JP5601861B2 - Manufacturing method of boron steel rolled annealed steel sheet - Google Patents

Description

  The present invention relates to a steel plate made of boron steel suitable for various machine parts including automobile parts, which has good workability, hardenability and weldability, and excellent in local ductility, and a method for producing the same.

  Since carbon steel (boron steel) with a small amount of B is excellent in hardenability, it is low cost in various parts applications that require sufficient quenching to the center of the plate thickness (becomes a martensite structure). Used as a material. Rolled and annealed steel sheet of boron steel (annealed hot-rolled steel sheet or cold-rolled steel sheet) is processed into a predetermined part shape by press forming such as punching and bending, and forging and cutting, and then quenched and tempered. Used after heat treatment such as induction hardening and carburizing and quenching. Moreover, welding may be performed with respect to such a hardened product. Therefore, the boron steel rolled annealed steel sheet, which is a raw material, is required to have characteristics corresponding to applications such as workability, hardenability, and weldability after quenching.

However, the following problems occur in the boron steel sheet.
(I) The solid solution B of the steel surface layer portion is reduced by high-temperature heating performed at the time of manufacturing the steel sheet or at the time of quenching after forming the part (de-B).
(Ii) The hardenability of the surface layer portion may decrease due to removal B, and the surface hardness of the quenched product may decrease.
(Iii) Particularly when the plate thickness is large, the cooling rate at the time of quenching is lowered, so that the reduction in surface hardness is easily promoted. In some cases, the cross-sectional hardness in the vicinity of the central portion of the plate thickness is also reduced, and the strength of the material itself may be insufficient.

  Countermeasures have been proposed for the reduction in surface hardness. For example, in Patent Document 1, boron steel having a C content of 0.15 to 0.35 wt% is used, and the carbon potential of the atmospheric gas in the protective atmosphere is 0.04 to 0.25% higher than the material carbon content. A method is disclosed that compensates for the hardenability deterioration of the surface portion due to de-B by setting and finely carburizing the surface of the component and then quenching. However, this method requires equipment for carburizing treatment. Further, this method cannot be applied to local quenching such as induction quenching and electron beam quenching for a short time.

  Patent Document 2 uses boron steel having a C content of 0.1 to 0.3% by mass, and prevents the formation of coarse grains during carburization by fine precipitation of carbonitrides of Ti and Nb. Further, Cr and Mo are added to compensate for the hardenability deterioration due to de-B, and the generation of an incompletely hardened structure formed to a depth of 0.2 to 0.7 mm from the surface is prevented. However, in this method, in order to finely precipitate carbonitrides of Ti and Nb, it is necessary to gradually cool the temperature range of 800 to 500 ° C. at a cooling rate of 1 ° C./sec or less, resulting in poor productivity. Further, no consideration is given to the decrease in surface hardness, which is a problem during quenching without carburizing.

JP-A-57-94516 JP 2001-303172 A

  As described above, the carburization treatment can be applied to the reduction in surface hardness during quenching of boron steel. However, the carburizing process has problems in capital investment, processing time, and processing cost, and is not always easily adopted in applications that require inexpensive parts. In addition, it is desired to obtain a processed product with as high a dimensional accuracy as possible in parts processing before quenching. In particular, when performing fine blanking punching, it is extremely advantageous to increase the dimensional accuracy by using a material having good “local elongation”. However, it is not easy to sufficiently improve local elongation as well as general ductility in a steel sheet for a quenched member, and it cannot be said that current materials necessarily meet the recent processing needs.

  In view of the current situation, the present invention is excellent in local elongation in addition to normal workability (ductility) as a raw steel plate before quenching, and has a high quenching hardness in the surface part of the component even when the plate thickness is thick during quenching. Is intended to provide a steel plate material that can secure good weldability after quenching.

  According to the inventors' research, steel sheets that meet the above objectives are adjusted to a specific range of component composition, and then the coiling temperature is defined in hot rolling, and the cooling rate until coiling is strictly controlled. And it turned out that it can implement | achieve by giving the annealing which has a predetermined | prescribed heat pattern.

That is, in the present invention, in mass%, C: 0.15 to 0.35%, Si: 0.50% or less, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0 0.020% or less, Cr: 0.20 to 1.00%, Al: 0.005 to 0.200%, Ti: 0.01 to 0.20%, B: 0.0005 to 0.0050%, N : A chemical composition having 0.0080% or less, the balance Fe and inevitable impurities, an X value determined by the following formula (1) of 17.00 or more, and a carbon equivalent Ceq determined by the following formula (2) of 0.500 or less. The total elongation T.E1 in the rolling direction is 38.0% or more, and the mark distance is 10 mm using notched tensile test pieces in which 2 mm V notches are formed at both edges in the center of the parallel part of the rolling direction JIS No. 5 tensile test piece. boron steel rolled annealed steel sheet local elongation Elv by tensile test ductile comprising 43.0% or more Hisage It is.
X value = 5.5C ^1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 2.3Cr) (1)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 (2)

The value of the content expressed in mass% of the element is substituted for the element symbol in the above formulas (1) and (2). The total elongation T.E1 is measured for a portion with a gauge distance of 50 mm by performing a tensile test using a JIS No. 5 tensile test piece collected so that the longitudinal direction is the rolling direction until it breaks at room temperature according to JIS Z2241: 1998. The value of elongation (%) at the time of breaking recorded is adopted. Further, the local elongation Elv is determined by using notched tensile test pieces in which 2 mmV notches are formed at both edges in the center of the parallel part of a JIS No. 5 tensile test specimen taken so that the longitudinal direction is the rolling direction, and the gauge distance is It is measured by the same method as T.El except for 10 mm. The 2 mm V notch can adopt the V notch shape defined by JIS in the Charpy impact test piece (FIG. 7B of number 1311 of JIS G0202-1987).
The thickness of the boron steel rolled annealed steel sheet of the present invention is, for example, 1.0 to 12.0 mm.

Moreover, as a manufacturing method of said boron steel rolling annealing steel plate,
Slab heating temperature: 1100 to 1300 ° C., finishing temperature: 800 to 900 ° C., average cooling rate from the end of finishing rolling pass to winding: 30 to 45 ° C./sec, winding temperature: 500 to 650 ° C. A step of hot rolling to perform hot rolling,
If necessary, with respect to the hot-rolled sheet, a step of performing cold rolling with a rolling rate of 60% or less to obtain a cold-rolled sheet,
A step of heat-treating the hot-rolled plate or the cold-rolled plate with a heat pattern of any one of the following conditions A and B,
A method of manufacturing a boron steel rolled annealed steel sheet having the following is provided.

[Condition A]
(A ₁ −30 ° C.) or more and less than A _{1 in} the austenite phase non-generation temperature range, 0.5 h or more soaking is maintained, and the temperature is raised to A ₁ or more (A ₁ + 50 ° C.) or less in the austenite phase generation temperature range Heat pattern in which soaking is maintained for 5 to 20 hours, and then gradually cooled from the holding temperature to at least (A ₁ -10 ° C.) at a cooling rate of 5 to 30 ° C./h.
[Condition B]
(A ₁ -30 ° C.) A heat pattern that cools after holding soaking for 0.5 h or more in the austenite phase non-generation temperature range of A ₁ or more and less than A ₁ .

However, the value determined by the following equation (3) is adopted as A ₁ .
A ₁ (° C.) = 723 + 29.1 Si−10.7 Mn + 16.9 Cr (3)

  The value of the content expressed in mass% of the element is substituted for the element symbol in the above formula (3). The term “hot rolled sheet” as used herein means a sheet in a hot rolled steel sheet that has not yet undergone heat treatment after winding. It may be pickled after hot rolling. “Cold-rolled sheet” means a cold-rolled steel sheet that has not yet been heat-treated.

  The boron steel rolled annealed steel sheet of the present invention is superior in the hardenability of the surface layer compared to the conventional boron steel rolled annealed steel sheet, and also has good workability (total elongation and local elongation) before quenching, and has high dimensional accuracy. It is advantageous in obtaining a product. It also has good weldability. There is no need to add a new manufacturing process at the manufacturing stage and the part manufacturing stage of the material steel plate. Therefore, the present invention contributes to improvement in strength, improvement in dimensional accuracy, and reduction in manufacturing cost in applications of various machine parts including automobile parts.

[Chemical composition]
Hereinafter, “%” in the chemical composition of steel in this specification means “% by mass” unless otherwise specified.
C is an element necessary for securing the core strength as a machine structural component. In order to ensure sufficient strength, a C content of 0.15% or more is required. However, if the C content increases, the workability after annealing decreases, and the weldability after quenching and tempering also decreases. In the present invention, in consideration of providing workability and weldability that can be widely applied to various machine parts including automobile parts, the C content is set to a range of 0.35% or less.

  Si is effective in deoxidizing steel and improving hardenability. However, if it is added excessively, the workability decreases. Further, it promotes the formation of a grain boundary oxide layer during annealing, and causes a reduction in fatigue characteristics of the member after tempering heat treatment (quenching and tempering). Therefore, the Si content is set to 0.50% or less.

  Mn is effective in improving deoxidation / desulfurization and hardenability, and a Mn content of 0.20% or more is ensured in order to sufficiently exhibit these functions. However, the excessive Mn content causes the annealed material to harden, causing a decrease in workability, and promotes weld cracking after quenching and tempering. As a result of various studies, the Mn content is limited to a range of 1.0% or less.

  P and S are elements that adversely affect the toughness of steel, and it is desirable that the content is small. As a result of the examination, in the component system of the present invention, P is allowed up to 0.030% and S is allowed up to 0.020.

  Cr has effects of improving hardenability and improving strength and wear resistance. However, if it is less than 0.20%, the effect is not sufficiently exhibited. On the other hand, the excessive addition of Cr hardens the annealed material and causes a decrease in workability, and also causes a weld crack after quenching and tempering. The Cr content is limited to a range of 1.00% or less.

  Al is a component contained as a deoxidizing material for steel, but has the effect of suppressing other abnormal growth of austenite grains during quenching by generating other AlN. In addition, this N fixing effect serves to suppress the formation of BN and promote the improvement of hardenability by solid solution boron. In order to fully exhibit these actions, it is necessary to ensure a total Al content of 0.005% or more. However, excessive Al content tends to cause a surface defect of the steel sheet and increases the cost. As a result of various studies, the Al content needs to be in the range of 0.200% or less, more preferably 0.100% or less.

  Ti combines with N in the steel and precipitates as TiN. In the present invention, precipitation of BN is prevented by addition of Ti, and solid solution boron effective in improving hardenability is secured. Ti also has the effect of refining the austenite grain size during quenching. In order to fully exhibit these actions, it is necessary to contain 0.01% or more of Ti. However, when the Ti content is increased, TiC is excessively precipitated and the workability is lowered. The Ti content is in the range of 0.20% or less.

  B is an element that significantly improves the hardenability by adding a small amount. Of the B present in the steel, in order to ensure a sufficient amount of solid solution boron effective for improving hardenability, it is necessary to contain 0.0005% or more of B. However, since excessive B content becomes a factor which inhibits the toughness of steel, B content is restrict | limited to the range below 0.0050%.

The X value defined by the following formula (1) is an index for evaluating the hardenability of the surface layer part on the premise that the amount of dissolved boron is reduced in the component system of the present invention.
X value = 5.5C ^1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 2.3Cr) (1)
As a result of detailed studies, the inventors have been able to significantly suppress a decrease in surface hardness in a quenching material obtained by performing a normal quenching treatment when the X value is adjusted to a component composition of 17.00 or more. I found.

  N is desirably as small as possible in order to sufficiently secure the above-mentioned solid solution B. As a result of various studies, N contamination is allowed up to 0.0008%.

The carbon equivalent Ceq defined by the following formula (2) is an index for evaluating the weldability after quenching.
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 (2)
As a result of various studies, when this Ceq exceeds 0.500, occurrence of weld cracks may be observed in the material after quenching. Therefore, in the use of quenched parts used for welding, it is important to have a chemical composition in which Ceq is adjusted to 0.500 or less.
The steel according to the present invention adjusted to the above chemical composition has a surface hardness of 350 HV or higher by a normal quenching process.

[Ductility]
The rolled steel sheet of the present invention has excellent hardenability capable of obtaining a surface hardness of 350 HV or more, and has good ductility that can be applied to various hardened part shapes. . Specifically, the total elongation T.El, which is a general ductility index, is 38.0% or more, and the local elongation Elv (previously described) that can evaluate local ductility is 43.0% or more. It has the characteristic. It is not always easy to maintain these two types of ductility indices at a high value at the same time. The local elongation Elv is an index that greatly affects the dimensional accuracy in fine blanking punching and the like. The present applicant disclosed in Japanese Patent Application No. 2009-063921 a technique for producing a boron steel sheet having a small difference between the surface hardness after quenching and the center thickness hardness. However, in a component-based boron steel having a surface hardness of 350 HV, a steel sheet in which the total elongation T.El and the local elongation Elv are maintained at the above high values has not been realized. The boron steel rolled steel sheet of the present invention having a ductility of T.El of 38.0% or more and Elv of 43.0% or more is dimensional accuracy when subjected to fine blanking punching processing, etc., compared with conventional materials. It will bring about improvement. T.El and Elv can be controlled mainly by the above-described chemical composition limitation and hot rolling condition limitation.

The boron steel rolled annealed steel sheet of the present invention can be manufactured by the following method.
(Hot rolling)
The slab heating temperature before hot rolling may be set to 1100 to 1300 ° C. similarly to general carbon steel. The finishing temperature (temperature of the hot rolling final pass) is 800 to 900 ° C. If it is below 800 ° C., the deformation resistance is increased, the sheet passing property is lowered, and it is difficult to secure a winding temperature described later. When it exceeds 900 degreeC, an austenite particle size will coarsen and the toughness of a hot rolled material will fall. The winding temperature is 500 to 650 ° C. If it is below 500 ° C., the hot-rolled material becomes hard and the productivity is lowered. If the temperature exceeds 650 ° C, the amount of pro-eutectoid ferrite increases, the distribution of carbide after annealing becomes non-uniform, and the lamellar spacing of pearlite increases, making it difficult to spheroidize carbide by annealing, and after annealing The workability of is reduced.

  In addition to these conditions, it is important to control the average cooling rate from the end of the finishing rolling pass to the winding in the range of 30 to 45 ° C./sec in order to achieve both high total elongation T.El and local elongation Elv. is there. When the average cooling rate is less than 30 ° C./sec, the amount of pro-eutectoid ferrite becomes too large, and the distribution of cementite after post-annealing becomes uneven. As a result, it becomes difficult to make the total elongation T.El and the local elongation Elv compatible with high values as described above. On the other hand, when the average cooling rate exceeds 45 ° C./sec, the hot-rolled material becomes hard and the productivity is lowered. After winding, pickling can be performed by a normal method.

(Cold rolling)
The hot-rolled sheet obtained under the above conditions can be cold-rolled as necessary to adjust the sheet thickness. It is desirable that the cold rolling rate be in the range of 60% or less. If the cold rolling rate becomes higher than that, the cost increases. Although the lower limit of the cold rolling rate is not particularly limited, it is advantageous to secure a cold rolling rate of 20% or more when emphasizing obtaining good sheet thickness accuracy.

[Annealing]
The rolled steel sheet that has been hot-rolled and cold-rolled as necessary is subjected to annealing.
In annealing, (a) the steel plate obtained in the previous step was soaked for 0.5 h or more in a temperature range of (A ₁ -30 ° C.) or more and less than A ₁ , and then (b) A ₁ or more (A ₁ + 50 ° C. ) In a temperature range of 0.5 to 20 h, and (c) adopt a heat pattern in which the temperature is gradually cooled from the holding temperature to at least (A ₁ -10 ° C.) at a cooling rate of 5 to 30 ° C./h. Can do.

Here, A ₁ is a temperature (° C.) substantially corresponding to the A ₁ transformation point, and in the target steel type of the present invention, the heat treatment condition can be managed by the value determined by the above equation (3).

The heating and holding in (a) may be approximately 24 h or less. The step (b) may be omitted. However, heating of (b) is performed after (a) to dissolve fine carbides and part of the carbides remain, and then the carbides are formed using the remaining carbides as a nucleus in the slow cooling process of (c). By making it grow, it becomes easier to obtain a spherical and coarse carbide form. It is desirable to carry out the slow cooling of (c) until the transformation is completed. For this reason, at least (A ₁ −10 ° C.) is gradually cooled while maintaining the predetermined cooling rate. Temperature to end the slow cooling, for industrial may be set between the up from _{(A 1 -10 ℃) (A} 1 -80 ℃). The subsequent cooling rate may be set arbitrarily. When the heating in (b) is omitted, the cooling is performed directly from the holding temperature in (a), but the cooling rate in this case does not need to be particularly managed.

The metal structure of the annealed steel sheet thus obtained is a coarse spherical carbide structure, but the dispersion of the spherical carbide is made uniform and softer than a normal annealed material. That is, by this annealing process, a rolled annealed steel sheet having excellent workability suitable for various machine parts including automobile parts can be obtained.
In addition, before using for this heat processing, appropriate structure | tissue control needs to be performed at the above-mentioned hot rolling stage.

As described above, the boron steel rolled annealed steel sheet of the present invention is obtained.
[Processing and quenching]
The steel plate of the present invention obtained as described above is processed into a predetermined machine part and then subjected to a quenching process. When processing parts, it is possible to set stricter processing conditions than before by utilizing good processability. A tempering process is usually performed after quenching. The parts that have undergone such tempering heat treatment are further subjected to welding as necessary.

Steel having the composition shown in Table 1 was melted, hot-rolled under the conditions specified in the present invention, pickled, and a hot-rolled sheet having a thickness of 6 mm was obtained. The hot rolling conditions were a heating temperature of 1150 to 1270 ° C, a finishing temperature of 815 to 880 ° C, a winding temperature of 550 to 610 ° C, and an average cooling rate from the finishing temperature to the winding temperature of 33 to 39 ° C / sec. . The pickled hot-rolled sheet obtained was annealed with a heat pattern of the following (A) that satisfies the regulations of the present invention to obtain a test material.
(A) Soaking soak at 715 ° C. 20h → Soaking soaking at 760 ° C. 10h → Cooling to 650 ° C. at 10 ° C./h→Cooling furnace

The following tests were performed using the specimens after annealing.
[Processability evaluation]
Two indices, total elongation T.El and local elongation Elv, were obtained as indices of workability. A material having a larger value of the local elongation Elv may be considered as a material suitable for fine blanking punching.
The total elongation T.E1 was obtained by conducting a tensile test using a JIS No. 5 test piece in the rolling direction. The gauge distance was 50 mm.
The local elongation Elv was determined by conducting a tensile test using notched tensile test pieces in which 2 mm V-notches were formed on both edges at the center of the parallel part on a JIS No. 5 test piece in the rolling direction as described above. In this case, the gauge distance is 10 mm.
The total elongation T.El was determined to be 38.0% or more, and the local elongation Elv was determined to be 43.0% or more.

[Evaluation of hardness after quenching]
Each specimen was soaked at 900 ° C. for 15 minutes and then quenched in an oil bath. The surface hardness of the quenched material and the cross-sectional hardness at the center of the plate thickness were measured with a Vickers hardness meter at a load of 10 kg. Those having a surface hardness of 350 HV or higher were judged to have good hardenability.

(Weldability)
About the said hardening material, the y-type weld crack test prescribed | regulated to JISZ3158-1993 was implemented. The welding conditions were a y-shaped test piece, current: 150 A, voltage: 20 V, and speed: 20 cm / min. Weld cracks were detected using a 10-fold magnifier, and the quality was determined by the presence or absence of cracks.
These results are shown in Table 2.

  All of the examples of the present invention exhibit excellent workability of T.El: 38.0% or more and local elongation Elv: 43.0% or more, and surface hardness of 350 HV or more is obtained after quenching. The property was also good.

  On the other hand, since No. 1 has a small amount of C, the surface hardness after quenching does not satisfy 350 HV. No. 3 and No. 8 were inferior in surface hardness after quenching because the amounts of Mn and Cr were relatively small and the X value was not satisfied. In No. 6, the local elongation Elv was low because the amount of Cr was too high. No. 9 has a carbon equivalent Ceq that is too high, the workability (T.El, Elv) is poor, and weld cracks also occur. No. 11 was inferior in workability (T.El, Elv) because the amount of Mn exceeded the range of the present invention. Since No. 16 and No. 19 had high carbon equivalent Ceq, weld cracks occurred. Since No. 21 had a high C content, workability (T.El, Elv) was poor, and further, the surface hardness after quenching was low because it did not satisfy the definition of the X value. In No. 22, the total elongation T.E1 was low because of the high Mn content.

The result of having investigated the influence by hot rolling, cold rolling, and annealing conditions using No. 12 steel of Table 1 is illustrated. Hot rolling finish temperature, coiling temperature, hot rolling with various changes in the average cooling rate from the end of the finishing rolling pass to winding, and then pickling, and some parts are cold rolled, Annealing was performed to prepare a specimen having a thickness of 6.0 mm. As the annealing conditions, the same conditions (A) as in Example 1 or the following conditions (B) were adopted. T.El and Elv were determined for the test material after annealing by the same method as in Example 1.
(A) Soaking soak at 715 ° C. 20 h → Soaking soak at 760 ° C. 10 h → Cooling at 10 ° C./h until 650 ° C. → Bool cooling (B) Soaking soak at 710 ° C. 20 h → 10 ° C./h until 650 ° C. Cooling → furnace cooling Table 3 shows the results.

All of the inventive examples exhibited good processability (T.El, Elv).
On the other hand, although test No. (3) has a finishing temperature and a coiling temperature within the scope of the present invention, the average cooling rate from the end of the finishing rolling pass to the coiling is small, so that after annealing. The distribution of carbides was uneven and the spheroidization rate was low, resulting in poor processability. In Test No. (7), the coiling temperature was higher than the range of the present invention, so the distribution of carbides after annealing was non-uniform and the spheroidization rate was low, resulting in poor workability.

Claims (4)

In mass%, C: 0.15 to 0.35%, Si: 0.50% or less, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.20 to 1.00%, Al: 0.005 to 0.200%, Ti: 0.01 to 0.20%, B: 0.0005 to 0.0050%, N: 0.0080% Hereinafter, with respect to the slab having a chemical composition in which the balance Fe and inevitable impurities, the X value determined by the following formula (1) is 17.00 or more, and the carbon equivalent Ceq determined by the following formula (2) is 0.500 or less. Slab heating temperature: 1100 to 1300 ° C., finishing temperature: 800 to 900 ° C., average cooling rate from the end of finishing rolling pass to winding: 30 to 45 ° C./sec, winding temperature: 500 to 650 ° C. A step of performing rolling,
With respect to the hot-rolled sheet, (A ₁ -30 ° C.) or more and less than A _{1 in} the austenite phase non-generated temperature range, hold for 0.5 h or more, raise the temperature, and A ₁ or more (A ₁ + 50 ° C.) or less. Annealing is performed in a heat pattern in which the temperature is maintained for 0.5 to 20 hours in the austenite phase generation temperature range, and then gradually cooled from the holding temperature to at least (A ₁ -10 ° C.) at a cooling rate of 5 to 30 ° C./h. Process,
The total distance T.E1 in the rolling direction is 38.0% or more, and the mark distance is 10 mm using notched tensile test pieces in which 2 mm V notches are formed at both edges in the center of the parallel part of the rolling direction JIS No. 5 tensile test piece. A method for producing a boron steel rolled annealed steel sheet having a local elongation Elv of 43.0% or more according to a tensile test.
However, the value determined by the following equation (3) is adopted as A ₁ .
X value = 5.5C ^1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 2.3Cr) (1)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 (2)
A ₁ (° C.) = 723 + 29.1 Si−10.7 Mn + 16.9 Cr (3) In mass%, C: 0.15 to 0.35%, Si: 0.50% or less, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.20 to 1.00%, Al: 0.005 to 0.200%, Ti: 0.01 to 0.20%, B: 0.0005 to 0.0050%, N: 0.0080% Hereinafter, with respect to the slab having a chemical composition in which the balance Fe and inevitable impurities, the X value determined by the following formula (1) is 17.00 or more, and the carbon equivalent Ceq determined by the following formula (2) is 0.500 or less. Slab heating temperature: 1100 to 1300 ° C., finishing temperature: 800 to 900 ° C., average cooling rate from the end of finishing rolling pass to winding: 30 to 45 ° C./sec, winding temperature: 500 to 650 ° C. A step of performing rolling,
A step of subjecting the hot-rolled sheet to cold rolling at a rolling rate of 60% or less,
Keeping soaked for 0.5 h or more in the austenite phase non-generated temperature range of (A ₁ -30 ° C.) or more and less than A _{1 with} respect to the cold-rolled sheet, raising the temperature to A ₁ or more (A ₁ + 50 ° C.) or less. Annealing is performed in a heat pattern in which the temperature is maintained for 0.5 to 20 hours in the austenite phase generation temperature range, and then gradually cooled from the holding temperature to at least (A ₁ -10 ° C.) at a cooling rate of 5 to 30 ° C./h. Process,
The total distance T.E1 in the rolling direction is 38.0% or more, and the mark distance is 10 mm using notched tensile test pieces in which 2 mm V notches are formed at both edges in the center of the parallel part of the rolling direction JIS No. 5 tensile test piece. A method for producing a boron steel rolled annealed steel sheet having a local elongation Elv of 43.0% or more according to a tensile test.
However, the value determined by the following equation (3) is adopted as A ₁ .
X value = 5.5C ^1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 2.3Cr) (1)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 (2)
A ₁ (° C.) = 723 + 29.1 Si−10.7 Mn + 16.9 Cr (3) In mass%, C: 0.15 to 0.35%, Si: 0.50% or less, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.20 to 1.00%, Al: 0.005 to 0.200%, Ti: 0.01 to 0.20%, B: 0.0005 to 0.0050%, N: 0.0080% Hereinafter, with respect to the slab having a chemical composition in which the balance Fe and inevitable impurities, the X value determined by the following formula (1) is 17.00 or more, and the carbon equivalent Ceq determined by the following formula (2) is 0.500 or less. Slab heating temperature: 1100 to 1300 ° C., finishing temperature: 800 to 900 ° C., average cooling rate from the end of finishing rolling pass to winding: 30 to 45 ° C./sec, winding temperature: 500 to 650 ° C. A step of performing rolling,
A process of annealing with a heat pattern to cool after holding soaking for 0.5 h or more in an austenite phase non-generated temperature range of (A ₁ -30 ° C.) or more and less than A _{1 with} respect to the hot rolled sheet,
The total distance T.E1 in the rolling direction is 38.0% or more, and the mark distance is 10 mm using notched tensile test pieces in which 2 mm V notches are formed at both edges in the center of the parallel part of the rolling direction JIS No. 5 tensile test piece. A method for producing a boron steel rolled annealed steel sheet having a local elongation Elv of 43.0% or more according to a tensile test.
However, the value determined by the following equation (3) is adopted as A ₁ .
X value = 5.5C ^1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 2.3Cr) (1)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 (2)
A ₁ (° C.) = 723 + 29.1 Si−10.7 Mn + 16.9 Cr (3) In mass%, C: 0.15 to 0.35%, Si: 0.50% or less, Mn: 0.20 to 1.00%, P: 0.030% or less, S: 0.020% or less, Cr: 0.20 to 1.00%, Al: 0.005 to 0.200%, Ti: 0.01 to 0.20%, B: 0.0005 to 0.0050%, N: 0.0080% Hereinafter, with respect to the slab having a chemical composition in which the balance Fe and inevitable impurities, the X value determined by the following formula (1) is 17.00 or more, and the carbon equivalent Ceq determined by the following formula (2) is 0.500 or less. Slab heating temperature: 1100 to 1300 ° C., finishing temperature: 800 to 900 ° C., average cooling rate from the end of finishing rolling pass to winding: 30 to 45 ° C./sec, winding temperature: 500 to 650 ° C. A step of performing rolling,
A step of subjecting the hot-rolled sheet to cold rolling at a rolling rate of 60% or less,
A step of annealing the cold-rolled sheet with a heat pattern to cool after holding for 0.5 h or more in an austenite phase non-generated temperature range of (A ₁ -30 ° C.) or more and less than A ₁ ,
The total distance T.E1 in the rolling direction is 38.0% or more, and the mark distance is 10 mm using notched tensile test pieces in which 2 mm V notches are formed at both edges in the center of the parallel part of the rolling direction JIS No. 5 tensile test piece. A method for producing a boron steel rolled annealed steel sheet having a local elongation Elv of 43.0% or more according to a tensile test.
However, the value determined by the following equation (3) is adopted as A ₁ .
X value = 5.5C ^1/2 (1 + 0.6Si) (1 + 4.1Mn) (1 + 2.3Cr) (1)
Ceq = C + Si / 24 + Mn / 6 + Cr / 5 (2)
A ₁ (° C.) = 723 + 29.1 Si−10.7 Mn + 16.9 Cr (3)