JPH0559970B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing a high-tensile galvanized steel sheet having ultra-deep drawability. More specifically, the present invention is a high-strength steel plate necessary for reducing the weight of automobiles, and is a high-tensile hot-dip galvanized steel sheet having ultra-deep drawability that enables thinning of difficult-to-form parts of automobile parts. This invention relates to a method for manufacturing steel plates. (Conventional technology) In recent years, demand for high-strength steel sheets has increased in the automobile industry due to the pursuit of improved fuel efficiency and safety associated with lighter vehicle bodies. In response to this demand, various high-strength steel plates have been developed and used not only as automobile materials but also in many other fields. Therefore, many proposals have been made to increase the tensile strength and provide even more advanced press formability to hot-dip galvanized steel sheets, which have traditionally been used for automobile parts due to their excellent anti-corrosion properties. Ta. As is well known, there are currently various methods for manufacturing hot-dip galvanized steel sheets, but the most common method is a method using a continuous hot-dip galvanizing line with an annealing furnace in the line (e.g. Sendzimir method). It was used in However, the hot-dip galvanized steel sheets obtained by these methods have poor press formability, that is, deep drawability, and cannot be sufficiently formed. Therefore, various measures have been proposed to improve the press formability of these materials. For example, in Japanese Patent Application Laid-Open No. 58-19465, when producing hot-dip galvanized steel sheets on a continuous hot-dip galvanizing line such as the Sendzimir method, a low C steel composition is used, and B and As needed
A method is disclosed for improving the aging characteristics and press formability of the obtained hot-dip galvanized steel sheet by adding one or more of Ti, Nb, Zr, and V. However, when the tensile strength is 35 kg or more, it is difficult to obtain ultra-deep drawability. Japanese Patent Application Laid-open No. 59-74232 describes ultra-low C steel with B,
In addition to adding Ti and Nb in combination, it has been proposed to perform low-temperature winding at 650 to 800℃ after hot rolling, then perform recrystallization annealing at a temperature below Ac ₃ points after cold rolling, and adjust cooling. ing. This is a steel type that utilizes strain aging properties due to B, but the tensile strength is 35
It is difficult to ensure ultra-deep drawability and low-temperature embrittlement resistance in kilo-class and above. JP-A No. 59-190332 discloses that when producing hot-dip galvanized steel sheets on a continuous hot-dip galvanizing line similar to the above-mentioned publication, B, Ti, and Nb are added in combination to ultra-low C steel. A method for obtaining a hot-dip galvanized steel sheet having ultra-deep drawability with good subsequent workability is disclosed. JP-A No. 59-193221 discloses a method in which B, Ti, and Nb are added in combination to ultra-low C steel, and the steel is continuously recrystallized annealed at a temperature below the Ac ₃ point. . With either method, it has been difficult to obtain a hot-dip galvanized steel sheet that has high strength, low-temperature toughness, and excellent press formability. (Problems to be Solved by the Invention) The purpose of the present invention is to provide a method for producing a high-tensile galvanized steel sheet that has a tensile strength of 35 to 45 kg and exhibits ultra-deep drawability and excellent low-temperature embrittlement resistance. It is to provide. (Means for Solving the Problems) Therefore, the present inventors conducted various studies in order to provide hot-dip galvanized steel sheets with high strength, ultra-deep drawability, and excellent resistance to low-temperature embrittlement. However, in order to improve the formability, we made it extremely low C, and in order to obtain high strength without deteriorating the formability, we mainly added Mn,
P is used as a solid solution strengthening element, Nb and Ti, which are compositional components related to low temperature brittleness resistance and formability, are adjusted appropriately, and an appropriate amount of B is added to improve low temperature brittleness resistance. Furthermore, by appropriately selecting the holding conditions in the low-temperature zone in a continuous galvanizing line, it is possible to produce ultra-deep-drawn high-strength hot-dip galvanized steel sheets by leaving an appropriate amount of solid solute C without deteriorating formability. I learned this and completed the present invention. Here, the gist of the present invention is that the weight%
So, C: 0.0005-0.0050%, Si≦0.60%, Mn: 1.0-2.5%, P: 0.010-0.080%, S≦0.015%, sol.Al: 0.010-0.100%, N≦0.0050%, B: 0.0002 ~0.0008%, Nb is Nb/C≧4, and Nb≦0.08%, and the content ratio of N to Ti and N and S are
Continuously cast molten steel with a composition that contains Ti within a range where the total content ratio of After hot and cold rolling, the obtained continuous casting slab was continuously annealed at a temperature of 730-900℃ in a continuous hot-dip galvanizing line, and then cold-held at a holding temperature of 430-500℃ for 20-120 seconds. This is a method for producing a high-strength hot-dip galvanized steel sheet that has excellent formability, ultra-deep drawability, and low-temperature embrittlement resistance, by hot-dip galvanizing the steel sheet. After the above-mentioned continuous annealing, cooling may be performed to the above-mentioned holding temperature of 430 to 500°C at a cooling rate of 1°C/sec or more, and the above-mentioned low-temperature holding may be so-called gradient cooling. In one embodiment of the present invention, after hot rolling,
Winding may be performed at a temperature of 400 to 680°C. (Function) In the present invention, the reason why the composition of each component in the steel is limited as described above will be described. (a) C0.0005 to 0.0050wt%: From the viewpoint of formability, the lower the C content is, the better; however, if it is too low, the amount of solid solution C in the steel will be insufficient, causing low-temperature brittleness, so the lower limit should be 0.0005wt. %, and on the other hand, if it is too large, moldability will deteriorate, so the upper limit is set to 0.005wt%. (b) Si≦0.60wt%: If Si is contained in steel in an amount exceeding 0.60wt%, the formability will be significantly reduced due to solid solution hardening, and furthermore, when alloying for galvanizing, the alloying speed will increase. Since this decreases, the upper limit is set at 0.60wt%. (c) Mn1.0-2.5wt%: Mn has the effect of preventing the occurrence of surface defects due to red heat embrittlement of steel during hot rolling due to its presence, but if its content is lower than 1.0wt% , since the predetermined high tension cannot be obtained, the lower limit is set to 1.0wt%. On the other hand, if the content is high, it means that a large amount of Mn alloy iron must be added to adjust the composition when tapping the steel in the converter, which lowers the casting temperature and makes casting difficult, so the upper limit is set at 2.5wt. %. Preferably 1.0
~2.0wt%. (d) P: 0.010 to 0.080wt%: The P component has solid solution strengthening and hardening that increases tensile strength due to its inclusion, so 0.010wt% or more is required, but if it is too large, elongation decreases due to grain boundary segregation. occurs and is the main cause of low-temperature embrittlement, so the upper limit should be set.
The content shall be 0.080wt%. Also, as with Mn, the drop in formability due to the addition of P is small despite the increase in strength. (e) S≦0.015wt%: The S component combines with Mn to form a nonmetallic inclusion called Mn that is easily stretched during solidification. This tends to become a starting point for defects such as cracks during press molding. Furthermore, since Ti and TiS are generated in the steel, the amount of Ti added increases, which is unfavorable in terms of cost. In the present invention, it is limited to 0.015wt% or less. (f) sol.Al (acid soluble Al): 0.010-0.10wt%: sol.Al is added to steel as a deoxidizing element,
When the content exceeds 0.10 wt%, the deoxidizing effect becomes saturated, and inclusions such as Al ₂ O ₃ are generated, which increase in number and deteriorate the surface quality. On the other hand, if it is less than 0.010wt%, the deoxidizing effect will be reduced and oxides such as MnO and SiO ₂ will be produced, which will cause clogging of the tundish nozzle and further deteriorate the Ti addition yield due to TiO ₂ . Therefore, the sol.Al content is preferably 0.010 to 0.10 wt%. (g) N≦0.0050wt%: N is an impurity element that inevitably mixes into steel, but Ti fixes it as TiN and improves formability. If there is a large amount of Ti added, the amount of Ti required for stabilization will increase.
Unfavorable in terms of cost. Therefore, the upper limit is set to 0.0050wt%. (h) (48/14N)≦Ti≦(48/14N+48/32S) The above formula determines the contents of N, S, and Ti that are fixed by Ti. Ti fixes N and S in steel as TiN and TiS and improves its formability, but if its content is low, it causes poor formability due to too much dissolved N, while if its content is high, It stuck until
The formation of TiC causes a shortage of solid solution C in the finished product, worsening its low-temperature embrittlement resistance. Therefore, it is preferable that the Ti content satisfies the above formula. (i) Nb/C≧4 and Nb≦0.080wt%: When Nb is dissolved in steel, it forms stable nitrides and carbides and improves formability. Particularly in the case of the present invention, the moldability is improved by the formation of carbides. The reason why the upper limit is set to 0.08 wt% is that the effect will be saturated even if added in excess of this value. Regarding the Nb/C ratio, if the ratio is less than 4.0, moldability deteriorates due to excessive solid solution C. Therefore, the lower limit of the Nb/C ratio is set to 4. (j) B0.0002-0.0008wt%: B preferentially segregates at grain boundaries in steel and prevents deterioration due to low-temperature brittleness of P, but if its content is too high, it increases hardness and reduces deep drawability. and moldability deteriorates. Therefore, the upper limit of B addition is
The content shall be 0.0008wt%. On the other hand, it prevents grain boundary segregation of P,
The effective lower limit for preventing deterioration due to low-temperature brittleness
0.0002wt%. In the present invention, the range of B addition is preferably 0.0002 to 0.0008 wt%. Next, the manufacturing conditions of the present invention will be explained in detail. Molten steel having the above-mentioned composition is melted by blowing in a converter, and then adjusting the composition at the time of tapping from the converter. Next, a CC slab is cast by normal continuous casting, and this slab is directly delivered without cooling, or directly delivered and then heated, or once reheated as a cold piece and then hot rolled. There is no particular limit to the heating temperature of this slab, but
It is preferable to set the temperature to the temperature required for normal hot rolling, that is, 1100°C or higher. In addition, the finishing temperature is preferably set to ₃ or more Ar points, since this produces a texture that is unfavorable to the drawability after cold rolling. Next, if the winding temperature is too low, poor flatness will occur, and plating adhesion and uniformity of plating amount will deteriorate, so the lower limit is usually set to 400°C. Furthermore, if the winding temperature is too high, the amount of scale generated will increase and the descaling efficiency during pickling will decrease, so the upper limit is set at 680°C. Preferably it is 500-660°C. During cold rolling, ordinary pickling is performed to remove scale from the surface of the steel sheet, and then cold rolling is performed. By increasing the cold rolling rate at that time, the r (rank hood) value, which is an index of drawability, is improved. Furthermore, in the present invention, since Ti, Nb, and B are added in combination, it is preferable to lower the recrystallization temperature by setting the rolling reduction to 50% or more. Continuous hot-dip galvanizing in the present invention is carried out by a method using a continuous hot-dip galvanizing line having an in-line annealing furnace such as the Sendzimir method. The holding temperature and holding time during this continuous annealing are performed above the recrystallization temperature to cause grain growth and obtain the desired TS. Furthermore, at this time, NbC is dissolved into Nb and C to form solid solution C. At this time, in order to bring about recrystallization, the heating holding temperature and holding time are set at 730°C or higher and for 10 seconds or more, to prevent excessive softening and deterioration of the hardening efficiency due to component addition (Si, P, Mn).
900℃ or less and 60 seconds or less. Also, the cooling rate after this heating and holding is Nb+C→
Prevents redeposition of NbC by NbC. That is, it is preferable to set the temperature to 1° C./sec or more in order to achieve low YP formability and to prevent low-temperature brittleness by leaving a small amount of solid solution C. The temperature for maintaining the low temperature after cooling (temperature for controlling the amount of solid solution C in the steel) is maintained at 430 to 500°C for 20 to 120 seconds. This low temperature maintenance does not necessarily have to be maintained at a constant temperature, and if necessary, so-called gradient cooling may be performed. For example, the temperature range of 430 to 500°C
May be ramp cooled for ~120 seconds. That is, during this low temperature maintenance, Nb+C→NbC+ C . In other words, the amount of solid solution of C is controlled,
If the holding temperature is too high than 500°C, NbC will precipitate, solute C in the steel will decrease, and low-temperature brittleness will deteriorate. Moreover, if this holding temperature is too low than 430°C, the amount of solid solution C will be too large, the YP value will increase, and the moldability will deteriorate. In addition, if this holding temperature is too low, when immersed in molten zinc for hot-dip galvanizing,
The temperature of molten zinc decreases, and the plating properties (adhesion,
uniformity) deteriorates. Therefore, the holding temperature
It is preferable to set it as 430-500 degreeC. Further, the holding time at this time is 20 to 120 seconds. After hot-dip galvanizing, alloying treatment may be performed as necessary. In addition, in the case of either hot-dip galvanizing alone or alloying treatment, temper rolling of less than 2.0% may be performed for the purpose of shape modification. According to the present invention, a high-strength hot-dip galvanized steel sheet is manufactured through the above-mentioned manufacturing conditions.First, the compositional characteristics of the present invention are that Ti+Nb is added to ultra-low C steel by continuous casting. By adding Ti, N and S are first fixed, and Nb
to fix C. This causes interstitial elements (C+
IF steel (Interstitial Free)
It becomes steel and has excellent formability with low YP, high value, and El value. In this way, in order to improve the formability and achieve high tensile strength, Mn,
Actively add P and Si. On the other hand, IF steel tends to be inferior in low-temperature brittleness, and addition of P embrittles grain boundaries and promotes this tendency. Therefore, in the present invention, B is added to suppress the embrittlement tendency due to P, and the amount of solid solute C is controlled appropriately by the above-mentioned high temperature annealing, cooling rate, and low temperature holding conditions. be. The attached drawing is a diagram showing the heat cycle in the present invention. As already mentioned, according to the present invention, prior to galvanizing, continuous annealing is performed at a temperature range of 730 to 900°C for 10 to 60 seconds,
After dissolving NbC, it is then temporarily cooled to a temperature range of 430 to 500 °C at a cooling rate of 1 °C/sec or more, and
The solid solution C is controlled by maintaining the temperature at a low temperature for 20 to 120 seconds.
In this case, the temperature range may be cooled gradiently.
In this regard, conventionally, hot-dip galvanizing, which will be described later, has been performed immediately after the above-mentioned temporary cooling. In the case of the present invention, after adjusting the amount of solid solute C by maintaining the temperature at a low temperature in this manner, continuous hot-dip galvanizing is performed at a temperature of, for example, 460°C. At this time, if the holding temperature is higher than 460°C, secondary cooling is performed during plating. After hot-dip galvanizing, alloying treatment is performed by raising the temperature to, for example, 530° C., if desired. Thus, according to the present invention, a high-strength hot-dip galvanized steel sheet with improved low-temperature brittleness is manufactured. Next, the present invention will be explained in more detail with reference to Examples. Examples Table 1 shows steel types A to E with chemical compositions according to the present invention.
and shows the chemical compositions of steel types F to I whose chemical compositions are compared with those of the present invention. Steel having the chemical composition shown in Table 1 was melted by adjusting its composition in a converter, and was made into a slab by continuous casting. Steels A to E in the same table are based on the examples of the present invention, and steels F to I are based on comparative examples. Next, these slabs were hot-rolled, cold-rolled, and hot-dip galvanized using a Sendzimire continuous hot-dip zinc line and a glazing line to produce hot-dip galvanized steel sheets according to the manufacturing conditions described below. Manufacturing conditions for high-strength hot-dip galvanized steel sheets 1 Hot rolling 1 Slab heating temperature 1100 to 1250°C 2 Finishing temperature 860 to 960°C 2 Cold rolling Rolling rate 50 to 90% Table 2 shows specific hot rolling windings The performance of the finished product is shown together with the conditions and pre-plating treatment conditions. In addition, the (YP) value, (TS) value, (El) value, () value, and transition temperature of low temperature brittleness of hot-dip galvanized steel sheets manufactured under such manufacturing conditions and having the chemical composition shown in Table 1. The various performances of plating adhesion are summarized in Table 2, and the transition temperature referred to here is the temperature at which impact fracture occurs after cylindrical drawing with a drawing ratio of 1.6, and the fractured surface becomes brittle. In other words, it is an index representing low temperature brittleness. The plating adhesion was determined by the plating adhesion test described below, which examines the presence or absence of peeling of the plating layer applied to the surface of the steel plate. Regarding the plating adhesion test, the metal material bending test (JIS Z2248) IT is bent, a tape is applied to the inner surface of the bend, the tape is peeled off, and the adhesion is determined by checking whether there is a plating layer attached to the tape and the condition of the peeled surface. was evaluated.

【table】

[Table] (Note) ○: Good plating adhesion, ×: Poor plating adhesion From Table 2, the steels A to E of the invention examples have better formability than the steels F to I of the comparative examples. The influence factor is low
An overall evaluation of YP, high Ts, El, transition temperature representing low-temperature brittleness, and plating adhesion reveals that its mechanical performance is excellent. To explain this based on the test results of comparative steels F to I, in comparative steel F, the chemical compositions C and B are higher in content than the chemical composition limited in the present invention.
The YP value became high and therefore the value became low.
Comparative steel G has higher Si and P contents than the present invention,
On the contrary, the Mn content is low, so the value is low, and therefore the zinc plating adhesion is poor. For comparison steel H, B
Since no addition is made, the transition temperature becomes high, resulting in poor low-temperature brittleness. The above three types of steel are manufactured under the same manufacturing conditions as in the present invention, but in the following Comparative Steel I, the chemical composition is the same as defined in the present invention, but the plating pretreatment conditions are different from the method of the present invention and are annealed. Since no subsequent low-temperature holding is performed, the transition temperature becomes high, and therefore low-temperature brittleness is poor. Thus, steels A to E produced according to the present invention
Judging from the above-mentioned performances comprehensively, the comparative steels F to I
It can be seen that the product performance is superior compared to that of the previous model. (Effects of the Invention) As explained above, the present invention provides press formability that has high TS value, low YP value, and high value by adding Mn, P, Ti, and Nb in a chemical composition in steel. By adding B to the chemical composition of the steel and performing a special heat cycle treatment using a continuous furnace for plating with Ti, Nb, and C,
The transition temperature is -40°C or lower, suppressing low-temperature brittleness, and realizing an innovative manufacturing method that improves the low-temperature brittleness of conventional galvanized steel sheets. By this manufacturing method, it is possible to obtain a high-tensile hot-dip galvanized steel sheet with excellent formability and ultra-deep drawability. Therefore, the present invention has great significance in view of the recent increasing trend toward thinner automobiles and the use of high tensile strength materials for difficult-to-form parts in order to reduce the weight of automobiles.

[Brief explanation of the drawing]

The accompanying drawing is a diagram showing the heat cycle in the present invention.

Claims (1)

[Claims] 1% by weight: C: 0.0005-0.0050%, Si≦0.60%, Mn: 1.0-2.5%, P: 0.010-0.080%, S≦0.015%, sol.Al: 0.010-0.100% , N≦0.0050%, B: 0.0002 to 0.0008%, Nb is Nb/C≧4 and Nb≦0.08%, and the content ratio of N to Ti and N and S are
Continuously cast molten steel with a composition that contains Ti within a range where the total content ratio of After hot and cold rolling, the obtained continuous casting slab was continuously annealed at a temperature of 730-900℃ in a continuous hot-dip galvanizing line, and then cold-held at a holding temperature of 430-500℃ for 20-120 seconds. A method for producing a high-tensile galvanized steel sheet having excellent formability, ultra-deep drawability, and low-temperature embrittlement resistance, which is then subjected to hot-dip galvanizing. 2. The method according to claim 1, wherein after continuous annealing, the material is cooled to the holding temperature of 430 to 500°C at a cooling rate of 1°C/sec or more. 3. The method according to claim 1 or 2, wherein winding is performed at 400 to 680°C after hot rolling.