JPS59143027A - Production of high-strength steel plate having good ductility and processability - Google Patents

Abstract

PURPOSE:To produce a high-tension steel plate having excellent ductility and processability by subjecting a steel plate to a soaking treatment under a specific condition and the succeeding cooling treatment. CONSTITUTION:The heating of a steel plate contg. <0.15% C, 0.2-3.5% Mn, 0.01-0.15% P, and <0.10% Al, or a steel plate further contg. 1 or >=2 kinds selected from a group A consisting of 0.1-1.5% Si, 0.1-1.0% Cr, 0.1-1.0% Mo, 5-100ppm B and a group B consisting of 0.01-0.1% Nb, 0.01-0.2% Ti and 0.01 0.2% V up to a soaking temp. of the Ac3 transformation point or above is accomplished at a heating rate of >=5 deg.C/sec at least from 600 deg.C up to the Ac3 transformation point and the steel plate is held for 10sec- 10min at said temp., whereby the steel plate is subjected to a soaking treatment. The average cooling rate between 600-300 deg.C in the succeeding cooling is made to the critical cooling rate CR( deg.C/sec) calculated by the equation ( I ) or above in the case of the steel having the former compsn. and said rate calculated by the equation (II) or above in the case of the steel having the latter compsn.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength steel plate with good ductility and workability, particularly a tensile strength of 60 k! j f / m4
The present invention relates to a low-cost manufacturing method for the above high-strength steel plate.

In recent years, from the viewpoint of automobile safety and weight reduction, tensile strength of 60 to 9 f /
- High tensile strength thin steel sheets and the like are increasingly being used. Materials used in such applications are required to have high tensile strength, good ductility and workability, and good spot weldability when assembling a vehicle body. Recently, mixed-structure steel sheets consisting of ferrite and low-temperature transformation products mainly consisting of martensite or bainite have been widely used as steel sheets that satisfy these requirements. However, in order to increase the strength of conventional mixed structure steel sheets, Mn, Sl, Nb, and T
It is necessary to add large amounts of elements such as i, which results in an increase in cost, and addition of large amounts of Mn and SI not only increases manufacturing costs but also tends to cause surface oxidation during continuous annealing. There was a problem of deteriorating spot weldability.

When a large amount of fcMn or the like is contained, a band-like structure is developed, which is thought to be caused by its segregation, and there is a problem in that workability such as bending and local ductility are deteriorated.

An object of the present invention is to provide a method for manufacturing a high-strength steel plate that has both ductility and good workability, and is inexpensive to manufacture, solving the problems of the prior art described above.

This object of the present invention is achieved by the following two inventions.

The gist of the first invention is as follows. That is, in terms of weight ratio, C: 015% or less, Mn: 0.2 to 3
．． 5%, P:001-015CH, Al:o, 10% or less, and the balance is Fe and unavoidable impurities. At least 600℃ when heated to a temperature of at least 600℃
The heating rate in the section from Ac to the inflection point is 51://S
The step of heating at eC or higher and the soaking temperature of 10
The critical cooling rate CR is calculated using the following formula (1), which is the average cooling rate in the temperature range of 600 to 300°C during the soaking step held for seconds to 10 minutes and cooling after the end of the soaking step.
A method for producing a high-strength steel sheet with good ductility and workability, comprising the step of cooling at a temperature of ('C/5ec) or higher.

logC(℃/s eC)=-1,73(Mn(%
)+-3,5P(%))+19s-(1) Next, the gist of the second invention is that C, Mn, which is the same as the first invention,
In addition to having the basic composition of P and At,
i-i, s%Crho, 1-1.0%, M.

: 0.1-1.0%, B: 5-1100pp, Group A and Nb: 0.01-o, Ichi, Ti: 0
．． 01~0, 2chi, V: 0.01~0.2%j, Q
A method for producing a high-strength steel plate containing one or more selected members of Group B, the remainder being Fe and unavoidable impurities, wherein the steel plate is heated to a soaking temperature of Ac3 transformation point or higher. Acs from at least 600℃
A step of heating at a heating rate of 5°C/sec or more in the section up to the transformation point, a soaking step of holding the temperature at the soaking temperature for 0 seconds to 10 minutes, and a cooling step of 60°C after the soaking step is completed.
The average cooling rate in the temperature range of 0 to 300°C is shown below (
2) A method for producing a high-strength steel sheet with good ductility and workability, comprising the step of cooling at a critical cooling rate CR (° C./5ee) or higher calculated by the formula.

LogCf% (c/sec) = -1,73
(Mn (%) + 0.26 S i (%) + 3.5
P (%) + 1.3 Cr (%) + 2.67 Mo (%
), :]+3.95 (2) However, in the case of B addition, (2
) Change 3.95 in formula to 3.40.

As summarized above, the present invention relates to soaking for 10 seconds to 10 minutes at a temperature above the Ac3 transformation point during annealing, and the heating range from 600°C to the Ac3 transformation point has not been previously disclosed. This is a method for manufacturing high-strength steel sheets with good ductility and workability by heating rapidly at a heating rate of 1.b and further controlling the cooling conditions after soaking.

First, the reason for limiting the components of the high-strength steel sheet of the present invention will be explained.

C: C is an important element as one of the basic components of steel, and strength can be increased at low cost by increasing C, but if it exceeds 0.15%, spot weldability will sharply decrease. Because of deterioration, the upper limit was limited to 0.15%.

Mn: Mn is a solid solution strengthening element and at the same time is a particularly important element for the formation of low temperature transformation products. Mn is required to be 0.1% or more to prevent hot embrittlement, but from the viewpoint of melting, 0.1% or more is required.
．． The lower limit was set at 2%. Also, if Mn exceeds 3.5%, spot weldability deteriorates like C, so the upper limit should be set at 3.5%.
It was set at 5%.

P: P is a ferrite-forming element that is inexpensive and has a large solid solution strengthening ability, and is advantageous as a reinforcing element.If it is less than 0.01%, the manufacturing cost will increase and there is no particular advantage, so the lower limit is set at 0.01%.
And so.

Next 20.059hC-1,51A4n-(0~0.8
)%P steel plates were spot welded and the ductility ratio, shear tensile strength and cross tensile strength of the welded part were investigated, and the relationship with the P content is shown in Figure 1. From the first point, if P exceeds 0.15%, the strength and ductility ratio of the weld will deteriorate rapidly, so the upper limit of P was limited to 0.15%.

At: At is necessary as a deoxidizing element, but if it exceeds 0.10%, it will form alumina clusters and deteriorate the surface quality, and the risk of hot cracking will increase, so the upper limit should be set at 0.10%. limited to.

The above-mentioned limited amounts of C, Mn, P, and Az are the basic components of the high-strength steel sheet of the present invention, and 8i and Cr are also included.

A group consisting of each element of Mo, B, Nb, Ti
The object of the present invention can also be effectively achieved in a high-strength steel sheet containing one or more selected from Group B consisting of each element of , V, in the following limited amounts. The reasons for limiting these selected elements are as follows.

Group A (8i, Or, Mo, B,): As is clear from the above formula (2), the elements of Group A lower the critical cooling rate in the cooling process during annealing necessary for forming a mixed structure, and at the same time The amount of low-temperature transformation products is increased, which has the effect of increasing strength.

In order to effectively exhibit this effect, 8i, Cr.

Each element of MO needs to be at least 0.1%, and B needs to be at least 5 ppm7.However, since adding too much Si will saturate the effect and increase costs, the upper limit should be 1.5% for Si, 1.5% for Cr, Mo
is limited to 1.0 ppm, B is limited to 100 ppm, and 8
i:0.1-1.5S, Cr:O11-1.0%, M
o: 0.1 to 1.096, B: limited to a range of 5 to 1100 pp.

Group B (Nb, Ti, V): Nb, Ti, and V are all carbonitride-forming elements, and have the effect of making crystal grains finer, increasing strength due to precipitates, or suppressing recrystallization of ferrite phase, etc. This has the effect of strengthening the material. However, these effects are not fully exhibited when each element is less than 0.011%, so the lower limit was set to 0.01% for each element. Also, if excessive addition saturates the effect and increases the cost, the upper limit is set at 0.1% for Nb and 0.2% for Ti and V.
and Nb: 0.01-0.1%, ji: 0, respectively.
．． 01 to 0.2%, v: o, oi to 0.2%.

It should be noted that each element of the A# and B groups above exerts its own effect when used alone, but the effects of each element are not diminished even if they are added in combination.

The steel of the present invention having the above-described limited composition is hot-rolled, pickled, cold-rolled and continuously annealed after being melted. Although hot rolling may be carried out under normal conditions, low temperature winding of 600° C. or lower is preferred in order to obtain high strength. Furthermore, by controlling heat treatment conditions in a limited manner as described below, high-strength steel plates with good ductility and workability can be manufactured at low cost.

Next, the reason for limiting the annealing conditions in the present invention will be explained. Annealing conditions are the most important requirement of the present invention. In order to obtain a steel plate with high strength and excellent ductility, it is advantageous to heat the steel sheet to a temperature above the A c 1 transformation point and below the AC 3 transformation point, soak it, and rapidly cool it to form a mixed structure of ferrite and martensite. However, when the amount of Mn increases, its segregation causes [A
If soaking is carried out at a temperature above the Cl transformation point and below the Ac3 transformation point, the final structure obtained will be band-like, and the workability such as bending and local ductility will be low. On the other hand, when heated in the austenite single phase region above the A c3 transformation point, soaked, and rapidly cooled, the resulting structure becomes a non-band-like mixed structure of mainly ferrite and bainite, and although the ductility is slightly reduced, it is still better than recovery annealed steel. It has good workability such as bending and local ductility. That is, steel with the chemical composition shown in Table 2 Table 1 was annealed at two temperatures as shown in Table 2, corroded with nital, and the micrographs are shown in Figure 2 (5) and (B). It was related.

In Figure 2 (A) and (B), Figure 2 (A) is AC7
It was annealed at 725°C above the transformation point, Ac, below the transformation point, and a strong band-like structure remained;
) is Ac and was annealed at 870° C. above the transformation point, and the band-like structure has disappeared. As for bendability, as shown in Table 2, the critical bending radius is 6 mm and 8 mm for 725°C annealing.
When annealed at 70°C, the result is Omm, and annealing at Ac or above the transformation point is better. Based on these results, in the present invention, the annealing temperature was limited to Ac, which was higher than the transformation point, and the ductility was slightly sacrificed ((()) to improve workability.

Next, regarding the heating rate, it is not difficult to achieve an average heating rate of 5° C./sec or more from room temperature to the soaking temperature in an actual annealing furnace. However, it is a well-known fact, as shown in FIG. 3, that the heating rate of a steel plate decreases as the temperature increases. In FIG. 3, the solid line indicates the steel plate temperature, and the dotted line indicates the average heating rate. Therefore, even if an average heating rate of 5°C/sec or more is achieved when heating a steel plate from a low temperature near room temperature to a soaking temperature above the Ac transformation point,
For example, a heating rate of 5°C/s in a high temperature range from 500 to 600°C to the target Ac, soaking temperature above the transformation point.
ec, and this tendency becomes more pronounced as the temperature increases.

The present inventors focused on the influence of the heating rate in such a high-temperature zone on the tensile properties after annealing, and conducted the following basic experiment. A 12 town thick cold rolled steel plate having the chemical composition shown in Table 1 was first heated at a heating rate of about 10°C/sec up to 600°C, which is considered to be sufficient for normal continuous annealing, and then subjected to Ac, transformation, etc. The heating rate was significantly changed until the soaking temperature reached 850°C, which is a single phase of austenite with a temperature of 85°C.
After soaking at 0°C for 1 minute, short-time annealing was performed by cooling at a cooling rate of 30°C/sec, and the tensile properties were investigated. The results are shown in Figure 4.

In FIG. 4, both tensile strength and yield stress increase as the heating rate increases, but elongation hardly decreases. Moreover, this effect is particularly remarkable when the heating rate is 5°C/sec or more, so from 600°C to A
The heating temperature in the section up to the C8 transformation point was limited to 5°C/sec or higher.

Next, the starting temperature T□ that requires rapid heating in the high temperature range
We considered this. That is, a cold-rolled steel sheet having the chemical composition shown in Table 1 is heated from room temperature to the rapid heating start temperature TH in 1.07 seconds as shown in FIG. 1 and 5℃/
sec, and after soaking at 850°C for 1 minute, it was cooled at 30°C/sec.

From Figure 6, 5'C/
It is clear that high strength can be obtained without deteriorating ductility by heat treatment at a rapid heating rate of sec or more, so the rapid heating start temperature T8 was limited to 600° C. or higher. It is obvious that the faster the heating rate is in both the low temperature range and the high temperature range, the better the quality of the material will be obtained.

The reason why the balance between strength and ductility is improved by heat treatment at a heating rate of 5 seconds or more to a soaking temperature of AC3 transformation point or higher in a high temperature range of 600° C. or higher as described above can be presumed as follows. That is, when considering the annealing of the steel with limited components of the present invention, the temperature of 600° C. almost corresponds to the recrystallization start temperature of ferrite grains. By increasing the heating rate in the region above that temperature and shortening the residence time between the recrystallization start temperature and the A C1 transformation point after cold rolling, the state of extremely fine recrystallized grains or complete recrystallization can be achieved. The austenite transformation begins when the A c 1 transformation point is reached without completing the heating process.By further heating to a temperature above the AC 3 transformation point in a short time, the austenite grain size that exists during soaking becomes smaller, and after cooling, the austenite transformation begins. As the austenite undergoes metamorphosis, it eventually becomes ferrite and bainite (
A microstructure (some of which contains martensite) is obtained.

It is thought that this microstructural refinement is effective in improving the balance between strength and ductility.

As mentioned above, the heating rate from about 600°C, which is the recrystallization start temperature, to at least the Ac3 transformation point, preferably to the soaking temperature, is 5 t? Heating at a rate of 1/sec or higher is one of the important factors for obtaining high-strength steel with good ductility.

In addition, the soaking time is 1 to complete the austenite transformation.
The soaking time was limited to 10 seconds to 10 minutes because it was necessary to hold the temperature for 0 seconds or more, and if it was held for more than 10 minutes, the austenite grains would become coarse.

The cooling rate after soaking is determined in order to obtain high strength and good ductility. In other words, the cooling rate is the critical cooling rate CR(t,/s e
c) It is necessary to cool down at or above.

(a) When only basic components of C, Mn, P, and A4 are contained in limited amounts (first invention) togcR ('c/5ec) = -1,73 (Mn (%)
-1-3,5P(%))+3.95 +・+ ++ +
+ (1) (b) In addition to the basic components of C, Mn, P, and At (: S i ).

Group A consisting of Cr, Mo, and B, and Nb, Ti, and V
When containing a limited amount of one or more selected from Group B (Second invention) Log CJ Hi1-
(107sec) =-1,73(Mn%+0.26
S i (%) → -3SP (%) + 130r (%)) -1
67Mo(%))+3.95-・-・--(2) However, in the case of B addition, 3.95 in equation (2) is changed to 3.40.

The reason why the cooling rate is limited as above is that the cooling rate is (1)
The critical cooling rate CR (c/5
If the temperature is less than 7-element doperite structure, high strength cannot be obtained, but the critical cooling rate CR (℃/5ec)
If it is above, the structure is usually composed of ferrite and bainite (including some martensite), which has high strength and good ductility.
Since workability is obtained, the cooling rate can be calculated using equation (1) or (
2) The cooling speed was limited to the critical cooling speed determined by equation (2).

Next, the cooling rate in the range of 600 to 300°C was specified. When cooling from the soaking temperature, 600°C
If the cooling rate between 600 and 300°C, which is sufficiently lower than the Ms point, is small, diffusion transformation will occur and have an adverse effect on the balance between strength and ductility. Therefore, the cooling rate between 600 and 300°C is Critical cooling rate C calculated by formula (2) Cc/5
ec) limited to the above.

As described above, the present invention limits the basic composition and the composition of selected additive elements, heats at a heating rate of 5 vSeC or more from 600°C to the Ac3 transformation point in annealing, and heats at a soaking temperature (2 Soak for 10 seconds to 10 minutes, and then cool to 600 to 300°C using formula (1) or (2).
) A microstructure consisting of ferrite and bainite containing some martinsite can be obtained by rapid cooling at a rate higher than the critical cooling rate determined by the formula (2), thereby obtaining a high-strength steel plate with high strength, good ductility, and workability. was completed.

Example Table 3: For steel having the four types of components shown in Table 2, the finish rolling temperature is 830-870°C, and the coiling temperature is 500-550°C.
Then, annealing was performed under the same heat treatment conditions as shown in Table 3, including heating rate from 600°C to Ac, transformation temperature, soaking temperature, and cooling rate from 600°C to 300°C. The yield stress (ys), tensile strength (TS), elongation, and bendability of these annealed steel plates were investigated, and the results are also shown in Table 3. The bendability was expressed using the critical bending radius shown below.

In Table 3, the sample material A1 of the invention example, the sample material 5 of the comparative example, the sample material 2 of the invention example and the sample material 7 of the comparative example are each soaked at a temperature of However, unlike the comparative example, the heating rate of the present invention example is 5° C./sec or more, so it can be seen that the strength can be increased without deterioration of elongation.

Furthermore, when comparing the sample material AI of the present invention example and the sample material shop 6 of the comparative example, the soaking temperatures are 870°C and 750°C, respectively.
The examples of the present invention, in which the soaking temperature is equal to or higher than the A c3 transformation point, have an excellent critical bending radius of 1, whereas the comparative examples, in which the soaking temperature is less than the A c3 transformation point, have a significantly poor bendability of 5.

As is clear from the above examples, the high-strength steel sheet with good ductility and workability according to the present invention is obtained by hot-rolling and cold-rolling a steel slab with a limited chemical composition in a conventional manner. When heating to the soaking temperature of 60
The section from 0°C to the Ac3 transformation point is rapidly heated at a heating rate of 5'c7SeC or higher, soaked for 10 seconds to 10 minutes, and cooled to 600 to 3
The 00°C section is cooled at a cooling rate higher than the critical cooling rate CR (c/5eC) that the inventors have determined as a function of the steel composition, and the steel structure is changed to a microstructure of bainite containing ferrite and some martinsite. By doing so, we have established a method for manufacturing a high-strength steel plate that has good ductility and workability, and has a strength of 6°1 (yf/-) or more.Also, the present invention has a low manufacturing cost and uses spot welding as a characteristic of steel. Because of its excellent properties, it can be widely used as a reinforcement system for automobiles, etc.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the P content and the tensile test results of spot welds in experiments to obtain the present invention, and Figure 2 (5
) and (b) are micrographs showing the metal structure when the annealing soaking temperature in the experiment for obtaining the present invention was below the A c3 transformation point and above the A c3 transformation point, respectively. A cooling pattern showing that the heating rate decreases as the temperature of the steel plate increases, and Figure 4 shows the cooling pattern from 600 to 850 during annealing in the experiment to obtain the present invention.
5 is a diagram showing the relationship between the heating rate between °C and the tensile test results, FIG. 5 is a diagram showing the heating and cooling pattern of the annealing method in the annealing experiment to obtain the present invention, and FIG. 6 is a diagram showing the relationship between the heating rate and the tensile test results. It is a diagram showing the relationship between rapid heating start temperature/IT□ and tensile test results in an experiment. Agent Patent Attorney Takeo Nakaji Figure 1 P amount (weight 2) Figure 2 (Bnd stand-1 J Keiko Figure 3 Figure 4 (0%ec) Figure 5 Figure 60 bvu TH

Claims (1)

[Claims] (1) C: 0.15% or less, Mn: 0.2~ by weight ratio
3.5chi, P: 0.01-0.15%, Al: 0.10
In the method for manufacturing a high-strength steel plate comprising Fe and unavoidable impurities, the steel plate lAc is heated to a soaking temperature of at least 600°C above its transformation point.
The heating rate in the section from Ac to transformation point is 5℃/se
The average cooling rate in the temperature range of 600 to 300 ° C. for the step of heating above c, the soaking step of holding at the soaking temperature for 10 seconds to 10 minutes, and the cooling after the soaking step is as follows (1) The critical cooling rate CR (
A method for producing a high-strength steel plate with good ductility and workability, comprising the step of cooling at a temperature of 1.5 C/sec or more. A! ogcR('C/5ec) = -1,73(Mn
(%)+3.5P(96):]+3.95 ・(1)
(2) C: 0.15% or less, Mn: 0.2~ by weight ratio
3.5 section, P: 0.01-0.15 section, AAI: 0.
Contains 10% or less, and furthermore, Si: 0.1-1.5%
, Cr20, 1~1.0chi, Mo:0.1~1.0
Group A and N consisting of H, B: 5-1001) pm
b: 0.01-0.1 Ti, Ti: 0.01-0.2
%, V: 0.01 to 0.2 In the method for manufacturing a high strength steel plate containing one or more selected from Group B consisting of 0.01 to 0.2 yen, the remainder being Fe and unavoidable impurities, the steel plate heating to a soaking temperature equal to or higher than the A c 3 transformation point at a heating rate of at least 5° C./sec or more in the section from at least 600° C. to the A c H transformation point, and 10 seconds at the soaking temperature. 600℃ during the soaking step for ~10 minutes and cooling after the soaking step.
The average cooling rate in the temperature range of ~300°C is as follows (2
) A method for producing a high-strength steel plate with good ductility and workability, comprising the step of cooling at a critical cooling rate CR (°C A6c) or higher calculated by the formula. logCR(″(,/5ec)” −1,73(Mn
(%) +〇、26Si (%)+3.5P(%)+1
．． 3Cr (%1 +2.67Mo (1)) +3.9
5 - (2) However, in the case of B addition, 3.9 of equation (2)
-5 & Change to 3.40.