JP2014028375A - Production method of high strength steel sheet superior in edge quality and gauge uniformity - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method of a high strength steel sheet superior in edge quality and gauge uniformity, the method enabling the high strength steel sheet having the excellent edge quality (without an edge crack) and the gauge uniformity to be stably produced.SOLUTION: The production method of the high strength steel sheet superior in the edge quality and the gauge uniformity, is provided in which a steel slab having a predetermined component composition is hot-rolled at a finish rolling finishing temperature of Arpoints or more, cooling is started within 2s after the hot rolling, the cooling is performed up to 600°C or less at an average cooling speed of 50°C/s or more, the slab is wound at a temperature of 400-550°C, cold rolling at a draft of 40% or more is performed, tension between respective rolling stands is set to 21.0 kgf/cmor more in the cold rolling, and a linear rolling load difference between the respective rolling stands except for the final rolling stand is also set to 0.25 tonf/mm or less.

Description

  The present invention relates to a method for producing a high-strength steel sheet suitable for automobile structural members, particularly a high-strength steel sheet having a tensile strength TS of 780 MPa or more and excellent edge quality and gauge uniformity.

  In recent years, for the purpose of ensuring the safety of passengers in the event of a collision and improving fuel efficiency by reducing the weight of the vehicle body, the application of high-strength steel sheets with a TS of 780 MPa or more and a thin plate thickness has been actively promoted ( For example, see Patent Document 1). In particular, recently, application of a high-strength steel sheet having an extremely high strength having TS of 980 MPa class and 1180 MPa class has been studied.

JP 2010-209392 A

  If the strength of the steel sheet is increased, edge cracks and gauge fluctuations are likely to occur during cold rolling. In the high-strength steel sheet described in Patent Document 1, no consideration is given to edge cracks and gauge fluctuations during cold rolling.

  The present invention has been made in view of the circumstances as described above, and is capable of stably producing a high-strength steel sheet having excellent edge quality (without edge cracks) and gauge uniformity, and edge quality and gauge. It aims at providing the manufacturing method of the high strength steel plate excellent in uniformity.

  In order to solve the above-mentioned problems, the present inventors have conducted intensive studies.

  In hot rolling of steel plates, the cooling rate of the edge (width end) of the steel plate is faster than the cooling rate of the width center (width center portion), and the edge of the steel plate (hot rolled steel plate) after hot rolling The hardness (strength) is larger than the hardness (strength) at the width center. Especially in the case of high-strength steel sheets, the hardness difference (strength difference) itself becomes considerably large. It was found that it was difficult to stretch compared to the center and edge cracks were likely to occur. In addition, the same phenomenon occurs in the longitudinal direction of the steel sheet, and in the case of a high-strength steel sheet, there is a hardness difference (strength difference) between the front and rear ends of the hot-rolled steel sheet (hot rolled steel sheet) and the central portion in the longitudinal direction. From the fact that it becomes considerably large, it was found that in the subsequent cold rolling, the front and rear end portions are less likely to be stretched than the central portion in the longitudinal direction, and gauge fluctuation (sheet thickness fluctuation) is likely to occur in the longitudinal direction.

  Therefore, in order to suppress edge cracks and gauge fluctuations (thickness fluctuations) that occur in the high-strength steel sheets as described above, in the steel sheets after hot rolling (the steel sheets before cold rolling), in the width direction and the longitudinal direction. It is necessary to reduce the hardness difference (strength difference), and in particular in the width direction, it is found that the strength difference (hardness difference) ΔTS between the width center strength TSc and the width edge strength TSe may be set as follows. It was.

    ΔTS <200 MPa (preferably <150 MPa)

  For that purpose, the idea was to reduce the hardness difference (strength difference) in the width direction and the longitudinal direction by using the microstructure of the steel sheet after hot rolling as a bainite-based structure. Specifically, it was found that the strength difference can be reduced by setting the bainite area ratio to 90% or more, the ferrite area ratio to 5% or less, and the pearlite area ratio to 5% or less.

  Moreover, in order to make edge quality favorable, it turned out that it is necessary to optimize the tension between each rolling stand as a cold rolling condition, and the rolling line load difference between each rolling stand except a final rolling stand.

  In this way, the present inventors stably produce high-strength steel sheets with excellent edge quality and gauge uniformity by optimizing the steel composition and hot rolling / cold rolling conditions. It has been found that this is possible and the present invention has been made.

  That is, the present invention has the following features.

[1] By mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.5%, Mn: 1.5 to 3.0%, P: 0.001 to 0.05% , S: 0.0001 to 0.01%, Al: 0.001 to 0.1%, N: 0.0005 to 0.01%, with the balance being composed of Fe and inevitable impurities The steel slab is hot-rolled at a finish rolling finish temperature of Ar ₃ or higher, starts cooling within 2 s after hot rolling, and cools to 600 ° C. or lower at an average cooling rate of 50 ° C./s or higher. Winding is performed at a temperature of 550 ° C., and cold rolling with a reduction rate of 40% or more is performed. In cold rolling, the tension between rolling stands is 21.0 kgf / cm ² or more and between each rolling stand excluding the final rolling stand. Edge quality, characterized in that the rolling line load difference is 0.25 ton / mm or less Method for producing a high strength steel sheet excellent in over-di uniformity.

  [2] The steel slab further contains, by mass%, Cr: 0.01 to 1.5%. The high-strength steel plate excellent in edge quality and gauge uniformity according to the above [1] Manufacturing method.

  [3] The steel slab further includes at least one of Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, and Cu: 0.01 to 2.0% by mass. The method for producing a high-strength steel sheet excellent in edge quality and gauge uniformity according to the above [1] or [2], comprising an element.

  In the present invention, a high-strength steel sheet having excellent edge quality and gauge uniformity can be stably produced.

  Details of the present invention will be described below. Note that “%” representing the content of component elements means “% by mass” unless otherwise specified.

(1) Component composition C: 0.05 to 0.2%
C is an important element for strengthening steel, has a high solid solution strengthening ability, and is an indispensable element for adjusting the hardness. If the C content is less than 0.05%, sufficient strength cannot be obtained. On the other hand, if the amount of C exceeds 0.2%, weldability deteriorates, and the formation of a segregation layer causes a decrease in formability. Therefore, the C content is 0.05 to 0.2%.

Si: 0.5 to 2.5%
Si needs to be 0.5% or more to obtain a sufficient strength by solid solution strengthening. Moreover, since the structure of the hot-rolled steel sheet is a bainite-based structure, the Si amount needs to be 0.5% or more. On the other hand, if the amount of Si exceeds 2.5%, the weldability deteriorates. Therefore, the Si content is 0.5 to 2.5%.

Mn: 1.5 to 3.0%
Mn is effective for preventing hot embrittlement of steel and ensuring strength by solid solution strengthening and structure strengthening. Moreover, it is effective in improving hardenability and making the steel sheet structure before cold rolling into a bainite single phase. However, in order to obtain these effects, the amount of Mn needs to be 1.5% or more. On the other hand, if the amount of Mn exceeds 3.0%, the steel sheet structure cannot be converted into a bainite single phase, and gauge fluctuation occurs. Therefore, the Mn content is 1.5 to 3.0%.

P: 0.001 to 0.05%
P is an element that has a solid solution strengthening action and can be added according to a desired strength. In order to obtain such an effect, the P amount needs to be 0.001% or more. On the other hand, if the amount of P exceeds 0.05%, weldability is deteriorated. Therefore, the P content is 0.001 to 0.05%.

S: 0.0001 to 0.01%
S segregates at the grain boundaries and embrittles the steel during hot working, and also exists as a sulfide and reduces local deformability. Therefore, the amount needs to be 0.01% or less, preferably 0.003% or less, more preferably 0.001% or less. However, the amount of S needs to be 0.0001% or more due to restrictions on production technology. Therefore, the S content is 0.0001 to 0.01%, preferably 0.0001 to 0.003%, more preferably 0.0001 to 0.001%.

Al: 0.001 to 0.1%
Al acts as a deoxidizer and is an effective element for the cleanliness of steel, and it is necessary to add 0.001% or more in the deoxidation step. On the other hand, when the Al content exceeds 0.1%, the surface properties are deteriorated. Therefore, the Al content is 0.001 to 0.1%.

N: 0.0005 to 0.01%
N is an element that degrades the aging resistance of steel. In particular, when the N content exceeds 0.01%, the deterioration of aging resistance becomes remarkable. The smaller the amount, the better. However, the amount of N needs to be 0.0005% or more due to restrictions on production technology. Therefore, the N amount is set to 0.0005 to 0.01%.

  The balance is Fe and inevitable impurities, but for the following reasons, Cr: 0.01 to 1.5%, Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: It is preferable that at least one element selected from 0.01 to 2.0% is contained alone or in combination.

Cr: 0.01 to 1.5%
Cr contributes to increasing the strength of the steel sheet by strengthening the structure, so it can be added as appropriate. In order to obtain such effects, the Cr amount needs to be 0.01% or more. On the other hand, if the amount of Cr exceeds 1.5%, the ratio of the second phase becomes too large, or Cr carbides are generated excessively, leading to a decrease in ductility. Therefore, the Cr content is 0.01 to 1.5%.

Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 2.0%, at least one kind Mo, Ni, and Cu serve as solid solution strengthening elements In addition, in the cooling process during annealing, the austenite phase is stabilized and contributes to increasing the strength of the steel sheet by forming a composite structure. In order to obtain such effects, the Mo amount, Ni amount, and Cu amount must each be 0.01% or more. On the other hand, if the Mo amount is 1.0%, the Ni amount is 2.0%, and the Cu amount exceeds 2.0%, the formability deteriorates. Therefore, the Mo amount is 0.01 to 1.0%, the Ni amount is 0.01 to 2.0%, and the Cu amount is 0.01 to 2.0%.

(2) Manufacturing conditions (2.1) Hot rolling Finish rolling finishing temperature: Ar ₃ points or more In order to make the microstructure of the steel sheet after hot rolling a bainite-based structure, it is necessary to finish the rolling in a single austenite phase. There is. For this reason, the finishing temperature of finish rolling is set to Ar ₃ points or more.

Cooling conditions after hot rolling: cooling to 600 ° C. or less at an average cooling rate of 50 ° C./s or more within 2 s When a time exceeding 2 seconds elapses before starting cooling after hot rolling, on the run out table A ferrite phase is likely to be generated non-uniformly, and a uniform microstructure of a hot-rolled steel sheet mainly composed of a bainite phase suitable in the present invention cannot be obtained. The same problem also occurs when the average cooling rate is less than 50 ° C./s or when the average cooling rate is not cooled to 600 ° C. or lower.

Winding temperature after hot rolling: 400-550 ° C
By setting the coiling temperature to 400 to 550 ° C., it is possible to obtain a microstructure of a hot rolled steel sheet mainly composed of a bainite phase suitable in the present invention.

  And according to said component composition and manufacturing conditions (hot rolling), the microstructure of the steel plate after hot rolling (steel plate before cold rolling) is made a bainite-based structure, the bainite area ratio is 90% or more, ferrite The area ratio can be 5% or less, and the pearlite area ratio can be 5% or less.

  In addition to bainite, ferrite, and pearlite, carbides such as martensite, tempered martensite, and cementite may be produced.If the above bainite, ferrite, and pearlite ratios are satisfied, the object of the present invention is achieved. Can be achieved.

(2.2) Cold rolling Rolling ratio during cold rolling: 40% or more When the rolling reduction ratio is less than 40%, it becomes the core of reverse transformation to the austenite phase when subsequently annealed. The total number of grain boundaries and dislocations per unit volume decreases, making it difficult to obtain a favorable microstructure in the final product.

Tension between each rolling stand at the time of cold rolling: 21.0 kgf / cm ² or more By setting the tension between each rolling stand at the time of cold rolling to 21.0 kgf / cm ² or more, the stretching amount in the width direction and the longitudinal direction can be increased. The difference is suppressed and the occurrence of edge cracks can be suppressed. The upper limit is preferably 23.0 kgf / cm ² or less from the viewpoint of preventing breakage due to over tension. The tension between the rolling stands can be controlled by rolling conditions such as the rolling reduction of each rolling stand.

Rolling line load difference between each rolling stand excluding the final rolling stand during cold rolling: 0.25 tonf / mm or less Rolling line load difference between each rolling stand excluding the final rolling stand during cold rolling is 0.25 tonf / By setting it to mm or less, it is possible to perform cold rolling without a rapid reduction fluctuation between the rolling stands. The rolling line load difference between the rolling stands can be controlled by rolling conditions such as the rolling reduction of each rolling stand. The reason for excluding the final rolling stand is that the occurrence of edge cracking cracks and gauge fluctuations causes a problem of the rolling load difference between the rolling stands on the front side of the final rolling stand.

  As described above, in the present invention, by optimizing the component composition and the manufacturing conditions (hot rolling, cold rolling), fluctuations in the tensile strength in the width direction are reduced and edge cracks are generated. In addition, it is possible to manufacture a high-strength steel sheet with greatly reduced plate thickness fluctuation (gauge fluctuation).

  As an example of the present invention, a 980 MPa class high strength steel sheet was manufactured.

  Steel No. 1 having the component composition shown in Table 1. A to G were melted by a converter, made into a slab by a continuous casting method, hot-rolled under the hot rolling conditions shown in Table 2, and then cold-rolled under the cold rolling conditions shown in Table 2.

In the hot rolling, cooling was started within 2 s after finishing the finish rolling, and after cooling to 600 ° C. at an average cooling rate shown in Table 2, winding was performed. The rolling reduction of cold rolling was 40%. Ar ₃ (° C.) in Table 1 was calculated by the following formula.

Ar ₃ (° C.) = 868−396 × [C] + 25 × [Si] −68 × [Mn] −21 × [Cu] −36 × [Ni] −25 × [Cr] −30 × [Mo]
Here, [M] represents the content (% by mass) of the element M in the steel.

  Moreover, in Table 2, the line load difference between stands shows the maximum value of the rolling line load difference of # 1- # 4 rolling stands.

  And the area ratio of ferrite, pearlite, and bainite with respect to the obtained hot-rolled steel sheet was corroded with 3% nital after polishing the plate thickness section parallel to the rolling direction of the steel plate, and the plate thickness 1/4 position ( 10 positions at a magnification of 2000 times using a SEM (scanning electron microscope) with respect to the steel sheet surface in a depth direction corresponding to ¼ of the plate thickness, and using the obtained tissue image, Media The area ratio of each structure (ferrite, pearlite, bainite) was calculated for 10 visual fields using Imagenet-Pro of Cybernetics, and the values were averaged. In the above structure image, ferrite has a gray structure, pearlite has a layered structure of ferrite and cementite (white), and bainite has a slightly white structure than ferrite.

  Moreover, the tensile test was performed based on JISZ2241 (1998) using the JIS5 test piece which sampled so that the tension direction might follow the rolling direction of a steel plate, and TS (tensile strength) was measured. The sample for the tensile test was taken from the position of the width center and 20 mm from the edge. Edge position TS−width center TS was set to ΔTS.

  Furthermore, about the edge crack with respect to the steel plate after the cold rolling obtained above, the operator visually confirmed both edge portions in the coil longitudinal direction. Moreover, the plate | board thickness of the width | variety center part was measured with the plate | board thickness meter (X-ray), and the maximum gauge fluctuation | variation amount was computed from the deviation with the cold rolling target plate | board thickness.

  The results are shown in Table 3.

  It can be seen that the steel sheets after cold rolling in the examples of the present invention are all high-strength steel sheets having no edge cracks and a maximum gauge fluctuation amount of 50 μm or less, and having excellent edge quality and gauge uniformity. In addition, after annealing, all the steel plates of the present invention examples have a TS of 989 to 1009 MPa, and a high strength steel plate of 980 MPa class is obtained.

  This confirmed the effectiveness of the present invention.

Claims (3)

In mass%, C: 0.05 to 0.2%, Si: 0.5 to 2.5%, Mn: 1.5 to 3.0%, P: 0.001 to 0.05%, S: A steel slab containing 0.0001 to 0.01%, Al: 0.001 to 0.1%, N: 0.0005 to 0.01%, with the balance being composed of Fe and inevitable impurities The finish rolling finish temperature is hot-rolled at Ar ₃ point or higher, starts cooling within 2 s after hot rolling, cools to 600 ° C. or lower at an average cooling rate of 50 ° C./s or higher, and 400 to 550 ° C. Winding is performed at a temperature, and cold rolling is performed at a rolling reduction of 40% or more. In cold rolling, the tension between rolling stands is 21.0 kgf / cm ² or more, and the rolling line load between the rolling stands excluding the final rolling stand. Edge quality and gauge characterized by a difference of 0.25 tonf / mm or less Method for producing a high strength steel sheet excellent one property.   The method for producing a high-strength steel sheet excellent in edge quality and gauge uniformity according to claim 1, wherein the steel slab further contains Cr: 0.01 to 1.5% by mass.   The steel slab further contains at least one element of Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, and Cu: 0.01 to 2.0% by mass%. The manufacturing method of the high strength steel plate excellent in edge quality and gauge uniformity of Claim 1 or 2 characterized by the above-mentioned.