JP2004270038A - High tensile strength hot dip galvannealed steel sheet having excellent plating property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength hot dip galvannealed steel sheet which has excellent workability in press forming or the like, and has excellent plating properties as well. <P>SOLUTION: The high tensile strength hot dip galvannealed steel sheet is obtained by coating the surface of a steel sheet having a composition comprising, by mass, 0.0005 to 0.0050% C, 0.3 to 2.5% (exclusive of 0.3%) Si, 0.1 to 2.5% Mn, 0.003 to 0.10% Ti, 0.003 to 0.10% Nb, 0.0005 to 0.0080% B, 0.040 to 0.18% P, ≤0.015% S, 0.005 to 0.10% Al and ≤0.0060% N, and also comprising Si and Mn in the ranges satisfying the relation in the following inequality (1), and the balance Fe with inevitable impurities with a hot dip galvannealed layer in which the content of Fe is 9 to 12%. The inequality (1) is denoted by 1.5 (%Mn)-2≤(%Si)≤2(%Mn). <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a high-tensile alloyed hot-dip galvanized steel sheet suitable for use in bending, press forming, drawing, and the like, mainly for use in automobile bodies.

2. Description of the Related Art In recent years, there has been an increasing demand for weight reduction of a vehicle body in order to improve fuel efficiency due to regulations on exhaust gas of automobiles due to environmental problems. Improving vehicle safety is also an important issue. Therefore, as one of measures for solving these problems, a high-tensile alloyed hot-dip galvanized steel sheet having a tensile strength of about 400 MPa or more and excellent press formability is required.
However, in general, the cold-rolled steel sheet tends to deteriorate in press formability, that is, the average r value and TS-El balance, and the surface characteristics such as plating characteristics also deteriorate as the tensile strength increases. Therefore, it is important to improve the plating properties and the press formability as well as to increase the tensile strength in order to use the product for automobiles.

Until now, various methods have been proposed for improving the press formability accompanying the increase in tension.
For example, as in the case of a high-strength cold-rolled steel sheet excellent in formability disclosed in Patent Document 1, a carbonitride forming component is used to improve workability and aging based on an ultra-low carbon steel with reduced C. There is disclosed means for adding a certain amount of Ti, Nb, etc., and for increasing the tensile strength mainly with Si, Mn, P which does not impair the workability. In particular, Si is an advantageous component that increases the tensile strength without deteriorating the workability represented by the average r value and elongation, but when a large amount of Si is contained, deterioration of the surface characteristics is unavoidable, and the plating characteristics However, there was a problem that the temperature was remarkably deteriorated.
JP-A-63-100158

For this reason, in the case of increasing the tensile strength of a steel sheet for alloyed hot-dip galvanizing, Patent Document 2 (High-strength cold-rolled steel sheet excellent in formability, hot-dip galvanized high-strength cold-rolled steel sheet, and a method of manufacturing them) As disclosed, a method in which Si is limited to 0.03% or less and P and Mn are mainly used as reinforcing components has been generally used. However, the addition of a large amount of P not only delays alloying of hot-dip galvanization, but also tends to cause secondary working embrittlement, particularly in extremely low carbon steel. In addition, although the influence of Mn on the plating characteristics is small, the plating characteristics start to deteriorate from the Mn content of 1% or more when Si is limited to 0.1% or less, and the transformation point decreases when the content is large. As a result, there is a problem that the hot-rolled sheet is hardened or does not recrystallize during annealing, which leads to deterioration of the material.
As described above, there is a limit in achieving high tension while maintaining workability and plating characteristics using only P and Mn.
JP-A-5-255807

  The present invention is intended to advantageously solve the above-mentioned problems, and proposes a high-tensile alloyed hot-dip galvanized steel sheet that is excellent in workability such as press forming suitable for use in automobiles and has excellent plating properties. The purpose is to do.

By the way, the present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, by effectively utilizing Si as a reinforcing component, and by specifying the correlation between Si and Mn, good plating characteristics and New knowledge has been obtained that a high-tensile alloyed hot-dip galvanized steel sheet with a tensile strength of 400 MPa or more that combines press formability can be obtained.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
C: 0.0005 mass% or more, 0.0050 mass% or less,
Si: 0.3 mass% or more, 2.5 mass% or less (excluding 0.3 mass%),
Mn: 0.1 mass% or more, 2.5 mass% or less,
Ti: 0.003 mass% or more, 0.10 mass% or less,
Nb: 0.003 mass% or more, 0.10 mass% or less,
B: 0.0005 mass% or more, 0.0080 mass% or less,
P: 0.040 mass% or more, 0.18 mass% or less,
S: 0.015 mass% or less,
Al: 0.005 mass% or more, 0.10 mass% or less and N: 0.0060 mass% or less, and Si and Mn are contained under the relationship of the following formula (1), and the balance is based on the composition of Fe and unavoidable impurities. A high-tensile alloyed hot-dip galvanized steel sheet having excellent galvanizing properties, comprising an alloyed hot-dip galvanized layer having a Fe content of 9 to 12% in the coating layer on the surface of the steel sheet.
Record
1.5 (maas% Mn) -2 ≦ (mass% Si) ≦ 2 (mass% Mn) ---- (1)

  This invention utilizes Si as a strengthening component in ultra-low carbon steel and specifies the mutual relationship between the contents of Si and Mn, thereby achieving high plating properties that are excellent in plating properties and suitable for press forming. The steel sheet according to the present invention is intended to obtain a tension alloyed hot-dip galvanized steel sheet, and can greatly contribute to reducing the weight of automobiles and improving safety.

First, the experimental results on which the present invention is based will be described.
C: 0.002 mass%, Ti: 0.04 mass%, Nb: 0.03 mass%, B: 0.002 mass%, P: 0.10 mass%, S: 0.006 mass%, Al: 0.04 mass%, and N: 0.002 mass%. The relationship between the Si and Mn contents and the plating properties of 0.75 mm cold-rolled steel sheets having different Si and Mn contents was investigated.
In addition, these steel sheets are subjected to hot rolling at a finish rolling temperature in the range of 820 to 910 ° C., followed by cold rolling in a rolling reduction range of 75 to 85% after pickling. After that, it was further annealed in a hot-dip galvanizing line at a temperature range of 780 to 890 ° C, quenched to a temperature range of 450 to 500 ° C, and immersed in a hot-dip galvanizing bath containing 0.13 mass% of Al for plating. After that, it is manufactured by performing an alloying treatment in a temperature range of 450 to 550 ° C (Fe content in the plating layer is about 10%). The plating characteristics were evaluated by visual judgment of appearance (non-plating) and judgment of powdering resistance by a 90 ° bending test.
Table 1 summarizes the results of an investigation on the relationship between the Si and Mn contents and the plating properties of these steel sheets.

As a result, as apparent from Table 1, when the mutual relationship between Si and Mn is in the range of 1.5 (mass% Mn) -2 ≦ (mass% Si) ≦ 2 (mass% Mn), the plating appearance and powder resistance It turned out that both ringing properties were good. If the content is out of the above range and at least one of Si and Mn is large, the plating characteristics deteriorate. The reason for this is not necessarily clear, but the oxide film formed on the steel sheet surface becomes SiO ₂ when Si is added alone and MnO when Mn is added alone, and both have poor wettability with molten zinc, whereas Si, Mn It is considered that when satisfies the above relational expression, an oxide film mainly composed of MnSiO ₃ is formed, and the wettability with molten zinc is improved.

Next, the reasons for limiting the component composition ranges in the present invention will be described.
C: 0.0005 mass% or more, 0.0050 mass% or less C is required to have a content of 0.0050 mass% or less in order to obtain a steel sheet having good workability, particularly good TS-El balance. It is desirable that the ultra-low carbon steel is preferably 0.0040 mass% or less, more preferably 0.0030 mass% or less. However, when a large amount of P is contained at the same time, when C is less than 0.0005 mass%, the secondary working brittleness is deteriorated. Accordingly, the C content is limited to 0.0005 mass% or more and 0.0050 mass% or less, but is preferably 0.0005 mass% or more and 0.0040 mass% or less, more preferably 0.0005 mass% or more and 0.0030 mass% or less.

Si: 0.3 mass% or more, 2.5 mass% or less (excluding 0.3 mass%)
Si has the effect of reducing the deterioration of workability and strengthening the steel.Since the purpose of the present invention is to use Si to strengthen the steel while maintaining the workability, when the tensile strength is 400 MPa class or more, The Si content needs to be 0.3 mass% or more. However, when the content exceeds 2.5 mass%, the secondary processing brittleness is deteriorated. Therefore, the Si content is limited to 0.3 mass% or more and 2.5 mass% or less (excluding 0.3 mass%).

Mn: 0.1 mass% or more, 2.5 mass% or less
It is desirable to reduce the Mn content from the viewpoints of workability and resistance to secondary working brittleness, but if it is less than 0.1 mass%, a sufficient reinforcing effect as a material for automobiles cannot be obtained. In addition, when a large amount of Si that raises the transformation point during hot rolling is contained, it becomes difficult to perform normal hot rolling in the austenite region, so that Mn that lowers the transformation point is preferably 0.2 mass% or more, It is more preferable to add 0.4 mass% or more to adjust the transformation point to an appropriate temperature range. On the other hand, if the content exceeds 2.5 mass%, it is difficult to improve the resistance to secondary working brittleness, and the steel is extremely hardened, so that cold rolling becomes difficult. Therefore, the Mn content is limited to 0.1 mass% or more and 2.5 mass% or less, but is preferably 0.2 mass% or more and 2.5 mass% or less, and more preferably 0.4 mass% or more and 2.5 mass% or less.

Ti: 0.003 mass% or more, 0.10 mass% or less
Nb: 0.003 mass% or more, 0.10 mass% or less
Ti fixes part or all of N, C, and S, and Nb fixes part or all of C, which is effective in securing workability and non-aging properties of ultra-low carbon steel. . However, if the content is less than 0.003 mass%, the effect is not obtained, so this value is set as the lower limit. On the other hand, in any case, when the content exceeds 0.1 mass%, the workability deteriorates conversely, so this value is made the upper limit. However, from the viewpoint of further improving the workability, when the C content is reduced to about 0.0020 mass% or less, the total of Ti and Nb is preferably 0.03 mass% or more, 0.10 mass% or less, more preferably 0.03 mass% or less. % Or more and 0.08 mass% or less.

B: 0.0005 mass% or more and 0.0080 mass% or less B segregates at the grain boundary and shows a remarkable effect on prevention of secondary working embrittlement. However, if the content is less than 0.0005 mass%, the effect is small, while 0.0080 mass%. %, The effect is not only saturated, but also the yield strength is increased and the elongation is reduced, thereby deteriorating the workability. Therefore, the content is limited to 0.0005 mass% or more and 0.0080 mass% or less. However, since the addition of B slightly deteriorates the workability, it is preferably 0.0005 mass% or more, 0.0050 mass% or less, and more preferably 0.0005 mass%, except that P containing more than 0.15 mass% is contained. Above, it is 0.0030 mass% or less.

P: 0.040 mass% or more and 0.18 mass% or less P has the effect of strengthening the steel and improving the workability, particularly the average r value, and is contained according to the desired strength. The effect becomes remarkable when the content is 0.040 mass% or more. On the other hand, when the content exceeds 0.18 mass%, the alloying of the plating is significantly delayed, and the material is deteriorated due to solidification segregation during casting. . Further, when the C content is reduced for the purpose of improving the material, the secondary working embrittlement resistance starts to deteriorate at a content of 0.12 mass% or more, and a large amount of B needs to be added at a content of 0.15 mass% or more.
Therefore, the P content is specified to be 0.040 mass% or more and 0.18 mass% or less, preferably 0.040 mass% or more and 0.15 mass% or less, more preferably 0.040 mass% or more and 0.12 mass% or less.

S: 0.015 mass% or less If S exceeds 0.015 mass%, a large amount of MnS precipitates and the workability is deteriorated. However, from the viewpoint of further improving workability, it is preferably at most 0.010 mass%, more preferably at most 0.008 mass%. It is desirable that the content be small, but if the content is less than 0.0005 mass%, it is not practical because the current technology significantly increases the production cost.

Al: 0.005 mass% or more, 0.10 mass% or less
Al is added as needed for deoxidation and precipitation fixation of N in steel. However, if the content is less than 0.005 mass%, inclusions increase and good workability cannot be obtained. On the other hand, if the content is too large, not only does the workability deteriorate, but also the surface properties deteriorate, so the Al content is limited to 0.005 mass% or more and 0.10 mass% or less. However, from the viewpoint of optimizing workability by the above mechanism, it is preferably 0.005 mass% or more and 0.06 mass% or less, more preferably 0.01 mass% or more and 0.06 mass% or less.

N: 0.0060 mass% or less N is a harmful component that is one of the main causes of strain aging. If the content is too large, a large amount of Ti or Al must be added, so the upper limit is 0.0060 mass%. However, from the viewpoint of further improving workability, the content is preferably 0.0040 mass% or less, more preferably 0.0030 mass% or less. It is desirable that the content is small, but if the content is less than 0.0003 mass%, it is not practical because the current technology significantly increases the production cost.

Next, preferable production conditions of the steel of the present invention will be described.
Steelmaking and casting conditions may be performed according to a conventional method.
The hot rolling finish temperature should be at least 750 ° C to improve the workability after cold rolling and annealing. At a temperature lower than this, the rolled structure in the hot-rolled sheet is remarkably left, which is disadvantageous for forming a texture advantageous for press formability. On the other hand, if the rolling is completed at a temperature exceeding 1000 ° C., the structure of the hot-rolled sheet becomes coarse, and a texture advantageous for press formability cannot be obtained. Therefore, it is preferable that the hot rolling finishing temperature be 750 ° C or more and 1000 ° C or less.
In cold rolling, sufficient workability cannot be obtained unless the rolling reduction is 60% or more. Therefore, the cold rolling reduction is advantageously 60% or more, preferably 70% or more. On the other hand, if the cold rolling reduction is 95% or more, the workability deteriorates. Therefore, the upper limit is preferably set to 95%.

  The plating is desirably performed in a continuous galvanizing facility. The recrystallization annealing temperature after the cold rolling may be 700 ° C. or more and 950 ° C. or less, but preferably 800 ° C. or more. After annealing, it is rapidly cooled to a temperature range of 380 to 530 ° C. If the quenching stop temperature is less than 380 ° C, non-plating occurs, while if it exceeds 530 ° C, the plating surface becomes uneven, which is not preferable. After quenching, it is subsequently immersed in a hot-dip galvanizing bath containing 0.12 to 0.145 mass% of Al for plating. If the Al content in the bath is less than 0.12 mass%, alloying proceeds excessively and the plating adhesion (powdering resistance) deteriorates. On the other hand, when it exceeds 0.145 mass%, non-plating occurs.

  Alloying by heating subsequent to plating is performed so that the Fe content in the plating layer is 9 to 12%. That is, the alloy is formed by heating at a temperature of 450 to 550 ° C. for 14 seconds or more and 28 seconds or less. If the heating temperature is less than 450 ° C. or the heating time is less than 14 seconds, alloying is not sufficiently performed, and the Fe content is less than 9%, and the flaking resistance is deteriorated. On the other hand, if the heating temperature exceeds 550 ° C. or the heating time exceeds 28 seconds, alloying proceeds excessively, and the powdering resistance deteriorates.

It is preferable that the bath temperature of the galvanizing bath be in the range of 450 to 490 ° C., and the temperature of the plate to be penetrated into the bath is not lower than the bath temperature and not higher than (bath temperature + 10 ° C.). If the bath temperature is lower than 450 ° C. or the intrusion plate temperature is lower than the bath temperature, solidification of zinc is promoted, and it becomes difficult to adjust the amount of plating. On the other hand, when the bath temperature exceeds 490 ° C or the intruding plate temperature exceeds (bath temperature + 10 ° C), the elution of iron from the steel sheet into the bath is promoted, and dross is formed, which tends to cause surface defects.
The properties of the present invention do not change even if the steel sheet obtained by the present invention is subjected to a special treatment such as applying a lubricant to the surface to further improve press formability, welding property, or corrosion resistance. Further, even if the steel sheet of the present invention (or the steel sheet obtained by subjecting the surface of the steel sheet of the present invention to a special treatment) is subjected to temper rolling for the purpose of shape correction, the characteristics of the present invention are not changed.

  Hot rolling was performed using a steel slab having the composition shown in Table 2 which was melted in a converter, then was pickled and then cold-rolled to a thickness of 0.8 mm. Plating was performed, and tensile properties and plating properties of the obtained steel sheets were investigated. The plating bath temperature ranges from 460 to 480 ° C, the intrusion plate temperature is higher than the plating bath temperature, the bath temperature + 10 ° C or lower. It was held.

  Investigation results such as the hot rolling finish temperature (FDT), cold rolling reduction, annealing temperature, quenching stop temperature, Al content in the plating bath, and Fe content in the plating layer after alloying were prepared as the above manufacturing conditions. The results are shown in Table 3.

Here, the tensile properties were measured using a JIS No. 5 test piece, and the plating properties were tested and evaluated in the same manner as in the above-described experiment.
From Tables 2 and 3, it can be seen that all of the applicable examples of the present invention are excellent in the TS-El balance represented by TS × El and also have excellent plating characteristics.

Claims (1)

C: 0.0005 mass% or more, 0.0050 mass% or less,
Si: 0.3 mass% or more, 2.5 mass% or less (excluding 0.3 mass%),
Mn: 0.1 mass% or more, 2.5 mass% or less,
Ti: 0.003 mass% or more, 0.10 mass% or less,
Nb: 0.003 mass% or more, 0.10 mass% or less,
B: 0.0005 mass% or more, 0.0080 mass% or less,
P: 0.040 mass% or more, 0.18 mass% or less,
S: 0.015 mass% or less,
Al: 0.005 mass% or more, 0.10 mass% or less and N: 0.0060 mass% or less, and Si and Mn are contained under the relationship of the following formula (1), and the balance is based on the composition of Fe and unavoidable impurities. A high-tensile alloyed hot-dip galvanized steel sheet having excellent galvanizing properties, comprising an alloyed hot-dip galvanized layer having a Fe content of 9 to 12% in the coating layer on the surface of the steel sheet.
Record
1.5 (maas% Mn) -2 ≦ (mass% Si) ≦ 2 (mass% Mn) ---- (1)