JPS6047324B2 - Improvement of hot rolled steel rods or rods - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hot-rolled steel rod or bar, and a method for making said steel rod or bar.

In order to develop a surface layer of bainite or martensite around the austenitic core, the rod or bar is superficially quenched as it exits the finishing table of the rolling mill, followed by There is a known method in which the core transforms into ferrite and carbide by cooling, and the surface layer is tempered by the heat transmitted from the core. For certain applications, rods or bars with stress corrosion or bainitic or martensitic surface layers are not suitable.

Such applications include anchors, rods that must be placed in contact with soil containing foreign substances (e.g. sulphates, nitrates and chlorides), or reinforcing bars that are embedded in concrete containing such foreign substances. included. Ferrite/pearlite microstructures are known to have better stress corrosion resistance than tempered martensite. According to one aspect of the invention, a method for manufacturing a copper rod or bar stock includes the following sequential steps.

That is, the raw material is rolled to a predetermined size, and the rolled rod or bar is partially quenched to reduce the temperature of the core of the rod or bar to a value below the critical point of austenite transformation. the rod or bar at said temperature for a sufficient period of time to produce a surface layer of ferrite-pearlite type structure. The partially cooled rod or rod is then rapidly quenched to reduce the temperature of its surface, and the martensite or bainite sandwiched between said surface layer and the core, which is still in the austenitic state. 6" to produce an annulus of acicular microstructure consisting of 6", and then the rod or bar is cooled to transform the austenite core into ferrite and carbide, or bainite, or martensite, or two of these components. or a combination of continuous processes, such as spraying water onto the surface of the rod or bar as it passes from the hot rolling mill, or using a quenching device containing a vigorous stream of water, such as a trough rod or bar. By feeding, the rod or bar is rapidly cooled.

Air cooling may be used for high alloy copper. Aside from this,
The rod or bar may be cooled by an air/water mist directed at the surface or by passing through a bath of liquid metal (eg lead) or aqueous solution (eg salt water). According to another aspect of the invention, the hot rolled steel rod or bar comprises an outer surface layer of ferritic-pearlite type, and the annulus immediately below said surface layer comprises martensite and/or bainite. It has an acicular) microstructure and a core of bainite, martensite, ferrite, and carbide, or a combination of two or more of these components.

The bainite ring may be sandwiched between the martensitic ring and the center. In the ferrite-pearlite transition, pearlite or ferrite with a collapsed pearlite structure is generated.

The invention will now be described by way of example with reference to the accompanying drawings, in which: FIG.

FIG. 1 illustrates cooling curves measured at three positions across the width of a round steel material manufactured according to the invention.

FIG. 2 is a cross-sectional view of a bar manufactured according to the present invention.

Figures 3 and 4 illustrate the hardness distribution resulting from bars produced according to the invention. FIG. 5 shows three types of microstructures of the bars showing the hardness distribution in FIG. 4.

The cooling curve illustrated in Figure 1 is a plot of temperature in degrees Celsius against time in seconds and shows the temperature at the surface of the bar, just below the bar surface, and at various times during the cooling process. It shows the temperature that exists in the center.

This cooling step is carried out to experimentally resemble the conditions experienced by hot rolled bars leaving the finishing table of a hot rolling mill. The curves are designated by reference numbers 1, 2 and 3, respectively. The rod material used in the experiment was 0.21% carbon.
Manganese 0.62%, Sulfur 0.06%, Phosphorus 0.012
%, a diameter of 20Tn! with a weight composition of 0.02% silicon and the balance iron and incidental impurities! ! Typical reinforcing bars of t were used. The bar is heated to a temperature of approximately 980°C,
Next, 3. Partially quenched in the hoe. At the end of this rapid cooling (time indicated on the horizontal axis of the graph), the temperature at the center of the bar exceeded the critical temperature for austenite transformation. The surface layer of the bar was held in the pearlite transition temperature range for about 1 second (time T2) to create a pearlite-ferrite microstructure in the surface layer.

Alternatively, the pearlite transition temperature conditions are maintained by intermittent water cooling of the bar (eg, by intermittent jets) or by feeding the bar through a series of water troughs. At the end of this cooling stage, the surface layer may also contain small amounts of bentonite. In order to cool the bar until both the surface layer and the intermediate layer are at a temperature below the temperature at which martensite is formed (time T3), once the desired surface layer is formed, the bar is further immersed in an aqueous salt solution. It is rapidly quenched by soaking in the liquid, air-cooled,
It was then rapidly quenched again by immersion in a saline solution for 1 second.

At this point, the temperature of the surface of the bar is approximately 180°C, and the temperature of the surface of the bar is 3°C. The temperature at the bottom was about 280°C and at the center of the bar it was about 500'C. These two rapid quenchings resulted in martensitic rings just below the surface layer consisting primarily of pearlite. The quenching step brought the bar to a homogenization temperature in air of about 300'C. During this air cooling, the heat emanating from the core tempered the middle layer of the bar and the core transformed into a combination of ferrite, carbide, and bainite. For rods and bars of different sizes and qualities, the above cooling times and temperature ranges will be different.

However, in each case curve 2 is always above the critical temperature before . Figure 3 shows the hardness distribution of the bars produced in the above experiment.

This hardness distribution is shown as a solid line and is labeled with the reference number 4. As is clear from Fig. 3, this bar has a diameter of 210~
A relatively soft pearlite surface layer with a hardness of 220HV100, an acicular microstructure containing martensite and bainite just below the surface with a hardness of about 320HV10, and a hardness of about 260HV10.
It was found to have a relatively soft core containing bainite and ferrite with a hardness of HV10. Also shown in FIG. 3 is the hardness distribution of bars of the same diameter and composition as above, but treated with a modified cooling process.

The hardness distribution of this bar is depicted in dashed lines in FIG. 3 and is labeled with the reference number 5. The cooling process of this second bar consisted of an initial quench for 6 seconds in a lead bath kept at a temperature of 450°C, followed by 1 second of air cooling, and 4 seconds of cooling in a -10°C hot water bath. Included rapid quenching. Bars produced by this cooling process were found to have a surface layer similar to that described above, but with an intermediate layer just below the surface layer of about 350 HV1.
The bar differed in that it had a hardness of 0, and the center of the bar had a hardness close to 400HV10. Therefore, hardness distribution 4
A bar with a hardness distribution of 5 has a relatively soft surface layer, a hard middle layer, and a relatively soft center, whereas a bar with a hardness distribution of 5 has a relatively soft surface layer, a hard middle layer, and a relatively soft center. It had layers and a relatively hard center. Figure 4 shows a bar of the same composition and diameter as above, but with hardness distribution 6, cooled by combined immersion in an aqueous solution of tap water and salt water.

From this figure, it becomes clear that this bar exhibits a hardness distribution similar to distribution 4 in FIG. Figure 5 shows that the hardness distribution is as shown in Figure 4.
It shows the microstructure across the bar shown in . This fine structure was obtained on the surface of the bar, at a position 2Tr0n below the bar surface, and at the center of the bar. This microstructure consists of a ferrite-pearlite microstructure on the bar surface and needles containing martensite and bainite just below the surface. It clearly shows the microstructure in the central part consisting of ferrite, carbide, and bainite. It goes without saying that by appropriately selecting the quenching medium and the quenching time, a range of different microstructures can be obtained in the width direction of the bar.

As illustrated in Figure 2, the rod or bar produced by the above method has an outer surface layer consisting essentially of pearlite and ferrite, an inner ring of martensite or bainite, and bainite or fetalite and carbide or mixtures thereof. It has a central part.

Such rods or bars have the advantage of retaining the corrosion resistance of pearlite-reinforced bars, and also have the advantage of increased strength and improved ductility. When rolling a heated bar, a dynamic state exists in the bar immediately after the final deformation in which the austenite recrystallizes.

This state is immediately followed by particle growth. The fine recrystallized grains tend to dislocate very quickly at temperatures around 600-700 DEG C., producing a pearlite-type microstructure. Therefore, when the surface of the bar is cooled to a temperature corresponding to the temperature at which the ferrite-to-pearlite type transition occurs, the above reaction is accelerated. Eventually, as a result of grain growth, the austenite stabilizes, thereby promoting the formation of martensite just below the bar surface. When the present invention is used in a heated rod mill, it is consequently preferable to quench the rod as soon as it emerges from the mill. It goes without saying that the above method can be applied to a wide range of steel compositions. but,
This method is mainly applicable to the production of simple carbon-manganese type steel rods or bars as follows. In the above-mentioned carbon-manganese type steel rod or bar, the stability of austenite allows the transition to ferrite-pearlite at the bar surface in a short time, while the stability is sufficient to transform to martensite-bainite at the semi-surface layer. have sex.

[Brief explanation of drawings]

FIG. 1 is a cooling curve of a steel bar manufactured by this method.

Claims (1)

[Claims] 1. Rolling the raw material to a predetermined size and partially quenching the rolled rod or rod to reduce the temperature at the core of the rod or rod to a value below the critical point of austenite transformation. the rod or bar is heated at said temperature for a sufficient period of time to produce a surface layer of ferrite-pearlite type structure, without reducing its surface area to a temperature at which a ferrite-pearlite type transformation occurs. rapidly quenching the held and partially cooled rod or bar to reduce the temperature of its surface;
The rod or bar is then cooled to produce an annulus of acicular microscopic composition of martensite or bainite sandwiched between the surface layer and the still austenitic core. 1. A process for producing steel rods or rods, characterized in that it comprises a continuous step of transforming parts into ferrite and carbide, or bainite, or martensite, or a combination of two or more of these components. 2. A method according to claim 1, wherein the first partial quenching is carried out by spraying water onto the surface of the bar as it leaves the hot rolling mill. 3. A method according to claim 1, wherein the first partial quenching is carried out by passing the bar through a trough containing a coolant. 4. The method according to claim 2, wherein the coolant is liquid lead. 5. The method according to claim 2, wherein the coolant is water. 6. The method of claim 2, wherein the coolant is salt water. 7. A method according to claim 1, wherein the first partial quenching is carried out by directing an air jet to the surface of the bar as it leaves the hot rolling mill. 8. Claims 1 to 8, wherein the rapid quenching is carried out by immersing the partially quenched bar in an aqueous solution.
The method described in any one of Section 7. 9. Said claim in which rapid quenching is achieved by treating the partially cooled bar in a series of separate quenching stages, each separated by a short period of time during which the bar is air cooled. The method according to any one of items 1 to 8. 10 outer surface layer of ferrite-pearlite type structure,
an annulus immediately inside said surface layer with an acicular microstructure consisting of martensite and/or bainite, and a core consisting of bainite or mardensite or ferrite and carbide or a combination of two or more of these components; A steel rod or bar containing parts.