JP2927166B2 - Manufacturing method of high carbon cold rolled steel sheet with excellent short-time hardenability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention provides a method for producing a high-carbon cold-rolled steel sheet in which the thermal deformation during quenching is extremely small and the required hardenability can be stably obtained in a short time.

[0002]

2. Description of the Related Art Blades, springs, washers, springs and other mechanical parts are made of high-carbon cold-rolled steel sheets (for example, JIS G331).
Using 1) as a material, it is manufactured through forming processes such as slitting, punching, bending, pressing, and cutting, followed by quenching, tempering, and other heat treatment processes. Among these processes, the steel maker manufactures the raw material, and the user performs the processing and heat treatment.

[0003] In this case, the material is required to have (a) good workability and (b) excellent strength and toughness after heat treatment.

In order to secure the former performance, spheroidizing annealing is generally performed. This is a process that has been performed for a long time, whereby the material is softened and workability is ensured. On the other hand, for the latter, measures are taken such as selecting high carbon steel, controlling alloying elements and impurities, and carefully selecting heat treatment conditions at processing manufacturers.

[0005] In addition to the above characteristics, it has recently been required to ensure hardenability by heating for a short time. This is a request for the purpose of saving labor and reducing costs by the user. Specifically, there is a demand for providing a material having sufficient performance even if the soaking time in the austenite region is shortened. Further, in such a case, it is strongly demanded that the shape after quenching, that is, no warping or twisting be provided.

[0006] The technology for responding to this demand is basically based on the idea that cementite grains during spheroidizing annealing are made as fine as possible and are dissolved in a short soaking time. The specific method is described in, for example, Japanese Patent Laid-Open No.
No. 7 discloses that after adjusting the chemical composition, the average particle size of the cementite particles present in the steel is reduced to 0.8 μm or less, and in order to realize such a state, cooling after hot rolling is performed. Selection of cooling rate and cooling end temperature in the process,
It is stated that cold rolling conditions and annealing conditions need to be specified. It is described that by such a method, sufficient hardening hardness can be obtained by soaking at 850 ° C. for 10 minutes.

[0007]

However, the method disclosed in Japanese Patent Application Laid-Open No. 4-116137 has the following problems.

(1) The soaking condition at 850 ° C. for 10 minutes is not enough. At present, shorter time is required, and soaking at lower temperature is desired.

(2) When quenching high-carbon steel produced by the method disclosed in Japanese Patent Application Laid-Open No. 4-116137 under low-temperature and short-time heating conditions, carbides may not be sufficiently melted and quenching hardness may be insufficient. is there.

(3) Further, when an actual operation is carried out, unevenness in baking is likely to occur under short-time soaking conditions. For this reason, shape defects such as twisting and warping may occur.

[0011]

The object of the present invention is to provide a concrete method for overcoming such problems, and the gist of the present invention is as follows: (1) C: 0.3 to 1.3 wt.%, Si ; 0.10-0.30wt%, Cr; 0.0
5 to 0.25wt%, Mn; 2.0wt.% Or less, Cu: 0.3wt% or less, Ni:
3.5 wt% or less, W: 2.5 wt% or less, Mo: 0.3 wt% or less, V: 0.
A step of preparing a steel containing 3 wt% or less; a step of hot rolling at a winding temperature (CT) of 460 to 650 ° C .; a step of descaling by pickling; Cold rolling within the range specified in the above, hydrogen 5% or more, dew point -1
In a reducing atmosphere of 0 ° C or less and oxygen of 100ppm or less, C <0.8w
t.% 640 ℃ ~ Ac1, C ≥0.8wt.% 640 ~ 75
Annealing temperature of 0 ° C, annealing process for 2 to 25 hours, and temper rolling are sequentially performed, and the surface roughness is 0.1 to 0.40 µm in Ra during cold rolling or temper rolling. This is a method for producing a high carbon steel sheet excellent in short-time hardenability, characterized by imparting a high hardness. However, for Mn, Cu, Ni, W, Mo, V, the case without addition is included.

[0012]

(Equation 2)

(2) Production of the high-carbon steel sheet excellent in short-time hardenability, in which intermediate annealing is performed at 640 to 720 ° C. for 2 to 10 hours after the hot rolling and before pickling and descaling. Is the way.

[0014]

The present inventor has conducted various studies and has found that these high-carbon steel sheets have sufficient workability as a raw material, and have sufficient hardenability under low-temperature, short-time soaking conditions. A method for producing a steel having a good shape without sometimes twisting or warping was studied. As a result, the following findings were obtained.

(1) The finer the cementite grains, the shorter the dissolution time in the austenite region. In addition, it becomes a solid solution even at a low temperature.

(2) In order to reduce the cementite grain size after spheroidizing annealing, it is important to select rolling conditions in cold rolling, and the rolling is performed in a specific area related to the finishing temperature of hot rolling. There is a need. It is preferable that the work roll diameter is small.

(3) In order to equalize the temperature in a short time, it is effective to control the surface condition and quickly absorb the radiant heat from the furnace.

Hereinafter, the reasons for limiting the chemical components and the production conditions of the high-carbon cold-rolled steel sheet according to the present invention as described above will be described.

(1) Chemical composition (a) In order to obtain hardness, hardenability and abrasion resistance required for a C steel sheet, it is necessary to add 0.3 wt.% Or more of C. On the other hand, if the C content exceeds 1.3 wt.%, Embrittlement after quenching is caused. Therefore, the C content is set to 0.3 to 1.3 wt.%.

(B) Si When the content of Si is less than 0.1 wt.%, The hardness at the time of quenching becomes insufficient.
If i exceeds 0.3 wt.%, it becomes hard after quenching and becomes brittle. For this reason, the Si content is set to 0.1 to 0.3 wt.%.

(C) Cr If the Cr content is less than 0.05 wt.%, Carbides during annealing during cold rolling become coarse, and hardenability in a short time is reduced. On the other hand, Cr is 0.
If it exceeds 25 wt.%, Solid solution of carbon is prevented during annealing, and spherical carbides are dispersed unevenly. For this reason, workability deteriorates and hardenability in a short time decreases. For this reason
The Cr content was 0.05 to 0.25 wt.%.

(D) Mn If Mn exceeds 2.0 wt.%, It hardens rapidly and the toughness deteriorates.
Therefore, when added, the Mn content is set to 2.0 wt.% Or less.

(E) Cu Cu is an element having an effect of facilitating removal of the surface scale of a hot-rolled steel sheet by pickling or the like. However, if it exceeds 0.3 wt.%, It becomes brittle after quenching. Therefore, when adding
0.3 wt.% Or less.

(F) Ni Ni has the effect of strengthening the ferrite matrix and increasing the tensile strength. However, if it exceeds 3.5 wt.%, Graphitization tends to occur, and increasing the amount is disadvantageous in terms of cost. Therefore, when adding, the upper limit is 3.5 wt.%
And

(G) WW has the effect of increasing the temper softening resistance,
If it exceeds, stable carbides are generated during annealing, which causes unevenness during quenching. Therefore, when adding, 2.
The upper limit is 5wt.%.

(H) Mo Mo has the effect of increasing the tempering softening resistance like W,
It is also an element effective for improving toughness and the like. However, 2.
When added in excess of 5 wt.%, Hardenability decreases. Therefore, when adding, the upper limit is 2.5 wt.%.

(I) VV has the effect of refining austenite grains, enhancing hardenability, and increasing tempering softening resistance and improving toughness. However, if it exceeds 0.3 wt.%, It tends to become extremely brittle after quenching. Therefore, when adding, the upper limit is 2.5 wt.%.

(2) Manufacturing Process In the present invention, a high-carbon cold-rolled steel sheet having excellent short-time hardenability is manufactured through the steps of hot rolling → cold rolling → spheroidizing annealing. If necessary, intermediate annealing is performed after hot rolling.

(A) Hot rolling conditions Hot rolling is performed under ordinary conditions. The slab heating temperature does not need to be particularly specified, and a conventional temperature range is sufficient.
Generally, a temperature in the range of 1050-1300 ° C is selected. In addition, the pass schedule and the rolling reduction may be carried out under conventional conditions. However, if the winding temperature is lower than 450 ° C., the material becomes hard and edge cracks are likely to occur in the subsequent cold rolling. As a result, the yield decreases. on the other hand
If the temperature exceeds 650 ° C., the lamellar spacing of the layered pearlite increases, and the fracture of cementite in the subsequent cold rolling process becomes insufficient. In such a case, even if the spheroidizing annealing is performed, the distribution of carbides tends to be non-uniform, and the carbides tend to coarsen rapidly. Therefore, the hardenability in a short time is deteriorated.

In such a case, the surface roughness, which will be described later, is reduced to some extent by defining the surface roughness, but if it exceeds 650 ° C., the desired performance of the present invention cannot be obtained. Therefore, the winding temperature is limited to 450 to 650 ° C.

(B) Intermediate annealing In order to further improve short-time hardenability, it is desirable to perform intermediate annealing. By such a treatment, part of the layered cementite after the hot rolling is melted into the ferrite and is divided, which has an effect of promoting the carbide crushing action in the subsequent cold rolling process. However, the annealing temperature was 64
If the temperature is lower than 0 ° C., the above effect cannot be obtained. On the other hand, if the temperature exceeds 720 ° C., the carbides become coarse and coarse, and the short-time hardenability is reduced. On the other hand, if the annealing time is less than 2 hours, the above effect cannot be obtained. If the annealing time exceeds 10 hours, the hardenability of short-time hardening due to coarsening of carbide aggregation is caused. Therefore, when performing the intermediate annealing, it is desirable to set the annealing temperature to 640 to 720 ° C. and the annealing time to 2 to 10 hours.

(C) Cold Rolling The hot rolled material is sent to a step of removing surface scale by means of pickling or the like as it is or after intermediate annealing as necessary. Subsequently, cold rolling is performed, and the rolling conditions here are an important point of the present invention.

In the cold rolling process, conditions are set such that the layered cementite produced by the hot rolling is crushed and finely dispersed, and the particle diameter of the spherical cementite produced in the final annealing process is on the order of submicrons. Must be set.

Therefore, the present inventors conducted studies from such a viewpoint, and found that the relationship between the hardenability and the cold rolling reduction was recognized with a specific relationship, and that the winding temperature had a great influence on the relationship. Has been found to exert. In other words, the higher the winding temperature, the higher the cold rolling ratio for ensuring hardenability. Specifically, the cold rolling ratio (CR) and the winding temperature (CT) satisfy the condition of Equation (3). Found that hardenability is ensured in the case of quenching.

[0035]

(Equation 3)

These relationships are derived from the relationship between the lamella spacing of pearlite and the degree of crushing of cementite due to reduction. As the pearlite spacing increases, the rolling reduction for crushing cementite as much as necessary is also increased. It indicates that it will be higher.

In order to finely crush cementite,
The higher the cold rolling reduction, the more advantageous. However, if the cold rolling reduction is too high, the material is hardened more than necessary and edge cracks occur. Cold rolling rate at which edge cracking occurs (CR)
Is also closely related to the winding temperature (CT), and the winding temperature (C
As the T) increases, the critical cold rolling ratio (CR) at which edge cracking occurs increases. It has been found that when the relationship satisfies the condition of Equation 4, edge cracking is prevented.

[0038]

(Equation 4)

FIG. 1 shows such a relationship. In the drawing, the preferable range of the winding temperature (CT) is also shown.

An effective technique for breaking up carbides during cold rolling is to introduce shear-type plastic deformation. In general, in cold rolling, shear-type plastic deformation is likely to be introduced, but the degree of crushing of carbides varies depending on the work roll diameter during rolling. That is, as the work roll diameter is smaller, the deformation at the time of rolling becomes shear plastic deformation, and the carbide is more finely broken. The finer the crushed carbides, the finer the carbides after spheroidizing annealing, and thus, the sooner the carbides dissolve in the matrix during the heating step during heat treatment after forming. Therefore, it is preferable that the work roll diameter be 300 mm or less.

(D) Annealing conditions (a) Atmosphere gas The material of the present invention contains Si and Cr, and seizure hardly occurs. However, when the hydrogen concentration is less than 5 vol.%, The temperature distribution becomes non-uniform. Seizure is likely to occur. Therefore, the hydrogen concentration in the atmosphere gas during annealing is specified to be 5 vol.% Or more.

On the other hand, when the oxygen concentration exceeds 100 ppm, a temper color is generated, and the yield decreases. Therefore oxygen is 100ppm
It is preferable to set the following. Further, even when the dew point exceeds -10 ° C, a temper color easily occurs and a decarburized layer is formed. The part of the decarburized layer has extremely deteriorated hardenability, which is extremely inconvenient for products in the application field targeted by the present invention. Therefore, the dew point should be specified at -10 ° C or less.

The remaining gas is an inert gas such as a nitrogen gas and an argon gas.

(B) Annealing temperature and time If the annealing temperature is lower than 640 ° C., the spheroidization of the carbide will be incomplete, and the formability will be deteriorated. On the other hand, if the annealing temperature is less than 0.8 wt.
If the temperature exceeds 750 ° C in the case of t.% or more, austenite is generated, and the amount of solid solution of carbide rapidly increases. For this reason, pearlite is generated again in the slow cooling process after soaking, and coarse carbides are generated. For this reason, the moldability is deteriorated, and the short-time soaking hardenability is extremely reduced. Therefore, when the annealing temperature is 640 ° C or more and the carbon content is less than 0.8 wt.%, The Ac1 point or less, and when the carbon content is 0.8 wt.% Or more,
It was specified as 750 ° C or less.

If the annealing time is less than 2 hours, the spheroidization of the carbide will be incomplete and the formability will be significantly deteriorated. On the other hand, if the annealing time exceeds 25 hours, the carbides will agglomerate and coarse, and the short-time soaking hardenability will deteriorate. Further, seizure is likely to occur. Therefore, the annealing time is limited to 2 to 25 hours.

(E) Temper rolling After the annealing, temper rolling is performed. The temper rolling conditions are not particularly specified. It can be carried out under conventional conditions. However, in the present invention, since the above-mentioned surface roughness is an important point, when the surface roughness is provided by temper rolling, it is necessary to control the roll surface so that Ra is in the above-mentioned range.

(F) Final Rolling After the temper rolling, cold rolling and annealing for increasing the ferrite grain size may be performed. Even if such processing is performed, the effects of the present invention are not impaired.

(G) Surface Roughness Surface roughness is an important point of the present invention, and is a key to enable rapid isothermal quenching. The material inserted into the furnace is heated mainly by radiation from a heat source, but it is necessary to reduce the amount of heat reflected from the material surface as much as possible and to absorb the heat effectively. As a means for realizing this, the present inventors have found that it is effective to make the surface roughness Ra 0.1 μm or more.

FIG. 2 shows the relationship between the surface roughness and the relative hardness (H). The relative hardness ratio (H) is, as shown in Equation 5, the hardness of a material that is quenched after holding at 800 ° C. for 5 seconds.
[(HRC) ac] is a value expressed as a percentage, divided by the hardness [(HRC) max] of the material quenched after holding it at a high temperature for a long time.

[0050]

(Equation 5)

The relative hardness (H) increases as the surface roughness increases, and reaches 90% or more when Ra exceeds 0.1 μm. On the other hand, when Ra exceeds 0.4 μm, flaws occur during product processing. Therefore, the surface roughness Ra is limited to the range of 0.1 to 0.4 μm.

If the surface roughness is within the above range, the effect of the present invention can be obtained, but the difference between the values on the front and back sides is 0.
It is desirable to be within 2 μm. By doing so, the penetration rates of carbides on the front and back sides during the short-time soaking are almost equal, and the shape defect (twist, warp, etc.) of the product after quenching and tempering is reduced.

FIG. 3 shows the relationship between the relative hardness rate (H) and the shape defect rate when quenched after holding at 800 ° C. for 5 seconds. The shape defect rate referred to here is a probability that a product after quenching and tempering is warped. As is apparent from the figure, when the short-time hardenability, that is, the relative hardness rate (H) increases, the shape defect rate decreases. In addition, the results of intermediate annealing after hot rolling and those without intermediate annealing are also shown, but the shape defect rate is lower when intermediate annealing after hot rolling is performed than when intermediate annealing is not performed. Is low.

The method for controlling the surface roughness within the above range is not particularly defined. The roughness may be imparted at any stage as long as it is a cold rolling step in the production process of the present material. For example, it may be provided at the time of temper rolling after cold rolling or at the time of final cold rolling of about 20% for coarsening ferrite.

[0055]

EXAMPLES SAE107 having the chemical composition shown in Tables 1 and 2
A high-carbon steel was manufactured under the same conditions as shown in Tables 1 and 2, and its performance was evaluated. The evaluation items were (1) the presence or absence of edge cracks (X: those with edge cracks, ○: those without edge cracks) (2) the presence or absence of temper color (X: those with temper color, ○: temper color (3) Presence or absence of decarburized layer (X: Decarburized layer was recognized,
○: No decarburized layer) (4) Relative hardness rate (Evaluates short-time hardenability, as described above) (5) Shape defect rate (warpage occurs in product after quenching and tempering) Ratio).

Nos. 1 to 14 satisfy the constitutional requirements of claim 1, and Nos. 7 to 12 also satisfy claim 2. These materials are excellent in short-time hardenability, are free from edge cracking, temper color, decarburized layer, and are excellent in wear resistance.

It can be seen that in Nos. 7 to 12 in which intermediate annealing was performed after hot rolling, the shape defect rate was further lower than that in the case where intermediate annealing was not performed.

No. 1 and No. 7 were hydrogen atmospheres during annealing.
Although the condition was set to 100%, the shape defect rate was lower than that of the others.

Further, a difference in surface roughness between the front and back sides, for example, N
o. Comparing 2 and 3, the relative hardness is the same, but R
No. 2 having a small difference between the front and back of a has a lower shape defect rate than No. 3.

No. 13 and No. 14 were No. 6 and No. 12 respectively at 20%
And cold annealed at 700 ° C. for 4 hours. Although these treatments are performed to coarsen the ferrite and soften the material, it has been shown that even if such treatments are performed, the properties are the same. This indicates that the excellent hardenability intended by the present invention can be obtained even when a process for coarsening and softening ferrite is provided.

On the other hand, in No. 15, the winding temperature at the time of hot rolling is lower than that of the first aspect, and an edge crack occurs. No. 20 has a higher winding temperature at the time of hot rolling than in the range of claim 1, a low relative hardness rate, and a high shape defect rate. Further, the dew point is lower than the range of claim 1, a tempered color is formed, and a decarburized layer is formed. No.
In No. 16, the cold rolling rate (CR) is lower than the range of claim 1, and a decrease in the relative hardness rate and an increase in the defective shape rate are recognized. No.
In No. 16, the oxygen concentration also exceeded the range of claim 1, and generation of a temper color was also recognized. No. 26 has a higher cold rolling ratio (CR) than the scope of claim 1, and has edge cracks.
In No. 17, the annealing temperature after cold rolling was lower than that of claim 1, and a decrease in the relative hardness and an increase in the shape defect rate were observed. In No. 18, the annealing time after cold rolling was lower than the range of claim 1, and a decrease in the relative hardness rate and an increase in the shape defect rate were observed. No. 22 has a higher cold rolling rate (CR) than the range of claim 1 in the annealing time. Similarly, No. 19 shows a decrease in the relative hardness rate and an increase in the defective shape rate. It is lower than the range of item 1, and a decrease in the relative hardness rate and an increase in the defective shape rate are observed, and generation of a temper color and a decarburized layer is also observed. In No. 21, the glossiness was higher than the range of claim 1, and a decrease in the relative hardness rate and an increase in the defective shape rate were recognized. No. 23 is the range of claim 1 where the Si content is
And sufficient hardness has not been obtained after quenching. No.
No. 24 has a low Cr content, and after annealing, carbides become coarse and
Poor penetration. No. 25 has a high Cr content and
Because the carbon was difficult to dissolve and the distribution of carbides became uneven,
Hardenability was insufficient. No. 27 was prepared under the conditions described in Japanese Patent Application Laid-Open No. 4-116137, but had a small amount of undissolved carbide and was inferior in abrasion resistance.

Tables 3 and 4 show the results of a similar experiment performed on SK5. As is clear from these tables, results similar to the results in Tables 1 and 2 performed on SAE1070 were obtained.

[0063]

[Table 1]

[0064]

[Table 2]

[0065]

[Table 3]

[0066]

[Table 4]

[0067]

According to the present invention, there is provided a method for producing a high-carbon cold-rolled steel sheet in which the hardenability in a short time is improved and the percentage of defective shapes is reduced. In addition, edge cracking during cold rolling, occurrence of temper collar during annealing, and generation of a decarburized layer are also prevented.

[Brief description of the drawings]

FIG. 1 is a diagram showing a relationship between a winding temperature during hot rolling, a cold rolling rate (CR) during cold rolling, and hardenability / edge cracking.

FIG. 2 is a diagram showing a relationship between surface roughness and relative hardness.

FIG. 3 is a diagram showing a relationship between a shape defect rate and a relative hardness rate.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Kazuzo Jiromaru, Inventor Nippon Kokan Co., Ltd. 1-2-1, Marunouchi, Chiyoda-ku, Tokyo (58) Field surveyed (Int. Cl. ⁶ , DB name) C21D 9 / 46 C21D 6/00 C21D 8/02 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.3 to 1.3 wt.%, Si; 0.10 to 0.30 wt%
, Cr: 0.05 ~ 0.25wt%, Mn: 2.0wt.% Or less, Cu: 0.3wt%
Hereinafter, a process of preparing steel containing Ni: 3.5 wt% or less, W: 2.5 wt% or less, Mo: 0.3 wt% or less, V: 0.3 wt.% Or less, and a winding temperature (CT) of 460 to 650 ° C. Hot rolling, pickling descaling, and cold rolling reduction (CR)
Cold rolling within the range specified in, and a reducing atmosphere of 5% hydrogen or more, a dew point of -10 ° C or less, and oxygen of 100ppm or less, 640 ° C to Ac1 with C <0.8wt.%, Ac ≥ 0.8wt. 6 for%
A step of performing soaking at a soaking temperature of 40 to 750 ° C. for 2 to 25 hours and a step of performing temper rolling are sequentially performed. At the time of temper rolling or cold rolling after temper rolling, Ra 0.1
A method for producing a high carbon steel sheet having excellent short-time hardenability, characterized by providing a surface roughness of from 0.4 to 0.4 μm. Where Mn,
For Cu, Ni, W, Mo and V, the case without addition is included. (Equation 1) 2. After the hot rolling and before pickling and descaling ,
The method for producing a high carbon steel sheet excellent in short-time hardenability according to claim 1, further comprising a step of performing intermediate annealing at 640 to 720 ° C for 2 to 10 hours.