JP2016211047A - Hot rolled steel sheet and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet having a characteristic specifically, a tensile strength of 440 MPa or more and less than 590 MPa, a yield ratio (YR) which is a ratio between tensile strength and 0.2% proof stress, of 80% or more, a total elongation (EI) of 30% or more and a hole expanding ratio (λ) of 60% or more.SOLUTION: The hot rolled steel sheet is provided that is composed of: a chemical composition containing, by mass%, C:0.06 to 0.15%, Si:0.01 to 0.5%, Mn:0.1 to 0.5%, P:0.03% or less, S:0.02% or less, sol.Al:0.001 to 0.5% and N:0.01% or less; and steel structure in which, by area ratio, ferrite occupies 90% or more of the structure and the balance occupies 5% or less and is composed of at least one kind of cementite, martensite or bainite, and in which percentage of acicular ferrite in the ferrite is 10% or more and average particle diameter of the ferrite is 10 μm or less. The hot rolled steel sheet has a mechanical characteristic specifically, a tensile strength of 440 MPa or more and less than 590 MPa, a yield ratio (YR) which is a ratio between tensile strength and 0.2% proof stress, of 80% or more, a total elongation (EI) of 30% or more and a hole expanding ratio (λ) of 60% or more.SELECTED DRAWING: None

Description

  The present invention relates to a hot-rolled steel sheet and a method for producing the same. More specifically, the present invention has a tensile strength of 440 MPa or more and less than 590 MPa, yield ratio (YR) of 80% or more, total elongation (El) of 30% or more, and hole expansion ratio (λ) of 60%. The present invention relates to a hot-rolled steel sheet having the above mechanical properties and a method for producing the same.

In recent years, increasing the strength of automobile body parts has been promoted from the viewpoint of improving fuel efficiency by reducing the weight of the vehicle body for the purpose of reducing CO ₂ emissions and stricter safety standards for collisions. Further, from the viewpoint of resource saving, it is desired to obtain high strength with a small alloy addition amount in the steel plate as the material of the body parts.

  Against this background, the shape of body parts is diversifying. For this reason, there is an increasing demand for hot-rolled steel sheets having not only high strength but also various properties such as press formability, weldability, and collision resistance. Excellent impact resistance is indispensable especially for undercarriage parts and structural members. When used in such applications, in addition to the desired ductility and hole expansibility, the ratio of tensile strength to 0.2% proof stress A high yield ratio (YR) is required.

  In general, as a steel sheet having high strength and high YR, the steel structure is a single phase structure such as ferrite, bainitic ferrite, bainite, martensite, etc., and solid solution strengthening such as Mn and Si and / or Ti, A technique of strengthening by precipitation strengthening with alloy carbides such as Nb and V or Cu is known.

  For example, Patent Document 1 discloses a high-yield-ratio high-strength hot-rolled steel sheet in which a composite carbide containing Mo and V, Ti or V carbide is dispersed and precipitated in a steel structure composed of ferrite with an area ratio of 95% or more, Patent Document 2 discloses a hot-rolled steel sheet that secures a high YR by uniformly dispersing V carbide with an area ratio of 95% or more of ferrite.

  However, since the hot-rolled steel sheets disclosed by Patent Documents 1 and 2 require extremely expensive alloy elements such as Mo and V in order to ensure high YR, they are not suitable for mass production from an economical viewpoint.

  Patent Document 3 discloses a hot-rolled steel sheet having both ductility, stretch flangeability and strength by making the steel structure a uniform bainitic ferrite single-phase structure over the entire thickness. However, since the total elongation (El) in the examples of Patent Document 3 is a value less than 30%, there is a risk of cracking in applications with severe ductility such as overhang and drawing.

  Patent Document 4 discloses a high-strength hot-rolled steel sheet that achieves both yield strength and toughness by using 90% or more of the steel structure as martensite and utilizing precipitation strengthening of TiC. However, since the present invention has an ultrahigh strength of TS900 MPa or more and the total elongation (El) is less than 30%, there is a possibility that cracking may occur in overhang or drawing.

  Furthermore, Patent Document 5 discloses that the steel sheet after hot rolling is subjected to primary water cooling, air cooling, and secondary water cooling in order, whereby the ferrite phase area ratio is 85% or more of the entire structure, and the bainite phase area ratio. 10% or less, an area ratio of other phases other than the ferrite phase and the bainite phase is 5% or less, and the area ratio of the acicular ferrite phase in the entire ferrite phase is 30% or more and less than 80%. A high strength hot rolled steel sheet is disclosed. This hot-rolled steel sheet has a hole expansion ratio λ of 125% or more, and has a TS of 490 MPa or more and less than 590 MPa and excellent workability. However, the hot-rolled steel sheet disclosed by Patent Document 5 has a high Mn content of 0.8 to 1.8% by mass. For this reason, since austenite is stabilized, a sufficient amount of ferrite cannot be ensured, and excellent ductility may not be ensured. In addition, the alloy cost is increased, which is not desirable from the viewpoint of resource saving.

Japanese Patent No. 4899881 JP2013-227597A Japanese Patent No. 5040197 JP 2014-47414 A International Publication No. 2012/020847 Pamphlet (Japanese Patent Laid-Open No. 2012-57250)

  The present invention has been made in view of such a current situation, and has excellent ductility and high yield ratio suitable for press formability and impact resistance required for automobile undercarriage parts and automobile body structural members. It aims at providing the hot-rolled steel plate which has it stably cheaply.

  Here, specific mechanical property targets in the present invention have a high strength with a tensile strength of 440 MPa or more and less than 590 MPa, a total elongation (El) of 30% or more, and a hole expansion ratio (λ) of 60% or more. In addition to having excellent ductility and stretch flangeability, the yield ratio (YR) represented by the ratio of tensile strength and 0.2% proof stress is 80% or more, and has excellent impact resistance characteristics.

The present inventors have intensively studied to solve the above problems.
In order to provide excellent total elongation (El) and hole expansion rate (λ), the inventors first assumed that the steel structure is a ferrite-based structure. At the same time, in order to develop a high yield ratio (YR), we aimed to control the grain size and morphology of the ferrite.

  Specifically, the present inventors have attempted to strengthen the grain refinement by refinement of the ferrite grain size by controlling the cooling start time and the cooling stop temperature after the finish rolling in the hot rolling process, and the ferrite. It has been found that by combining the strengthening of the structure by controlling the shape to an acicular form, it is possible to have the characteristics of high total elongation (El), hole expansion ratio (λ) and high yield ratio (YR).

  For example, as disclosed in Patent Document 5, it is generally known that the same effect can be obtained by adding a large amount of alloy elements such as Mn and Cr. Unlike the conventional method, the present invention can control the structure without adding a large amount of these alloy elements.

The present invention completed based on the above findings is as follows.
(1) By mass%, C: 0.06% to 0.15%, Si: 0.01% to 0.5%, Mn: 0.1% to 0.5%, P: 0.0. 03% or less, S: 0.02% or less, sol. Al: 0.001% or more and 0.5% or less and N: 0.01% or less, the chemical composition consisting of Fe and impurities, and cementite, martensite or 90% or more of ferrite in area ratio and the balance of 5% or less It consists of at least one kind of bainite, the proportion of acicular ferrite in the ferrite is 10% or more, a steel structure having an average grain size of ferrite of 10 μm or less, and a tensile strength of 440 MPa or more and less than 590 MPa, The yield ratio (YR), which is the ratio of tensile strength to 0.2% proof stress, is 80% or more, the total elongation (El) is 30% or more, and the hole expansion ratio (λ) is 60% or more. A hot-rolled steel sheet characterized by having mechanical properties.

  (2) The chemical composition further includes one or more selected from the group consisting of Ti: 0.02% or less, Nb: 0.02% or less, and V: 0.02% or less in terms of mass%. Item 1. A hot-rolled steel sheet according to item 1.

  (3) The chemical composition further includes one or more selected from the group consisting of Cr: 0.03% or less, Mo: 0.03% or less, and B: 0.01% or less in terms of mass%. The hot-rolled steel sheet according to item 1 or 2 contained.

  (4) The steel composition further includes one or more selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass%. The hot-rolled steel sheet according to any one of items 1 to 3, which is contained.

(5) The steel slab is subjected to finish hot rolling, and after rolling at Ar ₃ −10 ° C. or higher, a temperature range of 600 ° C. or higher and 740 ° C. or lower at a cooling rate of 100 ° C./second or higher within 0.2 seconds The product is cooled to a temperature of 20 ° C. or more and 40 ° C. or less from the cooling stop temperature, further cooled to a temperature range of 550 ° C. or less at a cooling rate of 45 ° C./second or more, and then wound. The method for producing a hot-rolled steel sheet according to any one of items 1 to 4.

However, Ar ₃ (° C.) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P], [% C], [% Si], [% Mn] , [% P] indicate the contents (mass%) of C, Si, Mn, and P, respectively.

  According to the present invention, while having a tensile strength of 440 MPa or more and less than 590 MPa, it exhibits excellent press formability of 30% or more of total elongation (El) and a hole expansion ratio (λ) of 60% or more and a yield ratio (YR) of 80%. A hot-rolled steel sheet and a manufacturing method having the above excellent impact resistance performance are provided.

  The hot-rolled steel sheet according to the present invention is optimal as a material for structural members used in automobiles and various industrial machines, particularly as a material for automobile undercarriage parts and vehicle body reinforcing members.

  Below, the hot-rolled steel sheet and its manufacturing method which concern on this invention are demonstrated. In the following description, “%” related to the chemical composition of the steel sheet is “% by mass”.

1. Chemical composition of steel sheet (1-1) C: 0.06% or more and 0.15% or less C is an element having an effect of increasing the strength of steel, and in the present invention, a tensile strength of 440 MPa or more and less than 590 MPa is ensured. It is an important element to do. If the C content is less than 0.06%, it is difficult not only to secure a tensile strength of 440 MPa or more, but also the processing time in the decarburization process is increased, resulting in an increase in manufacturing cost, which is industrially undesirable. . Therefore, the C content is 0.06% or more. The C content is preferably 0.08% or more. On the other hand, if the C content exceeds 0.15%, the ductility, hole expansibility, and yield ratio decrease significantly. Therefore, the C content is 0.15% or less. Preferably it is 0.13% or less.

(1-2) Si: 0.01% or more and 0.5% or less Si is a ferrite-forming element, which promotes the formation of ferrite after finish rolling and has the effect of strengthening the ferrite by solid solution strengthening, so is good It is an effective element for ensuring a high ductility and a high yield ratio. If the Si content is less than 0.01%, the ferrite-forming ability is reduced, and a predetermined ferrite content cannot be obtained, so that ductility is deteriorated and it is difficult to secure a strength of 440 MPa or more. Therefore, the Si content is 0.01% or more. Preferably it is 0.02% or more, More preferably, it is 0.1% or more.

  On the other hand, if the Si content exceeds 0.5%, the melting point of the firelite will increase significantly, the scale peelability in the hot rolling process will be significantly reduced, and a large amount of island scale patterns will remain on the product surface. There is a deterioration of the surface appearance. Therefore, the Si content is 0.5% or less. Preferably it is 0.3% or less. More preferably, it is 0.2% or less.

(1-3) Mn: 0.1% or more and 0.5% or less Mn is an austenite generating element and has an action of stabilizing austenite in the finish rolling step. Moreover, it has the effect | action which fixes S as MnS and suppresses the red hot embrittlement of steel by the production | generation of FeS. In order to obtain this effect, the Mn content is 0.1% or more. Preferably it is 0.14% or more. On the other hand, if the Mn content exceeds 0.5%, austenite is stabilized, so that a sufficient amount of ferrite cannot be secured, and excellent ductility and hole expansibility cannot be secured. In addition, the alloy cost is increased, which is not desirable from the viewpoint of resource saving. Therefore, the Mn content is 0.5% or less. Preferably it is 0.4% or less.

(1-4) P: 0.03% or less P is contained as an impurity, and has the effect of degrading the toughness of steel, and lowers the adhesion of the chemical conversion solution. If the P content exceeds 0.03%, the adverse effects due to the above action become significant. Therefore, the P content is 0.03% or less.

(1-5) S: 0.02% or less S is contained as an impurity, and has an action of forming sulfides in steel and deteriorating ductility and stretch flangeability. If the S content exceeds 0.02%, the adverse effect due to the above action becomes significant. Therefore, the S content is 0.02% or less. Preferably it is 0.01% or less.

(1-6) sol. Al: 0.001% or more and 0.5% or less Al has an effect of making the steel sound by deoxidation and promotes the formation of ferrite. sol. If the Al content is less than 0.001%, the above-described effects cannot be obtained. Therefore, sol. The Al content is 0.001% or more. Preferably it is 0.01% or more, More preferably, it is 0.03% or more.

  On the other hand, sol. Even if the Al content exceeds 0.5%, the effect of the above action is saturated, resulting in an increase in cost. Therefore, sol. The Al content is 0.5% or less. Preferably it is 0.3% or less.

(1-7) N: 0.01% or less N is contained as an impurity, and when its content is excessive, the bendability is significantly reduced. Therefore, the N content is 0.01% or less. The N content is preferably 0.008% or less.

(1-8) One or more selected from the group consisting of Ti: 0.02% or less, Nb: 0.02% or less, and V: 0.02% or less Ti, Nb, and V are C and It is an element that combines and finely precipitates as carbides, and has the effect of suppressing the coarsening and recrystallization of austenite grains in hot finish rolling, and has the effect of increasing the strength of the hot-rolled steel sheet. Therefore, you may contain 1 type, or 2 or more types of these elements.

  On the other hand, if each of Ti, Nb, and V is contained in an amount exceeding 0.02%, high temperature heating exceeding 1250 ° C. is required as a solution treatment for obtaining fine carbides that contribute to high strength. There is a risk of raising the unit. Therefore, the content of Ti, Nb, and V is set to 0.02% or less.

  In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ti: 0.001% or more, Nb: 0.001% or more, and V: 0.001% or more.

(1-9) One or more selected from the group consisting of Cr: 0.03% or less, Mo: 0.03% or less, and B: 0.01% or less Cr, Mo, B are all These are optional elements, all of which have an effect of enhancing the hardenability of steel and are useful for increasing the strength. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, if any of these elements is contained beyond the above range, the ductility is significantly lowered and the cost is increased, which is not desirable from the viewpoint of resource saving. Therefore, the content of each element is within the above range. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Cr: 0.02% or more, Mo: 0.01% or more, and B: 0.0001% or more.

(1-10) REM: 0.1% or less, Mg: 0.01% or less, and Ca: one or more selected from the group consisting of 0.01% or less REM (rare earth element), Mg, Ca Is an optional element, and all have the effect of improving ductility by finely spheroidizing oxides and sulfides. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, if any of REM, Mg, and Ca is contained exceeding the above range, a large amount of oxides and sulfides are formed in the steel, and the ductility is significantly reduced. Therefore, the content of each element is within the above range.

  In order to more reliably obtain the effect of the above action, it is preferable to contain 0.0001% or more of any of these elements.

Here, REM refers to a total of 17 elements of Sc, Y, and lanthanoid. In the case of lanthanoid, it is added industrially in the form of misch metal. In the present invention, the content of REM refers to the total content of these elements.
The balance other than the above is Fe and impurities.

2. Next, the reason for limiting the steel sheet structure related to the hot-rolled steel sheet of the present invention will be described. The steel sheet structure referred to here is a structure at a ¼ depth position of the plate thickness from the surface of the steel sheet, and indicates an average structure of the steel sheet.

(2-1) Ferrite: 90% or more in area ratio The ferrite in the present invention means both soft polygonal ferrite and relatively hard acicular ferrite. By containing 90% or more of these in the structure, the work hardening ability of the steel sheet can be improved and high total elongation (El) can be expressed, and the hole expansion ratio (λ) can be achieved by forming a uniform structure and reducing the heterogeneous interface. Can be increased.

  Acicular ferrite means a ferrite having a major axis / uniaxial ratio (aspect ratio) of ferrite grains of 3 or more, whereas polygonal ferrite means a ferrite having an aspect ratio of less than 3.

  If the area ratio of ferrite is less than 90%, the required work hardening ability cannot be obtained, and the total elongation (El) and the hole expansion ratio (λ) are deteriorated. When emphasizing the total elongation (El) and the hole expansion rate (λ), the area ratio of ferrite is preferably 95% or more.

(2-2) Remainder: 5% or less consisting of at least one of cementite, martensite or bainite As the second phase other than ferrite, at least one of cementite, martensite or bainite is present. These exist together with ferrite and act as a composite structure strengthening, and have an effect of increasing the strength of the steel sheet. If the balance exceeds 5%, the amount of ferrite for obtaining the total elongation (El) is insufficient and the hole expansion ratio (λ) is deteriorated, resulting in a decrease in the yield ratio (YR). Therefore, in the present invention, at least one of cementite, martensite, or bainite in the steel structure is defined as 5% or less in terms of area ratio. In the case where emphasis is placed on the total elongation (El) and the hole expansion ratio (λ), the area ratio of the remaining portion is preferably 2% or less.

(2-3) Ratio of acicular ferrite in ferrite: 10% or more Acicular ferrite is generated in the cooling process after finish rolling, but is known to have a lower transformation temperature than polygonal ferrite. Therefore, since the dislocation density to be introduced is higher than that of polygonal ferrite, the ferrite itself is strengthened, which contributes to increasing the strength of the material.

  When the proportion of acicular ferrite in the ferrite is less than 10%, the yield ratio (YR) specified in the present invention is below, so that sufficient impact resistance characteristics cannot be obtained. Accordingly, the proportion of acicular ferrite in the ferrite of the steel sheet structure is set to 10% or more. When emphasizing the yield ratio (YR), the proportion of acicular ferrite in the ferrite is preferably 20% or more.

(2-4) Average grain size of ferrite: 10 μm or less When the average grain size of ferrite exceeds 10 μm, the contribution of crystal grain refinement decreases, and the yield is lower than the strength specified in the present invention and has a high yield ratio (YR). It cannot be expressed. Therefore, the average particle diameter of ferrite is set to 10 μm or less. In the case where emphasis is placed on the collision resistance, the average particle diameter of the ferrite is preferably 7 μm or less.

  The lower limit of the average ferrite grain size is not particularly limited, but if the average ferrite grain size is less than 1 μm, the yield strength and tensile strength of the material may coincide with each other, and sufficient work hardening ability cannot be obtained and the total elongation (El ) May deteriorate. Therefore, the average particle size of ferrite is preferably 1 μm or more.

3. Next, the reason for limiting the mechanical properties of the hot-rolled steel sheet according to the present invention will be described.

(3-1) Tensile strength: 440 MPa or more and less than 590 MPa, yield ratio (YR) which is a ratio of tensile strength and 0.2% proof stress: 80% or more, total elongation (El): 30% or more, hole expansion rate (Λ): 60% or more When the tensile strength is lower than 440 MPa, it is easy to satisfy the mechanical characteristics in the first place, and it is not desirable for a vehicle body reinforcing member that requires collision performance. Further, if the tensile strength is 590 MPa or more, the total elongation (El) is deteriorated, and therefore, it is not preferable because the use of parts that can be applied is limited from the viewpoint of ensuring press formability. In the present invention, since it has the characteristics of total elongation (El) of 30% or more and hole expansion ratio (λ) of 60% or more, it has good press formability and exhibits a high yield ratio (YR) of 80% or more. There is an advantage that the degree of freedom in designing parts is increased even for members that require characteristics.

4). Steel strip manufacturing method The hot-rolled steel sheet according to the present invention can be manufactured by the following method.

  The steel having the above-described chemical composition is made into a steel ingot by a continuous casting method after being melted by a known means, or by decomposing and rolling after making it into an ingot by an arbitrary casting method, etc. It is said.

  In the continuous casting process, in order to suppress the occurrence of surface defects due to inclusions, it is preferable to cause an external additional flow such as electromagnetic stirring in the molten steel in the mold.

5. Manufacturing method of steel plate (5-1) The steel piece is subjected to finish hot rolling, and the rolling is completed at Ar ₃ -10 ° C or higher.

  The steel ingot or steel slab may be reheated once it has been cooled and subjected to hot rolling, or the steel ingot in a high temperature state after continuous casting or the steel slab in a high temperature state after decomposition rolling, or It may be subjected to hot rolling by keeping it warm or carrying out auxiliary heating.

  In the hot rolling, it is preferable to use a lever mill or a tandem mill. In particular, from the viewpoint of industrial productivity, it is more preferable that at least the final several passes be a rolling using a tandem mill.

  The temperature of the steel slab used for hot rolling is not particularly limited as long as the temperature becomes an austenite single phase, but is preferably 1060 ° C. or more from the viewpoint of securing the rolling temperature, and from the viewpoint of suppressing scale loss. Is preferably 1300 ° C. or lower.

Upon performing hot rolling, the rolling temperature and _Ar 3 -10 ° C. or higher. Ar ₃ here is a temperature calculated from the components in the steel represented by the following formula, and [% C], [% Si], [% Mn], and [% P] are C, The content (mass%) of Si, Mn, and P is shown.

Ar ₃ (℃) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P]
When rolling is completed at less than Ar ₃ −10 ° C., ferrite transformation occurs during rolling, and the structure in which processed ferrite is mixed becomes remarkable, so that the total elongation deteriorates. Moreover, since the rolling texture of the hot-rolled steel sheet develops excessively, the hole expansion rate may deteriorate. Therefore, the hot rolling finish is the _Ar 3 -10 ° C. or higher. When importance is attached to hole expansibility, the rolling temperature is preferably Ar ₃ or higher.

  (5-2) Cool to a temperature range of 600 to 740 ° C. at a cooling rate of 100 ° C./second or more within 0.2 seconds, and allow to cool until the temperature drops from a cooling stop temperature to a temperature of 20 to 40 ° C.

  The hot-rolled steel sheet is cooled to a temperature range of 600 ° C. or higher and 740 ° C. or lower at a cooling rate of 100 ° C./second or less within 0.2 seconds after completion of hot rolling, and is 20 ° C. or higher and 40 ° C. from the cooling stop temperature. Allow to cool to below temperature.

  By performing the cooling as described above, the release of the strain introduced by hot rolling is suppressed, and then the ferrite transformation using the strain as a driving force occurs by allowing to cool, and the steel structure intended in the present invention Can be obtained.

  When the cooling start time after rolling exceeds 0.2 seconds, or when the cooling rate is lower than 100 ° C./second, the release of processing strain is promoted, and the ferrite transformation in the high temperature region is promoted. Becomes coarse. Since the release of the processing strain is suppressed as the cooling rate is increased, it is more preferably 600 ° C./second or more, and still more preferably 800 ° C./second or more, in order to make the above effect manifest.

  Further, even when the cooling start time and the cooling rate are within the range defined by the present invention, when the cooling temperature range exceeds 740 ° C., the polygonal ferrite transformation at a high temperature is promoted, and the acicular ferrite defined by the present invention The amount of ferrite and the particle size of ferrite cannot be ensured. As a result, the tensile strength of 440 MPa or more and the yield ratio (YR) of 80% or more specified in the present invention cannot be satisfied. Therefore, the upper limit of the temperature range for cooling is 740 ° C. The upper limit of the preferred temperature range is 730 ° C.

  On the other hand, when the temperature range to be cooled is below 600 ° C., bainite is excessively generated, and a necessary amount of ferrite cannot be secured, so that the total elongation (El) and the hole expansion rate (λ) are deteriorated. Further, if the temperature drop due to cooling is less than 20 ° C. or more than 40 ° C., the ferrite transformation becomes insufficient or the ferrite transformation is promoted too much, and the amount of acicular ferrite and the ferrite particle size specified in the present invention Can not be secured. Therefore, the lower limit of the temperature range for cooling is 600 ° C. The lower limit of the preferred temperature range is 640 ° C.

(5-3) Cool to a temperature range of 550 ° C. or lower at a cooling rate of 45 ° C./second or more and wind up.
After cooling, if the cooling rate to a temperature range of 550 ° C. or lower falls below 45 ° C./second, ferrite grains become coarse, so that the yield ratio (YR) of 80% or more cannot be satisfied. Therefore, after cooling, cooling is performed at a cooling rate of 45 ° C./second or more to a temperature range of 550 ° C. or less.

  Note that the upper limit of the cooling rate is not specified because it is sufficient that the cooling rate does not cause a decrease in tensile strength due to the coarsening of ferrite grains. In practice, the maximum cooling rate that can be cooled by the cooling device used is the upper limit of the cooling rate.

  From the above process, the coiling temperature is 550 ° C. or lower. When it is desired to increase the tensile strength, the cooling stop temperature may be lowered. In this case, the cooling stop temperature before winding may be 500 ° C. or lower, preferably 200 ° C. or lower.

The present invention will be described more specifically with reference to examples.
After steel having the chemical composition shown in Table 1 was melted in a 150 kg high-frequency vacuum melting furnace, rough rolling was performed in a temperature range of 1000 ° C. or higher to obtain a slab having a thickness of 30 mm and a width of 180 mm.

This slab was heated to a temperature of 1250 ° C. and subjected to a homogeneous heat treatment for 1 hour. Hot rolling was performed using a test hot rolling apparatus so that the total reduction ratio was 91%, and the plate thickness was 2.6 mm. Except for some comparative examples, 5-pass multi-pass hot rolling was performed under the condition that the rolling completion temperature was Ar ₃ points −10 ° C. or higher.

  After rolling, except for some comparative examples, primary cooling was performed to a predetermined temperature range at a cooling rate shown in Table 2, and the mixture was allowed to cool for about 5 seconds in that temperature range to be lowered to a temperature of about 25 ° C. Then, secondary cooling was performed to cool to the coiling temperature. A hot-rolled steel sheet was obtained under these hot rolling conditions.

The following test was implemented with respect to the obtained hot-rolled steel plate.
1) Evaluation of steel structure The type of steel structure of the steel sheet was observed using a SEM after corroding a cross section parallel to the rolling direction of the steel sheet with a Nital reagent. The observation was performed at a magnification of 1000 and the number of visual fields at 10 locations. After identifying the type of each phase with an SEM image, the area ratio of each phase, the ferrite particle size, and the proportion of acicular ferrite in the ferrite were derived using particle analysis software.

2) Evaluation of mechanical properties After removing the scale of the surface by hydrochloric acid pickling, the obtained hot-rolled steel sheet was subjected to the following tensile test and hole expansion test.

2-1) Tensile test A JIS No. 5 tensile test was taken from the direction perpendicular to the rolling direction of each steel plate. The test method conformed to JIS Z2241. The tensile test measured 0.2% proof stress, tensile strength TS, and total elongation El. YR was a value obtained by dividing 0.2% proof stress by tensile strength TS.

2-2) Hole expansion test A 100 mm square base plate was cut out from each steel plate and punched to 10 mmφ at the center of the base plate by a universal testing machine. The punching clearance was approximately 12% in accordance with the Japan Iron and Steel Federation standard (JFST1001-1996). A hole expansion test was performed on the base plate. The hole expansion test was performed by a method according to the Japan Iron and Steel Federation standard (JFST1001-1996), and each sample was measured three times under the same conditions, and the average value was taken as λ.

  The test results are summarized in Table 3.

  Specimen No. which is the present invention. 1 to 11 satisfy the requirements of the steel composition and steel structure defined in the present invention, have a high strength with a tensile strength of 440 MPa or more and less than 590 MPa, a total elongation (El) of 30% or more, and a hole expansion ratio ( λ) has an excellent press formability of 60% or more, and also has an excellent impact resistance property of a yield ratio (YR) of 80% or more.

On the other hand, the test material No. As for Nos. 12 to 14, although the steel composition and the steel structure satisfy the provisions of the present invention, since the rolling completion temperature was lower than Ar ₃ -10 ° C. defined in the present invention, the ferrite in which the processing strain during rolling remained was developed. The total elongation and the hole expansion rate deteriorated significantly.

  Specimen No. 15 and 16, since the time from the completion of rolling in the hot rolling process to the start of cooling exceeded 0.2 seconds specified in the present invention, the ferrite particle diameter was excessively coarsened, and the ferrite particle diameter specified in the present invention was reduced. In addition, the proportion of acicular ferrite also decreases. No. 15 has a tensile strength of less than 440 MPa and cannot satisfy the strength range defined in the present invention. In No. 16, the yield ratio (YR) was below the range specified in this specification, resulting in poor impact resistance performance.

  Specimen No. No. 17, because the cooling rate from the completion of rolling in the hot rolling process to the cooling stop temperature was below the specified range of the present invention, and the ferrite was rapidly coarsened, so the tensile strength was less than 440 MPa, and the specified range of the present invention Could not meet.

  Specimen No. 18 and 19, both the time from the completion of rolling in the hot rolling process to the start of cooling and the cooling rate from the cooling stop temperature are not within the specified range of the present invention, and the ferrite grain size and the acicular ferrite ratio are the present invention. Therefore, the tensile strength of 440 MPa or more and the yield ratio (YR) of 80% or more could not be satisfied.

  Specimen No. No. 20, the time from the completion of rolling in the hot rolling process to the start of cooling exceeds 0.2 seconds specified in the present invention, and the cooling stop temperature is higher than the specified range of the present invention, so the ferrite grain size becomes coarse In addition, the yield ratio (YR) was less than the specified range of the present invention because of the low proportion of acicular ferrite.

  Specimen No. No. 21, because the cooling stop temperature in the hot rolling process is higher than the specified range of the present invention, the ferrite grain size becomes coarse and the proportion of acicular ferrite is low, so the tensile strength and the yield ratio (YR) are It was less than the specified range of the present invention.

  Specimen No. No. 22, because the cooling stop temperature in the hot rolling process is lower than the specified range of the present invention, bainite develops excessively, and the amount of ferrite specified in the present invention cannot be secured, so that the total elongation (El) and hole expansion The rate (λ) was significantly deteriorated, and mechanical characteristics within the specified range of the present invention were not obtained.

  Specimen No. In No. 23, the coiling temperature in the hot rolling process exceeded the temperature specified in the present invention, and the ferrite grains were coarsened, so the tensile strength was lower than the range specified in the present invention.

  Specimen No. No. 24 exceeds the range of Mn content specified in the present invention, bainite is excessively generated, and the necessary ferrite content cannot be obtained. Therefore, the tensile strength exceeds the range of 440 MPa or more and less than 590 MPa specified in the present invention. As a result, the total elongation (El) and the hole expansion (λ) were less than the values specified in the present invention, and the ductility was poor. Moreover, the yield ratio (YR) was less than the specified range of the present invention.

  Specimen No. No. 25 exceeds the range of the Mn content specified in the present invention, bainite is excessively generated, and the necessary ferrite content cannot be obtained. Therefore, the total elongation (El) and the hole expansion (λ) are also included in the present invention. As a result, the ductility was poor.

  Furthermore, the test material No. No. 26 was lower than the range of the C content specified in the present invention, and the tensile strength was lower than the range of 440 MPa to less than 590 MPa specified in the present invention.

Claims (5)

In mass%, C: 0.06% to 0.15%, Si: 0.01% to 0.5%, Mn: 0.1% to 0.5%, P: 0.03% or less , S: 0.02% or less, sol. Al: 0.001% or more and 0.5% or less and N: 0.01% or less, the chemical composition consisting of the balance Fe and impurities,
It consists of at least one of cementite, martensite or bainite with an area ratio of 90% or more and the balance of 5% or less, and the proportion of acicular ferrite in the ferrite is 10% or more. A steel structure having 10 μm or less;
The tensile strength is 440 MPa or more and less than 590 MPa, the yield ratio (YR), which is the ratio between the tensile strength and the 0.2% proof stress, is 80% or more, and the total elongation (El) is 30% or more. A hot-rolled steel sheet having mechanical properties such that the expansion ratio (λ) is 60% or more.   The chemical composition further comprises one or more selected from the group consisting of Ti: 0.02% or less, Nb: 0.02% or less, and V: 0.02% or less in terms of mass%. Item 2. The hot rolled steel sheet according to Item 1.   The chemical composition further comprises one or more selected from the group consisting of Cr: 0.03% or less, Mo: 0.03% or less, and B: 0.01% or less in mass%. The hot rolled steel sheet according to claim 1 or 2.   The steel composition further comprises one or more selected from the group consisting of REM: 0.1% or less, Mg: 0.01% or less, and Ca: 0.01% or less in mass%. The hot-rolled steel sheet according to any one of claims 1 to 3. The steel slab is subjected to finish hot rolling, and after rolling at Ar ₃ −10 ° C. or higher, it is cooled to a temperature range of 600 ° C. or higher and 740 ° C. or lower at a cooling rate of 100 ° C./second or higher within 0.2 seconds. And cooling to a temperature of 20 ° C. or more and 40 ° C. or less from the cooling stop temperature, further cooling to a temperature range of 550 ° C. or less at a cooling rate of 45 ° C./second or more, and then winding. The method for producing a hot-rolled steel sheet according to any one of claims 1 to 4.
However, Ar ₃ (° C.) = 905-455 [% C] -38 [% Si] -62 [% Mn] +472 [% P], [% C], [% Si], [% Mn] , [% P] indicate the contents (mass%) of C, Si, Mn, and P, respectively.