MXPA06004994A - Hot-rolled steel plate excellent in chemical treatment characteristics and method for production thereof - Google Patents

Abstract

A hot-rolled steel plate which contains, in mass%, 0.03%to 0.15%of C, 0.8 to 3.0%of Si, 0.5 to 3.0%of Mn, 0.07%or less of P, 0.01%or less of S, 0.015 to 0.1%of Al, 0.001 to 0.008%of N, and optionally contains Ti, Nb or the like in addition to the above, wherein an oxide present on the surface thereof has an Si concentration of 3.5%or less and an Mn concentration of 3.5%or less. It is preferred that the surface of the steel plate has an average roughness Ra of 3.0µm or less and that the number of pits formed by pickling is five or less in average in a square having a side of 10µm. The scale on a steel plate after hot rolling is pickled by immersing the steel plate in a solution having an HCl concentration of 7 to 15%and an Fe ion concentration of 4 to 12%at a solution temperature of 80 to 98°C for 40 seconds or longer. The hot-rolled steel plate is a high strength hot-rolled steel plate having an enhanced content of Si, and allows the homogeneous formation of a chemical coating film over the whole surface thereof, without the addition of a new step in the production of the steel plate, and also allows the employment of an easier method for quality control.

Description

HOT LAMINATED STEEL SHEET EXCELLENT IN CHEMICAL CONVERTIBILITY AND METHOD OF PRODUCTION OF THE SAME TECHNICAL FIELD The present invention relates to an excellent hot-rolled steel sheet in chemical convertibility which, when chemically converted to a sheet of priming steel, is formed with a conversion coating homogeneously on its entire surface, and a method of production thereof. BACKGROUND TECHNIQUE When coating auto body parts by electrolytic deposition of metals or electrodeposition and on the other hand when coating metal surfaces, the practice is to prime the surfaces by chemical conversion. The chemical conversion covers a metal surface by an inert conversion coating to improve the adhesion and corrosion resistance of the coating formed thereon. In addition, from the point of view of reducing the weight of automobiles and guaranteeing safety, a thin gauge, high strength steel blade is used. For chassis parts etc., a hot-rolled steel sheet is less expensive than a cold-rolled steel sheet that is being used. The hot-rolled steel sheet is produced through the steps of hot rolling and pickling. In the pickling step, the oxide residue on the surface of the steel sheet is removed by pickling with hydrochloric acid. The following proposals have been made in the past with respect to a hot-rolled, high-strength steel sheet, improved in its chemical convertibility and a method of producing it. For example, Japanese Patent Publication (A) No. 11-50187 discloses a hot-rolled, high-strength steel sheet where the Si concentration ratio on the surface and inside of the steel sheet is made from 1.3 or less to eliminate problems of deterioration of chemical convertibility and deterioration resulting from corrosion resistance after coating. As a means of obtaining such a Si concentration ratio, crushing of the hot-rolled steel sheet after pickling etc. is shown to reduce the Si-oxides present on the surface. In addition, Japanese Patent Publication (A) No. 10-1748 discloses a hot-rolled, high-strength steel sheet, where the Vicker hardness ratio of the surface and the interior of the steel sheet is made from 0.98 or less to improve chemical convertibility and the ability to be worked. This covers the steel that contains Ti. The deposits on the surface of the steel sheet are produced carbides instead of Ti TiO2 oxide, which cause the deterioration of the chemical convertibility and therefore the previous ratio of hardness is obtained. It is considered that Ti02 forms fine deposits adapted and increases the hardness of the steel sheet, while the TiO is incompatible and reduces the hardness of the steel sheet. As means for this, the hot rolling conditions are shown. Japanese Patent Publication (A) No. 11-50187 Japanese Patent Publication (A) No. 10-1748 DESCRIPTION OF THE INVENTION When hot-rolled steel sheet is primed by chemical conversion, in particular with steel with a high content of Yes, locations called "bald spots" can be observed under a microscope where a conversion coating is not formed. It is observed that such locations are oxidized when examined by the naked eye. Even when no oxidation is observed, after coating, problems arise such as the release of the coating along with the passage of time. The Japanese Patent Publication (A) No. 11-50187 limits the Si concentration ratio of the surface and the inside of the steel sheet to a specific range, while the technique of the Publication of Japanese Patent (A) No. 10-1748 limits the hardness ratio of the surface and the interior to a specific range to improve chemical convertibility. For this reason, when these techniques are applied to a production line of a hot-rolled steel sheet, it is necessary to measure the inside of the steel sheet and raise a question in the measurement for quality control. It should be noted that in the first case, the value measured in a ground position of 0. 5 mm from the surface is made with the Si concentration of the interior, while in the latter case, the value measured in a position of a depth of 1/4 of the thickness of the surface is made with the hardness of the interior. The technique of Patent Document 2 covers steel containing Ti with a content of Si reduced to 0.8% by mass or less and is a special technique that controls the state of the deposits by means of hot rolling conditions. Therefore, the problem to be solved by the present invention is to provide a sheet of hot rolled steel, high strength, high in Si content, which allows a conversion coating to be homogeneously formed on the entire surface of the steel sheet in primer, without adding any new step in the production of steel sheet, and to facilitate quality control. Therefore, the inventors carried out intensive studies in the improvement of chemical convertibility and as a result they took note of the oxide concentration of the surface of the steel sheet and of the properties of the surface of the steel sheet, in particular of the relief or roughness of the surface, and discovered that by defining the Si and Mn concentrations of the oxides on the surface of the steel sheet and limiting the pitting or roughing corrosion in the pickling to a specific range, the chemical convertibility was greatly improved . The present invention applies this discovery in concrete form and provides an excellent hot rolled steel sheet in chemical convertibility, produced through a step of hot rolling and pickling, comprising, in mass%, C: 0.03 to 0.15% Yes: 0.8 to 3.0%, Mn: 0.5 to 3.0%, P: 0.07% or less, S: 0.01% or less, Al: 0.015 to 0.1%, N: 0.001 to 0.008%, and the residue of Fe and impurities unavoidable, the oxides on the surface of the steel sheet having, in% mass, an Si concentration of 3.5% or less and a concentration of Mn of 3.5% or less. In addition, an excellent hot-rolled steel sheet in chemical convertibility, produced through a hot-rolling and pickling step, is provided, comprising, in% mass, C: 0.03 to 0.15% Si: 0.8 to 3.0%, Mn: 0.5 to 3.0%, P: 0: 07% or less, S: 0.01% or less, Al: 0.015 to 0.1%, N: 0.001 to 0.008%, and one or both of Ti: 0.02 to 0.3% and Nb : 0.01 to 0.5%, Cu: 0.2 to 1.8% and Ni: 0.1 to 2.0%, Mo: 0.05 to 0.5%, B: 0.0002 to 0.006%, and Ca: 0.0005 to 0.005% alone or in combination, and one residue of Fe and the unavoidable impurities, the oxides on the surface of the steel sheet having, in% mass, a concentration of Si of the 3. 5% or less and a Mn concentration of 3.5% or less. In the above steel sheets of the present invention, the average roughness Ra of the surface of the steel sheet is 3.0 μm or less and the number of holes of a diameter of 1 μm to 0.3 μm caused by the pickling is of a average of 5 or less in squares of the surface of the steel sheet when it is divided into squares of 10 μm per side. In addition, to solve the above problem, the method of the present invention is a method of producing an excellent heat-laminated steel sheet in chemical convertibility, characterized, in a pickling step by producing the hot-rolled steel sheet of the present invention, immersing the sheet in an aqueous solution having, in mass%, a concentration of HCl of 7 to 15%, an ionic concentration of Fe of 4 to 12%, and a residue of metal ions different from that of Fe and impurities, at a solution temperature of 80 to 98 ° C for 40 seconds or more. In addition, it is a method of producing an excellent hot-rolled steel sheet in characterized chemical convertibility, in a pickling step by producing the hot-rolled steel sheet of the preferred aspects of the present invention, by dipping the sheet into an aqueous solution having, in% mass, a concentration of HCl of 7 to 15%, an ionic concentration of Fe of 4 to 12%, and a residue of metal ions different from that of Fe and impurities, at a temperature of solution from 80 to 95 ° C for a time of a range of 40 sec. or more, even when the HCl concentration (% by mass) x time of immersion (sec.), reaches 520 or less. In addition, in the above methods of the present invention, the aqueous solution preferably includes, in% by mass, 0.5 to 5% of HNO3. BEST MODE FOR WORKING THE INVENTION In the present invention, the ingredients of the steel sheet are limited to the above ranges to obtain a high strength and high viability that allow to use parts of automobile chassis and obtain an excellent chemical convertibility. The reasons for limitation are as follows. The percentages of all the elements are in percent by mass.

C: If it is less than 0.03%, the elongation becomes low, while if it is above 0.15%, the corrosion resistance drops. Yes: If it is less than 0.8%, the strength and elongation become lower, while if it is above 3.0%, the pickling capacity drops. Mn: If it is less than 0.5%, the elongation drops, while if it is above 3.0%, the pickling capacity drops. Q: If it is above 0.07%, the ability to expand the holes drops and the elongation and other mechanical properties fall. S: If it is above 0.01%, the corrosion resistance drops. Al: If it is less than 0.015%, oxides of Si and Mn are easily formed on the surface of the steel sheet and the chemical convertibility drops, while if it is above 0.1%, the corrosion resistance drops. N: If it is less than 0.001%, the chemical convertibility drops, while if it is above 0.008%, the elongation drops. The steel sheet of the present invention may also include, in addition to the above ingredients, as necessary, the following ingredients alone or in combination. In addition to improving resistance, one or both of Ti and Nb can be added. In this case, if the Ti is less than 0.02%, there is little action in improving the resistance by the formation of carbides and the effect of the improvement of the mechanical strength can not be assured. Even when more than 0.3% is added, the effect of the increase in resistance becomes saturated. Nb: If less than 0.01%, there is little action to improve the strength and the effect of improving the mechanical strength can not be assured by its addition. Even when more than 0.5% is added, the effect of the increase in resistance becomes saturated. In addition to increasing the strength, Cu can be added and, according to the needs, the steel can be heated to a temperature of approximately 450 to 650 ° C for heat treatment. In this case, if the Cu is less than 0.2%, the effect is smaller, while even when more than 1.8% is added, the effect becomes saturated. When adding Cu, Ni is added together to avoid cracking of the steel sheet in the hot working time. The effect of this Ni is exhibited when it is present in 0.1% or more and saturates by 2.0%. In addition, when increasing the resistance, Mo can be added. In this case, if the Mo is less than 0.05%, there is little action to improve the resistance by the formation of carbides and the effect of the improvement of the mechanical resistance can not be assured. due to its addition. Even when more than 0.5% is added, the effect of the increase in resistance becomes saturated. In addition, it is possible to reduce aging due to nitrogen and improve the hole expansion property by adding B. This effect is exhibited by adding B to 0.0002% or more and saturating by 0.006%. In addition, it is possible to add Ca to avoid a drop in the hole expansion property, due to the formation of MnS. This effect is exhibited by adding Ca to 0.0005% or more and saturating by 0.005%. In the hot-rolled steel sheet of the present invention, the oxides on the surface of the steel sheet comprised of the above composition of ingredients have, in% mass, an Si concentration of 3.5% or less and a concentration of Mn of 3.5% or less. The hot-rolled steel sheet, produced through the step of hot rolling and pickling, it is detached from the surface oxide residue by pickling, although with a steel sheet having a higher Si content, even when the apparent oxide residue is completely removed, the oxides will remain partially. The present invention solves the problem of chemical convertibility by modifying the oxides to the previous state. The chemical conversion is performed by removing any oil deposited on the surface of the steel sheet by degreasing, then immersing the sheet in a chemical conversion solution for a predetermined time. By this treatment, the Fe ions of the steel sheet are dissolved in the conversion solution, by reacting the ingredients of the solution, and forming a large number of converted crystal beads, formed by the compounds including Fe , Zn, P, 0, etc. These develop and form coatings that cover the entire surface of the steel sheet. At this time, it is considered necessary to make fine, converted crystals of 10 μm or smaller crystals, uniformly deposited on the entire surface. If the state of non-deposition is poor and locations of "bald spots" of deposition are present, problems of poor adhesion of the coating at the time of application or a fall in corrosion resistance after coating will arise. If the steel sheet becomes high in the Si content, the amount of oxides with high Si content in the surface residues after hot rolling will be increased. With the usual pickling with hydrochloric acid, it will easily remain on the surface of the steel sheet. If the steel sheet that has residual oxides with a high Si content is chemically converted onto it, it will easily present locations of "bald spots" of non-deposition. From this phenomenon, in locations of residual oxides with high Si content, it is believed that the bald spots are formed because of the delay in the dissolution of Fe ions and the delay in the reaction to form crystal grains converted to the moment of chemical conversion. In addition, bald spots with residual oxides with high Mn content also occur easily. In the hot rolled steel sheet of the present invention, even if oxides remain on the surface of the steel sheet after pickling, since the oxides have an Si concentration of 3.5 mass% or less, and a concentration of Mn of 3.5% by mass or less, there is no delay in the dissolution of Fe ions in the chemical conversion. Therefore, the development of cores to the same degree as the locations without oxides to form fine, converted crystal grains of 10 μm or smaller, the surfaces of the oxides are converted in their entirety, a conversion coating comprising of fine grains of converted glass, uniformly deposited on the entire surface of the steel sheet, and the formation of bald spots can be prevented. The surface conditions of the steel sheet of the present invention can be judged by identifying the oxides by EPMA from the distribution of oxygen on the surface of the steel sheet etc. and analyzing its concentration of yes and its concentration of Mn. Si and Mn on the surface of a steel material are usually analyzed by EPMA at an acceleration voltage of 15 kV. In this case, the concentration is detected to a depth of approximately 3 μm from the most superficial position of the steel sheet. However, even under these conditions, caused by the thickness of the oxide layer of the surface and the roughness of the surface etc., sometimes even the information of parts deeper than 3 μm is detected. In some cases, the base metal includes Si and Mn. In the present invention, the analysis values of Si and Mn by EPMA at an acceleration voltage of 15 kV should be 3.5 mass% or less. The concentration does not have to be just the oxides. It is confirmed that if the steel sheet surface is in such condition, the chemical convertibility is good. In the stainless steel sheets of the present invention, even when a coating comprised of fine grains of converted glass is formed, uniformly on the complete steel sheet by chemical conversion, oxidation sometimes occurs after chemical conversion. The inventors investigated in detail the steel sheets oxidized in this way and the non-oxidized steel sheets. As a result, they learned that the roughness of the surface and the micro-holes of the steel sheet are related to oxidation. The micro-holes were formed by the perforation caused by the pickling. When there is a greater surface relief on the surface of the steel sheet or when there is a large number of micro-holes present, when rinsing the steel sheet submerged in and lifted out of the chemical conversion solution, the chemical conversion solution is probably remains in the gaps causing the ion dissolution of Fe of the steel sheet and leading to oxidation. In addition, if the average roughness Ra of the surface of the steel sheet is 3.0 μm or less and the number of holes a due to the pickling is an average of 5 or less in squares of the surface of the steel sheet divided into squares of 10 μm per side, the inventors learned that there is no oxidation after the chemical conversion. An average of 3 or less is more preferable. "Hole" means a hole with a diameter from μm to 0.3 μm. Oxidation is judged by observation by the naked eye just after rinsing and drying after chemical conversion. The steel sheet that does not oxidize just after drying will also not be oxidized later. Estimating the pitting corrosion and the average roughness Ra of the steel sheet surface, the pitting corrosion was measured by cutting a sample of a total width x length of approximately 500 mm from the steel sheet and measuring pitting corrosion on the steel sheet. the surfaces of three locations, that is, the positions of 150 mm from the two axes and from the center in the direction of the amplitude, in ranges of 100 μm x 100 μm divided into 10 μm squares per side. Considering the average roughness Ra of the surface of the steel sheet, the average roughness Ra was measured in the same locations. The average rugosity Ra was measured based on the arithmetic average roughness method of JIS B0601.

The measuring device for the average roughness Ra is preferably a probe type rugosimeter. A "SURFTEST SV-400" Mitsutoyo was used for the measurement. Next, the method of the present invention is a pickling method for producing the above-mentioned steel sheets of the present invention. The pickling conditions to make the oxides on the surface of the steel sheet containing, in% mass, an Si concentration of 3.5% or less and an Mn concentration of 3.5% or less, are to immerse the sheet in a aqueous solution having, in mass%, a concentration of HCl of 7 to 15%, an ionic concentration of Fe of 4 to 12%, and a residue of metal ions different from that of Fe and impurities at a solution temperature of 80 to 98 ° C for 40 seconds or more. Pickling under these conditions can be carried out in the usual pickling step of the hot rolled sheet. The residue on the surface of the steel sheet is suitably removed and an excellent hot-rolled steel sheet in chemical convertibility is obtained. If the concentration of HCl is less than 7%, the ionic concentration of Fe is less than 4%, the temperature of the solution is lower than 80 ° C, or the immersion time is less than 40 seconds, the oxides with a concentration of Si and a concentration of Mn exceeding 3.5% will remain on the surface of the steel sheet. If the concentration of HCl is above 15%, the ionic concentration of Fe is above 12%, or the temperature of the solution is above 98 ° C, the roughness of the surface of the steel sheet, caused by the pickling and drop of chemical convertibility. Preferably, it is effective to make the temperature of the solution 85 to 95 ° C for pickling. Further, in the method of the present invention, the pickling conditions for making the average roughness Ra of the surface of the steel sheet of 3.0 μm or less and to make the number of holes caused by the pickling an average of 5 or less in the squares of the steel sheet surface divided into squares of 10 μm per side, further limits the above conditions of the present invention. The conditions are the immersion of the leaf at a solution temperature of 80 to 95 ° C for a time of a range of 40 seconds or more until the HCl concentration (mass%) x immersion time (sec.) it reaches 520 or less. The pickling under these conditions can be carried out in the usual pickling step of the hot rolled sheet. The scale on the surface of the steel sheet is suitably removed and an excellent hot-rolled steel sheet in chemical convertibility is obtained. If the temperature of the solution exceeds 95 ° C or if the immersion for a time of a range where the HCl concentration (mass%) x immersion time (sec.) Exceeds 520, the roughness Ra of the surface of the steel sheet after pickling will exceed 3.0 μm, the number of holes caused by pickling will end up exceeding the previous range, and oxidation will likely occur after chemical conversion. In addition, it is also effective to add nitric acid to the pickling solution and make the HN03 concentration 0.5 to 5%. In this case, the effect of pickling is stimulated by HNO3. When adding HN03, it is preferably effective to have the temperature of the solution 80 to 90 ° C for the pickling. If the concentration of HNO3 is less than 0.5%, no effect appears, while if it is above 5%, it results in a rough surface. EXAMPLES Hot-rolled steel sheets were collected from the ingredients shown in Table 1, under the conditions shown in Table 2, then their chemical convertibility was judged. The comparative examples of Table 1 are outside the range of the present invention in the ingredients marked by the asterisks. The hot-plate heating temperature in the hot rolled was 1200 ° C, the hot rolled finishing temperature was 880 ° C, and the sheets were cooled on a rolling surface of hot inlet rolls below 390 ° C, then rolled at 390 ° C, then cooled to room temperature. The pickling was performed by immersing samples cut from the leaves in a test pickling tank. The asterisk marks in Table 2 indicate conditions unrelated to the conditions of the method of the present invention. In addition, "ct" in Table 2 is the value of the HCl concentration (% by mass) x immersion time (sec.). Table 3 shows the results. The concentrations of Si and Mn were analyzed by EPMA using an acceleration voltage of 15 kV. The chemical conversion was performed on samples cut from the leaves using an analysis tank by a method similar to the current chemical conversion. That is, each leaf sample was defatted, immersed in a surface adjustment solution for 30 seconds, then immersed in a chemical conversion solution (PBWL35 made by Japan Parkerizing) for treatment for 30 sec. rinsed and dried. The chemical convertibility was judged to be looking for bald spots by means of observation by an SEM of the surface of the steel sheet given the conversion coating and looking for oxidation by means of observation by the naked eye just after drying. In addition, the chemical properties of the steel sheet are shown. In Table 3, from No. 1 to No. 6 and No. 11 to No. 26 of the examples of the invention, they were free of bald spots and oxidation after chemical conversion and exhibited excellent chemical convertibility. From No. 18 to No. 26 they had special elements added to them. From No. 18 to No. 23 exhibited improved tensile strength. No. 18 showed the addition effect of Ti, No. 19 and No. 20 the addition of Ti and Nb, No. 21 and No. 22 the addition of Cu and Ni, and No. 23 the addition of Mo. No. 24 and No. 25 exhibited an improvement in the hole expansion ratio, caused by the addition of Ca, while No. 26 exhibited an improvement in the hole expansion ratio, caused by the addition of B. The examples of the invention had pickling conditions, as shown in conditions A to E of Table 2, of an HCl concentration x immersion time (ct) of 520 or less. No oxidation could be observed even in locations where coatings comprised of fine grains of converted glass were formed. Nos. 7 to 10 of the comparative examples had pickling conditions outside the conditions of the present invention. Conditions F of No. 7 had an insufficient immersion time, conditions of No. 8 had a low solution temperature, conditions H of No. 9 had a low concentration of HCl, and all had an Si concentration of oxides exceeding 3.5% resulting in bald spots after chemical conversion. Conditions I of No. 10 had a high ionic concentration of Fe, a concentration of Si and a concentration of Mn of the oxides exceeding 3.5%, bald spots after chemical conversion, and oxidation.

From No. 27 to No. 33 of the comparative examples had ingredients of steel sheets outside the range of the present invention. No. 27 had a high amount of C and showed oxidation. No. 28 had a high amount of Si and a Si concentration of surface oxides that exceeded 3.5% while 29 had a high amount of Mn and a concentration of Mn of surface oxides that exceeded 3.5. %. Each exhibited bald spots and oxidation. No. 30 had a high amount of S, while No. 31 had a low amount of A. In both, although Si and Mn concentrations of surface oxides were low, there was oxidation. No. 32 had a low amount of N. Even though the Si and Mn concentrations of the surface oxides were low, there were bald spots and oxidation. No. 33 had a high amount of P.

Even though the concentrations of Si and Mn were low, there was oxidation. INDUSTRIAL APPLICABILITY The present invention improves the chemical convertibility without requiring the reduction of the Si content as in the prior art, thus providing a hot-rolled, high-strength, high-strength steel sheet used to reduce weight and ensure safety in automobiles etc., where even when other additive elements are not used, the resistance and viability are not impaired. In addition, the sheet can be produced through the hot rolling step and the pickling step by adjusting only the pickling conditions. In addition, the concentration of Yes and the concentration of Mn of the surface of the steel sheet need only be done within suitable ranges, so that quality control is also facilitated.

Table 1 * The comparative examples are outside the range of the present invention in the underlined ingredients.

Table 2 * The comparative examples are outside the range of the present invention in the underlined ingredients.

Table 3 1) The Ra is measured based on the average roughness method, arithmetic of JIS B0601. The measuring device was a "SURFTEST SV-400" Mitsutoyo. 2) When dividing the surface of the steel sheet into a square grid of 10 μm per side Good: Number of parts with holes of 5 or less Poor: Number of parts with holes of 6 or less

Claims (1)

1. CLAIMS 1.- Excellent hot-rolled steel sheet in chemical convertibility, produced through a step of hot rolling and pickling, characterized in that it comprises, in% by mass, C: 0.03 to 0.15% Si: 0.8 to 3.0%, Mn: 0.5 to 3.0%, P: 0.07% or less, S: 0.01% or less, Al: 0.015 to 0.1%, N: 0.001 to 0.008%, and the residue of Fe and the unavoidable impurities, the oxides on the surface of the steel sheet having, in mass%, an Si concentration of 3.5% or less and an Mn concentration of 3.5% or less. 2.- Excellent hot rolled steel sheet in chemical convertibility, produced through a step of hot rolling and pickling, characterized in that it comprises, in mass%, C: 0.03 to 0.15% Si: 0.8 to 3.0%, Mn : 0.5 to 3.0%, P: 0.07% or less, S: 0.01% or less, Al: 0.015 to 0.1%, N: 0.001 to 0.008%, and one or both of Ti: 0.02 to 0.3% and Nb: 0.1 to 0.5%, Cu: 0.2 to 1.8% and Ni: 0.1 to 2.0%, Mo: 0.05 to 0.5%, B: 0.0002 to 0.006%, and Ca: 0.0005 to 0.005% alone or in combination, and a residue of Fe and the unavoidable impurities, the oxides on the surface of the steel sheet having, in% mass, an Si concentration of 3.5% or less and a concentration of Mn of 3.5% or less. 3.- Excellent hot-rolled steel sheet in chemical convertibility as established in the claim 1 or 2, characterized in that an average roughness Ra of the surface of the steel sheet is 3.0 μm or less and a number of pitting corrosions of a diameter of 1 μm to 0. 3 μm caused by pickling is an average of 5 or less in squares of the surface of the steel sheet when it is divided into squares of 10 μm per side. 4. A method of producing an excellent hot-rolled steel sheet in chemical convertibility, characterized in that, in a pickling step when the hot-rolled steel sheet of claim 1 is produced or 2, immersing the sheet in an aqueous solution having, in mass%, a concentration of HCl of 7 to 15%, an ionic concentration of Fe of 4 to 12%, and a residue of metal ions different from that of Fe and impurities, at a solution temperature of 80 to 98 ° C for 40 sec., or more. 5. A method of producing an excellent hot-rolled steel sheet in chemical convertibility, characterized in that in a pickling step when the hot-rolled steel sheet of claim 3 is produced, immersing the sheet in an aqueous solution which has, in% mass, a concentration of HCl of 7 to 15%, an ionic concentration of Fe of 4 to 12%, and a residue of metal ions different from that of Fe and impurities, at a solution temperature of 80 at 95 ° C for a time of a range of 40 sec. or more, until the HCl concentration (mass%) x immersion time (sec.) reaches 520 or less. 6. An excellent production method of the hot-rolled steel sheet in chemical convertibility, as set forth in claim 4 or 5, characterized in that said aqueous solution includes, in% by mass, 0.5 to 5% of HN03.