JP2006002172A - Steel sheet for door member - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet for door member which can be formed to a target sheet thickness by cold rolling and where it requires a long period of time to drill a through hole. <P>SOLUTION: The steel sheet for door member has a composition containing, by mass, 0.4 to 1.2% C and 0.2 to 2.0% Mn and also has a pearlite structure containing ≥90 area% pearlite and having ≤150 nm lamellar spacing. Further, one or more kinds among, by mass, ≤2.0% Si, ≤1.6% Cr, ≤0.3% Mo, ≤0.3% V, ≤0.1% Nb and ≤0.007% B can be incorporated. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a steel plate for a door member which is used for a door member such as a residence, particularly a door member around a keyhole and which is difficult to drill with a tool such as a drill.

A galvanized steel sheet having a thickness of about 0.6 to 1.0 mm is used for a door member such as a residence. Galvanized steel sheet is excellent in workability and can be formed into a door shape that meets the needs, and various patterns can be applied by painting. In addition, the use of high Cr austenitic stainless steel with excellent corrosion resistance has been studied as a door member that maintains a beautiful appearance for a long period of time.
Since the door is required to have a function of preventing entry from the outside in addition to processability, designability, and durability, a lock that cannot be easily unlocked with tools is attached. Corresponding to the advancement of measures to prevent unlocking, drilling is performed using a punch and a punch that opens a hole with an electric drill in the door member around the keyhole and inserts a special tool from the perforated part to the inside of the door. Increasing the number of cases of illegal intrusion into residences by expanding the hole diameter to the extent that the arm enters the door and then unlocking it with the hand turned from the hole to the inside of the door.

  As countermeasures against the intrusion that unlocks the door from inside the door by drilling the door member, there is a method of using a material that is difficult to drill in the door outer plate, a structure in which a member that is difficult to drill is incorporated in the door wall around the keyhole There has been a strong demand for steel sheets that have been studied and have excellent drillability. Since the probability of seeing the intrusion site increases as much as it takes time to pierce the door member, at the present stage, “it takes more than three times longer than conventional materials to drill a hole” is a steel plate for door members It is a requirement standard.

There is a safe material as a material expected to be diverted to a door member steel plate. The material for safes is a material with improved hard drillability and fusing resistance, a material in which ceramics are clad between alloy steels (Patent Document 1), and a low alloy tool steel with hard drillability is austenitic heat resistant steel. A material clad in (Patent Literature 2), a material in which hard-to-cut stainless steel is built up on the surface of a hard-to-drill Mn steel plate (Patent Literature 3), and the like are known. However, all of them are expensive materials because they are made into products by cladding or build-up, and there is a problem in recycling waste materials with materials clad with ceramics.
JP-A-6-166136 Japanese Patent Laid-Open No. 5-92280 Japanese Unexamined Patent Publication No. 9-21276

  The present invention has been found as a result of investigating and examining materials with excellent drilling workability in order to protect against illegal intrusion by opening a door and unlocking from the inside. It is an object of the present invention to provide a steel plate for a door member that satisfies the required standard “It takes more than three times as long as a conventional material to make a drill hole” by adjusting the metal structure without depending on the build-up.

  The steel plate for door members of the present invention has a composition containing C: 0.4 to 1.2 mass%, Mn: 0.2 to 2.0 mass%, pearlite: 90 area% or more, and lamellar spacing: 150 nm or less. It has a pearlite structure. The steel plate for door members is further Si: 2.0 mass% or less, Cr: 1.6 mass% or less, Mo: 0.3 mass% or less, V: 0.3 mass% or less, Nb: 0.1 mass % Or less, B: 0.007 mass% or less can be included. Both P and S as impurities are adjusted to 0.05 mass% or less.

We investigated and examined the relationship between the steel grade and the requirement standard of "It takes more than 3 times longer than conventional materials to make a drill hole" and found that the same steel grade is softer than bainite and tempered martensite. It was excellent in difficult drilling workability. Pearlite is a structure obtained by appropriate management of hot rolling conditions, and does not require a heat treatment step like bainite or tempered martensite. Moreover, since the hardness is relatively low, it is also advantageous that it can be formed into a predetermined shape by cold rolling or cold working.
The pearlite structure is a lamellar structure in which ferrite and cementite are arranged in layers, and hard cementite exhibits high resistance to drill penetration. Although the drillability is improved as the pearlite area ratio is increased, the drillability is further improved by densifying the lamellar structure of ferrite / cementite.

The densification of the lamellar structure is expressed by the cementite lamella spacing, and when the lamella spacing is 150 nm or less, the time required for drilling is dramatically increased. The relationship between the lamella interval and the drilling time can be explained as follows.
Pearlite is a lamellar structure in which layered ferrite phases and cementite (Fe ₃ C) are alternately arranged. In the same steel type, the lower the transformation temperature, the narrower the lamellar spacing becomes, and both ferrite and cementite become thin and hard. Cementite has a hardness of about 1200 HV, which is harder than the cutting edge of the drill that comes into contact with the hole, and acts as a resistance against drilling. However, since it is brittle, if it cracks due to contact with the drill, it tends to brittle. Therefore, when the drill blade advances in parallel with the thickness direction of cementite, it is thinner and more cementite (in low temperature range) than the structure in which thick and few cementite (generated by pearlite transformation in high temperature range) is dispersed. The structure in which the pearlite transformation is distributed) has a greater load against drilling and improves drillability.

Hard cementite is usually brittlely cracked without plastic deformation, but exhibits plastic deformation behavior when it is very thin at 25 nm or less. When plastic deformation of cementite is possible, the contact surface of the drill blade is covered, and the contact frequency between the drill blade and cementite increases, which also improves difficult drilling workability.
Considering the relationship between the thickness of hard cementite and the fracture behavior, a pearlite structure with thin cementite and narrow lamellar spacing is advantageous for difficult drilling workability. In particular, when the lamella spacing is 150 nm or less, the cementite thickness is 25 nm or less, and the hard cementite exhibits plastic deformation behavior, so that difficult drilling workability is improved.

In the steel sheet for door members of the present invention, the amounts of C and Mn are adjusted so that a structure of pearlite: 90% by area or more can be obtained after hot rolling.
As the C content increases, a pearlite structure tends to be generated, and 0.4 mass% or more is effective. However, since an excessive amount of C content causes excessive equipment load during cold rolling, the upper limit of the C content is set to 1.2% by mass.

Mn exhibits the effect of delaying the austenite → pearlite transformation. In order to start the pearlite transformation at the constant temperature holding temperature, the lower limit of the Mn content is set to 0.2% by mass, and in order to shorten the time until the end of the pearlite transformation, the upper limit of the Mn content is set to 2.0% by mass.
Although P and S contained in the steel plate for door members are harmful components to toughness and the like, adverse effects can be suppressed by regulating to 0.05% by mass or less.

As other components, Si, Cr, Mo, V, Nb, Ti, B or the like may be added as necessary.
Si is a component added as a deoxidizer at the molten steel stage, but as the content increases, the steel material is hardened and the elongation and drawing are reduced. In applications, it is preferable to limit the upper limit of Si content to 2.0% by mass.
Cr is a component that increases the hardenability of the steel material and increases the hardness. However, since excessive addition causes generation of bainite, the upper limit is 1.6 mass%.

Mo and V precipitate as carbides and are effective for hardening the steel, but the generated carbides do not dissolve sufficiently in the austenitizing temperature range and show a tendency to suppress the formation of pearlite. The upper limit is mass%.
Nb has the effect of refining the prior austenite grain size to elongate and improve the drawing, but the addition effect is saturated at 0.1% by mass.
Ti is an effective component for densifying the pearlite structure, but excessive addition is economically disadvantageous, so the upper limit is set to 0.2% by mass.
B is a component that suppresses the formation of ferrite and facilitates the formation of pearlite. However, excessive addition degrades hot workability and toughness, so 0.007 mass% is the upper limit.

  A steel material adjusted to a predetermined composition and composition is a steel plate having the required characteristics of a door member by hot rolling at a finishing temperature of 800 to 950 ° C., a cooling rate of 20 degrees / second or more, and a winding temperature of 600 ° C. or less. To be manufactured. Since the pearlite transformation is a transformation that occurs from the austenite phase, the finishing temperature is set in the temperature range of 800 to 950 ° C. to ensure hot rolling in the austenite region. At a slow cooling rate, a large amount of pro-eutectoid ferrite or pro-eutectoid cementite is generated and the pearlite area ratio is lowered. Therefore, a target pearlite structure is generated by securing a cooling rate of 20 ° C./second or more. Moreover, in order to densify the pearlite structure, the winding temperature is set to 600 ° C. or lower.

  A steel sheet having a pearlite structure by hot rolling under appropriate conditions exhibits excellent hard workability although the hardness is relatively low. Low hardness means that the subsequent cold rolling process has a smaller load than bainite and martensite and can be manufactured at low cost. Moreover, the lower limit plate | board thickness which can be manufactured with a normal hot-rolling installation is more than the plate | board thickness (about 1 mm or less) generally used for a door member use. Therefore, when applied to such a door member, it is necessary to reduce the plate thickness by cold rolling, but it is possible to cold-roll with a relatively small load because of its low hardness.

  In order to satisfy the required characteristics as a door member, it is necessary to form a pearlite structure of pearlite: 90 area% or more and lamella spacing: 150 nm or less by hot rolling. As the soft ferrite decreases, the difficult drilling workability improves, and the required hard drilling workability is obtained with pearlite: 90 area% or more. On the other hand, when the lamellar spacing is 120 nm or less, the influence of densification of the pearlite structure becomes remarkable, and the steel sheet becomes extremely difficult to drill.

  A steel material having the composition shown in Table 1 was melted and then hot-rolled to a plate thickness of 1.5 mm. Hot rolling was performed in the range of finishing temperature: 800 to 900 ° C., winding temperature: 500 to 650 ° C., and tempered to a pearlite structure having different hardness and lamellar spacing.

A test piece was cut out from each hot-rolled steel sheet and subjected to a drilling test. In the drilling test, a new general-purpose straight drill (JIS B4301) with a blade diameter of 10 mm, a tip angle of 118 degrees, and a groove length of 95 mm was used, and a load to hold the drill in the length direction (drill load load): 196 N was applied. Rotational speed: While rotating the drill at 600 rpm, the time (drill penetration time) required for the hole having the same size as the drill diameter was measured.
The case where no through-hole was formed even when 120 seconds passed from the start of drilling was evaluated as “no penetration”, and the drill penetration time of steel A corresponding to the current material was 17 seconds, so the drill penetration time: 60 More than a second (> 17 seconds × 3) was evaluated as “difficult drilling workability”.

  As can be seen from the results of the investigation in Table 2, pearlite: 90 area% or more, lamellar spacing: 150 nm or less of the pearlite structure test piece, any drill penetration time is 60 seconds or more, it is difficult to satisfy the required characteristics Has drillability. The pearlite area ratio was determined by observing the metal structure with a scanning electron microscope at a magnification of 500 times, classifying it into a ferrite structure and a pearlite structure, and performing image processing. The lamella spacing was also measured using a scanning electron microscope, taking pearlite metal structures at various positions at a magnification of 1500 times, measuring the lamella spacing for 10 pearlite colonies from the photograph, and lamellar spacing among the 10 data. It was obtained by averaging the five data from the smallest of.

On the other hand, steel type A with insufficient C content had a soft ferrite-based structure with little formation of pearlite. Therefore, the drill penetration time is as short as 17 seconds, and the required characteristics as a door member are not satisfied. Even when the same steel type was used, if the pearlite area ratio was too low or the lamella spacing was too large, through holes were formed in a short time that did not reach 60 seconds (test numbers 2, 5, and 12).
As is clear from this comparison, it can be confirmed that the required characteristics as the door member are satisfied by adjusting the pearlite: 90 area% or more and the lamella spacing: 150 nm or less.

Steels B, C, D, E, and F in Table 1 were hot rolled to a thickness of 3.5 mm under the conditions of test numbers 2, 4, 5, 7, 9, and 10 in Table 2, respectively, and then cold rolled. The plate thickness was finished to 1.5 mm. In any case, since the hardness was relatively low, a large hot rolling load was not required.
The test piece cut out from each cold-rolled steel sheet was subjected to the same drilling test as in Example 1, and the drill penetration time was measured. As seen in the test results of Table 3, in steel materials satisfying pearlite: 90 area% or more and lamellar spacing: 150 nm or less in the structure before cold rolling, drill penetration time: 60 seconds or more even after cold rolling The processing characteristics were satisfied. On the other hand, in steel materials that do not satisfy pearlite: 90 area% or more or lamellar spacing: 150 nm or less, holes were formed in a short time within 60 seconds, and the drilling properties were inferior (test numbers 1 and 3).

  As explained above, a steel plate having a pearlite structure adjusted to pearlite: 90 area% or more and lamella spacing: 150 nm or less has a drill penetration time three times longer than that of a conventional material used as a door member. And used as a door member that can sufficiently withstand drilling. In addition, since the hardness is relatively low, it can be formed to the target plate thickness without requiring an excessive load even during cold rolling, and processing at the time of attachment is also possible. Therefore, illegal intrusion using a special tool is prevented, and the safety of the living space is achieved.

Claims (2)

  C: 0.4 to 1.2% by mass, Mn: 0.2 to 2.0% by mass, P: 0.05% by mass or less, S: 0.05% by mass or less, pearlite: 90% by area or more And a lamellar spacing: a steel plate for a door member having a pearlite structure of 150 nm or less.   Furthermore, Si: 2.0 mass% or less, Cr: 1.6 mass% or less, Mo: 0.3 mass% or less, V: 0.3 mass% or less, Nb: 0.1 mass% or less, Ti: 0 The steel plate for door members according to claim 1, comprising one or more of 2 mass% or less and B: 0.007 mass% or less.