JPS61106716A - Manufacture of bar steel having superior toughness at low temperature - Google Patents

Abstract

PURPOSE:To obtain the bar steel which holds superior toughness even at very low temps. lower than -120 deg.C, merely by regulating the rolling process of the steel as well as the chemical composition of the steel, obviating the necessity of the addition of large amounts of expensive alloying elements and of finding a complicated means. CONSTITUTION:The billet consists of, by weight, 0.02-0.1% C, <0.5% Si, 1.1-2.5% Mn, 0.15-0.5% Mo, 0.01-0.1% Nb, 0.01-0.1% Al, and the balance Fe with inevitable impurities, and further contains >=1 kind among 0.05-0.3% Cu, 0.05-1.2% Ni, 0.05-1.2% Cr, 0.01-0.05% Ti, and 0.0005-0.003% B. This billet is heated up to 1,000 deg.C maximum, hot-rolled at <850 deg.C finishing temp. and at >60% cumulative draft at <=880 deg.C, and air-cooled to room temp.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing a steel bar that exhibits high strength and good toughness even in an extremely low temperature environment, particularly at -120°C or lower.

(Prior art) In recent years, the demand for steel bars for reinforced concrete used in low-temperature environments, such as reinforced concrete structures in cold regions and polar regions, reinforced concrete freezers, and tanks for liquefied gases such as LNG and LPG, has been increasing. It has been showing its direction around 1.

Conventionally, 9% Ni steel and high Mn austenitic steel have been developed as low-temperature reinforcing bars used in such environments.・
,,17 pa 4″°゛′″f″6°t L゛f a b
l1i(it5 ft @ *, rc @ G #
Since it is included in IT, it could only be used for extremely limited purposes.

On the other hand, for shell structures, etc., reinforcing bars specified in JIS G3112 (i.e., yield strength: about 42 to 43 kgf / mA, heated to 1100 to 1250 °C, finishing temperature: 100' to about 900 °C) (manufactured by inter-rolling) are used, but these are intended to be used at room temperature or at temperatures higher than that.
When exposed to the above-mentioned low temperatures, especially extremely low temperatures of -100° C. or lower, there are concerns about toughness.

Therefore, recently, LPG tanks that can reach temperatures below -60℃ as mentioned above, and ethylene or LNG tanks that can reach temperatures as low as -100℃ have been developed to exhibit a certain level of high strength and toughness even when exposed to extremely low temperatures. Although there has been competition to develop steel bars that can withstand high temperatures, the current situation is that no steel bar with satisfactory cryogenic properties has yet been obtained.

(Objective of the Invention) From the above-mentioned viewpoint, the present inventors have developed a high-strength and high-strength product that can be used in extremely low-temperature environments below -120°C, for which demand is expected to become stronger in the future. As a result of conducting research to create a steel bar that exhibits toughness without adding large amounts of expensive alloying elements, we came to the following findings as shown in far~(C1).

(al) In particular, specific amounts of Mn, Mo, and Nb are added to a low carbon steel whose C content is adjusted to 0.02 to 0.10% (hereinafter, % indicating a component ratio is % by weight), and When this is subjected to low-temperature heating and hot rolling at a low finishing temperature, a fine mixed ferrite-heinite structure with an average grain size of 10 μm or less and preferably a volume percentage of heinite of 10 to 30% can be obtained as rolled. It is possible to realize steel materials with extremely good strength and low-temperature toughness.

(bl Furthermore, when the steel material obtained in this way is subjected to tempering treatment at a specific temperature, the yield strength is improved by 5 to 1
In other words, by strictly controlling the chemical composition and rolling conditions of the steel, and further combining tempering treatment at a specific temperature if necessary, we can see an increase in strength of about 0 kqf/product, and further improve the low-temperature toughness. It is possible to produce steel bars at low cost that have excellent low-temperature performance that was previously unobtainable.

(Structure of the Invention) The present invention has been made based on the above findings: C: 0.02 to 0.10%, Si: 0.5% or less, Mn: 1.10 to 2.50%, Mo: 0.15-0.50%, Nb: 0.010-0.100%, Al:
Cu: 0.010 to 0.100%, and if necessary, further Cu: 0.05 to 0.30%
, Ni: 0.05-1.20%, Cr: 0.05-1.20%, Ti: 0.01-0.05%, B i O, 0005-0.0030%. After heating a steel billet with a composition consisting of , the balance being Fe and unavoidable impurities to a temperature of up to tooo Celsius, the cumulative rolling reduction rate in the temperature range of finishing temperature = 850℃ or less and 880℃ or less: By hot rolling 60% or more and then air cooling to room temperature, or further tempering at 500 to 700°C if necessary, we can produce a steel bar with extremely excellent low-temperature properties, with an impact fracture transition temperature of 1120°C or less. It is characterized by the fact that it produces

Next, in the method for manufacturing a steel bar of the present invention, the reason why the composition ratio of the steel and the rolling/heat treatment conditions are numerically limited as described above will be explained.

A. Ingredient composition of steel a) C The C component is included in order to impart a predetermined strength to the steel bar, but if its content is less than 0.02%, the desired strength cannot be obtained; If the C content exceeds 10%, barlite structure will be mixed into the steel bar structure, resulting in deterioration of toughness, so the C content should be set at 0.02 to 0.
It was set at 10%.

b) Si The Si component is an effective element for deoxidizing steel, and is usually 0°1
The amount of addition is 5 to 0.35%. However, when deoxidizing is carried out in step 81, Si addition is not necessarily necessary, and if it is added in excess of 0.5%, hot workability will be adversely affected. For this reason, the Si content was determined to be 0.5% or less.

1. c) Mn is an element necessary for the sulfurization of steel, and also acts as a solid solution in the steel matrix to improve the strength of the steel material, as well as imparting a certain hardenability to the steel material. It also has

In order to generate a fine mixed structure of ferrite and haynite under the rolling conditions of the present invention and impart the desired strength and low-temperature properties to the steel, the Mn content must be 1.10% or more. On the other hand, if the Mn content exceeds 2.50%, segregation becomes significant and toughness and weldability deteriorate, so the Mn content was set at 1.10 to 2.50%.

d)M.

The Mo component is an extremely effective element for imparting strength without impairing the toughness of the steel.In addition, in the case of the method of the present invention, the hardenability of the steel is adjusted to form ferrite in the rolled state. It is also an indispensable element for obtaining a fine mixed structure of bainite and bainite.If the MOC content is less than 0.15%, the above effect will not be fully exhibited, but on the other hand, if the MOC content exceeds 0.50%, Since the above effect would be saturated, the MOC content was set at 0.15 to 0.50%.

e) Nb The Nb component is used to obtain the fine mixed structure of ferrite and bainite found in making the present invention.
1' is an essential element whose content is 0.01
If it is less than 0%, it becomes difficult to prevent coarsening of austenite grains during the heating stage of the steel billet before rolling (heating stage at 1000°C or less), and eventually the fine mixed structure of ferrite and bainite is stabilized. you won't be able to get it. On the other hand, if the Nb content exceeds 0.100%, the effect of suppressing austenite grain coarsening will be saturated and this will only lead to an increase in the cost of steel materials.
．． It was set as 100%.

f) A(2) In addition to deoxidizing the steel, the AlC component also prevents austenite grains from coarsening during the heating stage of the steel billet before rolling.
It has the same effect as Nb mentioned above. If the Al content is less than o, oto%, the above effects will not be sufficiently exhibited,
On the other hand, if the Al content exceeds o.
It was set as 00%.

g) Cu The Cu component has the effect of increasing the strength of steel with almost no adverse effect on the toughness of the steel, and is therefore included when it is necessary to further improve the strength of the steel material. If the amount is less than 0.05%, the desired effect cannot be obtained, and if the content exceeds 0.30%, the hot workability of the steel will be impaired.
The content was determined to be 0.05 to 0.30%.

h) Ni The Ni component is an element that is particularly effective in improving the low-temperature toughness of steel, so when further improvement in low-temperature toughness is required, it should be added in an amount of 0.05% or more so that the effect becomes noticeable. However, if the Ni content exceeds 1.20%, the cost of the steel material will increase, and the rate of occurrence of hydrogen defects such as white spots will increase during manufacturing, so the Ni content should be reduced to 0. It was set at 0.05 to 1.20%.

1) Cr 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 is not achieved. On the other hand, if the content exceeds 1.20%, it will cause deterioration of cold workability.
The content was determined to be 0.05 to 1.20%.

j) Ti The Ti component, like Nb and Al, has the effect of refining austenite crystal grains, and is an effective element for obtaining a fine mixed structure of ferrite and bainite, so it is included as necessary. However, its content is 0.
If the content is less than 0.01%, the desired effect cannot be obtained. On the other hand, if the content exceeds 0.05%, the Ti carbonitrides present in the steel will become coarser and the number of Ti carbonitrides will also increase. This causes deterioration of hot workability. Therefore, the Ti content is 0.01 to 0.
It was set at 0.5%.

k) B The B component has the effect of improving the hardenability of steel when added, so it is added when it is necessary to further increase the strength of the steel material, but its content is If the B content is less than 0.0005%, the desired effect cannot be obtained; on the other hand, if the B content exceeds 0.0030%, hot workability deteriorates.
It was set at 0.0030%.

B. Rolling and heat treatment conditions a) Rolling in heating temperature Rolling + Jij If the heating temperature exceeds 100°C, the austenite grains during heating will become coarse even if steel having the chemical composition specified in the method of the present invention is used. ,
As rolled, fine ferrite and heinite ′l□1
Mixed fibers cannot be obtained. Therefore, the desired low-temperature toughness cannot be achieved, so it is essential that the upper limit of the rolling oil heating temperature is 1000°C.

On the other hand, even if the heating temperature is lowered, there is no negative effect on the low-temperature properties, but if it is set too low, the load on the rolls during billet rolling becomes excessive and productivity deteriorates. It can be said that heating at ℃ is appropriate.

b) Rolling temperature and reduction rate In order to impart the specified strength and toughness to steel, hot rolling,
In particular, it is necessary to repeat deformation by pressure and recrystallization in a temperature range of 880° C. or lower to make the austenite grains finer.

At this time, if the cumulative reduction rate at 880°C or lower is less than 60%, the desired refinement cannot be achieved, so the cumulative reduction rate in the temperature range of 880°C or lower is determined to be 60% or higher.

C) Finishing temperature If finish rolling is performed at a temperature exceeding 850°C, the desired microstructure cannot be achieved and the desired high toughness steel cannot be obtained, so the finishing temperature is set at 850°C or lower. Ta.

Note that if the finishing temperature is too low, steel having the chemical composition specified in the method of the present invention will be rolled in the non-recrystallized region of austenite, resulting in anisotropy in mechanical properties due to the development of texture. Therefore, the finishing temperature is 850
It is desirable to set the temperature to about 750°C.

If the component steel subject to the present invention is rolled under the above-mentioned hot rolling conditions, a fine mixed structure of ferrite and heinite can be obtained as it is rolled.

d) Tempering temperature The steel bar produced under the rolling conditions of the present invention and having the chemical composition specified by the method of the present invention has a fine mixed structure of ferrite and henite even after being rolled. Accordingly, if the tempering treatment is then performed within the range of 500 to 700°C, the yield stress will further increase and the toughness will be further improved.

In this case, if the tempering temperature is less than 500'C, the above-mentioned 91 fruits will not be able to fully develop, and on the other hand, if the tempering temperature is less than 700'
If the temperature exceeds 00°C, ferrite and bainite recrystallize during tempering, resulting in collapse of the microstructure and deterioration of toughness. Therefore, the tempering temperature was set at 500 to 700°C.

Next, the present invention will be explained using Examples and in comparison with Comparative Examples.

First, steels 1 to 38 having the compositions shown in Table 1 were made by the 1ff18 melting method, and then 16
A steel piece of Qmm square was made into a rolled material.

Next, this steel piece was heated to 950°C and hot rolled at a cumulative reduction rate of 90% below 880°C and a finishing temperature of 800°C to form a round bar with a diameter of 25+ am.

After finish rolling, the round bar material was allowed to cool to room temperature.

For each as-rolled round bar obtained in this way,
Microstructural observations, tensile tests, and impact tests were conducted.

In addition, in microstructural observation, ferrite, bainite, and pearlite in the as-rolled material were determined, and the grain size was also measured.

In the tensile test, the parallel part; JI with a diameter of 14111111
A No. S 4 test piece was cut out from the rolled material, and the yield strength, tensile strength, elongation (calculated using gauge length: 50+nm) and reduction of area with respect to 0.5 pearlite total elongation were measured.

The impact test was conducted using a JIS No. 4 (2mm V notch) Charpy test piece, and the absorbed energy at -120°C [SiE-, □. ] and the impact fracture transition temperature (brittle-ductile fracture transition temperature) [νTrs).

The results obtained are shown in Table 2.

As is clear from the results shown in Table 2, the steel bars that have the chemical composition (Steels 1 to 27) that satisfy the conditions of the method of the present invention and that are manufactured by the method that satisfies the conditions of the method of the present invention are It exhibits a fine mixed structure of [ferrite + heinite] with a grain size of 10 μm or less, and a yield strength of 40 kgf/
mA or more and vE-IL. shows a value close to 30 kgf-m, indicating that the strength and toughness are extremely excellent. Further, vTrs is also lower than -120°C in all cases, and it is clear that brittle fracture does not occur even at a temperature of -120°C.

On the other hand, even if the rolling conditions in the method of the present invention are met, the steel bars whose composition does not satisfy the conditions of the present invention (to which steels 28 to 38 are applied) are discharged. The value of the)·,
, 1 is low・Trs is also on the high temperature side from -120℃ to 0℃・It is found that brittle fracture occurs at −120℃ and the toughness is poor, and the yield strength is also 40 kgf/change. It is clear that the strength is unstable.

Refreshing Plate I Using 160 mm square steel pieces of Steel 1 shown in Table 1, round bars with a diameter of 25 mm were manufactured under various rolling conditions.

After finish rolling, the round bar material was allowed to cool to room temperature.

The microstructure, strength and toughness of the obtained round bars were investigated in the same manner as in Example 1, and the results are shown in Table 3.

The results shown in Table 3 also show that even if steel with the composition specified by the method of the present invention is used, strength and/or toughness will deteriorate if the rolling conditions are outside the range of the method of the present invention. The yield strength is 40.0 kgf/min.
It is clear that vTrs does not meet the target value of a temperature lower than 1120°C.

] Bokumachi Naoshiru 1 Using 160 mm square steel pieces of steel 1, 12 and 24 shown in Table 1, heating temperature for both pieces: 950°C Cumulative reduction rate below 880°C: 90% Finishing temperature 2800
After producing a round bar with a diameter of 251° C. by hot rolling at 30° C., it was tempered as shown in Table 4 by holding the bar at 480 to 720° C. for 1 hour and then allowing it to cool in the atmosphere.

The microstructure, strength and toughness of the IM round bars were examined in the same manner as in Example 1, and the results are also shown in Table 4.

The following can be seen from the results shown in Table 4.

That is, when the tempering temperature is 480° C., the yield strength and vTrs hardly change compared to the as-rolled material, and the effect of tempering is not recognized.

However, in the tempering temperature range of 500 to 700°C, the yield strength increases significantly, and at the same time vTrs also shows a significant decrease)
・11),,j ("T0゛ru・9maru・It is clear that both the strength and toughness of the round bar are significantly improved by the treatment that satisfies the requirements of the present invention.

However, it is clear that when tempering is performed at a temperature exceeding 700° C., the microstructure becomes coarse and the strength decreases, and the toughness also deteriorates.

(Overall Effects) As described above, according to the present invention, the chemical composition and rolling method of steel can be changed without adding large amounts of expensive alloying elements or taking particularly complicated measures. This brings about industrially useful effects, such as the ability to produce steel bars with high strength and toughness that are sufficiently satisfactory even in cryogenic environments below -120°C at low cost.

Claims (2)

[Claims] (1) In weight%, C: 0.02 to 0.10%, Si: 0.5% or less, Mn: 1.10 to 2.50%, Mo: 0.15 to 0.50%, Nb: 0.010 to 0.100%, Al: 0.010 to 0.100%, and if necessary, further Cu: 0.05 to 0.30%, Ni: 0.05 to 1.20%, Also contains one or more of Cr: 0.05-1.20%, Ti: 0.01-0.05%, B: 0.0005-0.0030%, the balance being Fe and unavoidable impurities. After heating a steel slab with a composition of A method for manufacturing a steel bar with excellent low-temperature toughness, which is characterized by air cooling until the temperature reaches 100 degrees. (2) In weight%, C: 0.02 to 0.10%, Si: 0.5% or less, Mn: 1.10 to 2.50%, Mo: 0.15 to 0.50%, Nb: 0.010 to 0.100%, Al: 0.010 to 0.100%, and if necessary, further Cu: 0.05 to 0.30%, Ni: 0.05 to 1.20% , Cr: 0.05 to 1.20%, Ti: 0.01 to 0.05%, and B: 0.0005 to 0.0030%, the balance being Fe and unavoidable impurities, After heating a steel billet with a composition consisting of: up to a maximum temperature of 1000°C, it is hot rolled at a finishing temperature of 850°C or less and a cumulative reduction rate of 60% or more in a temperature range of 880°C or less, and then A method for producing a steel bar with excellent low-temperature toughness, which comprises air cooling to room temperature and then further tempering at 500 to 700°C.