to below AC1 point. ing.

However, although the low-temperature reinforcing bars manufactured in this manner have low-temperature toughness, the low-l crystal toughness of the weld heat-affected zone is significantly deteriorated due to the influence of heat during welding. In other words, due to the heat effect of welding, the weld heat affected zone
-, the grain size becomes coarser, and bainite is formed in some parts of the structure, resulting in a decrease in toughness.

[Problems to be Solved by the Invention] In view of the problems of the conventional low-temperature reinforcing bars explained in ``-'', the present inventor has conducted intensive research and as a result of repeated examinations, the content of steel and the content ratio, and
By specifying the heat treatment process, we developed a manufacturing method for low-temperature reinforcing bars with excellent weldability that can prevent deterioration of the low-temperature toughness of the weld heat-affected zone.

[Means for Solving the Problems] The method for producing a low-temperature reinforcing bar with excellent weldability according to the present invention is characterized by: C0.02-0. ]2wt%, S i 0.05-0.
5wt%, Mn 0.5-2.0wt%, Ni 0.2
~1.5wt%, Ti 0.04~0.15wt%, N
0.005wt% or less, Al 0.01-0.07
wt%, B 0.0005 to 0.0030 wt%, and the balance substantially consists of Fe, 850 to 1050
After heating to a temperature of ℃, rolling is performed, and the finishing temperature is 780 to 68℃.
0℃, finishing at 500℃ after finishing at a rolling rate of 15% or more
7 to 10° C./sec or more to form a martensite and bainite structure.

The manufacturing method of the low-temperature cylindrical rod having excellent weldability according to the present invention will be described in detail below.

First, the components and content ratios of the steel used in the method for manufacturing a low-temperature reinforcing bar with excellent weldability according to the present invention will be explained.

C is an element that guarantees the strength of a reinforcing steel bar, and if the content is less than 0.02 wt%, this effect cannot be obtained, and if the content exceeds 0.12 wt%, the low temperature toughness decreases. Therefore, the C content is set to 0.02 to 0.12 wt%.

The purpose of Si is to deoxidize, and if the content is less than 0.05 wt%, the deoxidizing effect will be small, and if the content exceeds 0.5 wt%, the toughness will deteriorate. Therefore, the Si content is set to 0.05 to 0.5 wt%.

Mn is an element that ensures strength and hardenability, and if the content is less than 0.5 wt%, this effect will be small, and if it is too large, the toughness will decrease due to the combination of B content, so the upper limit of the content is 2. Set it to 0wt%. Therefore, the Mn content is 05~
The content shall be 2.0 wt%.

Ni is an element that improves low temperature toughness, and the content is 0.
, less than 2 wt%, this effect is small;
If the content exceeds wt%, the effect will be saturated and it will become expensive. Therefore, the Ni content is set to 0.2 to 1.5 wt%.

Ti is an element that fixes N, and the content is 0.04wt
%, this effect is small, and if it is less than 0. If the content exceeds 15vt%, the toughness will deteriorate. Therefore, the Ti content is set to 0.04 to 0.15 wt%.

N is an element that inhibits the hardenability of B, and its content is 0.
．． If B is contained in an amount exceeding 0.05 wt%, the hardenability effect of B is small. Therefore, the N content is set to 0.005 wt% or less.

AI is an element necessary for sufficient deoxidation and crystal grain refinement, and if the content is less than 0.01 wt2, such effects will be small; If it is contained in a large amount exceeding Oht%, the surface quality will deteriorate. Therefore, the AI content is 0.
01 to 0.07 wt%.

B is an element for improving hardenability, and if the content is less than 0.0005 wt%, this effect is small, and
If the content exceeds 0.0030 wt%, hot workability will deteriorate. Therefore, the B content is 0.0005 to 0.0
030wt%.

Next, the rolling process of the method for manufacturing a low-temperature reinforcing bar with excellent weldability according to the present invention will be explained.

It is desirable that the rolling heating temperature is low in order to prevent coarsening of austenite crystal grains, but if the heating temperature is too low, AIN will increase and it will be easy to crack during rolling, so the rolling heating temperature should be 850~850°C. The temperature shall be 1050°C.

As for the finish rolling temperature, low temperature finish rolling is effective for refining crystal grains, but if the temperature is too low, distortion will remain in the crystal grains and deteriorate low temperature toughness, so the lower limit is 680.
The finishing rolling temperature is set at 680 to 780°C, since crystal grains become coarse when the temperature exceeds 780°C.

The finishing rolling rate is required to be 15% or more, and if the rolling rate is low, coarse grains will result, which will deteriorate low-temperature toughness.

After finish rolling, the grain size becomes fine with a grain size number of 10 or more, making it easy for the crystal grains to grow rapidly. Rapid cooling is performed at a cooling rate of 10° C./second or more to refine the crystal grains and prevent formation of a pearlite structure.

In this way, since the crystal grains in the weld heat-affected zone of the produced low-temperature reinforcing bar (J welding) do not undergo the heat influence, deterioration in low-temperature toughness can be prevented.

"Example" An example of the J8 method for manufacturing a low-temperature reinforcing bar with excellent weldability according to the present invention will be described.

Examples Steels having the components and content ratios shown in (1) of Table 1 were melted, and after blooming into steel slabs, they were rolled under the 4-rolling conditions shown in (2) of Table 1.

The heating temperature can be controlled by water cooling during rolling.
The final rolling rate was set to 20%, and the finishing method was carried out using a 25 mm diameter bar, which was heated to a temperature of 500°C by cooling with water after exiting the finishing roll. The average cooling rate was adjusted to 50°C/sec and 20'C/sec. Note that the cooling rate when water cooling is not performed (J average is 3° C./sec.

Table 1 (2) shows the results of a tensile test and a 2mmV notch glossy bee impact test on rolled steel bars, as well as a 2mmV notch glossy bee impact test on welded heat-affected zones of rolled steel bars.

From this Table 1, it can be seen that the weldability of the present invention (42) The reinforcing bar manufactured by a method other than the manufacturing method of excellent low-temperature reinforcing bar has a high weldability of vTrs, -60°C or lower, and a welding heat-affected zone. v'l
'rs is also high at -40℃ or more, but the manufacturing method of low-temperature reinforcing bars with excellent weldability according to the present invention
It can be seen that it has excellent low temperature toughness.

17 = "Effects of the Invention" As explained above in page 4, the method for manufacturing the low-temperature reinforcing bar with excellent weldability according to the present invention has the configuration described in -1-, so when welded during use, This has the effect that the welding heat at the welding point and the low-temperature toughness of the welding area are significantly improved.

Claims (1)

[Claims] C0.02~0.12wt%, Si0.05~0.5w
t%, Mn0.5-2.0wt%, Ni0.2-1.5
wt%, Ti0.04-0.15wt%, N0.005
wt% or less, Al0.01-0.07wt%, B0.0
005 to 0.0030 wt%, the remainder being substantially F
The steel material consisting of e is heated to a temperature of 850 to 1050°C and then rolled, finished at a finishing temperature of 780 to 680°C and a finishing rolling rate of 15% or more, and then cooled to 500°C at a rate of 10°C/second or more to produce martensite and bainite. A method for producing a low-temperature reinforcing bar with excellent weldability characterized by a microstructure.