JPH08269541A - Production of hot rolled high carbon steel plate excellent in hardenability and workability - Google Patents

Abstract

PURPOSE: To produce a hot rolled high carbon steel plate excellent in hardenability and workability by successively applying hot rolling, coiling, and annealing to a high carbon steel stock of specific composition under respectively specified conditions. CONSTITUTION: A stock of a high carbon steel, having a composition consisting of, by weight, 0.2-1.3% C, 0.1-1.0% Si, 0.05-2.0% Mn, <=0.05% P, <=0.05% S, 0.01-0.2% Al, <=0.05% N, and the balance Fe with inevitable impurities, is heated at 1000-1300 deg.C and roughed at a temp. between the Ar, point and 950 deg.C at >=50% draft, and rolling is finished at >=600 deg.C. Subsequently, finish rolling is done at a temp. between 600 deg.C and <Ar1 at 10-<30% draft. At this time, rolling is done at >=0.15 coefficient of friction at least at the final stand of a finish rolling mill. Further, the resulting hot rolled plate is coiled at 450-700 deg.C and then held at 500-700 deg.C for 10sec-8hr to undergo annealing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high carbon hot-rolled steel sheet suitable for use in structural steel, steel for machine parts, tool steel and the like.

[0002]

2. Description of the Related Art Hot-worked carbon steel is manufactured under normal manufacturing conditions, namely, heating temperature: 1200 to 1300 ° C, hot rolling finishing temperature: 800 to 900 ° C, coiling temperature: 400 to 650 ° C. In this case, the structure generally has a layered carbide called pearlite. Since such a layered carbide deteriorates workability and causes quenching failure and toughness deterioration during heat treatment such as quenching, it is common to make the carbide spherical in the next step.

As a method of spheroidizing a layered carbide, conventional methods include a. A method in which a batch furnace is used for heating for a long time to a temperature just below A ₁ point and then cooling, b. After heating to a temperature just above the A ₁ point, but such a method of slowly cooling the vicinity of A ₁ point is known, both required a long processing time of up to several tens of hours.

Therefore, as a remedy for the above problem, c. After hot working, above the dynamic recrystallization temperature of ferrite, A
After addition of machining at _one point temperatures below, 630 ° C. or higher, a method of holding 5 hours 5 minutes to a temperature of less than ₁ point A (JP-B-63-1
No. 4045), and d. A method of transforming into pearlite, bainite, martensite, etc. during hot working, then rapidly heating, and then working in a temperature range of Ac ₃ point or less, and then performing the treatments a and b in a relatively short time. (JP-A-63-86814 and 63-86815)
No. 63-89617).

[0005]

However, the above-mentioned c. In the method of (3), not only heating is required during processing, but it is difficult to control the temperature, and the above d. Each of these methods left problems, where the subsequent processing still required a considerable amount of time. In addition, although uniformly refining the carbide in the plate thickness direction is advantageous for general cold workability, hardenability, and toughness of the steel plate, burrs are generated during punching and so-called precision There was a problem that the punching workability was poor and the bending workability was not sufficient.

The present invention has been developed in view of both the problems in the production of the above-mentioned fine spheroidized carbide-dispersed steel and the problems in the characteristics thereof, and it is a so-called hot rolling process in a factory and a short time. It has the same characteristics as fine and uniform spherical carbide-dispersed steel only by the annealing process, and also has the problems of conventional fine carbide-dispersed steel such as workability, especially precision punching workability and bending workability. It is an object of the present invention to propose a method for producing a high-carbon hot-rolled steel sheet which is advantageously improved.

[0007]

Means for Solving the Problems Now, as a result of intensive studies to achieve the above object, the inventors completed the transformation from austenite to layered pearlite before or during finish rolling in the hot rolling step, It is extremely effective for achieving the intended purpose that after rolling processing, rolling at high temperature, and then annealing for a short time, it is very effective in achieving the intended purpose. The finding that only a certain proportion of the steel sheet surface layer structure with respect to the plate thickness direction can be made into a fine carbide dispersed structure, and punching workability and bending workability, which were problems of conventional steel plates, can be advantageously improved. Got

The present invention is based on the above findings, and its gist is as follows. 1) C: 0.2-1.3 wt%, Si: 0.1-1.0 wt%, M
n: 0.05 to 2.0 wt%, P: 0.05 wt% or less, S: 0.05w
A high carbon steel material containing t% or less, Al: 0.01 to 0.2 wt%, N: 0.05 wt% or less, and the balance of Fe and unavoidable impurities is heated to a temperature of 1000 to 1300 ° C. and then Ar. ₃
Rolling is carried out at a rolling reduction of 50% or more in the temperature range of at least 950 ° C and below 950 ° C.
Finishing rolling at a rolling reduction of 10% or more and less than 30% in a temperature range of 600 ° C or more and less than ₁ point of Ar, and at least the final stand of the finishing mill is rolled under a friction coefficient μ of 0.15 or more. Then, wind it in the temperature range of 450 to 700 ° C, and then in the temperature range of 500 to 740 ° C for 10 s.
A method for producing a high-carbon hot-rolled steel sheet excellent in hardenability and workability, characterized by performing annealing for holding for ec to 8 hours.

2) In the above 1), the material composition of the material is C: 0.2 to 1.3 wt%, Si: 0.1 to 1.0 wt%, Mn: 0.
05-2.0 wt%, P: 0.05 wt% or less, S: 0.05 wt% or less, Al: 0.01-0.2 wt%, N: 0.05 wt% or less, and Ni: 0.1-5.0 wt%, Cr: 0.1 ~ 5.0 wt
% Mo: 0.1-1.0 wt%, B: 0.0005-0.0100 wt% One or more selected from the rest, the balance Fe
And a method for producing a high carbon hot-rolled steel sheet which has an inevitable impurity composition and is excellent in hardenability and workability.

[0010]

The present invention will be described in detail below. First, the reason why the chemical composition of steel is limited to the above range will be described. C: 0.2 to 1.3 wt% In the present invention, it is necessary to form a uniform layered carbide as a premise, but if the C content is less than 0.2 wt%, ferrite is first formed in the cooling process from austenite, so that the entire surface is uniform. Layered carbides (pearlite) are not formed, while if C exceeds 1.3 wt%, coarse net-like carbides are first produced during the cooling process from austenite, and thus uniform fine spherical carbides cannot be obtained. Limited to ~ 1.3 wt%. The preferable content range is 0.4 to 1.
It is 0 wt%.

Si: 0.1-1.0 wt% Si is not only useful for deoxidation but also contributes effectively to the improvement of hardenability, but if the content is less than 0.1 wt%, the effect is poor, while 1.0 wt% %, The steel plate becomes hard and becomes brittle, so the content was limited to the range of 0.1 to 1.0 wt%. The preferred content range is 0.15 to 0.50 wt%.

Mn: 0.05 to 2.0 wt% Mn is added to improve strength or hardenability, but if the content is less than 0.05 wt%, solute S increases and brittleness occurs during hot working. However, the Mn content is limited to the range of 0.05 to 2.0 wt% because if it exceeds 2.0 wt%, the toughness decreases. The preferable content range is 0.4 to 1.0 wt%.

P: 0.05 wt% or less If P exceeds 0.05 wt%, grain boundary embrittlement easily occurs, so it is necessary to set P to 0.05 wt% or less. More preferably 0.
It is less than 01wt%.

S: 0.05 wt% or less S exceeds 0.05 wt%, since the toughness is remarkably deteriorated, so S must be 0.05 wt% or less. More preferably 0.
It is less than 01wt%.

Al: 0.01 to 0.2 wt% Al is added as a deoxidizing agent, but if the content is less than 0.01 wt%, it has no effect. On the other hand, if it exceeds 0.2 wt%, the cost increases. Not only that, but it also causes embrittlement of the steel sheet, so the range was limited to 0.01 to 0.2 wt%. From the viewpoint of cost performance, the preferable range is 0.04 to 0.1 wt%.

N: 0.05 wt% or less N may be positively added and used for strengthening.
If added in excess of 0.05 wt%, the steel plate becomes brittle, so
It should be below 05wt%. If no particular strengthening is required, it is preferably 0.01 wt% or less.

Although the essential components have been described above, the reinforcing components described below can also be contained in the present invention, if necessary. Ni: 0.1 to 5.0 wt% Ni is added as needed because it has the effect of improving the hardenability and toughness of the steel sheet, but if it is less than 0.1 wt%, the addition effect is poor, while 5.0 wt% If added in excess of%, the effect will be saturated and only the cost will increase, so the content was made 0.1-5.0 wt%. The preferred content range is 0.15 to 2.0 wt%.

Cr: 0.1-5.0 wt% Cr is added as necessary because it has effects of improving corrosion resistance and suppressing graphitization of carbides, but if less than 0.1 wt%, the addition effect is poor. If added in excess of 5.0 wt%, the steel becomes hard and brittle, so the range was set to 0.1 to 5.0 wt%. The preferred content range is 0.1-1.0 wt%
Is.

Mo: 0.1 to 1.0 wt% Mo not only enhances hardenability and effectively contributes to improvement of wear resistance, but also improves "tempering embrittlement" which occurs at a specific tempering temperature. However, if it is less than 0.1 wt%, its effect is poor.
Even if added in excess of 1.0 wt%, the effect will reach saturation and only cost will increase, so the content was made 0.1-1.0 wt%. The preferred content range is 0.10-0.5 wt%
Is.

B: 0.0005 to 0.0100 wt% B has the effect of enhancing the hardenability and is added as necessary, but if it is less than 0.0005 wt%, its effect is poor, while if it exceeds 0.0100 wt%. Then, the steel hardens and becomes brittle, so the range was made 0.0005 to 0.0100 wt%. The preferred content range is 0.0010 to 0.0030 wt%.

Next, the manufacturing method according to the present invention will be described. Both the continuous casting method and the ingot-casting method are suitable for the production of the raw material slab. Next, heat treatment is performed prior to hot rolling, but it is necessary to realize complete solution treatment by heating before hot rolling, and from this viewpoint, 1000 to 1300
Limited to the range of ° C. The preferred heating temperature range is 11
50 to 1250 ° C.

In the hot rolling, particularly in the rough rolling process, prior to the finish rolling, layered carbides (pearlite) having small equal intervals are formed.
Is important for uniform production. In order to generate pearlite after rolling, the rough rolling temperature needs to be the Ar ₃ transformation point or higher. On the other hand, in order to increase the nucleation sites of pearlite, the rough rolling temperature must be 950 ° C or lower and the reduction rate must be 50% or higher.

Before or after the subsequent finish rolling, after being transformed into pearlite, in the finish rolling step, pearlite is applied to each of the surface layer portions on the front and back surfaces of the steel sheet in an area of 1/10 or more and 1/4 or less of the sheet thickness. Divide. The divided region has a structure in which fine spheroidized carbide is dispersed in ferrite by the subsequent self-annealing, but the ratio to the plate thickness should be 1/10 or more of the plate thickness, and the fine carbide dispersed structure layer As a result, excellent cold workability such as hardenability, toughness and bending workability cannot be obtained. On the other hand, even if the region where the thickness exceeds 1/4 of the plate thickness is made to have a fine carbide dispersion structure, not only the dramatic improvement in the above-mentioned characteristic effects cannot be expected, but also problems such as burr formation at the time of punching occur. It occurs newly. From this, it is effective that the region forming the fine carbide dispersion structure in which the fine spheroidized carbide is uniformly dispersed in the ferrite is within the range of 1/10 or more and 1/4 or less of the plate thickness. At this time, the spheroidization rate of the carbide is 90% or more,
The particle size is preferably 0.10 μm or less. The spheroidization rate is important for ensuring workability, and
%, The effect becomes remarkable, and by setting the carbide grain size to 0.10 μm or less, quench hardening can be obtained even under a short-time heat treatment condition.

As described above, in order to make only the surface layer portion have a fine carbide dispersion structure, the following may be carried out. Since the pearlite transformation takes place before the completion of finish rolling, the finish rolling finish temperature is the Ar ₁ transformation point (usually around 700 ° C regardless of the steel type).
Less than On the other hand, if the finish rolling temperature is lower than 600 ° C, the load during rolling increases, so the lower limit of the finish rolling finish temperature is set to 600 ° C. In addition, the reduction ratio of finish rolling in the temperature range of 600 ℃ or more and less than Ar ₁ point must be large enough to divide the pearlite in the steel sheet surface layer, and it should be 10% or more. When the ratio is 30% or more, the amount of strain for dividing the pearlite in the surface layer is too large, which causes adverse effects such as defective shape.
The rolling reduction in finish rolling was limited to the range of 10% or more and less than 30%.

By the way, it is important to carry out the hot finish rolling under non-lubrication or low lubrication conditions such that at least the final stand of the finish rolling mill has a friction coefficient μ of 0.15 or more, preferably 0.20 to 0.40. Is.
This is because it is possible to effectively apply shear strain only to the surface layer of the steel sheet specified by the present invention after rolling under such a non-lubricated or low-lubricated condition, and effectively divide the pearlite in this region. Because you can do it.
When finish rolling is performed under conditions that deviate from the above-mentioned lubrication conditions and reduction rates, the surface layer structure in the prescribed region will have an appropriate spheroidization rate, even if winding treatment is performed under appropriate conditions thereafter. Moreover, it is impossible to obtain a spherical carbide having an appropriate particle size. It should be noted that the last stand is sufficient as the stand to be rolled under the non-lubrication condition or low lubrication condition of μ ≧ 0.15 as described above. The same non-lubricated or low-lubricated rolling may be applied to the stand.

In the winding step following the finish rolling, the divided pearlite in the surface layer portion is spheroidized. Here, the winding temperature is 4
If the temperature is less than 50 ° C, the spheroidization rate: 90% or more and the particle size of the spherical carbides: 0.05 μm or less cannot be achieved. The coiling temperature is limited to the range of 450 to 700 ℃, because it causes excessive temperature and decarburization. The range of 550 to 650 ° C is preferable.

After the above-mentioned winding step, further 500-
By performing short-time annealing at 740 ℃ for 10 seconds to 8 hours,
It is possible to achieve softening by coarsening the carbide to the extent that the hardenability is not impaired, improving bending workability, and improving workability such as extending the life of the punching die. Here, 500
Annealing below ℃ or less than 10 seconds does not provide sufficient softening, whereas annealing above 740 ° C or over 8 hours regenerates pearlite and coarsens carbides, which impairs hardenability. The conditions are 10 seconds to 8 hours in the temperature range of 500 to 740 ° C. The preferred temperature is 600-
700, the time is 60 seconds to 2 hours.

[0028]

【Example】

-Example 1 JIS standard SK5 equivalent (a steel in Table 1, C: 0.79 wt%, S
i: 0.24wt%, Mn: 0.35wt%, P: 0.02wt%, S: 0.005
wt%, Al: 0.05 wt%, N: 0.01 wt%: Ar ₃ = 700 ° C)
After heating to 1200 ° C. using the above material, hot rolling and annealing were performed under various conditions shown in Table 2 to obtain a high carbon hot rolled steel sheet. The steel sheet structures thus obtained were all two-phase structures of ferrite and carbide. The characteristics of the structure, that is, the ratio of the layer thickness to the plate thickness of the spherical carbide dispersion structure region of the surface layer, the spheroidization rate of the carbide in the region of the surface layer to the plate thickness of 10% (ratio of major axis / minor axis is 1.0). The ratio of the number of carbides in the range of to 1.5 to the total number of carbides), particle size (arithmetic mean of major axis and minor axis), and the ratio of the spherical carbide dispersion layer on one side of the front and back surfaces to the plate thickness are investigated. ° The minimum bending radius that does not cause cracks in the bending test (compare radius R divided by plate thickness t) and clearance are 15
The burr height (barrel height) after punching was measured as%. The results are also shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

FIG. 1 shows an example of the invention manufactured under the conditions of No. 1 in Table 1 and conditions of No. 3 (annealing conditions are outside the scope of the invention).
The results of examining the relationship between each heating time and the surface hardness when heating-quenching at 750 ° C. for the comparative example manufactured in No. 3 and the comparative example manufactured under the No. 3 condition (the hot rolling condition is outside the invention range) Indicates. In addition, as shown in FIG.
30 mm for samples in which the ratio of the spherical carbide dispersed phase to the plate thickness is variously changed for steel of K-5 equivalent composition
The results of comparing the heights of burrs at the shearing portion when punching (clearance: 15% of plate thickness) to φ are shown.

As is clear from Tables 1 and 2, it is understood that the steel sheets produced according to the method of the present invention all have good bending properties and punching properties and have good workability. It is considered that the reason why such good characteristics were obtained by the method of the present invention is that the size, shape and distribution of the carbide were properly controlled. Further, as shown in FIG. 1, the steel sheet manufactured by the method of the present invention has a good short-time hardenability, and it can be seen that it can sufficiently cope with induction hardening. Further, as is clear from FIG. 2, the burr height of the steel sheet of the present invention is remarkably reduced as compared with the conventional material in which the entire plate thickness has a fine carbide dispersed structure.

Example 2 JIS standard S35C equivalent (steel b in Table 1, C: 0.35 wt%, S
i: 0.27wt%, Mn: 0.80wt%, P: 0.016wt%, S: 0.0
05 wt%, Al: 0.05 wt%, N: 0.01 wt%, Ar ₃ = 750
After heating to 1100 ° C., hot rolling and annealing were performed under the conditions shown in Table 3 to obtain a high carbon hot rolled steel sheet. Table 3 shows the results of an examination of the characteristics and characteristic values of the obtained steel sheet structure in the same manner as in Example 1.

[0034]

[Table 3]

As is clear from Table 3, the product manufactured according to the present invention exhibits excellent workability with excellent bending properties and punching properties, and the structure composed of ferrite and carbide has a predetermined plate thickness of the steel plate surface layer. The ratio is a fine and uniform spherical carbide dispersion structure.

Example 3 JIS standard SK2 equivalent (c steel in Table 1, C: 1.20 wt%, S
i: 0.23 wt%, Mn: 0.32 wt%, P: 0.018 wt%, S: 0.0
05 wt%, Al: 0.05 wt%, N: 0.01 wt%, Ar ₃ = 730
C.) material, after heating to 1250 ° C., hot rolling and annealing were performed under the conditions shown in Table 4 to obtain a high carbon hot rolled steel sheet. Table 4 shows the results of investigating the characteristics and characteristic values of the obtained steel sheet structure in the same manner as in Example 1.

[0037]

[Table 4]

As is clear from Table 4, the products manufactured according to the present invention show good workability with excellent bending properties and punching properties, and the structure composed of ferrite and carbide also has a predetermined plate thickness of the surface layer of the steel plate. The ratio is a fine and uniform spherical carbide dispersion structure.

Example 4 Using the materials d steel to i steel in Table 1, after heating to 1250 ° C., Table 5
Hot rolling and annealing were performed under the conditions shown in to obtain a high carbon hot rolled steel sheet. Regarding the characteristics and characteristic values of the obtained steel sheet structure,
Table 5 shows the results of the investigation conducted in the same manner as in Example 1.

[0040]

[Table 5]

As is clear from Table 5, the products manufactured according to the present invention show excellent workability with excellent bending properties and punching properties, and the structure composed of ferrite and carbide also has a predetermined plate thickness of the steel plate surface layer. The ratio is a fine and uniform spherical carbide dispersion structure.

[0042]

As described above, according to the present invention, it is possible to form a fine carbide dispersion structure in which spherical carbides are finely and uniformly dispersed only in the surface layer of the steel sheet only by the hot rolling step and the short-time annealing step. , Of course, has excellent hardenability,
It is possible to obtain a high-carbon hot-rolled steel sheet excellent in precision punching workability and bending workability, which have been problems in the past.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between heating time and surface hardness in short-time heating-quenching.

FIG. 2 is a graph showing the relationship between the ratio of the fine carbide dispersed structure layer to the plate thickness and the hardenability and punching workability of the steel plate.

Claims (2)

[Claims] 1. C: 0.2-1.3 wt%, Si: 0.1-1.0
wt%, Mn: 0.05 to 2.0 wt%, P: 0.05 wt% or less, S: 0.05 wt% or less, Al: 0.01 to 0.2 wt%, N: 0.05 wt% or less, and the balance Fe and unavoidable impurities After heating the high carbon steel material having the composition of 1000 to 1300 ° C, rough rolling is performed at a rolling reduction of 50% or more in the temperature range of Ar ₃ points or more and 950 ° C or less, and then the rolling end temperature is 60.
Finish rolling at a rolling reduction of 10% or more and less than 30% in a temperature range of 0 ° C or more and 600 ° C or more and less than Ar ₁ point.
At least the final stand of the finishing mill is rolled under the condition that the friction coefficient μ is 0.15 or more, and then 450
A method for producing a high-carbon hot-rolled steel sheet excellent in hardenability and workability, which comprises winding in a temperature range of to 700 ° C and further annealing at a temperature range of 500 to 740 ° C for 10 seconds to 8 hours. 2. The composition according to claim 1, wherein the composition of the material is C: 0.2 to 1.3 wt%, Si: 0.1 to 1.0 wt%, Mn: 0.05 to 2.0 wt%, P: 0.05 wt% or less, S: 0.05 wt%. % Or less, Al: 0.01 to 0.2 wt%, N: 0.05 wt% or less, and Ni: 0.1 to 5.0 wt%, Cr: 0.1 to 5.0 wt% Mo: 0.1 to 1.0 wt%, B: 0.0005 to 0.0100. Contains one or more selected from wt%, with the balance being Fe
And a method for producing a high carbon hot-rolled steel sheet which has an inevitable impurity composition and is excellent in hardenability and workability.