JPS6233289B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides a wire rod with excellent cold workability and toughness, which has a fine spheroidized structure in which carbide particles are dispersed in a matrix having fine ferrite grains as hot-rolled;
This article relates to a method for manufacturing carbon steel or alloy steel materials such as steel bars. Carbon steel or alloy steel wire rods, steel bars, etc., when hot-rolled, have a structure unsuitable for cold working such as cold heading and cutting, so they are usually subjected to spheroidizing annealing,
It is necessary to perform heat treatment such as softening annealing or normalizing. Spheroidizing annealing is a heat treatment in which a rolled coil or steel bar is heated for several hours or more and then cooled to make some or all of the cementite in the steel into a spherical or nearly spherical form. Headability and cutting workability are improved. Softening annealing is a process similar to spheroidizing annealing performed in a short time to obtain an incomplete spheroidized structure or to remove processing distortion. , is a process that normalizes the non-uniform structure of the as-rolled product, mainly for the purpose of improving machinability. However, in order to improve hot-rolled material to the desired material properties through these heat treatments, it is necessary to heat the material for a long time from approximately 600℃ to approximately 900℃, followed by cooling, and the treatment requires strict temperature control. Control must be applied, which requires a large amount of thermal energy and complicated work. The present invention has been made to solve the above-mentioned difficulties, and by controlling the steel heating temperature in the hot rolling process of carbon steel or alloy steel, the steel material temperature during hot rolling, and the cooling rate after hot rolling finishing, As rolled, it has a fine spheroidal structure with carbide particles dispersed in a matrix with fine ferrite grains, and after hot rolling, it is equivalent to or equivalent to those subjected to these heat treatments without any special heat treatment as described above. This made it possible to provide material properties that exceed these. That is, the present invention is a carbon steel or alloy steel, and has a fine spheroidized structure and has excellent cold workability and toughness, and a carbon steel or alloy steel that is heated to a temperature of 650 to 850°C A mixed structure of ferrite and undecomposed carbide or a mixed structure of austenite and undissolved carbide is formed, and then the hot rolling end temperature is set at 950°C or less to 700 to 950°C to prevent the above carbide from dissolving. After hot rolling, it was cooled in a temperature range of 750 to 650°C at a cooling rate of 40°C/min or less to create a fine spheroidal structure with fine ferrite grains and finely dispersed carbides. Provides a method for manufacturing hot rolled steel. The present invention will be explained in detail below. According to the present invention, the steel material is first heated prior to rolling, and its structure is made into a mixed structure of ferrite and undissolved carbide divided into spheres or rods, or a mixed structure of austenite and undissolved carbide. This heating is carried out in the temperature range from 650 to 850 °C (generally between the _A1 and _A3 transformation points), but in the case of heating above the _A3 transformation point, the carbide base In order to prevent dissolution and disappearance into the austenite, it is necessary to carry out the process in a relatively short time so that austenitization does not progress too much. usually,
Heating before hot rolling is carried out by holding the steel at a high temperature above the _A3 transformation point (e.g. about 1000°C or above) for a relatively long time so that the structure is sufficiently austenitized and the carbides are completely dissolved. The present invention is characterized in that it is carried out at a relatively low temperature and a mixed structure in which carbides coexist is formed. This is intended to cause the carbide to remain until after the hot rolling process, and to form a fine spheroidal structure using the carbide as a nucleus by controlled cooling after the hot rolling process. In addition, above 2
There is no difference in the material properties of the final hot-rolled steel regardless of the mixed structure of the seeds, but a mixed structure of austenite and undissolved carbides is easier to hot-roll and is more difficult to roll during rolling. This is preferable because it requires less motor load. The steel material, which is heated to have a predetermined mixed structure containing carbides, is then subjected to hot rolling. This hot rolling is carried out at a temperature of 950° C. or lower so as not to cause dissolution of carbides. Generally, in hot rolling steel materials,
The temperature of the steel increases due to processing strain, and the degree of this increase becomes more pronounced as the rolling speed increases. For example, if the finish rolling speed exceeds about 20 m/sec, the temperature rise will be large enough to cause dissolution of carbides. In such cases, it is necessary to provide appropriate cooling to prevent excessive rise in steel material temperature. As a cooling means for this purpose, for example, intermediate water cooling during hot rolling or finishing water cooling after finish rolling is used. The necessity of cooling means also differs depending on the steel type. For example, in steel materials containing a large amount of Cr such as SCr4, carbides that are difficult to dissolve are formed, so if the finishing rolling speed exceeds about 25 m/sec. However, there is no need to apply forced cooling. The hot rolling finish temperature is 700-950°C. If the temperature exceeds 950°C, undissolved carbides will dissolve into the base as described above, and a fine spheroidal structure will not be obtained in the wire after final slow cooling. In addition, 700℃ is near the _A1 transformation point of steel, and at temperatures below 700℃, the deformation resistance during rolling is large, making rolling difficult. Spheroidized tissue cannot be obtained. Steel products that have been hot-rolled for a predetermined period of time are subjected to controlled cooling following completion of finish rolling. If the cooling rate is relatively fast in this cooling process, a large amount of fine pearlite will be generated, which will hinder cold heading, etc. Especially in steel materials with high quench hardenability that contain Mo, Cr, Ni, etc., martensite and benthite structures will occur. appears. This martensite and bainite structure is unsuitable for any cold working. Moreover, coarse pearlite has a structure suitable for cutting, but is not suitable for wire drawing or cold heading. In this way, the optimal adjusted cooling conditions after hot rolling differ depending on the steel type and the type of subsequent cold working, but generally the temperature range is 750 to 650°C and the cooling rate is 40°C/40°C.
By controlling the cooling to less than 10 minutes, the formation of a structure unsuitable for cold working is avoided, and a preferable steel structure similar to that of the spheroidized annealed material is provided. There is no essential limit to the lower limit of the cooling rate, but the time required to complete the transformation is always long, which hinders production at an industrial pace, so it should be approximately 0.5
It is appropriate to set the lower limit to ℃/min. FIG. 1 schematically shows a specific example of the rolling method according to the present invention. The steel material heated to a temperature of 650 to 850° C. in the heating furnace 1 and formed into a mixed structure containing carbides is subjected to predetermined hot rolling in the rolling mill 2. If the temperature rise is significant due to processing strain during hot rolling, the steel material is cooled by the intermediate water cooling means 3 or the finishing water cooling means 4 so that the steel material temperature does not exceed 950°C. The steel material M that has been finish rolled is wound up by a winding machine 5. After coiling, coils, steel bars, etc. are subjected to controlled cooling so that the cooling rate in the temperature range of 750 to 650°C does not exceed 40°C/min. For example, as shown in FIG. 2, this controlled cooling is carried out by forming a steel wire M into a spiral shape using a laying head 6, and sequentially feeding it into an annealing furnace 8 using a conveyor 7 at an appropriate transfer speed. Alternatively, the steel wire M may be cooled at a cooling rate of This can also be carried out by charging the wire bundle M' into the slow cooling furnace 8' for a predetermined period of time, and then extracting it from the furnace. The lehr-cooling furnaces 8 and 8' can be provided with appropriate heat generating means as necessary to make them complete atmospheric furnaces, or a predetermined cooling rate can be given by maintaining a certain degree of atmosphere in the furnace by utilizing the sensible heat of rolling the steel wire. You can do it like this. Note that depending on the steel type and the desired material properties, it is also possible to obtain a predetermined adjusted cooling effect without particularly providing a lehr. As described above, by controlling the temperature before and during rolling and adjusting the cooling after rolling according to the present invention, fine carbides are arranged in the rolling direction in a matrix of fine ferrite grains as pseudo-spherical carbides, as shown below. A dispersed tissue is obtained. The steel thus obtained has material properties equal to or better than those of these heat-treated materials, has high strength, and has excellent cold workability and toughness, even without subsequent heat treatment such as spheroidizing annealing and softening annealing. The reason why good material properties can be obtained as hot-rolled without any additional heat treatment is because, as mentioned above, complete austenitization is suppressed in the heating stage before rolling, and rolling is done with carbides remaining. Moreover, controlled cooling prevented the appearance of structures such as martensite and lower bainite, resulting in the formation of a mixed structure consisting of carbide, ferrite, and a small amount of pearlite in a form close to granular, or a mixed structure of carbide and ferrite. This is because the temperature of the steel material during rolling is controlled much lower than usual, resulting in a significantly finer grain size. Figure 5 (photograph substituted for drawing) [ ] shows steel type SCM21.
(C0.13~0.18%, Si0.15~0.35%, Mn0.6~0.85
%, Cr0.9-1.2%, Mo0.15-0.3%) by the method of the present invention, heated to a temperature of 715℃, subjected to intermediate water cooling and finishing water cooling, and then rolled.
Microscopic structure (magnification 400) of a steel wire rod (wire diameter 8 mmφ) obtained by cooling in the temperature range of 750 to 650 °C at a cooling rate of 0.5 °C/min. This is a microscopic structure (magnification: 400) of a steel wire (wire diameter: 8 mmφ) obtained by subjecting the wire to a preheating temperature of 1080° C. and then annealing it to form a spheroid in a continuous annealing furnace.
As is clear from the comparison between the two, the steel wire produced by the method of the present invention (photo []) has a structure in which finer carbides are more uniformly dispersed than that of the normal spheroidized annealed material (photo []), and moreover, the steel wire rod produced by the method of the present invention (photo []) has a structure in which finer carbides are more evenly dispersed than that of the ordinary spheroidized annealed material (photo []). Since it has a shape closer to a spherical shape, it has excellent cold workability, and it can be seen that it is easy to process bolts and parts by cold heading, for example. In addition, according to the measurement results of the above-mentioned test materials, the normal spheroidized annealed material has a tensile strength of 49 kg/
mm ² and hardness (Hv) of 140, whereas the steel material produced by the method of the present invention has a tensile strength of 53 Kg/mm ² and hardness (Hv).
151, and has slightly higher levels of strength and hardness. The method of the present invention is applicable to various carbon steels and alloy steels. Its component composition may be arbitrary depending on the use and required material properties, but C:
At least 0.03% is required for the formation of carbides. Note that if the C is less than 0.03%, the mechanical properties and cold workability of the hot rolled steel are good even with normal hot rolling, but in the present invention, as mentioned above, C0.03% or more is required to form carbides. In order to achieve this, special hot rolling as described above is carried out to improve mechanical properties and cold workability. On the other hand, if the C content exceeds 1.2%, a structure in which undissolved carbides are spheroidized and dispersed cannot be obtained, and the mechanical properties and cold workability of the hot rolled steel material cannot be improved to the extent that it can withstand cold working such as cold heading. It becomes impossible to improve. For this reason, the upper limit is set at 1.2%. Alloy steel, for example, Si2.5% or less, Mn2.0%
Below, one or more of Ni4.5% or less, Cr3.5% or less, Mo0.7% or less, as grain adjustment elements
Ti0.2% or less, Zr0.05% or less, Nb0.05% or less,
It may contain one or more elements of 0.15% V or less, 0.1% or less Al, and other appropriate elements for improving the necessary material properties. Impurities may be present within the range normally allowed for carbon steel and alloy steel; for example, P and S may each be contained in an amount of 0.04% or less. Examples of such steel types include piano wire,
Hard steel wire rods (SWRH types), various SC materials, SMn materials,
Examples include structural steel materials such as SMnC materials, SCr materials, SCM materials, SNC materials, and SNCM materials. Next, the present invention will be specifically explained with reference to Examples. Example 1 SCr4 (C0.38-0.43%, Si0.15-0.35%, Mn0.6
~0.85%, P0.03% or less, S0.03% or less, Cr0.9~
1.2%) and heated to a billet temperature of 730℃,
The steel billet temperature was adjusted to 900°C or less by intermediate water cooling or finishing water cooling, and the steel piece was rolled to a wire diameter of 8 mmφ. After winding, the steel was cooled in the temperature range of 750 to 650°C at a cooling rate of 0.8°C/min. The microscopic structure of the obtained wire is shown in FIG. It can be seen that the spherical carbides are in a good condition with fine and uniform dispersion. Comparing this structure with that shown in Fig. 5 (steel type SCM21), the difference is that in addition to undissolved carbides and ferrite, some pearlite appears. This is due to the slightly high C content and the fact that the adjustment cooling after rolling was performed at a slightly faster cooling rate, resulting in a tensile strength of 60 Kg/mm ² and a hardness of Hv164 (H _Br 156). It was slightly higher than the above values, and showed a value similar to that of softened annealed materials. By the way, the hardness of steel material has a certain correlation with its machinability. Figure 4 shows the hardness (H _Br ) of various steel materials.
This is a graph [according to the SAE Handbook (1964)] showing the relationship between H _Br and machining rate (%).
160 shows the best machinability. As mentioned above, the steel wire obtained in this example had a hardness of H _Br 156, indicating excellent machinability. Example 2 SWRCH43K (C0.40~0.46%, Si0.10~0.35
%, Mn0.6~0.9%), SCM21 (C0.13~0.18%,
Si0.15~0.35%, Mn0.6~0.85%, Cr0.9~1.2%,
Mo0.15~0.3%), SCM3 (C0.33~0.38%, Si0.15
~0.35%, Mn0.6~0.85%, Cr0.9~1.2%,
Mo0.15~0.3%), SCR4 (C0.38~0.43%, Si0.15
~0.35%, Mn0.6~0.85%, Cr0.9~1.2%), each steel billet was heated to 750℃, and then subjected to intermediate water cooling or finishing water cooling to reduce the temperature during rolling to 830℃.
Adjusted to below ℃ to make wire rod, 750 to 650℃ after winding.
was cooled at a cooling rate of 1.5°C/min. The wire diameter and tensile strength of each of the obtained steel wires are shown in Table 1 in comparison with those of the spheroidized annealed material and the normal as-rolled material.

[Table] As shown in Table 1 above, it can be seen that the steel materials produced by the method of the present invention have mechanical properties closer to those of spheroidized annealed materials than ordinary as-rolled materials.
In addition, in the case of any type of steel, if the cooling rate after rolling is made slower, mechanical properties comparable to those of spheroidized annealed material can be obtained. Example 3 SCR4 (C0.38-0.43%, Si0.15-0.35%,
Mn0.6~0.85%, Cr0.9~1.2%), SCM21 (C0.13
~0.18%, Si0.15~0.35%, Mn0.6~0.85%,
After heating a steel billet (Cr0.9~1.2%, Mo0.15~0.3%) to 780℃, it is subjected to intermediate water cooling and finishing water cooling to adjust the temperature during rolling to below 900℃, and the wire diameter is 6mmφ. After rolling and winding, the material was cooled in a temperature range of 750 to 650°C at a cooling rate of about 2°C/min. The tensile strength (Kg/mm ² ) and reduction of area (%) of each of the obtained steel wires are shown in Table 2 in comparison with those of a conventionally rolled material and a spheroidized annealed material.

[Table] As shown in Table 2 above, the hot-rolled steel produced by the method of the present invention has a tensile strength close to that of spheroidized annealed steel and a significantly higher reduction of area than ordinary as-rolled steel. Moreover, it can be seen that the result is higher than that of the spheroidized annealed material. In addition, a separate test has confirmed that the critical cracking rate of cold heading is equivalent to or better than that of spheroidized annealed material. As described above, according to the present invention, various carbon steels and alloys can be produced without special heat treatment such as spheroidizing annealing after rolling, and which have material properties equivalent to or better than these heat-treated materials as hot-rolled. Steel materials can be obtained, which have particularly excellent cold workability and toughness, so cold heading, cutting, etc. can be easily carried out, and the processing yield can be increased and the tool life can be improved. Furthermore, since the heat treatment described above is not required, the manufacturing process is simplified, and at the same time, the manufacturing cost is significantly reduced due to the energy saving effect. Note that it is also possible to add heat treatment such as spheroidizing annealing to the steel material produced by the method of the present invention. The processing time in this case is about 1/2 to 1/3 of the processing time normally required for those heat treatments, and an extremely good structure is provided, and material properties such as cold workability are further significantly improved. .

[Brief explanation of the drawing]

Fig. 1 is a process explanatory diagram showing a specific example of the manufacturing method of the present invention, Figs. 2 and 3 are explanatory diagrams showing a specific example of the adjustment cooling means after rolling, and Fig. 4 is the relationship between hardness and cutting rate. The graphs shown in Figures 5 [] to [] are photographs substituted for drawings showing the microscopic structure of steel materials. The main symbols in the drawings are as follows. 1: heating furnace, 2: rolling mill, 3: intermediate water cooling means, 4: finishing water cooling means, 5: winding machine, 7: conveyor, 8, 8': slow cooling furnace.

Claims (1)

[Claims] 1 Carbon steel or alloy steel containing 0.03 to 1.20% C is heated to a temperature of 650 to 850°C to form a mixed structure of austenite and undissolved carbide or a mixed structure of ferrite and undissolved carbide, and this is heated to 950°C. After hot rolling in the following temperature range with a hot rolling finish temperature of 700 to 950°C to prevent dissolution of the carbide, the temperature range of 750 to 650°C is set at a cooling rate of 40°C.
A method for producing a hot-rolled steel material with excellent cold workability and toughness, characterized by forming a fine spheroidal structure with fine ferrite grains and finely dispersed carbide particles by cooling at a temperature below ℃/min. .