WO2015146173A1 - High-carbon hot-rolled steel sheet and method for producing same - Google Patents

Abstract

The present invention provides a high-carbon hot-rolled steel sheet in which a steel having B added thereto is used as a raw material, said steel sheet exhibiting consistently excellent hardenability even when annealing is carried out in a nitrogen atmosphere, and exhibiting excellent workability having, prior to hardening treatment, a hardness of HRB 73 or less and a total elongation of 39% or more. Provided is a high-carbon hot-rolled steel sheet that: has a composition containing, by mass%, C: 0.20-0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol.Al: 0.10% or less, N: 0.0050% or less, and B: 0.0005-0.0050%, and further containing 0.002-0.030%, in total, of one or more of Sb, Sn, Bi, Ge, Te, and Se, wherein the proportion of the B content taken up by solid-solution B is 70% or more; has a microstructure comprising ferrite and cementite, wherein the cementite density within the ferrite grains is 0.08/µm² or less; and has a hardness of HRB 73 or less and a total elongation of 39% or more.

Description

High carbon hot rolled steel sheet and manufacturing method thereof

The present invention relates to a high carbon hot rolled steel sheet and a method for producing the same. In particular, the present invention relates to a high-carbon hot-rolled steel sheet to which B is added, which has a high effect of suppressing nitriding in the surface layer, is excellent in workability and hardenability, and a method for producing the same.

Currently, automotive parts such as gears, transmission parts, seat recliner parts, etc. are manufactured by processing hot-rolled steel sheets, which are carbon steel materials for machine structures specified in JIS G 4051, into desired shapes by cold working. In many cases, it is manufactured by quenching in order to ensure the desired hardness. For this reason, the hot-rolled steel sheet used as a raw material is required to have excellent cold workability and hardenability, and various steel sheets have been proposed so far.

For example, Patent Document 1 discloses that hardness is increased when induction hardening is performed by heating at an average heating rate of 100 ° C./second, holding at 1000 ° C. for 10 seconds, and rapidly cooling to room temperature at an average cooling rate of 200 ° C./second. Is a medium carbon steel sheet for cold working, having a mass ratio of 500 HV to 900 HV, C: 0.30 to 0.60%, Si: 0.06 to 0.30%, Mn: 0.3 -2.0%, P: 0.030% or less, S: 0.0075% or less, Al: 0.005-0.10%, N: 0.001-0.01%, Cr: 0.001- 0.10%, or Ni: 0.01 to 0.5%, Cu: 0.05 to 0.5%, Mo: 0.01 to 0.5%, Nb: 0.01 to 0.5%, Ti: 0.001 to 0.05%, V: 0.01 to 0.5%, Ta: 0.01 to 0.5%, B: 0 001-0.01%, W: 0.01-0.5%, Sn: 0.003-0.03%, Sb: 0.003-0.03%, As: 0.003-0.03% In which the average diameter d of the carbide is 0.6 μm or less, the spheroidization rate p of the carbide is 70% or more and less than 90%, the average diameter dμm of the carbide and the spheroidization rate p of the carbide Disclosed is a medium carbon steel sheet for cold work that satisfies d ≦ 0.04 × p-2.6, or a medium carbon steel sheet for cold work whose hardness before cold work is 120 HV or more and less than 170 HV. Has been. Patent Document 1 discloses, as a method for producing such a cold-working medium carbon steel sheet, a hot steel in which the steel having the above-described chemical composition is kept at 1050 to 1300 ° C. and then rolled at 750 to 1000 ° C. Rolled, then cooled to 500 to 700 ° C. at a cooling rate of 20 to 50 ° C./s, cooled to a predetermined temperature at a cooling rate of 5 to 30 ° C./s, wound up, and held under predetermined conditions Thereafter, annealing at a temperature of 600 ° C. or higher and Ac ₁ −10 ° C. or lower is disclosed.

Further, in Patent Document 2, by mass, C: 0.20% to 0.45%, Si: 0.05% to 0.8%, Mn: 0.5% to 2.0% P: 0.001% to 0.04%, S: 0.0001% to 0.006%, Al: 0.005% to 0.1%, Ti: 0.005% to 0.2 %: B: 0.001% or more and 0.01% or less and N: 0.0001% or more and 0.01% or less, or Cr: 0.05% or more and 0.35% or less, Ni: 0.01% to 1.0%, Cu: 0.05% to 0.5%, Mo: 0.01% to 1.0%, Nb: 0.01% to 0.5% Hereinafter, V: 0.01% to 0.5%, Ta: 0.01% to 0.5%, W: 0.01% to 0.5%, Sn: 0.00%. Contains one or more components of 03% to 0.03%, Sb: 0.003% to 0.03%, and As: 0.003% to 0.03%, from the surface layer A boron-added steel sheet having an average solid solution B concentration of 10 ppm or more in a region up to a depth of 100 μm is disclosed. Further, in Patent Document 2, when annealing is performed in an atmosphere mainly composed of nitrogen, a phenomenon called nitrogen absorption occurs, and B, which is an important element from the viewpoint of hardenability, binds to N in steel during annealing. Thus, it is disclosed that BN is formed and the solid solution B is reduced, and the effect of improving the hardenability by B cannot be ensured. In Patent Document 2, in order to ensure hardenability, the solid solution B in the region from the surface layer to a depth of 100 μm needs to be 10 ppm or more, and for that purpose, heating and annealing processes in the manufacturing process are required. It is disclosed that it is important to suppress the influence of the atmosphere. Patent Document 2 discloses, as a method for producing such a boron-added steel sheet, a steel having the above component composition is heated at 1200 ° C. or less, hot-rolled at a finish rolling temperature of 800 to 940 ° C., and then cooled. After cooling at 20 ° C./s to 650 ° C. or less, cooling at 20 ° C./s or less, winding at 400 to 650 ° C., pickling, and after dehydration at 95% or more of hydrogen and up to 400 ° C. It is disclosed that annealing is performed at a temperature of 660 ° C. or higher and Ac ₁ or lower in an atmosphere of −40 ° C. or lower with a dew point of 20 ° C. or lower and 400 ° C. or higher. Further, Patent Document 2 further describes cold rolling after the pickling, or cold rolling after the annealing, and further annealing at a temperature of Ac ₁ to Ac ₁ + 50 ° C. to Ac ₁ -30 ° C. Slow cooling and the like are disclosed.

Japanese Patent No. 5048168 Japanese Patent No. 4782243

In order to obtain good cold workability, the high carbon hot rolled steel sheet is required to have relatively low hardness and high elongation. For example, for automotive parts and the like that have been manufactured in a plurality of processes such as hot forging, cutting and welding in the past and integrally formed with a cold press, the hardness is 73 or less in Rockwell hardness HRB, The characteristic that the total elongation (El) is 39% or more is required. Moreover, excellent hardenability is desired for such a high carbon hot-rolled steel sheet having relatively low hardness and high elongation and good workability, for example, Vickers hardness of HV440 or higher after water quenching. Is desired.

In the technique of Patent Document 1, the average diameter of the carbide is set to 0.6 μm or less in order to ensure quench hardening ability in induction hardening at an average heating rate of 100 ° C./second, but the C content is 0.3 to 0.00. In steels containing a large amount of C, such as 6%, the average particle size of carbides is made as fine as 0.6 μm or less, so the density of carbides tends to increase, the strength tends to increase, and workability may be reduced. In addition, the manufacturing method includes two-stage cooling, such as cooling to 500 to 700 ° C. at a cooling rate of 20 to 50 ° C./s after hot rolling and then cooling to a cooling rate of 5 to 30 ° C./s. There is a problem that it is difficult to manage the cooling control because of the control.

Even in the technology of Patent Document 2, after the hot rolling, cooling is performed in two stages of cooling at a cooling rate of 20 ° C./s or more until 650 ° C. or less, and then cooling at 20 ° C./s or less. There was a problem that it was difficult to manage the cooling control. Furthermore, in the technique of Patent Document 2, 0.5% or more of Mn is added to improve the hardenability. Although Mn improves hardenability, it increases the strength of the hot-rolled steel sheet itself by solid solution strengthening and increases the hardness.

On the other hand, B is known as an element that improves hardenability by adding a small amount. However, as described in Patent Document 2, annealing is performed in an atmosphere mainly composed of nitrogen, which is generally used as an atmospheric gas. Then, there was a problem that the solid solution B decreased and the effect of improving the hardenability by B could not be obtained. In Patent Document 2, such a problem is solved by annealing in an atmosphere containing 95% or more of hydrogen or an atmosphere in which the hydrogen is replaced with an inert gas such as Ar. The cost of heat treatment using is increased. Moreover, it is unclear whether this technique alone can suppress nitrogen absorption by annealing in a nitrogen atmosphere.

In order to solve the above-mentioned problems, the present invention uses a steel with a Mn content lower than that of conventional steel and added with B, and even if annealing is performed in a nitrogen atmosphere, excellent quenching can be achieved. It is an object of the present invention to provide a high carbon hot-rolled steel sheet having excellent workability such that the hardness is 73 or less in HRB and the total elongation is 39% or more before quenching, and a method for producing the same. .

The inventors of the present invention made an intensive study on the relationship between the manufacturing conditions, workability, and hardenability of high carbon hot-rolled steel sheets containing B, with a Mn content of 0.50% or less and a lower Mn content than conventional steel. As a result, the following knowledge was obtained.
i) The hardness and total elongation (hereinafter also simply referred to as elongation) of the high carbon hot-rolled steel sheet before quenching are greatly affected by the cementite density in the ferrite grains, with a hardness of 73 or less in HRB and 39% or more. In order to obtain the total elongation (El), the cementite density in the ferrite grains needs to be 0.08 pieces / μm ² or less.
ii) The cementite density in the ferrite grains is greatly affected by the finish rolling temperature in the hot rolling finish rolling and the cooling rate from 700 to 700 ° C. after the finish rolling. If the finish rolling temperature is too high or the cooling rate is too low, in the steel sheet after hot rolling, a steel sheet having a structure with pearlite and a predetermined proeutectoid ferrite volume fraction cannot be obtained, and spheroidizing annealing is performed. It becomes difficult to reduce the cementite density later.
iii) By adding at least one of Sb, Sn, Bi, Ge, Te, and Se to the steel, even when annealing is performed in a nitrogen atmosphere, nitriding is prevented and a decrease in the amount of dissolved B is suppressed. High hardenability.

The present invention has been made on the basis of such knowledge, and the gist thereof is as follows.

[1] By mass%, C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005 to 0.0050%, and one or more of Sb, Sn, Bi, Ge, Te, Se in total 0.002 to 0.030% content, the balance is composed of Fe and inevitable impurities, the proportion of the solid solution B content in the B content is 70% or more, and consists of ferrite and cementite, A high carbon hot-rolled steel sheet having a microstructure with a cementite density of 0.08 pieces / μm ² or less in ferrite grains, a hardness of 73 or less in HRB, and a total elongation of 39% or more.

[2] The high-carbon hot-rolled steel sheet according to [1], further containing, by mass%, one or more of Ni, Cr, and Mo in a total amount of 0.50% or less.

[3] The above [1] or [2] wherein the average diameter of all cementite in the structure composed of ferrite and cementite is 0.60 μm or more and 1.00 μm or less, and the average diameter of cementite in the ferrite grains is 0.40 μm or more. ] The high carbon hot-rolled steel sheet described in any one of the above.

[4] By mass%, C: 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005 to 0.0050%, and one or more of Sb, Sn, Bi, Ge, Te, Se in total A steel containing 0.002 to 0.030% and the balance being composed of Fe and inevitable impurities, after hot rough rolling, finish rolling at finish rolling temperature: Ar ₃ transformation point or more and 870 ° C. or less, 700 Is cooled at an average cooling rate of 25 ° C./s or more and 150 ° C./s or less, and coiled at a winding temperature of 500 ° C. or more and 700 ° C. or less, so that pearlite and pro-eutectoid ferrite with a volume ratio of 5% or more can be obtained. A method for producing a high carbon hot-rolled steel sheet, in which the steel sheet is annealed below the Ac ₁ transformation point.

[5] The method for producing a high carbon hot-rolled steel sheet according to the above [4], wherein the steel further contains, in mass%, one or more of Ni, Cr, and Mo in a total amount of 0.50% or less.

According to the present invention, a high carbon hot rolled steel sheet having excellent hardenability and workability can be produced. The high carbon hot-rolled steel sheet of the present invention is suitable for automotive parts such as gears, transmission parts, and seat recliner parts, which require cold workability for the raw steel sheet.

Hereinafter, the high carbon hot-rolled steel sheet and the manufacturing method thereof according to the present invention will be described in detail. Note that “%”, which is a unit of component content, means “% by mass” unless otherwise specified.

1) Composition C: 0.20 to 0.40%
C is an important element for obtaining strength after quenching. When the C content is less than 0.20%, HV440 or more cannot be obtained with the desired hardness, specifically the hardness after water quenching, by heat treatment after forming into a part. For this reason, C content needs to be 0.20% or more. On the other hand, if the C content exceeds 0.40%, the steel sheet becomes hard and cold workability deteriorates. Therefore, the C content is set to 0.40% or less. In order to obtain high quenching hardness, the C content is preferably 0.26% or more. By setting the C content to 0.32% or more, HV440 or more can be stably obtained with water quenching hardness, which is more preferable.

Si: 0.10% or less Si is an element that increases the strength by solid solution strengthening. As the Si content increases, it hardens and the cold workability deteriorates, so the Si content is made 0.10% or less. Preferably, the Si content is 0.05% or less, more preferably 0.03% or less. Since Si lowers the cold workability, the lower the Si content, the better. However, since excessively reducing Si increases the refining cost, the Si content is preferably 0.005% or more.

Mn: 0.50% or less Although Mn is an element that improves hardenability, it is also an element that increases strength by solid solution strengthening. If the Mn content exceeds 0.50%, the steel sheet becomes too hard and cold workability is lowered. In addition, since a band structure due to segregation of Mn develops and the structure becomes non-uniform, variations in hardness and elongation tend to increase. Therefore, the Mn content is 0.50% or less. Preferably, the Mn content is 0.45% or less, more preferably 0.40% or less. Although the lower limit is not specified, in order to suppress the precipitation of graphite and obtain a predetermined quenching hardness by solid solution of all C in the steel sheet during quenching heating, the Mn content is 0.20% or more. It is preferable that

P: 0.03% or less P is an element that increases the strength by solid solution strengthening. If the P content exceeds 0.03%, the steel sheet becomes too hard and cold workability is reduced. Moreover, since the strength of the grain boundary is lowered, the toughness after quenching deteriorates. Therefore, the P content is 0.03% or less. In order to obtain excellent toughness after quenching, the P content is preferably 0.02% or less. P decreases the cold workability and toughness after quenching, so the lower the P content, the better. However, if P is reduced more than necessary, the refining cost increases, so the P content is 0.005% or more. preferable.

S: 0.010% or less S is an element that has to be reduced in order to form sulfides and to reduce the cold workability of the high carbon hot-rolled steel sheet and the toughness after quenching. If the S content exceeds 0.010%, the cold workability and the toughness after quenching of the high carbon hot-rolled steel sheet are significantly deteriorated. Therefore, the S content is set to 0.010% or less. In order to obtain excellent cold workability and toughness after quenching, the S content is preferably 0.005% or less. Since S decreases cold workability and toughness after quenching, the S content is preferably as low as possible. However, if S is reduced more than necessary, the refining cost increases, so the S content is 0.0005% or more. preferable.

sol. Al: 0.10% or less sol. When the Al content exceeds 0.10%, AlN is generated during the heating of the quenching process, the austenite grains are excessively refined, the generation of the ferrite phase is promoted during cooling, the structure becomes ferrite and martensite, and after the quenching Hardness decreases. Therefore, sol. Al content shall be 0.10% or less. Preferably, sol. Al content is 0.06% or less. Note that Al has a deoxidizing effect, and in order to sufficiently deoxidize, sol. The Al content is preferably 0.005% or more.

N: 0.0050% or less When the N content exceeds 0.0050%, BN is formed more than necessary, so that the amount of dissolved B decreases. In addition, the formation of BN and AlN more than necessary causes the austenite grains to become too fine during heating in the quenching process, and the formation of a ferrite phase is promoted during cooling, so the hardness after quenching decreases. Therefore, the N content is 0.0050% or less. Preferably, the N content is 0.0045% or less. The lower limit is not particularly defined, but N forms BN and AlN as described above. If appropriate amounts of BN and AlN are formed, these nitrides moderately suppress austenite grain coarsening during heating in the quenching process and improve toughness after quenching, so the N content is 0.0005% or more. Is preferred.

B: 0.0005 to 0.0050%
B is an important element that enhances hardenability. Under the condition of the cooling rate after finish rolling in the hot rolling of the present invention, when the B content is less than 0.0005%, the amount of solid solution B that delays the ferrite transformation is insufficient, so that sufficient hardenability is achieved. Improvement effect cannot be obtained. Therefore, the B content needs to be 0.0005% or more, and is preferably 0.0010% or more. On the other hand, when the B content exceeds 0.0050%, the recrystallization of austenite after finish rolling is delayed. As a result, the rolling texture of the hot-rolled steel sheet develops and the in-plane anisotropy of the mechanical property value of the steel sheet after annealing increases. As a result, ears are liable to occur in the drawing, and the roundness is lowered, which tends to cause problems during molding. For this reason, it is necessary to make B content 0.0050% or less. From the viewpoint of improving hardenability and reducing anisotropy, the B content is preferably 0.0035% or less. Therefore, the B content is set to 0.0005 to 0.0050%. More preferably, the B content is 0.0010 to 0.0035%.

In the present invention, in addition to the above-described optimization of the B content, it is important to control the amount of the solid solution B that contributes to improving the hardenability. When B contained in the steel plate is in a solid solution state of 70% or more, that is, when the ratio of the solid solution B content in the total B content (B content) in the steel plate is 70% or more, The excellent hardenability intended by the present invention is obtained. Therefore, the ratio of the solid solution B amount in the B content is set to 70% or more. Preferably, the ratio of the solid solution B content to the B content is 75% or more. In addition, the ratio of the solid solution B amount in the B content means {(solid solution B amount (mass%)) / (total B content (mass%))} × 100 (%).

0.002 to 0.030% in total of one or more of Sb, Sn, Bi, Ge, Te, Se
Sb, Sn, Bi, Ge, Te, Se are all elements that have an effect of suppressing nitriding from the steel sheet surface. In the present invention, one of Sb, Sn, Bi, Ge, Te, Se is used. It is necessary to contain the above. Further, when the total content of these elements is less than 0.002%, a sufficient nitriding suppression effect is not recognized. For this reason, one or more of Sb, Sn, Bi, Ge, Te and Se are contained in a total amount of 0.002% or more. Preferably, the total content of Sb, Sn, Bi, Ge, Te, Se is 0.005% or more. On the other hand, even if the content of these elements exceeds 0.030% in total, the nitriding suppression effect is saturated. Moreover, since these elements tend to segregate at the grain boundaries, if the total content of these elements exceeds 0.030%, grain boundary embrittlement may occur. For this reason, in the present invention, one or more of Sb, Sn, Bi, Ge, Te, and Se are contained in a total amount of 0.030% or less. The contents of Sb, Sn, Bi, Ge, Te and Se are preferably 0.020% or less in total.

As described above, the N content is 0.0050% or less, and at least one of Sb, Sn, Bi, Ge, Te, and Se is contained by 0.002 to 0.030% in total. Even when annealed in a nitrogen atmosphere, the nitriding from the steel sheet surface is suppressed, the increase in the nitrogen concentration in the steel sheet surface layer is suppressed, and the average amount of nitrogen contained in the range of 150 μm depth from the steel sheet surface in the sheet thickness direction, The difference from the average amount of nitrogen contained in the entire steel sheet can be 30 ppm by mass or less. Moreover, since nitriding can be suppressed in this way, even in the case of annealing in a nitrogen atmosphere, the ratio of the solid solution B content in the B content in the steel sheet after annealing can be 70% or more. .

When the difference between the average nitrogen content contained in the range of 150 μm depth from the steel sheet surface in the thickness direction and the average nitrogen content contained in the whole steel sheet exceeds 30 mass ppm, it is formed in the steel sheet surface layer portion. The difference between the amount of BN and AlN and the amount of BN and AlN formed near the center of the steel plate thickness increases. In that case, troubles such as the inability to obtain uniform hardness after quenching occur. Therefore, it is necessary to suppress the difference between the average nitrogen content contained in the range of 150 μm depth from the steel plate surface to the plate thickness direction and the average nitrogen content contained in the whole steel plate to 30 mass ppm or less.

The remainder other than the above is Fe and inevitable impurities, but in order to further improve the hardenability, one or more of Ni, Cr, and Mo may be contained. In order to obtain such an effect, it is preferable that at least one of Ni, Cr, and Mo is contained and the total content is 0.01% or more. On the other hand, since these elements are expensive, when one or more of Ni, Cr, and Mo are contained, the total content needs to be 0.50% or less. Preferably, the content of these elements is 0.20% or less in total.

2) Microstructure In the present invention, in order to improve cold workability, it is necessary to perform annealing (spheroidizing annealing) to spheroidize cementite after hot rolling to obtain a microstructure composed of ferrite and cementite. Note that spheroidization refers to a state in which cementite having an aspect ratio (major axis / minor axis) ≦ 3 occupies 90% or more by volume with respect to the total cementite. In particular, in order that the Rockwell hardness is 73 or less in HRB and the total elongation is 39% or more, the cementite density in the ferrite grains needs to be 0.08 pieces / μm ² or less. Hereinafter, the cementite density is also referred to as the number density of cementite grains.

Number density of cementite grains in ferrite grains: 0.08 / μm ² or less The steel sheet of the present invention comprises ferrite and cementite. If the number density of cementite grains in the ferrite grains is high, it becomes hard due to dispersion strengthening and the elongation decreases. In order to set the hardness to a predetermined value or less and the elongation to a predetermined value or more, the number density of cementite grains in the ferrite grains needs to be set to 0.08 particles / μm ² or less. The number density of cementite grains in the ferrite grains is preferably 0.07 / μm ² or less, and more preferably 0.06 / μm ² or less. The cementite diameter present in the ferrite grains is about 0.15 to 1.8 μm in the major axis, and is a size that has a slight effect on the precipitation strengthening of the steel sheet. Therefore, by reducing the number density of the cementite grains in the ferrite grains The strength can be reduced. Since cementite at the ferrite grain boundary hardly contributes to dispersion strengthening, the number density of cementite grains in the ferrite grain is defined as 0.08 / μm ² or less. In addition to the above ferrite and cementite, even if the remaining structure such as pearlite is inevitably generated, the effect of the present invention is not impaired as long as the total volume ratio of the remaining structure is about 5% or less. , It may be contained.

Average diameter of all cementite: 0.60 μm or more and 1.00 μm or less and average diameter of cementite in ferrite grains: 0.40 μm or more Steel sheets whose average diameter of cementite in ferrite grains is less than 0.40 μm are cementite in ferrite grains Since the number density of grains increases, the hardness of the steel sheet after annealing may increase. In order to make the hardness below a desired value, the average diameter of cementite in the ferrite grains is preferably 0.40 μm or more. More preferably, the average diameter of cementite in the ferrite grains is 0.45 μm or more.

The cementite at the ferrite grain boundary tends to be coarser than the cementite in the ferrite grain, and in order to make the average diameter of the cementite in the ferrite grain 0.40 μm or more, the average diameter of the entire cementite is 0.60 μm or more. There is a need. Preferably, the average diameter of all cementite is 0.65 μm or more. On the other hand, if the average diameter of all cementite exceeds 1.00 μm, the cementite may not be melted during heating in a short time such as induction hardening, and the hardness may not be less than the desired value. The average diameter of all cementite is preferably 1.00 μm or less. More preferably, the average diameter of all cementite is 0.95 μm or less. The average diameter of the cementite can be determined by observing the microstructure by SEM, measuring the long diameter and short diameter of the cementite grains, and measuring the average diameter of all cementite and the average diameter of cementite in the ferrite grains.

If the ferrite grain size becomes too coarse, the hardness decreases, but the improvement in elongation may be saturated. Therefore, the average grain size of ferrite in the structure composed of ferrite and cementite should be 12 μm or less. Is preferable, and 9 μm or less is more preferable. On the other hand, if the average particle diameter of ferrite is less than 6 μm, the steel sheet may be hardened, so the average particle diameter of ferrite is preferably 6 μm or more. The particle size of the ferrite can be measured by observing the microstructure with an SEM.

3) Mechanical properties The steel sheet of the present invention requires excellent workability in order to form automotive parts such as gears, transmission parts, and sheet recliner parts by cold pressing. In addition, it is necessary to increase the hardness by quenching to impart wear resistance to the parts. For that purpose, in addition to improving hardenability, it is necessary to reduce the hardness of the steel sheet to HRB 73 or less and increase the elongation to make the total elongation (El) 39% or more. The lower the hardness of the steel sheet, the better from the viewpoint of workability, but there are parts that are partially quenched, and the strength of the original sheet may affect the fatigue characteristics. For this reason, as for the hardness of a steel plate, HRB60 excess is preferable. In addition, said HRB can be measured using a Rockwell hardness meter (B scale). The total elongation is 10 mm / min with a tensile tester of Shimadzu AG10TB AG / XR using a JIS No. 5 tensile test piece cut in a direction of 0 ° (L direction) with respect to the rolling direction. It can be done by measuring the broken sample.

4) Manufacturing conditions The high carbon hot-rolled steel sheet of the present invention is made of steel having the above composition, and is desired by hot rolling after hot rough rolling and finish rolling at a finish rolling temperature of Ar ₃ transformation point or higher and 870 ° C. or lower. After finishing rolling, the steel sheet is cooled to 700 ° C. at an average cooling rate of 25 ° C./s to 150 ° C./s, and the winding temperature is 500 ° C. to 700 ° C. A steel sheet having 5% or more pro-eutectoid ferrite is produced, and then subjected to spheroidizing annealing at an Ac ₁ transformation point or less. In addition, it is preferable that the rolling reduction in finish rolling shall be 85% or more.

Hereinafter, the reasons for limitation in the method for producing a high carbon hot-rolled steel sheet of the present invention will be described.

Final rolling temperature: Ar ₃ transformation point or more and 870 ° C. or less In order to reduce the number density of cementite grains in the ferrite grains after annealing to 0.08 particles / μm ² or less, pearlite and pro-eutectoid ferrite with a volume ratio of 5% or more are used. It is necessary to spheroidize the hot-rolled steel sheet having a microstructure. In hot rolling in which finish rolling is performed after hot rough rolling, when the finish rolling temperature is higher than 870 ° C., the proportion of pro-eutectoid ferrite decreases, and the number density of predetermined cementite grains cannot be obtained after spheroidizing annealing. . Moreover, the cementite particle diameter and the ferrite particle diameter after spheroidizing annealing are likely to be coarsened. For this reason, finish rolling temperature shall be 870 degrees C or less. In order to sufficiently increase the proportion of pro-eutectoid ferrite, the finish rolling temperature is preferably 850 ° C. or lower. On the other hand, if the finish rolling temperature is less than the Ar ₃ transformation point, coarse ferrite grains are formed after hot rolling and after annealing, and the elongation is significantly reduced. Therefore, the finish rolling temperature and Ar ₃ transformation point or more. Preferably, the finish rolling temperature is 820 ° C or higher. The finish rolling temperature is the surface temperature of the steel sheet.

Average cooling rate from the finish rolling temperature to 700 ° C .: 25 ° C./s or more and 150 ° C./s or less To make the number density of cementite grains in the ferrite grains after annealing 0.08 pieces / μm ² or less, pearlite, It is necessary to spheroidize the hot-rolled steel sheet having a microstructure having pro-eutectoid ferrite of 5% or more by volume ratio. Since the temperature range from finish rolling to 700 ° C. in hot rolling corresponds to the temperature range where ferrite and pearlite transformation start temperatures exist, the proeutectoid ferrite fraction in the steel sheet after hot rolling is 5% or more by volume. In order to achieve this, the cooling rate from the finish rolling temperature to 700 ° C. is an important factor. If the average cooling rate in the temperature range from finish rolling to 700 ° C. is less than 25 ° C./s, the ferrite transformation hardly progresses in a short time, and the paride fraction becomes higher than necessary. The fraction of precipitated ferrite cannot be obtained. In addition, the formation of coarse pearlite makes it difficult to obtain a desired steel sheet structure after spheroidizing annealing. Therefore, the average cooling rate in the temperature range from finish rolling to 700 ° C. is set to 25 ° C./s or more. Further, in order to obtain the number density of cementite grains in the ferrite grains after spheroidizing annealing of 0.07 / μm ² or less, it is preferable to set the pro-eutectoid ferrite fraction to 10% or more by volume. The average cooling rate is preferably 30 ° C./s or more. More preferably, the average cooling rate is 40 ° C./s or more. On the other hand, when the average cooling rate exceeds 150 ° C./s, it is difficult to obtain pro-eutectoid ferrite. Therefore, the average cooling rate from 700 ° C. after finish rolling is set to 150 ° C./s or less. Preferably, the average cooling rate is 120 ° C./s or less. More preferably, the average cooling rate is 100 ° C./s or less. The temperature is the surface temperature of the steel sheet.

Winding temperature: 500 ° C. or higher and 700 ° C. or lower The steel sheet after finish rolling is wound into a coil shape at a winding temperature of 500 ° C. or higher and 700 ° C. or lower after cooling as described above. When the coiling temperature exceeds 700 ° C., the structure of the hot-rolled steel sheet becomes coarse and a desired steel sheet structure cannot be obtained after annealing, and the strength of the steel sheet becomes too low to be coiled into a coil shape. It may be deformed by its own weight, which is not preferable for operation. Therefore, the coiling temperature is 700 ° C. or less. Preferably, the coiling temperature is 650 ° C. or lower. On the other hand, when the coiling temperature is less than 500 ° C., the steel sheet structure becomes fine, the steel sheet becomes hard, the elongation becomes small, and the workability decreases. Therefore, the coiling temperature is 500 ° C. or higher. Preferably, the winding temperature is 550 ° C. or higher. The winding temperature is the surface temperature of the steel plate.

Microstructure of steel sheet after hot rolling: Structure having pearlite and pro-eutectoid ferrite of 5% or more in volume ratio In the present invention, after spheroidizing annealing described later, it consists of ferrite and cementite, and the cementite grains in the ferrite grains A steel sheet having a microstructure with a number density of 0.08 pieces / μm ² or less is obtained. The microstructure after spheroidizing annealing is greatly affected by the microstructure of the steel sheet after hot rolling. By making the microstructure of the steel sheet after hot rolling into a structure having pearlite and 5% or more pro-eutectoid ferrite in volume ratio, it can be made a desired structure after spheroidizing annealing, Become. In addition, in a steel sheet that does not have pearlite or has a pro-eutectoid ferrite fraction of less than 5% by volume, a predetermined number density of cementite grains can be obtained after spheroidizing annealing below the Ac ₁ transformation point. Therefore, the steel sheet strength is increased. Therefore, the microstructure of the steel sheet (hot-rolled steel sheet) obtained by hot rolling, cooling and winding under the above-described conditions is a structure having pearlite and a pro-eutectoid ferrite of 5% or more by volume ratio. Preferably, the structure is composed of pearlite and pro-eutectoid ferrite with a volume ratio of 10% or more. In order to obtain a more uniform structure after annealing, the fraction of pro-eutectoid ferrite is preferably 50% or less by volume.

Annealing temperature: Ac ₁ transformation point or less Annealing (spheroidizing annealing) is performed on the hot-rolled steel sheet obtained as described above. When the annealing temperature exceeds the Ac ₁ transformation point, austenite precipitates, and a coarse pearlite structure is formed in the cooling process after annealing, resulting in a non-uniform structure. Therefore, the annealing temperature is less Ac ₁ transformation point. Although the lower limit is not particularly defined, the annealing temperature is preferably 600 ° C. or higher, more preferably 700 ° C. or higher, in order to obtain the desired number density of cementite grains in the ferrite grains. As the atmospheric gas, any of nitrogen, hydrogen, and a mixed gas of nitrogen and hydrogen can be used, and these gases are preferably used, but Ar may be used and is not particularly limited. The annealing time is preferably 0.5 to 40 hours. By setting the annealing time to 0.5 hours or more, the target structure can be stably obtained, the hardness of the steel sheet can be set to a predetermined value or less, and the elongation can be set to a predetermined value or more. The annealing time is preferably 0.5 hours or longer. More preferably, it is 8 hours or more. Further, if the annealing time exceeds 40 hours, the productivity is lowered and the manufacturing cost is likely to be excessive. Therefore, the annealing time is preferably 40 hours or less. The annealing temperature is the surface temperature of the steel sheet. The annealing time is a time during which a predetermined temperature is maintained.

In addition, in order to melt the high carbon steel of the present invention, both a converter and an electric furnace can be used. Further, the high carbon steel thus melted is made into a slab by ingot-bundling rolling or continuous casting. The slab is usually heated and then hot rolled. In addition, in the case of the slab manufactured by continuous casting, you may apply the direct feed rolling which heats as it is or keeps heat in order to suppress a temperature fall. Moreover, when heating and rolling a slab, it is preferable to make slab heating temperature 1280 degrees C or less in order to avoid the deterioration of the surface state by a scale. In hot rolling, since finish rolling is performed at a predetermined temperature, the material to be rolled may be heated by a heating means such as a sheet bar heater during hot rolling.

Steels having chemical composition compositions of steel numbers A to H shown in Table 1 were melted and then cooled under the hot rolling conditions shown in Table 2 and then cooled, wound, and made into hot rolled steel sheets. In addition, the cooling rate shown in Table 2 is an average cooling rate from after finish rolling to 700 ° C. Next, pickling and annealing (spheroidizing annealing) in a nitrogen atmosphere (atmosphere gas: nitrogen) under the annealing conditions shown in Table 2, a hot rolled steel sheet having a thickness of 4.0 mm and a width of 1000 mm (heat (Annealed sheet) was manufactured. The hot rolled annealed sheet thus manufactured was examined for hardness, elongation, and microstructure. Moreover, the microstructure of the hot rolled steel sheet before annealing was also investigated. The results are shown in Table 2. The Ar ₃ transformation point and the Ac ₁ transformation point shown in Table 1 were obtained by Formaster.

Hot rolled annealed sheet hardness (HRB)
A sample was taken from the center of the plate width of the steel sheet after annealing, and measured at five points using a Rockwell hardness meter (B scale) to obtain an average value.

Total elongation of hot-rolled annealed sheet (El)
Using a JIS No. 5 tensile test piece cut from the annealed steel sheet in a direction of 0 ° (L direction) with respect to the rolling direction, a tensile test was performed at 10 mm / min with a Shimadzu AG10TB AG / XR tensile tester. The fractured samples were butted together to determine the elongation (total elongation).

Microstructure The microstructure of the hot-rolled steel sheet before annealing (the microstructure of the hot-rolled sheet) was observed by SEM, and the type of the structure and the fraction of proeutectoid ferrite were determined. The fraction of pro-eutectoid ferrite was divided into locations other than the ferrite region and the ferrite region, and the area ratio was determined by determining the proportion of the ferrite region, and this value was defined as the volume fraction of the pro-eutectoid ferrite. In addition, in the hot-rolled steel sheet before annealing shown in Table 2, the presence of pearlite is confirmed by the above SEM observation.

The microstructure of the steel sheet after annealing (the microstructure of the hot-rolled annealed sheet) is obtained by cutting and polishing a sample taken from the center part of the sheet width, applying nital corrosion, and using a scanning electron microscope, 1/4 of the sheet thickness. Using structure photographs taken at a magnification of 3000 at five locations, the type of the structure was observed, and the number of cementites that were not on the grain boundaries and whose major axis was 0.15 μm or more was measured. Dividing by the area of the field of view of the photograph, the cementite density in the ferrite grains (number density of cementite grains) was determined. The cementite diameter was determined by measuring the long diameter and short diameter of each cementite grain using the above structure photograph, and determining the average diameter of all cementite and cementite within the grain. For the ferrite grain size, the average grain size was calculated by obtaining the grain size using the above structure photograph.

In addition, regarding the steel sheet after annealing (hot rolled annealed sheet), the difference between the N content of the surface layer of 150 μm and the average N content in the steel sheet and the ratio of the solid solution B content in the B content were determined as follows. The results are shown in Table 2.

The difference between the average N amount of the surface layer 150 μm and the average N amount in the steel sheet Using a sample taken from the center of the sheet width of the steel sheet after annealing, the average N amount of the surface layer 150 μm and the average N amount in the steel sheet were measured, and the surface layer 150 μm The difference between the average N amount and the average N amount in the steel sheet was determined. Here, the average N amount of the surface layer of 150 μm is the average N amount contained in the range from the steel plate surface to the depth of 150 μm in the plate thickness direction. The average amount of N in the surface layer of 150 μm was determined as follows. That is, the cutting was started from the surface of the collected steel sheet, the steel sheet was cut to a depth of 150 μm from the surface, and the cut piece generated at this time was collected as a sample. The amount of N in this sample was measured and used as the average amount of N on the surface layer of 150 μm. The average N amount of the surface layer of 150 μm and the average N amount in the steel sheet were determined by measurement by an inert gas melting-thermal conductivity method. The difference between the average N content of the surface layer 150 μm obtained in this way (the N content in the range of 150 μm depth from the surface to the surface) and the average N content in the steel sheet (N content in the steel) is 30 ppm by mass or less. If it is, it can be evaluated that the nitrification can be suppressed.

The ratio of the amount of solute B in the B content A sample was taken from the center of the plate width of the steel plate after annealing. BN in the steel was extracted with 10% by volume Br methanol, and the B content precipitated as BN was subtracted from the total B content in the steel to obtain the solid solution B amount. The ratio of the solute B amount to the total B content (B content) contained in the steel is {(solid solution B amount (mass%)) / (total B content (mass%))} × It calculated | required by 100 (%). If this ratio is 70 (%) or more, it can be evaluated that the fall of the amount of solute B can be suppressed.

Steel plate hardness after quenching (quenching hardness)
Moreover, the steel plate after annealing was made into the original plate, the three types of quenching processes were performed as follows, the steel plate hardness (quenching hardness) after quenching was investigated, and hardenability was evaluated. The results are shown in Table 2.

A flat plate test piece (width 15 mm × length 40 mm × plate thickness 4 mm) is taken from the center of the plate width of the steel plate (original plate) after annealing, and is immediately cooled with water by holding the plate test piece at 870 ° C. for 30 s. Quenching was performed by a method (water cooling), a method of holding at 870 ° C. for 30 s and immediately cooling with 120 ° C. oil (120 ° C. oil cooling). For the quenching characteristics, the hardness of the cut surface of the test piece after the quenching treatment was measured with a Vickers hardness tester under the condition of a load of 1 kgf to obtain an average hardness, which was defined as the quenching hardness.

Further, a disk test piece (55 mmφ × plate thickness 4 mm) is taken from the center of the width of the steel plate (original plate) after annealing, and induction-quenched (heated at a heating rate of 200 ° C./s, water-cooled after reaching 1000 ° C.) Also quenching treatment was performed. At this time, the hardness of the cut surface of the test piece at the outermost peripheral portion of the test piece was measured at two points with a Vickers hardness tester under a load of 0.2 kgf to obtain the average hardness, and this was set as quenching hardness.
The quenching hardness which was kept at 870 ° C. for 30 s and was water-cooled and 120 ° C. oil-cooled satisfied both the water-cooled hardness and the 120 ° C. oil-cooled hardness in the conditions of Table 3 and was induction-hardened. When the hardness satisfied the induction hardening hardness shown in Table 3, it was determined to be acceptable (◯) and evaluated as having excellent hardenability. Also, if any of the hardness after holding at 870 ° C. for 30 s after water cooling, oil cooling at 120 ° C., and hardness after induction quenching water cooling does not satisfy the conditions shown in Table 3, it will be rejected (×) and hardenability It was evaluated as inferior. In addition, Table 3 represents the quenching hardness according to the C content that can be evaluated as having sufficient quenchability from experience.

From Table 2, the hot-rolled steel sheet of the present invention example has a microstructure composed of ferrite and cementite, and the cementite density in the ferrite grains is 0.08 pieces / μm ² or less, and the hardness is 73 or less in HRB. It can be seen that since the total elongation is 39% or more, the cold workability is excellent and the hardenability is also excellent.

 
 

Claims (5)

1. C. 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005 to 0.0050%, and one or more of Sb, Sn, Bi, Ge, Te, Se in total 0.002 to 0.030% content, the balance is composed of Fe and inevitable impurities, the proportion of the solid solution B content in the B content is 70% or more, and consists of ferrite and cementite, A high carbon hot-rolled steel sheet having a microstructure with a cementite density of 0.08 pieces / μm ² or less in ferrite grains, a hardness of 73 or less in HRB, and a total elongation of 39% or more.
2. Furthermore, the high carbon hot-rolled steel sheet according to claim 1, further comprising 0.50% or less in total of one or more of Ni, Cr, and Mo in mass%.
3. 3. The high carbon according to claim 1, wherein an average diameter of all cementites in the structure composed of ferrite and cementite is 0.60 μm or more and 1.00 μm or less, and an average diameter of cementite in the ferrite grains is 0.40 μm or more. Hot rolled steel sheet.
4. C. 0.20 to 0.40%, Si: 0.10% or less, Mn: 0.50% or less, P: 0.03% or less, S: 0.010% or less, sol. Al: 0.10% or less, N: 0.0050% or less, B: 0.0005 to 0.0050%, and one or more of Sb, Sn, Bi, Ge, Te, Se in total A steel containing 0.002 to 0.030% and the balance being composed of Fe and inevitable impurities, after hot rough rolling, finish rolling at finish rolling temperature: Ar ₃ transformation point or more and 870 ° C. or less, 700 Is cooled at an average cooling rate of 25 ° C./s or more and 150 ° C./s or less, and coiled at a winding temperature of 500 ° C. or more and 700 ° C. or less, so that pearlite and pro-eutectoid ferrite with a volume ratio of 5% or more can be obtained. A method for producing a high carbon hot-rolled steel sheet, in which the steel sheet is annealed below the Ac ₁ transformation point.
5. The method for producing a high carbon hot-rolled steel sheet according to claim 4, wherein the steel further contains, by mass%, one or more of Ni, Cr, and Mo in a total amount of 0.50% or less.