JP4438614B2 - High-strength hot-rolled steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and surface properties and having a tensile strength of 590 MPa or more, and a method for producing the same.

  With the recent trend toward lower fuel consumption in the automobile industry, from the viewpoint of reducing vehicle weight, steel sheets, which are the main structural material, are required to be thinner and stronger. At the same time, in order to reduce the manufacturing cost of automobiles, inexpensive hot-rolled steel sheets are used for parts that are not directly visible from the outside, such as lower parts such as lower arms. Since such parts are subjected to hole expansion after blanking in addition to normal press molding, steel sheets are often required to have high strength and at the same time ductility and stretch flangeability (local ductility).

  Conventionally, as a steel sheet having high ductility and high strength, dual phase steel (hereinafter referred to as “DP steel”) in which a martensite structure is dispersed in a ferrite structure is known. However, DP steel is known to be inferior in stretch flangeability because the interface between the two becomes a starting point of microcracks and voids during hole expansion due to the hardness difference between the ferrite phase and the martensite phase.

In Patent Document 1, steel excellent in stretch flangeability due to a two-phase structure of ferrite and bainite is proposed, but Cr is an essential addition, and there remains a problem in chemical conversion treatment.
Patent Document 2 and Patent Document 3 propose steel having a steel structure with strength mainly composed of fine bainite and stretch flangeability. However, these steels have a problem in ductility because high strength is realized by a bainite phase having a high dislocation density.

  On the other hand, Patent Document 4 proposes a steel plate having excellent stretch flangeability composed of three phases of ferrite, bainite, and martensite. According to this, it is said that a high strength steel having both stretch flangeability and ductility can be obtained, but there is a fear that the steel plate performance becomes unstable due to the variation in the volume fraction of the three phases.

  Further, Patent Document 5 discloses a steel whose ferrite single-phase structure is strengthened with Mo carbide or metal carbide containing Mo, but it is a problem in terms of manufacturing cost because it is essential to add expensive Mo. There is.

Furthermore, for structural high-tensile steel sheets having a strength of 590 MPa or more, an appropriate amount of Si is often added from the viewpoint of both strength and ductility, but when a steel material containing Si is heated, FeO covering the steel sheet surface and base steel and FeO-Fe ₂ SiO ₄ eutectic compound layer near the interface is formed, two are intricately engaged, because it can not be completely removed by conventional descaling, during the rolling process Fe ₂ O ₃ becomes a scale (island scale) that is unevenly distributed on the surface of the steel material. This island scale causes uneven cooling temperature of the steel sheet in the cooling process of the steel sheet after finish rolling, impairs the stability of the steel sheet characteristics in the coil, and as an indented scale ridge (island scale ridge) after pickling Since it remains, it tends to be the starting point of fatigue failure. Moreover, in the steel plate which does not perform pickling (a steel plate with a scale), an island-like scale is not removed but it remains on the steel plate surface as a red scale, and surface properties are impaired.

  Patent Document 6 discloses a method for improving the surface properties by uniformly generating scale on the entire surface of the steel sheet by adding a large amount of Si to the steel sheet, but excessive Si addition is applied to the structural material for automobiles. In contrast, there is a problem that the corrosion resistance is deteriorated because the chemical conversion processability is deteriorated. On the other hand, if the amount of Si is decreased, it is difficult to uniformly generate the scale on the entire surface, and the surface property is deteriorated. There was a problem that decreased.

JP-A-57-101649 JP 2000-109951 A JP 2003-112204 A JP-A-6-293910 JP 2002-322540 A Japanese Patent Laid-Open No. 3-79718

  An object of the present invention is an inexpensive hot-rolled steel sheet having a tensile strength of 590 MPa or more, excellent in ductility and stretch flangeability, and substantially free of island-like scales and red scales on the steel sheet surface, and its It is to provide a manufacturing method.

  As a result of intensive studies to solve the above problems, it has been found that by making the area ratio of ferrite in the steel sheet structure 90% or more, the stretch flangeability can be greatly improved and at the same time compatible with high ductility. .

  Therefore, we focused on precipitation strengthening by fine dispersion of metal carbide as a means to increase the strength of the originally soft ferrite single-phase structure, and further investigated the component system and manufacturing conditions that achieve both strength, ductility and stretch flangeability. As a result, a steel material containing Ti and Nb is substantially composed of a ferrite single phase generated in a temperature range of 600 to 780 ° C. after hot finish rolling, and Ti or Nb single or composite carbonized precipitates are finely dispersed inside. It was found that the obtained steel has an excellent balance between strength, ductility and stretch flangeability.

  Furthermore, the island-shaped scale that was difficult to remove with conventional Si-added steels by descaling the rough bar after rough rolling to an appropriate temperature by reheating etc. according to the addition balance of Si, P, and Al in the steel The present invention was completed by obtaining the knowledge that can be easily removed from the steel sheet surface. Therefore, the present invention is as follows.

  (1) Steel composition is mass%, C: 0.02-0.18%, Si: 0.01-1.5%, Mn: 0.2-2.0%, P: 0.10% Hereinafter, S: 0.007% or less, Al: 0.02 to 2.0%, N: 0.01% or less, Ti: 0.01 to 0.3%, and Nb: 0.01 to 0.1% The balance is composed of Fe and impurities, and satisfies the following formulas (1) and (2). The steel structure contains ferrite in an area ratio of 90% or more, and has mechanical properties. However, tensile strength: 590 MPa or more, product of tensile strength (MPa) and hole expansion rate (%): 65000 MPa ·% or more, product of tensile strength (MPa) and total elongation (%): 15400 MPa ·% or more A high-strength hot-rolled steel sheet characterized by substantially not having island-shaped scales and red scales on the steel sheet surface.

0.8 ≦ (C / 12) / (Ti / 48 + Nb / 93) ≦ 3.0 (1)
0.1 ≦ (Al + 5P) / Si (2)
Here, C, Ti, Nb, Al, P, and Si in the formula represent the content (unit: mass%) of each element.

  (2) The steel composition is one type selected from the group consisting of Ca: 0.0010 to 0.0070% and Mg: 0.0010 to 0.0070% in mass% instead of part of Fe Alternatively, the hot-rolled steel sheet according to (1) above, which contains two kinds.

(3) By mass%, C: 0.02 to 0.18%, Si: 0.01 to 1.5%, Mn: 0.2 to 2.0%, P: 0.10% or less, S: 0.007% or less, Al: 0.02-2.0%, N: 0.01% or less, Ti: 0.01-0.3% and Nb: 0.01-0.1%, and the balance Fe and impurities, and the Sonetsu rolling a steel ingot or steel slab after the temperature T _SRT defined in the following formula (3) having a steel composition satisfying the following (1) and (2) Applying a rough bar, and finishing the rolling bar at the Ar ₃ point or higher after descaling the rough bar in a state where the obtained rough bar is at the temperature T _BRT defined by the following equation (4) Hot rolling to obtain a hot rolled steel sheet, and the average cooling rate of 30 ° C./second or more after completion of finish rolling of the obtained hot rolled steel sheet 600-780 cooled to a temperature range of ° C., then the temperature range in to stay more than 10 seconds 180 seconds or less, the method of producing a high strength hot-rolled steel sheet shall be the features to take subsequent winding.

0.8 ≦ (C / 12) / (Ti / 48 + Nb / 93) ≦ 3.0 (1)
0.1 ≦ (Al + 5P) / Si (2)
T _SRT ≧ 1600 + 100ln (Ti × C) (3)
T _BRT ≧ 1050-36.4ln ((Al + 5P) / Si) (4)
Here, C, Ti, Nb, Al, P and Si in the formula represent the content of each element (unit: mass%), and the unit of T _SRT and T _BRT is ° C.
(4) The steel composition is one type selected from the group consisting of Ca: 0.0010 to 0.0070% and Mg: 0.0010 to 0.0070% in mass% instead of part of Fe. Alternatively, the method for producing a high-strength hot-rolled steel sheet according to (3) above, which contains two types.

Here, the “ferrite” in the present invention refers to a BCC structural phase excluding a BCC structural phase in a bainite structure having a lath shape.
Further, in the present invention, “substantially free of island-scale scales or red scales” means that after hot rolling, the surface is not subjected to cutting, mechanical grinding and surface removal by surface cutting. The area ratio of the island scale ridges and the red scale for portions excluding the portions of 100 mm from the width direction end and 30 m of the length direction end is 3% or less for each of the front and back surfaces of the hot-rolled steel sheet. Say something. Accordingly, steel plates that are sheared, trimmed, or slit-processed using such a hot-rolled steel plate as a raw material are also “substantially free of island-like scale ridges or red scales” and are included in the present invention.

The dimensions of the hot-rolled steel sheet in the present invention are preferably those having a width of 1000 mm or more and a length of 200 m or more.
The “temperature” in the present invention is the surface temperature of the steel material.

  The steel sheet of the present invention has excellent surface properties, high strength and high ductility, and excellent stretch flangeability. Therefore, it is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for structural members represented by automobile members and undercarriage parts.

  In addition, since it can be manufactured stably and in large quantities at a low cost, it is possible to achieve a remarkable industrial effect.

Next, the reason why the present invention is limited as described above will be described. In this specification, “%” indicating “steel composition” is “mass%”.
[Steel composition]
C: 0.02-0.18%
In order to secure a tensile strength of 590 MPa or more, the lower limit of the C content is 0.02%. On the other hand, excessive addition of C delays the ferrite transformation after rolling and impairs stretch flangeability by lowering the ferrite volume fraction, so the upper limit of the C content is 0.18%. The preferred lower limit is 0.03% and the preferred upper limit is 0.1%.

Ti: 0.01 to 0.3%
Ti is an important element in the present invention. By adding Ti, N in the steel can be fixed and stretch flangeability can be improved, and it is effective for precipitation strengthening of steel by combining with C to become fine TiC carbide. For this reason, the lower limit of the Ti content is 0.01%. A preferred lower limit is 0.05%. On the other hand, if added excessively, coarse carbonitrides are formed in austenite and the mechanical properties of the steel sheet are deteriorated. For this reason, the upper limit of the Ti content is set to 0.3%. A preferred upper limit is 0.2%

Nb: 0.01 to 0.1%
Nb is an important element in the present invention. By adding Nb, Nb (C, N) is formed in the steel, austenite grains are refined to increase the nucleation sites of ferrite, and the coarsening of the metal structure is suppressed. For this reason, the minimum of Nb content shall be 0.01%. On the other hand, excessive Nb addition forms coarse Nb (C, N) in the slab and impairs the mechanical properties of the steel sheet. For this reason, the upper limit of the content is set to 0.1%. A preferable upper limit is 0.05%.

(C / 12) / (Ti / 48 + Nb / 93): 0.8 to 3.0
When the value of (C / 12) / (Ti / 48 + Nb / 93) is less than 0.8, the addition efficiency of Ti and Nb for improving the strength is low, resulting in cost loss. On the other hand, if the value of (C / 12) / (Ti / 48 + Nb / 93) exceeds 3.0, the stretch flangeability deteriorates due to coarse cementite. Therefore, C, Ti and Nb are contained so as to satisfy the following formula (1).
0.8 ≦ (C / 12) / (Ti / 48 + Nb / 93) ≦ 3.0 (1)
Preferably the range of formula (1) is 0.9 to 2.5.

Mn: 0.2 to 2.0%
It is an element effective for improving the strength by solid solution strengthening, and also lowers the transformation temperature from austenite to ferrite, so that the finishing temperature in hot rolling can be lowered, and the strength can be improved by refining the ferrite grain size. Is an effective element. For this reason, the lower limit of the Mn content is 0.2%. Excess addition delays ferrite transformation after finish rolling and promotes formation of a hard second phase, so the upper limit of the Mn content is 2.0%. From the viewpoint of balance between strength and stretch flangeability, the preferred lower limit is 1.0%, and the preferred upper limit is 1.5%.

Si: 0.01 to 1.5%
Si is an element that enables strength to be increased while maintaining a relatively large ductility, and further has an effect of promoting ferrite transformation and suppressing the coarsening of cementite in steel. For this reason, the lower limit of the Si content is set to 0.01%. From the viewpoint of strength and ductility, the content is preferably 0.2% or more. Excessive addition impairs the chemical conversion treatment with phosphoric acid to zinc phosphate applied to the product surface, so the upper limit of the content is 1.5%. From the viewpoint of balance between strength and ductility, the lower limit of the preferable content is 0.2%, and the upper limit of the preferable content is 1.3%.

Al: 0.02 to 2.0%
Al is an important element in the present invention. In order to have substantially no island-like scales and red scales on the steel sheet surface, it is necessary to remove the island-like scales by descaling the coarse bar after rough hot rolling at a predetermined temperature. Al has the effect of lowering the predetermined temperature. In order to perform the descaling at the predetermined temperature, it is generally effective to heat the rough bar after rough rolling. In such a case, it is possible to lower the heating temperature of the rough bar by adding Al. And effective in ensuring descale stability and cost reduction. For this reason, the lower limit of the Al content is set to 0.02%.

On the other hand, excessive addition causes the Ar ₃ point to be raised excessively, leading to an increase in finishing temperature and impairing the production stability, and the stretch flangeability deteriorates due to coarse inclusions containing Al. For this reason, the upper limit of Al content is made 2.0%.

  Furthermore, Al has the effect of promoting ferrite transformation, and it is difficult to cause degradation of stretch flangeability due to coarsening of precipitates, or by slow cooling, air cooling or holding at a constant temperature for a short time with a relatively low manufacturing load. It is an effective element that makes it possible to obtain a desired sufficient ferrite phase. For this reason, it is desirable to make it contain in the range of more than 0.05 to 0.3% in order to reduce the manufacturing load and achieve both strength and stretch flangeability. More desirably, it is 0.12 to 0.3%.

P: 0.10% or less In order to contribute to securing the strength, it may be added. However, since it segregates at the grain boundary to cause embrittlement, the upper limit of the content is 0.10%.

(Al + 5P) / Si: 0.1 or more Si causes the generation of island scales due to the formation of FeO—Fe ₂ SiO ₄ during heating. On the other hand, Al and P have the effect of improving the scale peelability when descaling the coarse bar by lowering the melting point of Fe ₂ SiO ₄ and finally suppressing the island scale flaws or red scale on the steel sheet surface. Have. Considering the descaling temperature in actual operation, Si, Al, and P are further contained so as to satisfy the following formula (2) in order not to have island-like scales or red scales.
0.1 ≦ (Al + 5P) / Si (2)

Ca: 0.0010 to 0.0070, Mg: 0.0010 to 0.0070
Addition of Ca and / or Mg improves the balance between strength and stretch flangeability, so it may be added. If the content is less than 0.001%, the effect cannot be obtained. Therefore, when added, the lower limit of the content of at least any one element is set to 0.001%. On the other hand, when the content exceeds 0.0070%, the efficiency with respect to the added amount is lowered and the cost in the refining process is increased. For this reason, the upper limit of content is 0.0070%, respectively.

S: 0.007% or less Since it is an impurity that binds with Mn or the like to form coarse sulfide inclusions and significantly impairs workability, the content is made 0.007% or less. More preferably, it is 0.003% or less.

N: 0.01% or less An impurity element that impairs workability, and its content is 0.01% or less. More preferably, it is 0.006% or less.

[Organization]
The hot-rolled steel sheet according to the present invention contains 90% or more of ferrite in area ratio with respect to the entire structure. If the area ratio of ferrite is lower than 90%, stretch flangeability is significantly deteriorated. Considering the balance between stretch flangeability and ductility, the area ratio of ferrite is preferably 95% or more. In the remaining structure, bainite, cementite, etc. remain. They are at most 10%, usually 2-3%, and do not substantially affect the properties.

[Production conditions]
Temperature of steel ingot or billet before rough hot rolling:
The steel ingot or steel slab before rough hot rolling dissolves coarse carbonitride that inhibits stretch flangeability, and in order to avoid deterioration of mechanical properties such as strength reduction and stretch flangeability, Ti and The temperature is equal to or higher than the T _SRT defined by the following formula (3) according to the C content.
T _SRT ≧ 1600 + 100ln (Ti × C) (3)

Steel ingots or steel _slabs at temperatures below T _SRT must be heated to temperatures above T _SRT before rough hot rolling, but this step involves dissolving solid carbonitrides as described above. Therefore, when the steel ingot obtained by continuous casting is subjected to rough hot rolling without cooling to less than T _SRT , heating is not necessary. Although the upper limit of T _SRT is not particularly defined, the upper limit is preferably set to 1400 ° C. from the viewpoint of durability of the heat resistant wall in the furnace and yield due to scale loss.

Coarse bar temperature during descaling:
The rough bar after the rough hot rolling has the following (4) depending on the contents of Si, Al and P in the steel so that the steel plate surface does not substantially have island-like scales and red scales. Descaling is performed at the temperature T _BRT defined by the equation.
T _BRT ≧ 1050-36.4ln ((Al + 5P) / Si) (4)

For Si-containing steels, a FeO-Fe2SiO4 eutectic compound layer that is difficult to remove (descaling) during heating in the hot rolling process is formed on the steel sheet surface, and then becomes an island scale by cooling. By making the coarse bar at a temperature equal to or higher than the above T _BRT according to the contents of Si, Al and P, FeO and Fe ₂ SiO ₄ are in a semi-molten state, and peeling removal (descaling) becomes easy. is there.

Although there is no particular limitation on the means for bringing the rough bar to a temperature equal to or higher than the above-mentioned B _BRT , industrially, for example, the use of an induction heating device or the like installed between the rough rolling roll stand group and the finish rolling roll stand group may be used. Suitable. Of course, the heating temperature of the steel slab or ingot before rough hot rolling may be set to a high temperature to ensure a temperature _{equal to} or higher than the above T _BRT, but the load on the heating furnace increases and the fuel cost increases. Therefore, a coarse bar heating apparatus as in the above example is desirable. There are no particular restrictions on the descaling means / equipment, but in practice, injection of high-pressure water by spraying is suitable.

The finish rolling is performed in the austenite temperature range, and the finish temperature of the finish rolling only needs to be Ar ₃ point temperature or higher. The finish rolling finish temperature is more preferably as close as possible to the Ar ₃ point temperature. This is because the effect of accelerating ferrite transformation due to processing strain introduced during rolling is enhanced. The Ar ₃ point of the steel defined in the present invention is approximately 750 ° C. to 980 ° C.

After finishing the finish rolling, the steel sheet is cooled to a temperature range of 600 to 780 ° C. at an average cooling rate of 30 ° C./sec or more, and then stayed in the temperature range for more than 10 seconds and 180 seconds or less to obtain a steel plate.
The reason for prescribing the cooling rate is to avoid this when the average cooling rate is slower than 30 ° C./sec, because the stretch flangeability deteriorates due to pearlite precipitation during cooling.

  The reason why the cooling stop temperature is set to a temperature range of 600 to 780 ° C. is that when cooling is stopped at a temperature higher than 780 ° C., the metal carbide precipitated in the ferrite is coarsened, and the balance between strength and stretch flangeability deteriorates. The reason why the lower limit is set to 600 ° C. is that when cooling is stopped at a temperature lower than this, a bainite phase is formed and ductility deteriorates. In view of the balance between strength and stretch flangeability, the stop temperature is more preferably 650 ° C. or higher and 730 ° C. or lower.

  With respect to the staying time in the temperature range, if the staying time is 10 seconds or less, a sufficient amount of ferrite cannot be obtained, and ductility and stretch flangeability deteriorate, so that the staying time exceeds 10 seconds. However, if the stay exceeds 180 seconds, coarse carbides or carbonitrides are formed, and the balance between strength and stretch flangeability is impaired. Preferably it is 35 seconds or less.

  In many cases, during the production using an actual production line, the hot-rolled steel sheet is wound into a steel strip, and the winding temperature at this time is 500 ° C. or less from the viewpoint of mechanical property balance and operational stability. It is desirable to do. When it winds up at temperature higher than 500 degreeC, the metal carbide in steel will grow coarsely and stretch flangeability will deteriorate.

Of steels having the steel composition shown in Table 1, steel types A, B, D, and F were made into slabs by continuous casting, and then steel pieces were collected from the vicinity of the center of the slab and hot forged to a thickness of 30 mm. Thereafter, the slab heating temperature was changed variously, and the slab heated at each temperature was hot-rolled with a small test tandem mill, and the finished rolling temperature was set to the Ar ₃ temperature or more of each steel type. A 6 mm steel plate was obtained.

  The steel sheet after finish rolling is immediately water-cooled, cooled and stopped within the range defined by the present invention, and then cooled to a predetermined temperature again, and then simulated the heat history after winding the actual hot-rolled coil. Winding simulation was performed with a slow cooling furnace to obtain a hot rolled steel sheet.

  Regarding the mechanical properties of the obtained steel sheet, the tensile properties and stretch flangeability were investigated by the following methods. Tensile properties are measured with JIS No. 5 test pieces. Stretch flangeability is 100 mm square test pieces. A punched hole with a clearance of 12% and a diameter of 10 mm is formed in the center, and a conical punch with a tip angle of 60 ° is used. This hole was expanded and evaluated by a limiting hole expansion rate calculated from a hole diameter when a crack generated at the edge of the hole penetrated in the plate thickness direction. Moreover, the cross section in the rolling direction was etched, and the structure was observed with a microscope.

Table 2 shows the results of investigating the influence of the slab heating temperature on the mechanical characteristics described above, but compared with the trial numbers 1, 2, 5, and 6 in which the slab heating was performed at a temperature satisfying the T _SRT defined in the present invention. , Run No. 3 and 4 were carried out slab heating at temperatures below T _SRT is low strength, poor mechanical properties balance.

Of steels having the chemical composition shown in Table 1, steel types A, B, C, D, E, F, G and steel types H, I and K were made into slabs by continuous casting, and then steel pieces were collected from the vicinity of the slab center. Then, hot forging was performed to a thickness of 30 mm. Thereafter, the slab is heated at a temperature that satisfies the _TSRT defined by the present invention, and then rolled with a test tandem mill at a finish rolling temperature higher than the Ar ₃ point of each steel grade, and a thickness of 2.6 mm. Steel plate was obtained.

  Immediately after finish rolling, cool with water spray, cool to the predetermined cooling stop temperature, stay at that temperature for a predetermined time, cool to the predetermined temperature again, and after winding the actual hot rolled coil Winding simulation was performed with a slow cooling furnace simulating the heat history of a hot rolled steel sheet.

  Regarding the mechanical properties of the obtained steel sheet, the tensile properties and stretch flangeability were investigated by the following methods. Tensile properties are measured with JIS No. 5 test pieces. Stretch flangeability is 100 mm square test pieces. A punched hole with a clearance of 12% and a diameter of 10 mm is formed in the center, and a conical punch with a tip angle of 60 ° is used. This hole was expanded and evaluated by a limiting hole expansion rate calculated from a hole diameter when a crack generated at the edge of the hole penetrated in the plate thickness direction. Moreover, the cross section in the rolling direction was etched, and the structure was observed with a microscope.

  The results are summarized in Table 3. According to Table 3, the test number 7 cooled after rolling at a cooling rate outside the claimed range of the present invention has a reduced strength and does not satisfy the strength specification of the present invention. Moreover, compared with the steel plate obtained by the trial numbers 8-16 according to the claims of the present invention exhibiting good strength × hole expansion rate value and strength × total elongation value, the steel plates according to the trial numbers 17-22 are stronger. × The hole expansion rate value is low and the strength / stretch flangeability balance is poor. In particular, with respect to the cooling stop temperature and the residence time, the trial numbers 17 to 19 manufactured outside the scope of the present invention did not reach the 90% ferrite area ratio defined in the present invention, and compared with the above-described present invention examples. The expansion rate is low, and the strength × hole expansion rate value is significantly degraded.

Of the steels having the chemical composition shown in Table 1, steel types A, B, D, E, F, and steel type J were made into slabs by continuous casting, and then steel plates were manufactured on an actual production line. After slab heating at a temperature higher than _TSRT defined in the present invention, the slab extracted from the heating furnace was roughly rolled by a tandem mill to obtain a rough bar. The rough bar is reheated to a specified temperature (rough bar temperature) with a heating device installed between the rough rolling mill group and the finish rolling mill group, and then finish rolling is performed at an Ar ₃ temperature or higher. Thus, a hot rolled steel sheet having a thickness of 2.6 mm and a width of 1200 mm was obtained. The steel sheet after finish rolling was water-cooled and air-cooled at a predetermined temperature (cooling stop temperature) for a predetermined time (residence time), and then water-cooled again to 500 ° C. or lower, and wound with a down coiler to obtain a steel sheet coil.

  Regarding the coverage of island scale ridges and red scales, island scale ridges are indented ridges formed by removing the red scale after the steel plate pickling, so the red scale of the steel plate before pickling and the island shape after pickling The position and area ratio of scale ridges are almost the same. Therefore, if the red scale area ratio before pickling is investigated, the area ratio of island scale ridges after pickling can be obtained. The coverage of red scale on the surface before pickling was investigated, and the coverage of island scale ridges and red scale was obtained.

  In the obtained hot rolled steel sheet before pickling, the coverage of the steel sheet surface consisting of a portion excluding 100 mm from the end in the rolling width direction and a portion excluding 30 m from the start and end in the longitudinal direction rolling is 3%. The following cases were judged as good (surface properties in Table 4), and when over 3%, they were judged as poor (surface properties x in Table 4). The survey was conducted on the rolling front and back surfaces, and the surface properties were judged on the surface with the higher red scale coverage.

Furthermore, JIS5 tensile test specimens were collected from the hot-rolled steel sheet after surface observation, and the tensile characteristics were investigated. Also, for stretch flangeability, a 100 mm square test specimen was collected, and the diameter was 12% at the center with a clearance of 12%. With a 10mm punched hole, this hole is expanded with a conical punch with a tip angle of 60 °, and the critical hole expansion rate calculated from the hole diameter when the crack generated at the edge of the hole penetrates in the thickness direction. evaluated. Moreover, according to Table 4 which etched the cross section of the rolling direction and observed the structure | tissue with the microscope, according to this invention, the trial numbers 23-27 which implemented manufacture with the actual manufacturing line are good intensity | strength x hole expansion rate value and The test number 28 and the test number 29 out of the range show the strength × total elongation value, but the ferrite area ratio does not reach 90% and the strength × hole expansion rate value is low and the strength × total elongation value is also low. Inferior in strength / stretch flangeability balance and strength / ductility balance.

In addition, the trial numbers 30 to 33, which were carried out at a coarse bar heating temperature lower than the T _BRT specified in the present invention, remained on the steel plate surface with a red scale exceeding 3%, and were inferior in surface properties, but according to the present invention. The trial numbers 23 to 27 manufactured under the conditions have substantially no red scale on the surface of the steel sheet, have good surface properties, and realize strength / ductility balance and stretch flangeability balance.

Claims (4)

Steel composition is mass%, C: 0.02-0.18%, Si: 0.01-1.5%, Mn: 0.2-2.0%, P: 0.10% or less, S : 0.007% or less, Al: 0.02 to 2.0%, N: 0.01% or less, Ti: 0.01 to 0.3% and Nb: 0.01 to 0.1% And the balance Fe and impurities, and satisfying the following formulas (1) and (2), the steel structure contains 90% or more of ferrite by area ratio, and the mechanical properties are tensile Strength: 590 MPa or more, product of tensile strength (MPa) and hole expansion rate (%): 65000 MPa ·% or more, product of tensile strength (MPa) and total elongation (%): 15400 MPa ·% or more, hot After rolling, the surface is not subjected to cutting, mechanical grinding of the surface, and surface removal by hot-melting In this regard, the area ratios of the island scale ridges and the red scale for the portions excluding the portions of 100 mm from the width direction end and 30 m of the length direction end are 3% for each of the front and back surfaces of the hot-rolled steel sheet. high-strength hot-rolled steel sheet, characterized in der Rukoto below.
0.8 ≦ (C / 12) / (Ti / 48 + Nb / 93) ≦ 3.0 (1)
0.1 ≦ (Al + 5P) / Si (2)
Here, C, Ti, Nb, Al, P, and Si in the formula represent the content (unit: mass%) of each element. The steel composition is one or two selected from the group consisting of Ca: 0.0010 to 0.0070% and Mg: 0.0010 to 0.0070% by mass% instead of part of Fe. The high-strength hot-rolled steel sheet according to claim 1, comprising: In mass%, C: 0.02 to 0.18%, Si: 0.01 to 1.5%, Mn: 0.2 to 2.0%, P: 0.10% or less, S: 0.007 %: Al: 0.02 to 2.0%, N: 0.01% or less, Ti: 0.01 to 0.3% and Nb: 0.01 to 0.1%, the balance Fe and consists impurities, and the following (1) and (2) subjected to Sonetsu rolling a steel ingot or slab having a steel composition which satisfies after the temperature T _SRT defined in the following (3) the crude Finishing hot rolling in which the rolling completion temperature is Ar ₃ or higher after descaling the rough bar in a state where the obtained rough bar is at the temperature T _BRT defined by the following equation (4). To obtain a hot-rolled steel sheet, and after the finish rolling of the obtained hot-rolled steel sheet is completed, the average cooling rate of 30 ° C./second or more is 60. 780 is cooled to a temperature range of ° C., then the temperature range in to stay more than 10 seconds 180 seconds or less, the method of producing a high strength hot-rolled steel sheet shall be the features to take subsequent winding.
0.8 ≦ (C / 12) / (Ti / 48 + Nb / 93) ≦ 3.0 (1)
0.1 ≦ (Al + 5P) / Si (2)
T _SRT ≧ 1600 + 100ln (Ti × C) (3)
T _BRT ≧ 1050-36.4ln ((Al + 5P) / Si) (4)
Here, C, Ti, Nb, Al, P and Si in the formula represent the content of each element (unit: mass%), and the unit of T _SRT and T _BRT is ° C. The steel composition is one or two selected from the group consisting of Ca: 0.0010 to 0.0070% and Mg: 0.0010 to 0.0070% by mass% instead of part of Fe. The manufacturing method of the high intensity | strength hot-rolled steel plate of Claim 3 characterized by the above-mentioned.