EP3617333A1

EP3617333A1 - Method for manufacturing a hypereutectoid steel product by thermomechanical processing

Info

Publication number: EP3617333A1
Application number: EP19193739.0A
Authority: EP
Inventors: Heimo Roselli
Original assignee: Roselli Oy
Current assignee: Roselli Oy
Priority date: 2018-08-27
Filing date: 2019-08-27
Publication date: 2020-03-04
Also published as: FI128299B; FI20185698A1

Abstract

The object of the invention is a method for manufacturing a low-alloy hypereutectoid steel product by thermomechanical processing, in which
• an iron raw material free of additives is chosen or produced,
• carbon in a concentration of 0.8-2.0 % is added to the molten iron,
• the blank is cast rapidly under cooling in order to generate carbide nuclei that are small in size and evenly dispersed,
• a series of cycles is performed, in which the working starting temperature is set at approximately 800 °C, constantly above the A₁ point and
• the product is worked into its final shape
• the blank is quenched by rapid salt water cooling.

Description

The object of the invention is a method for manufacturing a low-alloy hypereutectoid steel product by thermomechanical processing, in which

an iron raw material is chosen or produced,
carbon in a concentration of 0.8-2.0 % is added to the molten iron,
a blank is cast,
a series of cycles is performed, in which the working starting temperature is set at approximately 800 °C, constantly above the A₁ point and
the blank is quenched.

High-carbon steels are currently invariably used with a carbon content of up to approximately 2.0 %. With high carbon concentrations, a considerable hardness is imparted to the steel, but at the expense of brittleness. Conventional high-carbon carbon steel, the carbon content of which is approximately 1.4 %, obtains during quenching a hardness of approximately 70 on the Rockwell C scale. Such steel is also very durable as a result of the action of the carbides therein, but nevertheless so brittle that its potential uses are quite limited. Typical uses are, inter alia, pulling tools, cutting blades, that are not subjected to impact stresses or high temperatures, and razor blades.
In view of these hardness values, several other uses would also be possible if the steel were made more resilient. With the processing means in accordance with the invention, low-alloy, high-carbon steels are obtained that are substantially more resilient than normal, the applicability of the hard steel grades thus improves, and very useful steels, the carbon content of which is as high as 2 %, can be manufactured with the method in accordance with this invention. The hardness and durability of such carbon steels are in a class of their own. A problem of steels with a high carbon content is the formation of graphite, which then coalesces with additives to form carbides, such as chrome carbides. Figure 1 shows a sectional view of a conventional carbon steel. The grain boundaries are clearly visible.
The cast steel products require "normalization" in order to remove the coarse cast structure. Normalization is further used to eliminate inhomogeneities in forged objects as well as phenomena (inter alia grain growth) caused by an uneven heating of welded or flame-cut products. By means of normalization, the structure can again be rendered "fine-grained" and its mechanical properties simultaneously restored.
During normalization, the steel is heated to an austenitic phase and is kept there until the "austenite" is homogenized and subsequently air-cooled. During normalization, a resilient and finely distributed "ferritic-perlitic" microstructure is achieved with the non-alloy structure steels.
The object of the invention is to provide a pure, in particular additive-free high-carbon steel product that has good strength properties. The characterizing features of the invention are indicated in the claims.
The correct temperature of austenization annealing is important for normalization, as it is for other thermal processes. If austenization occurs at a temperature that is too high, grain growth is the consequence. For this reason, the austenization temperature with normal carbon steels can only be above an approximately 50 °C A₃ point.
A steel processing method similar to this invention is disclosed in the inventor certificate SU 456841 , but it only applies to a carbon content of over 2 %. The document also does not disclose any information beyond the processing of the blank.
Of the known thermomechanical processes, the closest to the initial processing in accordance with the invention is an HTMT process in which a blank with a carbon content of 0.5% is subjected to high-temperature thermomechanical processing.
U.S. patent no. 3459599 (Grange ) discloses a method for thermomechanically quenching and tempering steel. In this process, the steel is worked vigorously slightly above the A₁ temperature and finally slightly below the latter. The steel contains chrome and manganese. The patent US3178324 of the same applicant discloses a hypereutectoid, multi-cycle quenching and tempering always down to room temperature, but without working. In order to attain a fine-grained steel without alloy materials, the object is heated and cooled drastically above the A₁ point, so that the structure changes repeatedly between the martensitic and austenitic states while a heating to a temperature that accelerates grain growth is avoided. The cooling phase each time is rapid so that the change in state occurs in the ferritic direction.
The characterizing features of the invention are indicated in the claims. Although all stages of the invention are known separately, the invention is a series of stages to be executed with precision, each having its significance and no one stage being omissible.
Expensive alloy materials can be avoided - with inexpensive materials unique results are achieved. Alloy materials react in different ways to heat and some are poisonous. A product in accordance with the invention is natural and thus easy to recycle.
In the method according to the invention, excess carbon is collected into evenly dispersed small carbide pockets. This is achieved by means of a rapid cooling and a small amount of sulphur and/or phosphorus, which form carbide nuclei in the absence of "better" additives. The evenly dispersed small carbide pockets participate during the series of cycles in working as well as in the A₁ lattice transformation.
The method in accordance with the invention requires a remarkable level of expertise. The hot working of the blank for attaining the product occurs within a narrow temperature range - heating momentarily and precisely to approximately 800 °C. Heating does not reach an austenitic phase (point A_cm), but the A₁ point is crossed. In the series of cycles, during working, the cooling is not particularly rapid so that the structure does not become heavily martensitic at any given time.
The weldability of the product obtained with the method according to the invention is outstanding and a welding rod can be made from the same.
The cutting ability of the product is outstanding (the cutting blade keeps its sharpness - shiny finish).
The thickness of the product to be quenched is advantageously 2 - 20 mm, most advantageously 5 - 12 mm. The weight of the object can also vary significantly, typically 50 g - 30 kg, as the thinnest dimension is the most significant. A continuous process is also possible, e.g. a process for a 400 - 1200 mm steel band requires several consecutive furnace-correction-roller assemblies, after which there is a quenching unit, from which the product is either wound onto a large drum or cut into sheets.
The strength and resilience of the obtained steel are of the highest quality (test object 3400 MPa/ 250 kJ/m²). It is thus possible to temper or rejuvenate and improve resilience at the expense of hardness. Figure 2 shows a sectional view of a steel product in accordance with the invention. The grain boundaries are not visible to the naked eye.
This steel is, apart from its high carbon content, by composition a conventional fine-grained steel. Alloy materials are not at all necessary, as the processes in accordance with the invention impart a fine granularity, strength and hardness. The most important aspect for its properties is the combined multi-stage thermal and working processing or thermomechanical processing.
According to one explanation, the fine-grained and evenly dispersed carbides break up the lattice cells during the series of cycles, from which the freed atoms move to the grain boundaries and plug them. The success of the final result can be checked by checking whether or not the grain boundaries are visible to the naked eye.
The cooled object is in an austenitic + cementite phase and the nuclei of the hypereutectoid carbides are produced in this case either inside the crystals or at the grain boundaries under the influence of the working. The formation of adverse carbide networks at the grain boundaries is substantially reduced for this reason while working is continued. Carbides are formed more evenly both inside the crystals and at the grain boundaries.
It is advantageous to use only iron made from an ore base as the raw material, as the impurities of some recycled scrap have an adverse effect on the success of the process. The carbon content is 0.8-2.0 % depending on the desired properties. For good hot-working properties, the sulphur and phosphorus content must be kept low, but a small quantity of the same is required for the formation of the carbide nuclei.
The invention is illustrated in the following with the help of examples.
The manufacture of a pure steel product by thermomechanical processing here means that there is nothing else in the blank besides iron and carbon. The concentrations of additives in the raw material are very low. The sulphur content is nevertheless 0.007 - 0.038% and/or the phosphorus content is in the range of 0.005 - 0.026 %, so that the carbides with the most advantageous nuclei are formed. Aluminium and silicon are detrimental in this regard. Their concentrations must be extremely slight.
First, a raw material is chosen or produced that is free of vanadium, wolfram, molybdenum, chrome and nickel, the raw material, however, thus containing small quantities of sulphur and/or phosphorus for forming the carbide nuclei. It is possible to select a pure scrap metal while avoiding adverse additives that cannot be removed by means of refining processes.
The composition of raw ore is known and does not contain any adverse compounds. By means of a refining process, an ideal raw material is obtained. Any hydrogen in the raw ore must be removed. It causes hydrogen occlusions.
In a subsequent stage, carbon in a concentration of 0.8 - 2.0 %, most advantageously 1.6 - 1.9 %, is added to the molten iron. Afterwards, the blank is cast and cooled rapidly, a 5 kg object with salt water or at a corresponding speed, whereupon the mentioned carbide nuclei are formed so as to be small in size and evenly dispersed.
Casting is carried out at a rather low temperature, at 1500 °C. It is left to cool rapidly, as it is not possible to exploit the heat of the cast. The speed of cooling can be fixed to be the same as that of a chill cast of a 5kg pellet with a cross section of 50 mm x 100 mm, ±30%. If the blank does not turn out, it must be heated again to over 1140 °C, generally in the range of 1000 - 1220 °C and the process started again from the beginning.
After casting, 5 - 15 working cycles are performed in a series of cycles as follows:

The working starting temperature is set at approximately 800 °C, constantly above the A₁ point,
the temperature of the blank alternates between above and below the A₁ point at least five times while the heated blank is worked, whereupon it cools down below the A1 point and is again brought above the A₁ point by heating, whereupon the alternation of the face-centered cubic and body-centered cubic and the working caused by the carbides causes the removal of the grain boundaries.

After and during this series of cycles, the blank is worked into its final shape. During this series of cycles, the temperature is above the A₁ point, but is below the A₁ point right at the end in order to preserve its properties. Then the blank is quenched into its final shape by heating it to approximately 780 °C, after which it is immediately rapidly quenched, blank having a cross section of 5 mm x 100 mm by salt water, ±30%, or by a method producing a corresponding cooling speed.
5 kg object by salt water, or a method producing a corresponding cooling speed. The quenched blank can be tempered in a known manner.
A thin object can also be quenched with oil. How the quenching is realized depends on the intended use and the composition of the object. If quenching is performed at the end, the final result is clearly martensitic. If less quenching is performed, the final result is bainitic or contains residual austenite. Quenching can optionally be performed as a surface quenching with quenching reaching a depth of 2 - 15 mm.

Claims

A method for the manufacture of a hypereutectoid pure steel product by thermomechanical processing, in which, in order to obtain a fine-grained steel, a steel product without additives is heated and cooled back and forth above and below the A₁ point while simultaneously working the same, so that the structure alternates between different states while a heating to a temperature that accelerates grain growth is avoided,
characterized in that
• an alloy-free iron raw material free of vanadium, wolfram, molybdenum, chrome and nickel is chosen or produced, which still contains small quantities of sulphur and/or phosphorus for forming the carbide nuclei,

• carbon in a concentration of 0.8 - 2.0 %, most advantageously 1.6 - 2 %, is added to the pure molten iron,

• a blank is cast, which is rapidly cooled, similar to a chill cast of a 5kg pellet with a cross section of 50 mm x 100 mm, ±30%, or at a corresponding speed, whereupon the mentioned carbide nuclei are formed so as to be small in size and evenly dispersed,

• 5 - 15 working cycles are performed in a series of cycles as follows:
• the working starting temperature is set at approximately 800 °C, constantly above the A₁ point,

• the temperature of the blank alternates between above and below the A₁ point at least five times while the heated blank is worked, whereupon it cools down below the A₁ point and is again brought above the A₁ point by heating, whereupon the alternation of the face-centered cubic and body-centered cubic and the working caused by the carbides causes the removal of the grain boundaries, and after this series of cycles:
• the blank is further worked into its final shape, advantageously above the A₁ point in order to preserve its properties, and

• the blank is quenched in the final form of the product by heating it to approximately 780 °C, after which it is immediately rapidly quenched, blanks having a cross section of 5 mm x 100 mm by salt water, ±30%, or with a method producing a corresponding cooling speed.
The method according to claim 1, characterized in that the quenched blank is tempered/rejuvenated in a manner known per se.
The method according to claim 1 or 2, characterized in that a pure ore free of additives is chosen as the raw material, the ore being purified of carbon and impurities by a conversion process.
The method according to claim 1, characterized in that selected pieces of scrap metal in which there is no vanadium, wolfram, molybdenum, chrome or nickel, are chosen as the raw material.
The method according to any of claims 1 - 4, characterized in that the sulphur content is 0.007 - 0.038 %.
The method according to any of claims 1 - 5, characterized in that the phosphorus content is 0.005 - 0.026 %.
The method according to any of claims 1 - 6, characterized in that the mentioned series of cycles or the heating and cooling of the blank occurs together with a working mainly in the austenitic phase.
The method according to any of claims 1 - 7, characterized in that the thickness of the product to be quenched is advantageously 2 - 20 mm, most advantageously 5 - 12 mm.
The method according to any of claims 1 - 7, characterized in that the weight of the product is in the range of 50 g - 30 kg.
The method according to any of claims 1 - 8, characterized in that manufacturing occurs continuously, wherein a, most advantageously 400 - 1200 mm, steel band is subjected to a process having several consecutive furnace-correction-roller assemblies, after which there is a quenching unit, from which the product is wound onto a drum or cut into sheets.
The method according to any of claims 1 - 10, characterized in that the quenching occurs as a surface quenching with the quenching reaching a depth of 2 - 15 mm.