JP4452254B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel sheet suitable for applications that require a beautiful surface, such as an automobile.

  Hot-rolled steel sheets used for automobiles and industrial machines are generally manufactured through a rough rolling process and a finish rolling process. FIG. 5 is a diagram schematically showing a conventional method for producing a hot-rolled steel sheet. 6 (a) to 6 (e) are cross-sectional views showing the state of the steel sheet in each step shown in FIG. 5 in the order of steps, FIG. 6 (a) after rough rolling, and FIG. 6 (b) after descaling. FIGS. 6C and 6D show the state of the product after the finish rolling, and FIG. 6E shows the state of the product after the finish rolling. As shown in FIG. 5, in the manufacturing process of a hot-rolled steel sheet, first, a slab obtained by continuously casting molten steel adjusted to a predetermined composition is rolled by a roughing mill 101, and then a plurality of rolling stands 102 a to 102-. A hot rolling steel sheet 107 having a predetermined thickness is obtained by hot rolling with a finish rolling mill 103 constituted by 102 g.

  In such a conventional method for producing a hot-rolled steel sheet, as shown in FIG. 6A, a scale 109 made of oxide is generated on the surface of the steel sheet 108 in the rough-rolled steel sheet after the rough rolling. . Since the scale 109 causes wrinkles (scale wrinkles) in the subsequent finish rolling, in the conventional method of manufacturing a hot-rolled steel sheet, the high-pressure water 106 is directed from the nozzle 105 of the descaling device toward the rough-rolled steel sheet 104. , After removing the scale on the surface of the steel plate 108, finish rolling is performed (see, for example, Patent Documents 1 and 2).

For example, in the method for removing scale of a hot-rolled steel sheet described in Patent Document 1, when the pressure of water when spraying from the spray nozzle is P (kg / cm ² ), the collision pressure of water on the surface of the hot-rolled steel sheet The power consumption of the water supply pump is minimized by adjusting the flow rate of water and the distance between the tip of the injection nozzle and the steel plate so that the value is equal to or greater than A (kg / cm ² ) obtained by the following formula 1. To maintain and reduce operating costs.

  Moreover, in the rolling method of the hot-rolled steel sheet described in Patent Document 2, between the descaling device and the first stage rolling stand of the finishing mill and / or the first stage rolling stand of the finishing mill and the first rolling stand. Generation of scale wrinkles is suppressed by forcibly cooling the center portion in the width direction of the hot-rolled steel sheet that is higher than the critical temperature for generating scale wrinkles between the second rolling stand.

Japanese Patent No. 2780600 Japanese Patent Publication No. 60-48241

  However, even if descaling is performed after rough rolling to remove the scale on the surface, the scale is generated again in the subsequent finish rolling process, and there is a problem that scale wrinkles are generated in the product. Specifically, as shown in FIG. 6C, the scale 109 generated by hot rolling in the first stage rolling stand (hereinafter also referred to as the first stand) 102a is shown in FIG. As shown, it grows between a first stand 102a and a second-stage rolling stand (hereinafter also referred to as a second stand) 102b. Then, the scale 109 on the grown surface is pushed into the steel plate during rolling in the second and subsequent rolling stands. As a result, as shown in FIG. 6 (e), a scale layer 111 having a non-uniform thickness is formed on the surface of the finished product (hot-rolled steel sheet), which is the cause of scale wrinkles and appearance defects. Become.

  Fig.7 (a) is sectional drawing which shows the normal part of a hot-rolled steel plate, FIG.7 (b) is sectional drawing which shows the collar part of a hot-rolled steel plate. As shown in FIG. 7A, a normal hot-rolled steel sheet has a scale layer 111 uniformly formed on the surface, but as shown in FIG. The thickness of the scale layer 111 is thick, and there is also a part that is 1.3 times or more of the normal part.

  Moreover, when the water cooling apparatus is installed between the 1st stand and the 2nd stand of a finishing rolling process like the technique described in patent document 2, and the width direction center part of the steel plate is forcedly cooled, steel plate Although the surface of the glass is temporarily cooled, there is a problem that the temperature rises due to the recuperation and the scale is likely to be easily returned. Even if the thickness of the scale generated between the first stand and the second stand becomes uniform by this method, scale growth, fragmentation and embedding occur after the second stand. There is a problem that it cannot be completely prevented. Furthermore, in this method, since the outermost surface portion of the steel plate is forcibly cooled, there is also a problem that the characteristics of the product (hot rolled steel plate) after finish rolling are deteriorated.

  The present invention has been devised in view of the above-described problems, and provides a method for producing a hot-rolled steel sheet capable of improving the surface shape by suppressing the occurrence of scale wrinkles during hot rolling. For the purpose.

The manufacturing method of the hot-rolled steel sheet according to the first invention of the present application is mass%, C: 0.04 to 0.35%, Si: 0.001 to 2.5%, and Mn: 0.1 to 2.5%. In a method for producing a hot-rolled steel sheet, a rough-rolling step of roughly rolling a slab to obtain a rough-rolled steel sheet, and hot-rolling the rough-rolled steel sheet with a finishing rolling mill comprising a plurality of rolling stands The surface rolling temperature of the steel sheet immediately after passing through the first stage rolling stand is T (° C.), and after passing through the first stage rolling stand, when the time until the rolling stand stage was K (s), the value of S ₁ which is defined by the following equation (2) is less than 1.95, is rolled in the second stage rolling stands The thickness of the scale on the surface of the steel sheet is 10 μm or less.

The manufacturing method of the hot-rolled steel sheet according to the second invention of the present application is, in mass%, C: more than 0% and 0.02% or less, Si: 0.001 to 0.9%, and Mn: 0.1 to 2.5. %, A rough rolling step of roughly rolling a slab to obtain a rough rolled steel sheet, and hot rolling the rough rolled steel sheet by a hot rolling mill comprising a plurality of rolling stands. A finishing rolling step for obtaining a steel plate, wherein the finishing rolling step is performed by setting the surface temperature of the steel plate immediately after passing through the first stage rolling stand to T (° C.) and passing through the first stage rolling stand. when the time until the rolling stand of the second stage was K (s), the value of S ₁ which is defined by the following equation (3) and 3.01 or less, rolling in the second stage rolling stands The thickness of the scale of the surface of the steel sheet to be made is 25 μm or less.

  In the method for producing a hot-rolled steel sheet according to the first and second inventions of the present application, the slab is further in% by mass, Cr: 0.02-0.5%, Cu: 0.02-1.6, and Ni. : It may contain at least one element selected from the group consisting of 0.02 to 1%.

According to the present invention, depending on the steel composition, the surface temperature T (° C.) of the steel sheet immediately after passing through the first stage rolling stand and the second stage rolling stand after passing through the first stage rolling stand are reached. adjusting the value of S ₁ which is defined by the time K (in seconds) until the thickness of the scale to be generated until the roll stand of the second stage of the finish rolling step, the following thickness scale defects occur Therefore, a hot-rolled steel sheet having an excellent surface shape can be produced.

  The best mode for carrying out the present invention will be described below with reference to the drawings. This inventor investigated the factor which the thickness nonuniformity generate | occur | produces in the hot-rolled steel plate after a finish rolling process, and acquired the knowledge shown below. 1A to 1C are cross-sectional views schematically showing changes in scale, and FIG. 1A is a diagram between a first stage rolling stand and a second stage rolling stand. FIG. 1B shows the state after the third stage rolling stand, and FIG. 1C shows the state after the third stage rolling stand. As shown in FIG. 1A, when the scale grows between the first stand and the second stand, a bulge 110 may occur on the surface of the steel plate. When the bulge 110 is generated on the surface in this way or the thickness of the scale exceeds a specific value, the scale 109 on the surface is divided by rolling on the second stand as shown in FIG. 1B. Further, as shown in FIG. 1C, after the third stage rolling stand (hereinafter also referred to as the third stand), the scale 109 is pushed into the steel plate 108, and the above-described FIG. A scale layer 111 having a non-uniform thickness as shown in FIG.

  That is, when a steel sheet with a scale attached to the surface is rolled, the scale is also stretched as the steel sheet is stretched, but when the scale exceeds a certain thickness, a portion that cannot be stretched is generated and the scale is divided. For this reason, when the scale thickness is thick, even if the thickness is uniform, the scale is divided in the hot rolling at the next rolling stand, resulting in uneven thickness of the scale. Moreover, the limit thickness by which the scale produced | generated on the steel plate surface is parted by rolling changes with additive elements. It is presumed that this is because the adhesion between the steel sheet and the scale changes due to the generation of CO gas due to oxidation of C and the additive elements concentrated on the surface. Then, due to the change in adhesion, a circular bulge of about 0.2 to 30 mm is partially generated on the surface of the scale. Since the portion where the surface is swollen has a different ductility from the other portions, the thickness unevenness occurs in the hot rolling at the next rolling stand.

Therefore, the present inventor conducted intensive studies in order to solve the cause of the unevenness of the scale thickness of 3 μm or more that partially occurs due to the scale division in the middle of rolling and the change in adhesion between the steel sheet and the scale, In the case of a steel plate with a normal composition, the scale thickness is 10 μm or less, and the temperature and the holding time between the first stand and the second stand are optimized, so that the scale is divided and the adhesion between the steel plate and the scale. We found that both changes can be prevented. And in order to satisfy these, when the surface temperature of the steel plate immediately after passing through the first stand is T (° C.), and the time from passing through the first stand to the second stand is K (seconds), The inventors have found that it is effective to set the value of S ₁ defined by the following formula (4) to less than 1.95, and have reached the first invention of the present application.

Furthermore, in the case of a low C steel sheet having a C content of 0.02% or less, the present inventor sets the scale thickness to 25 μm or less, and further sets the value of S ₁ defined by the above formula (4) to 3. By setting it to 01 or less, it was found that both the scale division and the adhesiveness change between the steel plate and the scale can be prevented, and the second invention of the present application was achieved.

  First, a method for manufacturing a hot-rolled steel sheet according to the first embodiment of the present invention will be described. In the following description, mass% in the composition is simply described as%.

  FIG. 2 is a diagram schematically showing a method for producing a hot-rolled steel sheet according to this embodiment. 3 (a) to 3 (d) are sectional views showing the state of the steel plate in each step shown in FIG. 2 in the order of steps, and FIGS. 3 (a) to 3 (c) are views of the first stand and the second stand. Meanwhile, FIG. 3 (d) shows a state between the second stand and the third stand. In addition, in FIG. 2, components other than the work roll in the rough rolling mill 1 and each rolling stand 2a-2g are abbreviate | omitting illustration. As shown in FIG. 2, in the manufacturing method of the hot-rolled steel sheet of this embodiment, first, it manufactures by continuous casting etc., for example, C: 0.04-0.35%, Si: 0.001-2. 5%, Mn: 0.1 to 2.5%, P: more than 0% to 0.05%, S: more than 0% to 0.02%, Al: 0.01 to 0.1% and N : A composition comprising more than 0% and 0.01% or less, the balance being Fe and inevitable impurities, the thickness of which is, for example, about 250 mm, heated, and then roughly rolled by the roughing mill 1 For example, a roughly rolled steel plate 4 having a length of about 40 mm is obtained.

Next, high pressure water 6 is sprayed from the nozzle 5 of the descaling device toward the rough rolled steel plate 4 to remove the scale generated on the surface of the rough rolled steel plate 4. And the rough rolling steel plate 4 is hot-rolled by the finish rolling mill 3 in which seven rolling stands 2a to 2g are arranged at substantially equal intervals in the rolling direction of the steel plate, for example, hot rolling having a thickness of 2 to 5 mm. Get a steel plate. At this time, by adjusting the surface temperature T (° C.) of the steel plate immediately after passing through the first stand 2a and the time K (seconds) from passing through the first stand 2a to reaching the second stand 2b, the above formula ( the value of _{S 1} obtained by 4) be less than 1.95. The surface temperature T (° C.) of the steel sheet immediately after passing through the first stand 2a can be adjusted by, for example, water cooling before inserting into the first stand, the rolling speed, and the like. The passage time K (seconds) from the first stand 2a to the second stand 2b can be adjusted by, for example, the rolling speed.

Thereby, as shown to Fig.3 (a)-(c), since the growth of the scale 9 produced | generated on the surface of the steel plate 8 can be suppressed between the 1st stand 2a and the 2nd stand 2b, 2nd The thickness of the scale 9 in the steel plate 8 rolled by the stand 2b can be 10 μm or less, and the occurrence of swelling can be prevented. On the other hand, in the case of performing finish rolling under a condition that the value of S ₁ obtained by the above equation (4) becomes 1.95 or more, the thickness of the scale 9 exceeds 10μm between the first stand 2a and the second stand 2b End up.

  The effects described above are such that the C content in the steel plate 8 is 0.04 to 0.35%, the Si content is 0.001 to 2.5%, and the Mn content is 0.1 to 2.5. % Is obtained. Hereinafter, the reason for limiting the numerical value of the steel composition will be described.

C: 0.04 to 0.35%
C is an element necessary for ensuring the strength of the steel sheet. However, when the C content is less than 0.04%, it becomes difficult to meet the demand for high strength. On the other hand, if the C content exceeds 0.35%, the generation of wrinkles due to the scale becomes violent, which is not aesthetically preferable. Therefore, the C content is 0.04 to 0.35%.

Si: 0.001 to 2.5%
Si is an element necessary for ensuring the strength of the steel sheet. For this reason, when the strength is not required, it is not necessary to add it positively. However, if the Si content is less than 0.001%, the manufacturing cost increases, which is not preferable. On the other hand, when the Si content exceeds 2.5%, wrinkles due to scale become remarkable, and it is impossible to prevent wrinkles from occurring even by the method of the present invention. Therefore, the Si content is set to 0.001 to 2.5%.

Mn: 0.1 to 2.5%
Mn has the effect of ensuring the strength of the steel sheet and preventing cracking during hot rolling due to S in the steel. However, when the Mn content is less than 0.1%, the effect of preventing cracking during hot rolling cannot be obtained. On the other hand, when the Mn content exceeds 2.5%, wrinkles due to scale become remarkable. Therefore, the Mn content is 0.1 to 2.5%.

  Moreover, in the manufacturing method of the hot-rolled steel sheet of this embodiment, in addition to each said component, at least 1 sort (s) of element selected from the group which consists of Cr, Cu, and Ni may be added in steel. .

Cr: 0.02-0.5%, Cu: 0.02-1.6, Ni: 0.02-1%
Cr, Cu and Ni have the effect of improving the effect of reducing scale wrinkles. However, if the content of these elements is less than 0.02%, the effect cannot be obtained. On the other hand, whether the Cr content exceeds 0.5% or the Cu content exceeds 1.6%. If the Ni content exceeds 1%, the addition effect is saturated. Therefore, when adding Cr, Cu and / or Ni, the Cr content is 0.02 to 0.5%, the Cu content is 0.02 to 1.6, and the Ni content is 0.02 to 1, respectively. %.

  Furthermore, in the manufacturing method of the hot-rolled steel sheet of the present embodiment, in order to improve the material of the steel material, Ti: 0.15% or less, Nb: 0.15% or less, At least one element selected from the group consisting of B: 0.05% or less, Ca: 0.01% or less, and V: 0.15% or less can also be added.

  In addition, it is not necessary to specifically limit about steel components other than said each component, For example, P, S, N, Al, etc. may be contained. In that case, P is an element inevitably contained in the steel as an impurity, but if its content exceeds 0.05%, a non-uniform pattern may occur on the surface of the steel sheet, so The amount is desirably regulated to 0.05% or less. Similarly, S and N are elements inevitably mixed in the steel as impurities. However, when the S content exceeds 0.02% or the N content exceeds 0.01%, the ductility of the steel sheet is increased. Therefore, it is desirable to regulate the S content to 0.02% or less and the N content to 0.01% or less. Also, Al has an effect of deoxidizing molten steel, but if its content is less than 0.001%, a sufficient deoxidation effect cannot be obtained, and if it exceeds 0.1%, it becomes an oxide of Al. Since the resulting wrinkles are likely to occur, the Al content is preferably 0.01 to 0.1%.

In the method for manufacturing a hot-rolled steel sheet according to this embodiment, the surface temperature T (° C.) of the steel sheet immediately after passing through the first stand 2a and the time K (seconds) from passing through the first stand 2a to reaching the second stand 2b. adjust the, the value of S ₁ obtained from the equation (4) is hot rolled under conditions of less than 1.95, the thickness of scale on the surface of the steel sheet is rolled by the second stand 2b Can be made 10 μm or less. As a result, it is possible to prevent the occurrence of swelling between the first stand 2a and the second stand 2b, and it is possible to prevent the occurrence of scale breakage during hot rolling in the second stand 2b. A hot-rolled steel sheet having no surface defects and a good surface shape can be obtained.

Next, a method for manufacturing a hot-rolled steel sheet according to the second embodiment of the present invention will be described. As described above, when the amount of C in the steel is a low C steel having a predetermined amount or less, scale wrinkles are less likely to occur than the steel composition in the first embodiment described above, and therefore, before rolling in the second stand. it is possible to widen the allowable range of the thickness of the scale in the steel sheet, it can be wider range of values of S ₁ obtained by the above equation (4). Therefore, in the present embodiment, a hot-rolled steel sheet is manufactured using a slab in which C in steel is reduced and the addition amount of other elements is optimized.

  In the method for manufacturing a hot-rolled steel sheet according to this embodiment, similarly to the method for manufacturing a hot-rolled steel sheet according to the first embodiment shown in FIG. 2, the hot-rolled steel sheet is subjected to a rough rolling process, a descaling process, and a finish rolling process. Is manufactured. 4 (a) to 4 (c) are cross-sectional views showing the state of the steel plate between the first stand and the second stand in the order of steps, and FIG. 4 (d) is between the second stand and the third stand. It is sectional drawing which shows the state of a steel plate. In the method for producing a hot-rolled steel sheet according to the present embodiment, first, as shown in FIG. 2, it is produced by continuous casting or the like, for example, C: more than 0% and 0.02% or less, Si: 0.001-0. .9%, Mn: 0.1 to 2.5%, P: more than 0% to 0.007% or less, S: more than 0% to 0.004% or less, Al: 0.01 to 0.1% and N: More than 0% and 0.01% or less, and a slab having a composition composed of Fe and inevitable impurities is heated, and then roughly rolled by a roughing mill 1 to obtain a rough rolled steel sheet 4.

Next, high pressure water 6 is sprayed from the nozzle 5 of the descaling device toward the rough rolled steel plate 4 to remove the scale generated on the surface of the rough rolled steel plate 4. And the hot rolled steel plate 7 is obtained by hot-rolling the rough rolled steel plate 4 with a finishing mill in which seven stages of rolling stands 2a to 2g are arranged at substantially equal intervals in the rolling direction of the steel plate. At this time, the surface temperature T (° C.) of the steel plate immediately after passing through the first stand 2a and the second after passing through the first stand 2a by the same method as the manufacturing method of the hot-rolled steel plate of the first embodiment described above. adjust the time K and, in seconds, before reaching the stand 2b, the value of _{S 1} obtained by the above equation (4) to 3.01 or less.

Thereby, as shown to Fig.4 (a)-(c), the scale 9 produced | generated on the surface of the steel plate 8 by the hot rolling in the 1st stand 2a grows until it reaches the 2nd stand 2b. However, the scale thickness immediately before the second stand 2b shown in FIG. 4C is suppressed to 25 μm or less. On the other hand, if the value of S ₁ obtained by the above equation (5) is subjected to finish rolling under conditions of greater than 3.01, the thickness of the scale to produce the steel sheet surface exceeds 25 [mu] m.

  The effects described above are such that the C content in the steel plate 8 exceeds 0% and is 0.02% or less, the Si content is 0.001 to 0.9%, and the Mn content is 0.1 to 2. Obtained when 5%. Hereinafter, the reason for limiting the numerical value of the steel composition will be described.

C: More than 0% and 0.02% or less As described above, in the hot-rolled steel sheet of the present embodiment, the C content is reduced in order to suppress the occurrence of scale flaws. Specifically, when the C content exceeds 0.02%, and the finish rolling under the condition value of S ₁ obtained by the above equation (4) is 3.01 or less, 25 [mu] m scale thickness Even if it is below, generation of scale wrinkles cannot be prevented. Therefore, the C content is restricted to 0.02% or less.

Si: 0.001 to 0.9%
As described above, Si is an element necessary for securing the strength of the steel sheet, but it is not necessary to add it positively when strength is not required. However, if the Si content is less than 0.001%, the manufacturing cost increases. On the other hand, in the case of low C steel as in this embodiment, when the Si content exceeds 0.9%, the condition that the value of S ₁ obtained by the above formula (4) is 3.01 or less. Thus, even if the finish is rolled and the scale thickness is 25 μm or less, generation of scale wrinkles cannot be prevented. Therefore, the Si content is set to 0.001 to 0.9%.

  The reason for limiting the numerical value of the Mn content in the method for manufacturing a hot-rolled steel sheet according to this embodiment is the same as that for the method for manufacturing a hot-rolled steel sheet according to the first embodiment described above.

  Moreover, in the manufacturing method of the hot-rolled steel sheet of this embodiment, in addition to each said component, Cr: 0.02-0.5%, Cu: 0.02-1.6 and Ni: 0.02-1 At least one element selected from the group consisting of% can be added. Thereby, the scale wrinkle reduction effect can be improved.

  Furthermore, in the manufacturing method of the hot-rolled steel sheet of this embodiment, in order to improve the material of the steel material, Ti: 0.15% or less, Nb: 0.15%, in addition to the above components, as necessary. Hereinafter, at least one element selected from the group consisting of B: 0.05% or less, Ca: 0.01% or less, and V: 0.15% or less may be added.

  Furthermore, it is not necessary to specifically limit steel components other than the above components, and for example, P, S, N, Al, and the like may be included. In that case, P: 0.05% or less, S: 0.02% or less, N: 0.01% or less, Al: 0.01, similarly to the manufacturing method of the hot-rolled steel sheet of the first embodiment described above. It is desirable to set it to -0.1%.

In the method for producing a hot-rolled steel sheet of the present embodiment, the C content is restricted to 0.02% or less, a slab made of low-C steel is used, and the surface temperature T of the steel sheet immediately after passing through the first stand 2a. and (° C.), and adjust the time K (in seconds) prior to a second stand 2b after passing through the first stand 2a, conditions the value of S ₁ obtained from the equation (4) is 3.01 or less Since the thickness of the scale on the surface of the steel plate rolled by the second stand 2b is 25 μm or less, the occurrence of swelling between the first stand 2a and the second stand 2b is prevented. In addition, it is possible to prevent scale separation in hot rolling in the second stand 2b. As a result, a hot-rolled steel sheet having a uniform scale thickness, no scale wrinkles, and a good surface shape can be obtained.

  In addition, in the manufacturing method of the hot-rolled steel sheet of the first and second embodiments described above, the case where hot rolling is performed by a finish rolling mill provided with seven rolling stands has been described. The finishing mill is not limited, and a plurality of rolling stands may be provided. Moreover, in the manufacturing method of the hot-rolled steel sheet of these 1st and 2nd embodiment, it is not necessary to specifically limit about processes other than the process mentioned above, According to the use, quality, etc. of the hot-rolled steel sheet manufactured. The hot rolling conditions other than those described above are not particularly limited as long as they are within the range normally employed.

Examples of the present invention will be described below. First, as an example of the present invention, a hot-rolled steel sheet was produced by the manufacturing method of the first and second embodiments described above. Specifically, a slab having a composition shown in Table 1 below and having a thickness of 250 mm was heated to 1220 ° C., and then roughly rolled to a thickness of 35 mm by a rough rolling machine to prepare a rough rolled steel sheet. Next, water is sprayed toward the rough rolled steel sheet at a pressure of 13.6 MPa to remove the scale formed on the surface, and then the steel sheet is immediately passed between 7 rolling stands and hot rolled. Rolling was performed to produce a hot-rolled steel sheet having a thickness of 3.2 mm. At this time, by adjusting the surface temperature T (° C.) of the steel plate immediately after passing through the first stand 2a and the time K (seconds) from passing through the first stand 2a to reaching the second stand 2b, the above formula ( 4) by changing the value of S ₁ obtained, the scale thickness of the second stand 2b just before the presence of blisters, as well as to confirm the presence or absence of scale defects in the manufactured hot-rolled steel sheet. The results are summarized in Table 2 below. Table 2 below also shows the surface temperature T (° C.), the passage time K (seconds), and the S ₁ value in each example and comparative example. The balance in the steel composition shown in Table 1 below is Fe and inevitable impurities. Moreover, the underline in the following Table 1 and Table 2 shows that it is outside the scope of the present invention.

  As shown in Tables 1 and 2 above, no. 1-No. 7, no. 12, no. 13, no. 16, no. 18-No. 21 and no. 32-No. 34 is an example of the first embodiment of the present invention. 22, no. 23, no. 26-No. 28, no. 31, no. 35-No. The 38 hot-rolled steel sheets are comparative examples of the first embodiment of the present invention. No. 8-No. 11, no. 14, no. 15 and no. No. 17 hot-rolled steel sheet is an example of the second embodiment of the present invention. 24, no. 25, no. 29 and No. The 30 hot-rolled steel sheet is a comparative example of the second embodiment of the present invention.

As shown in Table 2, although the steel composition of the slab is within the range specified in the claim 1, hot under the condition that the value of S ₁ obtained by the above equation (4) is 1.95 or more Rolled Comparative Example No. 22, no. 23, no. 26-No. 28 and no. The hot rolled steel sheet No. 31 had scale wrinkles on the surface even when the scale thickness was 10 μm or less (No. 23, No. 28). Further, Comparative Example No. 1 in which the steel composition of the slab deviates from the range defined in claim 1. 35-No. 38 hot rolled steel sheet is obtained by hot rolling at a value of 1.95 or more conditions _{S 1} (No.37, No.38) as well, under the condition that the value of _{S 1} is less than 1.95 Scale flaws were also generated in the hot rolled products (No. 35, No. 36).

In contrast, a slab of steel composition within the range specified in the claim 1, carried out to hot rolling under conditions where the value of S ₁ is less than 1.95 cases No. 1-No. 7, no. 12, no. 13, no. 16, no. 18-No. 21 and no. 32-No. All of the 34 hot-rolled steel sheets had no surface defects during hot rolling, and the surface shape was good. In particular, in Example No. 1 in which Cr, Cu and / or Ni was added to steel. 12, no. 13, no. 16 and no. 18 hot-rolled steel sheet had a better surface shape than the hot-rolled steel sheets of the other examples.

Although the steel composition of the slab is within the range specified in the claim 2, comparative example the value of S ₁ is exceeds 3.01 No. 24, no. 25, no. 29 and No. The hot rolled steel sheet of No. 30 had scale wrinkles on the surface even when the scale thickness was 25 μm or less (No. 25, No. 29). In contrast, a slab of steel composition within the range specified in the claim 2, carried out to hot rolling under conditions where the value of S ₁ is a 3.01 or less Example No. 8-No. 11, no. 14, no. 15 and no. The hot rolled steel sheet No. 17 did not generate scale wrinkles during hot rolling and had a good surface shape. In particular, Example No. in which Cr, Cu and / or Ni is added to steel. 14, no. 15 and no. The hot rolled steel sheet No. 17 had an excellent surface shape.

(A)-(c) is sectional drawing which shows the change of a scale typically, (a) is between a 1st stage rolling stand and a 2nd stage rolling stand, (b) is 2nd Between the stage rolling stand and the third stage rolling stand, (c) shows the state after the third stage rolling stand. It is a figure which shows typically the manufacturing method of the hot rolled sheet steel of the 1st Embodiment of this invention. (A)-(d) is sectional drawing which shows the state of the steel plate in each process shown in FIG. 2 in order of a process, (a)-(c) is between a 1st stand and a 2nd stand, (d) is. The state between a 2nd stand and a 3rd stand is shown. (A)-(c) is sectional drawing which shows the state of the steel plate between the 1st stand and the 2nd stand in the manufacturing method of the hot-rolled steel plate of the 2nd Embodiment of this invention in order of a process, (d) FIG. 5 is a cross-sectional view showing a state of a steel plate between a second stand and a third stand. It is a figure which shows typically the manufacturing method of the conventional hot-rolled steel plate. (A)-(e) is sectional drawing which shows the state of the steel plate in each process shown in FIG. 5 in order of a process. (A) is sectional drawing which shows the normal part of a hot-rolled steel plate, (b) is sectional drawing which shows the collar part of a hot-rolled steel plate.

Explanation of symbols

1,101 Rough rolling mill 2a-2g, 102a-102g Rolling stand 3,103 Finish rolling mill 4,104 Rough rolled steel plate 5,105 Nozzle 6,106 High pressure water 7,107 Hot rolled steel plate 8,108 Steel plate 9,109 Scale 110 Swelling 111 Scale layer

Claims (3)

In the method for producing a hot-rolled steel sheet containing, in mass%, C: 0.04 to 0.35%, Si: 0.001 to 2.5% and Mn: 0.1 to 2.5%,
A rough rolling step of roughly rolling a slab to obtain a rough rolled steel sheet;
A finish rolling step of obtaining the hot-rolled steel sheet by hot rolling with a finish rolling mill composed of a plurality of rolling stands.
In the finish rolling step, the surface temperature of the steel sheet immediately after passing through the first stage rolling stand is T (° C.), and the time until it passes through the first stage rolling stand and reaches the second stage rolling stand. when was the K (s), is less than 1.95 the value of S ₁ which is defined by the following equation (a), it the thickness of scale on the surface of the steel sheet is rolled at the second stage rolling stands A method for producing a hot-rolled steel sheet, wherein the thickness is 10 μm or less.

In a method for producing a hot-rolled steel sheet containing, in mass%, C: more than 0% and 0.02% or less, Si: 0.001-0.9% and Mn: 0.1-2.5%,
A rough rolling step of roughly rolling a slab to obtain a rough rolled steel sheet;
A finish rolling step of obtaining the hot-rolled steel sheet by hot rolling with a finish rolling mill composed of a plurality of rolling stands.
In the finish rolling step, the surface temperature of the steel sheet immediately after passing through the first stage rolling stand is T (° C.), and the time until it passes through the first stage rolling stand and reaches the second stage rolling stand. the when the K (s), the value of S ₁ which is defined by the following equation (a) and 3.01 or less, the thickness of scale on the surface of the steel sheet is rolled at the second stage rolling stands A method for producing a hot-rolled steel sheet, characterized by being 25 μm or less.

  The hot-rolled steel sheet is further at least 1% selected from the group consisting of Cr: 0.02-0.5%, Cu: 0.02-1.6 and Ni: 0.02-1%. The method for producing a hot-rolled steel sheet according to claim 1 or 2, comprising a seed element.

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