JP2012020309A - Hot rolled steel plate superior in coating corrosion resistance and fatigue property, and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel plate superior in coating corrosion resistance and fatigue property, wherein, even if an electrodeposition baking finish is applied to the hot rolled steel plate having scale layers, a fine chemical conversion film can still be formed thereon without losing adhesion between the scale and a base iron, and to provide a method for manufacturing the same.SOLUTION: The hot rolled steel plate is characterized in that the volume fraction of magnetite in the scale layers is 60% or more, the average crystal grain size of the magnetite is 3 μm or less, and the average roughness of the scale/base iron interface is 1.5 μm or less by the average roughness Ra.

Description

  The present invention relates to a hot-rolled steel sheet having a scale layer, which has excellent corrosion resistance and fatigue properties after electrodeposition coating, and a method for producing the same, and is particularly suitable as a material for automobiles, truck frames, members, chassis, and the like. The present invention relates to a hot-rolled steel sheet excellent in paint corrosion resistance and fatigue characteristics and a method for producing the same.

  Usually, a hot-rolled steel sheet has a scale layer of about 10 μm mainly composed of iron oxide on the surface after being wound as a coil. When such a hot-rolled steel sheet is used for a member such as an automobile or a truck, an electrodeposition baking coating may be applied to the surface of the steel sheet with a scale layer for the purpose of corrosion prevention. It has been a problem that post-corrosion resistance cannot be obtained.

  On the other hand, in addition to corrosion resistance after coating, fatigue characteristics are also required for steel sheets used in automobiles, truck frames, chassis, and the like. In general, it is well known that fatigue characteristics are strongly influenced by the roughness of the steel sheet surface. For example, as described in Patent Document 1, the surface is smoothed by sufficiently performing descaling before finish rolling. There is known a method for improving fatigue characteristics. However, when a steel sheet is produced by this method, the fatigue characteristics are good, but the corrosion resistance after painting of the material may be inferior with the scale attached, and both the fatigue characteristics and the corrosion resistance after painting have been demanded.

In general, when a steel sheet with a scale layer is coated, the corrosion resistance after painting is "(1) Adhesion between scale and ground iron" and "(2) Pretreatment of electrodeposition coating. It is thought that it depends greatly on the chemical conversion process to be performed. Examples of techniques for improving the adhesion of the scale include, for example, a method in which the structure of the scale layer is mainly magnetite (Fe ₃ O ₄ ) (see, for example, Patent Documents 2 to 4), and a method for reducing the scale (eg, patent References 3 to 7) and a method of reducing the ratio of MnFe ₂ O ₄ in the scale layer (see, for example, Patent Document 8) are disclosed.

  However, in the above-described conventional technology, although the adhesion between the scale and the base iron is improved, when a chemical conversion treatment, which is a pretreatment of electrodeposition coating, is performed on a steel plate with a scale, a good chemical conversion treatment film is not formed. Then, there was a problem that the adhesion with an electrodeposition coating film provided thereafter was lowered and the corrosion resistance after coating was deteriorated. In addition, when descaling before finish rolling is performed with a high-pressure water descaling apparatus (for example, see Patent Documents 1 and 9) in order to reduce the scale, chemical conversion processability cannot be sufficiently obtained. As a result, there was a problem that the adhesion of the electrodeposition coating film was lowered and the coating corrosion resistance was deteriorated.

JP 09-137249 A JP 09-271806 A JP 2000-87185 A JP 2002-143905 A Japanese Patent Application Laid-Open No. 07-252593 JP 09-272918 A JP 11-277105 A JP 2004-346416 A JP 2000-015323 A

  The present invention has been made in view of the above problems, and even when electrodeposition baking coating is applied to a hot-rolled steel sheet having a scale layer, the adhesion between the scale and the ground iron is not impaired, and An object of the present invention is to provide a hot-rolled steel sheet that is capable of forming a good chemical conversion coating film and has excellent coating corrosion resistance and fatigue characteristics, and a method for producing the hot-rolled steel sheet.

The inventors first investigated in detail the influence of the scale structure on the chemical conversion treatment performed as a pretreatment for electrodeposition coating of steel sheets. As a result, the film formed on the steel sheet surface by the chemical conversion treatment shows a better form as the magnetite (Fe ₃ O ₄ ) fraction in the scale is higher and the particle size of the magnetite is finer. I found it. And it discovered that the hot-rolled steel plate in which the chemical conversion treatment film of the favorable form was formed became favorable in the corrosion resistance after electrodeposition coating (refer the graph shown in FIG. 1).

  Next, the present inventors conducted extensive studies on the conditions for refining the magnetite grains in the scale layer. As a result, as shown in the graph of FIG. 2, the magnetite crystal grains are not refined when finishing hot rolling is performed in a state where descaling has been completely performed as is conventionally performed. I found out. On the other hand, when finishing rolling is started in a state where a scale having a thickness within a predetermined range exists, and when an appropriate amount of strain is applied to the scale within a predetermined temperature range, the steel sheet is formed after cooling the steel plate. It was found that the magnetite crystals were refined. Although the cause of the refinement of magnetite crystals is not clear, the fine defects introduced by the addition of strain in the scale mainly composed of wustite (the scale formed during high-temperature finish rolling is mainly wustite) However, it is thought that it may function as a transformation nucleus of magnetite formed during cooling.

On the other hand, when the inventors repeated diligent experiments, if the scale thickness at the start of finish rolling is too large, the magnetite fraction decreases and the coating corrosion resistance tends to decrease, and the scale / base metal interface roughening occurs. As a result, it became clear that the fatigue characteristics decreased.
Furthermore, the present inventors investigated the influence of manufacturing conditions on the magnetite fraction. As a result, it was found that, in addition to the scale thickness at the start of finish rolling, the finish rolling temperature, the coiling temperature, and the cooling rate between 650 and 300 ° C. are influential factors.

As a result of the above experiments, the present inventors optimized the hot rolling conditions to optimize the scale structure and crystal grain size, and the thickness of the scale / base metal interface. As a result, it was clarified that the coating corrosion resistance after the electrodeposition coating could be secured and that the tires also had good fatigue characteristics, and the present invention was completed.
That is, the gist of the present invention is as follows.

[1] The volume fraction of magnetite in the scale layer is 60% or more, the average crystal grain size of the magnetite is 3 μm or less, and the roughness of the scale / base metal interface is 1.5 μm or less in terms of the average roughness Ra. A hot-rolled steel sheet with excellent paint corrosion resistance and fatigue characteristics.
[2] By mass%, C: 0.2% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 2 0.0% or less, Cr: 3.0% or less, the balance having a component composition consisting of Fe and inevitable impurities, the heat excellent in coating corrosion resistance and fatigue characteristics according to the above [1] Rolled steel sheet.
[3] The hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to [1] or [2] above, wherein a bending fatigue limit ratio is 0.45 or more.
[4] A method for producing a hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to any one of [1] to [3], wherein an average scale thickness at the start of finish rolling is 3 to 30 μm. The steel sheet surface temperature is 30% or more within the range of 800 to 980 ° C., and the finish rolling finish temperature is 800 ° C. or more. Then, the manufacturing method of the hot-rolled steel plate excellent in the coating corrosion resistance and fatigue characteristics characterized by winding at 300-650 degreeC.
[5] A method for producing a hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to any one of [1] to [3], wherein an average scale thickness at the start of finish rolling is 3 to 30 μm. The steel sheet surface temperature is 30% or more within the range of 800 to 980 ° C., and the finish rolling finish temperature is 800 ° C. or more. Then, the manufacturing method of the hot-rolled steel plate excellent in the coating corrosion resistance and fatigue characteristics characterized by cooling between 300-650 degreeC with an average cooling rate of 5 degrees C / min or less.

According to the hot-rolled steel sheet having excellent corrosion resistance and fatigue characteristics according to the present invention, the above-described configuration enables adhesion between the scale and the base iron even when the electrodeposition baking coating is applied to the hot-rolled steel sheet having the scale layer. It is possible to form a good chemical conversion film without impairing the properties, and excellent coating corrosion resistance and fatigue durability can be obtained. As a result, the thickness of the component plate was set in anticipation of the thickness reduction due to corrosion in the conventional steel plate, whereas the hot rolled steel plate of the present invention provides excellent coating corrosion resistance. It is possible to reduce the thickness of the vehicle or the truck or the like. Moreover, when the steel plate is thin, the steel material is required to have high fatigue strength. However, the hot-rolled steel plate of the present invention has excellent fatigue characteristics, and is therefore extremely suitable for reducing the weight of the member.
Moreover, according to the method for producing a hot rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to the present invention, by adopting the above procedure and conditions, it is possible to produce a hot rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics. It becomes possible.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically illustrating a hot-rolled steel sheet excellent in coating corrosion resistance and fatigue characteristics according to the present invention and a method for producing the same, and affects the volume fraction of magnetite in a scale layer and magnetite grains on the corrosion resistance after electrodeposition coating baking It is a graph which shows the relationship with a diameter. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram schematically illustrating a hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to the present invention and a method for producing the hot-rolled steel sheet. It is a graph which shows the relationship with a diameter.

  Hereinafter, an embodiment of a hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics and a method for producing the same according to the present invention will be described in detail with reference to the drawings as appropriate. The present embodiment will be described in detail in order to better understand the purpose of the hot rolled steel sheet having excellent coating corrosion resistance and fatigue properties and the manufacturing method thereof according to the present invention, and unless otherwise specified, the present invention. It is not intended to limit.

[Hot rolled steel sheet]
The hot rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to this embodiment has a volume fraction of magnetite in the scale layer of 60% or more and an average crystal grain size of the magnetite of 3 μm or less. The interface is roughly configured with an average roughness Ra of 1.5 μm or less.

"Volume fraction of magnetite in the scale layer"
In the hot-rolled steel sheet of the present invention, the volume fraction of magnetite in the scale layer is a very important factor for ensuring the corrosion resistance after coating. When the magnetite fraction in the scale layer is less than 60%, it becomes difficult to form a good chemical conversion coating film. As a result, the adhesion with the electrodeposition coating performed on the chemical conversion coating is lowered, and the corrosion resistance is deteriorated. For this reason, in this invention, the volume fraction of the magnetite in a scale layer was prescribed | regulated to less than 60%. In the present invention, it is more preferable that the volume fraction of magnetite in the scale layer is 85% or more from the viewpoint of further improving the corrosion resistance.

"Average grain size of magnetite"
In the hot-rolled steel sheet of the present invention, the average crystal grain size of magnetite is a very important factor in securing the corrosion resistance after coating. If the crystal grain size of magnetite exceeds 3 μm, it becomes difficult to form a good base chemical conversion film, and the corrosion resistance after electrodeposition coating deteriorates. Therefore, the appropriate range was set to 3 μm or less. The average crystal grain size of magnetite in the present invention is more preferably 2 μm or less.

Note that the magnetite described in the present invention, an oxide having a spinel crystal structure consisting of the chemical formula of Fe ₃ O _4. In addition, in the crystal structure, even if atoms such as Mn, Al, Ti, etc. are partially substituted at the atomic positions of Fe, the effect on coating corrosion resistance is not changed, but if the substitution rate by other atoms exceeds 30%, scale cracks Therefore, the upper limit of the substitution rate with other atoms in the Fe position is set.

"Roughness of scale / geite interface"
The interface roughness between the scale and the ground iron is one of the indexes for judging the fatigue characteristics and is an important factor in the present invention. When the average roughness Ra of the scale / base iron interface exceeds 1.5 μm, the fatigue characteristics are remarkably lowered. Therefore, the appropriate range is set to 1.5 μm. In addition, the average roughness Ra of the scale / base iron interface in the present invention is more preferably 1.3 μm or less.

"Steel component"
In the hot-rolled steel sheet of the present invention, when the volume fraction of magnetite in the scale layer, the average crystal grain size of the magnetite, and the average roughness of the interface between the scale / base metal are within the above range, the steel components are It is more preferable to control as follows. That is, the hot-rolled steel sheet of the present invention is, in mass%, C: 0.2% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.00. It is more preferable that the composition contains 02% or less, Al: 2.0% or less, and Cr: 3.0% or less, with the balance being Fe and inevitable impurities.
Hereinafter, each component which comprises a steel component is demonstrated.

“C: Carbon” 0.2% or less In the present invention, when the amount of C exceeds 0.2%, the ratio of the pearlite structure and the volume fraction of cementite increase, and the scale / steel interface roughness is smoothed. However, since good fatigue characteristics cannot be obtained, the appropriate range of C is limited to 0.2% or less. Further, the lower limit of the amount of C is not particularly limited, but if it is less than 0.0003%, the production cost increases, so 0.0003% is a substantial lower limit.

“Si: silicon” 2.0% or less In the present invention, when the amount of Si exceeds 2.0%, descaling property decreases, and as a result, the scale thickness before finish rolling increases, and after hot rolling Since the roughness of the scale / base metal interface increases and the fatigue characteristics deteriorate, the appropriate range is set to 2.0% or less. Further, since Si affects the coating corrosion resistance through the influence on the magnetite fraction, from the viewpoint of this coating corrosion resistance, it is more preferable to set it to 0.5% or less. Further, the lower limit of the amount of Si is not particularly limited, but if it is less than 0.001%, the manufacturing cost increases, so 0.001% is a substantial lower limit.

“Mn: Manganese” 3.0% or less Mn is an element used for securing the strength of steel. However, when it exceeds 3.0%, the adhesion of the scale is reduced and the volume fraction of magnetite is reduced. As a result, the corrosion resistance after coating also decreases, so the appropriate range is made 3.0% or less. Moreover, although the minimum of the amount of Mn is not specifically limited, Since manufacturing cost will increase when it is less than 0.001%, 0.001% is a substantial minimum.

“P: Phosphorus” 0.1% or less P is used for securing the strength of steel. However, if the P content exceeds 0.1%, the coating corrosion resistance decreases, so the appropriate range is made 0.1% or less. Further, the lower limit of the amount of P is not particularly limited, but if it is less than 0.001%, the manufacturing cost increases, so 0.001% is a substantial lower limit.

“S: Sulfur” 0.02% or less S is an element that affects the fatigue characteristics of the base material. However, if the S content exceeds 0.02%, good fatigue properties cannot be obtained even if the scale / base metal interface roughness is smoothed, so the appropriate range is made 0.02% or less. Further, the lower limit of the amount of S is not particularly limited, but if it is less than 0.0003%, the production cost increases, so 0.0003% is a substantial lower limit.

“Al: Aluminum” 2.0% or less Al is used for deoxidation and structure control of a steel sheet. However, if the Al content exceeds 2.0%, the magnetite crystal grains become coarse and the coating corrosion resistance decreases, so the appropriate range is made 2.0% or less. Moreover, the lower limit of the amount of Al is not particularly limited, but if it is less than 0.001%, the manufacturing cost increases, so 0.001% is a substantial lower limit.

“Cr: Chromium” 3.0% or less Cr is used for controlling the structure of a steel sheet. However, if Cr is contained in excess of 3.0%, the magnetite fraction is reduced and the adhesion of the scale is lowered. As a result, the corrosion resistance after coating is also lowered. The following. Further, the lower limit of the Cr amount is not particularly limited, but if it is less than 0.001%, the manufacturing cost increases, so 0.001% is a substantial lower limit.

  In addition, the steel component in this embodiment does not have limitation in particular about another element, You may contain various elements suitably for intensity | strength adjustment.

"Bending fatigue limit ratio"
In the hot-rolled steel sheet of the present invention, the volume fraction of magnetite in the scale layer, the average crystal grain size of the magnetite, the average roughness of the interface between the scale / base metal, and the steel components are specified in the above ranges. Furthermore, the bending fatigue limit ratio is more preferably 0.45 or more.
Here, the bending fatigue limit ratio described in the present invention is a value obtained by dividing the fatigue limit by TS, and is a value indicating the fatigue characteristics of the hot-rolled steel sheet. If this bending fatigue limit ratio is 0.45 or more, fatigue fracture does not occur practically, so the range of the fatigue limit ratio is limited to 0.45 or more.

[Method for producing hot-rolled steel sheet]
Next, a method for producing a hot-rolled steel sheet having the above-described structure and excellent in coating corrosion resistance and fatigue characteristics will be described.
In the method for producing a hot rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to the present invention, after performing descaling so that the average scale thickness at the start of finish rolling becomes 3 to 30 μm, the steel sheet surface temperature: 800 to 980 ° C. The rolling reduction within the range of 30% or more, and finish rolling at a finish rolling end temperature of 800 ° C. or higher, followed by winding at 300 to 650 ° C.
In the present invention, in producing a hot-rolled steel sheet having the above-described configuration, the process up to finish rolling is performed in the same procedure and conditions as described above, and then between 300 to 650 ° C. at an average cooling rate of 5 ° C./min or less. It can be set as the method of cooling.
Hereinafter, each procedure and conditions prescribed | regulated with the manufacturing method of the hot rolled sheet steel of this invention are demonstrated.

  First, a slab composed of the above components is heated, and then rough rolling and finish rolling are sequentially performed. At this time, the slab heating condition and the rough rolling condition are not particularly limited, and each conventionally used condition can be adopted.

  In the present invention, the average scale thickness at the start of finish rolling is an important factor influencing the coating corrosion resistance and fatigue characteristics after hot rolling. Here, in the conventional manufacturing method, it is general to perform descaling completely before finish rolling. However, when descaling is completely performed and the average scale thickness at the start of finish rolling is less than 3 μm, a fine magnetite crystal cannot be obtained after hot rolling, so that a good chemical conversion film cannot be obtained. Corrosion resistance after electrodeposition coating deteriorates. On the other hand, if the average scale thickness at the start of finish rolling exceeds 30 μm, the unevenness of the scale / base iron interface after finish rolling becomes large and fatigue characteristics deteriorate, and the magnetite fraction decreases and the scale and Through the decrease in adhesion, it also causes deterioration of paint corrosion resistance. For this reason, in the manufacturing method of the present invention, the appropriate range of the average scale thickness at the start of finish rolling is limited to 3 to 30 μm.

  In addition, the descaling method performed before finish rolling is not particularly limited. However, the degree of descaling treatment changes according to the steel composition and the steel plate temperature at the time of descaling, so by changing the water pressure, the amount of water and the injection angle according to these steel components and the steel plate temperature, Adjust the scale thickness after descaling.

  In finish rolling, the surface temperature and the amount of strain applied during rolling are important factors that affect the crystal grain size of magnetite after cooling. When the finish rolling is performed under the condition that the surface temperature of the steel sheet is less than 800 ° C., the scale is crushed and voids are formed in the scale. As a result, the volume fraction of magnetite is lowered. On the other hand, if the finish rolling is performed under conditions where the steel sheet surface temperature exceeds 980 ° C., the magnetite does not become finer after cooling. For this reason, in the manufacturing method of this invention, the appropriate range of the steel plate surface temperature was limited to 800-980 degreeC.

In addition, in the finish rolling, if the cumulative rolling reduction within the above appropriate temperature range is less than 30%, the magnetite refinement effect cannot be obtained. For this reason, the appropriate range of the cumulative rolling reduction within the above appropriate temperature range is limited to 30% or more. In the present invention, the cumulative rolling reduction within the appropriate temperature range is more preferably 60% or more.
The cumulative rolling reduction described in the present invention is the following expression {(t0−tf) / when the initial plate thickness is t0 and the plate thickness after rolling is tf with respect to rolling performed within the above temperature range. This is the amount obtained by t0 × 100}.

On the other hand, if the finish rolling finish temperature is less than 800 ° C., the magnetite fraction decreases, and as a result, the coating corrosion resistance decreases. For this reason, in this invention, the appropriate range of finish rolling completion temperature was restrict | limited to 800 degreeC or more.
In finish rolling, roll rolling is usually performed a plurality of times. Therefore, as long as the conditions include rolling with a cumulative rolling reduction of 30% or more within the above temperature range, rolling under other conditions may be performed. I do not care.

  Next, in the production method of the present invention, the temperature at which the steel strip that has been subjected to finish rolling is wound is an important factor that affects the volume fraction of magnetite in the scale layer and the magnetite particle size. When the winding temperature of the steel strip is less than 300 ° C., the transformation to magnetite does not occur sufficiently, so that good coating corrosion resistance cannot be obtained. On the other hand, when the coiling temperature of the steel strip exceeds 650 ° C., the particle size of the magnetite becomes coarse. For this reason, in the manufacturing method of this invention, the appropriate range of the winding temperature of a steel strip was restrict | limited to the range of 300-650 degreeC. Moreover, in this invention, it is more preferable that the upper limit of coiling temperature shall be 590 degrees C or less from a viewpoint of maximizing a magnetite fraction.

  In the present invention, when the method of cooling the steel plate at 300 to 650 ° C. after finishing rolling is adopted, the cooling rate between 300 and 650 ° C. is determined by the volume fraction of magnetite in the scale layer. And an important factor affecting the magnetite particle size. If the average cooling rate within this temperature range exceeds 5 ° C./min, the transformation to magnetite does not occur sufficiently, and the magnetite particle size is not sufficiently refined. For this reason, in this invention, after finishing rolling, the suitable range of the average cooling rate at the time of cooling a steel plate between 300-650 degreeC was restrict | limited to 5 degrees C / min or less.

  The volume fraction of magnetite may be measured by measuring the surface of the hot-rolled steel sheet by X-ray diffraction or by measuring the cross section of the steel sheet by EBSD (electron beam backscattered electron diffraction). The average crystal grain size of magnetite can be obtained as a nominal grain size by measuring 100 or more crystal grains by EBSD method in the cross section of the steel sheet.

According to the hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to the present invention as described above, the scale can be applied to the hot-rolled steel sheet having the scale layer even when electrodeposition baking is applied. It is possible to form a good chemical conversion treatment film without impairing the adhesion between the steel and the ground iron, and excellent coating corrosion resistance and fatigue durability can be obtained. As a result, the thickness of the component plate was set in anticipation of the thickness reduction due to corrosion in the conventional steel plate, whereas the hot rolled steel plate of the present invention provides excellent coating corrosion resistance. It is possible to reduce the thickness of the vehicle or the truck or the like. Moreover, when the steel plate is thin, the steel material is required to have high fatigue strength. However, the hot-rolled steel plate of the present invention has excellent fatigue characteristics, and is therefore extremely suitable for reducing the weight of the member.
Moreover, according to the method for producing a hot rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to the present invention, by adopting the above procedure and conditions, it is possible to produce a hot rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics. It becomes possible.

  Hereinafter, examples of the hot-rolled steel sheet excellent in coating corrosion resistance and fatigue characteristics according to the present invention will be given and the present invention will be described more specifically, but the present invention is not limited to the following examples from the beginning, The present invention can be implemented with appropriate modifications within a range that can be adapted to the gist of the following, and these are all included in the technical scope of the present invention.

In this example, first, steels A to P having chemical components shown in Table 1 below were cast, and then the slab was reheated within a range of 1050 to 1250 ° C. and subjected to rough rolling.
Next, the remaining thickness of the scale was changed using a descaling apparatus, and finish rolling was performed under the conditions shown in Table 2 below. Thereafter, a winding process was performed at a predetermined temperature, or a process of changing the cooling rate between 650 ° C. and 300 ° C. by continuous cooling was performed.

And the evaluation test which is demonstrated below was done about the hot-rolled steel sheet of the example of the present invention obtained by the above-mentioned procedure, and a comparative example.
First, the volume fraction of magnetite in the scale layer is quantified by the X-ray diffraction method, and the crystal grain size of the magnetite present in the scale layer is determined by separating the magnetite phase by the EBSD method. The diameter was measured.
Further, the roughness of the scale / base iron interface was measured by the method described in JIS 0601B for the surface after removing the scale by pickling, and evaluated by the arithmetic average roughness Ra.
In addition, the tensile properties of the steel plates were measured under the condition that a JIS No. 5 test piece was taken from each steel plate and the tensile direction was perpendicular to the rolling direction (C direction).
Further, the fatigue properties of the steel sheet were evaluated by the plane bending fatigue test under the condition of stress ratio = −1 according to the method described in JIS Z2275, and evaluated at the fatigue limit of 10 million times. Fatigue limit ratio was calculated from (tensile strength)}.

  As for the coating corrosion resistance, first, a steel sheet with a scale layer was degreased, and after a zinc phosphate treatment (chemical conversion treatment) as a pretreatment, a cationic electrodeposition coating was performed at a thickness of 25 μm. And after giving a linear wrinkle to the electrodeposition coating surface, according to the method of JISZ2371, the 200h salt spray test (SST test) is performed, and the coating film peeling at the time of performing a tape peeling test after this test The width was measured. Then, the film peeling width of 2 mm or less was evaluated in two stages as “◯ (corrosion resistance OK)” and the film exceeding 2 mm as “× (corrosion resistance NG)”.

Table 1 below shows a list of steel components, and Table 2 below shows the analysis results of the scale layer present in the produced hot-rolled steel sheet, scale / base metal interface roughness, tensile strength (TS), fatigue characteristics, and coating. A list of evaluation results of corrosion resistance is shown. In Table 2 below, each heading indicates the following item.
t _scale : Scale thickness at the start of finish rolling (mm)
Red: Cumulative rolling reduction between 800 and 980 ° C. (%)
FT: Final finish rolling temperature (° C)
CT: Winding temperature (° C)
CR: Average cooling rate between 300-650 ° C. (° C./min)
f _v mag: volume fraction of magnetite in the scale layer (%)
dmag: average particle diameter of magnetite (μm)
Ra: Arithmetic average roughness (μm) of scale / base metal interface

  As shown in Table 2, the volume fraction of magnetite in the scale layer within the range specified by the present invention, the average crystal grain size of the magnetite, and the scale / base iron were prepared under the conditions defined by the present invention. Each of the hot-rolled steel sheets of the examples of the present invention controlled to the average roughness of the interface between them had a fatigue limit ratio of 0.46 or more, and the evaluation of coating corrosion resistance was “◯”. Thereby, it became clear that the hot-rolled steel sheet of the present invention is excellent in coating corrosion resistance and fatigue characteristics.

  On the other hand, the hot-rolled steel sheet of the comparative example shown in Table 2 is any one of the volume fraction of magnetite in the scale layer, the average crystal grain size of magnetite, and the average roughness of the interface between the scale / base metal. However, since the specified range of the present invention was not satisfied, at least one of coating corrosion resistance and fatigue characteristics was inferior.

Test Nos. A-3, D-2, and I-2 are those in which descaling is sufficiently performed and finish rolling is started with a small initial scale thickness, and the magnetite crystal grains are large and the coating corrosion resistance is NG. This is an example.
In addition, in test numbers A-2 and H-2, the scale thickness before finish rolling was excessive as compared with the specified range of the present invention. This is an example in which the coating corrosion resistance is evaluated as NG because the magnetite fraction is small.
In addition, although test numbers A-4, F-2, and J-2 had an appropriate scale thickness before finish rolling, no distortion was imparted to the scale during rolling, so that the magnetite crystal grains were not refined, This is an example in which the coating corrosion resistance is NG.

Test No. A-6 is an example in which the final finish rolling temperature was low, the scale was broken, the magnetite fraction was reduced, and the coating corrosion resistance was NG.
Test No. A-5 is an example in which the corrosion resistance was NG because the transformation from wustite to magnetite did not occur sufficiently because the coiling temperature was below the appropriate range.
Test number F-3 is an example in which the coiling temperature was equal to or higher than the appropriate range, so that the magnetite crystal grains were coarsened and the coating corrosion resistance was NG.
Moreover, since the steel components were not appropriate for test numbers K-1, L-1, M-1, N-1, O-1, and P-1, the magnetite fraction and the magnetite particle size were appropriate accordingly. This is an example in which the coating corrosion resistance is NG.

  From the results of the examples described above, even when the hot-rolled steel sheet of the present invention and the manufacturing method thereof are subjected to electrodeposition baking coating on a hot-rolled steel sheet having a scale layer, the adhesion between the scale and the base iron It is clear that it is possible to form a good chemical conversion film without impairing the coating, and to have excellent coating corrosion resistance and fatigue characteristics.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to provide the hot-rolled steel plate excellent in coating corrosion resistance and a fatigue characteristic suitable as raw materials, such as a frame, a member, and a chassis of a motor vehicle or a truck, for example. In this way, by applying the present invention to members such as automobiles and truck frames, members, chassis, etc., it is possible to fully enjoy the benefits such as improved corrosion resistance and fatigue strength after painting, and weight reduction. Industrial effect is extremely high.

Claims (5)

  The volume fraction of magnetite in the scale layer is 60% or more, the average crystal grain size of the magnetite is 3 μm or less, and the roughness of the scale / base metal interface is 1.5 μm or less in terms of the average roughness Ra. Hot rolled steel sheet with excellent coating corrosion resistance and fatigue characteristics. % By mass
C: 0.2% or less,
Si: 2.0% or less,
Mn: 3.0% or less,
P: 0.1% or less,
S: 0.02% or less,
Al: 2.0% or less,
The hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to claim 1, wherein Cr: 3.0% or less, and the balance has a composition composed of Fe and inevitable impurities.   The hot-rolled steel sheet having excellent coating corrosion resistance and fatigue characteristics according to claim 1 or 2, wherein a bending fatigue limit ratio is 0.45 or more. A method for producing a hot-rolled steel sheet having excellent coating corrosion resistance and fatigue properties according to any one of claims 1 to 3,
After performing descaling so that the average scale thickness at the start of finish rolling is 3 to 30 μm,
Steel sheet surface temperature: The cumulative rolling reduction within the range of 800 to 980 ° C. is 30% or more, and finish rolling finish temperature is 800 ° C. or more.
Then, the manufacturing method of the hot-rolled steel plate excellent in the coating corrosion resistance and fatigue characteristics characterized by winding at 300-650 degreeC. A method for producing a hot-rolled steel sheet having excellent coating corrosion resistance and fatigue properties according to any one of claims 1 to 3,
After performing descaling so that the average scale thickness at the start of finish rolling is 3 to 30 μm,
Steel sheet surface temperature: The cumulative rolling reduction within the range of 800 to 980 ° C. is 30% or more, and finish rolling finish temperature is 800 ° C. or more.
Then, the manufacturing method of the hot-rolled steel plate excellent in the coating corrosion resistance and fatigue characteristics characterized by cooling between 300-650 degreeC with an average cooling rate of 5 degrees C / min or less.

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