EP2006398B1

EP2006398B1 - Process for producing steel material

Info

Publication number: EP2006398B1
Application number: EP07741346.6A
Authority: EP
Inventors: Hitoshi Kataoka; Hirotaka Eguchi
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2006-04-11
Filing date: 2007-04-10
Publication date: 2014-12-17
Anticipated expiration: 2027-04-10
Also published as: CN101421424B; JPWO2007119722A1; WO2007119722A1; KR101074297B1; EP2006398A4; CN101421424A; KR20080112287A; EP2006398A1; JP5088633B2

Description

TECHNICAL FIELD

The present invention relates to a process for producing a steel material having a pearlite nose at which transformation completes for not less than 30 minutes. More particularly, the invention relates to a process for producing a steel material, including a first hot working step a second, hot working step, and process annealing of a semi-product steel material (semi-finished steel product) between the first and the second hot working steps.

BACKGROUND ART

In a case where a steel material is subjected to a first hot working step such as hot forging or hot rolling and then is subjected to a second hot working step such as hot forging or hot rolling, its metallographic structure changes into a hard martensitic structure when it is kept in the atmosphere to cool after the completion of the first hot working step step, if the steel material has extremely high hardenability. A delayed crack then occurs in a short time. There is, therefore, a need to provide an independent step of shortly and carefully process annealing on the steel material in a heating furnace after the first hot working step and before the second hot working step such as hot forging or hot rolling in order to form the metallographic structure into pearlite and reduce the hardness of the steel material.
More specifically, for example, referring to Fig. 2 showing a heat history in the above-described conventional process including, sequentially, a first hot working step, process annealing, and a second hot working step, the worked steel material (semi-product steel material) is cooled (2 in Fig. 2) after the completion of a hot working step 1 such as hot forging or hot rolling. The steel material is promptly introduced into a heating furnace (25 in Fig. 2) for an annealing step 3. In the annealing step 3, the steel material is heated to and kept at a desired temperature not less than the Ac3 point (26 in Fig. 2) under control to completely transform it into austenite. The steel material is controlled to be then cooled in such a slow cooling rate (27, 28, 29 in Fig. 2) as to sufficiently cause pearlitic transformation even if the semi-product steel material has extremely high hardenability, thereby completing process annealing. The process-annealed semi-product steel material is adjusted to have a low hardness as well as the adjustment of the metallographic structure, thus providing the material to be subjected to hot working in the second step.
JP 11302725 discloses a tool steel annealing method which involves granulation annealing in a furnace after reaching particular point between pearlite nose temperature and Ac1 transformation temperature.
As another means of performing slow cooling for generating pearlitic transformation, application of a special cooling chamber provided with a heat insulating material is proposed in JP-A-8-260058 (see Patent document 1).
Patent document 1: JP-A-8-260058

SUMMARY OF THE INVENTION

As descried above, in the case of manufacturing a steel material having such an alloy composition that is particularly high in hardenability, a hot worked semi-product steel material has been temporarily cooled and thereafter subjected to special process annealing, since it is difficult to control a temperature of the material.
Specifically, it has been technically common that the semi-product steel material after the hot working step is allowed to pass the transformation point repeatedly in the process annealing to form the pearlite structure and reducing the hardness without generating any cracks in the steel material. For this reason, the hot working forms a stable austenitic structure, and then the steel material is cooled into a martensitic transformation region or a bainitic transformation region. There has been a need to perform the process annealing so as to repeat transformation, in which the semi-product steel material is thereafter introduced into a heating furnace to be completely transformed into austenite at not lower than the Ac3 point, and then carefully cooled slowly.
When the cooling rate is lowered to about 3.3°C/minute by applying a cooling chamber as an ordinary cooling method, as described in JP-A-8-260058 which discloses a technique for slowly cooling in a special cooling chamber, a fault such as a crack is prevented and the steel material having the pearlite transformed metallographic structure. For an steel in accordance with JIS SKD61 which has hardenability much higher than those of ordinary steel materials and has a transformation completion point at a pearlite nose of not shorter than e.g. 30 minutes, however, cooling at a rate of about 3°C/minute is ineffective to reduce the hardness and obtain an annealed state.
Thus, for a steel having hardenability much higher than those of ordinary steel materials and having a transformation completion point at a pearlite nose of not shorter than e.g. 30 minutes, there have not been established techniques to improve the efficiency of the independent process annealing between the first hot working step and the second hot working step, which has been considered indispensable in the conventional art, and the technique to reliably cause pearlitic transformation and reduce the hardness.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a process for producing a steel material, which does not use any furnace but has the same effect as process annealing using a furnace, in the case of process annealing a alloy steel material having a pearlite nose transformation completion point of not shorter than 30 minutes and having extremely high hardenability.
The present invention is achieved in consideration of the above-described problems.
The present invention provides a process for producing a steel material having a pearlite nose transformation completion point of not shorter than 30 minutes, the process including a first hot working step, a second hot working step and process annealing performed between the first and the second hot working steps, the process annealing including steps of introducing a semi-product steel material after the first hot working step into a heat-insulation vessel, recuperating heat in the steel material, and holding the semi-product steel material at a temperature within a range of the pearlite nose transformation completion point plus/minus 20°C for not shorter than 30 minutes with use of transformation latent heat from the semi-product steel material, so that the semi-product steel material is transformed into pearlite
Preferably, the process for producing the steel material includes holding the semi-product steel material introduced into the heat-insulation vessel at the temperature of the pearlite nose transformation completion point plus/minus 20°C for not shorter than 2 hours.
More preferably, the process for producing the steel material includes holding the semi-product steel material introduced into the heat-insulation vessel at the temperature within the pearlite nose transformation completion point plus/minus 10°C for not shorter than 2 hours. Further preferably, the maximum surface temperature of the semi-product steel material when introduced into the heat-insulation vessel is within the range between the transformation completion point at pearlite nose plus 100°C and the transformation completion point at pearlite nose minus 200°C.
Further preferably, the hardness of the semi-product steel material after the process annealing is not higher than 300 HB, according to the process for producing the steel material.
The process for producing the steel material according to the invention is particularly favorable for the semi-product steel material having a weight of not less than 500 kg.
The process for producing the steel material according to the invention is particularly desirable for a semi-product steel material having a chemical composition containing, by mass percent: 0.10 to 2.0% of C; not more than 2.0% of Si; not more than 2.0% of Mn; 1.0 to 15.0% of Cr; and not more than 10.0% of Mo; at least one of not more than 4.0% of Ni, not more than 4.0% of V, not more than 20.0% of W, and not more than 10.0% of Co; and the balance being substantially Fe.
According to the method of process annealing a steel material having a pearlite nose transformation completion point of not shorter than 30 minutes of the invention, the temperature of the steel material is maintained by introducing the steel material into the heat-insulation vessel and utilizing pearlite transformation latent heat of the steel material, instead of using a heating furnace, thereby achieving the same effect as that of process annealing.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a heat pattern diagram showing an example of processing annealing according to the present invention;
Fig. 2 is a heat pattern diagram showing an example according to conventional process annealing;
Fig. 3 is a diagram showing a pearlite nose transformation completion point;
Fig. 4 is a diagram showing a TTT curve indicating a pearlite nose transformation completion time of SKD61;
Fig. 5A is a schematic front view showing a non-enclosed state of an example of a heat-insulation vessel according to the present invention;
Fig. 5B is a schematic side view of the heat-insulation vessel shown in Fig. 5A;
Fig. 5C is a schematic front view showing an enclosed state of the heat-insulation vessel shown in Fig. 5A;
Fig. 5D is a schematic side view of the heat-insulation vessel shown in Fig. 5C;
Fig. 6A is a schematic front view showing a non-enclosed state of another example of the heat-insulation vessel according to the present invention;
Fig. 6B is a schematic side view of the heat-insulation vessel shown in Fig. 6A;
Fig. 6C is a schematic front view showing an enclosed state of the heat-insulation vessel shown in Fig. 6A;
Fig. 6D is a schematic side view of the heat-insulation vessel shown in Fig. 6C;
Fig. 7A is a schematic front view showing a non-enclosed state of still another example of the heat-insulation vessel according to the present invention;
Fig. 7B is a schematic side view of the heat-insulation vessel shown in Fig. 7A;
Fig. 7C is a schematic front view showing an enclosed state of the heat-insulation vessel shown in Fig. 7A;
Fig. 7D is a schematic side view of the heat-insulation vessel shown in Fig. 7C; and
Fig. 8 is a schematic diagram showing a further example of the heat-insulation vessel according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described below in detail.
A process for producing a steel material having a pearlite nose transformation completion point of not shorter than 30 minutes includes, for example, a melting step of casting a molten steel to obtain a steel ingot, a step of performing hot working a certain number of times on the steel ingot obtained by the melting step, and process annealing performed between executions of hot working. The hot-worked and process annealed material may be subjected to heat treatment such as annealing or quenching and tempering.
Process annealing according to the invention is defined as annealing performed between the first and the second hot working steps as described above. For example, in a case of process for producing a steel material including hot pressing, first annealing, hot forging (cogging), second annealing, hot rolling, and third annealing, the first and second annealings correspond to the process annealing according to the invention.
The invention is applied to production of a steel material having a pearlite nose transformation completion point of not shorter than 30 minutes and having high hardenability, for which independent process annealing under strict temperature control has been considered indispensable.
The steel material having a pearlite nose transformation completion point of not shorter than 30 minutes includes e.g. alloy tool steels such as SKD11, SKD61 and SKT4. The steel material annealing method according to the invention is more advantageously applied to a steel material having a pearlite nose transformation completion point of not shorter than 60 minutes, more preferably, to a steel material having a pearlite nose transformation completion point of not shorter than 90 minutes, and further preferably to a steel material having a pearlite nose transformation completion point of not shorter than 2 hours.
The pearlite nose transformation completion point may be measured as described below.
A steel material of e.g. SKD61 is transformed to have an austenitic metallographic structure, quenched to a certain temperature not higher than the A1 point and held at the temperature. After a certain time period from the start of holding at the temperature, pearlitic transformation starts. With a lapse of time, the transformation ends. When the material is cooled before the completion of the transformation, the remaining austenite is transformed into martensite. Therefore, the time period before the end of the transformation can be obtained by measuring the hardness.
These experiments are made at different temperatures, and the transformation end points are plotted on a graph and connected by a curved line, thereby obtaining a diagram (TTT curve) such as shown in Fig. 4. In this curve, a portion at the shortest-time where the hardness decreases abruptly at a certain temperature is the pearlite nose transformation completion point. No austenitic structures are recognized there.
For example in the case of SKD61, pieces of steel material heated to a hot forging temperature are introduced into furnaces at temperatures set every 25°C steps in a range from 700 to 775°C and are held for 2, 5, 15 and 24 hours. The pieces of steel material held at the temperatures and for the periods as described above are taken out of the furnaces and cooled by air, and the hardness of the steel material is measured. The pearlite nose transformation completion point is in the vicinity of at 750°C for 5 hours, that is shortest-time point in the points where the hardness decreases abruptly.
Description will be made regarding the pearlite nose transformation completion point of not shorter than 30 minutes with reference to Fig. 3. In the TTT curve indicated by the holding temperature (°C) on the ordinate and the holding temperature (minutes) on the abscissa, a pearlite nose transformation completion point located at a holding time of 30 minutes is referred to as a pearlite nose transformation completion point of 30 minutes (a curved line represented by a dotted line in Fig. 3), and a pearlite nose transformation completion point located on the longer-holding-time side including 30 minutes is referred to as a pearlite nose transformation completion point of not shorter than 30 minutes (a curved line represented by a solid line in Fig. 3).
According to the invention, specified process annealing is performed on the above-described steel material having a pearlite nose transformation completion point of not shorter than 30 minutes and having good hardenability. The reason for the process annealing method specified in the invention will be described in detail with reference to a heat pattern shown in Fig. 1.
After a hot working step (1 in Fig. 1) such as hot forging or hot rolling, the semi-product steel material is cooled (2 in Fig. 1: a post-hot-working cooling step). In the course of the post-hot-working cooling step, the semi-product steel material is introduced into a heat insulation vessel and an annealing step 3 is started. The semi-product steel material is introduced into the heat insulation vessel to recuperate the surface temperature of the semi-product steel material (21 in Fig. 1).
Recuperation is defined as follows. The semi-product steel material introduced into the heat insulation vessel is increased in the surface temperature through heat conduction from an inner portion of the semi-product steel material and radiant heat from the walls of the heat insulation vessel, and the temperature difference between the internal temperature of the semi-product steel material and the surface temperature of the semi-product steel material is reduced. This is a preparation to temperature holding with use of transformation latent heat of the semi-product steel material.
Subsequently, the temperature is held (22 in Fig. 1) with the transformation latent heat of the semi-product steel material in the heat insulation vessel. The invention utilizes heat generated when the semi-product steel material transforms to keep the temperature of the material. That is, the present invention keeps the temperature of the semi-product steel material in a constant temperature range for a certain time period utilizing transformation latent heat of the semi-product steel material to transform the material to pearlite. For this purpose, the held temperature range and the time period are extremely important.
According to the invention, held temperature is at a temperature within a range of the pearlite nose transformation completion point plus 20°C and the pearlite nose transformation completion point minis 20°C for not shorter than 30 minutes. Holding in such a particular temperature range enables obtaining an annealed state, which has not been obtained. Moreover, according to the invention, the efficiency is extremely high as a result of finding that transformation latent heat in the semi-product steel material can be utilized, which will be described below in detail.
The reason why the held temperature is selected as the range within the pearlite nose transformation completion point plus/minus 20°C according to the invention, is that pearlite transformation is completed in a shorter time period in this range and the hardness can be reduced by utilizing transformation latent heat. A temperature range exceeding the pearlite nose transformation completion point plus 20°C requires a longer time to complete the pearlitic transformation and is, therefore, uneconomical, and a problem also arises that the metallographic structure easily becomes coarser. A temperature range below the pearlite nose transformation completion point minus 20°C also requires a longer time to complete pearlitic transformation and is, therefore, uneconomical, and entails a problem that pearlitic transformation cannot occur and progress, resulting in insufficient reduction of the hardness. The preferable temperature range is within a range of the pearlite nose transformation completion point plus/minus 10°C.
The reason why the holding time is selected to be not shorter than 30 minutes is because pearlitic transformation may not complete if the holding time is shorter than 30 minutes, and, in such a case, softening will be insufficient due to insufficient precipitation of carbides and a fault such as a crack will be generated. A preferable holding time at the temperature is not shorter than 1 hour. A more preferable holding time is not shorter than 2 hours. A further more preferable range of the holding time is 2 to 24 hours.
By keeping the temperature within the pearlite nose transformation completion point plus/minus 20°C for not shorter than two hours, the hardness after process annealing can be reduced to not higher than 300 HB, which is preferable for process-annealed material. The hardness after process annealing is preferably not higher than 270 HB, more preferably not higher than 250 HB.
The maximum surface temperature of the semi-product steel material, when the semi-product steel material is introduced into the heat insulation vessel, is in a range between the pearlite nose transformation completion point plus 100°C and the pearlite nose transformation completion point minus 200°C.
If the semi-product steel material is introduced at a temperature exceeding the pearlite nose transformation completion point plus 100°C, the temperature of the semi-product steel material will become excessively high by recuperation of heat from the semi-product steel material and the metallographic structure will easily have coarser grains. This coarsening of the structure is noticeable in a temperature range not lower than the pearlite nose transformation completion point plus 200°C. Therefore, the preferable upper limit of the temperature at which the semi-product steel material is introduced into the heat insulation vessel, is determined to be the pearlite nose transformation completion point plus 100°C in consideration of recuperation of heat from the semi-product steel material.
When the temperature of the semi-product steel material surface exceeds the pearlite nose transformation completion point plus 200°C after the material has been introduced, it is preferable to open a portion of the heat insulation vessel described below in order to limit the increase in temperature of the material.
The lower limit of the maximum surface temperature of the steel material, when the material is introduced into the heat insulation vessel, is determined to be the pearlite nose transformation completion point minus 200°C. This is because heat-recuperation is difficult to heat the semi-product steel material to the temperature within the pearlite nose transformation completion point plus/minus 20°C in the heat insulation vessel if the maximum surface temperature of the steel material is lower than the pearlite nose transformation completion point minus 200°C. As a preferable temperature range at which the semi-product steel material is introduced into the heat insulation vessel, the maximum temperature of the material surface is between the pearlite nose transformation completion point plus 50°C and the pearlite nose transformation completion point minus 150°C. More preferably, it is between the pearlite nose transformation completion point plus 50°C and the pearlite nose transformation completion point minus 100°C. The maximum surface temperature of the steel material refers to the temperature of a region where the temperature is highest in the entire surface of the steel material.
The reason why the semi-product steel material surface is focused in the invention is because heat during hot working remains inside the semi-product steel material and the temperature is lowest at the surface exposed to the external atmosphere. Therefore, the temperature of the semi-product steel material surface is defined according to the invention. The temperature of the material surface may be measured, for example, by a radiation thermometer.
The heat insulation vessel referred to in the invention is, for example, a box-like member or lid-like member with which the semi-product steel material is covered.
The heat insulation vessel has no heating sources. It may be of any structures which enables the semi-product steel material to be transformed into pearlite with the transformation latent heat from the material by holding the material in a certain temperature range for a certain time period, for example, by providing a heat insulating member inside of the box-like or lid-like member to form an enclosed space. The heat insulation vessel may be provided with holes or the like, e.g., for insertion of a thermocouple thermometer, for visually checking of the color tone of the steel material in the vessel or an opening having an openable/closable structure for adjusting the temperature of the material surface. It is not necessarily required to form an enclosed space completely isolated from the external atmosphere.
The heat insulation vessel 7 may have a structure shown in schematic front and side views in Figs. 5A to 5D, in which a semi-product steel material 4 is placed on a heat insulation vessel base 5 and a heat insulation vessel upper lid 6 is put over the base 5 to form an enclosed space surrounding the material 4. Alternatively, it may have a structure shown in schematic front and side views in Figs. 6A to 6D, in which the semi-product steel material 4 is placed in a heat insulation vessel lower lid 8 and a heat insulation vessel upper lid 6 is put to form an enclosed space surrounding the material 4. Alternatively, it may have a truck-type structure shown in Fig. 8, in which the semi-product steel material 4 is placed on a heat insulation vessel base 5 and a heat insulation vessel 7 with wheels 10 capable of traveling on rails 11 is caused to move in the direction of the arrow to form an enclosed space surrounding the material 4. In this case, the vessel 7 may be fixed and the base provided with wheels may be moved.
The structure of the heat insulation vessel may be designed by considering the shape and weight of the semi-product steel material, according to the invention. For example, in a case where the semi-product steel material is in a form of a round bar, for which collapse of a stack possibly occurs, the vessel may have a structure shown in Figs. 7A and 7C, in which the lower lid 7 is formed to have a triangular section where round bars of the material 4 are loaded in the lower lid 7 and the upper lid 6 is put thereon. In this case, it is preferable to provide a rollover prevention member 9 for the lower lid for preventing rollover of the lower lid 8. While the rollover prevention member 9 is illustrated to have columnar form by way of example in Figs. 7A to 7D, they may have an M-shaped or V-block formed section.
In a case of large sized semi-product steel material, such as those hot-forged from steel ingots, it is advantageous to use a heat insulation vessel having a structure shown in Figs. 5C and 5D, Figs. 6A to 6D or Figs. 7A to 7D in terms of workability and working safety. Among the schematic figures in the accompanying drawings, Figs. 5A and 5B, Figs. 6A, and 6B and Figs. 7A and 7B show states where enclosed spaces are not formed, while Figs. 5C and 5D, Figs. 6C and 6D and Figs. 7C and 7D show states where enclosed spaces are formed.
According to the invention, the annealing step 3 is executed, in which a temperature of the semi-product steel material is kept by using the above-described heat insulation vessel, and the semi-product steel material is taken out from the vessel (23 in Fig. 1) to be subjected to slow cooling (24 in Fig. 1), thus completing the annealing step 3.
The invention is particularly preferably applied to a large-sized piece of semi-product steel material having a weight of not less than 500 kg. This is because the large-sized piece of the material having a weight of not less than 500 kg ensures a necessary amount of heat for keeping a temperature for recuperation with transformation latent heat for a sufficient time for pearlitic transformation. The above-described effect may be further obtained for a large piece of the semi-product steel material having a weight of not less than 1 ton. It is further effective for a piece of not less than 4 ton.
According to the invention, as described above, the semi-product steel material is held at a temperature by utilizing pearlitic transformation latent heat so as not to form a martensitic metallographic structure. Thus, substantially same effect is achieved as the conventional process annealing using a furnace.
"Steel material" (or steel product) referred to in the invention is a steel material worked into a desired shape by working such as rolling, forcing or drawing. "Semi-product steel material" referred to in the invention corresponds to "billet" in the JIS terminology and is a material to be formed into the above-described steel product by hot working. For example, "semi-product steel material" includes "slab", "bloom", "billet", "sheet bar" or a circular billet having a diameter larger than 130 mm in the JIS terminology.
When the packing rate of the semi-product steel material in the heat insulation vessel is not less than 15%, the above-described advantage of the invention can be obtained more reliably.
The reason is described below. If the packing rate is not less than 15%, the heat amount of the semi-product steel material is large enough and the time before recuperation is therefore shortened. As a result, the effect of reliably keeping the temperature within the range of the pearlite nose transformation completion point plus/minus 20°C can be obtained. Conversely, if the packing rate is lower than 15%, the amount of loaded material is small, which is disadvantageous in terms of economy. Since the heat amount of the material is small, the time taken before recuperation becomes long. Further, it may be necessary to improve the effect of thermally insulation of the vessel.
A preferable upper limit of the packing rate is 95%. If 100% is reached, the vessel is necessary to have the same size as that of pieces of the material, and there is no freedom in designing the size of pieces of the material, resulting in a low usability. It is, therefore, desirable to determine a preferable upper limit of 95%.
The process for producing a semi-product steel material according to the invention is effective for alloys belonging to the category of tool steel in JIS standard. The process is particularly effective for alloys having a composition described below. Note that content of each element below is denoted by mass percent.

C: 0.10 to 2.0%

The reason for determine the carbon content to be 0.10 to 2.0% is as follows. Carbon may not be diffused into grains so that no carbides are precipitated in grains and pearlitic transformation does not generate as desired, if the carbon content is lower than 0.10 %. Preferably, the carbon content is not lower than 0.1%. If the carbon content exceeds 2.0%, the amount of carbides is in excess and the toughness is reduced. Preferably, the carbon content is 0.20 to 0.60%.

Si: not more than 2.0%

Silicon is added as a deoxidizer at the time of melting. If a large amount of silicon is added, the toughness is reduced. According to the invention, therefore, the silicon content is not less than 2.0%. Preferably, the content is 0.15 to 1.20%.

Mn: not more than 2.0%

Manganese is added as a deoxidizer and desulfurizing agent at the time of melting. If a large amount of manganese is added, the toughness is reduced. According to the invention, therefore, the manganese content is not more than 2.0%. Preferably, the content is 0.30 to 1.00%.

Cr: 1.0 to 15.0%

Chromium increases the hardenability and improves the tensile strength and toughness. However, if a large amount of chromium is contained, the toughness is conversely reduced. According to the invention, therefore, the chromium content is 1.0 to 15.0%. Preferably, the content is 1.0 to 13.0%.

Mo: not more than 10.0%

Molybdenum improves the hardenability. Also, molybdenum forms a fine carbide through tempering to increase the high-temperature tensile strength. However, if a large amount of molybdenum is contained, the toughness is conversely reduced. Therefore, the molybdenum content is not more than 10.0%. Preferably, the content is 0.20 to 5.0%.
Nickel, Vanadium, Tungsten and Cobalt described below are selective elements. One or more of them are contained.

Ni: not more than 4.0%

Nickel increases the hardenability and improves the toughness. However, if a large amount of nickel is contained, the transformation point is lowered, so that the high-temperature strength is reduced. Therefore, if nickel is contained, the content is not more than 4.0%, preferably not more than 2.0%.

V: not more than 4.0%

Vanadium makes crystal grains finer and increases the toughness. Vanadium forms a high-hardness carbonitride through tempering to increase the tensile strength. However, if a large amount of vanadium is added, the toughness is reduced. Therefore, if vanadium is contained, the content is not more than 4.0%, preferably 0.10 to 1.10%.

W: not more than 20.0%

Tungsten increases the hardenability. Also, tungsten forms a fine carbide through tempering to form the high-temperature tensile strength. However, if a large amount of W is contained, the toughness is reduced. Therefore, if tungsten is contained, the content is not more than 4.0 %, preferably 0.10 to 1.10%.

Co: not more than 10.0%

Cobalt increases the red-heat hardness and increases the high-temperature tensile strength. However, if a large amount of cobalt is added, the toughness is reduced. Therefore, if cobalt is contained, the content is not more than 10.0%.

The balance being substantially Fe

According to the invention, the balance other than the elements specified above is substantially Fe. Naturally, impurities may be inevitably contained. For example, since niobium and titanium are elements effective in making crystal grains finer, they may be contained in a range of not more than 0.20% such that the toughness is not disadvantageously reduced.
Aluminum is an element which makes diffusion of carbon faster and is effective in promoting precipitation of carbides during pearlitic transformation. Therefore, aluminum may be contained in a range of not more than 0.20%.

Examples

The invention will be further described in detail with respect to examples thereof referring to Fig. 1.

Three alloys: JIS SKD61 (steel No. 1), JIS SKT4 (steel No. 2) and JIS SKD11 (steel No. 3) were produced by melting in the atmosphere to obtain ingots having chemical compositions shown in Table 1.

[Table 1]

(mass%)
No.	C	Si	Mn	Cr	Ni	Mo	V	balance	JIS steels
1	0.40	1.0	0.4	5.0	-	1.4	0.7	Fe and inevitable impurities	SKD61
2	0.55	0.2	0.8	1.2	1.6	0.3	0.1	Fe and inevitable impurities	SKT4
3	1.50	0.2	0.4	11.7	-	0.8	0.2	Fe and inevitable impurities	SKD11

The three ingots were hot-forged to obtain semi-product steel materials, and test pieces are taken to measure a pearlite nose transformation completion point.
Measurement of a pearlite nose transformation completion point on the semi-product steel material (JIS steel SKD61) having the chemical component No. 1 was performed as follows. Test pieces were heated to a hot forging temperature (1150°C), thereafter were introduced into furnaces at temperatures set every 25°C steps in a range from 700 to 775°C to be held for 2, 5, 15 and 24 hours. After the isothermal holding, the test pieces were taken out from the furnaces and cooled by air and the hardness thereof was measured. A point at which the hardness was reduced at the shortest-time holding was estimated as a pearlite nose transformation completion point. The pearlite nose transformation completion point of the semi-product steel material No. 1 was in the vicinity of a point at 750°C for 5 hours.
Measurement of a pearlite nose transformation completion point on the semi-product steel material (JIS steel SKT4) having the chemical component No. 2 was as follows. Test pieces were heated to a hot forging temperature (1150°C), thereafter were introduced into furnaces at temperatures set every 25°C steps in the temperature range from 600 to 750°C to be held for 2, 5, 10, 24 and 48 hours. After isothermal holding, the test pieces were taken out from the furnaces and cooled by air and the hardness was measured. A point at which the hardness was reduced at the shortest-time holding was estimated as a pearlite nose transformation completion point. The pearlite nose transformation completion point of the semi-product steel material No. 2 was in the vicinity of a point at 650°C for 10 hours.
Measurement of a pearlite nose transformation completion point on the semi-product steel material (JIS steel SKD11) having the chemical component No. 3 was as follows. Test pieces were heated to a hot forging temperature (1150°C), thereafter were introduced into furnaces at temperatures set every 25°C steps in the temperature range from 675 to 775°C to be held for 2, 5, 10 and 24 hours. After isothermal holding, the test pieces were taken out from the furnaces and cooled by air and the hardness was measured. A point at which the hardness was reduced at the shortest-time holding was estimated as a pearlite nose transformation completion point. The pearlite nose transformation completion point of the semi-product steel material No. 3 was in the vicinity of a point at 725°C for 2 hours.
Next, process annealing was carried out by using the hot-forged semi-product steel materials having the chemical compositions Nos. 1 to 3. The size of pieces of the hot-forged semi-product steel material A having the composition No. 1 was 430 mm (t) x 430 mm (w) x 3000 mm (1) x 2 pieces, and the weight thereof was about 8600 kg. The size of the hot-forged semi-product steel material B having the composition No. 2 was 520 mm (t) x 830 mm (w) x 2400 mm (1) x 2 pieces, and the weight thereof was about 8000 kg. The size of the hot-forged semi-product steel material C having the composition No. 3 was 370 mm (t) x 370 mm (w) x 3500 mm (1) x 2 pieces, and the weight thereof was about 7500 kg. These semi-product steel materials correspond to "billet" or "bloom" in the JIS terminology. These materials are formed into steel products by being hot forged after the process annealing.
Concrete hot forging-process annealing conditions are described below.
The semi-product steel product No. A was subjected to hot forging at 1250°C (1 in Fig. 1), and then proceeded to the post-hot-working cooling step (2 in Fig. 1) in air. Preparation for process annealing 3 was made during the post-hot-working cooling step by mounting the steel material 4 on heat insulation vessel base 5 as shown in Figs. 5A and 5B. The maximum surface temperature of the semi-product steel was measured with a radiation thermometer and the steel material 4 was covered with insulation vessel upper lid 6 at 620°C, thereby completing introduction of the material 4 into the vessel 7. The packing rate of the material in the vessel was 35.6%.
After introducing the semi-product steel material, the temperature of the surface of the material was measured with a sheath thermocouple attached to the vessel. After heat recuperation to 800°C in the vessel (21 in Fig. 1), the semi-product steel material was held at the temperature within the pearlite nose transformation point plus/minus 20°C (730 to 770°C) for 5 hours to be transformed into pearlite (22 in Fig. 1). The material was then taken out from the vessel at 500°C (23 in Fig. 1) and cooled in air (24 in Fig. 1).
The semi-product steel product No. B was subjected to hot forging at 1250°C (1 in Fig. 1), and then proceeded to the post-hot-working cooling step (2 in Fig. 1) in air. Preparation for process annealing 3 was made during the post-hot-working cooling step by mounting the semi-product steel material 4 on heat insulation vessel base 5 as shown in Figs. 5A and 5B. The maximum surface temperature of the semi-product steel was measured with a radiation thermometer and the steel material 4 was covered with insulation vessel upper lid 6 at 600°C, thereby completing introduction of the material 4 into the vessel 7. The packing rate of the material in the vessel was 33.2%.
After introducing the semi-product steel material, the temperature of the surface of the material was measured with a sheath thermocouple attached to the vessel. After heat recuperation to 700°C in the vessel (21 in Fig. 1), the semi-product steel material was held at the temperature within the pearlite nose transformation point plus/minus 20°C (630 to 670°C) for 15 hours to be transformed into pearlite (22 in Fig. 1). The material was then taken out from the vessel at 500°C (23 in Fig. 1) and cooled in air (24 in Fig. 1).
The semi-product steel product No. C was subjected to hot forging at 1150°C (1 in Fig. 1), and then proceeded to the post-hot-working cooling step (2 in Fig. 1) in air. Preparation for process annealing 3 was made during the post-hot-working cooling step by mounting the semi-product steel material 4 on heat insulation vessel base 5 as shown in Figs. 5A and 5B. The maximum temperature of the semi-product steel surface was measured with a radiation thermometer and the steel material 4 was covered with insulation vessel upper lid 6 at 680°C, thereby completing introduction of the material 4 into the vessel 7. The packing rate of the material in the vessel was 30.7%.
After introducing the semi-product steel material, the temperature of the surface of the material was measured with a sheath thermocouple attached to the vessel. After heat recuperation to 850°C in the vessel (21 in Fig. 1), the semi-product steel material was held at the temperature within the pearlite nose transformation point plus/minus 20°C (705 to 745°C) for 4 hours to be transformed into pearlite (22 in Fig. 1). The material was then taken out from the vessel at 500°C (23 in Fig. 1) and cooled in air (24 in Fig. 1).
A comparative examples, semi-product steel materials No. D of JIS steel SKD61, No. E of JIS steel SKT4 and No. F of JIS steel SKD11 were hot-forged and thereafter cooled adjusting the cooling rate to 50°C/h. It was found that it passes on the shorter-time side of the pearlite nose transformation completion point from the cooling rate, the TTT curve and the hardness.
The semi-product steel materials used in the comparative examples had the same composition as those according to the invention, and the same hot forging conditions and the same sizes after hot forging as those according to the invention.
As examples for conventional art, steel materials were subjected to annealing under the conditions shown in Fig. 2. A conventional semi-product steel materials No. G is of a material according to JIS steel SKD61, No. H is of JIS steel SKT4, and No. I is of JIS steel SKD11.
The conventional semi-product steel materials were subjected to the hot forging step 1, cooled by air (2 in Fig. 2), and introduced into a heating furnace (25 in Fig. 2), followed by the annealing step 3. In the annealing step, the steel material was heated to and held at a temperature not lower than the Ac3 point (26 in Fig. 2), completely transformed into austenite, and then slowly cooled to complete the process-annealing. The dotted line in Fig. 2 indicates the Ac3 point.
The semi-product steel materials used in the comparative examples and those according to the examples of the invention had the same compositions, the same hot forging conditions and steel product sizes after hot forging. The annealing temperature of not lower than the Ac3 point and the holding time for the semi-product steel material were 870°C x 5h for No. D, 750°C x 5h for No. E, and 870°C x 5h for No. F.

Table 2 shows the results of the metallographic structure and hardness of the semi-product steel materials of the examples according to the invention, the comparative examples and the examples of the conventional art after process annealing. The holding times of the semi-product steel materials introduced into the heat insulation vessel at the temperature within the pearlite nose transformation completion point plus/minus 10°C were 2.5 hours for No. A, 7.5 hours for No. B, and 2 hours for No. C.

[Table 2]

Semi-product steel material No.	JIS steel	Cooling rate at pearlite nose transformation completion point plus/minus 20°C (°C/h)	passing time at pearlite nose transformation completion point plus/minus 20°C (h)	Metallographic structure after process annealing	Hardness after process annealing (HBW)	Remark
A	SKD61	8.0	5.0	Pearlite	207	the invention
B	SKT4	2.7	15.0	Pearlite	229
C	SKD11	10.0	4.0	Pearlite	248
D	SKD61	50.0	0.8	Pearlite + martensite	444	Comparative example
E	SKT4	50.0	0.8	Pearlite + martensite	477
F	SKD11	50.0	0.8	Pearlite + martensite	495
G	SKD61	20.0	2.0	Pearlite	201	Example of conventional art
H	SKT4	20.0	2.0	Pearlite	223
I	SKD11	20.0	2.0	Pearlite	207

As is apparent from Table 2, the metallographic structures and the harnesses after process annealing of the semi-product steel materials Nos. A, B, and C to which the present invention was applied and are equivalent to the semi-product steel materials Nos. G, H, and I of the conventional art when compared between corresponding materials.
The semi-product steel materials Nos. D, E, and F according to the comparative example which are cooled at higher cooling rates and having passed on the shorter-time side of the pearlite nose transformation completion point have different metallographic structures from the semi-product steel materials Nos. A, B, and C to which the process annealing of the present invention was applied, and have higher hardnesses in comparison between corresponding materials.
According to the process of the invention, as described above, process annealing can be performed without using a heating furnace.
After process annealing, subsequent-step hot working (hot forging) was performed successfully on the semi-product steel materials Nos. A, B, and C to which the present invention was applied and the semi-product steel material Nos. G, H, and I to which process annealing in the conventional process was applied, thereby these steel materials were worked into steel products.

INDUSTRIAL APPLICABILITY

According to the process for manufacturing a steel material of the present invention, temperature is maintained by using pearlitic transformation latent heat from the steel material, thereby obtaining substantially the same effect as that of process annealing using a heating furnace, without making the metallographic structure a martensite. Therefore, according to the present invention, a steel material can be annealed, for example, by utilizing a time for transporting it on the sea or land, so that distribution of steel materials is facilitated and it contributes to energy saving.

Claims

A process for producing a steel material having a pearlite nose transformation completion point of not shorter than 30 minutes, the process comprising:
a first hot working step (1),

a second hot working step, and

an annealing process (3) performed between the first hot working step (1) and the second hot working step, the annealing process (3) including the steps of:
introducing a semi-product steel material (4) into a heat insulation vessel (7) after the first hot working step (1),

recuperating (21) heat in the steel material (4), and

holding (22) the semi-product steel material (4) at a temperature within a range of a pearlite nose transformation completion point plus/minus 20°C with use of transformation latent heat from the semi-product steel material (4) for transforming into pearlite.
The process according to claim 1, wherein the temperature of the semi-product steel material (4) introduced into the heat insulation vessel (7) is held (22) at a temperature within the range of the pearlite nose transformation completion point plus/minus 20°C for not shorter than 2 hours.
The process according to claim 1 or 2, wherein the temperature of the semi-product steel material introduced into the heat insulation vessel (7) is held (22) at a temperature within a range of the pearlite nose transformation completion point plus/minus 10°C for not shorter than 2 hours.
The process according to any one of claims 1 to 3, wherein a maximum surface temperature of the semi-product steel material (4), when the semi-product steel material (4) is introduced into the heat-insulation vessel (7), is within a range between the pearlite nose transformation completion point plus 100°C and the pearlite nose transformation completion point minus 200°C.
The process according to any one of claims 1 to 4, wherein the semi-product steel material has a weight of not less than 500 kg.
The process according to any one of claims 1 to 5, wherein the semi-product steel material consists of, by mass percent:
0.10 to 2.0% of C;

up to 2.0% of Si;

up to 2.0% of Mn;

1.0 to 15.0% of Cr;

up to 10.0% of Mo;

at least one of up to 4.0% of Ni, up to 4.0% of V, up to 20.0% of W, and up to 10.0% of Co; and

the balance being substantially Fe.