JP2001081528A - High carbon steel strip excellent in cold workability and hardenability and method for producing the same - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To develop a high carbon steel strip having both cold workability and hardenability for sheet products such as automobile parts and electric parts, and a method for producing the same. SOLUTION: In weight ratio, C: 0.15 to 0.75%, S
i: 0.30% or less, Mn: 0.20 to 1.60%, sol.Al: 0.05%
Less than N, 0.0060% or less, and 5 ≦ sol.Al / N ≦ 20, and the cementite average particle diameter in steel is 0.5.
2.080 μm, the spheroidization ratio ≧ 80%, and the average cementite dispersion interval θsp in the steel defined by the formula (1) satisfies the relationship of the formula (2) with respect to the average ferrite grain size d. High carbon steel strip with a microstructure that is excellent in cold forgeability and hardenability. Carbide dispersion spacing ^{θsp (μm) = {10 6} /(3.14 × θn} 0.5 × 2.3 ··· (1) θn , the number of 1 mm ² per cementite 1.0 × θsp + 1.0 <d ( μm) <1.0 × θsp + 10. 0 ... (2)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a spinning process,
The present invention relates to a steel strip capable of satisfying both the formability in cold working such as rolling and the hardenability in quenching, and a method for producing the same.

[0002]

2. Description of the Related Art Driving system parts for automobiles that are used with increased strength by quenching / tempering or heat treatment such as austempering are conventionally provided with S35C to S7 specified in JIS G3311.
0C or a high carbon steel having a high C content such as SCM415 to SCM440, and containing a plurality of alloy components as necessary, forged by hot or cold, and further by cutting if necessary. It was processed into the shape of a part, and was used with necessary strength by heat treatment.

[0003] However, with the recent development of working techniques, an efficient manufacturing method in which hot forging or cutting is omitted by simply cold working a high carbon steel strip as it is has become widespread. In such a processing method, when a thin steel sheet of the above-described conventional steel type is used, work hardening behavior due to compression processing in the thickness direction and elongation that governs subsequent formability are insufficient, and cracks occur during processing. High probability of doing.

[0004] Further, even if the working is successfully performed by adjusting the alloy components, there have been many problems that the strength is insufficient in the heat treatment. Therefore, there has been a demand for a thin steel strip having a composition having a high heat treatment strength and a high formability, and a manufacturing method capable of stably manufacturing the thin steel strip.

[0005] Regarding the improvement of the cold workability, Japanese Patent Application Laid-Open Nos. 4-202629, 6-271935, 8-3687 and 9
-157758, 10-1552757, 11-80884, 11-
Each publication of No. 140544 specifies the conditions for limiting the spheroidized cementite structure or the annealing conditions for its formation. However, these findings alone do not always make it possible to stably apply the newly popularized cold working to steel products manufactured from steel strips.

Further, the hot rolling conditions for these high carbon steel strips are disclosed in Japanese Patent Publication No. 48609/1990 and Japanese Patent No. 2611455.
(1997), or some of them have been disclosed in JP-A-4-41618. However, in these prior arts, it has been proposed to stably perform cold rolling mainly by miniaturizing the pearlite of a hot-rolled steel strip, and as described above, steel products manufactured from the steel strip have been proposed. On the other hand, it does not always match the purpose of stably applying the newly popularized cold working.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to achieve both moldability and hardenability in cold working.
An object of the present invention is to provide a high carbon steel strip capable of obtaining a high elongation and high n and r values and a method for producing the same.

More specifically, the object of the present invention is to provide a specific C
In order to provide good cold workability as a steel strip of the amount, elongation ≧ −12 × C% + 34, n value ≧ −0.08 × C% + 0.22, r value ≧ −0.
An object of the present invention is to provide a high carbon steel strip satisfying a level of 3 × C% + 0.9 and a method for producing the same.

[0009] The morphology of spheroidized cementite obtained in the above-mentioned prior art was not always able to improve cold workability to desired properties. In addition, in the steel strip obtained by the conventional manufacturing method, the work hardening during the deformation in cold working often causes breakage when subjected to a deformation of a specific workability or more. There is a need for a steel material that can secure sufficient elongation even below.

Therefore, it has been an important issue to find a carbide structure capable of exhibiting high elongation even under work hardening conditions and an optimum condition of a ferrite structure of a matrix. Therefore, a more specific object of the present invention is to provide a high-carbon steel strip having a carbide structure capable of exhibiting high elongation even under work-hardening conditions, and an optimum manufacturing condition for a ferrite structure of a parent phase realizing the same. It is an object of the present invention to provide a method for producing a high carbon steel strip.

[0011]

The present inventor has focused on the fact that the cracking factor during cold working is caused by a decrease in elongation due to work hardening. The curing index n value was examined. Furthermore, spheroidized cementite and ferrite structure conditions capable of securing a predetermined n value or elongation are defined.

On the other hand, when the C content in the steel increases, the elongation decreases and the cold workability deteriorates, but the microstructure condition that can exhibit the best workability at the specific C content is defined as a parameter of the C content. It has been decided. Specifically, in order to make the dispersion interval of the spheroidized cementite in the steel strip as uniform as possible, the dense band-like structure of the spheroidized cementite expanded in the rolling direction observed as a metallographic feature of the high carbon steel strip. (Hereinafter referred to as a pearlite band) was specified under the tissue condition in which it was suppressed as much as possible.

In particular, since the pearlite band changes its form depending on the amount of C and other alloy components, the most influential C
It was decided to define parameters that could be defined by quantity. In addition, regarding the production method for obtaining the above-mentioned optimum spheroidized cementite and ferrite structure conditions, particularly, hot rolling conditions and annealing conditions capable of optimizing the spheroidization ratio and dispersion interval of the cementite are specified.

As a result of these studies, in order to secure the n value during the cold working, there are the following optimum metallographic conditions according to the amount of carbon and manufacturing conditions that can be adapted to the metallographic structure. I found something.

(1) The required hardness of the high carbon steel strip after heat treatment;
In securing the formability that can withstand cold working, the conditions of the alloy components are specified as follows. C: 0.15 to 0.75%,
Si: 0.30% or less, Mn: 0.20 to 1.60%, sol.Al: 0.05%
Less than, N: 0.0060% or less, and 5 ≦ sol.Al / N ≦ 20, and, if necessary, Cr: 0.2-1.2%, Mo: 0.05-1.0%,
One or more of Ni: 0.05 to 1.2%, V: 0.5 to 0.5%, Ti: 0.05%, and B: 0.0005 to 0.0050% (however, Ti and B are always added as a composite).

(2) Next, the optimum conditions for the amount of carbon for obtaining the above-described effect of suppressing cracking during cold working and for the metal structure under different alloy components are as follows.
0.5 to 2.0 μm, spheroidization ratio ≧ 80% is satisfied.
It has been found that the average carbide dispersion interval θsp in steel specified by (1) has a metallographic structure that satisfies the relationship of equation (2) with respect to the C content in steel and the average ferrite grain size d. .

Carbide dispersion interval θsp (μm) = ｛10 ⁶ /(3.14×mean θn｝ ^0.5 × 2.3 (1) where θn is the number of cementite per 1 mm ^2. In the region of the site with a thickness of 1/4 depth, a 100 × 100 μm field of view was divided into 16 parts, and the number of cementite was measured by 2,000 times magnification observation with a scanning electron microscope corroded by nital after polishing the cross section, This value is converted to the number in the area of 1 mm ² (θn) and the unit is n / mm ²
The average value of 16 visual fields is defined as an average θn (n / mm ² ).

1.0 × θsp + 1.0 <d <1.0 × θsp + 10.0 (2) At this time, the carbide is mainly cementite, and the carbide at this time, that is, the spheroidization ratio% of cementite is the major axis / minor axis. The occupation ratio of the spherical tissue satisfying <5 is shown.

Further, assuming that the maximum value of θn measured in the 16 visual fields is θnmax and the minimum value is θnmin, θnmax, θnmin
A high carbon steel strip with excellent cold workability was found, in which the correlation of the cementite density ratio defined in equation (3) was established.

Θnmin / θnmax> √C% (3) Further, as a production method, in order to uniformly disperse spheroidized cementite and maintain the relationship of the formula (2), the finishing temperature in hot rolling is range TF satisfies the formula (4) satisfies the subsequent cooling rate TC until the winding is (5), subsequently Ac ₁ -
Performing the box annealing in the temperature range of _{50 ℃ ~ Ac 1 + 40 ℃} , found that further, it is effective to repeat the annealing temperature range of cold rolling and 650 ° C. to Ac ₁ once or more than once Was.

1270 + 25 × (C%) − 500 × (C%) ^0.1 <Tf <1270 + 25 × (C%) − 500 × (C%) ^0.1 + 60 (4) 600 · ｛1-0.1 · (1 −C%) ² ｝ <TC <600 · [1-0.1 · ｛(1−C%) ² −0.9｝] (5)

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the steel composition and the production conditions in the present invention as described above will now be described.

A. Component Content Ratio of Material Steel (a) C The C content must be contained in a specific amount for the purpose of improving wear resistance and fatigue strength of the steel product after heat treatment. In the present invention, the tensile strength after heat treatment such as quenching / tempering or austempering, and further, if necessary, carburizing treatment or the like can be ensured to be 100 kgf / mm ² or more (Vickers hardness Hv: 300 or more) and after heat treatment. 210 kgf / mm ² (Vickers hardness 600) to ensure toughness
In order to facilitate the cold working after spheroidizing annealing, the upper limit is set to 0.75% and the lower limit is set to 0.15%. Preferably, the C content ranges from 0.20 to
0.70%.

(B) Si In the present invention, 0.30% is added as an upper limit in order to avoid a decrease in fatigue strength due to Si oxide after heat treatment. Preferably, it is 0.20% or less.

(C) Mn The addition of Mn requires 0.20% or more for the purpose of ensuring hardenability during heat treatment, tempering for improving toughness, and increasing the austempering temperature. But 1.60%
If the addition exceeds 3, hardening of the steel sheet in hot rolling occurs, and production such as pickling or cold rolling becomes difficult. For this reason, the range of the added amount of Mn is set to 0.60 to 1.60%. Preferably, it is 0.60 to 0.75%.

(D) sol.Al In the present invention, in order to ensure cold workability, the balance between the particle sizes of cementite and ferrite is defined. At this time, the ferrite grain size is specified, and in order to satisfy this condition, an appropriate amount and an appropriate size of Al-based nitride in the steel,
This causes AlN to precipitate. Al is inevitably contained in the deoxidation process of the steelmaking stage, but if Al is excessively contained, AlN
Becomes coarse, and the effect of controlling ferrite grain growth intended by the present invention cannot be obtained. On the other hand, even if Al is excessively reduced, Al
Since there is no precipitation of N, it is difficult to control the ferrite grain size, which is the object of the present invention. For this reason, less than 0.05% is blended. Preferably, the content is 0.005% or more.

(E) N N is an impurity element inevitably contained in steel.
To control the ferrite grain size of the high carbon steel strip for the purpose of the present invention, it is necessary to contain a specific amount and to contain sol.Al in a specific ratio. As a result of various evaluations, when the ratio of sol.Al/N to sol.Al is 5 or more and 20 or less, the ferrite particle size control intended in the present invention can be performed. N is 0.00
When the content exceeds 60%, even if the ratio with the above sol.Al is maintained,
Since the cold workability decreases, such as a decrease in the n value, the upper limit of the addition amount is set to 0.0060%.

(F) Cr The high carbon steel strip according to the present invention is formed as a steel part by cold working, and then, if necessary, is quenched and tempered with carburizing, or is subjected to austempering to increase its strength. Enhanced. At this time, in order to increase the strength or the toughness, an alloy element may be appropriately contained. Among them, Cr has a large effect of improving strength and toughness. Therefore, it is added in the range of 0.2 to 1.2%.
Below this range, a sufficient strength / toughness improving effect cannot be obtained. On the other hand, exceeding this range not only saturates the effect, but is economically undesirable. Therefore, when Cr is added, it is contained in the range of 0.2 to 1.2%. Preferably, it is 0.2-0.6%.

(G) Mo Mo is effective in increasing the strength and the toughness. Therefore, Mo is added in the range of 0.05 to 1.0% as necessary.
Below this range, the effect of improving strength and toughness cannot be obtained.
On the other hand, exceeding this range not only saturates the effect, but is economically undesirable. Therefore, when Mo is added, it should be contained in the range of 0.05 to 1.0%.

(H) Ni Ni is effective in increasing the strength and the toughness. Therefore, Ni is added in the range of 0.05 to 1.2% as necessary.
Below this range, a sufficient strength / toughness improving effect cannot be obtained. On the other hand, exceeding this range not only saturates the effect, but is economically undesirable. Therefore, when adding Ni, it is to be contained in the range of 0.05 to 1.2%.

(I) V V is effective in increasing the strength even if a small amount of V is added.
If necessary, it is added in the range of 0.05 to 0.50%. Below this range, a sufficient strength improving effect cannot be obtained. On the other hand, exceeding this range not only saturates the effect, but is economically undesirable. Therefore, when V is added, it is contained in the range of 0.05 to 0.50%.

(J) Ti and BB are added in an appropriate amount because they have the effect of improving the hardenability in quenching, the accompanying strength, and the toughness. At this time, the effect of B is impaired by N in the steel, and the addition of B itself makes it difficult to control the ferrite grain size, which is the object of the present invention. Is added to Ti. At this time, B is 0.0005
In the range of 0.0050%, the Ti content is 0.005 to 0.05
%.

If B exceeds this range, the hardening property and toughness deteriorate rather, and if B is less than this range, there is no effect. Also, it is desirable that Ti be contained at a ratio of about 10 times the B content.

B. Metal Structure of Steel Strip Subject to the Present Invention (k) Grain Size and Spheroidization Rate of Carbide The form of carbide precipitation greatly affects the cold workability.
In the present invention, the aim is to improve the cold workability of a high carbon steel strip, and the form of the spheroidized carbide and its dispersion interval affect the n value that governs the cold workability.

The morphology of the spheroidized carbide is determined by polishing the cross section of the steel strip and corroding it with nital, and then using a scanning electron microscope to examine a 100 μm width × 100 μm depth area of a quarter of the plate thickness from the steel strip surface. Spheroidized carbide has an average particle size
0.5 to 2.0 μm, the spheroidization ratio ≧ 80% is satisfied.

The spheroidization ratio% was defined as the occupation ratio of the spherical structure satisfying the relation of major axis / minor axis <5 when nital corrosion was observed to all carbides. Spheroidization ratio of spheroidized carbide
If it is less than 80%, the elongation is small in addition to the n value, and the cold workability is lower than that of the steel strip specified in the present invention. Also, the carbide particle size
If it is less than 0.5 μm, the n value is low and the cold workability is low. On the other hand, if it exceeds 2.0 μm, elongation decreases and cold workability deteriorates.

(1) Particle Number Density of Carbide, Dispersion Interval, and Ferrite Particle Size It is conventionally known that the dispersion interval of carbide has a strong influence on the strength and elongation of a steel strip. As the carbide becomes coarser, the dispersion interval becomes wider, the tensile strength decreases, the elongation increases, and the material becomes softer.
In the case of the present invention, the carbide is mainly cementite, and will be described below simply as cementite.

Thus, the cold workability, which is the object of the present invention,
For example, to improve cold forgeability, it is advantageous to increase the dispersion interval. However, if the dispersion interval of the cementite is excessively increased, the cementite becomes extremely coarse, and the cementite is easily broken during the deformation in the cold working, and the material itself is liable to be destroyed by this breakage.

Further, when cementite becomes spheroidized during annealing, it is said that the dispersed form of the spheroidized structure governs the grain growth of ferrite, which is the parent phase. If the ferrite grains are excessively fine, they have high hardness and yield strength and are not suitable for cold working. On the other hand, when it is excessively large, elongation is reduced and breakage is likely to occur in cold working.

Further, in the carbon steel strip which is an object of the present invention,
When the ferrite grain size is controlled in an appropriate range, the n value that has an effect on improving the cold workability can be maximized. Therefore, in defining the cementite dispersion interval, first, the number of cementite grains θn is 1/4 of the average thickness of the steel strip,
In the region of the area of, a 100 × 100 μm field of view was divided into 16 parts, and after polishing the cross section, it was corroded with nital and
The value obtained by measuring the number of cementite by observing at twice magnification is converted into the number per 1 mm ² , and the unit is n / mm ² .

Assuming that θn measured as described above is uniformly dispersed around 1 mm ² , the average value is calculated as the average θn
And Here, the average dispersion interval of carbides, that is, cementite, was defined as θspμm = (10 ⁶ /3.14×average θn) ^0.5 × 2.3.

On the other hand, when the average grain size of ferrite obtained by the same measurement method is d (μm), d has an influence on the n value in cold working as described above, and thus d is in the optimal numerical range. Need to be identified.

At this time, d is governed by the dispersion interval of cementite as described above, and Al is described by the action of the chemical composition.
In addition to being governed by N, it is governed by hot rolling, cold rolling, and annealing conditions. That is, while d cannot be defined only by the cementite dispersion interval, the cementite dispersion interval θsp and the ferrite grain size d must be controlled in the optimal numerical range.

In the present invention, the relationship between the dispersion interval θsp of cementite and the ferrite grain size d is defined in the following numerical range based on the results of an investigation based on a plurality of samples.
It has been confirmed that even in a plurality of steel types having different C contents, a good n value can be obtained with the component composition.

1.0 × θsp + 1.0 <d <1.0 × θsp + 10.0 Within the specified range of θsp and d, d is 1.0 × θsp + 1.0
At ~ 2.0, it is assumed to be appropriate for hole expansion, etc., where local elongation is required. When d is 1.0 × θsp + 2.0 to 4.0, high suitability for spinning, rolling and deep drawing with compression is assumed. Further, d is 1.0 × θsp +
In the range of 4.0 to 10.0, high suitability for cold working requiring high elongation is assumed.

(M) Variation ratio of particle number density of carbide (θnmin / θ
nmax) θn in the steel obtained by the above-described measuring method is governed by the form of pearlite formed by the hot-rolled steel strip. The steel of C content which is the object of the present invention becomes a pearlite band in which a structure of ferrite and pearlite are mixed in the hot-rolled steel strip. At this time, depending on the temperature conditions of the hot rolling, ferrite and pearlite tend to spread as a layered structure in the rolling direction.

Further, in order to improve cold workability,
Even if the cementite is spheroidized by spheroidizing annealing to soften the cementite, the distribution form of the cementite may have a non-uniform precipitation density depending on the trace of the layered structure of ferrite and pearlite in hot rolling.

In order to improve the cold workability, which is an object of the present invention, such a layered structure that has developed in the rolling direction or an unevenly dispersed morphology of spheroidized cementite can be used to increase the elongation and deformability during cold working. In some cases, the anisotropy is accompanied by non-uniform deformation, and in some cases, a break occurs during cold working in a specific orientation.

Therefore, in the present invention, in order to make the degree of dispersion of the spheroidized cementite uniform to a specific level or more, the maximum value of the number of cementite grains per visual field in the observation region of the metal structure is set to θnmax , The minimum value was defined as θnmin. As described above, if the difference in the density of cementite is small, the effect of improving the cold workability can be obtained.
It is necessary to increase the numerical value of in / θnmax.

However, as the amount of C increases, the amount of cementite dispersed in the ferrite increases, and the difference in the amount of precipitation decreases. That is, as the C amount increases, θ nm
in / θnmax tends to increase, but even if the C content increases, there is an optimum θnmin / θnmax in the steel.

Therefore, according to the present invention, θnmin / θ
nmax is defined by a function of C. As a result of studying the optimum value, by setting θnmin / θnmax to be larger than ΔC%, cold workability and anisotropy in cold work can be improved. The effect was confirmed.

C. Effect of Prescribed Conditions on Manufacturing Method (n) Finishing Temperature TF in Hot Rolling As described above, the present inventor has set forth the correlation condition between the cementite dispersion interval and the ferrite grains, and also the cementite precipitation density. It has been found that homogenization is effective for improving the cold workability, but such a metal structure can be easily ensured by setting the hot rolling conditions.

Recognition that the uniform dispersion of cementite is particularly important, suppression of the pearlite band is an important factor.
It is considered that segregation of pro-eutectoid ferrite formed during hot rolling and alloy components in a slab as a raw material is a major factor.
However, there is little clear and sufficient knowledge on securing uniform pearlite in such a carbon steel strip, and especially the conditions of the finishing temperature in hot rolling that can realize the metallographic structure found this time have been established. Absent.

According to the present invention, a plurality of steel materials were hot-rolled under various conditions, and the morphology of cementite when subsequently subjected to spheroidizing annealing was measured. As a result, it was found that it is effective to increase the finishing temperature TF (° C.) as the C content decreases.

Conventionally, in order to suppress the precipitation of pearlite bands in steel, it is effective to perform finish rolling at a specific temperature higher than the Ar ₁ point temperature. The variation in the morphology of the pearlite band precipitated from austenite due to the change in deformation resistance and the variation in the precipitation density of cementite after spheroidizing annealing cannot always be arranged in correlation with the Ar ₁ point temperature.

When the correlation between the amount of C and TF (° C.) is arranged, it can be approximated within a temperature condition range of 1270 + 25 × (C%) − 500 × (C%) ^0.1 . Therefore, in the present invention,
The finishing temperature TF (° C) is 1270 + 25 x (C%)-500 x (C%) ^0.1 <Tf <1270 + 25 x (C
%) − 500 × (C%) ^0.1 + 60.

However, according to this equation, at a high C composition,
Since the wear of the rolls and the like in the rolling process becomes apparent due to the decrease in the finishing temperature, it is desirable to avoid an excessive decrease in the temperature in the material region exceeding 0.6% C.

(O) Winding temperature TC in hot rolling As described above, the correlation condition between the cementite dispersion interval and the ferrite grains, which is effective for improving the cold workability, and the uniformity of the cementite precipitation density. In the hot rolling conditions, not only the finishing temperature but also the winding temperature has an important effect. The above-mentioned finishing temperature is particularly effective for controlling the pearlite band, whereas the spheroidization ratio and particle size of cementite greatly depend on the winding temperature TC.

According to the present invention, when a plurality of steel materials were hot-rolled under various conditions and subsequently the morphology of cementite was measured after spheroidizing annealing, TC was found to decrease with decreasing C content. It turned out to be effective. From various conditions, the relationship between the C content and TC that can be adapted to this tissue condition is summarized as 600 · {1-0.1 · (1-C%) ² }, and considering the temperature range that can be adapted, 600 · ｛ 1−0.1 ・ (1−C%) ² ｝ <TC <600 ・ [1−0.1 ・
By controlling the temperature within the range of {(1-C%) ² −0.9}], the metallographic structure of the present invention is secured.

Further, according to the present invention, alloy elements such as Mn and Cr are appropriately contained as necessary. However, if the total amount of the alloy elements is increased, the pearlite becomes excessively fine, and the strength of the hot-rolled steel strip is increased. Therefore, it is desirable to adopt a relatively high temperature within the range of the specified temperature condition in order to alleviate the hindrance in the rolling and the accompanying sheet passing process due to the excessive rise.

[0061]

EXAMPLES Example 1 Steel Nos. 1 to 20 shown in Table 1 were melted in a laboratory to produce steel ingots, heated at 1200 ° C. for 1 hour, and then heated at a finishing temperature and a winding temperature shown in Table 2. Rolled to a steel strip with a thickness of 2.5 mm and a width of 200 mm,
After the winding, slow cooling was performed at a cooling rate corresponding to the actual hot-rolled coil cooling of the production line at 20 ° C / h. After cooling in this way, after descaling by pickling, in a hydrogen atmosphere
Annealing at 740 ° C. for 8 hours was carried out so as to make cementite spheroidized, and the mechanical properties at this time were measured.

Further, a 25 mm square test piece was machined from this steel strip, heated at 870 ° C. for 30 minutes in an Ar atmosphere, quenched in 80 ° C. oil, and subsequently tempered at 420 ° C. × 40 minutes. The hardness and mechanical properties after the heat treatment were measured. These results are summarized in Table 3.

In this example, the elongation at the time of annealing was 24% as a criterion for judging the excellent cold workability and hardenability, which is the object of the present invention.
As described above, the criterion was adopted in which the n value was 0.16 or more, the r value was 0.7 or more, and the hardness after tempering was Hv250 or more.

As the metallographic structure, all steels showed a metallographic structure within the range of the present invention, but when the alloy component exceeded the range of the present invention, there were cases where effective characteristics could not be obtained. For example, steel No. 1 having a C content lower than the range of the present invention has insufficient hardness after tempering, and a steel having a C content exceeding the range of the present invention.
For No. 6, the growth was below the criteria. In steel No. 7, the Mn exceeded the upper limit, so the n value was below the criterion. Steel No. 8 has a heat treatment hardness lower than the reaction standard because Mn is below the lower limit of the criterion.

In addition, steel No. 15 has Cr and steel No. 16 has Mo.
However, in steel No. 17, the elongation was less than the criterion because V exceeded the upper limit of the range of the present invention. Steel No. 18 has Al and N balances, and steel No. 19 has Al and N balances and Ti and B balances outside the range of the present invention.
As a result, a steel strip having excellent cold workability and heat treatment hardness can be obtained for the first time within the component range specified in the present invention.

Example 2 Steel Nos. 2 to 5 shown in Table 1 (see Table 4) were prepared at 1200 ° C. ×
After heating for 1 hour, hot rolling was performed at the finishing temperature (TF) and winding temperature (Tc) shown in Table 5, and then slowly cooled at 20 ° C / h, which is the cooling rate equivalent to the cooling of coils in actual production lines. And board thickness 2.5
mm steel strip. After the subsequent pickling, annealing was performed in a hydrogen atmosphere at a temperature shown in Table 5 for 6 hours to measure the mechanical properties, and the same test piece as in Example 1 was prepared. After soaking for one minute, tempering was performed at 420 ° C. for 60 minutes to measure the hardness. The results at this time are also shown in Table 4.

Next, the mechanical properties of each test piece were evaluated in the same manner as in Example 1. The results are summarized in Table 6. The criterion at this time was to change according to the C amount. The reason is that the range of elongation, r value, n value changes according to the amount of C, and the value fluctuates under the hot rolling and annealing process conditions, so in order to define the superiority of the process conditions, This is because it became necessary to add a criterion according to the C amount.

The following numerical values were used as criteria for judging the superiority of the mechanical properties obtained in the present invention. Elongation ≧ −12 × C% + 34, n value ≧ −0.08 × C% + 0.22, r value ≧
−0.3 × C% + 0.9 As a result, in any of the steels, the process 1 in which the hot-rolling finish temperature falls below the specified temperature is such that the cementite density ratio specified in the formula (3) falls below the condition specified in the present invention. The r value in the 90 ° direction is low. Further, in Process 2 in which the temperature is lower than the specified winding temperature, cementite and ferrite are fine, the spheroidization ratio is low, and the n value is lower than the criterion. In Process 7, which exceeds the upper limit of the hot rolling finish specified temperature, the spheroidization ratio is low, and the elongation and the n value are small. Further, in Process 8, which exceeds the upper limit of the specified winding temperature, the cementite density ratio defined by the expression (3) is lower than the range of the invention, and the r value in the rolling 90 ° direction is low. In Process 10, which exceeds the upper limit of the specified annealing temperature, the diameter of cementite,
Ferrite particle size, spheroidization ratio is out of the range of the present invention, elongation,
The r value is below the criterion.

From the results of these investigations, the steel strip having the metallurgical structure and the production method specified in the present invention shows high elongation, n value, and r value at which good cold workability can be expected.

Example 3 Steels No. 20 to No. 24 shown in Table 7 were heated at 1200 ° C. for 1 hour and then hot-rolled at a finishing temperature (TF) and a winding temperature (Tc) shown in Table 8. Thereafter, the steel strip was slowly cooled at a cooling rate of 20 ° C./h, which is equivalent to the cooling of the coil in an actual production line, to obtain a steel strip having a thickness of 2.5 mm. After the subsequent pickling, annealing was performed in a hydrogen atmosphere at a temperature shown in Table 3 for 6 hours, followed by cold rolling to a thickness of 1.5 mm and annealing in a hydrogen atmosphere at 650 ° C. for 20 hours.

A test piece was prepared in the same manner as in Example 1, and after tempering at 870 ° C. for 20 minutes in an Ar atmosphere, tempering at 420 ° C. for 60 minutes was performed, and the hardness was measured (see Table 8). The mechanical properties of each test piece were evaluated in the same manner as in Example 1.
Shown in The criterion at this time was changed according to the amount of C as in Example 2 as described below.

Elongation ≧ −12 × C% + 34, n value ≧ −0.08 × C% +
0.22, r value ≧ −0.3 × C% + 0.9 As a result, in any of the steels, the process 1 in which the hot-rolling finish temperature falls below the specified temperature is such that the cementite density ratio specified in the formula (3) falls below the invention-specified condition. The r value in the rolling 90 ° direction is low. In Process 2 in which the temperature is lower than the specified winding temperature, cementite and ferrite grains are fine, the spheroidization ratio is low, and n
The value is below the criteria. In Process 7, which exceeds the upper limit of the specified finishing temperature, the spheroidization ratio is low, and the elongation and the n value are small.
Further, in process 8 exceeding the winding upper limit temperature, (3)
The cementite density ratio defined by the formula is below the range of the invention, and the r value in the direction of rolling 90 ° is low. In Process 10 exceeding the upper limit of the annealing temperature, the grain size of cementite, the grain size of ferrite,
The spheroidization ratio is out of the range of the present invention, and the elongation and the r value are below the criteria.

From the results of these investigations, the steel strip having the production method and metallographic structure specified in the present invention shows high elongation, n value, and r value at which good cold workability can be expected.

[0074]

[Table 1]

[0075]

[Table 2]

[0076]

[Table 3]

[0077]

[Table 4]

[0078]

[Table 5]

[0079]

[Table 6]

[0080]

[Table 7]

[0081]

[Table 8]

[0082]

[Table 9]

[0083]

According to the present invention, a high carbon steel strip having high elongation, n value and r value and high quenching / tempering hardness required for cold workability can be obtained, and efficient production of parts such as automobiles can be obtained. There is expected.

Claims (6)

[Claims] [Claim 1] C: 0.15 to 0.75%, Si: 0.30% or less, Mn: 0.20 to 1.
60%, sol. Al: less than 0.05%, N: 0.0060% or less, and 5
≦ sol.Al / N ≦ 20, having a carbide average particle diameter in the steel of 0.5 to 2.0 μm, and a spheroidization ratio represented by an occupation ratio of a spherical structure satisfying a major axis / minor axis <5 ≧ 80% satisfied, expression
The average carbide dispersion interval θsp in the steel specified in (1) is the cold forgeability and quenching having a metal structure satisfying the relationship of the formula (2) with respect to the C content and the average ferrite grain size d in the steel. High carbon steel strip with excellent properties. Carbide dispersion interval θsp (μm) = ｛10 ⁶ /(3.14 × average θn｝ ^0.5 × 2.3 · · (1) where θn is the area of the steel strip surface layer at a depth of 1/4 thickness After polishing, it was corroded with nital and observed at a magnification of 2000 with a scanning electron microscope. The field of 100 × 100 μm was divided into 16 parts, and the value of the measured number of carbide particles was converted to the number in the 1 mm ² area. The unit is n / mm ² , and the average value of 16 visual fields is the average θn (n / mm ² ): 1.0 × θsp + 1.0 <d (μm) <1.0 × θsp + 10.0 (2) 2. The correlation of the cementite density ratio defined by the equation (3) is established between θnmax and θnmin, where θnmax is the maximum value and θnmin is the minimum value of θn measured in the 16 fields of view. The high carbon steel strip according to claim 1 having excellent cold workability. θnmin / θnmax> √C% (3) 3. The steel composition further comprises: Cr: 0.2 to 1.2.
%, Mo: 0.05-1.0%, Ni: 0.05-1.2%, V: 0.05-0.
50%, Ti: 0.005 to 0.05%, and B: 0.0005 to 0.0050
% Or more selected from the group consisting of
(Ti and B are simultaneously blended).
Or the high carbon steel strip described in 2. 4. C: 0.15 to 0.75% by weight, Si: 0.
3% or less, Mn: 0.20 to 1.60%, sol.Al: less than 0.05%,
N: 0.0060% or less and a steel having a steel composition containing 5 ≦ sol.Al / N ≦ 20, the finishing temperature range TF satisfies the formula (4), and then the winding temperature TC becomes the formula (5). A method for producing a high carbon steel strip that performs satisfactory hot rolling and then performs box annealing in a temperature range of Ac ₁ −50 ° C. to Ac ₁ + 40 ° C. 1270 + 25 × (C%) − 500 × (C%) ^0.1 <Tf <1270 + 25 × (C%) − 500 × (C%) ^0.1 + 60 ・ ・ ・ (4) 600 ・ ｛1-0.1 ・ (1-C% ) ² ｝ <TC <600 ・ [1-0.1 ・ ｛(1-C%) ² -0.9｝] ・ ・ ・ (5) 5. After performing the hot rolling or further performing the box annealing, cold rolling and 650 ° C. to Ac ₁
The method for producing a high carbon steel strip according to claim 4, wherein the annealing in the temperature range is repeated once or twice or more. 6. The steel composition is as follows: Cr: 0.2-1.2%, Mo: 0.0
5 to 1.0%, Ni: 0.05 to 1.2%, V: 0.05 to 0.50%, Ti:
One or more members selected from the group consisting of 0.005 to 0.05% and B: 0.0005 to 0.0050% (however, Ti and B
The method for producing a high carbon steel strip according to claim 4, further comprising: