JPH1096031A - Manufacture of high carbon steel sheet, and manufacture of parts - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide methods of manufacture of a steel sheet, suitable for use in the manufacture of automobile safety parts against collision by a press quenching process, and the parts. SOLUTION: A steel, which has a composition consisting of, by weight, 0.20-0.40% C, 0.15-0.35% Si, 0.20-0.40% Mn, 0.20-0.50% Cr, 0.003-0.0030% B, 0.005-0.05% Ti, <=0.1% Nb, 0.05-0.10% sol.Al, and the balance Fe with inevitable impurities and satisfying 0.3×C×1/2×(1+4×Mn)×(1+2×Cr)×[1+0.24×(0.9-C)]>0.7, is hot-rolled under the conditions of a finish rolling temp. not lower than the Ar3 point and a coiling temp. of <=620 deg.C, descaled, and then heated at a temp. in the range between the Ac1 point and (Ac1 point +30 deg.C) for 1-20hr and cooled down to a temp. below (Ac1 point -30 deg.C) at <=20 deg.C/h cooling rate to undergo annealing, by which the high carbon steel sheet can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet suitable for, for example, a reinforcing material for an automobile bumper or a door, that is, a safety component for ensuring safety of an occupant in a collision accident. And a method for manufacturing a safety component.

[0002]

2. Description of the Related Art A high-strength thin steel plate is press-formed into a bumper reinforcing material for an automobile or a reinforcing material at the time of collision inside a door (also referred to as a door impact beam or a door guard bar). What was done is used. Such components (hereinafter, abbreviated as "safety components") for ensuring the safety of the occupant in the event of a car collision increase the safety of the vehicle if it is rugged, but this increases the weight of the vehicle body and reduces fuel consumption. There is a problem. However, these safety components are hardly subjected to stress during normal running, and need only be effective in the event of a collision. Therefore, the strength is set based on a concept different from that of a normal mechanical structure component that is assumed to be constantly stressed. That is, in order to reduce the weight as much as possible while maintaining sufficient strength, safety parts made of steel having the highest possible strength have been applied.

[0003] When steel pipes are applied to safety parts from the balance of strength, weight and economy, in order to increase the strength of steel,
After processing into a predetermined shape, quenching and tempering are often performed. However, steel pipes have a drawback in that their shapes cannot be freely set. On the other hand, if a thin steel sheet is press-formed, it can be formed into an arbitrary shape to some extent. For this purpose, a thin steel sheet which can be processed and has a strength exceeding 980 N / mm2 has been developed.

[0004] However, the application of a high-strength thin steel sheet is limited in formability by press forming due to poor workability, the dimensional accuracy of the processed part is poor, and the press die is severely worn. Furthermore, since the residual stress generated by molding is large, the phenomenon of cracking when left for a while in the state after press molding, coupled with the high strength of steel,
That is, delayed fracture occurs. This increases in frequency as the strength increases. In addition, it is also conceivable to use a high-C annealed thin steel sheet, form it, and then perform a heat treatment such as quenching and tempering to obtain the required strength. However, when the C content is increased, the workability is deteriorated and it becomes difficult to form into the required shape Moreover, since it is a molded product of a thin steel sheet, it is likely to be deformed during heat treatment.
As described above, many problems remain in using a thin steel sheet molded product as a safety component and increasing the strength.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a high carbon thin steel sheet for obtaining a part having excellent cold workability and a strength exceeding 1150 N / mm2 after heat treatment, for example, press quench. Another object of the present invention is to provide a method for manufacturing a vehicle safety part by quenching by press quenching in which a steel part is press-formed and constrained by a mold using the steel sheet.

[0006]

Means for Solving the Problems When a safety part is made of a thin steel sheet, in order to obtain an arbitrary shape with high precision, an annealed high carbon steel sheet is press-formed and then subjected to a heat treatment such as quenching and tempering to obtain a required strength. There is a way to get it. However, many safety components have a hat-shaped cross section or a beam-like shape with a shape close to that of a hat, and since it is a thin plate, there is a high risk of being greatly deformed by heat treatment after molding. Therefore, for thin steel sheets with a thickness of about 6 mm or less, a press quench method is adopted as a heat treatment means to achieve both the accuracy and high strength of this part shape by pressing it into a predetermined shape by press forming and then quenching while constraining it with a mold. It was decided to. Further, after quenching, tempering is usually performed. However, in order to reduce costs by omitting a process, it was considered that the quenched steel can be used as it is.

[0007] The press quench method is different from ordinary quenching,
Since the cooling rate tends to be slow, it is desirable to increase the hardenability of steel in order to obtain sufficient strength or hardness by such quenching. Elements that significantly increase the hardenability are C, Mn, Cr, Mo and the like.
Since n greatly reduces the workability of steel, it is necessary to keep its content as low as possible when considering the pressing of thin steel sheets. However, since the strength after the heat treatment is substantially determined by C, the range of the content of C is considerably limited. When a small amount of B is added, the hardenability can be improved without impairing the workability when the content of C is small.

In such a limited C content range,
First, an additive element capable of improving hardenability without impairing workability, its content range, and annealing conditions for ensuring workability were investigated. In addition, the conditions of the presquench were also examined. As a result, it has been clarified that by limiting the chemical composition range and the manufacturing method, a desired shape and dimensional accuracy can be ensured, and a high-strength safety component can be obtained. That is, the gist of the present invention is as follows.

(1) C: 0.20 to 0.40% by weight, S
i: 0.15 to 0.35%, Mn: 0.20 to 0.40%, Cr: 0.20 to
0.50%, B: 0.0003-0.0030%, Ti: 0.005-0.05
%, Nb: 0.1% or less, sol. Al: 0.05 to 0.10%,
A steel containing C, Mn, and Cr satisfying the following formula, and the balance consisting of unavoidable impurities and Fe, is hot-rolled by finish rolling at an Ar3 point or higher and winding at 620 ° C or lower, and descaled. After that, it is heated in the temperature range from Ac1 point to “Ac1 point + 30 ° C.” for 1 to 20 hours, and then “Ac1 point−
A method for producing a high-carbon thin steel sheet, comprising performing annealing at a cooling rate of 20 ° C./h or less to a temperature below “30 ° C.”.

[0010] 0.3 x C (%) 1/2 x [1 + 4 x Mn (%)] x [1 + 2 x Cr (%)] x {1 + 0.24 x [0.9-C (%)]}> 0.7 ... (2) C: 0.20 to 0.40% by weight, Si: 0.15 to 0.35
%, Mn: 0.20 to 0.40%, Cr: 0.20 to 0.50%, B: 0.0003
-0.0030%, Ti: 0.005-0.05%, Nb: 0.1% or less, sol. Al: 0.05-0.10%, C, Mn, and Cr
Is hot rolled by finish rolling at an Ar3 point or higher and winding at 620 ° C or lower, and after descaling,
1 to within the temperature range from point c1 to “Ac1 point + 30 ° C”
Heat for 20 hours, then perform annealing at a cooling rate of 20 ° C / h or less to a temperature below “Ac1 point -30 ° C”, and after cold rolling at a rolling reduction of 20% or more, at a temperature of Ac1 point or less. A method for producing a high carbon thin steel sheet, comprising annealing.

0.3 × C (%) 1/2 × [1 + 4 × Mn (%)] × [1 + 2 × Cr (%)] × {1 + 0.24 × [0.9-C (%)]}> 0.7 ... The steel sheet manufactured by the method (1) or (2) can be used as a material for various parts to be heat-treated after press forming. Among them, it is suitable as a material for safety parts for automobiles, and its manufacturing method includes the following (3).

(3) After forming the thin steel sheet manufactured by the method (1) or (2) into a predetermined shape, the thin steel sheet is heated to a temperature of 850 ° C. or more, and constrained by a mold at 500 to 450 ° C. After cooling to the temperature range at a cooling rate of 80 to 150 ° C / s, 20 to 150 ° C
Heat treatment by a press quench method of cooling to a normal temperature of 100 ° C. or less at a cooling rate of 1150 N / s.
/ Production method of automobile crash safety parts having tensile strength of not less than / mm2.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION In implementing the present invention, the form and range of conditions and the reasons for setting them will be described below. The contents of the chemical components of the following steels are all% by weight.

I, Chemical Composition of Steel C: If the content of C is less than 0.20%, sufficient strength cannot be obtained in quenching at a relatively low cooling rate such as pre-quenching. On the other hand, if the content exceeds 0.40%, workability is not improved even if sufficient annealing is performed, and it becomes difficult to form a part having a rather complicated shape and good dimensional accuracy. Therefore, the content range of C is set to 0.20 to 0.40%.

Si: added because it has a deoxidizing effect and is effective in suppressing inclusions. However, when the content is too large, the workability of the steel sheet is lowered and the surface properties are deteriorated, so that too much content is not preferable. Therefore, its content range is 0.
15 to 0.35%.

Mn: An effective and economical additive element for improving the hardenability, but has an unfavorable effect on the workability which is the object of the present invention.
0.4%. In addition, we want to utilize as much as possible as far as the processability is not deteriorated. Therefore, its content range is 0.2-0.4%.

Cr: Since hardenability can be enhanced without greatly increasing the hardness of the annealed material, Cr is added in an amount of 0.2% or more. However, too much addition still increases the hardness and deteriorates the workability, so it is limited to at most 0.5%. That is, the content range of Cr is set to 0.2 to 0.5%.

B: In the range where the amount of C is not so large as in the steel of the present invention, the addition of a small amount of B greatly improves the hardenability and hardly affects the hardness of the annealed material. use. If the content of B is 0.0003% or less, there is no effect on the hardenability improvement. On the other hand, if the B content exceeds 0.003%, the effect of the hardenability improvement is saturated and the steel becomes brittle. 0.003%.

Ti: N in steel combines with B to form a nitride of B, thereby reducing the effect of improving the hardenability of B. Therefore, in order to fully exert the effect of B, T
i is contained in a small amount. If the amount is too small, N cannot be fixed sufficiently, and if it is too large, the steel is hardened and the toughness is deteriorated.

Nb: Nb does not need to be particularly added. However, when quenching by press working, coarse parts may be formed during heating of quenching at a part having a high degree of working. Since this coarsening causes deformation during quenching, Nb is added when it is necessary to suppress coarsening. However, an excessive content degrades the workability of the steel sheet or makes the austenite grains too fine to deteriorate the hardenability, so the content should be at most 0.1%.

Sol. Al (acid-soluble Al): added for the purpose of deoxidizing steel, fixing N, and improving the yield of Ti addition. Since the addition of Ti is desirably as small as possible from the viewpoint of deterioration in workability of the plate, a large amount of Al is added to fix N. At least 0.05% sol.Al for this purpose
It must be contained, but if it is too much, it hardens the steel, so the upper limit is 0.1%. That is, the content range of sol.Al is 0.05 to 0.1%.

Of the unavoidable impurities mixed into the steel, S becomes nonmetallic inclusions, deteriorating the workability and deteriorating the toughness. Therefore, the content of S is desirably 0.015% or less.
Similarly, since N also becomes TiN or AlN and deteriorates workability and toughness, it is preferable to set N to 0.008% or less.

Also, since it is a thin plate, the size effect is small, but the cooling rate of quenching cannot be increased in order to reduce quenching and deformation while being restrained by a mold. Therefore, the content ranges of C, Mn, and Cr satisfy the following formula so that quenching can be sufficiently performed even at a low cooling rate.

0.3 × C (%) 1/2 × [1 + 4 × Mn (%)] × [1 + 2 × Cr (%)] × {1 + 0.24 × [0.9−C (%)]}> 0.7 ... II, Thin steel sheet manufacturing conditions Hot rolling finish temperature and winding temperature: In hot rolling using steel having the above chemical composition as a thin steel sheet, the finish rolling finish temperature is set to Ar3 point or higher. A temperature close to the Ar3 point is desirable. If the temperature is below the Ar3 point, not only does the deformation resistance increase, but it changes unstablely, making it difficult to control the sheet thickness. In addition, crystal grains of the steel sheet may grow abnormally. If the finishing temperature is too high away from the Ar3 point, the coil may be hardened during winding and become hard.

The winding temperature is 620 ° C. or less. this is,
This is to promote the spheroidization of cementite in the subsequent annealing process and to make it uniform. However, if the winding temperature is too low, the obtained hot-rolled coil becomes abnormally hard or the dispersion of cementite during spheroidizing annealing deteriorates, so that it is preferably up to about 520 ° C.

Spheroidizing annealing of hot-rolled sheet: Spheroidizing annealing is applied to a steel sheet obtained by hot rolling so as to have sufficient workability. Spheroidizing annealing may be performed under generally used conditions,
Particularly, in the case of the steel having the chemical composition range of the present invention, the steel is heated to a temperature range from the Ac1 point to the Ac1 point + 30 ° C. for 1 to 20 hours, and then cooled to the Ac1 point−30 ° C. at a cooling rate of 20 ° C./h or less. Conditions. This is because the combination of the winding temperature of 620 ° C. or lower and the spheroidizing annealing conditions was optimal for obtaining sufficient workability in the steel having a relatively low C content, which is the object of the present invention.

Cold Rolling and Annealing: When a thinner sheet thickness or stricter sheet thickness accuracy is required, after hot rolling, the sheet is further cold-rolled and annealed to obtain a sheet product. In this case, there is also a method of performing spheroidizing annealing after cold rolling after hot rolling,
Spheroidizing annealing is performed on a hot-rolled steel sheet before cold rolling because cold rolling becomes easy and the plastic anisotropy r value preferable for press working is improved. The spheroidizing annealing conditions are the same as in the above method.

The rolling reduction of the cold rolling is set to 20% or more. 20%
If it is less than 1, the shape of the sheet after rolling is unstable, and the strain cannot be sufficiently removed by annealing after cold rolling. It is desirable that the press workability after annealing is good.
%. After the cold rolling, annealing is performed at a temperature equal to or lower than the Ac1 point.
The method of annealing may be either a continuous annealing method or a box annealing method applied to a normal cold-rolled steel sheet. If the annealing temperature exceeds the Ac1 point, cementite will re-dissolve and the workability will be significantly reduced. If the annealing temperature is too low, the rolling distortion is not sufficiently released, so that the temperature is preferably 640 ° C. or higher.

III. Manufacturing conditions for safety parts Press quench conditions: The object of the present invention is to obtain parts having a steel plate strength of 1150 N / mm2 or more by press quench after forming. Press quench method for parts 8
Heat to a temperature of 50 ° C or higher and hold it in a mold to 500 ~ 4
After cooling to a temperature range of 50 ° C at a cooling rate of 80 to 150 ° C / s, cool at a cooling rate of 20 to 100 ° C to room temperature of 100 ° C or less.

First, a part formed from a thin steel sheet is heated to an austenite region, that is, 850 ° C. or higher. In this case, it is sufficient that the temperature reaches the austenite range. If the temperature is too high, coarsening and oxidation become remarkable.
Next, cooling with oil or the like, that is, quenching is performed while constrained by a mold. However, too fast cooling causes heat treatment distortion, and on the other hand, the cooling rate may not be increased due to the constraint of the mold. Depending on the shape, if the distortion can be suppressed by slowing the cooling rate, quenching may be performed without restraint. Also, if the cooling rate is reduced, baking will not occur and the desired strength will not be obtained. Therefore, the rate of rapid cooling from the austenite region is set to 80 to 150 ° C / s.

When the temperature falls to the temperature range of 500 to 450 ° C.,
Reduce the cooling rate and cool to room temperature. The cooling rate at a temperature lower than 500 to 450 ° C. is in the range of 20 to 100 ° C./s, because if it is too slow, the strength becomes insufficient, and if it is too fast, the strain increases. The shape may be corrected by further pressing while the temperature is below 500 to 450 ° C. and the temperature is high. In addition,
Tempering does not have to be particularly performed, but may be performed if necessary to correct when the strength is too high or to improve toughness.

[0032]

【Example】

[Example 1] Ten steel types shown in Table 1 were melted in a converter to form a continuous cast slab. After heating to 1250 ° C, finish hot rolling was started from 1150 ° C, and the finishing temperature and winding shown in the table were obtained. The hot rolling was completed at the temperature to obtain a thin steel plate with a thickness of 2 mm. The thin steel coil which has been pickled and descaled is heated in a box-type annealing furnace in a hydrogen atmosphere to the annealing temperature shown in the table at a heating rate of 50 ° C / h, and at that temperature, the time average shown in the table is obtained. Heated. Thereafter, the mixture was cooled from the soaking temperature to 680 ° C. at a cooling rate of 10 ° C./h, and then allowed to cool to room temperature. A JIS No. 13 type B tensile test piece was sampled from the obtained steel sheet, and the strength, elongation and r value were measured.

[0033]

[Table 1]

Table 2 shows the test results. In Test No. 1 using steel A, the content of C is out of the range of the present invention, and shows low ductility and high ductility, but the strength after press quench is not sufficient.
In Test No. 2 of Steel B, the content of each chemical composition was within the range of the present invention and the workability was good, but the value of the formula was out of the range defined by the present invention, which was also sufficient after the press quench. High strength cannot be obtained. In Test 4 of steel D having an Nb content exceeding the range specified in the present invention, the tensile strength was 460 N /
Exceeds mm2 and elongation is less than 30%, resulting in poor workability. Test Nos. 7, 8 and 9 are all based on steel G having a composition range defined by the present invention, but the annealing temperature is changed from Ac1 point defined by the present invention to "Ac1 point + 30 ° C."
Test No. 7 or 9 which deviated from the temperature range up to the above had a strength after annealing exceeding 500 N / mm2. Test Nos. 10 and 11 with steel H or I all have high strength, because the C content is too high. In comparison with these, the test methods 3, 5, 6 and 8 according to the production method of the present invention range, the strength is low, the elongation is large, the workability is good,
Sufficient strength can be obtained by the press quench method from the C content and hardenability.

[0035]

[Table 2]

Example 2 Six types of steel shown in Table 3 were melted in a converter to form a continuous cast slab, and the slab was heated to 1250 ° C., and rolling was started. 550 ° C
The hot rolling was completed. The pickled and descaled coil was annealed in a box annealing furnace in a hydrogen atmosphere at a soaking temperature of 740 ° C. for 12 hours, cooled from the soaking temperature to 680 ° C. at a cooling rate of 10 ° C./h, and allowed to cool. These coils were further cold-rolled to have a finished steel sheet having a thickness of 1.2 mm, and then annealed in a hydrogen atmosphere using a box-type annealing furnace. JIS13B type tensile test pieces were sampled from the annealed coil, and the strength, elongation and r value were measured.

[0037]

[Table 3]

Table 4 also shows the conditions of hot rolling, cold rolling and annealing, and the results of a tensile test of the obtained steel sheet.
As can be seen from these results, in the hot-rolled steel sheet, the r value, which is an index of the drawability of press forming, was always less than 1, but by performing cold rolling and annealing, a value exceeding 1 was obtained. And the drawability is improved.

Test No. 12 in Table 4 shows excellent press workability, but the C content is low and sufficient strength cannot be obtained after press quench. Test Nos. 13, 14 and 15 are the same steels with different rolling reductions, but those with a rolling reduction of 60% have better r-value and elongation than other 30% or 85%, It can be seen that there is an optimal value for. Test number 1
Test numbers 7, 18 and 19 are the same steels with different annealing temperatures after cold rolling, but the annealing temperature exceeds the A1 point.
In the case of 19, the r value and elongation decrease, the strength increases, and the workability is poor. In Test No. 21, the strength was high and the elongation was low, because the C content exceeded the range defined by the present invention.

[0040]

[Table 4]

Example 3 Steel F shown in Tables 1 and 2
A part (a) having the shape shown in FIG. 1 was formed by press working as one model of a part for automobile crash safety using a steel plate prototyped in Test No. 6. This molded product is heated to 880 ° C in a mildly carburizing inert atmosphere, put into a mold of the same type as in press molding, and injecting coolant into the mold while applying a holding pressure of 20 N / mm2. And quenched. The cooling rate is
The primary and secondary cooling rates were varied by primary cooling up to 490 ° C., secondary cooling below which temperature and by changing the coolant temperature and injection rate. After such a press quench, tempering was performed at 180 ° C. for 20 minutes.

As shown in FIG. 2, the "warp" and "twist" of the molded article were measured before and after the heat treatment, and the magnitude of the strain caused by the heat treatment was measured. Further, a JIS No. 13 type B tensile test piece was sampled from the vertical wall portion at the center in parallel with the longitudinal direction, and the strength was measured.

Table 5 summarizes the heat treatment conditions of the press quench, the magnitude of the strain, and the results of the tensile test after the heat treatment. As is clear from the comparison of these results, when the cooling rate is too fast, the strength is large but the strain due to the heat treatment is large. When the cooling rate is too slow, the strain is small but the strength is insufficient. Thus, by limiting the steel composition and controlling the annealing conditions after hot rolling, a high-carbon thin steel sheet excellent in press workability can be obtained, and further, after press forming using such a steel sheet, cooling. By performing the press quench with the speed within the range defined by the present invention, distortion is small and 1150
It can be seen that an automobile safety component having a sufficient strength exceeding N / mm @ 2 can be obtained.

[0044]

[Table 5]

[0045]

The present invention provides a method of manufacturing a thin steel sheet which is extremely suitable for use after being subjected to heat treatment after press forming, such as a safety part for an automobile, and a method of press-forming the thin steel sheet and performing heat treatment by a press quench method. The present invention provides a method for manufacturing a part having a required shape, dimensional accuracy, and strength, and is extremely significant in practical use.

[Brief description of the drawings]

FIG. 1 is a view showing the shape of a part molded as one model of a safety part for an automobile in a collision, (a) is a front view,
(B) is a bottom view and (c) is a side view.

FIG. 2 is a diagram showing locations where strain of a molded part is measured after heat treatment.

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Hiroyuki Nakagawa 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Osaka Sumitomo Metal Industries, Ltd.

Claims (3)

[Claims] (1) C: 0.20 to 0.40% by weight, Si: 0.
15-0.35%, Mn: 0.20-0.40%, Cr: 0.20-0.50
%, B: 0.0003 to 0.0030%, Ti: 0.005 to 0.05%, N
b: 0.1% or less, sol. Al: 0.05 to 0.10%, C, M
A steel having a content of n and Cr that satisfies the following equation, with the balance being inevitable impurities and Fe, hot rolled by finish rolling at an Ar3 point or higher and winding at 620 ° C or lower,
After descaling, it is heated in the temperature range from Ac1 point to “Ac1 point + 30 ° C” for 1 to 20 hours, and then “Ac1 point −30 ° C”
A method for producing a high-carbon thin steel sheet, wherein annealing is performed at a cooling rate of 20 ° C./h or less to a temperature lower than. 0.3 × C (%) 1/2 × [1 + 4 × Mn (%)] × [1 + 2 × Cr (%)] × {1 + 0.24 × [0.9-C (%)]}> 0.7 2. The weight ratio of C: 0.20 to 0.40%, Si: 0.
15-0.35%, Mn: 0.20-0.40%, Cr: 0.20-0.50
%, B: 0.0003 to 0.0030%, Ti: 0.005 to 0.05%, N
b: 0.1% or less, sol. Al: 0.05 to 0.10%, C, M
A steel having a content of n and Cr that satisfies the following equation, with the balance being inevitable impurities and Fe, hot rolled by finish rolling at an Ar3 point or higher and winding at 620 ° C or lower,
After descaling, it is heated in the temperature range from Ac1 point to “Ac1 point + 30 ° C” for 1 to 20 hours, and then “Ac1 point −30 ° C”
High-carbon, characterized by annealing at a cooling rate of 20 ° C./h or less until the temperature falls below 20 ° C., and then annealing at a temperature of Ac1 point or less after cold rolling at a rolling reduction of 20% or more. Manufacturing method of thin steel sheet. 0.3 × C (%) 1/2 × [1 + 4 × Mn (%)] × [1 + 2 × Cr (%)] × {1 + 0.24 × [0.9-C (%)]}> 0.7 3. The thin steel sheet produced by the method of claim 1 is formed into a predetermined shape, heated to a temperature of 850 ° C. or higher, and confined in a mold to a temperature range of 500 to 450 ° C.
After cooling at a cooling rate of 80 to 150 ° C./s, heat treatment is performed by a press quench method of cooling to a room temperature of 100 ° C. or less at a cooling rate of 20 to 100 ° C./s.
A method for producing a vehicle safety component having the above-described tensile strength.