JP2007239011A - Method for manufacturing galvannealed steel sheet having excellent surface characteristic - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a galvannealed steel sheet of mild steel which is superior in surface properties and is suppressible of the occurrence of streaky flaw. <P>SOLUTION: The method for manufacturing the galvannealed steel sheet comprises: employing a dead soft steel sheet containing, by mass%, ≤0.004% C as a base sheet; adjusting the content of S to 0.002 to 0.010%; and regulating a heating furnace charging temperature by an Mn content to regulate a hot rolling heating temperature, a finish rolling inlet side temperature and the highest temperature in a finish rolling mill train; wherein the occurrence of the surface defect of the streaky flaw is suppressed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  TECHNICAL FIELD The present invention relates to materials used for automobiles, particularly outer plates, and methods for producing the same, and more particularly to a method for producing an alloyed hot-dip galvanized steel sheet having excellent surface properties.

  The alloyed hot-dip galvanized steel sheet for automobiles is an ultra-low carbon steel sheet with a C content of approximately 0.004% or less, and an alloyed hot-dip galvanized steel sheet for the purpose of imparting rust prevention performance and lubrication performance after press forming. Is used. In particular, in recent years, the amount of use has increased since automobiles are required to have high durability. When used as the above-mentioned application, it is important to have excellent surface properties, that is, no streak defects (pattern defects) in the surface appearance.

  On the other hand, in improving the surface properties of the alloyed hot-dip steel sheet, for example, in JP-A-7-228944 (Patent Document 1) and JP-A-11-229039 (Patent Document 2), It is stated that the hot rolling conditions are regulated in order to suppress the bamboo leaf pattern that occurs when the base material is used.

Furthermore, JP 2001-181786 A (Patent Document 3) discloses a method for producing a high-strength hot-dip galvanized steel sheet, which regulates the amount of excess Ti, further regulates Mn to prevent a pattern system due to Mn segregation, Techniques for preventing brittleness are described.
Further, JP 2005-2363 A (Patent Document 4) has a streak by regulating the S content, the slab care method, the cutting temperature, etc. on the basis that S is 0.002% or less. Techniques for preventing defect prevention are described.

JP-A-7-228944 JP-A-11-229039 JP 2001-181786 A JP 2005-2363 A

  However, in the methods of Patent Document 1 and Patent Document 2, it is necessary to regulate the heating temperature at a low temperature, there is a problem that the hot rolling slab heating has a chance property, and the in-furnace time is prolonged. In Patent Document 4, the amount of standard S is too low, resulting in an increase in cost, and slab maintenance conditions and hot rolling milling temperature must be controlled in a complicated manner, resulting in production problems. It was. Moreover, if it is patent document 3, it is a manufacturing method aiming at a high strength steel plate, and did not present the easy manufacturing method of the mild steel in which the addition upper limit of the amount of Mn was decided on the material characteristic.

This invention solves such a problem, and it aims at providing the manufacturing method of the galvannealed steel plate of the mild steel excellent in the surface quality without a streak defect.
That is, in order to solve the above-mentioned problems, the inventor investigates the generated streak defects in detail, and at the same time, the components of the steel sheet, particularly the surface layer and the amount of S in the steel, the hot rolling conditions as production conditions, In particular, the relationship between heating temperature and finish rolling temperature and surface properties was examined in detail. As a result of investigations from various viewpoints, streak defects were mainly influenced by the surface particle size of steel materials, and it was reported that the surface temperature was controlled by lowering the extraction temperature. On the other hand, it was discovered that it occurred for two other reasons.

  One factor is that the scale peeling unevenness at the time of finish rolling is the starting point, and when scale remains as scale peeling unevenness, alloying is promoted only at the scale location, causing alloying unevenness, which occurs as streak defects. There was found. As a result of investigating the cause of this scale peeling unevenness, it is closely related to the surface enrichment degree of S amount affecting the finish rolling temperature condition and descalability and the components in the steel. We found the possibility that conditions to prevent drought exist.

  Another cause of streak defects is due to the remaining scale starting from hot brittleness during rough rolling in hot rolling, and it is particularly likely to occur when S is added above a certain threshold. At the same time, it was found out that soot was generated due to the charging temperature to the heating furnace, the amount of Mn, and the like. For the former factor, it is effective to increase S, and for the latter factor, it is effective to lower S. These two heating factors are contradictory contents, At the same time, it seemed difficult to produce a material that was compatible and of good quality.

  As a result of persistently investigating the influence of heating furnace entry side temperature and Mn amount, heating temperature, finish rolling entry side temperature, S amount and finish rolling temperature, the S amount range was regulated. The inventors have invented a production method capable of drastically suppressing the occurrence of both by regulating the amount by the heating inlet temperature and further setting the lower limit of the heating temperature. That is, the present inventors have found that a mild steel alloyed hot-dip galvanized steel sheet having excellent surface properties can be obtained by regulating S, Mn, charging temperature, heating temperature and finish rolling entry temperature.

  That is, in mass%, the amount of S is 0.002% or more and 0.010% or less, and the heating furnace charging temperature ≦ 69.3 × ln (Mn%) + 1075.7 is regulated. By making the heating furnace extraction temperature 1150 ° C. or higher and the finish rolling entry temperature 1040 ° C. or less, especially by making the maximum temperature in the finish rolling mill row not be in the range of 1000 to 1050 ° C., other steelmaking and heat It came to propose a manufacturing method that can suppress streak defects at the same time stably and inexpensively regardless of the manufacturing conditions.

  The present invention has been made based on the above findings, and the configuration thereof is as follows. (1) By mass%, C: 0.0005 to 0.004%, S: 0.002 to 0.010%, Al: 0.0005 to 0.100%, N: 0.0005 to 0.0050% , P: 0.003 to 0.030%, Mn: 0.03 to 0.30%, with the balance inevitable impurities and Fe in a state where the charging temperature T satisfies the following formula (1) It is charged in a heating furnace, heated in a range where the maximum temperature of the slab is 1150 ° C. to 1350 ° C., the slab is rolled by a roughing mill, and then finish rolling is performed at a finish rolling mill entry side temperature of 1040 to 1240 ° C. A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface properties.

(2) Production of an alloyed hot-dip galvanized steel sheet having excellent surface properties for finish rolling at a maximum temperature of 880 to 1000 ° C. or above 1050 ° C. to 1140 ° C. in the production method of (1). Method.
(3) In the above (1) or (2), as a component, B: 0.0001 to 0.0010%, Ti: 0.001 to 0.070%, Nb: 0.001 to 0.00. It exists in the manufacturing method of the galvannealed steel plate excellent in the surface quality containing 030%.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the alloyed hot-dip galvanized steel plate of the mild steel which was excellent in surface property and suppressed generation | occurrence | production of the streak defect can be provided.

Hereinafter, the details of the present invention will be described together with the reasons for limitation. First, the reasons for limiting the components will be described.
The C amount is set to 0.004% or less in order to ensure processability when used for automobile applications. Further, the reason why the S content is 0.010% or less is that, if it exceeds 0.010%, streak defects starting from rough rolling tend to increase under any conditions. That is, in an alloyed hot-dip galvanized steel sheet of mild steel in which elements other than Fe are reduced in order to improve workability, the amount of Mn that usually forms precipitates with S and can improve hot brittleness. Due to the small amount, cracking occurs when the brittle temperature region is reached during hot rolling. When this crack exists, the scale remains in the corresponding portion, and becomes a streak defect at the time of alloying of plating.

  In addition, this effect varies depending on the amount of Mn and the charging temperature, but in the case of 0.010% or more, in the present production method, the wrinkle generation rate becomes very high whatever the other conditions, so the upper limit is set. Regulated. The reason why the S content is 0.002% or more is to suppress streak defects caused by finish rolling. If it is below this threshold value, even if the heating temperature and the finish rolling entry temperature are devised, the defect in descaling in finish rolling occurs, scales remain, and streak patterns occur, so this is the lower limit.

Next, Mn will be described.
Mn becomes sulfide MnS during slab cooling, stabilizes in the slab, and does not form a solution if the heating temperature and heating time are about 300 min. When it is a sulfide, it does not affect the hot brittleness, but when the slab charging temperature is increased, the slab charging temperature T is T ≦ 69.3 × ln (Mn%) + 1075.7.
If the above condition is not satisfied, the sulfide MnS in the slab does not precipitate stably, causing hot brittleness and causing streak defects. Therefore, it is necessary to satisfy this relationship. The upper limit of the amount of Mn is preferably Mn: 0.30% from the viewpoint of workability. The reason why the lower limit of the amount of Mn is set to 0.03% is that it takes too much steelmaking cost to be lower than this.

  Since P and B are elements that affect the alloying speed, if added too much, alloying unevenness is caused by the overall alloying delay of the steel sheet, so this improvement effect can be sufficiently obtained. I can't. Therefore, the upper limit is regulated to P: 0.030% and B: 0.0010%. The reason why the lower limit is set to P: 0.003% and B: 0.0001% is that it takes too much steelmaking cost to be lower than this.

  Since Ti is an element that increases the alloying speed, if it is added too much, the alloying unevenness is caused by increasing the alloying speed of the overall steel sheet, so that the present improvement effect cannot be sufficiently obtained. Therefore, Ti: 0.070% is made the upper limit. The reason why the lower limit is set to 0.001% is that it takes too much steelmaking cost to be lower than this.

For Nb, if the amount added is too large, the target processability cannot be obtained, so the upper limit is regulated to 0.030%. The reason why the lower limit is set to 0.001% is that it takes too much steelmaking cost to be lower than this.
When manufactured under the regulation of only these conditions, the surface bite is difficult to achieve because the scale bite due to finish rolling increases. In order to suppress this, it is necessary to satisfy the following conditions for hot rolling.

Next, the important hot rolling conditions of the present invention will be described.
The slab heating temperature is one of the points of this manufacturing method. Normally, S added in steel produces a FeS liquid phase and maintains deskeiness, but since this does not form because the amount of S is reduced in this production method, descalability of finish rolling. There is concern about deterioration. For this improvement, it is necessary to secure the minimum amount of S only on the surface layer. MnS in steel is stabilized and does not form a solution if the current heating temperature and heating time are about 300 min. However, when the surface layer portion is heated at 1150 ° C. or higher, MnS is oxidized and MnO is converted to a concentrated S. Turn into. Thereby, if it is 1150 degreeC or more heating, the deskeability in finish rolling can be improved according to the S effect of a surface layer, and a streak defect can be prevented. The reason why the upper limit is set to 1350 ° C. is that when the temperature is higher than this, the scale melts and the generation of scale wrinkles increases.

Next, finish rolling conditions will be described.
The inventors have found that in the range where the maximum temperature in the finish rolling mill row exceeds 1000 ° C. to 1050 ° C., the scale becomes very easy to peel off, and the streak results are extremely deteriorated. When a fine scale is pushed into the surface state of the steel sheet due to an increase in scale thickness due to an increase in the amount of scale generated in finish rolling, as well as a change in peelability due to a difference in expansion coefficient between the scale and the base steel. It is presumed that the difference in roughness occurs or the scale itself causes uneven alloying at the time of alloy plating and is visually recognized as a streak defect. The scale thickness increases as it is rolled at a higher temperature, but is easily peeled off due to the difference in thermal expansion between the steel and the scale. When rolled at 1000 ° C. to 1050 ° C., it is estimated that the scale is thick and difficult to peel off. In particular, the highest temperature in the finish rolling mill row of 1020 to 1050 ° C. is particularly likely to cause wrinkles, so this temperature range must be avoided.

  The maximum temperature in the finishing rolling mill row needs to be calculated from the finishing rolling entry temperature and rolling conditions, and is difficult to manage in normal operation. Therefore, as a result of testing with various changes in temperature and rolling conditions, it was found that the temperature can be controlled almost at the finishing rolling entry temperature. Specifically, if the finish rolling entry temperature does not exceed 1040 ° C. and less than 1090 ° C., the maximum temperature in the finish rolling mill row of 1020 to 1050 ° C. can be substantially avoided.

  The basis for determining the upper and lower limits of the maximum temperature in the finish rolling mill row and the finish rolling entry side temperature is as follows. Since rolling becomes unstable when rolling below the transformation point, the lower limit of the maximum temperature in the finishing mill row was set to 880 ° C. Since the corresponding finish rolling entry temperature is 920 ° C., the lower limit of the finish rolling entry temperature is 920 ° C. The maximum temperature of the slab temperature was 1350 ° C., the corresponding upper limit of the finish rolling entry temperature was 1180 ° C., and the upper limit of the maximum temperature in the finish rolling mill row was 1140 ° C.

The details of not only the finish rolling conditions but also the fact that the amounts of S and Mn are closely related to the heating temperature and the finish rolling temperature will be described in detail.
First, C: 0.004% or less ultra-low carbon steel sheet with various changes in S content, Zn basis weight (per one side) 36-60 g / m ² , Fe% in Zn plating layer = about 10-11 The surface property of the plating layer under the condition of% was investigated.

  FIG. 1 shows the amount of S and the occurrence of streak defects caused by finish rolling, and FIG. 2 shows the relationship between the amount of S and the streak defects caused by rough rolling. FIG. 3 shows the influence of the amount of S for each finish rolling entry temperature, and FIG. 4 shows the relationship between the maximum temperature in the finish rolling mill row and streak defects. In addition, FIG. 5 shows the relationship between the heating temperature and streak defects, and FIG. 6 shows the relationship between the amount of Mn, the slab heating entry temperature, and the streak pattern. Here, the definition of occurrence indicates the ratio of the total length of the steel strip observed by visual observation to the total length of the steel plate observed by the total of the portions where the streak defects are observed. This value is directly related to the yield.

  As shown in FIG. 1, when only S is first focused, if the finish rolled steel sheet temperature is not regulated, the streak defect tends to increase when S becomes 0.010% or less. Show. In addition, as shown in FIG. 2, the occurrence of streak defects is abruptly reduced when the S amount is 0.010% or less. If it is normal, as a result of the above-mentioned two points of investigation, it is concluded that there are no conditions that are compatible at the same time.

However, as a result of performing a test focusing on the finish rolling entry temperature and the heating temperature, as shown in FIGS. 3 and 4, the S amount is 0.002% to 0.010% and the finish rolling entry temperature is By limiting the limit, especially the maximum temperature in the finish rolling mill row, both found a manufacturing method capable of preventing streak defects, and were able to manufacture a steel sheet with dramatically improved surface quality.
Next, as a result of evaluating Mn and slab heating with respect to the material with S ≦ 0.010%, it can be arranged in a relationship having a soot generation limit value, that is, the slab charging temperature T is
T ≦ 69.3 × ln (Mn%) + 1075.7
It was also found that the generation of soot almost disappears if

  Therefore, for the production of an alloyed hot-dip galvanized steel sheet characterized by having a hot-dip galvanized layer with a uniform appearance, the purity, S content, and Mn content of the steel sheet are limited, and the inlet temperature during finish rolling is set to be limited. It has been found that it is important to carry out hot rolling so that the maximum temperature in the finish rolling mill row is 1040 ° C. or less, particularly 1000 ° C. to 1050 ° C. is avoided. The original sheet is taken up at a predetermined coiling temperature and then cold-rolled, followed by hot dip galvanizing according to a conventional method, followed by alloying treatment.

  Various surface care slabs having the composition shown in Table 1 were heated to 1050-1300 ° C. and then hot-rolled to 4.0 mm, coiled, and then cold-rolled to 0.75 mm. Later, alloying hot dip galvanizing was performed. About the alloyed hot-dip galvanized steel sheet thus obtained, the incidence of streak defects in the plating layer was investigated. The results are shown in Table 2 together with the production conditions.

  As shown in Table 2, no. 1-5, no. 9, no. 12, no. 17, no. Nos. 19 to 20 are the present invention. 6-8, no. 10-11, no. 13-16, no. 18 is a comparative example. Comparative Example No. In Nos. 6 to 8, since the finish entry temperature and the finish rolling temperature are outside the conditions of the present invention, the generation rate of streak defects exceeds 1%. 10-11, or No. In No. 13, the slab heating temperature is low, or the finish rolling temperature is outside the conditions of the present invention, so that the generation rate of streak defects exceeds 1%.

  Comparative Example No. In Nos. 14 to 16, since the component composition conditions are outside the conditions of the present invention, the occurrence rate of streak defects exceeds 1%. Comparative Example No. No. 18 has a streak defect generation rate exceeding 1% because the finish entry temperature is outside the conditions of the present invention. On the other hand, No. 1 according to the present invention. 1-5, no. 9, no. 12, no. 17, no. Since 19-20 satisfy | fills the conditions of this invention, it turns out that the incidence rate of a streak defect is 1% or less.

It is a figure which shows the relationship between S amount and the rate of a streak defect resulting from finish rolling. It is a figure which shows the relationship between the amount of S, and the streak-like defect occurrence rate due to rough. It is a figure which shows the relationship between S amount according to finish rolling entrance temperature, and a stripe defect incidence. It is a figure which shows the relationship between the maximum temperature in a finishing rolling mill row | line | column, and a streak defect incidence. It is a figure which shows the relationship between heating temperature and a linear defect incidence. It is a figure which shows the relationship between Mn amount, slab heating entrance temperature, and a streak pattern.

Claims (3)

% By mass
C: 0.0005 to 0.004%,
S: 0.002 to 0.010%,
Al: 0.0005 to 0.100%,
N: 0.0005 to 0.0050%,
P: 0.003-0.030%,
Mn: 0.03 to 0.30%,
Then, the slab composed of the remaining inevitable impurities and Fe is charged into the heating furnace in a state where the charging temperature T satisfies the following formula (1), and the maximum slab temperature is in the range of 1150 ° C to 1350 ° C. An alloyed hot-dip galvanized steel sheet having excellent surface properties, characterized by heating and rolling the cast slab with a roughing mill and then finishing rolling at a finish rolling mill entry temperature of 920 to 1040 ° C or 1090 to 1180 ° C Manufacturing method.
T ≦ 69.3 × ln (Mn%) + 1075.7 (1) The manufacturing method of the alloyed hot-dip galvanized steel plate excellent in the surface property which finish-rolls by making the maximum temperature in a finishing rolling mill row into 880-1000 degreeC or more than 1050 degreeC-1140 degreeC in the manufacturing method of Claim 1. As a component in claim 1 or 2, further in mass%,
B: 0.0001 to 0.0010%,
Ti: 0.001 to 0.070%,
Nb: 0.001 to 0.030%,
A method for producing an alloyed hot-dip galvanized steel sheet having excellent surface properties.

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