JP6519011B2 - Hot rolled steel sheet and method of manufacturing the same - Google Patents

Description

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

BACKGROUND ART In recent years, from the viewpoint of improving fuel efficiency by reducing the weight of a vehicle body for the purpose of reducing CO ₂ emissions and tightening of collision safety standards, strengthening of body parts of automobiles has been promoted. In addition, from the viewpoint of resource saving, it is desired to obtain high strength with a small amount of alloy addition to steel plates which are materials of car body parts.

  Under such circumstances, the shapes of body parts are diversified. For this reason, the demand for a hot rolled steel sheet having not only high strength but also various properties such as press formability, weldability, and collision resistance is increasing. In particular, excellent impact resistance is essential for undercarriage parts and structural members, and in addition to the desired ductility and hole expandability when used for such applications, the ratio of tensile strength to 0.2% proof stress It is required that the yield ratio (YR) is high.

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

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

  However, 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 secure high YR, and therefore are not suitable for mass production from an economic 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 plate thickness. However, since the total elongation (El) in the example of Patent Document 3 is less than 30% in all cases, there is a possibility that a crack may occur in a ductile severe application 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 ultra-high strength of TS 900 MPa or more and the total elongation (El) is less than 30%, there is a possibility that a crack may occur in the stretching or drawing.

  Furthermore, in Patent Document 5, by sequentially performing primary water cooling, air cooling and secondary water cooling on the steel plate after hot rolling is finished, the area ratio of the ferrite phase occupying in the whole structure is 85% or more, the area ratio of the bainite phase 10% or less, area ratio of ferrite phase and other phases other than bainite phase is 5% or less, and the area ratio of acicular ferrite phase occupying in the entire ferrite phase is 30% or more and less than 80%. A strength hot rolled steel sheet is disclosed. This hot rolled steel sheet reliably obtains a hole expansion ratio λ of 125% or more, has a TS of 490 MPa or more and less than 590 MPa, and is excellent in workability. However, the hot rolled steel sheet disclosed by Patent Document 5 has a high Mn content of 0.8 to 1.8 mass%. Therefore, a sufficient amount of ferrite can not be secured to stabilize austenite, and excellent ductility may not be secured. Moreover, since it causes an increase in alloy cost, it is not desirable from the viewpoint of resource saving.

Patent 4899881 specification JP, 2013-227597, A Patent No. 5040197 JP, 2014-47414, A International Publication No. 2012/020847 pamphlet (Japanese Unexamined Patent Publication 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 collision resistance required for automotive underbody parts and structural members of automobile bodies. An object of the present invention is to provide a heat-rolled steel plate having the same inexpensively and stably.

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

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

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

  It is generally known that similar effects can be obtained by adding a large amount of alloy elements such as Mn and Cr as disclosed in, for example, Patent Document 5 for the above-mentioned structure control. The present invention, unlike the conventional method, can control the structure without adding a large amount of these alloying elements.

The present invention completed based on the above findings is as follows.
(1) In mass%, C: 0.06% or more and 0.15% or less, Si: 0.01% or more and 0.5% or less, Mn: 0.1% or more and 0.5% or less, 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, chemical composition including the balance Fe and impurities, and cementite, martensite or ferrite having a surface area ratio of 90% or more and the balance 5% or less A steel structure which is made of at least one of bainite, the ratio of acicular ferrite in the ferrite is 10% or more, and the average grain diameter of the ferrite is 10 μm or less, and the tensile strength is 440 MPa or more and less than 590 MPa. The yield ratio (YR), which is the ratio between tensile strength and 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 characteristics.

  (2) One or more selected from the group consisting of, in mass%, Ti: 0.02% or less, Nb: 0.02% or less, and V: 0.02% or less as the chemical composition. The hot rolled steel sheet according to item 1 which contains.

  (3) One or more selected from the group consisting of, in mass%, Cr: 0.03% or less, Mo: 0.03% or less and B: 0.01% or less as the chemical composition. The hot rolled steel sheet according to item 1 or 2 which contains.

  (4) 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 by mass% The hot rolled steel sheet according to any one of the items 1 to 3 which contains.

(5) The billet is subjected to finish hot rolling, and after completion of rolling at Ar 3-10 ° C. or more, cooling is started within 0.2 seconds, and the cooling rate is 100 ° C./s or more at 600 ° C. or more 740 ° C. It is cooled to the following temperature range, allowed to cool to a temperature of 20 ° C to 40 ° C from the cooling stop temperature, and further cooled to a temperature range of 550 ° C or less at a cooling rate of 45 ° C / sec or more, and then wound The manufacturing method of the hot rolled sheet steel of any one of 1 to 4 characterized by taking.

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

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

  The heat-rolled steel plate according to the present invention is most suitable as a material of structural members used in automobiles and various industrial machines, in particular as a component of automobile undercarriage parts and vehicle body reinforcing members.

  Hereinafter, a hot rolled steel sheet according to the present invention and a method of manufacturing the same will be described. In the following description,% relating to the chemical composition of the steel sheet is mass%.

1. Chemical composition of steel plate (1-1) C: 0.06% or more and 0.15% or less C is an element having an effect of enhancing the strength of steel, and in the present invention, a tensile strength of 440 MPa or more and less than 590 MPa is secured. It is an important element to If the C content is less than 0.06%, it will not only be difficult to secure a tensile strength of 440 MPa or more, but also the processing time in the decarburization step will be long, and the production cost will rise, which is not desirable industrially . Therefore, the C content is 0.06% or more. The C content is preferably 0.08% or more. On the other hand, when the C content exceeds 0.15%, the ductility, the hole expansibility, and the decrease in the yield ratio become remarkable. Therefore, the C content is made 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, and promotes formation of ferrite after finish rolling and has a function of strengthening ferrite by solid solution strengthening, which is favorable. It is an element effective to secure high ductility and high yield ratio. If the Si content is less than 0.01%, the ability to form ferrite decreases, and a predetermined amount of ferrite can not be obtained, and in addition to the deterioration of ductility, it becomes difficult to secure strength of 440 MPa or more. Therefore, the Si content is 0.01% or more. Preferably it is 0.02% or more, more preferably 0.1% or more.

  On the other hand, if the Si content exceeds 0.5%, the melting point of firelite increases significantly, the scale removability in the hot rolling process decreases significantly, and a large amount of island scale pattern remains on the product surface. Cause 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-forming element and has an effect of stabilizing austenite in the finish rolling process. In addition, S is fixed as MnS, and has an effect of suppressing the red heat brittleness of the steel due to the formation of FeS. In order to obtain this effect, the Mn content is at least 0.1%. Preferably it is 0.14% or more. On the other hand, if the Mn content exceeds 0.5%, a sufficient amount of ferrite can not be secured in order to stabilize austenite, and excellent ductility and hole expandability can not be secured. Moreover, it causes an increase in alloy cost, 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 deteriorating the toughness of the steel, as well as reducing the adhesion of the chemical conversion solution. When the P content exceeds 0.03%, the adverse effect due to the above-mentioned action becomes remarkable. Therefore, the P content is made 0.03% or less.

(1-5) S: 0.02% or less S is contained as an impurity, and forms a sulfide in the steel and has an action of deteriorating ductility and stretch flangeability. If the S content exceeds 0.02%, the adverse effect due to the above-mentioned action becomes remarkable. 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 action to strengthen the steel by deoxidation and promotes the formation of ferrite. sol. If the Al content is less than 0.001%, the above-described effect can not 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.

  Meanwhile, sol. Even if the Al content exceeds 0.5%, the effect due to the above action is saturated, leading to 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 the 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) Ti: 0.02% or less, Nb: 0.02% or less, and V: 0.02% or less One or more selected from the group consisting of Ti, Nb, and V are C and C It is an element that combines to form fine precipitates as carbides, and has an effect of suppressing coarsening of austenite grains and recrystallization in hot finish rolling, and has an action of increasing the strength of a hot-rolled steel sheet. Therefore, one or more of these elements may be contained.

  On the other hand, if each of Ti, Nb, and V is contained in excess of 0.02%, high temperature heating exceeding 1250 ° C. is required as a solution treatment to obtain fine carbides contributing to high strength. It may lead to a rise in credits. Therefore, the content of Ti, Nb and V is made 0.02% or less.

  In order to obtain the effect by the above action more reliably, 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) Cr: 0.03% or less, Mo: 0.03% or less and B: 0.01% or less One or more selected from the group consisting of Cr, Mo, and B are all It is an optional element, all of which have the effect of enhancing the hardenability of steel and are useful elements for strengthening. Therefore, one or more of these elements may be contained.

  However, if any of these elements is contained beyond the above range, the ductility is significantly reduced and the cost is increased, which is not desirable from the viewpoint of resource saving. Therefore, the content of each of these elements is in the above range. In order to obtain the effect by the above action more reliably, 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: One or more selected from the group consisting of RE: 0.1% or less, Mg: 0.01% or less and Ca: 0.01% or less REM (rare earth element), Mg, Ca Is an optional element, and both have the effect of improving ductility by finely spheroidizing oxides and sulfides. Therefore, one or more of these elements may be contained.

  However, when any one of REM, Mg, and Ca is contained beyond 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 in the above range.

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

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

2. Steel Structure of Steel Sheet Next, the reasons for limitation of the steel sheet structure relating 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 1/4 depth position of the sheet 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 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 hardenability of the steel plate can be enhanced, and high overall elongation (El) can be expressed, and moreover, the uniform structure becomes and the heterophase interface decreases, and the hole expansion ratio (λ) It is possible to raise

  The acicular ferrite means a ferrite having a ratio (aspect ratio) of the major axis to the uniaxial axis (aspect ratio) of 3 or more, while the polygonal ferrite means a ferrite having an aspect ratio of less than 3.

  When the area ratio of ferrite is less than 90%, the required work hardening ability can not be obtained, and the total elongation (El) and the hole expansion ratio (λ) deteriorate. When emphasis is placed on total elongation (El) and hole expansion ratio (λ), the area ratio of ferrite is preferably 95% or more.

(2-2) Remaining part: at least one of cementite, martensite or bainite of 5% or less As a second phase other than ferrite, at least one of cementite, martensite or bainite is present. These are present together with ferrite, act as a composite structure reinforcement, and have the effect of enhancing the strength of the steel sheet. If the remainder exceeds 5%, the amount of ferrite for obtaining the total elongation (El) is insufficient and the hole expansion ratio (λ) is deteriorated, which results 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 area ratio. When focusing 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 to Ferrite: 10% or More Acicular ferrite is formed in the cooling process after finish rolling, but is known to have a transformation temperature lower than polygonal ferrite. Therefore, since the dislocation density to be introduced is higher than polygonal ferrite, strengthening the ferrite itself contributes to the strengthening of the material.

  If the proportion of acicular ferrite in the ferrite is less than 10%, it is less than the yield ratio (YR) specified in the present invention, so that sufficient collision resistance can not be obtained. Therefore, the ratio of acicular ferrite in the ferrite of the steel sheet structure is made 10% or more. When emphasis is placed on the yield ratio (YR), the ratio of acicular ferrite to 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 a high yield ratio (YR) is obtained while falling below the strength specified in the present invention. It can not be expressed. Therefore, the average particle size of ferrite is set to 10 μm or less. When emphasis is placed on collision resistance, the average particle size of ferrite is preferably 7 μm or less.

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

3. Next, the reasons for limiting the mechanical properties of the hot-rolled steel sheet of 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 to 0.2% proof stress: 80% or more, total elongation (El): 30% or more, hole expansion ratio (Λ): 60% or more When the tensile strength is less than 440 MPa, it is not only easy to satisfy the above mechanical properties, but it is not desirable for a car body reinforcing member that requires collision performance. In addition, when the tensile strength is 590 MPa or more, the total elongation (El) is deteriorated, which is not preferable because the use of parts applicable from the viewpoint of securing the press formability is limited. In the present invention, it has characteristics of a total elongation (El) of 30% or more, a hole expansion ratio (λ) of 60% or more, good press formability, and a high yield ratio (YR) of 80% or more. There is an advantage that even in the case of members requiring characteristics, the degree of freedom in designing parts is increased.

4. Method of Producing Steel Piece The hot rolled steel sheet according to the present invention can be produced by the method described below.

  The steel having the above-described chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or made into a steel ingot by any casting method and then cracked by rolling etc. It is assumed.

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

5. Method for Producing Steel Plate (5-1) A billet is subjected to finish hot rolling, and rolling is completed at Ar ₃ −10 ° C. or higher.

  The ingot or billet may be reheated after being cooled and subjected to hot rolling, or the ingot in high temperature state after continuous casting or the billet in high temperature state after decomposing rolling or as it is, or It may be kept warm or subjected to auxiliary heating to be subjected to hot rolling.

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

  The temperature of the billet to be subjected to hot rolling may be any temperature that allows austenite single phase, and is not particularly limited. However, from the viewpoint of securing the rolling temperature, 1060 ° C. or higher is preferable, and from the viewpoint of scale loss suppression Preferably, the temperature is 1300 ° C. or less.

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

Ar ₃ (° C.) = 905-455 [% C]-38 [% Si]-62 [% Mn] + 472 [% P]
When rolling is completed below 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 is deteriorated. In addition, since the rolling texture of the hot-rolled steel sheet is excessively developed, the hole expansion rate may be deteriorated. Therefore, finish hot rolling is performed at Ar ₃ −10 ° C. or higher. When emphasizing the hole expandability, the rolling temperature is preferably Ar ₃ points or more.

(5-2) Cooling is started within 0.2 seconds, cooled to a temperature range of 600 to 740 ° C. at a cooling rate of 100 ° C./sec or more, and decreased to a temperature of 20 to 40 ° C. from the cooling stop temperature Allow to cool.

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

  By performing cooling as described above, the release of the strain introduced by hot rolling is suppressed, and by cooling thereafter, a ferrite transformation using the strain as a driving force occurs, and the steel structure aimed by the present invention You can get

  If the cooling start time after rolling exceeds 0.2 seconds, or if the cooling rate is less than 100 ° C./s, the release of working strain is promoted and the ferrite transformation in a high temperature range is promoted, so the steel sheet structure Become coarse. The release of the processing strain is suppressed as the cooling rate increases, so that the above effect is more preferably 600 ° C./s or more, more preferably 800 ° C./s or more.

  Further, even if the cooling start time and the cooling rate are in the range specified by the present invention, if the temperature range for cooling exceeds 740 ° C., transformation of polygonal ferrite at high temperature is promoted, and the acicular ferrite specified in the present invention The amount of ferrite and the grain size of ferrite can not be secured. 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 can not be satisfied. Therefore, the upper limit of the temperature range to be cooled is 740.degree. The upper limit of the preferred temperature range is 730 ° C.

  On the other hand, if the temperature range for cooling is lower than 600 ° C., bainite is excessively formed, the necessary amount of ferrite can not be secured, and the total elongation (El) and the hole expansion ratio (λ) deteriorate. In addition, if the temperature decrease 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 specified in the present invention or the ferrite particle diameter Can not secure. Therefore, the lower limit of the temperature range to be cooled is 600 ° C. The lower limit of the preferred temperature range is 640.degree.

(5-3) Cool to a temperature range of 550 ° C. or less at a cooling rate of 45 ° C./sec or more and take up.
After cooling, if the cooling rate to a temperature range of 550 ° C. or less is less than 45 ° C./sec, coarsening of ferrite grains occurs, and a yield ratio (YR) of 80% or more can not be satisfied. Therefore, after standing to cool, cooling is performed at a cooling rate of 45 ° C./sec or more to a temperature range of 550 ° C. or less.

  The upper limit of the cooling rate is not defined because it is sufficient if the cooling rate does not cause a decrease in tensile strength due to coarsening of ferrite particles. In effect, the maximum cooling rate that can be cooled by the cooling device used is the upper limit of the cooling rate.

  Since the above process is taken, the winding temperature becomes 550 ° C. or less. If 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 less, preferably 200 ° C. or less.

The invention will be more particularly described with reference to the examples.
After the 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 of 30 mm thick and 180 mm wide.

The slab was heated to a temperature of 1250 ° C. and subjected to homogeneous heat treatment for 1 hour. It hot-rolled so that a total rolling-reduction | draft ratio might be 91% using the hot rolling mill for test, and set plate | board thickness to 2.6 mm. Except for some comparative examples, 5-pass multipass hot rolling was performed under the condition that the rolling completion temperature was Ar ₃ point-10 ° C or higher.

  After rolling, except for some of the comparative examples, primary cooling was performed at a cooling rate shown in Table 2 to a predetermined temperature range, and was allowed to cool for about 5 seconds in that temperature range to be lowered to a temperature of about 25 ° C. After that, secondary cooling was performed to cool to the winding 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 a steel plate corroded the cross section parallel to the rolling direction of a steel plate with a nital reagent, and observed using SEM. The observation was performed at 10 fields of view at a magnification of × 1000. After identifying the type of each phase by a SEM image, the area ratio of each phase, the ferrite particle size, and the ratio of acicular ferrite in the ferrite were derived by particle analysis software.

2) Evaluation of mechanical properties The obtained hot-rolled steel sheet was descaled on its surface by hydrochloric acid pickling, and then subjected to the following tensile test and hole expansion test.

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

2-2) Hole Expansion Test A 100 mm square base plate was cut out of each steel plate and subjected to 10 mmφ punching at the center of the base plate using a universal testing machine. The clearance for punching was approximately 12% in accordance with the Japan Iron and Steel Federation Standard (JFST 1001-1996). An open hole test was performed on the blank. The hole spreading test was performed by a method according to the Japan Iron and Steel Federation Standard (JFST 1001-1996), 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 the steel structure defined in the present invention, and have 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 ( ) has an excellent press formability of 60% or more, and further has an excellent collision resistance property of a yield ratio (YR) of 80% or more.

On the other hand, test material No. As for 12 to 14, although the steel composition and steel structure satisfy the specification of the present invention, since the rolling completion temperature falls below Ar ₃ −10 ° C. specified in the present invention, the ferrite is developed with deformation strain during rolling. And the overall elongation and hole expansion rate significantly deteriorated.

  Test material No. Since the time from the completion of rolling to the start of cooling in the hot rolling process exceeds 0.2 seconds specified in the present invention, the ferrite particle size is excessively coarsened and the ferrite particle size specified in the present invention is In addition, the percentage of acicular ferrite decreases and the sample No. 1 In No. 15, the tensile strength is less than 440 MPa, and the strength range specified in the present invention can not be satisfied. In No. 16, the yield ratio (YR) was below the range specified in this specification, resulting in poor anti-collision performance.

  Test material No. In No. 17, since the cooling rate from the completion of rolling to the cooling stop temperature in the hot rolling process fell below the specified range of the present invention, the ferrite rapidly coarsened, so the tensile strength falls below 440 MPa, and the specified range of the present invention Could not meet.

  Test material No. In both of the steels 18 and 19, the time from the completion of rolling to the start of cooling and the cooling rate to the cooling stop temperature in the hot rolling process do not fall within the specified ranges of the present invention, and the ferrite grain size and acicular ferrite ratio The tensile strength of 440 MPa or more and the yield ratio (YR) of 80% or more could not be satisfied.

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

  Test material No. No. 21 has a tensile strength and a yield ratio (YR) because the ferrite grain size is coarsened and the proportion of acicular ferrite is low because the cooling stop temperature in the hot rolling step is higher than the specified range of the present invention. It did not reach the specified range of the present invention.

  Test material No. Since No. 22 is a cold stop temperature in the hot rolling process lower than the specified range of the present invention, bainite is excessively developed and the amount of ferrite specified in the present invention can not be secured, so the total elongation (El) The rate (λ) was significantly degraded, and mechanical properties within the specified range of the present invention were not obtained.

  Test material No. In No. 23, since the coiling temperature in the hot rolling step exceeded the temperature specified in the present invention, the ferrite grains became coarse, and the tensile strength was below the range specified in the present invention.

  Test material No. No. 24 exceeds the range of the Mn content specified in the present invention, and an excess of bainite is formed, and the necessary amount of ferrite can not be obtained, so the tensile force 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) become lower than the values specified in the present invention, and the ductility is poor. In addition, the yield ratio (YR) was less than the specified range of the present invention.

  Test material No. 25 exceeds the range of the Mn content defined in the present invention, bainite is excessively formed, and the required amount of ferrite can not be obtained, so the total elongation (El) and the hole expansion (λ) are also the present invention. As a result, the ductility was poor.

  Furthermore, the specimen No. No. 26 was lower than the range of C content defined in the present invention, and the tensile strength was less than the range of 440 MPa or more and less than 590 MPa defined in the present invention.

Claims (5)

C: 0.06% or more and 0.15% or less by mass, Si: 0.01% or more and 0.5% or less, Mn: 0.1% or more and 0.5% or less, 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,
The area ratio is at least one of cementite, martensite or bainite with 90% or more of ferrite and 5% or less of remainder, and the ratio of acicular ferrite in the ferrite is 10% or more, and the average particle diameter of ferrite is A steel structure having a size of 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 of the tensile strength to the 0.2% proof stress is 80% or more, and the total elongation (El) is 30% or more. A hot rolled steel sheet characterized by having mechanical characteristics having an expansion ratio (λ) of 60% or more.   The chemical composition further contains one or more selected from the group consisting of, by mass%, Ti: 0.02% or less, Nb: 0.02% or less and V: 0.02% or less The hot rolled steel sheet according to Item 1.   The chemical composition further comprises one or more selected from the group consisting of, by mass%, Cr: 0.03% or less, Mo: 0.03% or less and B: 0.01% or less The hot rolled steel sheet according to claim 1 or claim 2.   The steel composition further contains 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 by mass%. The hot rolled steel sheet according to any one of claims 1 to 3. The billet is subjected to finish hot rolling, and after completion of rolling at Ar 3-10 ° C. or higher, cooling is started within 0.2 seconds, and a temperature of 600 ° C. or more and 740 ° C. or less at a cooling rate of 100 ° C./s or more. It is cooled to the temperature range, allowed to cool down to a temperature of 20 ° C to 40 ° C from the cooling stop temperature, and further cooled to a temperature range of 550 ° C or less at a cooling rate of 45 ° C / sec or more and then wound up. The manufacturing method of the hot rolled sheet steel according to any one of claims 1 to 4, characterized by the above.
However, Ar3 (° C.) = 905-455 [% C] -38 [% Si] -62 [% Mn] + 472 [% P], and [% C], [% Si], [% Mn], [% P] shows the content (mass%) of C, Si, Mn, and P, respectively.