JPS6152350A - Large diameter, high strength rolled steel bar - Google Patents

Abstract

PURPOSE:To obtain a rolled steel bar which has a large diameter, high strength, the toughness and to which addition of expensive element for improving quenchability is suppressed as low as possible, by controlling pearlite transformation temp. in patenting large diameter steel bar. CONSTITUTION:The steel bar consists of, by weight 0.6-0.9% C, 0.25-2.0 Si, 0.5-2.0% Mn, 0.3-1.0% Cr and the balance Fe, and has >=20mm. diameter, >=120kg/mm.<2> tensile strength, >=20% reduction of area. In case the steel bar heated to austenite temp. range is cooled at equal rate for obtaining said bar, critical temp. in contact with pearlite transformation starting line in continuous cooling transformation curve thereof is denoted as Tc. Pearlite transformation is started in Tc-(Tc+40 deg.C) temp. range, and said bar is cooled to control so that max. temp. during said transformation is suppressed in <=Tc+80 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention aims to provide a large-diameter rolled steel bar with excellent strength and toughness.

[Conventional technology]

Conventionally, hot-rolled steel bars are cooled using a lead bath.
B patenting, air patenting, and hot water patenting are known, but there are problems with any of these methods. That is, although the Pb patenting method can produce rolled steel bars with excellent strength, the use of a lead bath causes a deterioration of the working environment and poses problems in terms of pollution. In addition, in the air patenting method and hot water patenting method, pb
Compared to the patenting method, cooling cannot be performed stably and rapidly, and therefore phase transformation cannot be promoted at low temperatures.

High-strength steel bars can also be obtained by quenching and tempering, but the so-called tempered martensitic steel produced in this way tends to have inferior delayed fracture characteristics compared to pearlite steel, so it is difficult to obtain from the viewpoint of reliability. It is necessary to increase the strength of pearlite steel.

However, in general, large-diameter steel rods, especially those with a diameter exceeding 20 mφ, have an extremely large amount of heat, so conventional air patenting and hot water patenting have a slow cooling rate, making it difficult to transform into pearlite at low temperatures. The tensile strength is 12 without adding a large amount of hardenable elements.
It was difficult to achieve a high strength of 0 kg/1 or more.

[Problem that the invention seeks to solve]

The present invention was made in view of such problems, and an object of the present invention is to provide a large-diameter steel rod having a diameter exceeding 20 mφ and having high strength and high toughness.

[Means for solving problems]

The present inventors were able to obtain a large-diameter high-strength rolled steel bar with minimal addition of alloying elements as described above, but the most preferable method for doing so is to control the pearlite transformation temperature. there were.

That is, in one embodiment of the present invention, when uniformly cooling a steel bar after hot rolling, if Tc is the critical temperature tangent to the pearlite transformation start line of the CCT curve of the steel bar, T
By starting pearlite transformation within the temperature range of c to c + 40°C and controlling cooling so as to suppress the maximum temperature during transformation to Tc + 80°C or less, the desired large diameter high strength rolled steel bar can be obtained. be.

The present invention will be described in detail below with reference to embodiments shown in the drawings.

Figure 1 shows an example of the weight% composition of 0.75C-0.81.
32 dragon φ containing Si-1,21Mn-0,80Cr
The pearlite transformation start line (Ps) and pearlite transformation end line (Pf) of the CCT curve (continuous cooling transformation curve) of the steel bar are shown.

As a result of our research efforts, the present inventors have found that when a steel bar heated to the austenite temperature range is cooled at a constant velocity, the critical temperature tangent to the pearlite transformation start line of the CCT curve is Tc, Tc cannot be obtained in the constant velocity cooling shown by the curve, and the curve at the beginning of the figure is a constant velocity cooling curve in which Tc is obtained. Then, by starting the pearlite transformation within the temperature range of Tc = Tc + 40°C and performing controlled cooling to suppress the high temperature of heat recovery due to pearlite transformation to below Tc + 80°C, the steel bar has a tensile strength of 120 kg/m.
We have found that the above can be obtained. That is, the cooling curve according to the present invention is as shown by C in FIG. Note that the case using the conventional air cooling method is shown as 2.

Here, the pearlite transformation start temperature is set to Tc=Tc+40°C because at a temperature lower than Tc, pearlite transformation does not occur and the martensite state occurs, and at a temperature higher than Tc+40°C, the required strength cannot be obtained.

In addition, the high temperature of recuperation due to pearlite transformation is Tc + 80°C.
The pearlite transformation start temperature was set as below: Tc~Tc+
This is because even if the temperature is within 40°C, if the maximum recuperation temperature is Tc+80°C or higher, the required strength cannot be obtained due to this heat generation.

In general, the finer the crystal grains, that is, the lower the rolling finish temperature, the better the toughness, but on the other hand, the finer the crystal grains, the lower the hardenability, so it undergoes pearlite transformation at high temperatures, resulting in a product that has both high strength and high toughness. It was difficult.

However, in the present invention, by performing forced cooling and controlling the pearlite transformation temperature, the grain size can be adjusted to the ASTM standard.
Even a rolled steel bar with a diameter of 8 or more can have a diameter of 201φ or more, a tensile strength of 120 kg/w” or more, and a reduction of area of 20% or more.

The above-described temperature control in the present invention is achieved by, for example, arranging water or mist nozzles on a rolled steel rod so as to spray it uniformly in the circumferential direction of the steel rod, and adjusting the amount of water and/or air. Furthermore, by spraying this continuously or intermittently, an arbitrary cooling rate can be given to control the starting temperature of pearlite transformation, and after the start of transformation, the maximum recuperation temperature can be controlled by spraying water or mist. do.

ASTM, Ilb, 8 by adjusting the rolling finishing temperature.
When the above-mentioned rolled steel rod is subjected to the above-mentioned controlled cooling, a steel rod with improved tensile strength and reduction of area can be manufactured.

〔Example〕

Example I A hot-rolled steel bar with a diameter of 32mm and containing the components shown in Table 1 was continuously cooled from 950°C at various cooling rates using water or a mist nozzle. At a cooling rate faster than 23°C/sec, martensite transformation occurred without pearlite transformation, and pearlite transformation occurred for the first time from 570°C at 23°C/sec.

Therefore, we also tested the tensile strength kg/w'' of the steel bars obtained by setting the above steel bar at TC = 570°C and varying the pearlite transformation start temperature and the maximum temperature during pearlite transformation as shown in Table 2. The results are summarized in Table 2, and Table 2 is further illustrated in FIG.

The vertical axis of FIG. 2 is the pearlite transformation start temperature (°C), the horizontal axis is the maximum temperature during pearlite transformation (°C), and the numbers in the figure represent the tensile strength. The shaded area in the figure is the temperature range of the present invention.

Table 1 Table 2 In this way, when uniform cooling is performed on a steel bar after hot rolling, if the critical temperature in contact with the pearlite transformation start line of the CCT curve of the steel bar is TC, then TC ~ TC
By starting pearlite transformation within the temperature range of +40℃ and suppressing the maximum temperature during pearlite transformation to TC+80℃ or less, tensile strength of 12 with a diameter of 20Bφ or more is achieved.
0 kg/wm” or more.

Example 2 Regarding a steel bar containing the components shown in Table 1 above and having a wire diameter of 32 mm, the rolling finishing temperature was varied from 750 to 1050°C, the pearlite transformation start temperature was 590°C, and the maximum temperature during transformation was 640°C. Forced cooling was performed using water or a mist nozzle so that the temperature reached ℃.

Table 3 shows the relationship between rolling finishing temperature and mechanical properties. The average value of 8 tests for each number is shown.

Table 3 In this way, austenite grains are
Hot-rolled steel bars with lh of 8 or more can also have a reduction of area of 20% or more by applying the above-mentioned controlled cooling, and have a diameter of 2.
Tensile strength 120 kg/am2 at 0++nφ or more
You can get more than that.

[Effects of the Invention] As described above, the present invention provides a rolled steel bar with a large diameter, high strength, and high toughness by controlling the pearlite transformation temperature and sound without adding expensive hardenability-improving elements. It is something.

[Brief explanation of drawings]

FIG. 1 is a diagram illustrating an example of the method for obtaining a rolled steel bar of the present invention, and shows a CCT curve of the steel bar, where Ps is the pearlite transformation start line of the CCT curve, Pf is the end line of the pearlite transformation,
Tc is the critical temperature at which it contacts Ps when cooled at a constant rate,
Curve C shows the cooling curve of the method of the present invention. Figure 2 shows Tc = 570°C in Example 2 of the present invention.
The pearlite transformation temperature control range (shaded area) to obtain the steel rod of the present invention in the case of , and the tensile strength of the obtained steel rod kg/Ryu2
represents. Haruma (seconds) Ding C 11! - Written amendment to the procedure April 1, 1985 1, Indication of the case 1988 Patent Application No. 174369 2, Name of the invention Large diameter high strength rolled steel bar 3, Person making the amendment Relationship to the case Patent issued Request Appointment Address: 5-15 Kitahama, Higashi-ku, Osaka City
Name (213) President of Sumitomo Electric Industries, Ltd. Tetsube 4, Agent Address: 6, Sumitomo Electric Industries, Ltd., 1-1-3 Shimaya, Konohana-ku, Osaka City, Column of scope of patent claims subject to amendment, and Column for detailed description of the invention. 7. Contents of amendment (1) The scope of claims in the specification will be corrected as shown in the attached sheet. (2) The same book, page 3, line 5, “The diameter is 20IIIRIφ
"..." should be corrected to "The diameter is 20 meters...". (3) The same Ministry, page 3, line 15, “The diameter is 2011Iwl.
φ..." to "diameter 20mm..."
” is corrected. (4) Page 4, line 13 of the same book, “Containing 321III
mmφ steel bar..." to "containing diameter 32II
Corrected to ``III Steel Bar...''. (5) Circular page 6, lines 5-6, “...
・Even if the diameter is 201111I+φ or more...
” to “...even if the diameter is 20mm or more...
...” is corrected. (6) The same book, page 7, line 2, “With a diameter of 32 mmφ...
..." is corrected to "The diameter is 32I11111...". (7) Same book, page 9, line 5, r wire diameter 32mmφ...
"..." is corrected to "diameter 32mw+...". (8) Same book, page 10, line 4, “Diameter 20III+
φ or more..." to "diameter 20- or more..."
...” is corrected. Claims: ``(1) Weight ratio: C: 0.6 to 0.9%, Si:
0.25-2.0%, Mn: 0.5-2.0%, C
It is a steel rod with a diameter of 200 or more, containing 0.3 to 1.0% r; the balance being Fe and unavoidable impurities, and having a tensile strength of 120kg/+n'' or more and a reduction of area of 20% or more. A large-diameter high-strength rolled steel bar. (2) When a hot-rolled steel bar is subjected to constant velocity cooling, the critical temperature at which it touches the pearlite transformation start line of the CCT curve of the steel bar is b: The pearlite transformation according to claim 1 is controlled and cooled so as to start pearlite transformation within a temperature range of Tc diameter high-strength rolled steel rod. (3) Adjust the rolling finishing temperature to achieve the ASTM grain size.
The large-diameter high-strength rolled steel rod according to claim 1 or 2, characterized in that eight or more rolled steel rods are subjected to controlled cooling at a time. ”

Claims (3)

[Claims] (1) Weight ratio: C: 0.6-0.9%, Si: 0.25
~2.0%, Mn: 0.5~2.0%, Cr: 0.3~
A steel rod with a diameter of 20 mmφ or more, containing 1.0%, the balance being Fe and unavoidable impurities, and having a tensile strength of 1.
A large-diameter high-strength rolled steel bar having a weight of 20 kg/mm^2 or more and a reduction of area of 20% or more. (2) When a hot-rolled steel bar is cooled at a constant speed, if To is the critical temperature tangent to the pearlite transformation start line of the CCT curve of the steel bar, pearlite will be produced within the temperature range of Tc to Tc + 40°C. The large-diameter high-strength rolled steel rod according to claim 1, which is controlled and cooled to initiate transformation and suppress the maximum temperature during transformation to Tc + 80°C or less. (3) Adjust the rolling finish temperature to achieve an ASTM grain size.
No. The large-diameter high-strength rolled steel rod according to claim 1 or 2, which is obtained by subjecting a rolled steel rod of 8 or more to controlled cooling.