JP6160655B2 - Hot rolled steel sheet and manufacturing method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet containing Si that is suitable for use in automobile members and the like, and has excellent chemical conversion properties, and a method for producing the same. The present invention also relates to a hot-rolled steel sheet that is also excellent in post-coating corrosion resistance evaluated in a test using salt water. The hot-rolled steel sheet of the present invention can be suitably used as a high-strength hot-rolled steel sheet having a tensile strength TS of 590 MPa or more.

  In recent years, there has been a strong demand for improving fuel efficiency of automobiles from the viewpoint of protecting the global environment. In addition, from the viewpoint of ensuring the safety of passengers in the event of a collision, there is a strong demand for increasing the strength of automobile bodies. In order to meet these demands, it has been actively promoted to increase the strength of a hot-rolled steel sheet used as a material for an automobile member and to reduce the thickness of the hot-rolled steel sheet at the same time as reducing the weight and increasing the strength of the automobile body. However, since many automobile members are manufactured by forming a steel plate, the steel plate used as the material is required to have excellent formability in addition to high strength.

  There are various methods for increasing the strength of a hot-rolled steel sheet. An effective means for increasing the strength without greatly impairing the formability is a solid solution strengthening method by adding Si.

When the hot-rolled steel sheet contains a large amount of Si, particularly 0.5% by mass or more of Si, SiO ₂ or Si—Mn is formed at the interface between the steel sheet surface and the oxide scale during slab heating, hot rolling, or subsequent annealing. A large amount of Si-containing oxide such as a composite oxide is formed. Since this Si-containing oxide deteriorates the scale peelability after rough rolling, when a large amount of the Si-containing oxide is formed, the scale is unevenly distributed on the surface of the hot-rolled steel sheet after finish rolling. (Island scale) is likely to occur. In such a portion, strong irregularities are formed on the surface of the hot-rolled steel sheet, and the scale tends to remain on the surface even after pickling. As a result, there is a problem that the chemical conversion property is remarkably lowered, and there is a problem that the coating film peels easily when exposed to a severe corrosive environment using salt water after electrodeposition coating, and the post-coating corrosion resistance is poor.

For example, Patent Document 1 discloses that the impact pressure per unit spray area is 0.2 MPa or more and 0.4 MPa or less before finish rolling, for the problem caused by the island-like scale pattern of such a Si-containing steel plate. And the technique which inject | pours the high-pressure water spray whose flow volume is 0.1 liter / min * mm < ² > or more and 0.2 liter / min * mm < ² > or less on the steel plate surface is disclosed.

In Patent Document 2, the temperature on the finishing mill side (entrance side) is controlled according to the amount of Si, and the collision pressure per unit area is 5.0 to 30.0 kgf / cm ² (4.9 to 29). A technique for performing descaling with high-pressure water at 0.4 MPa) is disclosed.

  Furthermore, in Patent Document 3, the Si concentration and Mn concentration of the surface of a hot-rolled steel sheet containing 0.8 to 3.0% of Si by mass% are controlled to 3.5% or less, and further, the surface roughness The chemical conversion coating is applied to the entire surface of the steel sheet by setting the Ra to 3.0 μm or less and the number of pitting by pickling to an average of 5 or less in each grid when the surface is divided into 10 μm squares. A hot-rolled steel sheet excellent in chemical conversion treatment that can be generated homogeneously is disclosed.

  Patent Document 4 discloses that a mass-% hot-rolled steel sheet containing 0.5 to 3.0% of Si is formed by oxidizing a Si-containing oxide formed on the steel sheet surface during hot rolling with nitric acid. The surface coverage of the iron-based oxide on the surface of the steel sheet is 40% or less by removing the iron-based oxide, which is inevitably removed during pickling with an oxidizing acid, by re-acid cleaning. A hot-rolled steel sheet having excellent chemical conversion properties and post-coating corrosion resistance is disclosed.

JP-A-4-238620 Japanese Patent No. 3241886 Japanese Patent No. 4206029 JP 2012-172181 A

  In the technique described in Patent Document 1, no consideration is given to chemical conversion treatment, and there is a problem that the effect of improving chemical conversion treatment and corrosion resistance after coating is not sufficient.

  In the technique described in Patent Document 2, it is necessary to control the entry temperature (FET) of finish rolling in accordance with the amount of Si, so that the rolling operation is complicated. Furthermore, the technique described in Patent Document 2 cannot be applied to a steel sheet having a Si content exceeding 1.0%. Moreover, in patent document 2, chemical conversion treatment property is not considered, and the improvement effect of chemical conversion treatment property and corrosion resistance after coating is not enough.

  Further, by using the technique described in Patent Document 3, simply reducing the Si concentration on the steel sheet surface is not sufficient for improving the chemical conversion property, and sufficient post-coating corrosion resistance cannot be obtained in a severe corrosive environment. There is a problem.

  Furthermore, the technique described in Patent Document 4 aims to improve the chemical conversion property of the hot-rolled steel sheet by strong pickling. However, in order to adapt this technology to a production line, a large facility modification is required, and this technology has a problem that the amount of pickling solution consumed increases and costs increase. Furthermore, the steel plate surface roughness may increase due to strong pickling, and the chemical conversion processability may be deteriorated.

  The present invention has been made in view of such circumstances, and has excellent chemical conversion treatment properties on the entire surface of the steel sheet and is excellent in post-coating corrosion resistance in a severe corrosive environment using salt water such as a salt spray test. It aims at providing a steel plate and its manufacturing method.

  In order to solve the above-mentioned problems, the inventors have conducted intensive research on surface properties that improve chemical conversion property and corrosion resistance after coating for high-strength hot-rolled steel sheets containing Si. As a result, the present inventors have found that the chemical conversion treatment property and the corrosion resistance after coating are remarkably improved by reducing the surface roughness and reducing the oxygen concentration on the surface, thereby completing the present invention.

  The present invention has been made based on the above findings, and the gist thereof is as follows.

  [1] It is characterized by containing Si in an amount of 0.5 to 5.0% by mass, an average surface roughness Ra of 2.0 μm or less, and an average oxygen concentration on the surface of 5.0% or less. Hot rolled steel sheet.

  [2] Further, by mass, C: 0.01 to 0.30%, Mn: 0.50 to 3.00%, P: 0.07% or less, S: 0.010% or less, Al: 0 The hot-rolled steel sheet according to [1], containing 0.105 to 0.100%, the balance being Fe and inevitable impurities.

  [3] Further, by mass%, containing one or more selected from Ti: 0.50% or less, Nb: 0.50% or less, and V: 0.50% or less The hot-rolled steel sheet according to [1] or [2].

  [4] A method for producing a hot-rolled steel sheet according to any one of [1] to [3], wherein hot-scaling is performed in which high-pressure water descaling with a collision pressure of 3.0 MPa or more is performed before finish rolling in hot rolling. A pickling process in which a hydrochloric acid concentration is 4.0 to 15% by mass, an iron ion concentration is 3.0% by mass or less, and a temperature is 50 to 90 ° C. for 40 seconds or more after the rolling step and the hot rolling step; A method for producing a hot-rolled steel sheet, comprising: after the pickling step, a time for the steel sheet to be exposed to the atmosphere is started within 4.5 seconds, and a water-washing step of washing the steel plate.

  [5] The method for producing a hot-rolled steel sheet according to [4], further comprising a drying step of drying the steel plate within 5 seconds after the water washing step.

  [6] The method for producing a hot-rolled steel sheet according to [4] or [5], wherein in the water washing step, the steel sheet temperature at the end of the water washing is 40 ° C. or less.

  According to the present invention, even when Si is contained in an amount of 0.5 to 5.0% by mass, the chemical conversion treatment property is good on the entire surface of the steel sheet, and furthermore, the corrosion resistance after coating is also good, and the problem of peeling of the coating film after coating. Can be suppressed. Therefore, the hot-rolled steel sheet of the present invention can be suitably used for a strength member of an automobile body.

  Details of the present invention will be described below. The present invention is not limited to the following embodiments. In the following description, the unit of the content of each element of the steel component composition, the unit of hydrochloric acid concentration, the unit of average oxygen concentration, the unit of Fe ion concentration in the pickling solution is “% by mass”, and Unless otherwise specified, it is simply indicated by “%”.

<Hot rolled steel sheet>
The hot-rolled steel sheet of the present invention is 0.5% by mass, contains Si in an amount of 0.5 to 5.0%, has an average surface roughness Ra of 2.0 μm or less, and an average oxygen concentration on the surface of 5.0% or less. It is.

  First, the component composition of the hot rolled steel sheet of the present invention will be described. The hot-rolled steel sheet of the present invention contains 0.5% to 5.0% Si by mass%. This invention is an invention which solves the problem which arises when Si containing oxide is formed in the steel plate surface. Moreover, the said subject by Si containing oxide arises by containing Si. From the above, the component composition of the hot-rolled steel sheet of the present invention only specifies that Si is contained.

Si: 0.5-5.0%
If Si is added, the strength of the steel can be increased without greatly impairing the workability. For this reason, Si is an effective element for achieving high strength of steel. However, Si is also an element that adversely affects chemical conversion treatment properties and post-coating corrosion resistance. In order to increase the strength by adding Si, the Si content needs to be 0.5% or more. On the other hand, if the Si content exceeds 3.0%, the effect of improving the surface properties for improving the chemical conversion treatment property according to the present invention is sufficient, but from the viewpoint of mechanical properties, the steel becomes hard and rollable. In some cases, the platenability (manufacturability) may be adversely affected, or the ductility of the steel plate itself may be reduced. Therefore, the Si content is preferably 0.5 to 3.0%. Note that the surface property improvement effect cannot be obtained when the Si content exceeds 5.0%.

  As described above, since the hot-rolled steel sheet of the present invention is an invention that solves the problems caused by containing Si, only Si is an essential component in the component composition of the hot-rolled steel sheet of the present invention, and other than the essential components. About the component of this, if it is the composition range which a normal hot-rolled steel plate has, it will be accept | permitted and it will not restrict | limit in particular.

  However, when the hot-rolled steel sheet of the present invention is used after being processed into an undercarriage member or the like of an automobile, as optional components, by mass%, C: 0.01 to 0.30%, Mn: 0.50 It is preferable to contain 3.00%, P: 0.07% or less, S: 0.010% or less, Al: 0.015-0.100%.

C: 0.01 to 0.30%
If the C content is less than 0.01%, the elongation is low. On the other hand, if C is contained excessively, the weldability is lowered, but if the C content is 0.30% or less, the weldability is not significantly lowered. From the above, the C content is preferably 0.01% or more and 0.30% or less.

Mn: 0.50 to 3.00%
If the Mn content is less than 0.50%, the elongation is low, and if it exceeds 3.00%, the pickling property is lowered. Further, excessive addition of Mn is not desirable because it causes an increase in raw material cost. From the above, the Mn content is preferably 0.50% or more and 3.00% or less.

P: 0.07% or less Since P is an element that impairs spot weldability, it is preferable that the P content is small. A P content of 0.07% or less is preferable because spot weldability is not significantly reduced.

S: 0.010% or less S is an element that may be inevitably mixed in steel. S is also a harmful component that precipitates as MnS and lowers the stretch flangeability of the steel sheet. In order to suppress a decrease in stretch flangeability, the S content is preferably 0.010% or less.

Al: 0.015-0.100%
Al is an element added as a deoxidizer in the steel making process. Moreover, Al is an element effective for separating nonmetallic inclusions that reduce stretch flangeability as slag. From the viewpoint of obtaining these effects, the Al content is 0.015% or more. However, excessive addition of Al causes an increase in raw material cost, so the upper limit of the Al content is preferably 0.100%.

  Moreover, the hot-rolled steel sheet of this invention is 1 type or 2 types chosen from Ti: 0.50% or less, Nb: 0.50% or less, V: 0.50% or less as arbitrary components other than the above. The above can be contained.

One or more selected from Ti: 0.50% or less, Nb: 0.50% or less, V: 0.50% or less Ti, Nb and V generate carbides and nitrides, and ferrite It is an element that suppresses the growth of the material, refines the structure, and improves formability, particularly stretch flangeability. In order to obtain these effects, Ti: 0.50% or less, Nb: 0.50% or less, and V: 0.50% or less are preferable. From the viewpoint of obtaining the above effects, Ti: 0.01% or more, Nb: 0.01% or more, and V: 0.01% or more are preferable.

  The balance other than the above essential components and optional components is Fe and inevitable impurities. Moreover, as above-mentioned, the component which a hot-rolled steel plate contains normally may be included. Inevitable impurities include O and N.

In the present invention, the average roughness Ra of the surface of the hot rolled steel sheet is adjusted to 2.0 μm or less, and the average roughness Ra of the surface is 2.0 μm or less. It is important to adjust the average oxygen concentration on the surface to 5.0% in order to improve the chemical conversion property and improve the corrosion resistance after coating. Here, the average oxygen concentration on the surface of the steel sheet was observed by using a scanning electron microscope to observe five randomly extracted visual fields at an acceleration voltage of 5 kV, a working distance of 8.5 mm, and a magnification of 500 times, and an energy dispersive X-ray spectrometer. Quantitative calculation from the X-ray spectrum obtained by analyzing by the above, and deriving by the method of obtaining the average value of the five visual fields. The oxygen concentration measured here is mainly the oxygen concentration of oxygen forming an oxide or hydroxide with Fe, Si, Mn and other elements contained in the steel plate surface, as will be described later. It greatly affects the chemical conversion of steel sheets. The point that this configuration is important for improving the corrosion resistance after painting will be described below.

  The chemical conversion treatment is performed by immersing the steel plate in a chemical conversion treatment solution for a predetermined time after removing oil adhering to the steel plate surface by degreasing treatment. By this treatment, Fe elutes from the steel sheet into the treatment liquid and reacts with the components in the chemical treatment liquid, resulting in a large number of chemical conversion crystal nuclei composed of compounds containing Fe, Zn, P, O, etc. Grows into a film covering the entire surface of the steel sheet. At this time, it is necessary to uniformly attach fine chemical conversion grains having an average crystal grain size of 10 μm or less over the entire surface. If the adhesion state of the chemical crystal grains is poor and there is a non-adhesion site called “skew”, problems such as poor adhesion of the coating film during coating and reduced corrosion resistance after coating occur.

  In particular, when the Si content of the hot-rolled steel sheet is high, the surface scale after rolling contains a large amount of Si-containing oxides, and island scales are liable to occur. Such a part forms a strong unevenness | corrugation in the hot-rolled steel plate surface, and it is easy to leave a scale on the surface even after pickling. Here, “high Si content” means that the Si content is 0.5% or more.

  In addition, even if the scale appears to be removed by pickling, fine oxides may be present in the recesses on the surface of the steel sheet, which inhibits the elution of Fe ions during the chemical conversion treatment, It is thought that it delays the formation reaction of and causes skein.

  Furthermore, even if the scale is completely removed by pickling, the pickling solution and the washing solution are likely to remain in the recesses in the subsequent washing and drying steps, and it may be difficult to dry, so a new oxide may be formed. is there.

  As described above, when a hot rolled steel sheet containing Si is used, in order to uniformly attach fine chemical conversion crystal grains having an average crystal grain size of 10 μm or less over the entire surface, even if an island scale is formed, It is necessary to be able to be removed (Problem 1), to be able to be removed even if the oxide is not formed in the fine recesses on the surface of the steel plate (Problem 2), and to be able to suppress the formation of a new oxide (Problem 3). . In order to solve the above problems, the surface roughness Ra of the hot-rolled steel sheet of the present invention is adjusted to 2.0 μm or less, and the average oxygen concentration is adjusted to 5.0% or less. Thus, if the chemical conversion property is improved, the post-coating corrosion resistance is also improved.

  If the surface roughness Ra is adjusted to 2.0 μm or less and the average oxygen concentration on the surface is adjusted to 5.0% or less, the island scale does not exist or the surface unevenness is mild even if it exists. Since the oxide is easily removed by pickling and oxidation of the steel sheet surface by the acid solution remaining in the recesses in the washing step and the washing water is less likely to occur, problems 1 to 3 are considered to be solved.

  Further, the lower limit of the surface roughness Ra is not particularly limited, but is preferably 0.3 μm or more because it can be produced by a normal production process. Further, the lower limit of the average oxygen concentration is not particularly limited, but it is preferably 0.5% or more because there is an inevitable natural oxide film.

<Method for producing hot-rolled steel sheet>
Next, a method for manufacturing a hot-rolled steel sheet will be described. As described above, in the hot-rolled steel sheet of the present invention, the surface roughness Ra is adjusted to 2.0 μm or less, and the average oxygen concentration on the surface is adjusted to 5.0% or less. In order to obtain such hot-rolled steel sheet surface, the strong scale peculiar to Si-containing steel sheet is removed (descaling) with a scale breaker after rough rolling, and the subsequent pickling and water washing processes are controlled appropriately. It is important to. The specific method will be described below.

  First, a slab having the above component composition is heated and hot-rolled by a normal method. The conditions for slab heating and hot rolling are not particularly limited, and may be set as appropriate.

  After hot rolling, in order to remove the strong primary scale peculiar to Si-containing steel, descaling treatment such as shot blasting and high-pressure water injection is performed before finish rolling so that island scales do not form after finish rolling. To do. By performing primary scale descaling strongly, it becomes possible to control the surface roughness Ra after pickling to 2.0 μm or less. Finish rolling is performed after the descaling. What is necessary is just to set the conditions of finish rolling suitably.

  “Strongly performs primary scale descaling” means descaling corresponding to a collision pressure condition of 3.0 MPa or more, preferably 3.0 to 7.0 MPa in high pressure water descaling.

  Subsequently, the scale is removed by hydrochloric acid pickling after a hot rolling process in which primary scale descaling is performed. At this time, if the concentration of hydrochloric acid in the pickling solution is less than 4.0%, it takes a long time to remove the scale, and if it exceeds 15.0%, the surface after pickling becomes rough. For this reason, the hydrochloric acid concentration is desirably in the range of 4.0% to 15.0%.

  The temperature of the pickling solution may be 50 to 90 ° C. However, when the temperature of the pickling solution is 70 ° C. or less, the scale removing ability is not sufficient and it takes a long time to remove the scale. In addition, when the temperature of the pickling solution is 90 ° C. or higher, cost increase due to volatilization of the acid solution and management become difficult. Therefore, the temperature of the pickling solution is desirably 70 ° C to 90 ° C.

  In a normal pickling line, about 4 to 12% of iron ions are contained in the pickling solution in order to continuously pass the steel plate, but in order to reduce the oxygen concentration on the steel plate surface after pickling, pickling is performed. The amount of iron ions in the liquid is preferably as small as possible. In order to realize the average oxygen concentration of the present invention, specifically, it is desirable that the iron ion concentration is 3.0% or less. When the Fe ion concentration increases due to the dissolution of Fe from the steel sheet, a part of the liquid is replaced with a new liquid, and the recovered acid liquid is treated again in a roasting furnace to recover the recovered acid from which Fe ions have been removed. The Fe ion concentration can be reduced by a method such as use.

  The pickling time is not particularly limited, but is preferably 40 seconds or longer, particularly preferably 40 seconds to 60 seconds. If the pickling time is 40 seconds or more, it is preferable because the scale can be completely removed. Even if the pickling time exceeds 60 seconds, the effect of the present invention does not improve, the yield decreases, and the acid solution deteriorates. Therefore, the pickling time is preferably 60 seconds or less.

  In addition, if the time for which the steel sheet is held in the atmosphere is long after pickling until the water washing step, the surface is oxidized and the oxygen concentration becomes high. Specifically, the time from pickling to rinsing (usually, since the steel plate is exposed to the atmosphere during the period from pickling to rinsing, this corresponds to the time during which the steel plate is exposed to the atmosphere) is 0 second to 4. It is desirable to be 5 seconds. As the water washing method, there are methods such as immersing the steel plate in washing water or spraying the washing water on the steel plate by spraying. Although the temperature of the water used for water washing is not specifically limited, since it is preferable that the steel plate temperature at the time of completion | finish of water washing will be 40 degrees C or less as follows, the temperature of the water used for water washing is preferable 40 degrees C or less, More preferably 10-35 ° C. The washing time is not particularly limited, but may be completed within 1 to 30 seconds from the viewpoint of the productivity of the steel sheet.

  Furthermore, when the steel plate temperature at the end of washing with water is high, there is a possibility that the steel plate is oxidized by the liquid film before the steel plate is dried, and the surface oxygen concentration becomes high. Therefore, it is desirable that the steel plate temperature at the end of washing with water is 40 ° C. or less. In addition, if water is present in the form of a liquid film on the surface of the steel sheet after washing with water, oxidation tends to proceed. Therefore, it is desirable to dry immediately after washing with water. Drying can be performed rapidly by a method such as blowing dry air. Specifically, it is desirable to dry within 5 seconds after completion of washing with water.

  As described above, in addition to completely removing the scale in the pickling process, it is important not to form a new oxide on the steel sheet surface in the subsequent water washing process and drying process.

  The present invention will be described in more detail with reference to examples.

  Table 1 shows the component compositions of the eight types of steel used as test materials. Table 2 (Table 2-1 and Table 2-2 are combined into Table 2) shows high-pressure water descaling conditions and pickling conditions during hot rolling. Table 2-2 is an example in which the conditions of the water washing after the pickling and the drying process were changed, and shows the conditions of the water washing and drying in addition to the high-pressure water descaling conditions and the pickling conditions during hot rolling.

  In hot rolling, slab heating was performed at 1250 ° C., then rough rolling was performed, descaling was performed, and finish rolling was performed at 910 ° C. Then, it wound up at the coiling temperature of 425 degreeC, and cooled to room temperature. Next, in the pickling performed, the prepared sample was made into small pieces of 70 mm × 150 mm size and immersed in an experimental pickling tank. Washing with water after pickling was performed by immersing the steel sheet in cleaning water, and drying after washing with water was performed by blowing dry air.

  The surface roughness of the hot-rolled steel sheet obtained as described above was measured according to JIS B 0601, and the arithmetic average roughness Ra was determined. Regarding the average oxygen concentration on the surface, using a scanning electron microscope, observe five randomly extracted visual fields at an acceleration voltage of 5 kV, a working distance of 8.5 mm, and a magnification of 500 times, and use an energy dispersive X-ray spectrometer. The oxygen concentration was quantitatively calculated from the obtained X-ray spectrum, and the average value of five fields of view was obtained to obtain the average oxygen concentration on the surface.

  Moreover, the test piece was extract | collected from the obtained hot-rolled steel plate, and chemical conversion property and the corrosion resistance after coating were determined.

  The evaluation method of chemical conversion property is as follows.

  A test piece of 70 mm × 150 mm size was collected from the manufactured hot-rolled steel sheet, degreased by Nippon Paint Co., Ltd .: degreased with Surf Cleaner EC90, washed with water, and then surface-adjusted with 5N-10 for 30 seconds. Then, it was immersed in a chemical conversion solution: EC1000R-1, treated at 40 ° C. for 120 seconds, washed with water and dried. The chemical conversion property was evaluated based on the presence or absence of staining by SEM observation of the steel sheet surface to which the chemical conversion film was applied. The chemical conversion film was observed by SEM at a magnification of 1000 times, and when a steel sheet surface not covered with zinc phosphate crystals was observed, it was determined that there was a skein.

  The evaluation method of the corrosion resistance after painting is as follows.

  Electrodeposition coating made by Kansai Paint Co., Ltd., which has been subjected to the above-mentioned chemical conversion treatment: Electrodeposition coating using PN-150, followed by baking in a furnace at 170 ° C. for 20 minutes for 20 μm electrodeposition A film was formed. Thereafter, the cut until reaching the steel plate with a cutter knife was applied in a cross shape, and this test piece was subjected to a neutral salt spray test prescribed in JIS Z2371: 2000 using a 5 mass% NaCl aqueous solution. A 960 hour salt spray test was conducted. Thereafter, the cross-cut portion was subjected to a tape peeling test using a cellophane tape having a width of 25 mm, and the one-side maximum peel width from the cut portion was measured. If the maximum peel width is 2.0 mm or less, it can be said that the post-coating corrosion resistance is good. The evaluation results obtained as described above are shown in Table 3 (Table 3-1 and Table 3-2 are combined into Table 3).

  From Table 3, in the examples of the present invention, good chemical conversion properties and post-coating corrosion resistance are obtained. On the other hand, in the comparative example, the chemical conversion property is inferior and the corrosion resistance after coating is not sufficient.

  The hot-rolled steel sheet of the present invention has not only excellent chemical conversion properties and post-coating corrosion resistance, but also high strength and excellent workability. It can also be suitably used as a material for members that require these characteristics.

Claims (4)

% By mass, containing 0.5 to 5.0% of Si, C: 0.08 to 0.16%, Mn: 1.44 to 2.10%, P: 0.07% or less, S: 0 0.010% or less, Al: 0.015 to 0.100%, Ti: 0.01 to 0.25%, Nb: 0.01 to 0.26% or less, V: 0.01 to 0.32% Containing, the balance consisting of Fe and inevitable impurities,
The average roughness Ra of the surface is 2.0 μm or less,
A hot-rolled steel sheet having an average oxygen concentration on the surface of 3.7 % by mass or more and 5.0% by mass or less. It is a manufacturing method of the hot-rolled steel sheet according to claim 1 ,
Before the finish rolling in hot rolling, a hot rolling step of performing high pressure water descaling with a collision pressure of 3.0 MPa or more,
After the hot rolling step, the hydrochloric acid concentration is 4.0-15 . A pickling step of 40% or more with an acid solution of 0 % by mass, an iron ion concentration of 3.0% by mass or less, and a temperature of 50 to 90 ° C .;
A method for producing a hot-rolled steel sheet, comprising: after the pickling step, a time for the steel plate to be exposed to the atmosphere is started within 4.5 seconds, and a water-washing step of washing the steel plate with water. Furthermore, it has a drying process which dries a steel plate within 5 second after the said water washing process, The manufacturing method of the hot rolled sheet steel of Claim 2 characterized by the above-mentioned. The method for producing a hot-rolled steel sheet according to claim 2 or 3 , wherein the water-washing step has a steel plate temperature of 40 ° C or less at the end of the water-washing.