JPS63195228A - Production of high r value hot rolled steel sheet having excellent longitudinal crack resistance - Google Patents

Abstract

PURPOSE:To improve the longitudinal crack resistance of a high r value hot rolled steel sheet, by adding specific ratios of B and Ca independently or in combination to a dead soft C-Al killed steel having a specific compsn. added with Ti and Nb alone or in combination. CONSTITUTION:The steel which contains, by weight, <=0.013% C, 0.05-0.35% Mn, <=0.03% S, <=0.02% P, 0.005-0.080% sol.Al, and 0.0005-0.005% B, is added with Ti or Nb at <=0.2% Ti and >=0.3% Nb and at such weight % as to attain formula I (where formula II) and consists of the balance Fe is used. While such steel is lubricated at 500-800 deg.C, the steel is rolled at 60-95% total draft and is then subjected to an annealing treatment or high-temp. coiling at 700-750 deg.C. The steel formed by incorporating 0.0005-0.01% Ca further into the above-mentioned steel and adding Ti and/or Nb at such weight % as to attain formula I (where formula III) may be used as well.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a high r-value hot rolled steel sheet, and in particular,
Ultra-low C-A j with excellent deep drawability! Killed-T1 or N
This invention relates to improvement of the standing resistance in steel with the sole addition of Ti and Nb or in steel with the combined addition of Ti and Nb.

[Conventional technology]

In recent years, the requirements for deep drawability (r value) of press-forming steel sheets, mainly for automobile parts, have become even more severe than before.

Research and development to improve this deep drawability is being carried out vigorously, and one example is an attempt to warm-roll ultra-low C-A1 killed steel to obtain a high r-value hot-rolled steel sheet. being done. In order to obtain high deep drawability also in this warm-rolled hot-rolled steel sheet, it is necessary to reduce solid solution C and Mn in the steel.

On the other hand, with the progress of today's steel manufacturing technology and the addition of C-fixed atoms such as Ti and Nb to steel, either singly or in combination, it has become possible to reduce the amount of solid solute C in steel to almost zero.

[Problem that the invention seeks to solve]

However, on the other hand, it is known that reducing the amount of solid solute C weakens the bonding force between atoms at grain boundaries and deteriorates the durability during secondary processing.
The same applies to a hot-rolled steel sheet obtained by hot-rolling a low C-AJ killed steel containing atomic equivalents or more of .

In view of these problems, the present invention aims to provide a method for producing a high r-value hot-rolled steel sheet with improved vertical resistance.

The inventor of the present invention has proposed the conventional extremely low C-Al killed-TI
Alternatively, as a result of intensive research and repeated studies on improving the longitudinal resistance of steel sheets with the addition of Nb alone or in combination with Ti and Nb, we have found that by adding B or Ca alone or in combination, The present invention was made based on the discovery that a high r-value hot-rolled steel sheet with excellent vertical resistance can be obtained.

[Means for solving problems]

That is, the first invention of the present application has C: 0.013% by weight or less, Mn: 0.05 to 0.35% by weight, S:
0.03 heavy economy or less, P j 0.02 weight% or less, s
Contains o1, A12: 0.005 to 0°080% by weight, B: 0.0005 to o, oos% by weight, Ti: 0.2% by weight or less, Nb: 0.3% by weight and C/12< (T + */4B+N b/9
3) <3×C/12 (However, T i * = T t-
48/14・N-48/32・S)
The remainder F after Ti or Nb was added singly or in combination.
s and unavoidable impurities, 500℃~8
Rolling is performed at a total reduction ratio of 60 to 95% with lubrication in the temperature range of 00°C, followed by annealing treatment or rolling at 700°C to
The winding is performed at a high temperature of 750°C.

Further, the second invention of the present application provides C: 0.013% by weight or less, M n : 0.05 to 0.35% by weight, and S: 0.
．． 03% by weight or less, P: 0.02% by weight or less, 5a
lt, AJ: 0. O05~o, oso overlay %, B
! 0.0005-0.005% by weight, Ca: 0.
0005 to O, 01% by weight, Ti: 0.2% by weight or less, Nb: 0.3% by weight or less, and C/12<(
Ti book/4B+ Nb/93) < 3 X C/
12 (1 day, T i * -T 1-48/14・
N -48/32 (S -32/40・Ca)) A steel made of the remaining F6 and unavoidable impurities to which Ti or Nb was added singly or in combination at a weight percent of 500°C to 800°C 60 with lubrication in the temperature range
Rolling is performed at a total reduction ratio of ~95%, and then annealing treatment or high-temperature winding at 700°C to 750°C is performed.

Here, the reasons for limiting the contained components and content ratios in the present invention will be explained. First, the first invention will be explained.

CTC is an element that has a large effect on deep drawability, and the lower the CTC content, the higher the deep drawability can be obtained.

Therefore, in order to obtain the desired high deep drawability, the C content should be 0.
0.013% by weight or less. On the other hand, the lower limit is determined by steelmaking technology, but currently it is o, ooi
It is about % by weight.

Mn: Like C, Mn is an element that affects deep drawability.
The lower the content, the better. However, if the content is less than 0.05% by weight, hot brittleness cannot be prevented, and if it exceeds 0.35% by weight, the deep drawability will deteriorate. Therefore, the Mn content is 0
．． 05 to 0.35% by weight.

T i, N b: T i, Nb
- As an additive element that improves the weight, especially *Ti
, Nb precipitates Tic, NbC and solidifies in steel? By fixing the capacity C, it is possible to make the amount of solid solute C even lower than the limit amount reduced by steelmaking technology. However, a large amount of Ti.

Since the addition of Nb extremely reduces solid solution C1, there is a risk of weakening the grain boundaries and causing secondary processing embrittlement.In addition, considering economic efficiency, TI is 0.2% by weight and Nb is 0%. The upper limit is .3% by weight. The precipitation of TiC can be controlled by Sl, but the range of the amount of TI that is possible for controlling this TiC precipitation is (T + ** + Nb) /C if the amount of Nb, which is a carbide forming element, is also taken into account. (However, T
The atomic weight ratio of i*-Ti-48/14·N-48/32 S is between 1 and 3. Therefore, Ti or Nb
Addition 11 (wt%) of one or both of C/12<
(T i * /48+Nb/93) < 3 X
The amount should satisfy C/12.

P: When P is added, there is a risk of brittle fracture and a risk of defective spont welding, which needs to be reduced, but due to the limitations of current steelmaking technology, the content should be 0.02% by weight or less.

SO3 has conventionally been known as a forming element for MnS inclusions that cause cracks during forming, and it is necessary to reduce the S content, which is set at 0.03% by weight or less.

BIB is an effective element for durability, and B is 0°000
By adding 5 to 0.005% by weight, sufficient standing resistance can be ensured. If the amount of B is less than 0.0005% by weight, the desired anti-sagging effect cannot be obtained; When the amount exceeds %, the effect becomes saturated. From this, in consideration of economic efficiency, the amount of B was determined to be from o,oos to 0.005% by weight.

A1: AJ is added to deoxidize the steel and fix free N by forming AIN. If N is insufficiently fixed, strain aging will appear. In order to effectively obtain the N fixing effect of /Ml, it is necessary to add o, oos in weight percent. On the other hand, if too much is contained, A ji
Since z causes Os-based inclusions to precipitate, which deteriorates ductility and also lowers economic efficiency, it is set to less than o, oso weight%.

Next, the reason for limiting the content components and content ratios in the second invention of the present application will be explained. In addition, C1P, Mn, S,
An! , Ti, and Nb are the same as those in the first invention, so their explanations will be omitted.

Although the detailed mechanism of Ca5Ca is unknown, it has the effect of increasing grain boundary bonding force. Also, by adding Ca, S forms CaS instead of MnS*
MnS is deformed during hot rolling and tends to crack later based on these inclusions, whereas CaS deforms during hot rolling and is less likely to crack, and the amount of Ca is If it is less than 0.001% by weight, the amount of CaS will increase, and the ductility will be reduced. Therefore, the amount of Ca is set to 0.0005 to 0.01% by weight.

Next, No. 0 of this application. Manufacturing conditions in the second invention will be explained.

In order to improve the deep drawability of a steel sheet, it is necessary to control the crystal orientation of the steel sheet so that the (111) plane is parallel to the sheet surface. The reason why rolling was carried out at a temperature range of 500° C. to 800° C. and a total reduction ratio of 60 to 95% is as follows. That is, when rolled under these conditions,
Rolling set &II\a belonging to ND//<111> and RD//<110> orientation group develops, and recrystallized texture with ND//<111> orientation group developed by subsequent annealing treatment or high temperature coiling. is obtained. These nyrite grains having many (111) orientations are responsible for increasing the r value (deep drawability) in the hot rolled sheet. On the other hand, when rolling at a temperature lower than 50'0°C, the r value decreases because the crystallization of Ti or Nb with C4pN is incomplete, and at a temperature exceeding 800°C, dynamic recrystallization occurs, resulting in texture is randomized, r
The value is lower than rolling at 500°C to 800°C.

The reason why lubrication is performed is to eliminate rolling non-uniformity in the thickness direction due to frictional force on the surface of the steel sheet.

Furthermore, after hot rolling during lubrication, annealing treatment or
The reason for performing high temperature coiling at 0°C to 750°C is that these heat treatments produce recrystallized grains with a developed (111) orientation in the crystal, resulting in hot rolling with excellent deep drawability. This is because steel plates can be obtained. Here, annealing includes not only continuous annealing and batch annealing but also heat treatment before the dipping step in a hot-dip galvanizing line. In addition, in this hot-dip galvanizing line, for example, from the viewpoint of homogeneous adhesion of molten zinc and temperature control during zinc adhesion, the temperature is set at 500°C to 8°C.
After raising the temperature to 50°C and maintaining soaking for 10 to 30 seconds,
It is cooled to 300°C to 500°C at a rate of 10°C/sec and immersed in a molten zinc bath. Further, in some cases, reheating treatment is performed at 600° C. for about 10 seconds to perform alloying treatment to improve corrosion resistance.

(Function) No. 0. In the second invention of the present application, solid solution C is reduced by adding Ti and Nb, and the crystal orientation is controlled by controlling the heat treatment conditions during warm region rolling to improve ductility. As a result, excellent deep drawability is obtained, and the amount of S is reduced to reduce the amount of MnS generated, which causes cracking. Since it is added in combination, excellent standing resistance can be obtained.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

Table 1 shows the chemical composition of the invention steel and comparative steel, in which invention steels 1 to 6 are the first invention steels, and invention steels 19 to 24 are the second invention steels.
mllli each according to the invention of .

In this example, after melting steel having the components and content ratios shown in Table 1, it was soaked at 300 to 850°C and rolled at a total reduction rate of 90% (10t-5t-It).

After that, vacuum annealing furnace treatment at 750°C for 1 hour assuming batch processing (see code A in Table 2), salt bath treatment at 850Xl for 5 minutes assuming CAL treatment (see code B in Table 2), and high temperature treatment. For winding method: 700℃~800℃
After rolling at 750°C x 30 minutes F
%C (see code C in Table 2) equivalent to high-temperature coiling, and the tensile strength, elongation, r value, Al (aging index), and brittle transition temperature were investigated.

Regarding the above-mentioned transition temperature, a test cup was prepared using a hot-rolled sheet subjected to bunching treatment with a blank of 145 and a drawing force α of 2.0, and a vertical cup cracking test shown in FIG. 1 was conducted.

In this test, a test cup 3 was placed over a conical punch 2 in a Freon 1 cooled with liquid nitrogen, and a load of 4 was applied from the bottom of the cup 3 in a temperature range from liquid nitrogen temperature to room temperature.
was added to expand the hole edge, and the transition temperature was measured from the brittle fracture rate at that time and was expressed in the third table. The results are shown in Table 2.

According to Table 2, the steel according to the present invention has tensile strength, elongation, and r value equal to or higher than that of the comparative steel, and has excellent deep drawability. In addition, the AI is higher than that of the comparative steel, and the transition temperature is lower than that of the comparative steel.
It can be seen that this improves the warp resistance.

Furthermore, regarding the steel type of invention f421, CAL.

A heat treatment equivalent to batch and high-temperature winding was performed, and the relationship between the finishing temperature and r value at that time was determined. The results are shown in Table 3 and Figure 2. According to this, a good r value is obtained in the finishing temperature range of the present invention (see A in FIG. 2) in any heat treatment.

〔Effect of the invention〕

As described above, according to the method for manufacturing a hot rolled steel sheet according to the first to third inventions of the present application, the amount of solid solute C is reduced by adding TI and Nb, and the crystal orientation is controlled by controlling the heat treatment conditions. In addition, by reducing the amount of S and increasing the resistance to vertical cracking, by adding Ca alone or in combination, a hot-rolled steel sheet with excellent deep drawability and resistance to vertical cracking can be obtained. effective.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the state of a vertical cup crack test for explaining the present invention in detail, and FIG. 2 is a diagram showing the relationship between finishing temperature and r(i[ in an example of the present invention. Patent Applicant Kobe Seirisho Co., Ltd. Agent Patent Attorney Get off Tsutomu Figure 1, Figure 2

Claims (2)

[Claims] (1) C: 0.013% by weight or less, Mn: 0.05-0
．． 35% by weight, S: 0.03% by weight or less, P: 0.02
% by weight or less, sol. Contains Al: 0.005 to 0.080% by weight, B: 0.0005 to 0.005% by weight,
Ti: 0.2% by weight or less, Nb: 0.3% by weight or less,
and C/12<(Ti*/48+Nb/93)<3×C
/12 (However, Ti*=Ti-48/14・N-48/
32・S), the total reduction rate is A high r-value hot-rolled steel sheet with excellent warp resistance, characterized in that it is annealed or coiled at a high temperature in a temperature range of 700°C to 750°C after 60 to 95% rolling. manufacturing method. (2) C: 0.013% by weight or less, Mn: 0.05-0
．． 35% by weight, S: 0.03% by weight or less, P: 0.02
% by weight or less, sol. Al: 0.005-0.080% by weight, B: 0.0005-0.005% by weight, Ca: 0
．． 0005 to 0.01% by weight, Ti: 0.2% by weight or less, Nb: 0.3% by weight or less, and C/12<
(Ti*/48+Nb/93)<3×C/12 (However,
Ti*=Ti-48/14・N-48/32 (S-32
/40・Ca)) by weight% of Ti or N
The steel consisting of the remainder Fe and unavoidable impurities to which b is added singly or in combination is rolled at a total reduction rate of 60 to 95% while being lubricated at a temperature range of 500 ° C to 800 ° C, and then annealed. Alternatively, a method for producing a high r-value hot-rolled steel sheet with excellent bending resistance, characterized in that high-temperature coiling is performed in a temperature range of 700°C to 750°C.