High carbon steel sheet and production method thereof

DETAILED DESCRIPTION OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy of tensile properties which is applied to parts formed into, for example, disk or cylinder with a high dimensional precision and then subjected to heat treatment such as quenching and tempering, and a method of producing the same.

[0003] 2. Description of Background Art

[0004] High carbon steel sheets have conventionally been applied to parts for machine structural use such as washers, chains or the like. Such high carbon steel sheets have accordingly been demanded to have good hardenability, and recently not only the good hardenability after quenching treatment but also low temperature—short time of quenching treatment for cost down and high toughness after quenching treatment for safety during services.

[0005] In addition, since the high carbon steel sheets have poor formability as compared with low carbon steel sheets and large planar anisotropy of mechanical properties caused by production process such as hot rolling, annealing and cold rolling, it has been difficult to apply the high carbon steel sheets to parts as gears which are conventionally produced by casting or forging with a high dimensional precision.

[0006] Therefore, it has been requested to improve the hardenability and the toughness of the high carbon steel sheets, and to reduce their planar anisotropy of mechanical properties.

[0007] The following methods have been proposed to meet the requirement.

[0008] (1) JP-A-5-9588, (the term “JP-A” referred to herein signifies “Unexamined Japanese Patent Publication”) (Prior Art 1): hot rolling, cooling down to 20 to 500° C. at a rate of 10° C./sec or higher so as to form fine pearlites, reheating for a short time, and coiling in order to accelerate spheroidization of carbides for improving the hardenability.

[0009] (2) JP-A-5-98388 (Prior Art 2): adding Nb and Ti to high carbon steels containing 0.30 to 0.70% of C so as to form carbonitrides for restraining austenite grain growth and improving the toughness.

[0010] (3) “Material and Process”, vol.1 (1988), p.1729 (Prior Art 3): hot rolling a high carbon steel containing 0.65% of C, cold rolling (reduction rate: 50%), batch annealing at 650° C. for 24 hr, subjecting to secondary cold rolling (reduction rate: 65%), and batch annealing at 680° C. for 24 hr for improving the workability; otherwise adjusting the chemical composition of a high carbon steel containing 0.65% of C, repeating the rolling and the annealing as above mentioned so as to graphitize cementites for decreasing the tensile strength, improving the r value and the elongation, and reducing the planar anisotropy of r-value to the same degree as low carbon steel sheets.

[0011] (4) JP-A-10-152757 (Prior Art 4): adjusting contents of C, Si, Mn, P, Cr, Ni, Mo, V, Ti and Al, decreasing S content below 0.002 wt %, so that 6 μm or less is the average length of sulfide based non metallic inclusions narrowly elongated in the rolling direction, and 80% or more of all the inclusions are the inclusions whose length in the rolling direction is 4 μm or less, whereby the planar anisotropy of toughness and ductility is made so small that the ratio of toughness and ductility in the rolling direction to those in the orthogonal direction to the rolling direction is 0.9 to 1.0.

[0012] (5) JP-A-6-271935 (Prior Art 5): hot rolling, at Ar3 transformation point or higher, a steel whose contents of C, Si, Mn, Cr, Mo, Ni, B and Al were specified, cooling at a rate of 30° C./sec or higher, coiling at 550 to 700° C., descaling, annealing at 600 to 680° C., cold rolling at a reduction rate of 40% or more, further annealing at 600 to 680° C., and temper rolling so as to reduce the planar shape anisotropy caused by heat treatment such as quenching and tempering.

PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] However, there are following problems in the above mentioned prior arts.

[0014] Prior Art 1: Although reheating for a short time, followed by coiling, a treating time for spheroidizing carbides is very short, and the spheroidization of carbides is insufficient so that the good hardenability might not be probably sometimes provided. Further, for reheating for a short time until coiling after cooling, a rapidly heating apparatus such as an electrically conductive heater is needed, resulting in an increase of production cost.

[0015] Prior Art 2: Because of adding expensive Nb and Ti in order to restrain the austenite grain growth, the production cost is increased.

[0016] Prior Art 3: Although the steel sheet of S65C having ferrite and cementite structure shows a high average r-value of around 1.3, the Δr=(r0+r90−2×r45)/4 is −0.47, which is a parameter of planar anisotropy of r-value, herein, r0, r45, and r90 shows a r-value of the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction respectively, and the Δmax of r-value being a difference between the maximum value and the minimum value among r0, r45, and r90 is 1.17. As a result, the planar anisotropy of r-value is large. In addition, two times of cold rolling and annealing cause an increase in production cost.

[0017] By graphitizing the cementites, the average r-value is further increased, the Δr decreases to 0.34 and the Δmax of r-value decreases to 0.85. The planar anisotropy of r-value is still large. In case graphitizing, since a dissolving speed of graphites into austenite phase is slow, the hardenability is remarkably degraded.

[0018] Prior Art 4: The planar anisotropy of toughness and elongation is considered, but the average r-value and n-value which are important parameters for the workability is not investigated.

[0019] Prior Art 5: The method for producing a high carbon steel sheet having a good dimensional precision at quenching and tempering is described, but no planar anisotropy is referred to.

[0020] The present invention has been realized to solve above these problems, and it is an object of the invention to provide a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy of tensile properties affecting workability, and a method of producing the same.

MEANS TO SOLVE THE PROBLEMS

[0021] The present inventors made a study on the high carbon steel sheet containing carbon 0.2% or more of carbon and chemical composition specified by JIS G 4051, JIS G 4401 or JIS G4802 to improve the hardenability, the toughness and the planar anisotropy of tensile properties, and found that it was effective to control the coiling temperature after hot rolling, the temperature of primary annealing, the cold rolling reduction rate, and the temperature of second annealing, or to reheat the sheet bar to Ar3 transformation point or higher before or during finish rolling for improving the structural uniformity in a thickness direction of steel sheet in addition to the control described above, whereby the existing condition of carbides precipitated in steel was optimized. Further, by the above means, the Δr decreased to −0.15 to 0.15 and the Δmax of r-value below 0.2.

[0022] The present invention has been be accomplished on the base of these findings. The first invention is a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy which contains chemical composition specified by JIS G 4051 (Carbon steels for machine structural use), JIS G 4401 (Carbon tool steels) or JIS G 4802 (Cold-rolled steel strips for springs), wherein more than 50 carbides having a diameter of 1.5 μm or larger exist in 2500 μm2, the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides is 80% or more, and the Δr being a parameter of planar anisotropy of r-value is more than −0.15 to less than 0.15, herein Δr=(r0+r90−2×r45)/4, and r0, r45, and r90 shows a r-value of the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction respectively.

[0023] The second invention is a method of producing a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy, comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at 620 to 680° C.

[0024] The third invention is a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy which contains chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, wherein more than 50 carbides having a diameter of 1.5 μm or larger exist in 2500 μm2, the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides is 80% or more, and the Δmax of r-value is less than 0.2, herein the Δmax of r-value is a difference between maximum value and minimum value among r0, r45 and r90.

[0025] The forth invention is a method of producing a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy, comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at a temperature T1 of 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C., wherein the temperature T1 and the temperature T2 satisfy the following formula (1),

1024−0.6×T1≦T2≦1202−0.80×T1 (1).

[0026] The fifth invention is a method of producing a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy, comprising the steps of: continuously casting into slab a steel containing chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, rough rolling the slab to sheet bar without reheating the slab or after reheating the slab cooled to a certain temperature, finish rolling the sheet bar after reheating the sheet bar to Ar3 transformation point or higher, or during reheating the rolled sheet bar to Ar3 transformation point or higher, coiling the finish rolled steel sheet at 500 to 650° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at a temperature T1 of 630 to 700° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or higher, and secondarily annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C., wherein the temperature T1 and the temperature T2 satisfy the following formula (2),

1010−0.59×T1≦T2≦1210−0.80×T1 (2).

[0027] The planar anisotropy means the maximum difference between tensile properties of the directions 0° (L), 45° (S) and 90° (C) with respect to the rolling direction.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The invention will be explained in detail as follows.

[0029] The first invention is a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy which contains 0.2% or more of carbon and chemical composition specified by JIS G 4051 (Carbon steels for machine structural use) , JIS G 4401 (Carbon tool steels) or JIS G 4802 (Cold-rolled steel strips for springs) , wherein more than 50 carbides having a diameter of 1.5 μm or larger exist in 2500 μm2, the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides is 80% or more, and the Δr being a parameter of planar anisotropy of r-value is more than −0.15 to less than 0.15, herein Δr=(r0+r90−2×r45)/4, and r0, r45, and r90 shows a r-value of the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction respectively.

[0030] Next, the reason of specifying the above features will be given.

[0031] (1) Number of carbides having a diameter of 1.5 μm or larger in 2500 μm2: more than 50, and ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides: 80% or more

[0032] Existing condition of carbides has a strong influence on the hardenability and the toughness after quenching treatment in which low temperature and short time heating is conducted. The effect of the carbide condition was first studied.

[0033] By making a steel having, by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%, hot rolling at a finishing temperature of 850° C., coiling at a coiling temperature of 560° C., pickling, primarily annealing at 640 to 690° C. for 40 hr, cold rolling at a reduction rate of 60%, and secondarily annealing at 610 to 690° C. for 40 hr, steel sheets were produced. Cutting out samples of 50×100 mm from the produced steel sheets, and heating at 820° C. for 10 sec, followed by quenching into oil at around 20° C., the average hardness was measured over 10 portions by Rockwell C Scale (HRc) to evaluate the hardenability. If the average HRc is 50 or more, it may be judged that the good hardenability is provided.

[0034]
FIG. 1 shows the relationship between carbide diameter and hardness in which more than 80% of carbides in 2500 μm2 have this diameter or less. When this carbide diameter is 0.6 μm or less, the hardness (HRc) is 50 or higher. That is, when 80% or more is the ratio of number of carbides having diameters ≦0.6 μm, the carbides are rapidly dissolved into the austenite phase, so that the hardenability is improved. However, if all the carbides have diameters ≦0.6, they are dissolved into the austenite phase at a short time heating and then the austenite grain is extremely coarsened.

[0035]
FIG. 2 shows the relationship between number of carbides having a diameter of 1.5 μm or larger in 2500 μm2 and austenite grain size. The decrease in number of carbides coarsens the austenite grain size. It is remarkable particularly when the number of carbides is below 50. The smaller is the austenite grain size, the higher is the toughness. Therefore, it is necessary to precipitate carbides having a diameter of 1.5 μm or larger, but the austenite grain size is not small unless at least 50 or more of the carbides exist in 2500 μm2.

[0036] The measurement of carbide diameter and number of carbides is not limited to a special method. However, it is preferable to observe carbides using a scanning electron microscope at 1500 to 5000 magnifications after polishing the cross section in a thickness direction of steel sheet sample and etching it, to take the photos, and to measure the carbide diameter and the number of carbides on the photos. The carbide diameter should be measured by averaging the diameters of the carbides observable on the photos. The measurement of the carbide diameter and the number of carbides should be conducted in an observation field area of 2500 μm2 or more, because if the observation field area is smaller than 2500 μm2, the number of observable carbides is too small, and therefore the diameter and the number of carbides could not be measured precisely. As to the upper limit of the observation field area, it is sufficient that the above condition of carbides is obtained at around 60% of cross section area in a thickness direction. The etching agent should preferably be a picral.

[0037] (2) Δr: more than −0.15 to less than 0.15

[0038] The high carbon steel sheet having a very low Δr of more than −0.15 to less than 0.15 can be applied to gear parts produced conventionally by casting or forging which need a high dimensional precision.

[0039] The above mentioned high carbon steel sheet can be produced by the method of the second invention comprising the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at 620 to 680° C. The details will be explained as follows.

[0040] (1) Coiling temperature: 520 to 600° C.

[0041] Since the coiling temperature lower than 520° C. makes pearlite structure very fine, carbides after the primary annealing are considerably fine, so that carbides having a diameter of 1.5 μm or larger cannot be produced after the secondary annealing. In contrast, exceeding 600° C., coarse pearlite structure is generated, so that carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. Accordingly, the coiling temperature is defined to be 520 to 600° C.

[0042] (2) Primary annealing: 640 to 690° C. for 20 hr or longer

[0043] The primary annealing is carried out on the coiled and descaled steel sheet in order to spheroidize carbides. If the primary annealing temperature is higher than 690° C., carbides are too much spheroidized, so that carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. On the other hand, being lower than 640° C., the spheroidization of carbides is difficult, so that carbides having a diameter of 1.5 μm or larger cannot be produced after the secondary annealing. Accordingly, the primary annealing temperature is defined to be 640 to 690° C. The annealing time should be 20 hr or longer for uniformly spheroidizing carbides.

[0044] (3) Cold reduction rate: 50% or more

[0045] In general, the higher the cold reduction rate, the smaller the Δr, and for decreasing the Δr sufficiently, the cold reduction rate of at least 50% is necessary. The upper limit is not always defined, but the cold reduction rate of 80% or less is preferable because the cold reduction rate of above 80% makes it difficult to handle a steel sheet.

[0046] (4) Secondary annealing: 620 to 680° C.

[0047] The cold rolled steel sheet is annealed for recrystallization. If the secondary annealing temperature exceeds 680° C., carbides are greatly coarsened, recrystallized grains grow markedly, the r-value of orthogonal direction to the rolling direction (C) becomes much higher than those of other directions (L or S), and the Δr increases. On the other hand, being lower than 620° C., carbides become fine, and recrystallized grains do not grow sufficiently, so that the workability decreases. Thus, the secondary annealing temperature is defined to be 620 to 680° C. For the secondary annealing, either a continuous annealing or a box annealing will do.

[0048] The third invention is a high carbon steel sheet having 0.2% or more of carbon and chemical composition specified by JIS G 4051 (Carbon steels for machine structural use) , JIS G 4401 (Carbon tool steels) or JIS G 4802 (Cold-rolled steel strips for springs), wherein more than 50 carbides having a diameter of 1.5 μm or larger exist in 2500 μm2, the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides is 80% or more, and the &max of r-value is less than 0.2. Here, the Δmax of r-value is a maximum difference between the maximum r-value and minimum r-value among r0, r45 and r90. The details will be explained as follows.

[0049] (1) Number of carbides having a diameter of 1.5 μm or larger in 2500 μm2: more than 50, and ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides: 80% or more

[0050] The reason for specifying the existing condition of carbides like this is, as described above in case of the first invention, that the austenite grain becomes fine by precipitating more than 50 carbides having a diameter of 1.5 μm or larger in 2500 μm2, improving the toughness, and the good hardenability is obtained by controlling the ratio of number of carbides having a diameter of 0.6 μm or less with respect to all the carbides to 80% or more.

[0051] (2) Δmax of r-value: less than 0.2

[0052] The high carbon steel sheet having a very low Δmax of r-value of less than 0.2 can be applied to gear parts produced conventionally by casting or forging which need a high dimensional precision.

[0053] The above mentioned high carbon steel sheet can be produced by the following method 1 or 2.

[0054] The method 1 comprises the steps of: hot rolling a steel having chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, coiling the hot rolled steel sheet at 520 to 600° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at a temperature T1 of 640 to 690° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or more, and secondarily annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C., wherein the temperature Ti and the temperature T2 satisfy the following formula (1).

1024−0.6×T1≦T2≦1202−0.80×T1 (1)

[0055] (1) Coiling temperature: 520 to 600° C.

[0056] As described in the method of the second invention, the coiling temperature lower than 520° C. cannot produce carbides having a diameter of 1.5 μm or larger after the secondary annealing. In contrast, exceeding 600° C., carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. Accordingly, the coiling temperature is defined to be 520 to 600° C.

[0057] (2) Primary annealing: 640 to 690° C. for 20 hr or longer

[0058] As described in the method of the second invention, the primary annealing is carried out on the coiled and descaled steel sheet in order to spheroidize carbides. If the primary annealing temperature is higher than 690° C., carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. On the other hand, being lower than 640° C., carbides having a diameter of 1.5 μm or larger cannot be produced after the secondary annealing. Accordingly, the primary annealing temperature is defined to be 640 to 690° C. The annealing time should be 20 hr or longer for uniformly spheroidizing carbides.

[0059] (3) Cold reduction rate: 50% or more

[0060] As described in the method of the second invention, for decreasing the Δr sufficiently, the cold reduction rate of at least 50% is necessary. The upper limit is not always defined, but the cold reduction rate of 80% or less is preferable from a view point of handling a steel sheet.

[0061] (4) Secondary annealing: 1024−0.6×T1≦T2≦1202−0.80×T1, and 620° C.≦T2≦680° C.

[0062] The secondary annealing condition should be controlled with the primary annealing condition to decrease the Δmax of r-value. Then, the effect of the primary annealing condition and the second annealing condition on the Δmax of r-value was investigated. The details will be described as follows.

[0063] By making a steel of, by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%, hot rolling at a finishing temperature of 850° C. and coiling at a coiling temperature of 560° C., pickling, primarily annealing at 640 to 690° C. for 40 hr , cold rolling at a reduction rate of 60%, and secondarily annealing at 610 to 690° C. for 40 hr, steel sheets were produced, and the Δmax of r-value was measured by a tensile test. FIG. 3 shows the effect of primary annealing temperature and secondary annealing temperature on the Δmax of r-value. As seen in FIG. 3, if the secondary annealing temperature T2 is (1024−0.6×T1) to (1202−0.80×T1), the Δmax of r-value is less than 0.2, and the planar anisotropy is small. Accordingly, the relation of 1024−0.6×T1≦T2≦1202−0.80×T1 is necessary. The Δmax of r-value is a maximum difference between the maximum r-value and minimum r-value among r0, r45 and r90.

[0064] The secondary annealing temperature has a strong influence on the carbide diameter and the carbide dispersion. If the secondary annealing temperature exceeds 680° C., carbides are coarsened, and carbides having a diameter of 0.6 μm or less cannot be produced. On the other hand, being lower than 620° C., carbides having a diameter of 1.5 μm or larger cannot be produced. Thus, the secondary annealing temperature T2 is defined to be 620 to 680° C. For the secondary annealing, either a continuous annealing or a box annealing will do.

[0065] Next, the method 2 will be explained.

[0066] The method 2 comprises the steps of: continuously casting into slab a steel containing chemical composition specified by JIS G 4051, JIS G 4401 or JIS G 4802, rough rolling the slab to sheet bar without reheating the slab or after reheating the slab cooled to a certain temperature, finish rolling the sheet bar after reheating the sheet bar to Ar3 transformation point or higher, or during reheating the rolled sheet bar to Ar3 transformation point or higher, coiling the finish rolled steel sheet at 500 to 650° C., descaling the coiled steel sheet, primarily annealing the descaled steel sheet at a temperature T1 of 630 to 700° C. for 20 hr or longer, cold rolling the annealed steel sheet at a reduction rate of 50% or higher, and secondarily annealing the cold rolled steel sheet at a temperature T2 of 620 to 680° C., wherein the temperature T1 and the temperature T2 satisfy the following formula (2),

1010−0.59×T1≦T2≦1210−0.80×T1 (2).

[0067] Detailed explanation will be made explained as follows.

[0068] (1) Reheating the sheet bar

[0069] By reheating the sheet bar, crystal grains are uniformed in a thickness direction of steel sheet during rolling, the dispersion of carbides after the secondary annealing is small, and the planar anisotropy of r-value becomes smaller. Concretely, the finish rolling of the sheet bar after reheating the sheet bar to Ar3 transformation point or higher, or during reheating the rolled sheet bar to Ar3 transformation point or higher is conducted. The reheating temperature should be Ar3 transformation point or higher to uniform the austenite grain and the structure. The reheating time should be at least 3 seconds.

[0070] (2) Coiling temperature: 520 to 600° C.

[0071] As described in the method 1, the coiling temperature lower than 520° C. cannot produce carbides having a diameter of 1.5 μm or larger after the secondary annealing. In contrast, exceeding 600° C., carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. Accordingly, the coiling temperature is defined to be 520 to 600° C.

[0072] (2) Primary annealing: 640 to 690° C. for 20 hr or longer

[0073] As described in the method 1, the primary annealing is carried out on the coiled and descaled steel sheet in order to spheroidize carbides. If the primary annealing temperature is higher than 690° C., carbides having a diameter of 0.6 μm or less cannot be produced after the secondary annealing. On the other hand, being lower than 640° C., carbides having a diameter of 1.5 μm or larger cannot be produced after the secondary annealing. Accordingly, the primary annealing temperature is defined to be 640 to 690° C. The annealing time should be 20 hr or longer for uniformly spheroidizing carbides.

[0074] (4) Cold reduction rate: 50% or more

[0075] As described in the method 1, for decreasing the Δr sufficiently, the cold reduction rate of at least 50% is necessary. The cold reduction rate of 80% or less is preferable from a view point of handling a steel sheet.

[0076] (5) Secondary annealing: 1010−0.59×T1≦T2≦1210−0.80×T1, and 620° C.≦T2≦680° C.

[0077] As described in the method 1, the secondary annealing condition should be controlled with the primary annealing condition to decrease the Δmax of r-value. Then, the effect of the primary annealing condition and the second annealing condition on the Δmax of r-value was investigated. The results will be described as follows.

[0078] By making a slab of, by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%, rough rolling, reheating the sheet bar at 1010° C. for 15 sec by an induction heater, finish rolling at 850° C., coiling at 560° C., pickling, primarily annealing at 640 to 700° C. for 40 hr, cold rolling at a reduction rate of 60%, and secondarily annealing at 610 to 690° C. for 40 hr, steel sheets were produced. Measurements were made on the (222) integrated reflective intensity in the thickness directions (surface, ¼ thickness and ½ thickness) by a X-ray diffraction method. As shown in Table 1, by reheating the sheet bar using an induction heater, the Δmax of (222) intensity being a maximum difference between the maximum value and the minimum value of (222) integrated reflective intensities in the thickness directions becomes small, and therefore the structure is more uniform in the thickness direction. FIG. 4 shows the effect of primary annealing temperature and secondary annealing temperature on the Δmax of r-value. In the method 1, as shown in FIG. 3, the Δmax of r-value is less than 0.2 when the secondary annealing temperature T2 is (1024−0.6×T1) to (1202−0.80×T1). On the other hand, by reheating the sheet bar using an induction heater in the method 2, the Δmax of r-value is further reduced to less than 0.15 in a wider range of T2, that is, 1010−0.59×T1≦T2≦1210−0.80×T1.

1TABLE 1Integrated reflective intensity (222)Reheating ofPrimarySecondary¼sheet barannealingannealingthick-½(° C. × sec)(° C. × hr)(° C. × hr)SurfacenessthicknessΔmax1010 × 15640 × 40610 × 402.812.952.890.141010 × 15640 × 40650 × 402.822.882.950.131010 × 15640 × 40690 × 402.902.913.020.121010 × 15680 × 40610 × 402.372.352.460.111010 × 15680 × 40650 × 402.402.362.470.111010 × 15680 × 40690 × 402.292.342.390.10—640 × 40610 × 402.703.012.900.31—640 × 40650 × 402.752.872.990.24—640 × 40690 × 402.812.903.050.24—680 × 40610 × 402.342.272.500.23—680 × 40650 × 402.392.232.510.28—680 × 40690 × 402.252.372.450.20

[0079] As described in the method 1, the secondary annealing temperature T2 is defined to be 620 to 680° C. to produce some carbides having a diameter of 0.6 μm or less and other carbides having a diameter of 1.5 μm or larger. For the secondary annealing, either a continuous annealing or a box annealing will do.

[0080] To produce the high carbon steel sheet of the present invention, a continuously cast slab may be hot rolled after being reheated or directly hot rolled after being cast. The sheet bar may be reheated by a bar heater. The bar heating is effective in a continuous hot rolling process using a coil box. In this process, the sheet bar may be also reheated before or after the coil box, or before and after a welding machine. For improving the sliding property, the high carbon steel sheet of the present invention may be galvanized through an electro-galvanizing process or a hot dip Zn plating process, followed by a phosphating treatment.

EXAMPLE

Example 1

[0081] This example relates to the high carbon steel sheet of the first invention.

[0082] By making a slab containing the chemical composition specified by S35C of JIS G 4051 (by wt %, C: 0.35%, Si: 0.20%, Mn: 0.76%, P: 0.016%, S: 0.003% and Al: 0.026%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 2, and temper rolling at a reduction rate of 1.5%, steel sheets of 1.0 mm thickness were produced. Herein, the steel sheet H is a conventional high carbon steel sheet.

2TABLE 2CoilingPrimaryColdSecondarySteeltemperatureannealingreductionannealingNumber of carbidesRatio of carbidesRemarksheet(° C.)(° C. × hr)rate (%)(° C. × hr)larger than 1.5 μmsmaller than 0.6 μm (%)RemarkA580650 × 4070680 × 408984PresentinventionB560640 × 2060660 × 408487PresentinventionC540660 × 2065640 × 408193PresentinventionD500640 × 4060660 × 406496ComparativeexampleE560710 × 4065660 × 4010358ComparativeexampleF540660 × 2040680 × 408684ComparativeexampleG550640 × 2060720 × 409861ComparativeexampleH620—50690 × 407470Comparativeexample

[0083] Then, carbide diameter, carbide dispersion, tensile properties, hardenability and austenite grain size were measured as follows. The results are shown in Table 3.

[0084] (a) Carbide diameter and carbide dispersion

[0085] The measurement of the diameter and the number of carbide was conducted in an observation field area of 2500 μm2 on the photos taken by a scanning electron microscope after polishing the cross section in a thickness direction of steel sheet sample and etching it.

[0086] (b) Tensile properties

[0087] JIS No.5 test pieces were sampled from the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction, and subjected to the tensile test at a tension speed of 10 mm/min so as to measure the tensile properties in each direction and the planar anisotropy. The Δmax of yield strength, tensile strength and elongation shown in Table 3 is a difference between the maximum value and the minimum value of each tensile property. The Δr in Table 3 was calculated by the equation Δr=(r0+r90−2×r45)/4, herein, r0, r45, and r90 shows a r-value of the directions of 0° (L), 45° (S) and 90° (C) with respect to the rolling direction respectively.

[0088] (c) Hardenability

[0089] Cutting out samples of 50×100 mm from the produced steel sheets, and heating at 820° C. for 10 sec, followed by quenching into oil at around 20° C., the average hardness was measured over 10 portions by Rockwell C Scale (HRc) to evaluate the hardenability. If the average HRc is 50 or more, it may be judged that the good hardenability is provided.

[0090] (d) Austenite grain size

[0091] The cross section in a thickness direction of the quenched test piece was polished, etched, and observed by an optical microscope. The austenite grain size number was measured following JIS G 0551.

3TABLE 3Hard-Auste-nesstineafterGrainMechanical properties before quenching quench-sizeSteelYield strength (MPa)Tensile strength (MPa)Total elongation (%)r-valueing(sizesheetLSCΔmaxLSCΔmaxLSCΔmaxLSCΔr(HRc)No.)RemarkA3953913934506502507535.736.435.90.71.060.971.040.045211.6Presentinven-tionB4054044117504498507935.836.836.21.01.120.981.230.105411.3Presentinven-tionC4094064148509505513835.236.435.31.20.981.191.05−0.095610.7Presentinven-tionD3693623708499496503930.129.331.01.71.160.921.330.16578.6Compa-rativeexampleE3703793759480484481436.936.036.40.91.150.961.470.184412.2Compa-rativeexampleF374377385114744804881435.734.636.31.71.250.961.460.205311.2Compa-rativeexampleG3723763797496493498538.037.737.70.31.140.941.640.234012.1Compa-rativeexampleH317334320175015165101536.534.635.51.91.120.921.350.164911.6Compa-rativeexample

[0092] As shown in Table 3, since the inventive steel sheets A-C have diameters and numbers of carbides within the range of the present invention, the HRc after quenching of these steel sheets is above 50 and the good hardenability is obtained. And the austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, since the Δmax of yield strength and tensile strength is 10 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and the Δr is more than −0.15 to less than 0.15, the planar anisotropy of tensile properties is very small.

[0093] In contrast, the comparative steel sheets D-H have a large Δmax of tensile properties or a large Δr. The steel sheet D of too low coiling temperature has a large Δmax of elongation of 1.7, a large Δr of 0.16, and poor toughness due to the coarse austenite grain caused by the small number of carbides having a diameter of 1.5 μm or larger. The steel sheet E of too high primary annealing temperature has a large Δr of 0.18 and poor hardenability due to the small number of fine carbides. The steel sheet F of too low cold reduction rate of 40% has a large Δmax of yield strength of 11 MPa, a large Δmax of tensile strength of 14 MPa, a large Δmax of elongation of 1.7%, and a large Δr of 0.20. The steel sheet G of too high secondary annealing temperature has a low HRc due to the large number of coarse carbides and a large Δr of 0.23. The conventional steel sheet H has a large Δmax of yield strength of 17 MPa, a large Δmax of tensile strength of 15 MPa, a large Δmax of elongation of 1.9%, and a large Δr of 0.16.

Example 2

[0094] This example relates to the method 1 for producing the high carbon steel sheet of the third invention.

[0095] By making a slab containing the chemical composition specified by S35C of JIS G 4051 (by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 4, and temper rolling at a reduction rate of 1.5%, 19 steel sheets of 2.5 mm thickness were produced.

4TABLE 4CoilingPrimaryColdSecondaryNumber ofRatio of carbidesSteeltemperatureannealingreductionannealingSecondary annealingcarbides largersmaller than 0.6 μmsheet(° C.)(° C. × hr)rate (%)(° C. × hr)range by the formula (1)than 1.5 μm(%)Remark1580640 × 4070680 × 40640-6805685Presentinvention2530640 × 2060680 × 40640-6805287Presentinvention3595640 × 4060680 × 20640-6806481Presentinvention4580660 × 4060660 × 40628-6746183Presentinvention5580680 × 2060640 × 40620-6586382Presentinvention6580640 × 4050660 × 40640-6805685Presentinvention7580640 × 4070640 × 40640-6805486Presentinvention8510640 × 2060680 × 40640-6803092Comparativeexample9610640 × 2060680 × 20640-6806861Comparativeexample10580620 × 4060680 × 40—3290Comparativeexample11580720 × 4060680 × 40—6865Comparativeexample12580640 × 1570680 × 40640-6805486Comparativeexample13580640 × 4030680 × 40640-6805884Comparativeexample14580660 × 2060620 × 40628-6746084Comparativeexample15580640 × 2060700 × 40640-6806673Comparativeexample16580640 × 4060690 × 40640-6806770Comparativeexample17580690 × 4060615 × 40620-6503388Comparativeexample18520640 × 2060640 × 20640-6804588Comparativeexample19620—50690 × 40—5167Comparativeexample

[0096] Carbide diameter, carbide dispersion, tensile properties, hardenability and austenite grain size were measured in the same way as shown in the Example 1. The results are shown in Table 5. Herein, the steel sheet 19 is a conventional high carbon steel sheet.

5TABLE 5Hard-Auste-nesstineafterGrainMechanical properties before quenchingquench-sizeSteelYield strength (MPa)Tensile strength (MPa)Total elongation (%)r-valueing(sizesheetLSCΔmaxLSCΔmaxLSCΔmaxLSCΔmax(HRc)No.)Remark13983944028506508513536.237.437.01.21.070.991.000.085411.1Presentinven-tion24104074125513512516436.838.036.81.21.021.011.110.105610.9Presentinven-tion33503483513470474472236.336.836.20.61.011.011.090.085111.6Presentinven-tion43953984049507506509336.637.537.30.91.090.991.010.105211.5Presentinvention53923974008502503501237.938.238.00.30.951.131.000.185111.5Presentinven-tion64013984079509509512337.537.938.51.00.941.071.020.135311.3Presentinven-tion74044014109510509512335.336.736.61.41.031.181.010.175511.0Presentinvention83743673747507505508329.928.431.32.91.171.011.430.42588.3Compa-rativeexample937138638015482491485927.125.026.72.11.140.931.310.384012.0Compa-rativeexample103953963994512512515327.025.428.22.81.270.981.280.30588.9Compa-rativeexample1137238438012484489485537.736.937.30.81.241.001.340.344212.0Compa-rativeexample12390384377134905004981029.024.929.44.51.190.941.290.355610.9Compa-rativeexample13372383390184804864931335.533.736.52.81.020.961.480.525311.3Compa-rativeexample144044014109510508513535.137.036.71.91.011.280.940.345211.4Compa-rativeexample1538538637610503501506537.536.836.41.11.281.001.310.314511.8Compa-rativeexample1638838937811504501507637.336.536.01.31.180.981.360.384311.9Compa-rativeexample1741040641711513510515535.336.736.51.41.021.260.920.34569.9Compa-rativeexample184124064159514511519835.136.536.31.40.971.220.880.34579.4Compa-rativeexample1932233532213510519514936.134.135.92.01.120.931.360.434312.0Compa-rativeexample

[0097] As shown in Table 5, since the inventive steel sheets 1-7 have diameters and numbers of carbides within the range of the present invention, the HRc after quenching of these steel sheets is above 50 and the good hardenability is obtained. And the austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, since the Δmax of yield strength and tensile strength is 10 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and the Δmax of r-value is less than 0.2, the planar anisotropy of tensile properties is very small.

[0098] In contrast, the comparative steel sheets have a large Δmax of tensile properties, or poor hardenability or toughness. The steel sheet 11 of too high primary annealing temperature has a large Δmax of r-value of 0.30. The steel sheet 13 of too low cold reduction rate of 30% has a large Δmax of yield strength of 18 MPa, a large Δmax of tensile strength of 13 MPa, and a large Δmax of r-value of 0.38. The steel sheet 16 of too high secondary annealing temperature has a low HRc of 43 due to the insufficient dissolution of carbides. The steel sheet 17 of too low secondary annealing temperature has poor toughness due to the coarse austenite grain caused by the large number of carbides having a diameter of 0.6 μm or less. The conventional steel sheet 19 has a large Δmax of r-value of 0.42.

Example 3

[0099] This example relates to the method 1 for producing the high carbon steel sheet of the third invention, too.

[0100] By making a slab containing the chemical composition specified by S65C-CSP of JIS G 4802 (by wt %, C: 0.65%, Si: 0.19%, Mn: 0.73%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 6, and temper rolling at a reduction rate of 1.5%, 19 steel sheets of 2.5 mm thickness were produced.

6TABLE 6CoilingPrimaryColdSecondaryNumber ofRatio of carbidesSteeltemperatureannealingreductionannealingSecondary annealingcarbides largersmaller than 0.6 μmsheet(° C.)(° C. × hr)rate (%)(° C. × hr)range by the formula (1)than 1.5 μm(%)Remark20560640 × 4070680 × 40640-6808686Presentinvention21530640 × 2060680 × 40640-6808288Presentinvention22595640 × 4060680 × 20640-6809482Presentinvention23560660 × 4060660 × 40628-6749083Presentinvention24560680 × 2060640 × 40620-6589283Presentinvention25560640 × 4050660 × 40640-6808785Presentinvention26560640 × 4070640 × 40640-6808386Presentinvention27510640 × 2060680 × 40640-6804493Comparativeexample28610640 × 2060680 × 20640-68010162Comparativeexample29560620 × 4060680 × 40—4791Comparativeexample30560720 × 4060680 × 40—10064Comparativeexample31560640 × 1570680 × 40640-6808387Comparativeexample32560640 × 4030680 × 40640-6808885Comparativeexample33560660 × 2060620 × 40630-6748984Comparativeexample34560640 × 2060700 × 40640-6809872Comparativeexample35560640 × 4060690 × 40640-6809970Comparativeexample36560690 × 4060615 × 40620-6504989Comparativeexample37600690 × 4050650 × 40620-6509677Comparativeexample38620—50690 × 40—10065Comparativeexample

[0101] Carbide diameter, carbide dispersion, tensile properties, hardenability and austenite grain size were measured in the same way as shown in the Example 1. The results are shown in Table 7. Herein, the steel sheet 38 is a conventional high carbon steel sheet.

7TABLE 7Hard-Auste-nesstineafterGrainMechanical properties before quenchingquench-sizeSteelYield strength (MPa)Tensile strength (MPa)Total elongation (%)r-valueing(sizesheetLSCΔmaxLSCΔmaxLSCΔmaxLSCΔmax(HRc)No.)Remark204124064137515518523834.235.735.21.51.040.960.970.086311.2Presentinven-tion214224194278524521526535.136.034.61.40.981.001.060.086411.0Presentinven-tion223653603635480483480334.535.034.10.90.970.981.070.106011.7Presentinven-tion234094094167518514519534.735.734.21.51.020.970.930.096111.6Presentinven-tion2440541041510511512512135.836.136.20.40.891.110.940.196011.6Presentinven-tion2541641242311519517523635.436.036.71.30.921.030.950.146211.4Presentinven-tion2641741442410521515524933.434.934.71.51.001.150.980.176311.1Presentinven-tion273853803888518515518328.224.828.23.41.220.961.280.32668.4Compa-rativeexample28385400395154895004931125.723.225.22.51.150.891.220.334812.2Compa-rativeexample294064104137519523526725.524.026.72.71.210.971.360.39669.0Compa-rativeexample3038439739413492500496835.834.635.61.21.200.901.180.305012.1Compa-rativeexample31405398389165005105111127.122.427.45.00.941.250.970.316411.1Compa-rativeexample32386396406204864975031733.731.934.82.90.811.170.940.366211.4Compa-rativeexample3341641242513521516523733.235.134.81.91.041.321.010.316111.5Compa-rativeexample3440239138814512510515535.734.834.31.41.220.971.340.375311.9Compa-rativeexample3540539539411514511517635.534.834.11.41.170.881.180.305112.0Compa-rativeexample3642041743114523519525633.334.834.51.51.001.260.930.336510.0Compa-rativeexample3737536337012482490485834.335.234.01.21.210.931.240.315611.8Compa-rativeexample38336350331195175285261134.532.433.82.11.100.831.290.444612.4Compa-rativeexample

[0102] As shown in Table 7, since the inventive steel sheets 20-26 have diameters and numbers of carbides within the range of the present invention, the HRc after quenching of these steel sheets is above 50 and the good hardenability is obtained. And the austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, since the Δmax of yield strength and tensile strength is 15 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and the Δmax of r-value is less than 0.2, the planar anisotropy of tensile properties is very small.

[0103] In contrast, the comparative steel sheets have a large Δmax of tensile properties, or poor hardenability or toughness. The steel sheet 30 of too high primary annealing temperature has a large Δmax of r-value of 0.26. The steel sheet 32 of too low cold reduction rate of 30% has a large Δmax of yield strength of 20 MPa, a large Δmax of tensile strength of 17 MPa, and a large Δmax of r-value of 0.39. The steel sheet 35 of too high secondary annealing temperature has a low HRc of 51 due to the insufficient dissolution of carbides. The steel sheet 36 of too low secondary annealing temperature has poor toughness due to the coarse austenite grain caused by the large number of carbides having a diameter of 0.6 μm or less. The conventional steel sheet 38 has a large Δmax of r-value of 0.46.

Example 4

[0104] This example relates to the method 2 for producing the high carbon steel sheet of the third invention.

[0105] By making a slab containing the chemical composition specified by S35C of JIS G 4051 (by wt %, C: 0.36%, Si: 0.20%, Mn: 0.75%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 8, and temper rolling at a reduction rate of 1.5%, 26 steel sheets of 2.5 mm thickness were produced.

8TABLE 8CoilingColdSecondaryRatio of carbidesReheating oftempe-Primaryreduc-Secondaryannealing rangeNumber ofsmaller thanSteelsheet barratureannealingtionannealingby the formulacarbides larger0.6 μmsheet(° C. × sec)(° C.)(° C. × hr)rate (%)(° C. × hr)(1)than 1.5 μm(%)Remark391050 × 15580640 × 4070680 × 40632-6805586Presentinvention401100 × 3 530640 × 2060680 × 40632-6805287Presentinvention41950 × 3595640 × 4060680 × 20632-6806481Presentinvention421050 × 15580660 × 4060660 × 40620-6806084Presentinvention431050 × 15580680 × 2060640 × 40620-6666282Presentinvention441050 × 15580640 × 4050660 × 40632-6805685Presentinvention451050 × 15580640 × 4070640 × 40632-6805486Presentinvention46—580640 × 4070680 × 40632-6805685Presentinvention47—530640 × 2060680 × 40632-6805386Presentinvention48—595640 × 4060680 × 20632-6806481Presentinvention49—580660 × 4060660 × 40620-6806183Presentinvention50—580680 × 2060640 × 40620-6666382Presentinvention51—580640 × 4050660 × 40632-6805685Presentinvention52—580640 × 4070640 × 40632-6805585Presentinvention531050 × 15510640 × 2060680 × 40632-6803092Comparativeexample541100 × 3610640 × 2060680 × 20632-6806761Comparativeexample55950 × 3580620 × 4060680 × 40—3289Comparativeexample561050 × 15580720 × 4060680 × 40—6865Comparativeexample571050 × 15580640 × 1570680 × 40632-6805586Comparativeexample581050 × 15580640 × 4030680 × 40632-6805884Comparativeexample591050 × 15580660 × 2060610 × 40620-6806084Comparativeexample601050 × 15580640 × 2060700 × 40632-6806674Comparativeexample611050 × 15580640 × 4060690 × 40632-6806670Comparativeexample621050 × 15580690 × 4060615 × 40620-6583388Comparativeexample631050 × 15520640 × 2060640 × 20632-6804588Comparativeexample641050 × 15620—50690 × 40—3387Comparativeexample

[0106] Carbide diameter, carbide dispersion, tensile properties, hardenability and austenite grain size were measured in the same way as shown in the Example 1. The results are shown in Table 9. Herein, the steel sheet 64 is a conventional high carbon steel sheet.

9TABLE 9Hard-Auste-nesstineafterGrainMechanical properties before quenchingquench-sizeSteelYield strength (MPa)Tensile strength (MPa)Total elongation (%)r-valueing(sizeIntegrated reflective intensity (222)sheetLSCΔmaxLSCΔmaxLSCΔmaxLSCΔmax(HRc)No.)Surface¼ thickness½ thicknessΔmaxRemark393983943984506508512636.537.437.00.91.070.991.020.085511.02.802.792.900.11Present invention404104074103514512516436.837.736.80.91.041.011.110.105610.92.852.923.000.15Present invention413513483503470474473436.436.836.20.61.031.011.090.085111.62.872.933.000.13Present invention423953984005508506509336.837.537.30.71.090.991.020.105311.42.722.802.840.12Present invention433953974005501503501237.938.238.10.30.951.091.000.145211.42.542.602.660.12Present invention444013994045509510512337.737.938.50.80.941.071.040.135311.32.852.932.990.14Present invention454044014054511509512335.736.736.61.01.031.151.010.145511.02.883.012.950.13Present invention463973944028506508513736.237.437.11.21.140.991.000.155411.12.752.903.030.28Present invention474094074125514512516436.838.036.91.21.021.011.140.165511.02.773.062.980.29Present invention483513483513470474469536.436.836.20.61.010.981.130.155111.62.792.743.020.28Present invention493953974049507505509436.637.537.20.91.130.961.010.175211.52.652.772.900.25Present invention503923964008502505501437.238.238.01.00.951.141.000.195111.52.482.582.750.27Present invention514033984079509505512337.537.738.51.00.941.121.020.185311.32.803.022.970.22Present invention524054014109510507512535.336.736.41.41.031.191.000.195411.12.832.803.040.24Present invention5337236437410507503508529.828.431.32.91.261.021.370.35588.32.812.882.960.15Comparative example5437138637915482491484927.125.026.32.11.270.981.270.294112.02.842.872.980.14Comparative example553923963997512509515627.225.428.22.81.331.041.360.32589.02.903.042.990.14Comparative example5637238538013484489486537.736.637.31.11.230.951.250.304212.02.202.282.320.12Comparative example57390384378124905004971028.824.929.44.51.160.891.200.315510.92.822.932.910.11Comparative example58372385390184804874931335.433.736.52.80.881.190.910.315311.32.832.902.980.15Comparative example594054014109510506513735.137.036.61.91.011.270.940.335211.42.732.792.860.13Comparative example6038338637610504501506537.536.936.41.11.180.941.290.354511.72.852.923.000.15Comparative example6138738937811503501507637.336.636.01.31.161.001.450.454411.92.822.962.930.14Comparative example6241040441713513507515835.336.736.11.40.871.170.880.29569.92.382.422.530.15Comparative example634114064159515511515835.136.536.01.41.021.321.000.32579.42.832.882.960.13Comparative example6432333532213510519513936.134.135.52.01.100.931.350.404312.02.332.392.480.15Comparative example

[0107] As shown in Table 9, since the inventive steel sheets 39-52 have diameters and numbers of carbides within the range of the present invention, the HRc after quenching of these steel sheets is above 50 and the good hardenability is obtained. And the austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, since the Δmax of yield strength and tensile strength is 10 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and the Δmax of r-value is less than 0.2, the planar anisotropy of tensile properties is very small. The heating after rough rolling is effective not only for reducing the planar anisotropy of tensile properties but also for making the structure uniform in a thickness direction.

[0108] In contrast, the comparative steel sheets have a large Δmax of tensile properties, or poor hardenability or toughness. The steel sheet 56 of too high primary annealing temperature has a large Δmax of r-value of 0.30. The steel sheet 58 of too low cold reduction rate of 30% has a large Δmax of yield strength of 18 MPa, a large Δmax of tensile strength of 13 MPa, and a large Δmax of r-value of 0.38. The steel sheet 61 of too high secondary annealing temperature has a low HRc of 44 due to the insufficient dissolution of carbides. The steel sheet 62 of too low secondary annealing temperature has poor toughness due to the coarse austenite grain caused by the large number of carbides having a diameter of 0.6 μm or less. The conventional steel sheet 64 has a large Δmax of r-value of 0.42.

Example 5

[0109] This example relates to the method 2 for producing the high carbon steel sheet of the third invention, too.

[0110] By making a slab containing the chemical composition specified by S65C-CSP of JIS G 4802 (by wt %, C: 0.65%, Si: 0.19%, Mn: 0.73%, P: 0.011%, S: 0.002% and Al: 0.020%) through a continuous casting process, reheating to 1100° C., hot rolling, coiling, primarily annealing, cold rolling, secondarily annealing, under the conditions shown in Table 10, and temper rolling at a reduction rate of 1.5%, 26 steel sheets of 2.5 mm thickness were produced.

10TABLE 10SecondaryReheating ofCoilingPrimaryColdSecondaryannealing rangeNumber ofRatio of carbidesSteelsheet bartemperatureannealingreductionannealingby the formulacarbides largersmaller than 0.6sheet(° C. × sec)(° C.)(° C. × hr)rate (%)(° C. × hr)(1)than 1.5 μmμm (%)Remark651050 × 15560640 × 4070680 × 40632-6808587Presentinvention661100 × 3 530640 × 2060680 × 40632-6808288Presentinvention67950 × 3595640 × 4060680 × 20632-6809482Presentinvention681050 × 15560660 × 4060660 × 40620-6808984Presentinvention691050 × 15560680 × 2060640 × 40620-6669183Presentinvention701050 × 15560640 × 4050660 × 40632-6808785Presentinvention711050 × 15560640 × 4070640 × 40632-6808386Presentinvention72—560640 × 4070680 × 40632-6808686Presentinvention73—530640 × 2060680 × 40632-6808387Presentinvention74—595640 × 4060680 × 20632-6809482Presentinvention75—560660 × 4060660 × 40620-6809083Presentinvention76—560680 × 2060640 × 40620-6669283Presentinvention77—560640 × 4050660 × 40632-6808785Presentinvention78—560640 × 4070640 × 40632-6808485Presentinvention791050 × 15510640 × 2060680 × 40632-6804493Comparativeexample801100 × 3 610640 × 2060680 × 20632-68010062Comparativeexample81950 × 3560620 × 4060680 × 40—4790Comparativeexample821050 × 15560720 × 4060680 × 40—10064Comparativeexample831050 × 15560640 × 1570680 × 40632-6808487Comparativeexample841050 × 15560640 × 4030680 × 40632-6808885Comparativeexample851050 × 15560660 × 2060610 × 40620-6808984Comparativeexample861050 × 15560640 × 2060700 × 40632-6809873Comparativeexample871050 × 15560640 × 4060690 × 40632-6809870Comparativeexample881050 × 15560690 × 4060615 × 40620-6804989Comparativeexample891050 × 15600690 × 2050650 × 40632-6809677Comparativeexample901050 × 15610—50690 × 40—9971Comparativeexample

[0111] Carbide diameter, carbide dispersion, tensile properties, hardenability and austenite grain size were measured in the same way as shown in the Example 1. The results are shown in Table 11. Herein, the steel sheet 90 is a conventional high carbon steel sheet.

11TABLE 11Mechanical properties before quenchingHardness afterAustetineIntegrated reflective intensity (222)SteelYield strength (MPa)Tensile strength (MPa)Total elongation (%)r-valuequenchingGrain size1/41/2sheetLSCΔmaxLSCΔmaxLSCΔmaxLSCΔmax(HRc)(size No.)SurfacethicknessthicknessΔmaxRemark654124064126515518521634.735.735.21.01.040.960.980.086411.12.872.822.970.15Presentinvention664224194245523521526535.136.035.10.90.981.021.060.086411.02.832.862.940.11Presentinvention673643603634480483481334.535.034.30.70.970.991.070.106011.72.852.902.970.12Presentinvention684094094156517514519534.735.734.71.01.020.960.930.096211.52.752.812.860.11Presentinvention694054104127511511512135.836.036.20.40.921.060.940.146111.52.582.642.710.13Presentinvention704164124219520517523635.936.036.70.80.891.030.960.146211.42.842.912.960.12Presentinvention714174144217521515521633.934.934.71.01.001.120.980.146311.12.852.992.950.14Presentinvention724114064137515519523834.235.735.31.51.080.930.970.156311.22.732.853.020.29Presentinvention734234194278523521526535.336.034.61.40.941.001.100.166311.12.763.032.970.27Presentinvention743653603625479483480434.635.034.10.90.950.981.120.176011.72.782.923.040.26Presentinvention754104094167517514519534.635.734.21.51.070.970.910.166111.62.692.822.960.27Presentinvention7640540841510511512514335.436.136.61.20.921.110.950.196011.62.502.642.750.25Presentinvention7741741242311518517523635.436.136.71.30.891.070.950.186211.42.813.032.990.22Presentinvention7841841442410520515524933.434.934.51.51.001.170.980.196211.22.792.873.030.24Presentinvention7938538039010518515520528.024.828.23.41.180.921.250.33668.42.832.872.960.13Comparativeexample80385400394154895004941125.723.225.02.51.120.881.220.344912.22.842.882.990.15Comparativeexample814064104159519522526725.324.026.72.71.181.011.420.41669.12.923.032.950.11Comparativeexample8238439739213492500497835.834.335.61.51.180.931.320.395012.12.222.262.340.12Comparativeexample83405397389165005095111127.022.427.45.01.240.901.270.376311.12.852.972.920.12Comparativeexample84386398406204864965031733.431.934.82.90.811.160.930.356211.42.882.943.020.14Comparativeexample8541841242513521516524833.235.134.51.91.021.230.860.376111.52.732.752.870.14Comparativeexample8640239338814512509515635.734.934.31.41.240.951.250.305311.82.842.872.990.15Comparativeexample8740639539412514510517735.534.734.11.41.110.861.190.335212.02.863.012.920.15Comparativeexample8842141743114523518525733.334.834.31.51.001.260.920.346510.02.402.422.540.14Comparativeexample8937536336912482490486834.335.434.01.41.170.991.400.415611.82.892.983.040.15Comparativeexample90338350331195175285241134.532.433.62.11.130.831.290.425411.92.372.402.500.13Comparativeexample

[0112] As shown in Table 11, since the inventive steel sheets 65-78 have diameters and numbers of carbides within the range of the present invention, the HRc after quenching of these steel sheets is above 50 and the good hardenability is obtained. And the austenite grain size of these steel sheets is small, and therefore the excellent toughness is obtained. In addition, since the Δmax of yield strength and tensile strength is 15 MPa or lower, the Δmax of the total elongation is 1.5% or lower, and the Δmax of r-value is less than 0.2, the planar anisotropy of tensile properties is very small. The heating after rough rolling is effective not only for reducing the planar anisotropy of tensile properties but also for making the structure uniform in a thickness direction.

[0113] In contrast, the comparative steel sheets have a large Δmax of tensile properties, or poor hardenability or toughness. The steel sheet 82 of too high primary annealing temperature has a large Δmax of r-value of 0.27. The steel sheet 84 of too low cold reduction rate of 30% has a large Δmax of yield strength of 20 MPa, a large Δmax of tensile strength of 17 MPa, and a large Δmax of r-value of 0.39. The steel sheet 87 of too high secondary annealing temperature has a low HRc of 52 due to the insufficient dissolution of carbides. The steel sheet 88 of too low secondary annealing temperature has poor toughness due to the coarse austenite grain caused by the large number of carbides having a diameter of 0.6 μm or less. The conventional steel sheet 90 has a large Δmax of r-value of 0.46.

Advantages

[0114] As explained above, the present invention enables to provide a high carbon steel sheet having excellent hardenability and toughness, and low planar anisotropy of tensile properties affecting workability. As a result, the high carbon steel sheet of the present invention is applicable to parts as gears which need a high dimensional precision. Since the parts as gears can be formed and subjected to quenching and tempering, their production cost becomes much lower as compared with parts as gears produced conventionally by casting or forging.

BRIEF DESCRIPTION OF THE DRAWINGS

[0115]
FIG. 1 shows the effect of the diameter of carbide before quenching on the hardness when a steel sheet of S35C was heated at 820° C. for 10 sec and then quenched into oil;

[0116]
FIG. 2 shows the effect of the number of carbides having a diameter of 1.5 μm or larger on the austenite grain size when a steel sheet of S35C was heated at 820° C. for 10 sec and then quenched into oil;

[0117]
FIG. 3 shows the effect of primary annealing temperature and secondary annealing temperature on the Δmax of r-value and the carbide dispersion in the method 1 for producing the high carbon steel sheet of the third invention; and

[0118]
FIG. 4 shows the effect of primary annealing temperature and secondary annealing temperature on the Δmax of r-value and the carbide dispersion in the method 2 for producing the high carbon steel sheet of the third invention.

	Number	Date	Country
Parent	09961843	Sep 2001	US
Child	10665865	Sep 2003	US

	Number	Date	Country
Parent	PCT/JP01/00404	Jan 2001	US
Child	09961843	Sep 2001	US

High carbon steel sheet and production method thereof

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