WO2011089845A1

WO2011089845A1 - Method for producing hot-rolled high carbon steel sheet

Info

Publication number: WO2011089845A1
Application number: PCT/JP2010/073881
Authority: WO
Inventors: 小林崇; 中村展之; 妻鹿哲也
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2010-01-22
Filing date: 2010-12-24
Publication date: 2011-07-28
Also published as: CN102712963A; JP5440203B2; JP2011149062A; KR20120112788A; CN102712963B; KR101390612B1

Abstract

Disclosed is a method for producing a hot-rolled high carbon steel sheet that contains not less than 0.5% by mass of C and has excellent workability and hardenability, while being reduced in variations of the characteristics in the steel sheet. Specifically disclosed is a method for producing a hot-rolled high carbon steel sheet, which is characterized by the following procedure: a steel bar having a composition that contains, in mass%, 0.5-1.0% of C, not more than 2.0% of Si, not more than 2.0% of Mn, not more than 0.03% of P, not more than 0.03% of S, not more than 0.08% of sol. Al and not more than 0.01% of N, with the balance made up of Fe and unavoidable impurities is hot rolled at a finishing temperature that is not less than the Ar₃ transformation point or the Ar_cm transformation point; then after primary cooling to a cooling stop temperature of 550-650˚C at an average cooling rate of not less than 60˚C/s, the steel sheet is allowed to cool for 1.0-10 s; then the steel sheet is subjected to secondary cooling to a cooling stop temperature of 500-600˚C at an average cooling rate of not less than 120˚C/s and then wound up; and following that, the steel sheet is annealed at a temperature not less than 640˚C but not more than the Ac₁ transformation point.

Description

Manufacturing method of high carbon hot rolled steel sheet

The present invention relates to a method for producing a high-carbon hot-rolled steel sheet, particularly high-carbon hot-rolled steel sheet that contains 0.5 mass% or more of C, has little variation in characteristics in the steel sheet, and is excellent in workability and hardenability. .

High carbon steel plates used for machine structural parts and tools are often subjected to quenching and tempering treatments for hardening after being cold-formed into various shapes. Therefore, high workability and hardenability are required for such high carbon steel sheets, and various techniques have been proposed so far.

For example, in Patent Document 1, in mass%, C: 0.30 to 1.20%, Si: 2.00% or less, Mn: 1.00% or less, P: 0.030% or less, S: 0 0.030% or less, Al: 0.01 to 0.08%, N: 0.010% or less, with the balance being substantially composed of Fe and inevitable alloy components, finishing temperature (Ac ₁ transformation) Point + 30 ° C.) After hot rolling as above, it is cooled to a temperature of 20 to 500 ° C. at a cooling rate of 10 to 100 ° C./s, and after holding for 1 to 10 s, a temperature of 500 ° C. to (Ac ₁ transformation point + 30 ° C.) A method for producing a high carbon thin steel sheet with good formability, which is reheated to a temperature range and wound in this temperature range, is disclosed.

Patent Document 2 discloses that steel containing 0.2 to 0.7% by mass of C is hot-rolled at a finishing temperature (Ar ₃ transformation point −20 ° C.) or higher, then the cooling rate exceeds 120 ° C./s and cooling is stopped. Cooling at a temperature of 650 ° C. or lower, winding at a coiling temperature of 600 ° C. or lower, pickling, and annealing at an annealing temperature of 640 ° C. or higher and an Ac ₁ transformation point or lower is produced. A method is disclosed.

In Patent Document 3, in mass%, C: 0.2 to 0.7%, Si: 2% or less, Mn: 2% or less, P: 0.03% or less, S: 0.03% or less, sol . A step of hot rolling a steel having a composition containing Al: 0.01% or less and N: 0.01% or less at a finishing temperature of (Ar ₃ transformation point −20 ° C.) or more to form a hot-rolled steel sheet; The step of cooling the hot-rolled steel sheet to a temperature of 650 ° C. or less at a cooling rate of 60 ° C./s or more and less than 120 ° C./s, and winding the hot-rolled steel sheet after cooling at a coiling temperature of 600 ° C. or less. a step, the hot-rolled steel sheet of the winding Tonochi, method of producing a high-carbon hot-rolled steel sheet is disclosed and a step of annealing at 640 ° C. or higher Ac ₁ transformation point of the annealing temperature.

JP-A-5-9588 JP 2003-13145 A JP 2008-156712 A

However, high carbon hot-rolled steel sheets produced by the production methods described in Patent Documents 1 to 3, particularly steel sheets containing 0.5% by mass or more of C tend to have large variations in characteristics within the steel sheets, and are not necessarily excellent. There is a problem that processability and hardenability cannot be obtained.

An object of the present invention is to provide a method for producing a high carbon hot-rolled steel sheet that contains 0.5% by mass or more of C, has small variations in characteristics in the steel sheet, and is excellent in workability and hardenability. To do.

The present inventors have earnestly studied a method for producing a high carbon hot rolled steel sheet containing 0.5 mass% or more of C, having small variations in characteristics in the steel sheet, and excellent in workability and hardenability. When cooling after hot rolling, it is cooled in a two-stage rapid cooling in which it is allowed to cool in a temperature range of 550 to 650 ° C., that is, cooled and wound in a rapid cooling-cooling-rapid cooling pattern, and then cementite spherical It has been found that it is effective to perform annealing for conversion.

The present invention has been made on the basis of such findings. In mass%, C: 0.5 to 1.0%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, sol. A steel slab having a composition containing Al: 0.08% or less, N: 0.01% or less, and the balance consisting of Fe and inevitable impurities is hot at a finishing temperature not lower than the Ar ₃ transformation point or Ar _cm transformation point. Rolled, first cooled to a cooling stop temperature of 550 to 650 ° C. at an average cooling rate of 60 ° C./s or more, allowed to cool for 1.0 to 10 s, and then cooled to an average cooling rate of 120 ° C./s or more to 500 to 500 A method for producing a high carbon hot-rolled steel sheet is provided, characterized by performing secondary cooling to a cooling stop temperature of 600 ° C. and winding, and then annealing at a temperature not lower than 640 ° C. and not higher than the Ac ₁ transformation point.

In the method for producing a high carbon hot-rolled steel sheet of the present invention, the average cooling rate during primary cooling is preferably 120 ° C./s or more. In addition, the sol. Al amount is 0.01% or less in mass%, or in steel slab, and in mass%, Cr: 0.1-2.0%, Mo: 0.1-1.0%, Ni : 0.1-2.0%, Cu: 0.1-1.0%, Ti: 0.01-0.10%, Nb: 0.01-0.10%, V: 0.01-0 10%, B: at least one element selected from 0.0005 to 0.0100% may be contained.

According to the present invention, it becomes possible to produce a high carbon hot rolled steel sheet containing 0.5% by mass or more of C, having small variations in characteristics in the steel sheet, and excellent in workability and hardenability. There is a remarkable effect.

The method for producing the high carbon hot rolled steel sheet according to the present invention will be described in detail below.

(1) Composition of steel slab Hereafter,% which is a unit of content of a component element shall mean the mass% unless otherwise indicated.

C: 0.5 to 1.0%
C is an essential element for increasing the strength of the steel sheet after quenching and tempering. If the amount of C is less than 0.5%, the strength required as a material for machine structural parts or tools cannot be obtained. On the other hand, if the amount of C exceeds 1.0%, the steel sheet becomes brittle and the workability is lowered. In addition, residual austenite tends to exist even after quenching, and the strength after heat treatment is saturated or reduced. Therefore, the C content is limited to 0.5 to 1.0%. Preferably it is 0.6 to 0.9%.

Si: 2.0% or less Si has an effect of deoxidizing steel and an effect of increasing the temper softening resistance after quenching, and therefore can be contained as necessary. However, the Si content also has the effect of graphitizing the cementite to lower the hardenability of the steel, so the Si content is limited to 2.0% or less. Preferably it is 0.5% or less.

Mn: 2.0% or less Mn has an effect of enhancing the hardenability of steel and can be contained as required. However, if Mn is contained excessively, the toughness and ductility of the steel are lowered, so the Mn content is limited to 2.0% or less. Preferably it is 1.0% or less.

P: 0.03% or less P has an effect of reducing the workability of the steel sheet and the toughness of the steel after the heat treatment, so the P content is limited to 0.03% or less. Preferably it is 0.02% or less.

S: 0.03% or less Since S has the effect of reducing the workability of the steel sheet and the toughness of the steel after the heat treatment, the S content is limited to 0.03% or less. Preferably it is 0.01% or less.

sol. Al: 0.08% or less Al is an element added for deoxidation of steel, and can be contained as necessary. However, as the content of Al, sol. Addition such that the Al content exceeds 0.08% leads to an increase in inclusions in the steel and a decrease in the workability of the steel sheet. Therefore, the content of Al is sol. The amount of Al is limited to 0.08% or less. Preferably it is 0.04% or less. Also, when the steel is kept at a high temperature, solute Al and solute N combine to form AlN in the steel, suppress the growth of austenite crystal grains during quenching heating, and reduce the hardenability of the steel sheet. There is a case. In particular, when the steel sheet is annealed in a nitrogen atmosphere, the above-described action becomes noticeable due to N entering the steel from the atmosphere. In order to avoid such a decrease in hardenability of the steel sheet due to the formation of AlN, the Al content is set to sol. More preferably, the Al content is 0.01% or less.

N: 0.01% or less A large amount of N may reduce the hardenability of the steel sheet through the formation of AlN in the steel. Therefore, the N content is limited to 0.01% or less. Preferably it is 0.005% or less.

The steel slab used in the present invention has the above-described component composition. Note that reducing the content of each element above the refining range to the extent that it is usually practiced, for example, to less than 0.001%, increases the manufacturing cost of the steel sheet, so there is a special reason. Not needed unless

The balance of the steel piece used in the present invention is Fe and inevitable impurities. However, even if one or more elements of Cr, Mo, Ni, Cu, Ti, Nb, V, and B are further added for the purpose of improving hardenability and temper softening resistance, the present invention The effect of is not impaired.

When these elements are added to the steel slab, Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, Ni: 0.1 to 2.0%, Cu: 0 0.1-1.0%, Ti: 0.01-0.10%, Nb: 0.01-0.10%, V: 0.01-0.10%, B: 0.0005-0.0100 % Content is preferable. When the content is less than the lower limit of each range, the effect of addition cannot be sufficiently obtained. In addition, when the content exceeds the upper limit of each range, the manufacturing cost is increased and the workability and toughness of the steel sheet may be lowered. More preferable content ranges are Cr: 0.1 to 1.0%, Mo: 0.1 to 0.5%, Ni: 0.1 to 1.0%, Cu: 0.1, respectively. ~ 0.5%, Ti: 0.01-0.05%, Nb: 0.01-0.05%, V: 0.01-0.05%, B: 0.0005-0.0050% is there.

Inevitable impurities include, for example, O, Sn, Pb and the like. Although it is desirable not to contain these elements as much as possible, a content of less than 0.01% is acceptable.

2) Production conditions In the method for producing a high carbon hot-rolled steel sheet according to the present invention, a steel piece having the above-described composition is hot-rolled at a finishing temperature equal to or higher than the Ar ₃ transformation point or Ar _cm transformation point, and 60 ° C./s. After the primary cooling to the cooling stop temperature of 550 to 650 ° C. at the above average cooling rate, it is allowed to cool for 1.0 to 10 s, and then to the cooling stop temperature of 500 to 600 ° C. at an average cooling rate of 120 ° C./s or more. Secondary cooling and winding up, followed by annealing at a temperature of 640 ° C. or more and Ac ₁ transformation point or less. In addition, it is preferable to remove the scale formed in the steel sheet surface layer by annealing or the like before annealing. The reason for limitation in the production conditions of the present invention will be described below.

Hot rolling finishing temperature: Ar ₃ transformation point or Ar _cm transformation point or higher When the hot rolling finishing temperature is lower than Ar ₃ transformation point or Ar _cm transformation point, the pro-eutectoid ferrite or pro-eutectoid cementite is partially precipitated. It is rolled and becomes a non-uniform steel sheet structure, and the uniformity of the characteristic in a steel plate falls. Therefore, the finishing temperature of hot rolling is not less than the Ar ₃ transformation point or Ar _cm transformation point.

The Ar ₃ transformation point or Ar _cm transformation point of the steel slab can be obtained, for example, by measuring a heat shrinkage curve in the cooling process from the austenite temperature range and by changing the curve.

Average cooling rate of primary cooling: 60 ° C / s or more Cooling stop temperature of primary cooling (cooling temperature): 550 to 650 ° C
The primary cooling after the hot rolling needs to be performed at an average cooling rate of 60 ° C./s or more up to the cooling stop temperature in the range of 550 to 650 ° C. after the hot rolling. In the present invention, the steel sheet structure after hot rolling is prepared into a structure mainly composed of uniform pearlite in order to avoid a decrease in manufacturability due to the generation of a low-temperature transformation phase while reducing variations in characteristics in the steel sheet. Therefore, it is essential to avoid coarse precipitation of ferrite and cementite in the cooling process after hot rolling. For this purpose, it is necessary to quickly firstly cool the hot-rolled steel sheet at an average cooling rate of 60 ° C./s or more to a cooling stop temperature of 550 to 650 ° C.

When the average cooling rate of primary cooling is less than 60 ° C./s, pro-eutectoid ferrite and pro-eutectoid cementite are remarkably generated during primary cooling, and a uniform microstructure is not formed. When the cooling stop temperature exceeds 650 ° C., a large amount of pro-eutectoid ferrite and pro-eutectoid cementite are generated after cooling stop even if cooling is performed at an average cooling rate of 60 ° C./s or more. On the other hand, when the cooling stop temperature is lower than 550 ° C., a low-temperature transformation phase such as bainite and martensite is partially generated, resulting in uneven cooling after the steel plate shape deteriorates, and consequently, variations in characteristics within the steel plate. There is sex. In order to surely avoid the generation of a proeutectoid phase and obtain a more uniform steel sheet structure, it is preferable to set the average primary cooling rate to 120 ° C./s or more.

Cooling time: 1.0 to 10 s
After the primary cooling, the steel sheet is allowed to cool for 1.0 to 10 seconds. By allowing to cool after the rapid cooling in the primary cooling, the pearlite transformation proceeds in a short time, and a uniform pearlite structure is formed. The main point of the present invention is to prepare the microstructure of the high carbon steel sheet before annealing into a structure mainly composed of uniform pearlite, and promotion of pearlite transformation by cooling is a very important role.

If the cooling time is less than 1.0 s, the above transformation promotion effect cannot be obtained sufficiently. A high carbon steel containing 0.5 mass% or more of C has high hardenability due to a high C content, and easily forms a low-temperature transformation phase. For this reason, if the cooling is performed for a short time of less than 1.0 s, the effect of promoting pearlite transformation is insufficient and the desired structure cannot be prepared. On the other hand, when the cooling time exceeds 10 s, the steel plate temperature rises due to transformation heat generation as the pearlite transformation progresses, and the pearlite generated in the latter stage of the cooling process becomes coarse, resulting in non-uniform characteristics in the steel plate. Invite. Therefore, the cooling time is limited to the range of 1.0 to 10 s. Preferably, it is 3 to 8 s. The term “cooling” means that the steel sheet is exposed to the atmosphere without forced cooling by water injection or the like. However, in order to wipe the cooling water injected in the primary cooling process, injecting a fluid such as compressed air toward the steel sheet for a short time may cause a sufficiently small cooling effect by the injection and impair the effect of the present invention. Not acceptable.

Secondary cooling average cooling rate: 120 ° C / s or more Secondary cooling cooling stop temperature (winding temperature): 500 to 600 ° C
The steel sheet after being allowed to cool for a predetermined time is cooled again at an average cooling rate of 120 ° C./s or more, and the cooling is stopped at a cooling stop temperature of 500 to 600 ° C. Since the temperature of the steel sheet after being allowed to cool is increased due to transformation heat generation, the steel sheet is cooled again to a temperature of 500 to 600 ° C. and wound up in order to suppress coarsening of the microstructure of the steel sheet. When the cooling stop temperature exceeds 600 ° C., coarse pearlite is likely to be generated, and the unevenness of the steel sheet structure cannot be completely avoided. On the other hand, when the cooling stop temperature is less than 500 ° C., a low-temperature transformation phase such as bainite or martensite is generated, the steel sheet is excessively hardened and the winding shape is deteriorated, and the workability is significantly reduced. The structure mainly composed of a low temperature transformation phase has an advantage that cementite is easily finely dispersed after annealing. However, a high carbon steel containing 0.5 mass% or more of C has a low C transformation phase due to a high C content. Since the hardness of the steel sheet is high and a decrease in the manufacturability and workability of the steel sheet cannot be allowed, the cooling stop temperature is limited to 500 ° C. or higher.

In order to increase the uniformity of the steel sheet structure, the average cooling rate after cooling must be 120 ° C./s or more. In the case of cooling by general water injection, since the temperature range of 500 to 600 ° C. is a region where the transition from film boiling to nucleate boiling starts, uneven cooling of the steel sheet is likely to occur. In such a temperature range, when water cooling is performed under the condition of mainly nucleate boiling so that the average cooling rate is 120 ° C./s or more, uneven cooling of the steel sheet is less likely to occur, and variation in characteristics in the steel sheet can be suppressed to a small level. it can. Water cooling with an average cooling rate of 240 ° C./s or more is more preferable.

Annealing temperature: 640 ° C. or more and Ac ₁ transformation point or less The hot-rolled steel sheet after winding is annealed in order to spheroidize cementite. At this time, when the annealing temperature is less than 640 ° C., cementite spheroidization does not proceed rapidly. Further, when the annealing temperature exceeds the Ac ₁ transformation point, the steel sheet structure is cooled after being partially re-austenitic during annealing, so that pearlite, that is, non-spheroidized cementite is mixed in the steel sheet structure after annealing. However, workability and hardenability are reduced along with the uniformity of characteristics in the steel sheet. Therefore, the annealing temperature is limited to a range of 640 ° C. or higher and Ac ₁ transformation point or lower. Preferably, it is less Ac ₁ transformation point 680 ° C. or higher.

In the present invention, since the structure of the hot-rolled steel sheet before annealing is prepared in a structure mainly composed of uniform pearlite, the spheroidization of cementite proceeds efficiently, so the time for holding the hot-rolled steel sheet at the annealing temperature About 10 hours or more. Desirably, it is 15 to 35 hours. The annealed steel sheet can be subjected to temper rolling as necessary for correcting the shape of the steel sheet or adjusting the surface properties.

The Ac ₁ transformation point of the steel sheet can be obtained from, for example, a curve change point by measuring a thermal expansion curve in a heating process from room temperature.

In the melting of high carbon steel used in the present invention, either a converter or an electric furnace can be used. The molten steel is made into a steel slab by continuous casting or ingot rolling after ingot forming. What is necessary is just to heat the steel slab before hot rolling to the temperature which can ensure a predetermined finishing temperature according to the capability of manufacturing equipment. The continuously cast steel slab may be hot-rolled directly or after heating for a short time without cooling to room temperature. It is also possible to additionally heat the steel slab during hot rolling with an induction heating device such as a bar heater or an edge heater.

Steel slabs A to L containing the elements shown in Table 1 and having the balance consisting of Fe and inevitable impurities, and having the Ar ₃ transformation point or Ar _cm transformation point and Ac ₁ transformation point shown in Table 1 Was made into a hot-rolled steel sheet having a thickness of 4.0 mm under the hot rolling conditions shown in Table 2, the scale was removed by pickling, annealing was performed in a nitrogen atmosphere under the annealing conditions also shown in Table 2, and then stretched. The steel sheets 1 to 24 were obtained by temper rolling at a rate of 0.5%. Each transformation point in Table 1 was determined by the method described above.

Samples for the thickness cross-section investigation parallel to the rolling direction were collected from each obtained steel plate, and the Vickers hardness and the average diameter of cementite in the plate thickness cross-section were measured as follows, and the dispersion of the characteristics in the steel plate And processability and hardenability were evaluated. The results are shown in Table 3.

Vickers hardness in the plate thickness section: The plate thickness section parallel to the rolling direction is mirror-polished, and 9.8 N (1 kgf) is compliant with the provisions of JIS Z 2244 at the 1/4 depth position of the plate thickness. Measured with test force. Each sample was measured five times or more, and the average value was defined as the Vickers hardness HV in the plate thickness section of the sample. This Vickers hardness HV is measured with samples taken at a total of 7 positions of 1/8, 1/4, 3/8, 1/2, 5/8, 3/4, and 7/8 of the steel plate width. The difference between the maximum value (HVmax) and the minimum value (HVmin) of the HV values at all 7 positions (ΔHV = HVmax−HVmin) and the average value (HVave) of the HV values at all 7 positions are obtained, and ΔHV / HVave The value was used as an index of the uniformity of characteristics in the steel sheet. When the value of ΔHV / HVave was 0.10 or less, it was evaluated that the variation in characteristics in the steel sheet was small.

The average diameter of cementite in the plate thickness cross section: After the sample thickness sample parallel to the rolling direction of the sample taken at 1/4 position of the steel plate width is mirror-polished and corroded with Picral, the thickness is 1/4 of the plate thickness. Using a structural photograph taken at a magnification of 5000 times with a scanning electron microscope, the geometric mean of the long diameter and short diameter of each cementite particle is the particle diameter of each cementite particle, and is within the field of view of the structure photograph. The average value of the cementite particles was defined as the average diameter d of the cementite of the steel sheet. The average diameter d of cementite can be used as an index of workability and hardenability because it is a measure of the amount of strengthening due to particle dispersion, the degree of stress concentration during processing, and the difficulty of decomposition during quenching heating. Was 0.5 to 2.0 μm, it was evaluated that it was excellent in workability and hardenability.

The steel plates of the examples of the present invention shown in Table 3 have a ΔHV / HVave value of 0.10 or less, which is an index of the uniformity of the properties in the steel plates, and the variations in the properties in the steel plates are small. The value of the average diameter d of cementite, which is an index of hardenability, is also 0.5 to 2.0 μm, and it is a high carbon hot rolled steel sheet excellent in workability and hardenability.

Claims

In mass%, C: 0.5 to 1.0%, Si: 2.0% or less, Mn: 2.0% or less, P: 0.03% or less, S: 0.03% or less, sol. A steel slab having a composition containing Al: 0.08% or less, N: 0.01% or less, and the balance consisting of Fe and inevitable impurities is hot at a finishing temperature not lower than the Ar 3 transformation point or Ar cm transformation point. Rolled, first cooled to a cooling stop temperature of 550 to 650 ° C. at an average cooling rate of 60 ° C./s or more, allowed to cool for 1.0 to 10 s, and then 500 to 500 ° C. at an average cooling rate of 120 ° C./s or more. A method for producing a high-carbon hot-rolled steel sheet, comprising: secondary cooling to a cooling stop temperature of 600 ° C. and winding, followed by annealing at a temperature of 640 ° C. or more and an Ac 1 transformation point or less.
The method for producing a high carbon hot-rolled steel sheet according to claim 1, wherein an average cooling rate during primary cooling is 120 ° C / s or more.
Sol. The method for producing a high carbon hot-rolled steel sheet according to claim 1 or 2, wherein the Al content is 0.01% by mass or less.
In addition to the steel piece, in mass%, Cr: 0.1 to 2.0%, Mo: 0.1 to 1.0%, Ni: 0.1 to 2.0%, Cu: 0.1 to 1 0.0%, Ti: 0.01-0.10%, Nb: 0.01-0.10%, V: 0.01-0.10%, B: 0.0005-0.0100% The method for producing a high carbon hot-rolled steel sheet according to any one of claims 1 to 3, wherein at least one selected element is contained.