JP6288321B2 - High strength hot rolled steel sheet and method for producing the same - Google Patents

Description

  The present invention is a high-strength hot-rolled steel sheet suitable for structural steel materials such as parts for transportation machinery such as automobiles and construction steel, and has a high strength of tensile strength (TS): 590 MPa or more. The present invention relates to a high-strength hot-rolled steel sheet excellent in stretch flangeability and fatigue characteristics and a method for producing the same.

From the viewpoint of global environmental conservation, for the purpose of reducing CO ₂ emissions, maintaining the strength of the car body while reducing the weight and improving the fuel efficiency of the car has always been an important issue in the automobile industry. . In order to reduce the weight of the vehicle body while maintaining the strength of the automobile body, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet used as a material for automobile parts. For example, the reduction in thickness and strength of steel plates for undercarriage parts of automobiles, which often use thick steel plates, can be expected to significantly reduce the weight of automobile bodies.

  On the other hand, since automobile parts made of steel plates, for example, suspension parts such as lower arms, are formed by burring or the like, excellent stretch flangeability is required for steel sheets for automobile parts. In addition, since bending stress continues to be generated in automobile parts due to vibration during running of the automobile, the steel sheet for automobile parts used as the material is also required to have excellent fatigue characteristics. For these reasons, high strength steel sheets that are excellent in stretch flangeability and fatigue properties are desired as steel sheets that are particularly applied to automobile underbody parts such as lower arms.

  Here, many studies have been conducted on high strength hot rolled steel sheets having both strength and workability, and various techniques have been proposed. Above all, the technology to increase the strength of the steel sheet by making the metal structure substantially ferrite single phase and precipitating fine carbides in the ferrite phase grains is to achieve both excellent strength and excellent flange flangeability. It is known to be a very useful technique.

  For example, Patent Document 1 relates to a high-strength hot-rolled steel sheet, and the composition of the steel sheet in mass% is C: 0.005% to 0.050%, Si: 0.2% or less, Mn: 0.8% or less, P: 0.025% or less, S: 0.01% or less, N: 0.01% or less, Al: 0.06% or less, Ti: 0.05% or more and 0.10% or less, S, N, and Ti are Ti ≧ 0.04 + (N / 14 × 48 + S / 32 × 48) (S , N, Ti: The content of each element (mass%) is included so that the balance is composed of Fe and inevitable impurities, and the steel sheet structure has an area ratio of 95% or more with respect to the entire structure of the ferrite phase. And a matrix in which fine carbides containing Ti and having an average particle diameter of less than 10 nm are dispersed and precipitated, and a structure in which the volume ratio of the fine carbides to the whole structure is 0.0007 or more has been proposed. In Patent Document 1, the steel sheet structure is a ferrite single-phase structure with excellent dislocation density and excellent workability. Further, by dispersing and precipitating fine carbides, the steel sheet is maintained while maintaining stretch flangeability. It is described that a high-tensile hot-rolled steel sheet having high strength, tensile strength of 590 MPa or more and excellent workability can be obtained.

JP 2012-26034 A

  However, in the technique proposed in Patent Document 1, the fatigue characteristics of the hot-rolled steel sheet are not studied. Therefore, according to the technique proposed in Patent Document 1, although a high-strength steel sheet excellent in stretch flangeability can be obtained, a high-strength steel sheet having fatigue characteristics is not always obtained.

  The present invention advantageously solves the problems of the prior art and is suitable as a material for automobile parts. It has a high tensile strength (TS) of 590 MPa or more, has excellent stretch flangeability, and further has fatigue characteristics. Another object of the present invention is to provide an excellent high-strength hot-rolled steel sheet and a method for producing the same.

In order to solve the above-mentioned problems, the present inventors diligently studied a technique for increasing the strength of a steel sheet while maintaining high workability and further greatly improving fatigue characteristics.
First, the inventors focused on hot-rolled steel sheets that have a ferrite phase with good workability as the main phase, and a technique for increasing the strength of hot-rolled steel sheets while maintaining excellent workability, particularly stretch flangeability. investigated. One technique for increasing the strength of a hot-rolled steel sheet having a ferrite phase as a main phase is, for example, a technique using a transformation structure such as martensite. However, when the steel sheet structure includes a soft phase and a hard phase as in the ferrite-martensite composite structure steel, voids are easily generated at the interface between the soft phase and the hard phase, and the stretch flangeability of the steel sheet is greatly deteriorated. As a technique for increasing the strength of a steel sheet while maintaining high stretch flangeability, it is preferable that the metal structure has a uniform strength.

  For the above reasons, the present inventors decided to increase the strength of the steel sheet by a method in which the hot-rolled steel sheet has a ferrite single-phase structure and the metal structure is uniformly strengthened. Examples of a technique for uniformly increasing the strength of the metal structure include, for example, a technique for increasing the strength of a ferrite single-phase steel sheet by solid solution strengthening of Si and Mn, A technique for precipitating fine carbides in the grains is effective.

  Next, the present inventors examined a technique for improving the fatigue characteristics of the steel sheet. In general, it is said that increasing the yield strength of a steel sheet is effective for improving fatigue properties. However, the fatigue properties of the steel sheet are not necessarily determined only by the yield strength. Therefore, the present inventors conducted a plane bending fatigue test on hot-rolled steel sheets having various ferrite single-phase structures and observed fatigue fracture surfaces in order to identify factors that affect the fatigue properties of hot-rolled steel sheets. . As a result, it was found that the deterioration of the fatigue characteristics of the hot-rolled steel sheet was largely due to the hard internal oxide layer near the steel sheet surface. And in the case of a hot rolled steel sheet having a ferrite single phase structure, when repeated bending load is applied, stress concentration occurs at the interface between a hard internal oxide layer near the steel sheet surface and a relatively soft ferrite, We have come to know that cracks have occurred starting from the internal oxide layer and fatigue fracture has progressed.

  From the above findings, it is presumed that a hot-rolled steel sheet having excellent fatigue characteristics can be obtained by increasing the yield strength while suppressing the formation of the internal oxide layer. Therefore, the present inventors examined a method for uniformly increasing the strength of the metal structure of the hot-rolled steel sheet having a single phase ferrite structure, suppressing the formation of an internal oxide layer, and further increasing the yield strength. .

  The internal oxide layer is mainly formed of Si or Mn oxide. Therefore, to suppress the internal oxide layer, it is effective to reduce the Si content and the Mn content of the steel sheet. However, when increasing the strength of a steel sheet by solid solution strengthening of Si and Mn, it is necessary to add a large amount of Si and Mn, and a thick internal oxide layer is inevitably generated. For this reason, it is difficult to improve the fatigue characteristics by the technique of increasing the strength of the steel sheet by solid solution strengthening of Si and Mn.

  Therefore, the present inventors use Ti precipitation strengthening and limit the content of Si, Mn, etc. to suppress the internal oxide layer, and to achieve both high strength and fatigue properties of the hot-rolled steel sheet. I thought. However, as a result of studying a method for suppressing the internal oxide layer of the hot-rolled steel sheet, it has been found that the internal oxide layer cannot be sufficiently suppressed only by limiting the content of Si, Mn and the like.

In response to such problems, the present inventors examined the hot rolling conditions when producing hot-rolled steel sheets in addition to limiting the content of Si, Mn, etc., and generated internal oxide layers. I tried to suppress it further. As a result, when the Si content is 0.40% or less and the Mn content is 1.20% or less, measures are taken to appropriately remove the oxide scale and further suppress the generation of oxide scale in the hot rolling process. As a result, it was found that precipitation of Si and Mn oxide near the steel sheet surface can be suppressed to 0.15 g / m ² or less. And it became clear that the fatigue characteristics of the hot-rolled steel sheet having a ferrite single-phase structure are greatly improved by suppressing the internal oxidation amount of the steel sheet surface layer to 0.15 g / m ² or less.

  In addition, the inventors tried to further improve the fatigue characteristics by increasing the yield strength of the hot-rolled steel sheet while achieving suppression of the internal oxide layer as described above. As a result, it was found that the yield strength of the hot-rolled steel sheet is dramatically increased by precipitating fine carbides containing Ti on the steel sheet and leaving a predetermined amount of solute C in the steel. Furthermore, as a result of studying the reason why the yield strength increases dramatically, dislocation motion is further greatly inhibited by solid solution C sticking to Cottrel, while dislocation is restricted by the fine carbide containing Ti. It was revealed that the yield strength was greatly improved.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: more than 0.020% and less than 0.060%, Si: less than 0.40%, Mn: more than 0.50% and less than 1.20%, P: 0.030% and less, S: 0.030% and less, Al: 0.10% and less, N : Contains 0.0100% or less, Ti: 0.050% or more and 0.110% or less, the balance is composed of Fe and inevitable impurities, the area ratio of ferrite phase is 95% or more, and the amount of solute C in steel 0.010% or more, carbide containing Ti is finely precipitated in the ferrite phase crystal grains, the carbide has an average particle size of less than 8 nm, the internal oxidation amount of the steel sheet surface layer portion is 0.15 g / m ² or less, high-strength hot-rolled steel sheet tensile strength is equal to or not less than 590 MPa.

[2] In the above [1], in addition to the composition, in addition to mass, REM, Zr, V, Nb, As, Cu, Ni, Sn, Pb, Ta, W, Mo, Cr, Sb, Mg, A high-strength hot-rolled steel sheet characterized by containing 1.0% or less in total of one or more selected from Ca, Co, Se, Zn, and Cs.

[3] The high-strength hot-rolled steel sheet according to [1] or [2], wherein the steel sheet surface has a plating layer.

[4] The steel material having the composition described in [1] or [2] is heated to an austenite single-phase region, subjected to hot rolling, accelerated cooling, winding, and cooling after the winding. To make a hot-rolled steel sheet,
In the hot rolling,
Descale with high-pressure water with impact pressure of 0.3 MPa or more before finish rolling,
The finish rolling is started within 3 s after the descaling,
The cooling between the stands which injects cooling water on the steel plate surface between any one or more stands of the finish rolling is performed,
The finish rolling temperature is 850 ° C or higher and 1100 ° C or lower,
The accelerated cooling is
Start within 5s after cooling between the stands,
The average cooling rate in the temperature range from 830 ° C. to 720 ° C. of the accelerated cooling is 30 ° C./s or more,
In the winding, the winding temperature is 520 ° C. or more and 720 ° C. or less,
The cooling after winding is a method for producing a high-strength hot-rolled steel sheet in which the average cooling rate in the temperature range from 500 ° C. to 200 ° C. is 10 ° C./h or more.

  According to the present invention, tensile strength (TS): suitable for structural steel materials such as automobile and other transportation machinery parts and construction steel materials, has high strength of 590 MPa or more and excellent stretch flangeability, In addition, a high-strength hot-rolled steel sheet having excellent fatigue characteristics can be obtained. Moreover, since the outstanding steel plate characteristic is acquired as mentioned above, this invention enables the further use expansion | deployment of a high intensity | strength hot-rolled steel plate, and has an industrial remarkable effect.

FIG. 1 is a diagram schematically showing a cross section near the surface of a hot-rolled steel sheet. FIG. 2 is a diagram showing the relationship between the amount of internal oxidation and the amount of solute C and the fatigue characteristics of a hot-rolled steel sheet.

Hereinafter, the present invention will be specifically described.
First, the reason for limiting the component composition of the hot-rolled steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% (mass%) unless there is particular notice.

C: more than 0.020% and less than 0.060%
C is an essential element for forming a carbide containing Ti in the steel sheet and increasing the strength of the hot-rolled steel sheet. If the C content is 0.020% or less, a carbide precipitation amount sufficient to make the tensile strength of the steel sheet 590 MPa or more cannot be obtained, and a tensile strength of 590 MPa or more cannot be obtained. On the other hand, if the C content exceeds 0.060%, surplus C that does not form Ti-containing carbides precipitates as cementite, and the stretch flangeability of the steel sheet decreases. Therefore, the C content is more than 0.020% and not more than 0.060%.

Si: 0.40% or less
Usually, Si is positively contained in a high-strength steel sheet as an effective element for improving the steel sheet strength without reducing ductility (elongation). However, Si is an element that deteriorates the fatigue characteristics of a steel sheet by forming an internal oxide layer near the surface of the steel sheet after the winding process following the cooling after the end of hot rolling when manufacturing a hot-rolled steel sheet. is there. Therefore, in the present invention, the Si content is limited to 0.40% or less for the purpose of suppressing the generation of Si oxide after winding and suppressing the formation of the internal oxide layer. The Si content is preferably 0.30% or less, more preferably 0.20% or less. Note that the Si content may be reduced to the impurity level.

Mn: more than 0.50% and less than 1.20%
Mn is a solid solution strengthening element and, like Si, is actively contained in ordinary high-strength steel sheets. However, if Mn is actively contained in the steel plate, an internal oxide layer is formed in the vicinity of the steel plate surface after the winding process, as in the case of Si, and the fatigue properties of the steel plate deteriorate. Therefore, in the present invention, the Mn content is 1.20% or less for the purpose of suppressing the generation of Mn oxide after winding and suppressing the formation of the internal oxide layer. The Mn content is preferably 1.00% or less.

  On the other hand, when the Mn content is 0.50% or less, the γ → α transformation point becomes high temperature, and it becomes difficult to refine the carbide containing Ti. The carbide containing Ti precipitates at the same time as the γ → α transformation in the cooling and winding process after finish rolling in the hot-rolled steel sheet manufacturing process, or in the ferrite. Here, when the [gamma]-> [alpha] transformation point becomes high temperature, the carbide containing Ti is precipitated in a high temperature region, so that the carbide becomes coarse. And it becomes difficult to make the carbide | carbonized_material containing Ti predetermined size, and desired steel plate intensity | strength will not be obtained. Therefore, the Mn content is more than 0.50%. The Mn content is preferably 0.60% or more, more preferably 0.80% or more.

P: 0.030% or less
P is a harmful element that segregates at the grain boundary to lower the elongation of the hot-rolled steel sheet, induces cracking during processing, and further degrades the impact resistance. Therefore, the P content is 0.030% or less.

S: 0.030% or less
S exists as MnS and TiS in steel. MnS and TiS promote the generation of voids during the punching process of hot-rolled steel sheets, and further serve as a starting point for the generation of voids during processing, thus reducing the stretch flangeability of the steel sheet. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and it is 0.030% or less. The S content is preferably 0.010% or less.

Al: 0.10% or less
Al is an element that acts as a deoxidizer. In order to obtain such an effect, it is desirable to contain 0.01% or more of Al. However, if the Al content exceeds 0.10%, it remains as an Al oxide in the steel sheet, and the Al oxide tends to agglomerate and become coarse, causing the stretch flangeability of the steel sheet to deteriorate. Therefore, the Al content is 0.10% or less. The Al content is preferably 0.05% or less.

N: 0.0100% or less
Since N exists as TiN in the steel, when the N content becomes large, the Ti content forming carbides decreases due to the presence of N, and the desired steel plate strength cannot be obtained. In addition, TiN promotes the generation of voids during the punching process of hot-rolled steel sheets, and further, since it becomes a starting point for the generation of voids during processing, it reduces the stretch flangeability of the steel sheet. For the above reasons, in the present invention, it is preferable to reduce the N content as much as possible, and it is set to 0.0100% or less. The N content is preferably 0.0060% or less.

Ti: 0.050% to 0.110%
Ti is an indispensable element for forming a carbide containing Ti to increase the strength of the steel sheet. When the Ti content is less than 0.050%, it is difficult to obtain a desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more). On the other hand, when the Ti content exceeds 0.110% and becomes excessive, when manufacturing a hot-rolled steel sheet, the carbide containing Ti tends to be coarsened after the winding process, and the desired hot-rolled steel sheet strength (tensile strength: 590 MPa or more) ) Is difficult to obtain. Therefore, Ti content shall be 0.050% or more and 0.110% or less. The Ti content is preferably 0.060% or more. Further, the Ti content is preferably 0.100% or less.

  Components other than the above are Fe and inevitable impurities. In addition to the above elements, the hot-rolled steel sheet of the present invention further includes REM, Zr, V, Nb, As, Cu, Ni, Sn, Pb, Ta, W, Mo, Cr, Sb, Mg, Ca, One or more selected from Co, Se, Zn, and Cs may be contained in a total of 1.0% or less. If the total content of these elements is 1.0% or less, the various properties of the hot-rolled steel sheet will not be adversely affected.

Next, the reason for limiting the structure of the hot rolled steel sheet and the amount of internal oxidation of the present invention will be described.
The hot-rolled steel sheet of the present invention has the above-described composition, and further, the area ratio of the ferrite phase is 95% or more, the solid solution C amount in the steel is 0.010% or more, The carbide containing Ti is finely precipitated, the carbide has an average particle diameter of less than 8 nm, and the internal oxidation amount of the steel sheet surface layer portion is 0.15 g / m ² or less.

Ferrite phase area ratio: 95% or more In the present invention, the formation of a ferrite phase is essential to ensure stretch flangeability of a hot-rolled steel sheet. In order to improve the stretch flangeability, it is preferable that the metal structure of the hot-rolled steel sheet is a ferrite single phase, but even if it is not a complete ferrite single phase, it is substantially a ferrite single phase, that is, the entire metal structure. If the ferrite phase is 95% or more in terms of the area ratio relative to the above, the above-described effects are sufficiently exhibited. Therefore, in the present invention, the metal structure of the hot-rolled steel sheet is a structure containing a ferrite phase of 95% or more in area ratio. Preferably it is 97% or more.

  In the hot-rolled steel sheet of the present invention, examples of the structure other than the ferrite phase that can be contained in the metal structure include cementite, pearlite, bainite, martensite, and retained austenite. When these structures are present in the metal structure, the stretch flangeability of the steel sheet is lowered. Therefore, it is preferable to reduce these structures as much as possible, but it is acceptable if the total area ratio with respect to the entire metal structure is 5% or less. Preferably it is 3% or less.

Solid solution C in steel: 0.010% or more C dissolved in steel adheres to dislocations whose movement is restricted by fine carbides, and further inhibits dislocation movement. This significantly increases the yield strength of the steel sheet and improves the fatigue characteristics. In order to fully exhibit such an effect, it is necessary to make the amount of solute C in steel 0.010% or more by mass%. The amount of solute C in the steel is preferably 0.012% or more.

However, if the amount of solute C in the steel is excessive, the amount of C that contributes to the formation of fine carbides (carbides containing Ti) decreases and the tensile strength TS of the steel sheet may decrease. The amount of C is preferably 0.03% or less by mass.
In the present invention, the amount of solute C is obtained by subtracting the amount of C (mass%) consumed by precipitation of carbide containing Ti, Fe ₃ C, etc. from the total amount of C (mass%) contained in the steel sheet. Value.

Carbide containing Ti In the present invention, the carbide finely precipitated on the hot-rolled steel sheet is a carbide containing Ti. When the hot-rolled steel sheet contains only Ti as a carbide constituent element, the carbide containing Ti is Ti carbide. In addition, when the hot-rolled steel sheet also contains carbide constituent elements (V, Nb, Mo, etc.) other than Ti, in addition to Ti carbide, a composite containing one or more of Ti, V, Nb, and Mo Carbides or those containing carbide constituent elements such as Zr and W in the carbides may be mentioned.

Average particle size of carbide containing Ti: less than 8nm In order to make the strength of hot rolled steel sheet tensile strength: 590MPa or more, the average particle size of carbide containing Ti is extremely important. In the present invention, Ti is included. The average particle size of the carbide is less than 8 nm. When carbide containing Ti finely precipitates in the ferrite phase crystal grains, the carbide acts as a resistance to dislocation movement that occurs when deformation is applied to the steel sheet, thereby increasing the strength of the hot-rolled steel sheet. However, as the carbides containing Ti are coarsened, the carbides are sparsely deposited, and the interval for stopping the dislocation is widened, so that the precipitation strengthening ability is lowered. And when the average particle diameter of the carbide | carbonized_material containing Ti will be 8 nm or more, sufficient steel plate reinforcement | strengthening capability for obtaining desired steel plate strength will not be acquired. Therefore, the average particle size of the carbide containing Ti is less than 8 nm. The average particle size of the carbide containing Ti is preferably 6 nm or less.

  In addition, although the effect of invention is not specifically limited, the fine precipitate (carbide containing Ti) in this invention may be observed so that it may be located in a line depending on the angle to observe. However, even in this case, the precipitates are actually randomly distributed in the plane where the rows of precipitates are observed, and when observed with a transmission electron microscope, the precipitates are often not observed in rows.

Internal oxidation amount of steel sheet surface layer (internal oxidation amount per side): 0.15 g / m ² or less As shown in Fig. 1, the surface of a hot rolled steel sheet (black skin material) containing Si or Mn is usually The oxide 4 is present in the region from the interface 3 between the black skin 1 and the ground iron 2 to a depth of about 10 μm, and an internal oxide layer is formed. In the case of the hot-rolled steel sheet of the present invention containing Ti, the oxide 4 is mainly an oxide of Si, Mn, Ti, or a composite oxide thereof. When a bending load is repeatedly applied to the hot-rolled steel sheet, the internal oxide layer causes stress concentration at the interface with the ferrite phase, which is the parent phase, and becomes a starting point of crack generation, and induces fatigue failure.

However, as a result of studies by the present inventors, it has been clarified that the above phenomenon (fatigue fracture) can be suppressed by setting the internal oxidation amount to 0.15 g / m ² or less. Therefore, in the present invention, the internal oxidation amount is set to 0.15 g / m ² or less. Internal oxidation amount is preferably 0.10 g / m ² or less, more preferably 0.05 g / m ² or less.
In the present invention, the amount of internal oxidation means the mass (g) of oxygen of oxide 4 present per unit area (1 m ² ) on one side of a hot-rolled steel sheet. In the present invention, the steel sheet surface layer means a region from the interface 3 between the black skin 1 and the ground iron 2 to a depth of 10 μm (surface layer part 10 in FIG. 1) in the black skin material, and a steel plate in the white skin material. It means a region from the surface to a depth of 10 μm, and in the case of a plating material, a region from the interface between the plating and the ground iron to a depth of 10 μm.

  By defining the composition, structure, and internal oxidation amount of the steel sheet surface layer as described above, it has the desired strength (tensile strength: 590 MPa or more), excellent stretch flangeability, and high fatigue characteristics. A strength hot-rolled steel sheet is obtained. Moreover, even if a plated layer is provided on the surface of the hot-rolled steel sheet of the present invention for the purpose of imparting corrosion resistance to the steel sheet, the above-described effects of the present invention are not impaired. In the present invention, the type of the plating layer provided on the surface of the steel sheet is not particularly limited, and any of an electroplating layer and a hot dipping layer can be applied. The alloy component of the plating layer is not particularly limited, and preferred examples include a galvanized layer and an alloyed galvanized layer, but are not limited thereto. By forming a plating layer on the surface, the corrosion resistance of the hot-rolled steel sheet is improved, and it becomes possible to apply it to automobile parts used in severe corrosive environments.

Next, the manufacturing method of the hot rolled steel sheet of the present invention will be described.
In the present invention, a steel material having the above composition is heated to an austenite single-phase region, hot-rolled, then cooled (for example, water-cooled), wound up to obtain a hot-rolled steel sheet. At this time, before the finish rolling of the hot rolling, the impact pressure is descaled with high-pressure water of 0.3 MPa or more, the finish rolling is started within 3 s after the descaling, and any one or more of the finish rolling is performed. The inter-stand cooling is performed by injecting cooling water onto the steel plate surface between the stands, the finish rolling temperature is set to 850 ° C. or more and 1100 ° C. or less, and the cooling is started within 5 s after the cooling between the stands, The average cooling rate to 720 ° C is 30 ° C / s or more, the winding temperature of the winding is 520 ° C to 720 ° C, and after the winding, the average cooling rate from 500 ° C to 200 ° C is 10 It is characterized by being cooled to at least ° C / h.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Thereafter, a slab is cast from molten steel, but it is preferable to use a continuous casting method to obtain a slab (steel material) from the viewpoint of productivity. However, the slab may be formed by a known casting method such as ingot-bundling rolling or continuous slab casting.

  The steel material obtained as described above is hot-rolled. In the present invention, the steel material is heated to an austenite single-phase region prior to hot rolling. If the steel material before hot rolling is not heated to the austenite single-phase region, the re-dissolution of carbides (carbides containing Ti) in the steel materials will not proceed, and fine precipitation of carbides containing Ti after rolling Cannot be realized.

  For the above reasons, in the present invention, prior to hot rolling, the steel material is heated to an austenite single phase region, preferably 1200 ° C. or higher. However, when the heating temperature of the steel material becomes higher than necessary, the yield decrease due to oxidation of the steel material surface becomes remarkable. Therefore, the heating temperature is usually preferably 1350 ° C. or lower. In addition, when hot rolling the steel material, if the steel material (slab) after casting has a temperature in the austenite single-phase region, the steel material is not heated or directly heated after being heated for a short time. You may roll.

  In hot rolling, first, a hot rough rolling is first performed on a steel material to form, for example, a sheet bar. Then, the sheet bar can be descaled and hot-finish rolled to form a hot-rolled steel sheet. In the present invention, the conditions for rough rolling are not particularly limited. In particular, when a thin slab casting method is employed, rough rolling may be omitted. Finish rolling is performed under the following conditions. In addition, the sheet | seat bar which carried out rough rolling of said steel raw material, and the hot-rolled steel plate which hot-rolled the sheet | seat bar are each described also as a steel plate below.

Descaling before finish rolling: high-pressure water having a collision pressure of 0.3 MPa or more In the present invention, it is necessary to suppress the formation of an internal oxide layer after winding described later. Here, in order to suppress the formation of the internal oxide layer, it is extremely effective to reduce the oxide scale (black skin) on the surface of the steel sheet during winding. This is because the easily oxidized elements such as Si and Mn are formed by taking oxygen from the oxide scale on the steel sheet surface after winding. Therefore, by reducing the thickness of the scale, the supply of oxygen can be reduced and the generation of the internal oxide layer can be suppressed. Further, by flattening the interface between the scale and the ground iron, the reaction area between the scale and the ground iron can be reduced, and the generation of the internal oxide layer can be suppressed.

  Therefore, in the present invention, the surface of the steel sheet after finish rolling is flattened and the internal oxide layer is suppressed by sufficiently descaling with an appropriate pressure before finish rolling. Descaling before finish rolling is performed by injecting high-pressure water onto the steel sheet surface. However, when the collision pressure of high-pressure water against the steel sheet surface is less than 0.3 MPa, the generation of the internal oxide layer cannot be sufficiently suppressed. Therefore, in the present invention, descaling is performed with high-pressure water having a collision pressure of 0.3 MPa or more before finish rolling. The collision pressure is preferably 0.5 MPa or more, and more preferably 1.0 MPa or more. However, if the impact pressure becomes larger than necessary, it is feared that the steel sheet surface layer portion undergoes ferrite transformation before finish rolling due to excessive cooling during descaling, and therefore the impact pressure is preferably 10 MPa or less.

Time until start of finish rolling after descaling: within 3 s Even if appropriate descaling is performed before finish rolling, new oxide scales are generated over time. However, if finish rolling is started within 3 s after the end of descaling before finish rolling, the adverse effects of the newly generated oxide scale can be ignored. Therefore, in the present invention, finish rolling is started within 3 s after descaling. Preferably, finish rolling is started within 3.0 seconds after descaling.

Cooling between stands Normally, even during finish rolling, generation of oxide scale on the surface of the steel sheet continues. Therefore, in this invention, the scale production | generation during finish rolling is suppressed by performing the cooling between stands which sprays a cooling water on the steel plate surface between stands. When the cooling water is sprayed on the steel plate surface between the stands, the steel plate surface is covered with the cooling water, and the steel plate surface can be prevented from being exposed to the atmosphere (oxygen). As a result, the reaction between the steel plate and oxygen is suppressed, and the generation of oxide scale can be reduced. Such an effect is sufficiently achieved by injecting cooling water between one or more stands. However, it is preferable to perform inter-stand cooling so that the surface temperature of the steel sheet does not become less than 850 ° C. during finish rolling.

  Although the present invention is not particularly limited, it is preferable to sequentially inject cooling water when the temperature of the steel sheet passing between the stands is 1150 ° C. or higher. Further, as will be described later, in the present invention, it is necessary to perform accelerated cooling (for example, water cooling) after completion of hot rolling (after completion of finish rolling) within 5 seconds after completion of cooling between stands. Therefore, in the present invention, a position (between stands) for performing the inter-stand cooling may be appropriately selected so that the accelerated cooling can be performed within 5 s after completion of the inter-stand cooling.

Finish rolling temperature: 850 ° C. or more and 1100 ° C. or less When the finish rolling temperature is lowered, ferrite transformation is promoted in cooling after rolling, and carbides are likely to be coarsely precipitated, making it difficult to obtain a predetermined steel sheet strength. For this reason, the finish rolling temperature, that is, the temperature on the finish rolling final stand exit side needs to be 850 ° C. or higher. Preferably it is 880 degreeC or more, More preferably, it is 920 degreeC or more. On the other hand, when the finish rolling temperature is higher than 1100 ° C., oxide scale is likely to be generated on the surface of the steel sheet, and thus the generation of the internal oxide layer cannot be sufficiently suppressed after winding. Therefore, the finish rolling temperature is limited to 1100 ° C. or lower. The finish rolling temperature is preferably 1050 ° C. or lower, more preferably 1000 ° C. or lower.

Time to start accelerated cooling after inter-stand cooling: within 5 s The above finish rolling temperature is in a temperature range where oxide scale is generated on the steel sheet surface. Therefore, in order to suppress the generation of the internal oxide layer due to the oxide scale, it is necessary to start the accelerated cooling such as water cooling immediately after the finish rolling to minimize the generation of the oxide scale. If the accelerated cooling is started within 5 seconds after the inter-stand cooling is completed, the newly generated oxide scale can be sufficiently reduced. Therefore, in the present invention, accelerated cooling is started within 5 s after inter-stand cooling.

Average cooling rate from 830 ° C to 720 ° C: 30 ° C / s or more In order to precipitate fine carbides (carbides containing Ti) with an average particle size of less than 8 nm, accelerated cooling and γ at the lowest possible temperature → It is necessary to make α transformation occur. When the average cooling rate in the temperature range from 830 ° C. to 720 ° C. is less than 30 ° C./s, the γ → α transformation occurs at a high temperature, and the carbide precipitated in the ferrite tends to be coarsened. Therefore, the average cooling rate in the above temperature range is 30 ° C./s or more. Preferably, it is 50 ° C./s or more. However, if the average cooling rate in the above temperature range becomes larger than necessary, it is difficult to control the coiling temperature and it may be difficult to obtain a stable strength. Therefore, the average cooling rate in the above temperature range is preferably 300 ° C./s or less. The average cooling rate referred to in the present invention refers to an average cooling rate from the start of cooling in a predetermined temperature range to the end of cooling.

Winding temperature: 520 ° C or higher and 720 ° C or lower Optimization of the winding temperature is important for precipitating fine carbides (carbides containing Ti) in ferrite and making the hot-rolled steel sheet a desired metal structure. is there. When the coiling temperature is less than 520 ° C., a low temperature transformation phase such as bainite is likely to occur, and it becomes difficult to make the metal structure substantially a ferrite single phase structure. On the other hand, when the coiling temperature exceeds 720 ° C., the reaction of Si and Mn in the steel deprives oxygen from the oxide scale on the surface of the steel sheet to form an oxide is promoted, and the internal oxidation amount is set to a predetermined amount or less. Becomes difficult. Moreover, since the coarsening of the carbide containing Ti is promoted during cooling of the hot-rolled coil after winding, it is difficult to make the average particle diameter of the carbide containing Ti less than 8 nm. Therefore, the winding temperature is set to 520 ° C. or higher and 720 ° C. or lower. The winding temperature is preferably 550 ° C. or higher. The coiling temperature is preferably 700 ° C. or lower.

After winding, average cooling rate from 500 ℃ to 200 ℃: 10 ℃ / h or more
Excess C that does not contribute to the formation of carbides containing Ti usually precipitates in the steel during cooling after winding. After winding, if the average cooling rate to 200 ° C is slower than 10 ° C / h, there is sufficient time for C dissolved in the ferrite to gather at the ferrite grain boundaries, so most of the excess C It precipitates as grain boundary cementite, and it becomes impossible to obtain a sufficient amount of solute C. Further, precipitation of grain boundary cementite may deteriorate the stretch flangeability of the steel sheet. Therefore, after winding, the average cooling rate from 500 ° C. to 200 ° C. was set to 10 ° C./h or more. The average cooling rate from 500 ° C. to 200 ° C. is preferably 15 ° C./h or more.

  The average cooling rate from 500 ° C. to 200 ° C. after winding is a value measured at the surface position of the inner plate width central portion of the hot rolled coil after winding. By controlling the average cooling rate of the surface position of the central part of the inner plate width of the hot rolled coil after winding within a predetermined range, the cooling rate inside the coil also becomes sufficiently high, and the solid solution in the steel plate extends over the entire length of the coil. C amount can be within the specified range. In addition, the average cooling rate from 500 ° C to 200 ° C of the hot-rolled coil after winding is as follows: accelerated air cooling by using a circulator, etc., cooling using heat of vaporization by water droplet (mist) spray, or water tank It can be adjusted to a desired value by performing accelerated cooling such as immersing the coil therein.

As described above, it has a desired structure, the amount of internal oxidation is limited to 0.15 g / m ² or less, tensile strength (TS): 590 MPa or more, stretch flangeability is good, and fatigue characteristics are high. A strength hot-rolled steel sheet is obtained. The high-strength hot-rolled steel sheet obtained by the present invention is suitable as a steel sheet for automobile undercarriage, and is suitable for press forming performed at room temperature. The high-strength hot-rolled steel sheet obtained by the present invention has excellent heat treatment characteristics, and is therefore suitable for warm forming in which a steel sheet before pressing is heated immediately from 400 ° C. to 750 ° C. and then press-formed immediately. . The thickness of the hot-rolled steel sheet in the present invention is not particularly limited, but is preferably 1.0 mm or more and 8.0 mm or less. In addition, the said desired effect is acquired even if the hot-rolled steel plate obtained by this invention is any case of a black skin material (material as hot-rolled) and a white skin material (hot-roll pickling material).

In the present invention, the hot-rolled steel sheet produced as described above may be plated to form a plating layer on the steel sheet surface. Even if the plating layer is formed, the effect of the present invention is not impaired. As the plating treatment, either electroplating or hot dipping can be applied. Further, the alloy component of the plating layer is not particularly limited, and a hot dip galvanized layer, an alloyed hot dip galvanized layer and the like can be mentioned as suitable examples, but of course, it is not limited thereto. Aluminum or aluminum alloy can also be plated.
Hot-rolled steel sheets that have been plated are also suitable for press forming materials that are performed at room temperature, and also for warm forming in which press forming is performed immediately after heating the steel sheet before pressing from 400 ° C to 750 ° C. is there.

  Molten steel was melted and continuously cast by a generally known method to obtain a slab (steel material) having a thickness of 300 mm having the composition shown in Table 1. These slabs are heated to 1250 ° C., roughly rolled, and subjected to finish rolling under the conditions shown in Table 2, accelerated cooling is performed after finishing rolling, winding is performed, and the coil after winding is further cooled to obtain a plate thickness. : 3.2 mm hot rolled steel sheet (black skin material).

Subsequently, except for some of the hot-rolled steel plates (steel plate No. S3), the hot-rolled steel plates obtained as described above were pickled to remove the surface scale, thereby obtaining a white skin material. In addition, some hot-rolled steel sheets (steel No. S7) are pickled to remove the surface scale, and then passed through a hot dip galvanizing line with an annealing temperature of 720 ° C, and a 460 ° C galvanizing bath (plating composition) : 0.15 mass% Al-Zn), and a hot-dip galvanized layer having an adhesion amount of 45 g / m ² per side was formed on the surface of the steel plate to obtain a hot-dip galvanized steel plate (GI material). Further, some hot-rolled steel plates (steel plate No. S11) were subjected to alloying treatment at 520 ° C. after forming a hot-dip galvanized layer as described above to obtain alloyed hot-dip galvanized steel plates (GA material).

  Samples are taken from the hot-rolled steel sheets (black skin material, white skin material, GI material, GA material) obtained above, and subjected to structure observation, tensile test, hole expansion test and fatigue test, and the ferrite phase area ratio The type and area ratio of the structure other than the ferrite phase, the average particle diameter, tensile strength, elongation, hole expansion ratio (stretch flangeability) and fatigue strength of the carbide containing Ti were determined. In addition, test pieces were collected from hot-rolled steel sheets (black skin material, white skin material, GI material, GA material), and the amount of solute C was measured by electrolytic extraction. Furthermore, test pieces were collected from hot-rolled steel sheets (black skin material, white skin material, GI material, GA material), the oxidation amount of the steel plate surface layer portion was quantified, and the internal oxidation amount of the steel plate surface layer portion was measured. Various test methods and measurement methods were as follows.

(I) Microstructure observation A test piece was taken from the obtained hot-rolled steel sheet, a cross section (L cross section) at a thickness of 1/4 position parallel to the rolling direction of the test piece was polished, corroded with nital, and then optical microscope. (Magnification: 400 times) and a scanning electron microscope (magnification: 2000 times) were taken for tissue photographs. Subsequently, using the photographed structure | tissue photograph, the kind of structure | tissue other than a ferrite phase and a ferrite phase and those area ratios were calculated | required with the image-analysis apparatus.

Moreover, the thin film produced from the plate | board thickness 1/4 position of the obtained hot-rolled steel plate was observed with the transmission electron microscope (TEM), and the average particle diameter of the carbide | carbonized_material containing Ti was calculated | required. The average particle size of carbides containing Ti was measured using a photograph taken with a transmission electron microscope (magnification: 300,000 times) for a minimum of 100 carbides (carbides containing Ti) in a total of 5 fields. The average value was defined as the average particle size. The carbide particle diameter is determined by measuring the maximum diameter d of the carbide (the diameter of the largest portion) and the diameter (thickness) t in the direction orthogonal thereto, and the arithmetic average value d _def = (d + t) / 2 As sought. Moreover, the identification of the carbide containing Ti was analyzed by EDX attached to TEM.

(Ii) Tensile test JIS No. 5 tensile test piece (JIS Z 2241) with the tensile direction in the direction perpendicular to the rolling direction (C direction) is taken from the obtained hot-rolled steel sheet and complies with the provisions of JIS Z 2241 The tensile test was performed, and the yield strength (YS), tensile strength (TS), and elongation (El) were measured.

(Iii) Hole expansion test From the obtained hot-rolled steel sheet, a test piece (size: 100 mm x 100 mm) was sampled, and a hole with an initial diameter d ₀ : 10 mmφ was punched into the test piece (clearance: test piece plate thickness) Of 12.5%). Using these test pieces, a hole expansion test was performed. That is, a conical punch having an apex angle of 60 ° is inserted from the punch side at the time of punching into the hole, the hole is expanded, and the diameter d of the hole when the crack penetrates in the thickness direction of the steel plate (test piece) is set. The hole expansion ratio λ (%) was calculated by the following formula. In addition, when the hole expansion ratio λ was 75% or more, it was evaluated that the workability (stretch flangeability) was good.
Hole expansion rate λ (%) = {(d−d ₀ ) / d ₀ } × 100

(Iv) Fatigue test A specimen was collected from the obtained hot-rolled steel sheet, and a plane bending fatigue test (double swing plane bending fatigue test) based on JIS Z 2275 (1978) was performed. The shape of the test piece was the No. 1 test piece (1-20) defined in JIS Z 2275 (1978). The maximum stress at which no fracture was observed until 10 ⁷ cycles was measured, and this stress was defined as fatigue strength. In addition, when the ratio of fatigue strength to the tensile strength measured in (ii) (fatigue strength / tensile strength), that is, the durability ratio was 0.40 or more, the fatigue characteristics were evaluated as good.

(V) Solid solution C amount in steel The solid solution C amount in steel was determined by subtracting the amount of C forming TiC and Fe ₃ C from the total amount of C contained in the hot-rolled steel sheet. The amount of C forming TiC and Fe ₃ C was experimentally determined by electrolytic extraction of the obtained hot-rolled steel sheet. Here, it was assumed that all the Fe contained in the extraction residue formed Fe ₃ C. In addition, the amount of C that forms TiC was calculated assuming that Ti that forms TiN and TiS, and Ti that excludes Ti in solid solution, all of Ti contained in the steel forms TiC. . At this time, it was assumed that N and S contained in the steel form all TiN and TiS. Moreover, the amount of solid solution Ti was calculated | required by quantifying the amount of Ti melt | dissolved in the electrolyte solution after electrolysis. In addition, the amount of element to contain was calculated | required by the chemical analysis of the obtained hot-rolled steel plate. The electrolytic extraction method was performed under the following conditions.

Test specimens were collected from the obtained hot-rolled steel sheet, and the current density was 20mA / cm ² in 10% AA electrolyte (10vol% acetylacetone-1mass% tetramethylammonium chloride / methanol). On the other hand, constant current electrolysis was performed. After the constant current electrolysis, the obtained electrolytic solution was filtered using a 200 nm pore size filter, and after the electrolytic residue remaining on the filter paper was appropriately treated, it was analyzed using an ICP spectroscopic analyzer. The amount of Fe was measured. Moreover, the electrolytic solution after filtration was analyzed using an ICP spectroscopic analyzer, and the amount of Ti in the electrolytic solution was measured. Based on the above assumption, the amount of dissolved C was calculated using the following equation.
Solid C content (% by mass)
= C (mass%)-{0.0717 x [quantitative Fe (g)] / [electrolytic mass (g)] x 100}
−0.251 × {[Ti (mass%)] − 3.42 × [N (mass%)] − 1.49 × [S (mass%)]
− ([Quantitative Ti (g)] / [electrolytic mass (g)] × 100)}

In the above formula, C, N, S and Ti mean the C content, N content, S content and Ti content (all by mass%) of the hot-rolled steel sheet (steel material). In the above formula, quantitative Fe is the amount of Fe in the electrolytic residue (g), quantitative Ti is constant current electrolysis and the amount of Ti in the electrolyte after filtration (g), and the electrolytic mass (g) is electrolyzed by constant current electrolysis. It is the mass (g) of the test piece made.
In deriving the above formula, the atomic weight of Fe is 55.85 (g / mol), the atomic weight of C is 12.01 (g / mol), the atomic weight of Ti is 47.88 (g / mol), and the atomic weight of N is 14.01 (g / mol). mol), and the atomic weight of S was 32.07 (g / mol). The electrolytic mass was determined by washing the electrolysis test piece after electrolysis, measuring the mass, and subtracting it from the test piece mass before electrolysis.

(Vi) Internal oxidation amount The internal oxidation amount of the surface layer portion of the hot-rolled steel plate was determined as follows.
A full-thickness test piece was collected from the obtained hot-rolled steel sheet. Further, for the black skin material and the plating material (GI material, GA material), the black skin formed on the front and back surfaces of the test piece (in the case of GI material, GA material, the plating layer) was chemically removed. The amount of oxygen per one side of the surface layer of the test piece was quantified by an impulse furnace-infrared absorption method. Specifically, the amount of oxygen M ₁ in the steel of the test piece (for the black skin material and the plating material, the test piece after peeling the black skin and the plating layer) was quantified by an impulse furnace-infrared absorption method. Next, the amount of oxygen M ₂ in the steel of the test piece whose front and back surfaces were mechanically polished by 10 μm was quantified by an impulse furnace-infrared absorption method. Then, subtract the oxygen amount M ₂ in the steel of the test piece after mechanical polishing of the front and back surfaces from the oxygen amount M _{1 in} the steel before mechanical polishing of the front and back surfaces, and convert it per unit area of one side. Thus, the amount of oxygen per one side of the surface portion of the test piece was determined.

  The results obtained as described above are shown in Table 3. Further, FIG. 2 shows the relationship between the amount of solid solution C obtained by the above (v) and the amount of internal oxidation obtained by the above (vi) and the durability ratio obtained by the above (iv). In FIG. 2, ◯ indicates the case where the durability ratio is 0.4 or more (substantially 0.40 or more), and x indicates the case where the durability ratio is less than 0.4.

As shown in Table 3, all the inventive examples have obtained hot-rolled steel sheets having high strength of tensile strength TS: 590 MPa or more and excellent workability of hole expansion ratio λ: 75% or more. In addition, all the hot-rolled steel sheets of the invention examples have excellent fatigue strength with an internal oxidation amount of 0.15 g / m ² or less and a durability ratio of 0.4 or more. Furthermore, as shown in FIG. 2, by setting the solid solution C amount to 0.010% or more in mass% and the internal oxidation amount of the steel sheet surface layer portion to 0.15 g / m ² or less, the durability ratio becomes 0.4 or more, and the fatigue characteristics are improved. It can be understood that an excellent hot-rolled steel sheet can be obtained. On the other hand, in a comparative example that is out of the scope of the present invention, a predetermined strength is not obtained, or workability (hole expansion rate λ) or fatigue characteristics is deteriorated.

1… black skin
2… Steel
3 ... Interface (Interface between black skin and steel)
4… Oxides
10… Surface layer

Claims (3)

% By mass
C: more than 0.020% and 0.060% or less, Si: 0.40% or less,
Mn: more than 0.50% and 1.20% or less, P: 0.030% or less,
S: 0.030% or less, Al: 0.10% or less,
N: 0.0100% or less, Ti: 0.050% or more and 0.110% or less,
Furthermore, one or more selected from REM, Zr, V, Nb, As, Cu, Ni, Sn, Pb, Ta, W, Mo, Cr, Sb, Mg, Ca, Co, Se, Zn, and Cs A total of 0.125% or less,
The balance has a composition consisting of Fe and inevitable impurities,
The area ratio of the ferrite phase is 95% or more, the amount of dissolved C in the steel is 0.010% or more, carbide containing Ti is finely precipitated in the ferrite phase crystal grains, and the average particle size of the carbide is A high-strength hot-rolled steel sheet having a structure of less than 8 nm, an internal oxidation amount of a steel sheet surface layer portion of 0.15 g / m ² or less, and a tensile strength of 590 MPa or more.   The high-strength hot-rolled steel sheet according to claim 1, further comprising a plating layer on the steel sheet surface. It is a manufacturing method of the high intensity | strength hot-rolled steel plate of Claim 1 or 2, and after heating a steel raw material to an austenite single phase area and performing a hot rolling, accelerated cooling, winding up, after this winding up When cooling to make a hot-rolled steel sheet,
In the hot rolling,
Descale with high-pressure water with impact pressure of 0.3 MPa or more before finish rolling,
The finish rolling is started within 3 s after the descaling,
The cooling between the stands which injects cooling water on the steel plate surface between any one or more stands of the finish rolling is performed,
The finish rolling temperature is 850 ° C or higher and 1100 ° C or lower,
The accelerated cooling is
Start within 5s after cooling between the stands,
The average cooling rate in the temperature range from 830 ° C. to 720 ° C. of the accelerated cooling is 30 ° C./s or more,
In the winding, the winding temperature is 520 ° C. or more and 720 ° C. or less,
The cooling after winding is a method for producing a high-strength hot-rolled steel sheet in which the average cooling rate in the temperature range from 500 ° C. to 200 ° C. is 10 ° C./h or more.

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