JPH0321608B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-strength thin steel sheets with good ductility and bending workability, particularly those with a tensile strength of 60 Kgf/mm ²
The present invention relates to a stable manufacturing method for the above high-strength steel plate. In recent years, high-strength steel plates with a tensile strength of 40 Kgf/mm ² or more have been frequently used for strong members such as bumpers and door guard bars from the viewpoint of vehicle safety and weight reduction. Materials used in such applications require not only high strength but also excellent ductility and bending workability. 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 meet these requirements. Currently, in such steel sheets, adding a large amount of Mn is essential for stabilizing austenite and eventually forming low-temperature transformation products, and the amount of Mn added is close to 2%. In such steel sheets, steel sheets with a mixed structure of ferrite and martensite with good ductility can be obtained by short-time heat treatment mainly in a continuous annealing furnace in the ferrite-austenite two-phase region, but the bending workability is affected by the addition of a large amount of Mn. The strong band structure causes deterioration, and the band structure also reduces local deformability and increases notch sensitivity, so these drawbacks have become a major problem depending on the parts used. To solve these problems, the band structure disappears.
There is a method of heat treatment in a temperature range above the Ar ₃ transformation point, which certainly improves bending workability, local deformability, and notch sensitivity, but it becomes a bainite-based structure and significantly deteriorates ductility. Furthermore, when heat treatment is performed in a temperature range above the Ac ₃ transformation point, the tensile properties change greatly due to fluctuations in the cooling rate, making it difficult to obtain a stable material. An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a stable method for producing a high-strength thin steel plate with excellent ductility, bending workability, and local ductility and low notch sensitivity. The above objects of the present invention are achieved by the following two inventions. The gist of the first invention is as follows. That is, in a method for producing a high-strength thin steel sheet containing C: 0.02% to 0.15%, Mn: 0.2 to 3.5%, P: 0.01 to 0.15%, Al: 0.10% or less, and the balance is Fe and unavoidable impurities in terms of weight ratio, The steel plate
A step of soaking for 3 to 60 seconds in a temperature range of Ac ₃ transformation point or more Ac ₃ transformation point + 50℃ or more, and ferrite/austenite in a temperature range of Ar ₃ transformation point or more or Ar ₁ transformation point or more after the soaking step The process of cooling to a two-phase region and holding in that temperature range for 20 seconds or more, and the average cooling rate in the temperature range of 600 to 300 ° C during cooling after the completion of the holding process, is the critical cooling rate calculated by the following formula (1) A method for manufacturing a high-strength thin steel sheet with good ductility and bendability, characterized by comprising a step of cooling to a temperature equal to or higher than CR (°C/sec). logCR (℃/sec) = -1.73 [Mn (%) + 3.5P (
%)]+3.95...(1) The gist of the second invention is as follows. That is, the same C, Mn, P as in the first invention,
In addition to the basic composition of Al, there is also a group A consisting of Si: 0.1-1.5% Cr: 0.1-1.0% Mo: 0.1-1.0% B: 5-100 ppm, and V: 0.01-0.20% Ti: 0.01-0.20% A high-strength thin steel plate containing one or more selected from Group B consisting of Nb: 0.01 to 0.10%, with the balance being Fe and unavoidable impurities is heat treated in the same manner as in the first invention. However, unlike the first invention, the average cooling rate in the cooling process is based on the critical cooling rate CR (° C./sec) calculated by the following equation (2). logCR (℃/sec) = 1.73 [Mn (%) + 0.26Si (%
)+3.5P(%) +1.3Cr(%)+2.67Mo(%)]+3.95...(2) However, in the case of B addition, change 3.95 in equation (2) to 3.40. First, the reason for limiting the components of the high-strength thin steel sheet of the present invention will be explained. C: C is an important element as one of the basic components of steel, and even if it is less than 0.02%, the mixed structure aimed at in the present invention can basically be obtained, but the Ac ₁ point increases rapidly and α
→The temperature range where the γ2 phase occurs becomes narrower, and as a result, it becomes very difficult to control the temperature during annealing, so set the lower limit.
It was set at 0.02%. On the other hand, if it exceeds 0.15%, spot weldability will deteriorate rapidly, so the upper limit was set at 0.15%. Mn: Mn is a solid solution strengthening element and at the same time essential for the mixed structure. The lower limit was set at 0.2% from the viewpoint of preventing hot brittleness and melting. On the other hand, as Mn increases, the critical cooling rate CR decreases, making it easier to obtain the desired mixed structure, but if it exceeds 3.5%, the spot solubility deteriorates, so 3.5% is set as the upper limit. P: P is a ferrite-forming element that is inexpensive and has high solid solution strengthening ability, and as the amount added increases, the critical cooling rate decreases.
It is preferable because it reduces CR, but if it is less than 0.01%, the effect is insufficient, and if it exceeds 0.15%, it impairs spot weldability, so it is limited to a range of 0.01 to 0.15%. Al: Al is necessary as a deoxidizing element, but excess Al
The upper limit was set at 0.10% because it forms alumina clusters, deteriorates the surface quality, and increases the risk of hot cracking. With the above limited amounts of C, Mn, P, and Al,
A basic component of the high-strength steel sheet of the present invention, and A
Each element of group Si, Cr, Mo, B, V as group B,
The object of the present invention can also be achieved more effectively in a high-strength steel sheet containing one or more of Ti and Nb in the following limited amounts. The reasons for these limitations are as follows. Group A Si, Cr, Mo, B: These elements of Group A are as clear from formula (2),
Both methods lower the critical cooling rate necessary for forming a mixed structure and at the same time increase the amount of low-temperature transformation products, resulting in an effect of increasing strength. For this result to be achieved, each element of Si, Cr, and Mo must be at least 0.1%, and B must be at least 5ppm.
The above is necessary, and excessive addition saturates the effect and increases costs, so the upper limit is set at 1.5% for Si, 1.5% for Cr,
Mo was limited to 1.0% and B to 100 ppm. Group B V, Ti, Nb: Each element in Group B is a carbonitride forming element, and has the effect of increasing strengthening by refining grains and suppressing recrystallization of precipitates or ferrite phase, but each element has the effect of increasing strength by 0.01%.
The effect is not sufficient if it is less than the lower limit.
Limited to 0.01%. In addition, excessive addition saturates the effect and increases costs, so V and Ti should be 0.20% or less.
Nb was limited to 0.10% or less. It should be noted that each of the elements of Group A and Group B exhibits their respective effects when used singly, but even if they are added in combination, their respective effects are not canceled out. According to the present invention, a high-strength thin steel plate with good ductility and bending workability can be produced at low cost by controlling heat treatment of steel whose composition is limited as described above in a limited manner as described below. In the present invention, continuous annealing is performed after hot rolling, pickling, and cold rolling. Hot rolling is carried out under normal conditions, but low temperature winding of 600°C or less is preferred in order to obtain high strength. Next, the reasons for limiting the annealing conditions in the present invention will be explained. The annealing conditions are the most important requirement of the present invention, and the heat treatment cycle of the steel plate in the present invention consists of the first step of maintaining the austenite single phase, the second step of maintaining the ferrite-austenite two-layer region, and the third step of rapid cooling. The process is divided into three stages. First, in the first step, it is essential to maintain the temperature at a temperature not lower than the Ac ₃ transformation point and not higher than the Ac ₃ transformation point + 50° C. for a period of not less than 3 seconds and not more than 60 seconds. This is to eliminate the band-like composition caused by the segregation of Mn by maintaining it in the austenite single phase region, and therefore the temperature at which it is maintained needs to be equal to or higher than the Ac ₃ transformation point. However, at high temperatures higher than the Ac ₃ transformation point +50°C, coarsening of grains progresses and deterioration of the surface properties of the steel sheet occurs, so the upper limit was set as the Ac ₃ transformation point +50°C. Further, the period of time for holding in the austenite single phase region is required to be 3 seconds or more in order for austenite reverse transformation to proceed sufficiently, while holding for more than 60 seconds is not preferable because it causes coarsening of the grains. Therefore, the first step is
The temperature was limited to above the Ac ₃ transformation point and below the Ac ₃ transformation point +50°C, and the time was limited to between 3 seconds and 60 seconds. In the second step, the steel plate that has been held in the austenite single-phase region in the first step is cooled and held in the ferrite-austenite two-phase region below the Ar ₃ transformation point and above the Ar ₁ transformation point for 20 seconds or more. It is. This is done to make ferrite appear in the austenite, where the band structure has disappeared, and to turn it into a ferrite-martensitic structure after rapid cooling.Holding for less than 20 seconds will not allow sufficient transformation, so the lower limit of time must be set. It was set to 20 seconds. Although the upper limit is not limited, there is no need to make it extra long, since the material quality will not be improved and production efficiency will be lowered even if it is made longer. In addition, the temperature needs to be below the Ar ₃ transformation point where ferrite appears, and the lower limit is below the Ar ₃ transformation point, since a mixed structure consisting of ferrite and martensite cannot be obtained after rapid cooling if it is below the Ar ₁ transformation point. Limited to Ar ₁ transformation point or above. Note that cooling in the second step after the completion of the first step may be carried out by any means, but in order to promote the production of ferrite, it is preferable to cool at a high rate in the sense of increasing the driving force for transformation. . Next, the third step is ferrite austenite 2
Cooling is performed from the phase zone, and the cooling rate is determined in order to obtain high strength and good ductility. That is, if the cooling rate is less than the critical cooling rate CR (° C./sec) determined by equations (1) and (2), a so-called ferrite-pearlite structure is formed and high strength cannot be obtained. On the other hand, if the cooling rate is equal to or higher than CR (°C/sec), a ferritic/martensitic structure containing bernite in a portion will be formed, resulting in high strength and good ductility.
The reason for limiting the cooling rate in the temperature range of 600 to 300℃ in rapid cooling is that when cooling from the ferrite-austenite two-phase region, if the cooling rate at temperatures below 600℃ is slow, bainite transformation or diffusion type transformation will occur. This is undesirable because the balance between strength and ductility is lost, and if it is not rapidly cooled to 300°C or less, which is sufficiently lower than the Ms point, bainite transformation or diffusion type transformation occurs, which is also unfavorable for the balance between strength and ductility. As described above, the present invention continuously heat-treats a steel plate with an appropriate alloy composition by first heating and maintaining the steel plate in a temperature range above the AC ₃ transformation point to eliminate the austenite single phase and band structure, and then converting it to Ar ₃ transformation. It is maintained in the ferrite-austenite two-phase region below the Ar ₁ transformation point, and then rapidly cooled to create a mixed structure consisting of ferrite and martensite (including a part of bainite) without a band structure, which improves bending workability. It is possible to produce a steel plate with good ductility and good ductility. The heat treatment cycle of the present invention is schematically shown in FIG. 1 and includes the following three stages. ()...Keeping the austenite single-phase region (1st step) ()...Keeping the ferrite-austenite two-layer region (2nd step) ()...Quick cooling (3rd step) Example 1 Finishing the steel with the composition shown in Table 1 Rolling temperature 830~870
℃ and a coiling temperature of 500 to 530℃, followed by cold rolling to form a cold rolled steel sheet with a thickness of 1.2 mm, as shown in Figure 2.
The seeds were subjected to heat cycle heat treatment. That is, A is annealing in the ferrite single phase region, and B is ferrite/
C is annealing in the austenite two-phase region, and C is annealing in the austenite single-phase region, and these are both comparative examples. On the other hand, D is a heat cycle that satisfies the conditions of the present invention.

[Table] After being held in the single-phase region for 30 seconds, it was cooled at a rate of 2°C/sec, then held in the ferrite-austenite two-phase area for 30 seconds, and then cooled at a rate of 30°C/sec. The mechanical properties of these heat-treated steel sheets were investigated and the results are shown in Table 2. Note that the critical bending radius is the minimum bending radius that does not cause cracking, and the notched tensile elongation was performed by machining a 2 mm V notch on a JIS No. 5 test piece at GL = 50 mm. From Table 2, it can be seen that the examples of the present invention have excellent ductility, good bendability, and excellent notch tensile properties. Example 2 Steel No. 2 in Table 1 was heated to 1.2 mm in the same manner as in Example 1.
It was rolled into a cold-rolled steel plate with a thickness of mm. This steel plate was held at 900℃, which is the austenite single phase region, for 30 seconds,
After that, the holding temperature T in the second step was set to 800, 750, 700,
The samples were cooled to four temperatures of 650°C at a rate of 5°C/sec, held at each holding temperature T for 200 seconds, and then cooled to room temperature at a rate of 40°C/sec. The tensile properties and bendability of these steel plates were investigated and are shown in Figure 3. In addition, 900

[Table] A steel plate that was held at ℃ for 30 seconds and then immediately cooled at a rate of 40 ℃/sec was also investigated for comparison, and the results are shown in Fig. 3 with a ◎ mark. In Figure 3,
It can be seen that the holding temperature T in the second step was held at 700 and 750°C, which is the ferrite-austenite two-phase region, and the material elongated and had excellent bendability. Example 3 Steel with the composition shown in Table 3 was made into a cold-rolled steel plate in the same manner as in Example 1, and this steel plate was used as the steel plate shown in Figure 2 (C).

[Table] Annealing was performed using two types of heat cycles: (1) and (D).
The tensile properties and bending properties of these annealed steel plates were investigated, and the results are shown in Table 4. From Table 4, those subjected to heat cycle (D), which is the heat treatment of the present invention, are as follows:

[Table] It can be seen that in all cases, the elongation is large without deteriorating the tensile strength, and the bendability can be improved at the same time. As is clear from the many examples described above, the present invention limits the alloy composition of the steel sheet, heats and holds the steel sheet above the Ac ₃ transformation point for 3 to 60 seconds during continuous annealing after hot rolling and cold rolling, and then heats the steel sheet with Ar. Ar below ₃ transformation point Hold at above ₁ transformation point for 20 seconds or more, then cool to 600 to 300℃
A high-strength thin steel sheet with good bending workability and excellent ductility is produced by rapid cooling with a limited cooling rate in the temperature range, and finally a mixed structure consisting of ferrite and martensite without a band structure. We were able to.

[Brief explanation of drawings]

Fig. 1 is a heat cycle diagram showing the heat treatment of the present invention, Fig. 2 is a heat cycle diagram showing various heat treatments, and Fig. 3 shows the influence of the holding temperature in the second step on tensile properties and bending properties. It is a line diagram.

Claims (1)

[Claims] 1. A high-strength thin steel sheet containing C: 0.02 to 0.15%, Mn: 0.2 to 3.5%, P: 0.01 to 0.15%, Al: 0.10% or less, and the balance being Fe and unavoidable impurities in terms of weight ratio. In the manufacturing method, the steel plate is
A step of soaking for 3 to 60 seconds in a temperature range of Ac ₃ transformation point or more Ac ₃ transformation point + 50℃ or less, and ferrite/austenite in a temperature range of Ar ₃ transformation point or more or Ar ₁ transformation point or more after the soaking step The process of cooling to a two-phase region and holding in that temperature range for 20 seconds or more, and the average cooling rate in the temperature range of 600 to 300 ° C during cooling after the completion of the holding process, is the critical cooling rate calculated by the following formula (1) A step of cooling at a temperature higher than CR (℃/sec),
A method for producing a high-strength thin steel plate with good ductility and bending workability, characterized by comprising: logCR (℃/sec) = -1.73 [Mn (%) + 3.5P (
%)] +3.95...(1) 2 Based on the weight ratio, C: 0.02% to 0.15% Mn: 0.2 to 3.5% P: 0.01 to 0.15% Al: 0.10% or less is the basic component, and Si: 0.1 to Selected from Group A consisting of 1.5% Cr: 0.1~1.0% Mo: 0.1~1.0% B: 5~100ppm and Group B consisting of V: 0.01~0.20% Ti: 0.01~0.20% Nb: 0.01~0.10% In the method for producing a high-strength thin steel sheet containing one or more of the following elements, the remainder being Fe and unavoidable impurities, the steel sheet is heated at a temperature range of 3 to 60° C. above the Ac ₃ transformation point and below the Ac ₃ transformation point + 50°C. a step of soaking for 2 seconds, and a step of cooling to a ferrite-austenite two-phase region in a temperature range of below Ar ₃ transformation point or above Ar ₁ transformation point after the soaking step, and holding it in the temperature range for 20 seconds or more; A step of cooling the average cooling rate in the temperature range of 600 to 300°C at a critical cooling rate CR (°C/sec) calculated by the following formula (2) during cooling after the completion of the holding step;
A method for manufacturing a high-strength thin steel plate with good ductility and bendability, characterized by comprising: logCR (℃/sec) = -17.3 [Mn (%) + 0.26Si (
%) + 3.5P (%) + 1.3 Cr (%) + 2.67 Mo (%)] + 3.95... (2) However, in the case of B addition, change 3.95 in equation (2) to 3.40.